Chapter 21 Design of implants and factors associated with implant failure (wear, loosening)
Tribology of natural and artificial joints
Kiran Singiseƫ and Paul A. Banaszkiewicz
Introduction#
A recent shift in emphasis with basic science from the IC Bis to try and link a topic into a clinical problem to make the subject more clinically relevant and less dry. A classic example is the clinical photograph of an explanted worn PE cup leading on to a discussion of wear.
A good understanding of tribological properft es helps the orthopaedic surgeon to choose the most suitable bearing solution for each individual patient.
Wear is an A-list topic with similar competency questions in the first part of a viva but unexpected or esoteric higher-order thinking questions in the second part. This is a method to keep the topic fresh with each diet of exams.
Structured oral examination question 1#
Clinical photograph of explanted poly cup with wear and cement
What do you see? Why has this happened?
This is a clinical picture of an explanted PE cup demonstrating acetabular wear (Figure 21.1). It may have been explanted because of associated aseptic loosening. Other causes for revision may include infection or duet o recurrent dislocation.
There is quite obvious wear seen on the inside of the acetabular cup, so what do you think has been the most likely cause for revision?
Aseptic loosening.
What do you mean by aseptic loosening?
Aseptic (i.e. not caused by infection) loosening refers to the failure of fixation a t the bone/implant interface, with resultant micro- or macromotion of the implant relative to the adjacent bone.
What is the difference between aseptic loosening and osteolysis.
[Silence.] They are quite similar processes [more silence]. Although the terms osteolysis and aseptic loosening are often used interchangeably, these processes are different. Aseptic loosening is an umbrella term that is used to describe total joint arthroplasty failure resulting from inadequate initial fixation, mechanical loss of fixation overtime, or biological loss of fixation caused by particle-induced osteolysis around the implant. Osteolysis refers to the host immunological response that results in implant loosening.
What do we mean by wear?
Wear is the removal of material from two surfaces underload, due to a sliding motion between them. Wear is the progressive loss of a bearing substance caused by mechanical or chemical (corrosion) action. There are several definitions of w ear. Learn one that you are happy with and stick to it.
What are the different types of wear that can occur?
There are two broad categories of wear, (1) mechanical and (2) chemical. Mechanical involves: Abrasive. Adhesive. Fatigue.
Erosive.
Chemical is independent of load and sliding distance:
Corrosion.
What are the various types of mechanical wear?
Two-body abrasive wear occurs with a hard (cobalt chrome) on soft (UHMWPE) bearing couple. Asperities on the hard bearing carve ridges (plough/cheese grater effect1) into the softer bearing. This generates new particles (w ear debris), which become third bodies. An example is between a metal femoral head and a polyethylene liner.
What do you mean by adhesive wear?
Adhesive wear occurs when opposing asperities of two surfaces bond with each other to form a junction .2 This junction is held by intermolecular bonds and generates friction. If these bonds are stronger than the cohesive strength of the weaker material, then fragments of the weaker material are sheared off.
What is fatigue w ear?
With fatigue w ear cyclical loading of one surface, at loads greater than the fatigue strength, leads to small cracks forming under the surface. Cracks propagate, joining together, and the loose material comes away from the surface. An example is delamination of the polyethylene inT KAs.
What is erosive wear?
Erosive wear occurs when hard particles travelling in fluid interposed between two surfaces remove some of the surface as they collide into it. An example would be ‘third-body’ wear from polyethylene wear particles/loose cement travelling in synovial fluid.
Corrosive wear?
Corrosive wear occurs with surface damage due to chemical reactions with the environment. Do not give a number when mentioning w ear mechanisms, i.e. do not say ‘wear can occur through five mechanisms (adhesive, abrasive, third body, fatigue corrosion)’. This is inviting tr ouble.3 Concentrate on the three big wear mechanisms (abrasive, adhesive and fatigue w ear) as these are what the examiners are most familiar with.
Although erosive and corrosive wear are less well covered in the textbooks, it is reasonable to briefly mention them and the examiners will decide if they want to further probe you for extra details.4
Third-body wear can be either classified as a subtype of abrasive wear or as a separate, distinct type of wear mechanism.
Erosive wear is classified as either a major or minor cause of mechanical wear depending on which textbook is read. It is unlikely that candidates will spend a large amount of viva time discussing this wear mechanism.
Can you drawout the different types of wear (Figure 21.2a–21.2c)?
In the exam it is difficult to draw if you are caught cold and need to work it out for the first timeT o be slick, the drawings need to be practised beforehand. Just as important is a succinct but clear explanation of each w ear mechanism while you are drawing.
What type of wear has occurred on the PE cup surface?
The majority of the wear pattern would be abrasive. Third-body abrasive wear is also a possibility. This is like having sand in one’s shoes.5 It is a form of abrasive wear that occurs when a hard particle becomes embedded in a soft surface. The particle acts very similarly to the asperity of a harder material in abrasive wear, removing material in its path. Hard third-body particles such as bone cement can produce damage to both the polyethylene articulating surface and the metal femoral head.
How is this wear mechanism in the hip different to that of the knee?
Adhesive and abrasive wear is more pronounced for THA while fatigue w ear (piting and delamination) is more problematic for TKA.
What is the RANKL pathway?
Candidates should be familiar with the RANKL pathway from the Part 1 SBI/EMI paper, but there is a world of difference in answering this topic in a viva exam. Osteolysis is a complicated subject with material sources sometimes lacking focused exam summaries. It is therefore important to go through a dry-run practice viv a to refine your answer. The main biological system that leads to osteolysis-induced resorption of bone is the receptor activator of nuclear factor-κB (RANK)/RANK ligand (RANKL) axis Activation of this system results in enhanced osteoclast recruitment and activity adjacent to bone implant surfaces, leading to osteolysis. Osteoprotegerin (OP Gis a decoy molecule that blocks RANK Land prevents bone resorption. The osteolytic response includes various cell types, such as osteoclasts, macrophages and osteoblasts/stromal cells. This is supported by the wide range of factors that are secreted by various cells, including cytokines, growth factors, metalloproteinases, prostanoids and lysosomal enzymes etc. (Figure 21.3).
Once macrophages are activated by particulate debris, they secrete various kinds of mediators to incite a complex cascade of events culminating in recruitment and maturation of osteoclasts, the bone-resorbing cells directly responsible for the pathogenic bone loss in osteolysis.
Other cell types involved in the production of cy tokines and inflammatory mediators include osteoblasts and fibroblasts. Matrix-degradativ e enzymes and chemokines are also released from several types of cells.
Osteoblasts can also phagocytose small particles causing adverse effects on viability, proliferation and function of osteoblasts as well as on osteoclasts. Research suggests that UHMWPE increases the release of RANKL from osteoblasts, while OP Gis significantly inhibited.
Although wear debris may consist of polyethylene, PMMA cement, or metal, by far the great majority of wear particles derives from polyethylene.
What factors affect the degree of osteolysis from wear particles?
Factors affecting osteolysis severity include: 1. Size of particles: Large particles escape active phagocytosis, being recognized as non-digestible foreign bodies. They fail to stimulate macrophages to produce high levels of pro-inflammatory and osteolytic cytokines. Particles within the br oad size range of 0.1–10.0 μm are phagocytosed by macrophages, leading to cellular activation. Those in the size range 0.1–1.0 μm are the most active. Very small submicron particles can escape phagocytosis and fail to stimulate macrophages to produce high levels of pro-inflammatory and osteolytic cy tokines. 2. Shape of particles (elongated particles are more active compared to round or spherical particles). 3. Volume of particles (the critical volume is 140 mm3/year). 4. Total number of particles. 5. Surface area. 6. Immune response to particles.
What areG ruen zones?
This is a widely used system in which the femoral component interface is considered in seven zones (Figure 21.4). These allow the location of cement fractures and of lucent lines either at the cement–bone or the cement–prosthesis interface.
It is the progressive changes seen in serial radiographs that are important in diagnosing osteolysis and femoral stem loosening.
Figure 21.1 Clinical photograph of explanted polyethylene cup demonstrating worn surface.
Figure 21.2a Wear mechanisms. Adhesive wear: opposing asperities bond to each other and shear off as one surface slides over the other.
Figure 21.2b Wear mechanisms. Abrasive wear: asperities on the harder material cut into the asperities of the softer material. The new particles become third bodies.
