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J Am Acad Orthop Surg, Vol 16, No suppl_1, July 2008, S111-S119.
© 2008 the American Academy of Orthopaedic Surgeons
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How have wear testing and joint simulator studies helped to discriminate among materials and designs?

Harry A. McKellop, PhD and Darryl D’Lima, MD, PhD

Dr. McKellop is Professor, Department of Orthopaedic Surgery, UCLA–Orthopaedic Hospital, and Director, The J. Vernon Luck Orthopaedic Research Center, Los Angeles, CA. Dr. D’Lima is Director, Orthopaedic Research Laboratories, Shiley Center for Orthopaedic Research and Education at Scripps Clinic, La Jolla, CA.

*The Implant Wear Symposium 2007 Engineering Work Group included Donald L. Bartel, PhD, Thomas D. Brown, PhD, Ian C. Clarke, PhD, Roy D. Crowninshield, PhD, Darryl D’Lima, MD, PhD, A. Seth Greenwald, DPhil(Oxon), Steven M. Kurtz, PhD, Jack Lemons, PhD, Michael T. Manley, PhD, Harry A. McKellop, PhD, Orhun K. Muratoglu, PhD, Ebru Oral, PhD, Lisa Pruitt, PhD, Clare Rimnac, PhD, Peter S. Walker, PhD, and Timothy Wright, PhD.

Dr. McKellop or a member of his immediate family has received research or institutional support from DePuy, Wright Medical Technology, and Exactech; has received royalties and miscellaneous nonincome support, commercially derived honoraria, or other nonresearch-related funding from DePuy; and is a consultant to or an employee of DePuy. Dr. D’Lima or a member of his immediate family has received research or institutional support from Stryker, DePuy, Smith & Nephew, and Zimmer, and has received miscellaneous nonincome support, commercially derived honoraria, or other nonresearch-related funding from Stryker and DePuy.


   Abstract
Top
 Abstract
Total Hip Arthroplasty
 Total Knee Arthroplasty
 Future Directions for Research
 Figures
 References
 
Historically, hip joint simulators most often have been used to model wear of a bearing surface against a bearing surface. These simulators have provided highly accurate predictions of the in vivo wear of a broad spectrum of bearing materials, including cross-linked polyethylenes, metal-on-metal, ceramic-on-ceramic, and others in development. In recent years, more severe conditions have been successfully modeled, including jogging, stair climbing, ball-cup micro separation, third-body abrasion, and neck-socket impingement. These tests have served to identify improved materials and to eliminate some with inadequate wear resistance prior to their clinical use. Simulation of the knee joint is inherently more complex than it is for the hip. It is more difficult to compare the results of laboratory tests with actual clinical performance, due to the lack of accurate in vivo measures of wear. Nevertheless, knee simulators, based on force control or motion control, have successfully reproduced the type of surface damage that occurs in vivo (eg, burnishing, scratching, pitting) as well as the size and shapes of the resultant wear particles. Knee simulators have been used to compare molded versus machined polyethylene components, highly cross-linked polyethylenes, fixed versus mobile bearings, and oxidized zirconia and other materials, under optimal conditions as well as more severe wear modes, such as malalignment, higher loading and activity levels, and third-body roughening. 


   Total Hip Arthroplasty
Top
 Abstract
 Total Hip Arthroplasty
Total Knee Arthroplasty
 Future Directions for Research
 Figures
 References
 
The ability of hip wear simulators to discriminate among alternative materials and designs for hip arthroplasty is best discussed in the context of the four distinct wear modes that occur in vivo1 (Figure 1). Mode 1 occurs when only the two bearing surfaces are articulating against each other, as intended by the designers. Most laboratory wear tests have simulated mode 1, using the loads and motions of normal walking. These studies have accurately predicted the typical clinical performance of a variety of bearing materials, most recently the highly cross-linked polyethylenes (Figure 2).2-9 Mode 1 tests also can be run under conditions simulating stair climbing, stumbling, and jogging, which can be valuable in detecting bearing materials that may be particularly subject to the higher loads that occur under these more adverse conditions.10-12 Hip simulators also have been successful in generating the type of wear particles (including shapes and size distributions) that are released from a prosthesis in vivo13-16 (Figure 3). This is important because the morphology of the particles affects the intensity of the osteolytic reaction.17,18


