Chapter 9 General principles and fracture biomechanics
Introduction#
Fracture biomechanics can be tough-going for most candidates and yet it is definitely an A-list topic.
Textbook chapters can be too complicated and detailed to understand whilst short note sections may appear incomplete as biomechanical assumptions have not been fully explained or the brevity of the notes makes them difficult to fully understand, nevermind encouraging any higher-order thinking.
We hope this chapter uncomplicates a difficult area of the syllabus that a lot of candidates find offputing.
Structured oral examination question 1#
IM nail biomechanics
This is a radiograph of a broken femoral nail that was used to fix a distal femoral shaft fracture (Figure 9.1). How can you prevent nail breakage? CANDIDATE 1 : I would insert the largest diameter solid nail that is available for use.
Not a good start. The answer isn’t particularly well thought out and is not scoring the candidate any marks.
Why would you use a solid nail? Most nails used in orthopaedics are hollow.
Because a solid nail will be stronger than a hollow nail. This is continuing on with a poor choice of imprecise terms and more importantly the candidate is missing scoring opportunities.
What do you mean by the term ‘stronger’?
Strong is the ability of a material to resist deformation. This is an incorrect definition and the vi vais going nowhere.
How does this relate to ultimate tensile strength (UTS)?
UTS is the highest stress observed on the stress versus strain diagram while the failure strength is the stress value at which the material eventually fails. Strong is an imprecise biomechanical term. A strong material has a high ultimate tensile strength. [Silence ...] Can I retract my last few statements and say that the torsional and bending rigidity of a solid nail will be greater than that of a hollow nail? The candidate is not geting past the opening questions Better to discuss factors predisposing to nail breakage. This will then lead onto discussions about area and polar moments of inertia, the biomechanical benefits of using larger-diameter nails for long bone fractures, the solid versus hollow nail dialogue, benefits of IM reaming, etc. Another road to journey down is patient factors that could predispose to nonunion and eventual nail breakage(smoking, alcohol, malnutrition etc) (Score 4.) CANDIDATE 2: An unstable fracture pattern (segmental or comminuted) or the use of a small unreamed diameter nail increases the risk of nail breakage. Distal femoral fractures have a high incidence of intramedullary nail breakage, especially if the fracture has been produced by high-energy trauma and the patient encouraged to weight bear early. Early weight bearing with delayed fracture healing increases the time over which cyclic stress may act to cause fatigue failure and nail breakage.
Technical errors in nail insertion, such as scoring the nail during locking, may weaken the nail or create stress risers. Excessive impaction during nail insertion duet o under-reaming of the medullary canal may also weaken the nail.
Intramedullary nails rarely break when no locking screws are used. Statically locked nails can produce high concentrations of stress at the proximal or distal end of the nail, predisposing it to breakage. Nail design changes such as increased material thickness around screw holes, cold forming which increases material strength reduce risk of nail breakage.
(Score 6.)
What do you mean by fatigue failure?
Failure of a material with repetiv e loading below the ultimate tensile strength.
What do we mean by S-N curves?
As mentioned cyclic loading of an object can result in fatigue failure. An S-N curve gives information about the number of cycles a material can endure for a given stress level.
Can you drawout an S-N curve?
(Figure 9.2)
What makes fatigue failure more likely?
In the presence of stress risers such as a hole, a sharp edge, an indentation, notch or scratch, the loads can go beyond the normal limit that is localized in that area. This can lead to crack propagation.
Why?
Crack propagation.
What do you mean by this?
Sorry, I am not sure. Score 6. For a score 8: The applied stress concentrates on residual material beneath a stress riser and means the number of cycles to failure is much lower than the fatigue strength. All stress risers greatly weaken a structure, stress risers (stress concentrators) produce increased local stresses that may be several times higher than those in the bulk of the material and may lead to local failure. Methods to produce a shorter working length(reaming for larger nail). Using a stainless steel rather than titanium nail (higher Youngs modulus) but long list of pros and cons for each material.
Figure 9.1 Lateral radiograph of left distal femur demonstrating a broken femoral nail.
Figure 9.2 S-N curve.
Structured oral examination question 2#
Area and polar moment of inertia
What do we mean by the terms second moment area and polar moment of inertia?
The second moment area of inertia provides a measure of how the material is distributed in the cross-section of an object relative to the load applied to it. The further away the material is from the centre of a beam, the greater its bending stiffness. The polar moment of inertia applies to a cylindrical structure and its ability to resist torsion.
How do the second moment area and polar moment area differ between a solid and a hollow nail?
For a solid nail the second moment area or second moment of inertia is directly proportional to the fourth power of the radius. I = πr4/4 For a hollow nail the second moment of area or second moment of inertia is directly proportional to the fourth power of the outer radius minus the fourth power of the inner radius. I = π(rO4 – rI4)/4 Nail wall thickness is equal to the difference between r0 = outer radius and r1 = inner radius.
Controversial topic
The assumption that if a nail has a thin wall then the inner radius is roughly equal to the outer radius, so the bending rigidity approximates to the third power of the outer radius, is inaccurate.
Hollow orthopaedic implants do not approximate to rO3.
In engineering, approximations are made for ‘thin’ cylinders, which is when the inner radius (r1)
approaches the outer radius (r0). The rO4 – rI4 term approaches rO3 as rOrI, hence the approximation.
However, in the case of orthopaedic implants, using the r03 approximation is inaccurate. Using a cylinder that thin has no practical use because of the forces it must resist. The wall thickness of a hollow nail must be optimal to be able to withstand bending stress and avoid sudden failing by buckling (local concentrated deformations). The implant also starts behaving as a curved sheet rather than a hollow cylinder .
A Synthes 13 mm nail (6.5 mm radius) has a 1.2 mm wall thickness. r0 = 6.5 mm, rI = 5.3 mm (which is
6.5 – 1.2). So (rO4 – rI4) = 1785 – 789 = 996. That’s the true value of the (rO4 – rI4) term, the true coefficient for moment of inertia Approximating r O3 is 6.53 = 275. 996 is 3.6 × 275, so to estimate a hollow nail to rO3 is a 360% underestimate which is quite significant.1
For a solid nail the polar moment of inertia (polar moment area) varies with the fourth power of its radius.
Jo = πr4/2
For a hollow nail the polar moment of inertia varies with the fourth power of the outer radius minus the inner radius.
Jo = π(rO4 – rI4)/2
Which type of nail, solid or hollow, biomechanically do we prefer to use?
A hollow nail is more efficient as less material can be used for equivalent values of bending and torsional rigidity.
Why?
The further the material is spread away from the neutral axis of the nail, the greater its rigidity against bending and torsional forces. (For score 7 candidates.)
Why are spiral fractures of the tibia more common in the lower third of the tibia even though the cortex is much thicker there?
The tibia resembles a cylinder. This site has a low polar moment of inertia even though the cortex is especially thick at this point, it has less resistance against torsional forces because of its smaller radius compared to the upper segment. Under a specific load the lower third segment will deform more than the upper segment of the tibia. The further amate rial is distributed away from the neutral axis of the structure, the greater the polar moment and therefore greater strength and rigidity against torsional stress.
(For score 8 candidates.)
What about the bending and torsional rigidity of a hollow nail with an open section?
There is very litile difference inbending rigidity between a solid hollow nail and an open nail section of the same diameter For torsional rigidity the situation is different and is a much greater affect. The difference in behaviour of open and closed nails against torsional stress is created by the circumferential discontinuity . Stress is transmift ed uniformly through the entire thickness of the cross-section of a closed-section hollow nail without any change indirection. The reis a change is stress direction in an open nail when its gap is reached with a significant fall in the magnitude of polar moment (Figure 9.3).
What about differences between the cross-sectional shape of a nail?
A cloverleaf cross-section nail improves torsional and bending rigidity compared to a round cross-section .
Why?
I am not sure.
Figure 9.3 With the open-section nail the reis reversion of the direction of torsional stress as shown by the arrows. In a closed hollow nail, the nail stress lines are in the same direction because of the continuity and therefore the nail is stronger against torsion.
Structured oral examination question 3#
IM nail
What does the picture show (Figure 9.4)?
This is a clinical picture showing an intramedullary nail. It looks as though this is a femoral antegrade nail as it has an anterior bow, it has multiple locking options proximally and it is cannulated distally. They are usually made of titanium. Be able to describe the typical features of an IM nail.
How does an IM nail function?
IM nails stabilize a fracture by acting as internal splints with load-sharing characteristics.
What do we mean by an internal splint?
Splintage is defined as a construct in which micromotion can occur between bone and implant, providing only relative stability without interfragmentary compression. Callus forms at the fracture site.
Do nails always act as a load-sharing device?
It depends on how it is used. Candidate waffle.
How is it used?
[Silence ...] In more comminuted fracture patterns that are not axially stable, a nail will have to transmit all the forces applied to the limb, so-called load-bearing. Ideally, a nail should be used as a load-sharing device, but incertain situations will be used as a load-bearing device.