Figure 21.2c Wear mechanisms. Fatigue w ear: cyclical loading causes accumulation of micr o-damage that breaks off as wear particles.
Figure 21.3 Model of interplay between macrophages, fibroblasts, lymphocyte, osteoclasts and osteoblasts in periprosthetic osteolysis. Particles may stimulate macrophages, fibroblasts and osteoblasts directly to induce RANK Land pro-inflammatory cytokines that can induce RANK LItis thought that T cells stimulated by the pro-inflammatory microenvironment may also promote osteoclast formations ynergized with TNF-α, by secreting IL-17. Thus, RANKL , TNF-α, IL-1, IL-6, IL-17 and M-CSF may mediate the differentiation of myeloid precursor cells into multinucleated osteoclasts.6
TRL = toll like receptors. Experimentally, polymethylmethacrylate (PMMA) and polyethylene (PE) particles have been shown to activate macrophages via the TRL pathway.
Figure 21.4 Gruen zones. Remember 1–7 starting a tGT and ending at calcar.
Structured oral examination question 2#
Wear and osteolysis
A clinical picture of a worn polyethylene cup is shown (Figure 21.5).
Mechanisms of wear.
Osteolysis.
Effective joint space.
What are the mechanisms of wear?
The main mechanisms of wear include abrasive, adhesive and fatigue w ear (see previous answer).
What do we mean by the term ‘osteolysis’?
Osteolysis is a cell-mediated biological process that results in the loss of bone as a direct response to stimulation of macrophages by biologically active particles. The core of the biological response that leads to osteolysis involves the receptor activator of NF-κB ligand [RANKL]–RANK axis for osteoclast precursors, resulting in their differentiation and maturation (see previous answer).
What about the importance of macrophages in the pathogenesis of osteolysis?
The cellular response that occurs in osteolysis is dominated by macrophages. Particles ranging from 0.1 to 10 μm in diameter undergo phagocytosis by macrophages. Once activated by particulate debris, macrophages secrete various kinds of mediators to incite a complex cascade of events culminating in osteoclast maturation. Pro-inflammatory mediators such as PGE2, TNF-α and IL-6 are generated in abundance by particle- challenged macrophages. The precise nature of stimulation of macrophages by particles remains unknown. However, it is thought that direct interactions between particle and cell surface are sufficient to activate osteoclastogenic signalling pathways. These interactions may include non-specific physical induction of transmembrane proteins or recognition of cell surface molecules by particles Recently, this phenomenon was explained with the role of toll-like receptor.
What do we mean by effective joint space7?
1. Schmalzried et al.8 coined the term ‘effective joint space’ to describe all periprosthetic regions that are accessible to joint fluid and thus particulate debris by the pumping action of the joint. 2. All periprosthetic regions that are accessible to joint fluid and its particulate debris. 3. The effective joint space is a concept that describes the entire volumetric area within a hip joint construct that can be infiltrated by PE wear particles and macrophages. Bone breakdown can occur anywhere within the effective joint space.
There are three definitions. Learn one ands tick with it. The presence of particulate matter in joint fluid will initiate a localized macrophage-induced phagocytosis and result in bone resorption. As fluid pressure propels joint fluid and thus particulate debris through the effective joint space, it will result in progressive bone loss. As bone is resorbed, a bigger sink is produced, encouraging even more flow (preferential flow) into that area, delivering more particles and causing more bone resorption. When sufficient bone has been resorbed, an osteolytic area can beseen on radiographs. If joint fluid is distributed more evenly in an interface, there will be slower resorption of bone accompanied by a fibroblastic response resulting in the radiographic appearance of linear (diffuse) bone loss.
How can you reduce effective joint space?
Reduction in the effective joint space may reduce the amount of osteolysis that can occur. The use of bone screws for fixation of acetabular shells is thought to create new voids in the acetabular bone that increase the effective joint space. Implanting acetabular shells without any screw holes in theory reduces the effective joint space. Cementing seals off the effective joint space. Using an Exeter stem that subsides into a centralizer blocks off any potential implan t/cement space. The use of circumferential proximal porous coated uncemented stem designs seals off the diaphyseal component of the femoral canal from the effective joint space and may reduce the amount of osteolysis occurring.
What new designs of hip replacements have been introduced to retard osteolysis by limiting the generation and spread of particulate debris?
Improved liner locking mechanisms reduce the amount of motion between shell and PE liner. It has been suggested wear debris can be produced at the interface between the metal acetabular shell and PE liner. NJR data with a ceramic on a highly cross-linked PE bearing couple compared to a more traditional metal-on-UHMWPE articulation. There have been improvements in the bearing articulation materials, particularly the development of first- and second-generation highly cr oss-linked PE. UHMWPE has much improved wear characteristics with reduced adhesive and abrasive wear in comparison to the older generation of PE. Improved results are being reported from traditional me tal on UHMWPE articulation. Candidates could be moved easily towards discussing improvements in PE manufacture, sterilization, shelf packaging, annealing versus heating , amorphous versus crystalline phase, etc. as part of an evolving viva on wear (advanced questions).
What are the risk factors for osteolysis?
Risk factors can be broken down into patien t-, surgical- and prosthesis-related factors. Patient factors include age at surgery and male gender. The evidence for association between increased body mass index and activity le vel is contradictory. Implant factors include prosthetic design, bearing couple, PE (manufacturing process, post- manufacturing sterilization, thickness of PE insert (knee) and liner (hip)). Surgical factors include cementing technique, correct prosthetic alignment anteversion, inclination prosthesis stability.
Figure 21.5 Clinical picture demonstrating worn surface of retrieved PE cup.
Structured oral examination question 3#
Wear in THA
Picture of aseptic loosening , what is wear, measures to reduce wear, particle size.
A radiograph of aseptic loosening hip is shown to the candidate (Figure 21.6).
What is wear?
Mechanical wear is the removal of material from two surfaces underload, due to the sliding motion between them.
What are the modes of wear of artificial joints?
The four modes of wear are: Mode 1. Generation of w ear material that occurs with motion between the two primary bearing surfaces, as intended by the designers. Mode 2. A primary surface rubbing against a secondary surface not intended as an articulating surface. Mode 3. Two primary bearing surfaces with interposed third-body particles. Mode 4. Two non-bearing surfaces rubbing together. McKellop’s classification. Do not confuse with the four Gruen modes of failure of cemented femoral stems or vice versa.9 The fundamental mechanisms of wear include adhesive, abrasive and fatigue w ear.
What measures can betaken to reduce wear?
This is a vague, non-specific question. It is easier to answer if you can turn the question around slightly and discuss factors that affect wear. One potential answer is to focus on PE. There is more than enough material to discuss that would use up the full 5 minutes of viva discussion if you are allowed to keep on talking (unlikely, but worth trying). So, a candidate’s leadin phrasing to discuss PE wear could be ‘The main type of wear particle implicated in osteolysis and loosening of total joint replacements is polyethylene. Methods to improve the wear characteristics of PE include ’: Manufacturing techniques. Sterilization techniques. Shelf life. Using first-/second-generation highly cr oss-linked PE. This may not work if the examiners have set questions the y are required to ask candidates for the viva topic and PE wear isn’t big on this list.
Another possible option ist o discuss:
Surgeon (technique) factors:
Implant selection, a voidance of implant malalignment, avoid impingement (increases wear), accurate restoration of mechanical axis joint, avoidance of debris contamination which will cause third bodywear.
Patient factors:
Weight (weight reduction).
Activity le vel (avoidance of excessive activities, eg. waterskiing, treadmill running, etc.).
Implant design.10 Decide on whether to discuss hips or knees or both.
Hips.
Offset. Decreasing offset increases joint reaction forces.
Choice of bearing couple (MoP, CoP, MoM).
Head size.
Knees.
Conformity.
Thickness of PE (minimum 8 mm). Thin PE predisposes to accelerated wear by delamination because of concentrated subsurface stress and fatigue w ear.
Femoral rollback.
Figure 21.6 Anteroposterior (AP) radiograph, pelvis, demonstrating loose right THA. Cement fracture and femoral stem and cup migration. Gruen mode 1a failure.
Structured oral examination question 4#
Wear in TKA
Polyethylene wear in TKA: discuss all factors.11
What does the picture show (Figure 21.7)? CANDIDATE 1: A worn tibial tray. CANDIDATE 2: A worn PE tibial insert. It looks like it is a CR-retaining ploy. There is evidence of severe delamination with uneven wear more pronounced medially. Discoloration is as a dark yellow tint representative of polyethylene oxidation. Discuss the factors associated with PE wear in TKA.