Figure 1
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  		Figure 1 The four wear modes for a total hip arthroplasty. A, In mode 1 (normal wear), only the intended bearing surfaces are in contact and undergoing wear. B, In mode 2 (subluxation wear), a bearing surface is wearing against a nonbearing surface (eg, due to subluxation of the ball from the socket). C, In mode 3 (third-body abrasive wear), the wear still is occurring between the two primary bearing surfaces, but with third-body abrasive particles interposed. In mode 4, the wear is occurring between two nonbearing surfaces, for example, neck-socket impingement (D) and backside wear (E) between the polyethylene cup and the metal shell. Many other sources of mode 2 and 4 wear are possible, and analogous wear modes occur with arthroplasties for other joints (eg, knee, ankle, shoulder, elbow, spine). (Courtesy of Ian Clarke, MD.)

 

Figure 2
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  		Figure 2 Comparison of clinical wear rates measured radiographically with an earlier prediction using a hip joint wear simulator. The clinical data for the highly cross-linked polyethylenes are plotted as a percentage of the wear rate of the control polyethylene in each study. All studies used gamma-sterilized polyethylene, with the exception of study A, which used gas-plasma–sterilized polyethylene (ie, non–cross-linked). The references for the individual data points are: A,2 B,3 C,4 D,5 E and G,6 F,7 H,8 and I.9

 

Figure 3
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  		Figure 3 Scanning electron micrograph of wear debris from an acetabular cup of conventional (gamma-sterilized) ultra-high–molecular-weight polyethylene that was generated in a hip joint simulator.16 This image demonstrates the shapes and sizes that are also typical of wear in vivo.

 
In mode 2, a bearing surface is wearing against a nonbearing surface, such as when the femoral head is slightly distracted from the socket during the swing phase of gait and comes into contact with the rim of the socket as load is reapplied (Figure 1, B). Fluoroscopic analyses have indicated that ball-socket distraction during normal gait typically is larger with a polyethylene socket than with a hard-on-hard bearing combination, presumably due to the "suction" that occurs with the smaller clearance of the hard-on-hard bearings.19,20 Nevertheless, retrieval analyses have indicated that microseparation frequently occurs with hard-on-hard bearings, typically producing a wear stripe on the femoral ball. This has been reproduced in hip simulators15,21,22 and may contribute to squeaking in vivo.23

Another, more serious type of mode 2 wear occurs when the femoral head is fully distracted from the socket (ie, dislocation) and is dragged across the rim of the metal acetabular shell. Such wear can be more damaging to a metal ball than to a harder ceramic ball; fortunately, once reduced, the metal-on-metal combination has the ability to self-polish,24,25 thereby partially restoring the original low wear rate. With a ceramic ball, a thin layer of metal from the shell is often smeared over the ceramic surface. The smear likely has little detrimental effect with ceramic-on-ceramic bearings, but it could lead to high wear with a ceramic ball bearing against a polyethylene cup.26 For practical reasons, few, if any, hip simulator tests have incorporated full distraction of the ball from the cup in a wear model.

In mode 3, third-body abrasive particles become entrapped between the two primary bearing surfaces (Figure 1, C). Third-body wear particles may directly abrade the ball and cup, or they may roughen the surface of the ball, indirectly increasing the rate of wear of the opposing polyethylene. Investigators have modeled mode 3 wear using two methods. The first is simply to place abrasive particles between the ball and cup.27,28 Although this may closely resemble the situation in vivo, controlling the location of the particles is difficult. In addition, if the particles become embedded in the polyethylene, wear measurements by weighing become unreliable. Another approach is to roughen the ball (eg, using sandpaper, tumbling in a grit) and then to articulate the damaged ball against the polyethylene cup under clean conditions, permitting accurate measurement of the resultant polyethylene wear.11,29-31 The disadvantage of this approach is that it does not include direct wear of the surfaces by the third-body particles, which may make it difficult to reproduce the precise distribution and severity of the type of damage that occurs in vivo.