Which is stiffer, a solid or a hollow nail?
Be careful with this question as it is poorly explained in some textbooks. The bending stiffness of a cylindrical cross-section is proportional to the fourth power of its radius as described by the second moment of area. In the case of a hollow cylinder, the bending stiffness is proportional to the fourth power of the outer radius minus the fourth power of the inner radius. As such, for any given material a hollow cylinder isless stiff than a solid cylinder of the same outer diameter. If, however, a constant volume of material is used for construction of an IM nail of fixed length, then the use of a hollow nail would allow a greater outer radius to be used, resulting in a stiffer nail.2
When plating a fracture what factors do you need to consider?
I would need to decide if I want to achieve primary or secondary bone healing.
If the fracture was significantly comminuted I would ideally choose to plate in bridging mode with fracture healing by indirect or secondary fracture healing with callus formation.
Simple fractures could be treated with interfragmentary compression.
Ideally, plate position should be on the tension side of the fracture. I would need to decide the length of plate itself, the number and relative position of screws needing to be inserted and the type of screws to be used (standard cortical screws, cancellous screws, locking screws, etc.).
The plate length should be 2–3 times higher than the overall fracture length in comminuted fractures and 8–10 times higher in simple fractures.
The plate screw density should be kept below a value of 0.5, indicating that less than half of the plate holes are occupied by screws.
Two screws (mono cortical or bi cortical) on each main fragment is the minimum number of screws needed to keep the plate bone constructions table. Such a construct will fail if one screw breaks due to overload or if the screw loosens, so it is generally advised to add another screw to each side of the fracture construct.
Plating offers two different fixation concepts – splinting and inter fragmentary compression.
Comminuted fractures are best treated using a splinting technique, because local bone and soft tissue de vascularization can be minimized; while in simple fractures interfragmentary compression is preferred as a stabilization tool.
When nailing the position, the length and diameter of the nail as well as the position of the locking bolts are more orless given and standardized by the local anatomy of the broken bone segment as well as the implant design (Table 9.1).
What is the working length of a plate?
The working length of a plate is defined as the distance across a fracture site between the two nearest points where the bone is fixed to the plate, e.g. the distance between the two screws closest to the fracture.
How is the working length of a plate altered?
The working length of a plate can be altered by changing screw position Screws placed close to a fracture create a short working length, which increases a plate’s construct stiffness Screws placed further from the fracture site increase the working length and produce a less-stiff construct that permits more motion a t the fracture gap.
What problems may arise if screws are placed too close to the fracture site?
There is the potential to create stress concentration with the risk of plate fracture. Placing screws further from the fracture site can better distribute the stress the plate experiences and decrease the risk for plate fatigue failure.
The addition of more than three screws per fragment does not significantly impact on a plate’s construct stiffness in axial loading. Adding an additional screw nearest to the fracture site provides the greatest increase in axial stiffness.
Why do we want to avoid over-torqueing of screws?
Over-torqueing of the screws should be avoided during insertion. The screw head can be destroyed. It may cause screw–bone interface failure (i.e. stripping torque).
What else?
As a screw is inserted into bone, the screw head compresses the plate against the bone with a force proportional to the torque applied to the screw. If the screw is tightened beyond the ultimate strength of the bone–screw interface, the screw threads will lose purchase in the bone and the screw will spin with liti le resistance. Although pull-out strength is related to the depth of the screw thread and quality of the bone, stripping the screw reduces the pull-out strength of the screw by more than 80%.
Why do we retighten screws before closure?
Before wound closure, all screws should bere tightened to allow time for stress relaxation of the screw–bone interface.
Figure 9.4 Titanium IM femoral nail.
Table 9.1 Characteristics of fixation.
Bi cortical
Self-tapping
Self-drilling
Standard cortical
Locking
Structured oral examination question 4#
Biomechanics IM nail
What is the working length of a nail?
Working length is defined as the length of a nail spanning the fracture site from its distal point of fixation in the proximal fragment to its proximal point of fixation in the distal fragment. More simply, it is the distance between the two points on either side of the fracture where the bone firmly grips the metal. Thus, working length is the unsupported portion of the nail between the two major bone fragments and reflects the length of a nail carrying the majority of the load across the fracture site.
What is the relationship between the working length of a nail and bending rigidity?
The bending rigidity of a nail is inversely proportional to the square of its working length.
What about torsional rigidity and working length?
Torsional rigidity is inversely proportional to a nail’s working length. If all else fails, a shorter working length means the greater the bending and torsional rigidity of a nail, i.e. a stronger fixation.
What factors affect working length?
The working length of a nail can be very variable depending on: Type of force (bending, torsion). When the bone bends at the fracture site the nail may become fixed to the bone by 3 point fixation. Type of fracture (fracture pattern) and if the fracture is reduced. Interlocking. This modifies the working length of a nail and increases torsional stability. Reaming. This prepares a uniform canal, allows a larger diameter nail to be used, improves nail/bone fixation and reduces the working length of the nail. A nail has a shorter working length inbending with fixation of a transverse fracture than when used to stabilize a comminuted fracture.
What affects bending rigidity?
Bending rigidity is affected by: 1. Material properties Young’s modulus of elasticA cobalt chromium nail has twice the bending stiffness as that of titanium. 2. Structural properties. (a) Length. (b) Second moment area (SMA) of the nail, which is a variable that describes the spatial distribution of amate rial within a structure. The SMA is affected by the organization and shape of the material.
For a solid circular nail, the bending rigidity is proportional to the fourth power of the nail’s radius.
SMA = π.r4/4
For a hollow nail the bending rigidity is very roughly proportional to the third power of the nail diameter.
Are you sure (see above)?
It is more accurate to say the bending rigidity is directly proportional to the fourth power of the outer radius minus the fourth power of the inner radius. SMA = π. (r40 – r14)/4 where r0 is the outer radius and r1 is the inner radius.
What affects torsional rigidity?
1. Material properties. 2. Structural properties. Torsional rigidity is proportional to the fourth power of the nail diameter. As lofted nail has a torsional rigidity of 1/50 that of a non-sloft ed nail.
And?
A nail with sharp corners or fluted edges resists torsional forces to a greater degree than a smooth-walled nail.
What is the difference between second area moment and polar moment for a nail?
The second moment area and polar moment area represent the relationship between bending and torsional rigidity of a nail and its cross-sectional dimensions. The greater the material is distributed away from the neutral axis of the structure, the greater is its area and polar moment and thus the strength and rigidity against bending and torsional stress. This may lead on to the examiners asking about the differences in second area moment and polar moment for a solid and hollow nail (see above). As mentioned previously, this is often poorly or inaccurately described in books or short notes bullet revision texts.
What happens to polar moment of inertia when a nail has a slot?
The polar moment of inertia is greatly reduced.
What about length of the nail, how does that affect bending rigidity?
The length of the nail between the forces working to bend it determines the length of the moment arm and therefore the magnitude of the bending momentA similar situation applies in cases of torsional forces.
How is bending and torsional stiffness related to working length?
The bending stiffness of a nail is inversely proportional to the square of its working length. The torsional stiffness is inversely proportional to its working length. The shorter the working length, the greater is the bending and torsional stiffness (rigidity) of the nail in the construct and the stronger the fixation.
How does medullary reaming affect working length?
Medullary reaming prepares a uniform canal and improves nail–bone fixation towards the fracture, thus reducing the nail’s working length.
What do you mean by stiffness?
Stiffness is defined as the slope of the curve in the elastic range on a stress–strain curve.
Are you sure?
Yes. The slope of the curve in the elastic range on a stress–strain curve is Young’s elastic modulus for a material. Stiffness is defined as the slope of a force versus displacement graph. Elastic modulus is the corresponding slope, but of a stress versus strain graph. Load is converted to stress and displacement to strain.
What material are IM nails made of?
They are made of either titanium or stainless steel. IM nails can be solid or hollow.
How can we reduce the stiffness of a nail?
One way of reducing stiffness ist o put a longitudinal slot in the wall of a nail. This makes it much more flexible, but does so at the cost of the nail losing overall bending and torsional strength. The slot allows the cross-section to be compressed when inserted into the medullary canal. Very stiff nails may damage the bone if there is any discrepancy between the shape of the nail and that of the bone.
What factors alter a nail’s axial, bending and torsional rigidity?
This can be divided into material and structural properties Parameters include cross- sectional geometry, nail length, the presence of a longitudinal slot and the elastic modulus of the material.
Structured oral examination question 5#
IM nails
Initial questions on area and polar moment of inertia, bending and torsional rigidity (see above).
How does the presence of a slot affect bending and torsional rigidity of a nail?
The presence of a slot reduces both the bending and torsional stiffness of a nail. A slot significantly affects torsional rigidity, but has much less effect on bending stiffness.
Why do we uses lofted nails?
It makes the nail easier to insert.