Ultra-high molecular polyethylene wear debris triggering osteolysis is one of the major causes of failure of TKA. Varus alignment of implants leads to accelerated medial PE wear and the risk of early failure. Varus placement of the tibial component > 3° leads to almost double the PE volumetric penetration rate. Backside wear between the plastic and me tal tray occurs due to micromotions occurring a t that interface. Snap-in capture mechanisms combined with manufacturing tolerances may lead to considerable motion under shear and torque. This is compounded by the roughness of the metal tibial tray, resulting in increased PE particle generation leading to osteolysis. This can be particularly severe in tibial tr ays with holes for fixation screws around which osteolytic lesions can develop. Additional component features that increase wear include thinner polyethylene inserts, some non- cemented tibial baseplates supplemented with tibial screws, and metal-backed patellar components. The manufacturing process of PE is important. Direct compression-moulded tibial components have better wear characteristics than components machined from either ram bar extrusion or sheet compression moulding. The change from sterilization of PE by gamma radiation in airt o an inert gas has resulted in much lower rates of osteolysis at 10 years post surgery. Sterilization of PE by gamma radiation in air creates free radicals that can react with oxygen when stored for an extended period of time in an o xygen-rich environment. This results in a subsurface band of highly oxidized polyethylene that results in decreased mechanical strength and a tendency to premature wear. Although the benefit of highly cross-linked PE in reducing wear rates in THA have been confirmed, there are concerns with its use in TKA. These include reduced strength, fatigue resistance and fracture toughness due to additional irradiation and thermal treatment. THA articulation occurs mostly as a sliding motion in a ball-and-sock et joint, while TKA articulation can occur as rolling, sliding and rotating.
The mechanism of wear between these two joints is different.THA wear is mostly due to micro-adhesion and abrasion, while TKA wear can be due to fatigue failure with delamination and piting.
Most supporting evidence for HXPLE in knee arthroplasty is derived from in-vitro wear simulator studies that show a reduction of w ear of up to 60%.
The literature is confusing as three recent mid-term, randomized clinical trials (mean follow-up: 2–5.9 years) comparing HXLPE and conventional UHMWPE bearings have all found no significant difference in clinical or radiological outcomes between the two bearings.12,13,14
However, NJR data from Australia have shown higher revision rates with non-XLPE. HXLPE had a lower cumulative percentage revision than conventional polyethylene at 5 years (4.0% vs 2.6%) and
10 years (5.8% vs 3.6%).
It is recognized that failed TKA have larger flake-shaped debris, which elicits a tissue response characterized by fewer macrophages. This is different from failed THA. This larger particle debris may be associated with delamination, piting and fatigue w ear.
A clinical picture of a tibial insert demonstrating a white subsurface oxidized band of PE is another classic lead-in prop to discuss PE wear in TKA. See also the cuting tool effect of machining of PE (Figure 21.8).
What are the wear mechanisms in TKA?
Three main wear mechanisms can beseen in TKA.15 These are adhesive, abrasive and surface fatigue. T ribo-chemical is sometimes mentioned as a fourth mechanism. Adhesive wear: the bonds formed between different materials are stronger than the specific material properties of either surface and therefore pullout fragments from one surface to another. Abrasive wear: a harder rougher material ploughs through a softer material. Surface fatigue: this is a process in which the material near to the surface is weakened by cyclic shear stresses or strains that exceed the fatigue strength of the material. Tribo-chemical wear: this is a process with a chemical basis that occurs at the interface between the articulating components and the environment. This friction mechanism initiates and propagates cracks at both the surface and subsurface.
Figure 21.7 Worn PE tibial tray. Delamination is seen as thin sheets of polyethylene separated from the surface.
Figure 21.8 Retrieval PE insert demonstrating classic white band defect of oxidation located 1–2 mm below the machined surface of PE.
What wear damage occurs at the tibial PE surfaces of aTKA?
Hood et al.16 described seven types of wear mechanism damage at the articulating surfaces of TKA: Burnishing (polishing). Scratching. Abrasion. Piting. Delamination. Third-body wear (embedded debris). Creep (surface deformation).
Burnishing: contact areas are polished due to a combination of abrasive and adhesive wear. This is a less-severe sign of wear, although submicrometre particle size generation can lead to macrophage activation and osteolysis.
Scratching: this is caused by abrasive wear. Differences in roughness and hardness between articulating surfaces lead to ploughing of the softer material.
Abrasion: characterized as a shredding of the polyethylene surface and classified as a mode of abrasive wear.
Pit inga mode of fatigue w ear that is characterized by the formation of millime tre-sized craters
(Figure 21.9). It is caused by cracks formed by repetiv e tensile and compressive stresses at the surface as the contact areas slide over the surface. It is considered to be a more benign wear mechanism that does not provoke an osteolytic response.
Third-body wear: wear debris can act as third-body particles, initiating w ear by rubbing at the bearing surfaces.
Delamination ( Figure 21.10): this is a severe form of fatigue w ear, involving the removal of sheets of polyethylene, and can result in catastrophic wear.
There is gross disruption of the material to a depth of 0.5 mm or more due to the formation and propagation of subsurface cracks. These cracks are thought to be due to the subsurface shear stresses that fluctuate indirection and magnitude.
Figure 21.9 Piting small cr ater-like surface defects.
Figure 21.10 Delamination of PE.
Creep (cold flow): the material deforms plastically without release of metal debris. More severe plastic deformation of the tibial insert may be an indication of malalignment or a mismatch of component sizes.
PE wear in knee arthroplasty occurs from a combination of rolling, sliding and rotation motions between the bearing surfaces which in due course leads to fatigue failure of the softer component (PE tibial insert resulting in piting and delamination .
Strategies to reduce polyethylene wear include the following.
Improving implant design.
Increased articular conformity increases the articular surface contact area, thereby reducing the subsurface PE contact stress per unit area. Congruent bearing designs lower the amount of cross-shear stresses.
–‘Double-dished’ geometry minimizes contact stresses and edge loading. The sagift al plane should be concave or dished and the individual medial and lateral tibial plateaus should also be dished in the coronal plane.
Improved locking mechanisms of modular tibial components to reduce potential back side wear.
Highly polished tibial baseplate.
Mobile bearings rotating pla foorm. A rotating yet flat PE bearing is matched against a highly polished cobalt chromium surface.
Monobloc tibial components. The PE bearing surface is direct compression-moulded to the tibial baseplate. This design is thought to eliminate backside wear, which may improve long-term survivorship inpatients receiving TKA.
All polyethylene tibial components have been used in an attempt to decrease or eliminate the problems associated with backside wear.
Improvements in the quality of ultrahigh-molecular-weight PE.
Improved sterilization techniques (gamma irradiation inert atmosphere).
Development of newer, highly cross-linked PE with the introduction of vitaminE and sequential annealing.
Refining surgical techniques.
Computer navigation.
Fellowship-trained surgeons.
Is there any evidence that all-polyethylene tibial components reduce wear (Figure 21.11)?
The theoretical benefits of using metal-backed tibial components include a more even distribution of w eight-bearing stresses to the underlying fixation interface and cancellous bone and a reduction in the potential polyethylene deformity caused by creep. A number of studies have failed to show any difference in survivorship between metal-backed and all-polyethylene tibial components. My understanding is that the newer types of one-piece polyethylene tibial components are best suited for elderly, low-demand patients where survival rates approach those of a metal- backed tibial component. With all-polyethylene tibial (AP T; Figure 21.11) the same amount of tibial resection allows for a thicker PE to be used, potentially increasing the lifetime of the prosthesis if wear rates are equivalent, as less tibial resection will be required to achieve the same poly thickness as a design with a metal tray, resulting in a larger metaphyseal surface area and the ability to use larger tibial component sizes, reducing the magnitude of contact stresses transmift ed across the joint while also preserving metaphyseal bone stock.17
What do we mean by conformity?
Contact stresses experienced at the PE surface are inversely proportional to the degree of conformity between the femoral condyle and the tibial PE insert. The greater the conformity at the articulating counter surfaces, the greater the contact area between these surfaces. This reduces detrimental subsurface contact stresses experienced by the PE.
So why don’t we just go with highly conforming knee designs?