In mode 4, two nonbearing surfaces are rubbing together under load. Unfortunately, many examples of this mode exist in modern hip prostheses, including backside wear between the acetabular liner and the shell (Figure 1, E), screw-shell fretting, neck-socket impingement (Figure 1, D), fretting at the Morse taper or other modular junctions, and pistoning at the stem-cement-bone interfaces, the latter caused by a debonded or loose prosthesis. Mode 4 wear is generally difficult to model in a hip simulator designed primarily to produce mode 1 wear. A more productive approach is to design a test apparatus and protocol specifically for the type of mode 4 wear being studied. For example, stem debonding and interface micromotion may be modeled using a full-scale mockup of a hip prosthesis implanted in a real or simulated femur and instrumented with micromotion sensors. Such tests are particularly valuable in conjunction with computational finite element modeling, which can identify the optimum locations for placing the micromotion sensors.32,33 The wear resulting from interface debonding and micromotion can then be quantified for various combinations of materials using an apparatus designed specifically for modeling fretting wear.34,35

State-of-the-art hip simulator studies have provided accurate and valuable predictions of the clinical performance of a variety of bearing materials under normal and adverse conditions. Examples include the validation studies in the development of highly cross-linked polyethylenes36-39 as well as evaluation of the effects of ball diameter,40,41 third-body abrasion,27,28,30,31 subluxation,21,22 rim impingement,39,42 (Figure 4), and metallurgy.12,43,44 Implant developers and orthopaedic surgeons should recognize that a given combination of bearing materials might have superior wear resistance in mode 1 yet be particularly susceptible to severe wear in modes 2, 3, or 4. Such trade-offs must be considered when choosing from among the possible combinations of materials and designs for prosthetic hips.


Figure 4
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  		Figure 4 Scanning electron micrographs of damage to the rim of ultra-high–molecular-weight polyethylene cups caused by impingement with the femoral component in a hip joint simulator. The cups were fabricated from polyethylene that had been gamma–cross-linked in the range used for conventional sterilization at 20 Mrad (A), 10 Mrad (B), and 2.8 Mrad (C). (Reproduced with permission from Holley KG, Furman BD, Babalola OM, Lipman JD, Padgett DE, Wright TM: Impingement of acetabular cups in a hip simulator: Comparison of highly cross-linked and conventional polyethylene. J Arthroplasty 2005; 20 [7 suppl 3]:77-86.)

 

   Total Knee Arthroplasty
Top
 Abstract
 Total Hip Arthroplasty
 Total Knee Arthroplasty
Future Directions for Research
 Figures
 References
 
Hip wear simulation has expanded our understanding of tribology of orthopaedic devices and provided relative rankings of bearing materials. Knee wear simulation is more challenging because of the increased degrees of freedom in the knee joint and the need for reduced articular conformity between bearing surfaces in order to provide adequate function for the patient. Imaging and radiostereometric analysis studies can provide reliable in vivo wear measurements for total hip arthroplasty. However, for total knee arthroplasty, implant retrieval analysis remains the primary source of in vivo validation for knee simulators.