What else?
Err ... When a solid nail is introduced into the medullary canal it makes room for its entry by compression of the surrounding bone tissue. Bone tissue exerts an equal and opposite force on the nail (Newton’s third law). EXAMINER : What about hoop stresses? Sorry? Hoop (expansion) stresses are generated in the bone when an IM nail is inserted. Hoop stresses are much higher when a solid or closed-section nail is introduced when compared toas loft ed nail. If hoop stresses are too high they can cause comminution or splintering of the bone.
What is the difference between stiffness and rigidity?
[Long silence ...] Sorry, I don’t know. Rigidity and stiffness are very similar concepts often used interchangeably to denote overall performance of a structure. There are however important distinctions between the two. Stiffness is amate rial property, a materials ability to resist deformation force divided by displacement. Rigidity is a structural property, a structures ability to resist deformation. Depends on amate rials stiffness and geometry construct. Rigidity incorporates both the type of material and its shape and size.
Why do we use interlocking screws (bolts)? What function do the y perform?
Interlocking screws help control torsion and axial loads placed on the nail. They provide rotational and longitudinal stability.
Historically, why were they introduced?
The use of interlocking screws expanded the indications for use of IM nails to include more proximal, more distal, and more unstable (highly comminuted, segmental) fractures.
What are the disadvantages of using interlocking screws?
Insertion requires reasonable technical skill to place. The holes in the nail act as stress risers. The weakest part of the nail to fatigue is a t or just proximal to the most proximal distal locking screw. There is an increased rate of nail breakage if the fracture is within 5 cm of these screws, or if the screw hole closest to the fracture is left unfilled. The closer the fracture is to the distal locking screws, the less cortical contact the nail has, which leads to increased stress on the locking screws and greater chance of screw breakage. The further the distal locking screw is from the fracture site, the more rotationallys table the fracture becomes because of friction of the nail within the medullary cavity and less chance of screw breakage. Screw holes closer to the end of the nail allow for the fixation of more proximal or distal fractures, but at the expense of stability of the construct.
When nailing a long bone, how do you decide on how many locking screws to use?
The number of interlocking screws used is based on fracture location, amount of fracture comminution, and the fit of the nail within the canal. Midshaft transverse femoral fractures have the greatest fixation stability because of isthmic cortical contact. Oblique and comminuted fractures rely on interlocking screws for stability, as do very proximal and very distal metaphyseal fractures, where the medullary canal widens and is filled with weaker cancellous bone. In general, one screw is sufficient for stable fractures.
How are locking screws (bolts) different to other screws used for fracture fixation?
Locking bolts have a wide core diameter and smaller outer thread diameter. The screw functions to reduce torsional stresses acting on the nail and is not designed to maximize pull-out strength.
What may occur if you place too many screws in multiple planes through a nail?
An IM nail allows fracture healing with relative stability even if a nail is statically locked. Minor movements occur between the nail and screw even in a static mode of nail fixation. Placing screws in multiple planes may lead to a reduction of this fragment toggle and hinder fracture healing.
What is the biomechanical effect of the orientation of the locking screw?
There are some studies that suggest oblique tibial locking screws increase the stability of proximal tibial fractures compared to transverse locking screws. Distally, however, there is litile difference. In the femur, studies suggest no difference in biomechanical behaviour with locking screw orientation.
What effect does multiple locking screw breakage have on fracture stability?
Premature failure of locking screws especially with unstable fracture patterns may lead to angulation shortening, malunion and IM nail migration.
How can you reduce the risk of locking screw failure?
Compared tousing a smaller screw a larger screw diameter increases fatigue resistance. A stainless steel screw has a different fatigue life than a titanium screw. Stainless steel is more ductile than titanium. Thread design and defects from the manufacturing or insertion process can also contribute to breakage. Some locking screws (bolts) may have surface defects caused by the machining of threads that could act as notches and contribute to the variability of fatigue life by a stress riser effect. Inserting a screw incorrectly can result in surface defects as well.
Why not insert the largest locking bolt possible to reduce the risk of locking bolt failure?
The largest diameter of locking bolt that can be used is limited by the diameter of the nail. Increasing the diameter of a locking bolt reduces the cross-section of the nail a t its hole, thereby predisposing to nail failure. Nail hole size should not exceed 50% of the nail diameter. Interlocking screws undergo four-point bending loads, with higher screw stresses seen at the most distal locking sites (Figure 9.5). Under axial load, and in the absence of cortical contact, bending of the screw and screw failure may occur.
Figure 9.5 Four-point loading on distal interlocking bolts. Four-point loads (arrows) acting on a distal interlocking screw.
Structured oral examination question 6#
Plates
The candidate is shown a laminated picture of a plate and asked to describe.
What is this?
This can be difficult to answer without having a pre-structured approach to the question. With a structure and an earlier rehearsal, it is relatively straighfoorward. Think in terms of: 1. Shape (semitubular, one-third tubular). 2. Width of plate (small, narrow, broad). 3. Shape of screw holes (round slots, oval slots). 4. Surface contact characteristics (L CP). 5. Intended site of application condylar plate). 6. According to the function (locking plate, but iress, neutralisation, bridging , compression). 7. Material (stainless steel titanium).
What happens to the moment of inertia if you place a rectangular beam 2 × 4 on its edge rather than on its side?
It is ‘stronger’ inbending when placed on its edge (2ʹʹ side) than on its flat (4ʹʹ) side, yet its cross-sectional area remains constant. A 2ʹʹ × 4ʹʹ beam on its edge has an area moment of inertia f our times greater than on its side and thus demonstrates a fourfold increase in rigidity. More simply put, for a rectangular beam where length, width of thickness is different, the area moment of inertia changes with the change in the orientation of beam with the plane of loading. In a bending mode of loading, plate strength and stiffness are dependent not only on cross- sectional area but also on the arrangement or distribution of material mass about the neutral axis (shape). The examiner is exploring whether a candidate understands basic biomechanical principles. The practical relevance is that when fixing a fracture, the bone surface chosen to apply a plate can affect mechanical construct stiffness. The bending stiffness of a bone plate is proportional to the thickness of the plate to the third power, whereas the bending stiffness is directly proportional to the width or elastic modulus of the plate. Therefore, changing the plate thickness has more effect upon stiffness than changing the plate width or material. A plate made of one material can have significantly different properties depending on its width, length, thickness and position of holes. The area moment of inertia (Ia) quantifies the bending resistance (stiffness) of a given cross- section, the larger the area moment of inertia, the greater the bending resistance, the less the stress produced within a structure under a given bending force.
This is expressed as the base (b) times the height or thickness cubed over 12.
Ia = bh3/12
Bending stiffness = EI a, where E is Young’s modulus, Ia second moment of area.
Candidates need to understand how the properties of a plate can change depending on its position on the bone, where screws are placed, and the loads it experiences indifferent areas of the body.
If you are plating a humerus fracture when would you use a 3.5-mm or 4.5-mm thickness plate?
The bending stiffness of a plate is proportional to the thickness of a plate to the third power. Therefore, the bending stiffness of a 4.5-mm plate will be more than twice the bending stiffness of a 3.5-mm plate (3.5 = 42,875, 4.5 = 91,125). Because of the large rotational forces placed on the humerus, it is best to use a 4.5-mm plate. The 4.5-mm plate is more staggered for screw placement compared to the 3.5-mm plate, which has a narrow area for screw insertion. This reduces the risk of postoperative fracture. In general, 3.5-mm plates should only be used in the supracondylar region unless the bone size is extremely small and there would be a worry that the screw diameter would be greater than 30% of the bone’s diameter of introducing a stress riser that will predispose to a postoperative fracture. The bone shape of the humerus is not flat and not plate-friendly and it can sometimes be difficult to fit a 4.5-mm plate onto the humerus. However, for examination purposes candidates should err on the side of caution and suggest usage of a 4.5-mm plate.
What causes a bone to break after fracture fixation?
There are two mechanisms. The first is a general difference between the stiffness of the plate and stiffness of the bone, i.e. ami smatch between the plate and bone stiffness. These cond reason is that the end of a plate can create an abrupt transition between the metal and bone resulting in a modulus mismatch that can lead to a stress riser. That is why some surgeons prefer to leave the last screw hole in a plate empty. This is particularly concerning if the end of the plate is in a high-stress region such as the subtrochanteric part of the femur. In this situation a longer plate should be used to bypass the high-stress area, especially if bone quality is poor.
What do we mean by working length of a plate?
The distance between the proximal and distal screw in closest proximity to the fracture is defined as the ‘working length’ of the plate.
Why is working length important?
Plate working length has been shown to influence construct stiffness, plate strain and cyclic fatigue properties of the plate.
What are the principles of plate fixation?