Highly conforming TKA designs significantly increase the stresses transmift ed to the fixation interface and increase the risk of early aseptic loosening. R oll-back is sacrificed, resulting in reduced range of knee movement. Conversely, reduction of contact area with low-conformity designs leads to accelerated PE wear and early TKA failure.
What about mobile bearings?
In-vitro wear studies have shown that mobile bearing produces less wear compared to fixed designs. This benefit, however, has failed to translate into improved clinical outcomes or survivorship.
What about surgeon factors?
Surgeon-controlled strategies recommended for reduction of PE w ear include meticulous attention to ligament balancing, reproduction of anatomic extremity alignment, restoration of the proper joint-line level, and balanced symmetrical flexion/extension gaps.
What else?
Ideally, a fellowship-trained arthroplasty surgeon.
What else?
GIRFT (geting it right first time). Regular appraisal/assessment including PROMS scores, scrutiny of NJR data including numbers and reasons for any revision knee surgery performed. Computer navigation.
Is there any evidence that computer navigation improves the accuracy of implant positioning?
There is some evidence that computer navigation reduces the number ofT KAs that have a coronal malalignment of more than 3°. Significant outliers are avoided.
Which paper?
I am not familiar with the specific papers, but the general literature suggests better alignment with navigation. This is a reasonable default statement if you don’t know the specifics of a paper. Try not to use it too often as this will irritate the examiners.
Figure 21.11 All-polyethylene tibial component (APT), posterior stabilized design.
Reference#
Bauwens K, Matihes G, Wich M, et al. Navigated total knee replacement. A meta-analysis. J Bone Joint
Surg [Am]. 2007;89A:261–269.
Although coronal malalignment is reduced, mean alignment and mechanical axis did not differ between navigated and conventional TKRA groups. The candidate trying to wrangle out of specifics!
Is there any evidence that computer navigation improves the survival of a knee prosthesis?
It is very difficult to prove that computer navigation definitely reduces the need for revision surgery. I am not familiar with the specific papers, but some NJR data from Australia demonstrate a lower revision rate in young patient sat around the 10-year mark. Most studies at mid-term follow-up have failed to show any substantial benefits in terms of functional outcomes, revision rates, patient satisfaction, or pa tien t-perceived quality of life. Recent Australian Registry data suggest there may be a small advantage, particularly in younger patients, as there is a small reduction in the rate of revision for loosening in this group.
What about the use of alternative bearings?
Some surgeons have begun using oxidized zirconium femoral components as a means of reducing polyethylene wear. This technology incorporates a zirconium oxide ceramic coating on a zirconium metal alloy femoral component. The surface is more scratch-resistant than cobalt chromium, lessening wear debris production. Another advantage of an oxidized Zr femoral component is that it is safe to use inpatients with a nickel sensitivity because there is no traceable nickel in this material.
Any long-term results reported?
Ten-year results were reported by Pinczewski et al. from Australia.18 They showed comparable rates of survival with other implants and excellent functional outcomes 10 years postoperatively. They did not show any reduction in revisions for PE wear and osteolysis. The main thrust of the paper appeared to be the high level of patient satisfaction with TKA using this bearing surface (WOMAC, KOOS).
Any concerns with the paper?
Single-surgeon series in a tertiary specialized referral centre that may not reflect the average standard knee arthroplasty surgeon practice.
Figure 21.12 Oxium-coated femoral knee implant. During manufacture, OXINIUM implants undergo a process that transforms the implant’s surface into a hard, ceramicized metal.
Structured oral examination question 5#
The candidate is shown a clinical picture of catastrophic PE failure in TKA (Figure 21.13).
Remember to structure your answer (1) PE thickness, (2) articular surface design, (3) knee kinematics, (4) PE manufacture, (5) PE sterilization and (6) surgical technique. This is a clinical photograph of an explanted tibial knee replacement component demonstrating catastrophic PE failure. Causes for this could include low conformity of implant design, inadequate PE thickness, use of low-quality PE ... The candidate started well, confidently going straight to the diagnosis of catastrophic PE failure and then was able to talk about possible causes. The candidate has put their cards on the table and the examiner can then decide where the viva goes next.
Figure 21.13 Catastrophic failure of PE tibial insert.
Structured oral examination question 6#
Charnley THA
Polyethylene cup: what is polyethylene, manufacturing advances, wear, what is the stem made from, stress–strain of the stem
What is polyethylene?
UHMWPE is a member of the polyethylene family of polymers with the repeat unit [C2H4]n, with n denoting the degree of polymerization. It is a linear (non-branching), semi-crystalline polymer which can be described as a two-phase composite of crystalline and amorphous phases. The manufacture of polyethylene is by the Ziegler process, which involves ethylene molecules being polymerized into a high molecular weight by compression-moulding and ram extrusion.
How is PE manufactured?
Manufacturing processes Ram bar extrusion with secondary machining. Polyethylene resin is simultaneously heated and pressurized within an evacuated chamber. As the solid polyethylene forms, it is extruded through an open extrusion port within the chamber. A primitive method of polyethylene manufacture, but cheap. Because the extrusion process is non-continuous, inconsistencies can be found within the solid polyethylene bar stock. Calcium stearate was initially used as a lubricant and release agent in the moulding process. This is no longer added to polyethylene. Calcium stearate crystals could be found between the particles of polyethylene, resulting infusion defects that became the point of crack initiation and propagation. Sheet compression moulding. This is a form of compression moulding used to manufacture large sheets of PE. The resin is poured onto a plate and these plates are brought together and heat is applied. After a specific heating cycle has been completed, the pressure is increased to a desired set point and the material is allowed to cool underpressure. Compression moulding into bars with secondary machining. This process produces rods rather than sheets of PE. Direct compression moulding.
The most advanced form of PE manufacture involves direct compression moulding from PE powder. The resin is directly moulded into the finished implant.
There is no secondary machining of the bearing surface. Best wear profile.
Direct compression moulding has a lower susceptibility to fatigue cr ack formation and propagation.
Hot isostatic pressing (HIPing) into bars.
Multistep conversion process of resin powder into stock material.
What do we mean by the cuting tool effect of polyethylene?
In a machined polyethylene insert, machine marks from the lathe create numerous micron- size grooves and shreds on the bearing surface. As the high-speed cuting la the removes PE, the remaining nearby PE is stretched. The microscopically stretched PE chains are more susceptible to radiation resulting in greater oxidation in this area. Stretching occurs in the amorphous areas of PE and is most pronounced in the PE 1–2 mm below the surface of the cut PE. One of the major advantages of direct compression moulding is that the surface of the moulded polyethylene insert is smooth, lacking machine marks or grooves.
What manufacturing advances have occurred with PE?
The advent of highly cross-linked PE (HXLPE) has been shown to improve wear rates in hip arthroplasty. Compared to standard PE, HXLPE has: Better wear resistance. The PE particles t end to be smaller in size and produce less osteolytic reaction. There is generally a decreased number of particles generated. Disadvantages include: Decreased tensile strength – the pulling force to break. Decreased fatigue strength – the maximum cyclic stress the material can withstand. Decreased fracture toughness – the force to propagate a crack. Decreased ductility – elongation without fracture. To score a 7 a candidate may need to discuss supportive literature of improved implant survivorship with HXLPE in THA. To score an 8, a candidate may need to discuss the controversies of HXLPE use in TKA.
What do we mean by cross-linking?
Cross-linking is a process that changes the mechanical and tribological properties of a polymer. By removing atoms or side groups from adjacent chains, covalent bonds can form that link chains together, inhibit relative molecular movement and as such modify the physical properties of the materials.
What methods are used for PE sterilization?
