Abstract
A recent shift in emphasis with basic science from the ICB is 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 properties 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.
Introduction
A recent shift in emphasis with basic science from the ICB is 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 properties 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
EXAMINER: What do you see? Why has this happened?
CANDIDATE: 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 due to recurrent dislocation.
EXAMINER: 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?
CANDIDATE: Aseptic loosening.
EXAMINER: What do you mean by aseptic loosening?
CANDIDATE: Aseptic (i.e. not caused by infection) loosening refers to the failure of fixation at the bone/implant interface, with resultant micro- or macromotion of the implant relative to the adjacent bone.
CANDIDATE: [Silence.] They are quite similar processes [more silence].
COMMENT: 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 over time, 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.
CANDIDATE: Wear is the removal of material from two surfaces under load, due to a sliding motion between them.
Wear is the progressive loss of a bearing substance caused by mechanical or chemical (corrosion) action.
COMMENT: There are several definitions of wear. Learn one that you are happy with and stick to it.
CANDIDATE: 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.
EXAMINER: What are the various types of mechanical wear?
CANDIDATE: 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 (wear debris), which become third bodies.
An example is between a metal femoral head and a polyethylene liner.
CANDIDATE: 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.
CANDIDATE: With fatigue wear 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 in TKAs.
CANDIDATE: 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.
CANDIDATE: Corrosive wear occurs with surface damage due to chemical reactions with the environment.
COMMENT: Do not give a number when mentioning wear mechanisms, i.e. do not say ‘wear can occur through five mechanisms (adhesive, abrasive, third body, fatigue, corrosion)’. This is inviting trouble.3
Concentrate on the three big wear mechanisms (abrasive, adhesive and fatigue wear) 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.
EXAMINER: Can you draw out the different types of wear (Figure 21.2a–21.2c)?
COMMENT: In the exam it is difficult to draw if you are caught cold and need to work it out for the first time. To be slick, the drawings need to be practised beforehand. Just as important is a succinct but clear explanation of each wear mechanism while you are drawing.
EXAMINER: What type of wear has occurred on the PE cup surface?
CANDIDATE: 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.
CANDIDATE: Adhesive and abrasive wear is more pronounced for THA while fatigue wear (pitting and delamination) is more problematic for TKA.
COMMENT: 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 viva 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 (OPG) is a decoy molecule that blocks RANKL and 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 cytokines and inflammatory mediators include osteoblasts and fibroblasts. Matrix-degradative 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 OPG is significantly inhibited.
Although wear debris may consist of polyethylene, PMMA cement, or metal, by far the great majority of wear particles derives from polyethylene.
EXAMINER: What factors affect the degree of osteolysis from wear particles?
CANDIDATE: 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 broad 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 cytokines.
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.
CANDIDATE: 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 21.3 Model of interplay between macrophages, fibroblasts, lymphocyte, osteoclasts and osteoblasts in periprosthetic osteolysis. Particles may stimulate macrophages, fibroblasts and osteoblasts directly to induce RANKL and pro-inflammatory cytokines that can induce RANKL. It is thought that T cells stimulated by the pro-inflammatory microenvironment may also promote osteoclast formation, synergized 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 at GT 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.
EXAMINER: What are the mechanisms of wear?
CANDIDATE: The main mechanisms of wear include abrasive, adhesive and fatigue wear (see previous answer).
EXAMINER: What do we mean by the term ‘osteolysis’?
CANDIDATE: 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).
CANDIDATE: 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.
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.
COMMENT: There are three definitions. Learn one and stick 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 be seen 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.
EXAMINER: How can you reduce effective joint space?
CANDIDATE: 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 implant/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.
EXAMINER: What new designs of hip replacements have been introduced to retard osteolysis by limiting the generation and spread of particulate debris?
CANDIDATE: 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 cross-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 metal on UHMWPE articulation.
COMMENT: 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).
EXAMINER: What are the risk factors for osteolysis?
CANDIDATE: Risk factors can be broken down into patient-, 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 level 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.
EXAMINER: What is wear?
CANDIDATE: Mechanical wear is the removal of material from two surfaces under load, due to the sliding motion between them.
EXAMINER: What are the modes of wear of artificial joints?
Mode 1. Generation of wear 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.
COMMENT: 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 wear.
EXAMINER: What measures can be taken to reduce wear?
COMMENT: 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 lead in 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 cross-linked PE.
This may not work if the examiners have set questions they are required to ask candidates for the viva topic and PE wear isn’t big on this list.
Another possible option is to discuss:
Surgeon (technique) factors:
Implant selection, avoidance of implant malalignment, avoid impingement (increases wear), accurate restoration of mechanical axis joint, avoidance of debris contamination which will cause third body wear.
Patient factors:
Weight (weight reduction).
Activity level (avoidance of excessive activities, e.g. water skiing, 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 wear.
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
EXAMINER: 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.
EXAMINER: Discuss the factors associated with PE wear in TKA.
CANDIDATE: 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 metal tray occurs due to micromotions occurring at 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 trays 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 air to 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 oxygen-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-socket 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 pitting.
Most supporting evidence for HXPLE in knee arthroplasty is derived from in-vitro wear simulator studies that show a reduction of wear 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, pitting and fatigue wear.
COMMENT: 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 cutting tool effect of machining of PE (Figure 21.8).
CANDIDATE: Three main wear mechanisms can be seen in TKA.15 These are adhesive, abrasive and surface fatigue. Tribo-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 pull out 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.
EXAMINER: What wear damage occurs at the tibial PE surfaces of a TKA?
CANDIDATE: Hood et al.16 described seven types of wear mechanism damage at the articulating surfaces of TKA:
Burnishing (polishing).
Scratching.
Abrasion.
Pitting.
Delamination.
Third-body wear (embedded debris).
Creep (surface deformation).
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.
Pitting: a mode of fatigue wear that is characterized by the formation of millimetre-sized craters (Figure 21.9). It is caused by cracks formed by repetitive 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 wear by rubbing at the bearing surfaces.
Delamination (Figure 21.10): this is a severe form of fatigue wear, 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 in direction and magnitude.
Figure 21.9 Pitting small crater-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 pitting and delamination.
Strategies to reduce polyethylene wear include the following.
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 sagittal 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 backside wear.
Highly polished tibial base plate.
Mobile bearings rotating platform. 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 base plate. This design is thought to eliminate backside wear, which may improve long-term survivorship in patients 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 vitamin E and sequential annealing.
Refining surgical techniques.
Computer navigation.
Fellowship-trained surgeons.
EXAMINER: Is there any evidence that all-polyethylene tibial components reduce wear (Figure 21.11)?
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