The principle is the conversion of tensile forces to a compression force on the convex side of an eccentrically loaded bone. This is achieved by placing a tension band (bone plate) across the fracture on the tension (or convex) side of the bone. Tension forces are counteracted by the tension band in this position and converted into compressive forces. The plate should be fixed to the tension side of a long bone to avoid fracture gapping. Plate stresses are significantly increased by gapping at the fracture site and may lead to fatigue failure of the plate. When a gap is left on the cortex opposite that to which the plate is attached, bending of the plate at the fracture site can cause the plate to rapidly fail. Gapping may also occur if a plate is not properly contoured during application. Slight prebending of a plate causes the ends of the opposite cortices to be driven together when the plate is applied. When apre bent plate is used, the inner screws are applied first and then the outer screws. Torsional and bending stiffness of a fracture construct can be significantly increased, and therefore, plate strain reduced, by increasing the length of the plate itself.
What about leaving a fracture gap opposite the plate?
This makes the plate a fulcrum and leads to increased stress at plate holes. The candidate has described the principles of tension band plating. As long bones are subjected to eccentric loading, a plate applied to the outer (convex) side counteracts tension forces and provides rigid internal fixation.
What happens if a plate is applied to the tension cortex, but the opposite cortex is defective?
The defective cortex cannot resist compression and the plate will undergo bending stresses and fail under axial load.
How do plate length and screw number affect the biomechanical stability of a plated construct?
The literature is slightly confusing in how plate length and screw number affect the biomechanical stability of a plated construct. There has been a trend towards fracture fixation using longer plates and fewer screws. Concerns that the plated construct would not provide sufficient construct stiffness and fracture rigidity compared with fixation using shorter plates and more screws have not been realized. The use of additional screws to a plated construct has the adverse effects of decreased bone vascularity, increased stress shielding, and possibly an increased risk of stress fracture.
What are the biomechanical differences between an IM nail and a plate?
This is a very broad question that ideally needs a well-thought out, comprehensive answer. Start off by first mentioning the function of both an IM nail and a plate . Simplify your answer by sticking to a non-locking plate, otherwise mentioning a locking plate may make your answer too complicated and confusing (Figure 9.6). A nail’s cross-section is round, resisting loads equally in all directions, where asa plate’s cross- section is rectangular, resisting greater loads in one plane compared to another (Figure 9.7). There are four main types of load acting on an IM nail. 1. Compression. 2. Tension. 3. Torsion. 4. Bending. Physiologic loading is a combination of all these forces. The primary function of the plate is to maintain alignment as an internal splint, and to create compression between the fracture ends such that bone can transfer some of the applied loads itself. A compression plate, tension bandora lag screw does this by generating compression across the fracture. Infixation constructs in which the plate–bone system can carry load, the compressed fractured bone carries a major part of the load.
Figure 9.6 Biomechanical differences between a plate and IM nail. Compared to an IM nail the bending moment (Force × Distance from force to implant) for a plate is greater due to the force being applied over a larger distance.
Figure 9.7 Biomechanical comparison of plate vs. IM nail. A nail’s cross-section is usually round, resisting loads equally in all directions. A plate’s cross-section is rectangular, resisting greater loads in one plane vs. the other.
What factors contribute to the overall biomechanical profile of an IM nail?
Sorry? Several factors contribute to the overall biomechanical profile and resulting structural stiffness of an IM nail. These include: Material properties: most nails are made from either stainless steel or titanium. Titanium nails are less stiff than stainless steel with a lower elastic modulus (closer to bone). Cross-sectional shape . Nail diameter affects the bending and torsional rigidity of a nail. For a solid circular nail, the bending and torsional rigidity is proportional to the fourth power of the nail radius. A larger diameter nail with the same cross-section is both stiffer and stronger than a smaller one. Diameter curves. Nails are contoured to accommodate IM curvature of long bones. Tibial nails have an 11° bend in the AP direction a t the junction of the upper third and lower two-thirds. It is called the angle of Her zogAn anterior bow determines how easily a femoral nail can be inserted as well as bone/nail mismatch that inturn influences the stability of fixation of the nail in the bone. A mismatch in the radius of curvature between the nail and the femur can lead to distal anterior cortical perforation. Length and working length (see previous questions).
What are the benefits of reamed nails?
Reaming allows the insertion of a lar ger-diameter nail that provides more rigidity inbending and torsion. It reduces the working length of the nail and improves stability.
Reaming increases the contact area between the nail and cortical bone.
You mentioned IM nails being curved. What is the reason for this?
IM nails are contoured to accommodate the curved intramedullary canal of a long bone.
Anything else?
A straight nail if inserted into a curved intramedullary canal will bend and produce stresses that may fracture the bone. Nails should be contoured to accommodate the curves.
Anything else?
A femoral nail is curved in an AP direction to conform to the curvature of the medullary canal. When fully inserted, a femoral nail should fit snugly and not produce any unnecessary stress.
What happens when you insert a femoral nail into the medullary canal?
The nail must bend somewhat to fit the curve of the intramedullary canal. An axial insertional force is necessary to insert the nail and this insertional force is maximal at three-quarters of the insertional length and decreases thereafter as the nail straightens out to adapt to the shape of the medullary canal.
What about hoop stresses generated when a femoral nail is inserted into the medullary canal?
The insertional axial force applied generates hoop stresses within the bone. Excessively large hoop stresses can lead to fracture propagation.
How do we reduce hoop stresses generated in the femur?
Avoid excessive force when impacting the nail in to the femur. Introduce the femoral nail in a controlled manner avoiding the use of a hammer if at all possible. Over-reaming the entry hole by 0.5–1 mm and using as lofted nail.
Anything else?
Not sure.
What about entry point?
Not sure. The most important factor affecting hoops tresses is femoral nail entry point. Avoid an excessive anterior or posterior entry point. If the insertion point is anterior to the central axis of the femur a large insertional force is required for nail insertion. This will generate excessively large hoop (expansion) stresses than can lead to a burst fracture of the femoral shaft. With a posterior entry point there will be loss of proximal fixation and a similar but much less severe increase inhoop stress generation with nail insertion.
IM nails have a straighter (larger) radius than the femoral canal with sometimes ami smatch inradius of curvature. This can lead to distal anterior cortical perforation.
Structured oral examination question 7#
Fracture healing
Fracture healing is a definite A-list topic. With a bit of revision and practice candidates should achieve a score 6 without too many difficulties. Most questions are very similar and cover the same material with only occasional deviation in to unknown territory midway through a viva.
1. Bone healing: types, types of stability, factors affecting it, cuting c ones diagram, different implants for stability, strain theory, what a bridge plating is and how it works.
2. Fracture healing, different types and stages.
Absolute versus relative stability options for fracture stabilization, fracture healing, factors affecting it evidence – did not know!).
How do fractures heal?
Fractures can heal by either director primary fracture healing or secondary fracture healing. The answer needs expanding, as the next obvious question would be to give an example of primary fracture healing. The type of fracture healing that occurs depends on the mechanical stability present at the fracture site. Fractures treated with open reduction in which inter fragmentary compression is achieved, such as with a lag screw or with a plate placed in compression mode, will heal by primary or direct fracture healing. The fracture fixation in this situation provides absolute stability. There is no motion a t the fracture site, and no callus is formed. The fracture heals through the formation ofos teonal cuting c ones and Haversian remodelling of the compressed cortical bone. Indirect or secondary fracture healing occurs with relative stability and movement at the fracture site. This type of fracture fixation occurs with fractures when treated with a castor brace, an intramedullary nail, or a plate placed in a bridging mode.
What do we mean by Perren’s strain theory?
Perren introduced the concept of strain in fracture healing. Fracture gap strain is defined as the relative change in the fracture gap (ΔL) divided by the original fracture gap (L). Tissue cannot be produced when strain conditions exceed the tissues train tolerance. The strain of cortical bone until it breaks is low, around 2%, while granulation tissue has a high strain tolerance of 100%. Rigid internal compression fixation, which minimizes strain, will lead to primary or direct fracture healing. Lamellar bone can tolerate up to 10% strain, and when this relative stability is present, the fracture heals with callus or secondary fracture healing.
Fracture healing will not occur when the strain at a fracture gap exceeds 10%.
What actually happens at the fracture gap?
In a narrow fracture gap, a defined distracting force will disrupt the few cells within it. The same force applied to a wider gap filled with granulation tissue will, however , only deform this tissue and not cause any rupture.
How does this influence the method of internal fixation you would chose to manage a fracture?
In a simple transverse or short oblique fracture, any deforming force is acting very locally on the single fracture gap that corresponds to a high concentration of stress. In a comminuted fracture with multiple fragments the same force will be distributed over a wide range of different fracture fragments or gaps (stress distribution). Applying Perren’s strain theory, a simple fracture type has ‘high strain’ and is best fixed by a method that produces absolute stability. A more comminuted fracture equates to a low-strain situation and can be managed with fixation that provides relative stability (bridging plate or IM nail).
So, we can leave large gaps at the fracture site if we are aiming for relative stability?