Sterilization methods can be divided into (1) non-energetic (nor adiation) and (2) energetic (using radiation methods. Ethylene oxide gas may leave residues harmful to tissues. Both ethylene oxide and gas plasma sterilization a void free radical production but do not allow cross-linking of PE. As such, they have a higher wear rate compared to cross-linked PE. Gamma sterilization in air: this makes PE susceptible to oxygenation. Irradiation in the presence of oxygen leads to chain scission of the polyethylene long chain and free radical generation a t the crystal surfaces. Gamma sterilization in an inert atmosphere: irradiation of PE ruptures PE bonds and creates free radicals. In the absence of oxygen, the free radicals will bond with an adjacent chain, resulting in cross-linking of PE. Cross-linked PE has improved resistance to adhesive and abrasive wear. Shelf ageing: even with gamma sterilization in an inert atmosphere, some free radicals remain in the PE. These free radicals are susceptible to oxidation. Avoidance of shelf ageing with improved packaging to stop oxygen from diffusing back into the PE through the packaging and causing oxidation. Highly cross-linked PE: First generation: high doses of gamma or electron beam radiation are used to promote an elevated cross- link density (i.e. covalent bonds) into UHMWPE. This results in an increase in wear resistance. Two different approaches are adopted to achieve oxidation resistance. 1. Annealing: this involves a single thermal treatment below the melting temperature of UHMWPE so that crystallinity and mechanical properties are preserved. However, the UHMWPE still contains residual free radicals with the potential to oxidize in vivo. 2. Remelting: pos t-irradiation remelting of the polymer above the crystalline transition. This strategy allows for elimination of free radicals up to undetectable levels, but at the expense of crystallinity changes and diminished mechanical properties. Second generation: use of alternative stabilization strategies, such as natural antioxidants (vitaminE), or sequential irradiation and annealing processes.
What is the Charnley femoral stem made of?
The original stem was a monoblock, flat-backed design with a polished surface manufactured out of EN58J stainless steel (Figure 21.14). The material was changed to 316 low- carbon vacuum-melted stainless steel in 1971 to improve corrosion resistance and fatigue properties.
Figure 21.14 Radiograph of Charnley total hip arthroplasty.
Score 7
Rates of stem fractures led to change in stem surface in 1969 to am aft finish using the vaquasheen process, which deliberately surface-hardened the metal to resist fracture. This was followed in 1974 by the introduction of the round-backed stem, which significantly increased the cross-sectio nal area of the stem over the flat-backed stem to increase stem rigidity and further resist fracturing.
In 1975, anteroposterior Cobra flanges were added to the stem to prevent the escape of cement at the level of the neck resection and also pressurize the cementin the femoral canal. This was designed to prevent subsidence of the stems in the femoral canal, as this was reported to be a significant problem related to stem failure.
These changes resulted in a shift in the behaviour of the stem to a composite beam where its predecessor had obeyed the taper-slip principle.
In 1982, the material was again changed to Ortron 90, a cold-worked stainless steel with high fatigue strength.
Gold medal
When Charnley changed his stem from the flatback to the Cobra design, the biomechanical characteristics were changed from a tapered polished (force-closed) to a shape-closed or composite beam biomechanical design. The biomechanical difference between the loading mechanisms of these two opposing philosophies was not appreciated at the time. A candidate may get asked about the different stem design philosophies of Charnley (composite beam) and Exeter (taper slip). If you are feeling very confident you can discuss the change in stem behaviour with change in Charnley stem.
What about the stress–strain curve of the stem?
The slope of the stress–strain curve of the stem would be the elastic modulus of stainless steel.
What are the risk factors for femoral stem fracture (Figure 21.15)?
Multiple risk factors for prosthetics tem fracture include: High BMI. Higher activity levels. Relatively young age. Male. Varus stem positioning. Relatively undersized stem relative topa tien t’s anatomy.
What is the incidence of Charnley stem fracture?
The first generation of Charnle y stainless steel femoral stems fractured in approximately 4.1% of patients with fatigue failure atiribut ed to insufficient stem cross-sectional area and inadequate cement support.
Improvements in stem design and metallurgy have markedly reduced the incidence of femoral stem fracture.
This is the basic science viva, so candidates are less likely to be asked how they would revise a broken femoral Charnley stem. This is material for the adult and pathology viva.
Figure 21.15 Explanted picture of broken Charnley femoral stem.
Structured oral examination question 7#
TK Aloose implant
Same discussion of wear, mechanics of loosening and biology of osteolysis as in previous questions.
Discussion about cement – what is it, materials, properties, etc.
Wedges for reconstruction of tibial defect, how wedges work, why wedges and not cement for buildup etc.
What types of tibial bone loss can occur with primary THA?
Large posteromedial asymmetrical osseous defects are often seen in the proximal tibia while performing a primary total knee arthroplasty (TK Ain severe varus knees. The majority of these are peripheral uncontained defects. Depending upon the size of the defect, these can be treated with cementoplasty, structural bone gratis or metallic wedges. Augmented prostheses with a built-up metal wedge are mechanically superior to cement alone in terms of resisting movement when loaded. I would prefer to use bone gratiing if possible as it is biological, cost-effectiv e and preserves bone stock for future revisions.
What about cement?
Cement is cost-effectiv e, but it cannot be used to address large bony defects. It is reasonable for use for small, shallow, limited defects in elderly patient sIt is difficult to pressurize it with large and/or uncontained bone loss. Results are mediocre as its resistance to shear stress and compression is low. Thermal necrosis of bone can occur as well as shrinkage when used in large quanties.
How do augments work for build-up of bone defects?
Metal augments allow rapid filling of bone defects that have been geometrically shaped with instruments. They provide stable support and transfer loading forces to the bone. Results are satisfactory. Using thick augments (30 mm) may result in a painful subcutaneous bulge in the tibia or in the distal femur. Thick augments can prevent bone–intercondylar spacer contact and thus limit stable rotation. When there is significant bone loss it is often difficult to achieve stable rotational control of the tibial or femoral implant using just a diaphyseal stem with augments. Therefore, to achieve stable fixation a pr ess-fit metaphyseal femoral sleeve can be used to enhance the rotational stability of the femoral component.
Anything else that can be used to deal with bone loss?
Extensive bone loss can beseen with revision TKA. Management options include: Metaphyseal sleeves (Figure 21.16). These are indicated in elderly patients where there is metaphyseal deficiency. A broach technique is used to prepare the bone for the press-fit implant.
Metaphyseal filling titanium cones (Figure 21.17a and 21.17b). A high variability of sizes and shapes allows a good adaptability of these modules to the metaphyseal bone deficiency, primarily for those types of cavities in which are liable cortical shell in the face of a metaphyseal endosteal bone defect is present.
My preferred option, if possible, ist o use bone gratiing , as it is biological, cost-effectiv e and preserves bone stock for future revisions. Its use is especially applicable in young patient sin whom a further revision operation is an ticipa ted. I would use impacted morsellized bone graft when the reis a contained bone loss larger than 10 mm. I would avoid its use if there was significant cortical bone loss or uncontained defects.
Megaprostheses: high complication rate but the surgical procedure and rehabilitation are rapid.
High rate of infection (~5%) that can often be followed by amputation. Best suited for elderly patients with very large bone loss or complex periprosthetic fractures.
Figure 21.16 Metaphyseal sleeve. The broad tibial sleeve contacts the remaining proximal tibial cortex impacting stability.
Figure 21.17a and 21.17b Metaphyseal filling titanium cones used to reconstruct metaphyseal bone loss in the tibia or femur.
Structured oral examination question 8#
Cementing technique THA
Picture of broken cement mantle (Figure 21.18) – reasons for this, mantle thickness, asked what would you inform your juniors about too litile or too much cement?
What are the reasons for a broken cement mantle?
Initiating events that result in cement failure are due to stresses experienced at the cement mantle that exceed the fatigue endurance limit of both the stem–cement interface and the cement material itself. The early development of stem–cement interface debonding (separation) and subsequent cement fracture are thought to be the initiating events of aseptic loosening. Cement mantle fractures are not benign; they are usually progressive, increasing in number and extent with time. It is important to reduce cement stresses so as to minimize the risk of cement debonding and fracture.
So how can high cement stresses be avoided?
By the creation of an optimally thick symmetric and homogeneous cement mantle.
So how do we achieve this?
Stresses experienced in the cement mantle have been shown to be highest at the stem tip and secondarily at the proximal–medial cement mantle. Stem malalignment produces non-uniform cement mantle thickness in key areas. Defects or voids in the cement mantle reduce bulk cement thickness and have a substantial effect on cement stresses. Variations in implant geometry (e.g. diameter and contour) and material have also been shown to affect stresses experienced in the cement mantle.
Can you be more specific19?
A varus femoral stem is associated with higher incidence of aseptic loosening. This results in a thin or non-existent cement mantle in the proximal medial and distal lateral zones. Large voids up to 5 mm in diameter are detrimental. The location of voids is important. Small voids in areas of the cement mantle known to experience high strains may result in premature fixation failure. A proximal–medial cement mantle greater than 10 mm orless than 2 mm in thickness is associated with a significant increase in cement fracture, radiolucent lines at the prosthesis–cement interface and progressive component loosening when compared to proximal–medial cement mantles that measure 2–5 mm in thickness.20
An asymmetrical distal cement mantle significantly increases the risk of implant failure.