Contemporary knee wear simulators can be broadly classified into two categories—predominantly force-controlled and predominantly displacement-controlled.45-48 Displacement-controlled wear simulators are easier to model because the kinematic inputs are easily measured in vivo. However, the forces generated by a displacement-controlled simulator may diverge substantially from in vivo conditions because of differences in implant design. Force-controlled simulators, however, use estimates of knee force magnitude and direction that have not yet been validated in vivo.49,50

The International Organization for Standardization (ISO) standards are the most widely used wear simulator protocols.47,48 These standards simulate the forces and kinematics generated during walking. Force-controlled machines simulate kinematics by applying forces and elastic bumpers (to simulate soft-tissue constraints). Attempts to validate these kinematics against those of cadavers mounted on the simulator50 and those of patients implanted with the same design49 have been at least partially successful. One form of validation is to establish whether the surface patterns of wear generated in vivo match those seen in retrievals. Electron microscopy has shown similarity between the patterns generated experimentally and those seen in retrievals51 (Figure 5). On visual inspection, wear is qualitatively similar among retrieved and wear-tested inserts, with burnishing as the primary mode of damage.52,53 The common secondary damage modes are scratching and pitting in the retrieved inserts. In the wear-tested inserts, scratching only is generally found. The surface area of the visible worn regions on retrievals is larger than that on wear-tested inserts, indicating the involvement of activities other than just walking.52


Figure 5
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  		Figure 5 A combination of rolling and sliding kinematics during in vitro simulation generated longitudinal, transverse, and random patterns of wear similar to those found on retrieved specimens. Longitudinal (A) and transverse (B) patterns shown on scanning electron microscope (SEM) images of polyethylene inserts after in vitro knee wear simulation. Longitudinal (C) and transverse (D) patterns shown on SEM images of retrieved polyethylene inserts. Arrows indicate anteroposterior orientation of the insert. (Adapted with permission from Tamura J, Clarke IC, Kawanabe K, et al: Micro-wear patterns on UHMWPE tibial inserts in total knee joint simulation. J Biomed Mater Res 2002;61:218-225.)

 
Osteolysis is a biologic response to wear debris that is dependent on the size and shape of the particles generated. Therefore, a validation step in knee simulation (as in hip simulation) is the ability of the simulator to generate wear particles similar to those recovered from tissues around knee replacements. The consensus is that wear particles retrieved from knee arthroplasties tend to be larger and more variable in size than those from around total hip arthroplasties.54-58 Differences in the distribution of wear particles by size also may vary depending on features such as increased conformity, design (mobile versus fixed), and bearing material.59-61 The percentage of particles >2 µm is greater in vivo than in vitro53,62 (Figure 6). However, because the smaller particles are more biologically active, the type of wear debris generated by in vitro testing reasonably approximates the debris generated in vivo.53,60-62


Figure 6
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  		Figure 6 A, Scanning electron micrograph of ultra-high–molecular-weight polyethylene (UHMWPE) particles from a total knee arthroplasty revised after 60 months in vivo (original magnification x5,000). Marker bar = 2 µm. Note the predominantly irregularly shaped particles; most are >2 µm. B, Scanning electron micrograph of UHMWPE particles from lubricants taken at 5 million cycles in the knee simulator (original magnification x5,000). Marker bar = 2 µm. Note the presence of smaller, rounded particles with minimal presence of large particles. (Reproduced with permission from Beaulé PE, Campbell PA, Walker PS, et al: Polyethylene wear characteristics in vivo and in a knee stimulator. J Biomed Mater Res 2002;60:411-419.)

 
Knee wear simulation has been used successfully to rank the performance of bearing materials under benign conditions. The improved wear performance of highly cross-linked polyethylene has been established. Highly cross-linked polyethylene is also more wear-resistant under conditions that include aggressive loading63 and accelerated aging64 (Figure 7). Other examples include reduced wear rates in polyethylene inserts tested against zirconia and zirconia-coated femoral components compared with cobalt-chrome alloy femoral components65,66 and reduced wear rates in compression-molded relative to machined polyethylene inserts.67


Figure 7
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  		Figure 7 Results of wear with aged warm irradiated with adiabatic melting (WIAM)-95 versus aged and unaged conventional highly cross-linked polyethylene. The wear rate for highly cross-linked polyethylene was significantly lower than that of either aged or unaged conventional polyethylene knee inserts. (Adapted with permission from Muratoglu OK, Bragdon CR, Jasty M, O’Connor DO, Von Knoch RS, Harris WH: Knee-simulator testing of conventional and cross-linked polyethylene tibial inserts. J Arthroplasty 2004;19:887-897.)