Small gaps can be left a t the fracture site incomplex fractures managed with relative stability and are usually tolerated. Larger gaps are less well tolerated and may result in delayed or non-union.
How big a fracture gap?
Persistent fracture gaps of over 2 mm in the tibia shaft are associated with delayed healing. If you have gone for absolute stability, then any persistent fracture gap should be avoided as this may predispose to non-union.
What do we mean by a stress riser?
A stress riser (concentrator) is a region of an object in which stresses are higher than in the surrounding material.
What causes crack growth?
Crack growth is heightened by stress corrosion, poor bone-to-bone contact at the fracture and if apa tien t has a large body mass.
What do we mean by stress corrosion?
Stress corrosion combines the effects of local growth of the crack resulting from cyclic loading with galvanic corrosion.
What is galvanic corrosion?
Galvanic corrosion occurs as a result of an electrochemical potential created by the contact of two different metals in an electrochemical medium that causes the release of ions from the metals. Galvanic corrosion can weaken the properties of a plate and screws, causing failure of fracture fixation and possible pain ands welling of the surrounding tissue.
That’s with a plate and screws, but we are discussing an isolated stress concentration propagating and expanding on a plate leading to fatigue failure.
In a fixed fracture, the dissimilar materials are the surface of the plate (e.g. stainless steel), which creates an oxide surface coating , and the same material exposed by the fatigue cr ack that has not yet developed the oxide film. The conductive fluid is saline found in the surrounding tissues. Galvanic corrosion can accelerate the failure of an implant, even when the implant is loaded well below its yield point, by increasing the rate at which the crack grows. This occurs because in addition to the mechanical propagation a t the site of the crack, material at the crack is being removed by the corrosion process.
Why do we tap before screw insertion?
Tapping is required so that the torque applied by the surgeon is converted into compression instead of cuting threads and overcoming the friction between the screw thread and the bone. With a pre-tapped hole, around 65% of the torque goes to produce compression and 35% to overcome the friction associated with driving the screw. When the hole is not tapped, only about 5% of the torque is used to produce compression, the rest going to overcome friction and to cut threads in bone.
So why do we sometimes a void tapping in cancellous bone?
In cancellous bone screw pull-out can become an issue, particularly in osteoporotic bone. Tapping reduces strength in cancellous bone in that running the tap in and out of the hole removes bone effectively increasing the diameter of the hole and reducing the amount of bone material that interacts with the screw threads.
What factors affect screw pull-out?
This is a classic predictable screw exam question Candidates need to have rehearsed and run through their answer beforehand.
Why is it important to do a final check and tighten all screws at the end of fracture fixation?
A screw holds the plate against bone partly by frictional contact, which depends on the frictional force generated between the undersurface of the plate and the bone. If any sliding occurs between the plate and the bone, the bending load will be transferred from the head of the screw into the plate, where screw plate contact occurs. Bending loads perpendicular to the axis of a screw, along with possible stress corrosion and freting corrosion, may cause a screw to fail in fatigue.
Structured oral examination question 8#
Fracture healing
How does fracture healing occur?
Fracture healing can occur by primary or secondary bone healing.
What is primary bone healing?
This requires close anatomical reduction with minimal movement at the fracture site (< 2% strain). In the initial stages, osteoblasts differentiate from mesenchymal cells and laydown woven bone in any gaps. Lamellar bone may be laid down directly if there are no gaps. Remodelling then occurs across the fracture site, with cuting c ones passing across the fracture site. Healing is slow.
What is gap healing?
This is a type of primary bone healing.
And?
In this process the fracture site is primarily filled by lamellar bone oriented perpendicular to the long axis, requiring a secondary osteonal reconstruction, unlike the process of contact healing. The primary bone structure is then gradually replaced by longitudinal revascularized osteons carrying osteoprogenitor cells which differentiate into osteoblasts and produce lamellar bone on each surface of the gap.
What is contact healing?
If the gap between bone ends isless than 0.01 mm and interfragmentary strain isless than 2%, the fracture unites by so-called contact healing. Under these conditions, cuting c ones are formed at the ends of the osteons closest to the fracture site. The tips of the cuting c ones consist of osteoclasts which cross the fracture line, generating longitudinal cavities a t a rate of 50–100 μm/day. These cavities are later filled by bone produced by osteoblasts residing at the rear of the cuting cone. This results in the simultaneous generation of a bony union and the restoration of Haversian systems formed in an axial direction. The answer is a bit out of sync as usually contact and then gap healing is described.
Can you draw a cuting c one for me, please?
See Chapter 20.
What is secondary bone healing?
Secondary healing (by callus) requires some motion a t the fracture site (> 2% but < 10%). It consists of both endochondral and intramembranous bone healing. Hard callus forms under the periosteum at the periphery. The callus undergoes a process of progressive stiffening. In the earlier, less stiff, stages it is more resilient to movement at the fracture site butless good at taking loads or resisting deformation. The strength of the healing fracture does not necessarily correlate with its stiffness.
What are the stages of secondary fracture healing by callus?
The stages of fracture healing include: Stage 1: First week. Haematoma formation with invasion of macrophages, leukocytes and lymphocytes. Proinflammatory cytokines (including IL-1 and IL-6 and tumour necrosing factor α), and peptide signal molecules (including BMP s, TGF-β and PDGF) recruit inflammatory cells and promote angiogenesis. Progenitor cells invade. The haematoma coagulates in between and around the fracture ends, and within the medulla forming a template for callus formation Granulation tissue forms. The acute inflammatory response peaks within the first 24 hand is complete after 7 days. Stage 2: 1 week to 1 month. Soft callus forms. In this stage, fibrous tissue, cartilage and woven bone form Chondroblasts and fibroblasts differentiate and form collagen (mainly type II) and fibrous tissue. Pr oteoglycans are produced, which suppress mineralization. The chondrocytes then release calcium into the EC Mand also protein-degrading enzymes that breakdown the proteoglycans, thus allowing mineralization to take place. Stage 3: 1–4 months. Hard callus forms. The soft callus is invaded by new blood vessels and chondroclasts breakdown the calcified callus, which is replaced by osteoid (type I collagen) formed by osteoblasts. The osteoid calcifies to form woven bone. The osteoid callus is stiffer than the soft chondroid callus. Stage 4: Remodelling – several years. The woven bone is remodelled to lamellar bone. The medullary canal reforms as the bone remodels in response to the stresses placed upon it.
Structured oral examination question 9#
Screws3
Introduction
This is an A-list 5-minute basic science or trauma viva question. Examiners would expect candidates to be well-versed with the basics of an answer. This material is asked in ST3 interviews and trauma meetings and covered in the mandatory AO Basic Principles of Fracture Management course, and also lots of FRCS
(Tr & Orth) revision textbook model answers or on websites.
The examiners usually probe and ask less-obvious questions if candidates have done well and are heading for a good pass (score 7/8). The other alternative is probing because the examiners had been expecting a better performance with greater detail. Lastly probing regardless of how well a candidate is doing as examiners are aware candidates know the routine questions/ answers and want to re-invent the topic.
We present an answer outline to be used as a guide only. This is a rather dull topic in print, only really coming to life in a live viva situation observing the to-and-fro questions and answers between examiner and candidate.
A dry run through of the topic is definitely more useful than multiplet extbook re-reads of the topic.
Candidates may be asked to draw a screw out or be shown a laminated diagram of a screw and asked to describe the various screw design features (Figures 9.8 and 9.9). Be prepared for both scenarios. Candidates need to practise drawing and describing out loud the various parts of a screw beforehand, otherwise in the exam if unpractised the y can appear quite unrehearsed and amateurish.
Can you describe the different parts of the screw and their function?
Figure 9.8 Candidate drawing of a screw.
Figure 9.9 Screw terms.
Head
This prevents sinking of the screw into the bone and provides a connection for a screwdriver.
The main slot designs (recess types) for a screwdriver are (1) single slot, (2) cruciate head, (3) Philips,
(4) recessed hexagonal head (hex head) and (5) Torx-6 stardriver (Figure 9.10).
A hexagonal head has six points of contact to increase torque, avoid slip and improve directional control. A stardriver maintains the advantage of the hex, but offers better resistance to stripping.
Figure 9.10 Slot for screwdriver.
Countersink
The countersink is the undersurface of the head and is either conical or hemispherical.
Runout
Transitional area between shaft and thread. Site of a stress riser and where a screw may break if incorrectly inserted.
Shaft
Smooth link between head and thread. Almost not present in a standard cortical screw.
Thread geometry
Most bone screws have asymmetrical threads, i.e. flat on upper surface and rounded underneath. This provides a wide surface for pulling and litile frictional resistance on the underside.
Thread pitch
The pitchis the distance between adjacent screw threads. With each full turn the screw advances by a distance equal to the distance between the threads.
The shorter the distance the ‘finer’ the pitch, the longer the distance the coarser the pitch. Cortical screws have a fine pitch and therefore a greater number of threads. Cancellous screws have a coarse pitch.