Inadequate centralization of the s tem or malrotation will result in excessively thinned areas of distal cement, increased cement strains and prosthesis bone contact.
Preserving < 2 mm of proximal–medial cancellous bone for 30 mm distal to the femoral neck cut increases cement mantle thickness and reduces proximal–medial cement strain, the incidence of cement fractures, and progressive implant loosening when compared to those cases in which > 2–5 mm of proximal–medial cancellous bone is retained.
Best results for femoral components allow for 2–5 mm proximal–medial thickness of cement mantle, less than 2 mm of proximal–medial cancellous bone thickness, a stem that fills more than half the distal part of the medullary canal and a stem in neutral orientation. Worst results for femoral components occur with a cement mantle thickness > 10 mm, a femur with more than 2 mm proximal–medial cancellous bone, a stem that fills half orless of the medullary canal and those in varus orientation. Charnley believed that cancellous bone was weak and incapable of significant load bearing capacity. He believed that remove lof weak proximal cancellous bone would improve the long-term fixation of ace men ted stem. Cement mantle stresses are mostly affected by stem design, stiffness and geometry. A stiffer stem reduces cement stress and therefore cobalt chromium is the generally preferred material for cemented THA. A thin layer of cement will occur if there is lack of removal of proximal–medial cancellous bone.
What would you inform your juniors about too litile or too much cement?
The femoral stems of hips that have a 2–5 mm thick cement mantle in the proximal medial region have a better outcome than stems implanted with a thicker(> 10 mm) or thinner (< 2 mm) cement mantle.
What about the cement, how can this be improved?
Improvements in the inherent properties of the cement increased strength, reduced britileness, improved interface adherence) to increase strain resistance and thus retard early debonding and microfractures should lead to improved long-term results from cement fixation.
What measures can betaken intra operatively to improve the quality of the cement mantle?
Figure 21.18 Anteroposterior radiograph showing the Exeter total hip replacement with radiolucent lines around the femoral component and fracture of the cement mantle.
1. Canal preparation
Use of correctly sized broaches that allow a mantle of adequate thickness, pulsatile lavage, and brushing and drying of the prepared canal before and during insertion.
Packing of the femoral canal with adrenaline-soaked swabs, hypotensive analgesia to reduce bleeding, suction catheter and avoidance of blood/cement occlusions.
2. Cement preparation
Centrifugation or vacuum-mixing to minimize pore formation and timing of cement injection to achieve optimal viscosity during insertion improves the cement mantle quality. Occlusion of the canal using a distal plug, retrograde filling of the canal and cement gun pressurization of the cement column with a tight proximal seal are essential in achieving an interdigitating , uniform and homogeneous cement mantle.
How can the cement mantle be optimized How can we obtain a high-quality cement mantle?
Methods forcement fixation optimization include: Cement gun pressurization (enhances interdigitation) of the cement column with a tight proximal seal (femoral pressurizer). Pulsed lavage (clean dry bone). Occlusion of the canal using a distal plug. Porosity reduction (v acuum-mixing) which leads to reduced stress points in cement. Cement mantle thickness > 2 mm. Stem centralizer (reduces risk of stem malpositioning to decrease stress on the cement mantle). Absence of cement mantle defects. Stiffer stem (results inless bending stress on cement mantle). Increase in stem modulus of elasticity results in elevated cement mantle stresses. Improvements in the mechanical properties of cement. This question approximates to the previous question, but needs as lightly different answer.
Have you heard of boneloc bone cement?
This is a bone cement that was withdrawn quite soon after introduction because of unacceptable revision rates with its use. Mean fracture toughness and mean tensile strengths were significantly lower than other conventional bone cements.
What is the ideal cement mantle thickness?
I would aim for a cement mantle thickness greater than 2 mm as any less than this increases the risk of cement mantle fracture. Score 8 candidates
Have you heard of the French paradox?
No, I am sorry I haven’t. The ideal cement mantle thickness is still uncertain Two philosophies about cement mantle thickness exist. Int heUK and USA, the first technique aims to produce a complete cement mantle of at least 2 mm in thickness and without ‘windows’. It is believed that ‘windows’ may allow debris to reach the interface and that thin cement mantles will be highly stressed and may fracture. The second technique, used in France, is the use of a thinner cement mantle in which the possibility of windows is accepted. This has been called the French paradox in which implantation of a canal-filling femoral component in a line-to-line manner is associated with a thin cement mantle.21
The reason for good results is thought to be the fact that a thin cement mantle in conjunction with a canal-filling stem was supported mainly by cortical bone and subjected to low stresses.
The discussion could move on to any number of topics related to cement use in arthroplasty surgery depending on how the viva is progressing. Barracks grading of cement.22,23 Generations of cementing technique. Categories of loosening of cemented stems (Harris). Exeter vs Charnley stem design.
Structured oral examination question 9#
Wear in TKA
What types of wear occur in TKA?
There are two main types of tibial component wear in TKA, (1) adhesive and abrasive wear and (2) fatigue damage (piting and delamination ). Fatigue w ear occurs during cyclic loading if the yield stress of polyethylene is exceeded (Figure 21.19). Piting and delamination are accelerated by the presence of free radicals which cause oxidation.
Fatigue w ear in tibial poly – what, why, where?
The appearance of fatigue w ear damage is primarily associated with cyclic compressive- tensile loading at the bearing surface, which generates subsurface tensile stresses that initiate and propagate cracks to form delamination and piting damage. Instances of fatigue w ear and fracture have repeatedly occurred over the history of UHMWPE use in TJR due to changes in molecular weight, fusion defects, crystallinity or cross-linking that can reduce the polymer’s resistance to crack initiation and growth. Fusion defects (manufacturing processes): the high melt viscosity of UHMWPE can prevent total consolidation of the powders and lead to fusion defects. Fusion defects can exist along particle boundaries, acting as cr ack nucleation sites for fatigue w ear (delamination orpit ing) under cyclic sliding contact. Direct compression moulding results in better consolidation of PE, which likely results in the reduction of defects at particle boundaries when compared to ram bar extrusion. Degree of cross-linking of PE: cross-linked PE has improved resistance to abrasive and adhesive wear mechanisms. By limiting the molecular mobility of chains in the amorphous region, cross-linking decreases the creep (cold flow) behaviour of UHMWPE and increases its resistance to wear debris liberation a t the contact surface. However, this also leads to a reduction in plas tic deformation processes of UHMWPE that manifests as a reduced resistance to fatigue cr ack propagation. This renders tibial PE inserts more susceptible to fatigue fracture or delamination w ear in the presence of high cyclic contact stresses or at sites of stress concentration, especially in a non-c onforming bearing surface. Sterilization technique: delamination and piting are common fatigue w ear mechanisms after gamma irradiation in air . Crystallinity of PE: increasing the overall percentage of crystallinity in UHMWPE improves its resistance to fatigue cr ack growth.
With melting of HCL PEther e is lowered crystallinity with improved wear characteristics but reduced mechanical properties. As such, edge loading or excessive PE post loading may result in macroscopic cracks.
Annealing avoids the reduction in the crystalline structure, but there is incomplete elimination of free radicals. Crystalline areas do not cross-link, resulting in an oxidation risk.
The JBJS review article ‘Osteolysis complicating total knee arthroplasty’24 provides a good framework for this viva topic. The starting framework for an ICB viva topic may involve basing the viva topic around a well-wrift en credible review article.
Figure 21.19 Tibial PE insert demonstrating fatigue failure. Fatigue failure is the formation of subsurface cracks in the polyethylene caused by cycles of loading and unloading of the joint, which then propagate and create particles that are shed into the joint space.
Structured oral examination question 10#
Osteolysis
The candidate is shown a picture of a THA with femoral osteolysis (Figure 21.20).
This is essentially a question on osteolysis and wear. The principle cell involved in osteolysis is the macrophage. Wear debris from prosthetic materials or bone cement is phagocytosed by macrophages causing release of various mediators. There is also evidence to suggesta minor role in phagocytosis for fibroblasts and osteoblast cells.
What size of PE material?