 
Because of the ongoing success of improved surgical techniques, implant design, biomaterials, sterilization methods, and packaging, most total knee arthroplasties do not have significant wear-related complications. Thus, measuring wear under benign conditions has become less relevant. Several attempts have been made to simulate more aggressive in vivo conditions, such as knee malalignment, soft-tissue tightness, higher forces, increased kinematic activity, and component roughening.45,63,68-71 Some of these wear protocols have been successful in generating severe damage and delamination in oxidatively aged polyethylene inserts that had been gamma-irradiated in air, which has become a default outcome for severe wear. However, these more aggressive wear protocols have yet to be universally accepted or standardized.

Highly cross-linked polyethylene is one example of a bearing material that has been successfully validated with hip wear simulation. Because of the harsher contact conditions in the knee, there is concern that damage, not wear, is most important, especially when coupled with oxidative aging. A few wear simulator reports suggest that highly cross-linked polyethylene may be safe in knee arthroplasty.63,64 However, the use of cross-linked polyethylene in total knee arthroplasty lags far behind that in hip arthroplasty.

Comparisons among implant designs have been less successful than comparisons among bearing materials because such comparisons require force-controlled wear simulators.52 Differences in articular conformity and constraint may artificially affect wear rates in designs tested in displacement-controlled simulators. A major design feature that has been extensively studied is the incorporation of a mobile bearing. A mobile bearing is conceptually attractive because of reduced contact stresses resulting from higher articular conformity coupled with increased tolerance for rotational malalignment. However, the presence of an additional articular surface provides a potential source for increased wear. Increased wear in mobile bearing designs has yet to be conclusively proved because of the technical difficulty in assessing regional wear in polyethylene inserts as opposed to global gravimetric changes.


   Future Directions for Research
Top
 Abstract
 Total Hip Arthroplasty
 Total Knee Arthroplasty
 Future Directions for Research
Figures
 References
 
Despite the overall success of hip joint simulators in predicting in vivo behavior of a wide variety of materials and bearing combinations, continual evaluation and refinement of the test protocols are paramount. This can best be achieved by close comparison of the predictions made using the simulators to the actual clinical behavior of new materials. For example, yttria-stabilized zirconia components experience a long-term phase transformation/surface degradation that is not predicted by current simulator test protocols. Improved methods are needed for recovering wear particles from test lubricants (particularly those in the nanometer size range), for characterizing their morphology, and for comparing their relative bioreactivity. There is need for improved understanding of the chemistry of the interaction between biologic lubricants (eg, bovine serum) and the bearing surfaces, especially to identify and eliminate artifacts that result from the necessarily simplified and accelerated laboratory test conditions.

Several areas of research would improve the value of knee wear simulation. More accurate in vivo force and kinematics data are needed for common activities of daily living, as well as for activities that could substantially affect wear because of the potential for increased stresses.72,73 Force-controlled simulators should be developed that could better simulate the passive soft-tissue constraints than do current machines. Sensitive and easily available markers of in vivo knee wear (similar to the radiographic measurements of hip wear) are required so that wear simulations can be validated with in vivo measurements. Finally, the size, morphology, distribution, and bioactivity of wear particles generated in vitro from knee simulators should approximate those generated in vitro. These validations are necessary because experimental reductions in wear may not directly result in clinical reduction of osteolysis, which has significant biologic contributions.


   Figures
Top
 Abstract
 Total Hip Arthroplasty
 Total Knee Arthroplasty
 Future Directions for Research
 Figures
References
 


   References
Top
 Abstract
 Total Hip Arthroplasty
 Total Knee Arthroplasty
 Future Directions for Research
 Figures
 References
 

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