A fine-pitched screw moves a smaller distance linearly for a given angular rotation, offers greater mechanical advantage, producing greater compression, and has more leverage than a coarse-pitched screw.
Thread depth
Thread depth is half the difference between thread diameter and core diameter. The thread depth determines the amount of contact with bone that inturn determines the resistance to pull-out. In cancellous bone a deeper thread is needed to increase the surface area to improve the purchase, as cancellous bone is weaker. This increases resistance to screw pullout.
Thread shape
The shape of thread may be V-thread (more stress at sharp corner), but iress threadless stress at the rounded corner), reverse but iress or square thread.
Lead
The lead is the linear distance travelled by a screw for one complete (360°) turn of the screw. If a screw is single-threaded, the lead is the same as the pitch. On a double-threaded screw, the lead is two times the pitch. This allows faster screw insertion, but consumes more torsional energy.
Diameters
Core diameter: narrowest diameter in the thread section. Solid section from which the threads project outwards. Also, a weak part of a screw. The size of drillbit used is equal to the core diameter. Torsional strength is proportional to the cube of the core diameter.
Sha diameḁter: diameter of shaft where there is no thread.
Outer or Thread diameter: the maximal thread width. The larger the outer diameter, the greater the resistance to pull-out.
Flutes
Channels that provide a route for removal swarf (bone debris).
Tip
Several different designs are available. The tip can be:
Non-self-tapping screw: smooth, conical tip. Needs pr e-drilling of a pilot hole and then use of a tap to create a channel/thread for insertion.
Self-tapping: needs pre-drilling of a pilot hole, but has cuting flutes for creating its own thread/channel incor tical bone that allows bone cutings to escape. In general, inferior bone-holding ability.
Self-drilling and self-tapping: tip will ma kea drill hole and will cut the channel for the thread.
Corkscrew tip: used in cancellous screws where the tip clears the pre-drilled hole.
Candidates may be stopped at any stage to be more closely questioned on a particular aspect of screw design.
What is the biomechanical definition of a screw?
There are several definitions. A screw is a mechanism that produces linear motion as it is rotated. A screw is a device that converts torsional force into an axial force. A screw is a mechanical device that converts a rotational movement (torque) into a linear movement (translation).
What do we mean by a screw’s purchase?
A screw’s hold in bone is referred to as purchase.
What do we mean by a screw pull-out strength?
The axial force required to remove a screw is referred to as its pull-out strength.
How can you maximize pull-out strength4?
The pull-out strength of a screw can be increased by increasing the contact surface area (interface) between screw threads and bone. This can be achieved by either: Increasing the outer diameter. Decreasing the inner (core) diameter. This effectively increases the width of the threads. Increasing the thread density (reducing the pitch). Enough for a 6 pass. Small-print stuff includes:
Increasing the number of threads engaged in the bone cortex (increased cortex thickness, bi cortical fixation).
Use a locking screw.
The ‘finer’ the pitch and the more turns the surgeon needs to make to insert the screw and the more turns of the spiral thread engage in a given depth of cortex. The more threads engaged in the cortex, the greater the pull-out strength (resistance).
What happens if the pitchis too small?
If the pitchis too small there is insufficient bone between individual threads.
How does the use of a locking screw maximize pull-out strength?
With a locking plate failure there is en-bloc pull-out of an interlocking screw system rather than sequential pull-out of conventional screws. A screw used in a locking plate requires resistance to bending both at the junction between the plate and screw and along the length of the screw. It doesn’t require a large pull-out strength. A locking screw as such has a relatively larger core diameter in relation to the thread diameter. However, in a locking screw fixation, a monobloc effect is produced, with all the screws forcing at the same time. This osteosynthesis method is much sturdier compared to sequential pull-out of conventional screws.
What surgeon factors can reduce screw pull-out strength?
Making too large a pilot hole. Repeated withdrawal and reintroduction of a screw causing damage to the negative threads in the bone tissue. Wobbling of the screwdriver handle during insertion. Poor technique or technical mistake.
Structured oral examination question 10#
A laminated photograph showing different types of screws (usually cancellous, cortical or cannulated) may be shown to a candidate. The candidate is then asked to discuss how the various screws differ in design features.
With the standardized viva format the days of candidates being handed over a screw to describe are over.
Describe the different types of screw.
Cortical and cancellous. The main differences relate to: Pitch. Cancellous screws have a larger pitch, greater thread depth and a smaller number of threads. Cortical screws have a smaller pitch, a greater number of threads and are designed to have better purchase incor tical bone. Tip. Cancellous screw tips are designed as a tapering spiral. These tips create their own threads in cancellous bone. As the tapered spiral advances, it pushes the spongy cancellous bone aside to thread its way into the bone. Cortical screws are usually blunt-ended. Core to outer diameter. Higher ratio in acor tical screw. Thread depth determines whether a screw is cancellous or cortical. Cortical screws have larger root diameter. Ratio of inner (c ore) diameter to outer (thread) for a 4.5mm cortical screw is 3/4.5 (66.7%) and for a 4 mm cancellous screw is 1.9/4 (47.5%) ~ 2/3rds to 1/2. Fully or partially threaded screw. Fully or partial threaded with cancellous. Fully threaded with cortical. Locking and non-locking. A locking screw has threads on the head to allow locking into plates and provide angular stability of the plate screw construct, increasing the pull-out strength of the screw. Ideal for use in osteoporotic bone. Cannulated and non-cannulated screws. Cannulated screws have a hollow core to allow placement over a guide wire. The hollow core weakens the screw, although clinically this is not often a problem. They are useful for accurate positioning near articular surfaces. Guide wire allows radiological check prior to screw insertion.
How do the design features of a particular screw relate to its function?
Cortical screw: acor tical screw is designed to gain maximal purchase in the bone cortices. Cortical bone is usually dense but has limited thickness. As such, to maximize purchase, cortical screws have a small pitch and their tips are designed to cut into dense cortical bone.
Cancellous screw: this screw is designed to gain purchase in cancellous bone, most commonly the metaphyses of long bones. Cancellous bone isless dense and spread out compared to cortical bone and to gain maximum purchase cancellous screws have wider threads and larger pitch. The screw threads cut their path in the bone when the screw is inserted (self-tapping screws) with the tip designed to press cancellous bone aside like a snowplough presses aside spread-out snow.
Locking screw: this has an additional set of threads around the head. It is used in combination with a locking plate that has reciprocal grooves around the plate holes. The locking screw locks into the plate. The plate/screw construct is more rigid than a non-locking construct and provides greater implant stability.
Cannulated screw: this has a canal through the central core in which a guide wire can be used to guide the position of the screw.
Locking screw (bolt). This is designed to control torsional and axial loads. A large core diameter (strength of screw and fatigue resistance) and no need for a large thread diameter as pullout resistance is not important.
What is the technique of lag screw fixation across a fracture site5?
This involves over-drilling the near-side object to a diameter slightly larger than the thread diameter, creating a gliding hole. The far object is drilled as normal to core diameter and tapped to the thread diameter. The screw thread only gains purchase in the far object, so when the head comes into contact with the near-side object it allows compression of the two objects. Drilling large lag (gliding) hole (near cortex). Drilling small (threaded) hole (far cortex). Countersinking. Measuring. Taping the far cortex with a protective tap sleeve. Need direction and wobble control. Insertion of screw. Reduction forceps should be removed just before final tightening of the screw. Be sure of your order. For example, if measuring takes place before countersinking then the length of the screw will be too long.
What about fracture reduction?
Normally, the fracture fragments should be reduced before the near cortex is drilled.
What do you mean by countersinking a screw?
When a screw is used without a plate, a countersink hole is created to reduce the risk of fracture as the screw is tightened. This disperses a high pressure over a wider area and reduces the prominence of the screw head.
What else?
Failure to countersink can result in very high stresses at the screw head/bone interface, causing microfractures, leading to screw loosening. Countersinking maximizes the contact between screw and bone to minimize stress.
What else?
Failure to perform proper countersinking causes an eccentric loading and lessens the degree of compression. In a very thin cortex it might also lead to slight displacement of the fragments because of the eccentric force.
So, what about a small fragment cortical screw, what size is this?
A small fragment screw has a thread diameter of 3.5 mm and a core diameter of 2.5 mm.
What about drill sizes for a lag screw?
3.5-mm drillbit (silver) for the gliding hole and 2.5-mm drillbit (gold) for the thread hole.
Is that for a large or small fragment cortical screw?
Small cortical screw. For a small cortical lag screw the near cortex is drilled with a 3.5-mm drill before the far cortex is drilled with a 2.5-mm drill, which is tapped with a 3.5-mm tap before inserting a 3.5-mm screw orientated perpendicular to the fracture to obtain compression.
What about a large fragment screw when it is used as a lag screw?