Studies have demonstrated that wear particles phagocytosed by macrophages elicited different responses depending on particle size Particles measuring < 5 μm in size generated a strong mononuclear macrophage response, whereas larger particles resulted in more multinucleated giant cells. There are many types of wear particles such as polyethylene, PMMA cement, alumina or metal. All have bioactivity and could therefore be involved in the events leading to osteolysis. Evidence suggests that PE wear particles are the most important factor in periprosthetic bone loss around articulations with PE linings. The debris generated from prosthetic w ear triggers a cascade of macrophage cytokines, such as interleukin-1-beta (IL-1β) and tumour necrosis factor-alpha (TNF-α), among others, resulting in osteoclastic bone resorption and eventually leading to osteolysis. Size: macrophages are most responsive to particles in the size range 0.2–7.0 μm, with larger particles of 90 μm evoking liti le response. Shape: the morphology of particles also appears to contribute to cellular responses, with UHMWPE debris with a roughened surface and a fibular shape provoking a greater response in terms of inflammatory cytokine production than particles with a smooth surface and a globular shape. Material properties (HXLPE vs UHMWPE): higher percentages of small wear particles (0.1 –1 μm range) are produced during laboratory wear of HXLPE than conventional PE. Smaller numbers of HXLPE particles compared to conventional PE are required to stimulate cytokine production from macrophages, possibly because of the higher percentage of smaller particles of HXLPE and the increased ability of cells to phagocytose smaller particles. HXLPE particles may interact with macrophages differently, producing more inflammation and osteolysis compared to conventional PE. The cellular mechanism of particle-induced osteolysis is that macrophages in the periprosthetic tissues phagocytose wear particles and become activated releasing an array of cytokines, leading to increased osteoclastic resorption of the adjacent bone and the production of the granulomatous tissue that fills the resorbed space.
The majority of bone resorption occurs from osteoclasts recruited to sites of osteolysis and activated by the osteoclastogenic molecules. To a limited degree, wear particle-activ ated macrophages present in granulomatous tissue participate directly in bone degradation in periprosthetic osteolysis.
The major pathway of osteoclastogenesis is the production of RANKL by osteoblastic stromal cells and binding of RANKL to its cognate receptor, RANK, on the surface of osteoclast precursors, stimulating these cells to differentiate into mature, active osteoclasts capable of resorbing bone. The natural antagonist of RANK Lis osteoprotegerin (OPG), whose role is to negatively regulate the activity of RANKL , and therefore bone resorption.
Other cell types, including fibroblasts, osteocytes and activated T cells, also produce RANK Land are capable of stimulating osteoclast ogenesis.
There is evidence of direct effects of PE particles on osteoblasts. Exposure of osteoblast-like cells toPE has been shown to induce changes in the rate of cell proliferation, to decrease alkaline phosphatase activity , and to increase the production ofos teoclastogenic mediators, such as PGE2, IL-6, GM-CSF, RANK Land nitric oxide.
Types of wear and modes of wear?
See previous viva questions.
Figure 21.20 Anteroposterior (AP) radiograph, pelvis, demonstrating femoral osteolysis.
Structured oral examination question 11#
Implant materials
Total knee arthroplasty components and the materials used. Discussions about the advantages of cobalt chrome versus stainless steel and then polyethylene manufacture and sterilization. All standard stuff covered. The implant biomaterials used in total knee arthroplasty include: Stainless steel. Cobalt chromium. Polyethylene. Uncemented implants. Titanium. Tantalum. Oxidized zirconium. Stainless steel The advantages of stainless steel include strength relatively ductile, easy availability, easy to process, easy to fabricate, acceptable biocompatibility and relatively cheap. We know its properties and these are consistent throughout. Traditionally , its corrosion resistance was poor, being susceptible topi ting , crevice, fatigue, freting , stress and galvanic corrosion. Its use in TK Awas restricted because other metallic alloys such as Ti-based and Co–Cr-based alloys exhibited superior mechanical (yield strength) and corrosion properties. Newer implant stainless steel contains a high chromium, molybdenum and nitrogen content, making it stronger and resistant to local corrosion. The presence of large amounts of nickel within stainless steel is a concern due to worries about Ni sensitivity . In recent years there have been attempts at replacing the nickel content of stainless steel with nitrogen to diminish the possibility of a nickel allergy developing. There are several methods that can be used to modify Young’s elastic modulus of stainless steel.
What does the 316L stand for in stainless steel?
The 3 stands for molybdenum (3%), 16 for nickel (16%) and L for low carbon (any stainless steel with less than 0.03% carbon). EXAMINER : Why low carbon? It is more resistant to corrosion in the body.
Cobalt chromium
The advantages of cobalt chromium are very good wear characteristics, fatigue strength (very strong), toughness and excellent corrosion resistance. Used as a bearing surface as it is very smooth and scratch resistant. It has a high Young’s modulus of elasticity and therefore risk of stress shielding. There is a worry it can provoke hypersensitivity reactions.
Titanium
Young’s modulus of elasticity is closer to bone; therefore, it is more ductile, with good corrosion resistance, ability to integrate with bone, inert, biocompatible and extremely strong. MRI-compatible. Self passivation. Poor wear characteristics, notch-sensitiv e and therefore not used as an articulation surface.
Expensive.
Can you give me an example of a titanium alloy used?
Titanium alloy 6AL4V (titanium 89%, aluminium 6%, vanadium 4%, others 1%). Vanadium does have potential ion reaction issues, especially if titanium is scratched. Tantalum: has a high resistance to corrosion, excellent biocompatibility and allows excellent osseointegration with host bone. Porous tantalum tibial c ones or metaphyseal sleeves have been used as a management option for severe tibial bone loss in revision knee surgery. Oxidized zirconium (OxZr): developed as an alternative bearing material for TJA. This can be used for femoral TKA components and shows significantly less PE wear than chromium. Low friction and high wear resistance. Higher-order thinking (HOT): puting this all together, cobalt–chromium alloys remain the predominant material (gold standard) used for TKA. There is a worry of increased PE tibial back side wear if titanium is used in place of cobalt–chromium. All poly tibia inserts were introduced to reduce wear and cost, but had poorer clinical results than conventional me tal tibia baseplates. For higher-order thinking, do not just reel off a list of biomaterial properties for each material, but try to match material properties to the function of TKA .
Structured oral examination question 12#
Wear and osteolysis
Acetabular cup. Explanted and worn.
What side is up? Divots on the other side may be from neck impingement.
How can you prevent wear? Implant factors/surgical factors.
Implant material, poly manufacturing process and direct compression moulding. Highly cross-linked with gamma irradiation in air vs. vitaminE. Shelf oxidation.
What about the head – we could use ceramics as less rough and better scratch profile.
Tell me about problems with wear – go through the whole RANK/RANKL discussion.
What side is up?
There is eccentric PE wear in the cup superiorly (Figure 21.21).
What is the difference between PE creep and wear (Figure 21.22)?
Creep is normal loading of the polyethylene cup and is superomedial. It is normal to see slight thinning in the area of the weight bearing as the plastic moulds itself . Abnormal loading leads to pressure more laterally, resulting in polyethylene wear on the superolateral side.
This worn bit here.
I pointed out divots on the outer side of the PE cup and said this could be due to neck impingement.
How do you prevent impingement?
Impingement in THA is both implant- and surgeon-dependent. The implant design factors are those that influence the femoral head–neck ratio aswell as features of acetabular design. The surgeon controls cup position with respect to inclination and anteversion and its depth in the osseous acetabulum. The surgeon also controls the level of the osseous femoral neck cut and the placement of the femoral component for the correct biomechanical restoration of the leg length and hip offset.
What factors affect the head–neck ratio?
The head–neck ratio is affected by femoral head size, femoral neck geometry, and the use of a shirt on the femoral head. Cam-type impingement can occur with use of a small head on a large circular taper or the use of a skirted femoral head. A trapezoidal-shaped neck is designed to create a better head–neck ratio , particularly with small heads.
What features increase acetabular impingement?
Features that increase acetabular impingement include the chamfer geometry of the rim of the polyethylene and the presence of an extended-rim (hooded) liner, particularly if the hood is incorrectly positioned in the hip. Surgeon factors include lateralizing the cup, especially if the cup is also placed in an excessive horizontal position, orf ailing to remove acetabular osteophytes that can impinge against the metal prosthetic neck or bony femur.
What do you mean by chamfer geometry?
The chamfer geometry is where the liner rim is sloped.
How do you reduce the chances of impingement occurring?
Correct restoration of femoral offset and leg length.
How do you ensure correct leg length and restoration of femoral offset?