For a large fragment screw this has a thread diameter of 4.5-mm and a core diameter of 3.1- mm. To be used as a lag screw we need two different drill sizes in order to create the gliding hole (near cortex) and a threaded hole (far cortex). This is score 5/score 6 material. Candidates are not scoring any marks.
What else?
We use a 4.5-mm drillbit for the gliding hole and a 3.2-mm pilot drillbit for the threaded hole.
What size of tap do we use?
The tap is 3.5 mm.
The tap size is 4.5 mm. The screw and tap are the same size. This is basic stuff that is being tested. If a candidate mentions using a 4.5-mm drill and a 3.5-mm tap they are heading for a poor fail (score 4) as they don’t understand the principles of lag screw fixation.
How do we measure the depth of the screw tract?
We use a depth gauge.
How?
We just measure the depth. It is important to engage the hook of the depth gauge against the obtuse edge of the exit hole, not the acute angled edge, otherwise the screw depth will be incorrect. Length reading will be too short. Longest distance allows maximum purchase.
What about a mini fragment set?
These contain cortical screws of size 1.5 mm and 2.0 mm.
What else?
The 1.5-mm screws are used to fix phalangeal fractures while the 2.0-mm screws are used to fix metacarpal fractures.
Drillbit for gliding hole and threaded hole and tap for a 1.5-mm cortical screw.
I don’t think they are used as a lag screw.
They can be used as a lag screw if needed.
For a 2-mm screw it is 2.0 mm for a gliding hole, 1.5 mm for a threaded hole and a 2-mm tap. Figure 9.11 gives comprehensive information on thread diameter, drillbit and tap diameter sizes.
What are the principles of the lag screw technique?
A lag screw allows compression of two fracture fragments as the screw thread only engages in the far cortex and slides through the near cortex. It is a technique, not a type of screw. Any screw can function as a lag screw. It provides interfragmentary compression. It results in absolute stability. The lag screw should compress fracture fragments together.
What are the conditions for interfragmentary compression?
The screw must glide through the near cortex.
Threads hold only in the far cortex.
Screw head should stop at the near cortex.
The maximum compression occurs at 90° to the fracture.
When does the lag screw principle fail?
Sorry, I am not sure what you mean. The near cortex needs to be over-drilled as otherwise the threads of the screw will engage both near and far cortex and no fracture compression will occur. Lag screws should not be used in comminuted fractures. They are ideally suited for simple fractures such as spiral or oblique. They should be inserted perpendicular to the fracture plane to generate the greatest interfragmental compression and reduce the risk of fracture displacement.
What are the functions of a scr ew6?
To produce interfragmentary compression. To attach implants to bone by compressing them onto the bone surface. To attach implants to bone, producing angular stability. To lock an intramedullary nail to the cortices. To block movement of a main fragment around an IM nail – Poller screws. AO teaching.
OK. Can you name me the different types of screw function?
I am not sure. Plate screw: preload and friction is applied to create a force between the plate and the bone. Lag compression screw (see above). Position screw: holds anatomical parts incorrect relation to each other without compression, i.e. thread hole only, no glide. For example, syndesmosis screw. Locking head screw: used with locking plate, threads in the screw head allow mechanical coupling to reciprocal threads in the plate and provide angular stability. Butir ess/antiglide screw: a lag screw may be applied in abut iress function a t the tip of the fragment.
Anchor screw: a point of fixation used to anchor a wire loop or strong suture. For example, K-wire fixation of medial malleolus fracture.
Push pull screw: a temporary point of fixation used to reduce a fracture by distraction and/or compression.
Reduction screw: a conventional screw is used through a plate to pull fracture fragments towards the plate, the screw may be removed or exchanged once alignment is obtained.
Pollar: screw is used as a fulcrum to redirect/guide an IM nail.
Candidates may be quizzed in more detail on each specific function.
Figure 9.11 Screws, drills, bits and taps.
Structured oral examination question 11#
Screw
The candidate is shown a picture of acor tical screw. What is a screw? What are the parts of the screw?
Why are there different thread diameters? How do you use a lag screw? Why do some screws now have threads in their heads?
Similar question to previous ones, but with a few different twists along the way.
What is a screw?
A screw is a device which converts rotational forces into linear motion [A O definition].
What are the parts of a screw?
A screw has four main functional components. Head. Shaft. The shaft is the smooth part of the screw between the head and the thread. The run-out is the spot where the shaft ends and the thread begins. Thread. Tip. This is your basic score 5/6 answer. Most candidates should aim to give a bit more detail in their answer to make sure of scoring at least a 6. Core diameter, thread diameter, pitch, etc. (see above).
What is the difference between thread diameter and thread depth?
Thread diameter is the maximum diameter of the threads. Thread depth is half of the difference between thread diameter and core diameter. The thread depth determines the amount of contact with the bones, which inturn determines the resistance to pull-out. The size of tap is equal to the thread diameter.
How do you use a lag screw7?
A slightly ambiguous question Possibly asking about what fractures are best suited for lag screw fixation Perhaps the question may lead on towards how you perform a lag screw fixation (see above).
Why do some screws now have threads in their heads?
These are locking screws. They increase screw pull-out strength and prevent sequential pull- out failure of a plate.
Structured oral examination question 12#
Lag screw
Lag screw, principles, how plates work, how nails work. Name drill sizes and how to insert a lag screw (3.5 and 4.5 mm cortex lag screw drillbits, taps, gliding hole and threaded hole)). Types of screw. Draw cross-section of a washer. How does a washer work?
How do plates work?
A bone plate transmits forces from one end of the bone to the other bypassing and therefore protecting the area of fracture. It also holds the fracture ends together maintaining alignment while the facture heals. As a screw is tightened against a plate it generates a compressive force between the plate and the bone. A reactionary friction force develops that is equal to the compressive force between the plate and bone.
What factors determine the success of a bone–plate fixation construct?
The success of a bone–plate fixation construct depends on several factors including: Plate thickness, geometry and material used: a stress on a plate that exceeds the elastic limit will lead to plate bending. Cyclical forces above the fatigue limit will eventually lead to plate failure if the fracture does not heal. A race against time for fracture healing versus hardware failure. Screw design, material, number and hold in bone: non-locking screws will usually loosen individually by toggling out. Locking screws fail en masse. Bone mechanical properties and bone health. Construct-placement of plate and direction of load. Compression between fragments: always attempt to apply a plate on the tension side of the bone and under compression.
What determines the strength of a plate?
BH3/12 B = base, H = height. Bending stiffness is proportional to the thickness (h) of the plate to the third power.
Why do we use a washer?
Washers spread the load applied by the head on the underlying cortex and are used to prevent the screw head from breaking through a thin cortex [AO]. Washers can be used with lag screw fixation to optimize compression and reduce the risk of unintentional intrusion of the screw head through cortical bone during screw insertion. The y are mainly used in metaphyseal bone or if the bone is osteoporotic.
Draw a cross-section of a washer.
A washer has two sides, a flat side and a concave side. The flat side of the washer rests on the bone while the concave (countersunk) side matches the undersurface of the screw head.
Structured oral examination question 13#
Design features of a screw
You are given a new screw from a rep. How would you appraise it? What would determine if YOU would consider using it or not?
You are given two types of self-tapping screws (one normal, one reverse-cuting as well as self-tapping) – describe these implants. Why would you use a reverse self-cuting screw?
You are given DC Sand DHS. What are these implants? Describe their differences. How would you apply them (exactly, with order of screw placement and why)? What are the principles and design specifications of a compression plate?
You have been given a new screw from a company representative How would you appraise it? What would determine if YOU would consider using it or not?
This is similar to the ‘given a new plate by a repand asked to evaluate’. This question is designed to test higher-order thinking. It is one level above a candidate talking through the various functional parts of a screw. What is the material of the screw titanium, stainless steel, bioabsorbable)? Titanium: high tensile and yield strengths, reduced stiffness, increased biocompatibility , Young’s modulus of elasticity closer to bone, diminished stress shielding MRI compatible, superior strength under the high cycle repeated load stresses. Titanium and titanium alloys are not notch-sensitiv e, which means that stress raisers have minimal effects on the mechanical properties of titanium implants. Titanium is immune to freting and local corrosion that is seen with stainless steel implants. Stainless steel: cheaper and easier to manufacture. Bioabsorbable: radiolucent, eliminates the need for hardware removal, reduced stress-shielding and allows a gradual load transfer to a healing fracture. Disadvantages include lower mechanical strength, higher cost and in some cases an undesired biological response. In simple terms, a screw should be made of a strong material that can withstand a heavy load. What is the size of screw to be used? Mini fragment, small fragment or large fragment screw. What function will it serve or simply, what is its mechanism of action? Conventional bone screw (cortical or cancellous). Locking bolt. Locking screw. Cannulated screw. Steer the question onto safe territory. Candidates may have a freehand into what screw function they can discuss or be pushed towards one particular direction. Unless you are very confident don’t mention small-prin t stuff such as but iress or anchor screws. Go for something that will allow you to keep talking and scoring marks.
Self-tapping. This is the ability of a screw to advance when turned, while creating its own thread. If a screw is not self-tapping it will generally be necessary to use a tap to cut a thread into the bone before screw insertion.
I would want to prevent screw failure and need to consider the following factors. Tensile strength (resistance to bending): this is directly proportional to the square of its core diameter – core diameter2. Torsional strength: proportional to the cube of its core diameter – core diameter3. A screw can break during insertion if the applied torsional load exceeds its torsional strength. Pull-out strength: I would need to decide on the pull-out strength of the screw. This depends on the thread diameter. It is affected by the density of the bone beneath the screw threads. Core diameter: the core diameter is a weak part of the screw. The smaller the core diameter, the greater the risk to shear off during insertion and removal. Stripping of the screw head: I would want to avoid stripping of the screw head. Mainly this is caused by the screwdriver incorrectly aligned with the screw axis. This means that the screwdriver is not aligned co-linearly with the screw axis and doesn’t obtain complete engagement in the screw head. A stardrive recess offers best resistance to stripping. I would certainly want to avoid use of a single-slot head. Titanium screws are more prone to stripping of the screw head than stainless steel. Countersink: I would want to know the shape of the countersink. A hemispherical undersurface is generally preferred because it allows a screw to be angulated in all directions within a washer or the screw hole of a plate while maintaining concentric contact between the screw and side of the plate. Score 8: The pull-out strength of a screw increases with increasing screw length, thread diameter, thread depth and bone strength. F = S × (L × π × D) × TSF F = pullout strengthS = ultimate shear strength of bone L = screw length D thread diameter The thread shape factor is defined as 0.5 plus the ratio of thread depth to thread pitch multiplied by a constant TSF = 05 + thread depth/thread pitch × K (constant)
The TS Fand thus screw holding strength increase whenever the thread depth becomes larger (larger threads and smaller root diameter).
Given two types of self-tapping screws (1 normal, one reverse-cuting aswell as self-tapping) – describe these implants. Why would you use a reverse self-cuting screw?
Self-tapping screws are produced with sharp cuting flutes at the leading end of the threaded portion of the screw. The flutes are milled into the thread blank. The flutes cut through bone and facilitate screw insertion. Tapping is the process of cutin gin ternal threads into the material. Tapping may reduce the torque needed to place the screw threads through dense cortical bone, but in cancellous bone it may decrease the ultimate holding and compressive power of the screw. Flutes are also milled into the opposite end of the threaded blank. These are called reverse-cuting flutes which allow the thread to cut its way out of the bone after fracture healing.
Can you give me an example of when to use a reverse self-cuting screw?
Reverse self-cuting screws are used to make screw removal easier. The screws are usually self-tapping screws with reverse-cuting flutes . An example would be a paediatric cannulated screw inserted over a guide wire used for a SUFE that will often require removal at a later date.
Given (shown aDCS and DHS implant. What are these implants? Describe their differences. How would you apply them (exactly, with the order of screw placement and why)? What are the principles and design specifications of a compression plate?
A DCS plate was initially designed for fixation of distal femoral fractures but can also be used to fix proximal femoral fracture sADHS implant is the ‘gold standard’ for intertrochanteric fracture treatment. The main difference between these two implants is the angle of the lag screw with respect to the plate (Figure 9.12). When used in the proximal femur, the DCS plate can only be used to treat stable fractures; i.e. fractures that can be directly reduced and anatomically reassembled to allow restoration of the bony medial but iress. Because the DCS plate has a 95° barrel angle, it does not allow for controlled compression. The design of the DCS plate can enhance fixation of selected stable subtrochanteric fractures because it permits stable fixation in the proximal fragment. The DCS plate has a 95° barrel angle, allowing it to enter the femur more proximally than th eDHS plate and allowing insertion of independent screws into the calcar. Its two round proximal plate holes permit insertion of two 6.5- mm cancellous bone screws. Using a 135° DHS plate to treat long oblique subtrochanteric fractures does not always allow fracture compression. With the 95° DCS plate stable fixation can be achieved by lagging the fracture through the plate because controlled collapse is not likely to occur.
Two 6.5-mm cancellous screws are inserted after lag screw insertion and seating of the plate
(Figure 9.13).
What size of drill do we use?
We drill a hole in the near cortex with a 4.5-mm drillbit. The 4.5/3.2 drill sleeve is fully seated into the plate hole and a 3.2-mm drillbit is used to drill into the far cortex. Measure, tap and then insert a 6.5-mm cancellous bone screw. This technique should prevent the drillbit from gliding along the calcar.
What do you mean by the drillbit gliding along the calcar?
Sorry, I am not sure. If a lag screw technique isn’t used the 3.2-mm drillbit will strike the endosteal aspect of the calcar obliquely and be deflected up the neck, where it may break. Over-drilling the outer cortex using a 4.5-mm drillbit and use of the drill sleeve through the hole will guide the 3.2-mm drill and prevent its deviation in the calcar. Th eDHS plate is fixed to the femur using 4.5-mm cortical screws. Aim for bi cortical fixation and start with the most distal hole.
When do we use a compression screw?
It allows further fracture impaction. It is useful in unstable fractures to prevent disengagement of the lag screw from the plate barrel.
Are there any concerns with using a compression screw?
A compression screw can cause stripping of the lag screw thread in porotic bone. A void excessive force and remove the compression screw after use.
Why?
I am not sure. A recent paper suggested retaining the compression screw as it helped to lessen mechanical failure of th eDHS by reducing the peak (von Mises) stress around the connection between the barrel and side plate.8
Figure 9.12 DCS plate for stable subtrochanteric fractures.
Figure 9.13 Insertion of two 6.5-mm cancellous bone screws through the proximal round holes of the DCS plate.
Structured oral examination question 14#
Design features of a plate
Plate design needs a bit of thought and pre-planning because if you are hit blind with this topic in a viva you may struggle and not respond well to questions. It gives the examiners the opportunity to focus in and test your knowledge on various subsections of biomechanics. You need to understand the management principles of your fracture, what you are trying to achieve with your fracture and what your plate is going to do to help you achieve this, and design the plate accordingly.
Design the perfect plate.
Score 4: I use a z plate because I know it well, the company sales representative is very supportive, we use this plate in our hospital and the company have been very competiv e in their pricing. This question is about principles of plate design and is used to test candidates for higher- order thinking skills. Plate implant procurement is usually agreed locally by an orthopaedic department based on many factors including cost, after sales support, educational training, bulk usage discount offered and performance. Stay clear of mentioning specific company implants.9 This is in contrast to referring to a cemented Exeter hip implant in a viva. This widely used implant isO DEP 10 A rated with 30 years survivorship results and most trainees are familiar with the kit. It is important to prepare for viva questions, but also important not to over-analyse your answers. It is also reasonable to mention established local hospital policies such as antibiotic orD VT prophylaxis in a viva. These protocols are usually decided through an appropriate local commift ee using a best evidence-based approach. ‘I would start antibiotics according to my local hospital antimicrobial guideline protocols.’ This may even sound quite official, so the examiners may be less likely to challenge you even if you are alit ile unsure about specifics.
Constructing an answer
Prepare a simple sentence and then divide your answer into various different headings and then work on the headings.
The design of my implant would require consideration of many factors which would include material properties structural properties interface fixation and modality of use.
1. Material properties
Candidates may end up discussing:
Youngs modulus.
Yield point.
Toughness.
Hardness.
Material properties are independent of shape.
My ideal material should be bio-inert, reliable, easily manufactured and sterilizable. It should be acceptably priced, have corrosion resistance and provide adequate ductility , toughness and hardness.
2. Structural properties
Bending stiffness.
Torsional stiffness.
Axial stiffness.
3. Interface fixation
Locking vs. non-locking plate.
4. Modality of use
Primary healing vs. secondary healing.
Anatomical reduction versus alignment.
Rigid versus relative fixation.
Load-sharing vs. load-bearing.
Combination.
Straight locking vs. variable angle.
Notes
1. Orthobullets, Ryan Eggers, Comment on structural properties.
2. The keywords are ‘a constant volume of material for a fixed length.’
3. Take hold of a screw and practise out loud naming its various components until flawless. This may take two or three attempts to become proficient in delivery.
4. The examiners may stop a candidate half-way through their description of a screw and ask this question.
5. Six steps in the lag screw technique.
6. The examiners may drill down for more detail onto a particular function.
7. The question is abit clums y. Better phrased as when to use a lag screw rather than how to use a lag screw.
8. Chang C-W, Chen Y-N, Li C-T, Peng Y-T, Chang C-H. Role of the compression screw in the dynamic hip–screw system: a finite-element study. Med Eng Phys. 2015;37(12):1174–1179.
9. Possibly NCB periprosthetic Zimmer system, as its use is very versatile for difficult complex periprosthetic femoral fractures.