It is essential to template the hip preoperatively and use a calliper-type pin device intra operatively to check leg length.
How can you prevent wear?
This is best discussed in terms of patien t-related factors, implant-related factors and surgical factors (see above). Implant material – the poly manufacturing process that has the best wear profile is direct compression moulding into the shape of the desired product. No secondary machining of polyethylene. HXPE with gamma irradiation in an inert atmosphere and vitaminE. Conventional PE used in hip arthroplasty was sterilized by gamma radiation in air , which offered the benefit of cross-linking but at the same time, this process produced free radicals that oxidized in air, leading to increased wear. High-dose gamma irradiation of polyethylene is not a sterilization process but a procedure to produce highly cross-linked PE. First-generation HXPE involves the addition of thermal processing (e.g. annealing or remelting) and gas sterilization (eg. ethylene oxide or gas plasma) or gamma sterilization in an oxygen barrier packaging with an inert gas (e.g. nitrogen or argon). Second-generation HXPE involves separate sequential g amma irradiation and annealing steps. The principle behind sequential steps is that smaller doses of radiation in summation can achieve the same levels of cross-linking without generating the number of free radicals created by a single large dose. VitaminE can also be added as a free radical scavenger.
Tell me about shelf oxidation.
Oxidative embritilemen t (characteristically identified as a subsurface white bandis atiribut ed to gamma sterilization in air , and subsequent long-term shelf storage in air has been recognized as a factor contributing to clinical failure (e.g. rim cracking and delamination).
How does femoral head size affect wear?
With conventional UHMWPE the larger the femoral head the greater the volumetric wear. The smaller the head size the greater the amount of linear wear. Frictional torque increases with head diameter, i.e. the moment arm of the rotating object. The wear of surfaces in rotational relative motion is directly related to frictional torque. Frictional torque [Nm] = Friction [N] × distance from centre of rotation [m] Wear of the bearing surface is directly related to frictional torque; therefore, a larger diameter femoral head produces more wear. Volumetric wear is proportional to the frictional torque of the THA. Therefore, an increase in femoral head size increases frictional torque and related volumetric wear.
Figure 21.21 PE cup demonstrating superior eccentric wear.
Figure 21.22 Difference between PE cup creep and wear.
This is the basis of Charnley’s LFA.
How can wear of a THA be measured?
Wear of the bearing surface of THA can be measured in two ways. 1. Linear wear: the thickness of the acetabular cup decreases as it wears with use. Linear wear is the change in the thickness of the acetabular cup with time. Linear wear [mm] = original thickness of acetabular cup [mm] – new shortest thickness of acetabular cup [mm] 2. Volumetric wear: this is the actual volume of wear of the acetabular component. Volumetric wear is related to linear wear by the equation Volumetric wear [mm3] = π × (radius of femoral head [mm])2 × linear wear [mm] As such, a larger diameter femoral head produces more volumetric wear for the same linear wear.
Can you tell me about the scratch profile of ceramic compared to metal (Figure 21.23)?
Best to draw this out if allowed (Figure 21.24). Scratches of the femoral head can lead to an increased rate of PE wear. Ceramics are harder and more resistant to scratching and damage by third-body wear particles than cobalt–chrome. Ceramic heads have superior surface characteristics. The surface is more wettable and better able to maintain surface lubrication than that of a metal head. Additionally , ceramic has a more rounded surface profile with fewer sharp ridges than a metal surface, thus making it better suited for a bearing surface.
Ceramic is chemically inert. In the aqueous environment of the body, passive oxide films form on the surface of metal femoral heads. This passive film is constantly sheared off and recreated during articulation, a process that increases the surface roughness over time, and also releases potentially damaging third-body particles in to the joint space. These consequences are avoided with ceramic heads.
Figure 21.23 Scratch profile of metal and ceramic heads.
Figure 21.24 Candidate drawing of scratch profile.
What factors affect wear of a hard-on-soft bearing surface?
For the head (hard-bearing surface): Surface roughness. Sphericity of head. Areas out of round are high stress points. These areas increase PE wear. Toughness (abrasive wear). Hardness (scratch resistance, adhesive wear). For PE (softer material): Manufacturing process. Sterilization process. Its modification by irradiation. PE shelf life.
References
1. Buzzword.
2. Buzzword again.
3. This applies in general to most of your answers. Candidates usually end up forget inga t least one item from a big numbered list and then appear a bit on the backfoot trying to remember it. Unnecessary.
4. If you decide to mention corrosion be careful, as it is chemical rather than mechanical. Although the examiners are more interested in the different types of mechanical wear, we would still mention it.
5. Analogies are helpful for the purpose of explanation or clarification. W e don’t think there are any major issues using them when appropriate for a principle or idea.
6. Lee S-S, Purdue PE, Nam J-S. Inflammatory periprosthetic bone loss. In Inflammatory Diseases – Immunopathology, Clinical and Pharmacological Bases. IntechOpen; 2012. DOI: 10.5772/25558. Available from: www.intechopen.com/books/inflammatory-diseases-immunopathology-clinical-andpharmacological-bases/biology-of-inflammatory-periprosthetic-bone-loss
7. Banaszkiewicz PA. Periprosthetic Bone Loss in Total Hip Arthroplasty: Polyethylene Wear Debris and the Concept of the Effective Joint Space, in Classic Papers in Orthopaedics. 2014, Springer. p. 85–87
8. Schmalzried T, Jasty M, and Harris WH. Periprosthetic bone loss in total hip arthroplasty. Polyethylene wear debris and the concept of the effective joint space. JBJS. 1992;74(6):849–863.
9. Quite common in practice viva sin underprepared candidates a few weeks away from siting the actual part 2 exam.
10. This may be the main thrust of the topic.
11. This is a gift.
12. Kim Y-H, Park JW, Kim JS, Lee JH. Highly crosslinked-remelted versus less-crosslinked polyethylene in posterior cruciate-retaining TKAs in the same patients Clin Orthop Rel Res. 2015;473(11):3588–3594.
13. Kindsfater KA, Pomeroy D, Clark CR, Gruen TA, Murphy J, Him denS. In vivo performance of moderately crosslinked, thermally treated polyethylene in a prospective randomized controlled primary total knee arthroplasty trial. J Arthropl. 2015;30(8):1333–1338.
14. Lachiewicz PF, Soileau ES. Is there a benefit to highly crosslinked polyethylene in posterior-stabilized total knee arthroplasty? A randomized trial. Clin Orthop Rel Res. 2016;474(1):88–95.
15. Rules need to be broken if needs be.
16. Hood RW, Wright TM, Burstein AH. Retrieval analysis of total knee prostheses: a method and its application to 48 total condylar prostheses. J Biomed MatRes Part A. 1983;17(5):829–842.
17. Doran J, YuS Smith D, Iorio R. The role of all-polyethylene tibial components in modern TKA. J Knee Surg. 2015;28(05):382–389.
18. AhmedI, Salmon LJ, Waller A, Watanabe H, Roe JP, Pinczewski LA. Total knee arthroplasty with an oxidised zirconium femoral component. Bone Joint J. 2016;98(1):58–64.
19. Dennis DA, Lynch CB. Optimizing the femoral component cement mantle in total hip arthroplasty. Orthopedics. 2005;28(8):S867–871.
20. Ebramzadeh E, Sarmiento A, McKellop HA, et al. The cement mantle in total hip arthroplasty: analysis of long-term radiographic results. J Bone Joint Surg Am. 1994;76:77–87.
21. El Masri F, Kerboull M, Kerboull L, Courpied JP, Hamadouche M. Is the so-called ‘French paradox’ a reality? Bone Joint J. 2010;92(3):342–348.
22. Barrack RL, Mulroy R, Harris WH. Improved cementing techniques and femoral component loosening in young patients with hip arthroplasty. A 12-year radiographic review. Bone Joint J. 1992;74(3):385–389.
23. Banaszkiewicz PA. Improved cementing techniques and femoral component loosening in young patients with hip arthroplasty: A 12-year radiographic review. In Classic Papers in Orthopaedics. 2014, Springer. p. 31–34.
24. Gilbert TJ, Anoushiravani AA, Sayeed Z, Chambers MC, El-Othmani MM, Saleh KJ. Osteolysis complicating total knee arthroplasty. JBJS Rev. 2016;4(7): pii: 01874474-201607000-00001. doi: 10.2106/JBJS.RVW.15.00081.