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in-brief |
Dr. Wright is F.M. Kirby Chair of Orthopaedic Biomechanics, Hospital for Special Surgery, New York, NY. Dr. Greenwald is Director, Orthopaedic Research Laboratories, Lutheran Hospital, a Cleveland Clinic hospital, Cleveland, OH. Dr. Lemons is Professor of Prosthodontics & Biomaterials, University of Alabama at Birmingham, Birmingham, AL. Dr. Oral is Instructor, Orthopaedic Surgery, Harvard Medical School, Harris Orthopaedic Biomechanics and Biomaterials Laboratory, Massachusetts General Hospital, Boston, MA.
*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. Wright or a member of his immediate family has received research or institutional support from Zimmer, Synthes Spine, and Smith & Nephew, and has received royalties from Mathys. Dr. Greenwald or a member of his immediate family serves as a consultant to or is an employee of DePuy, Medtronic, Smith & Nephew, Link Orthopaedics, and Wright Medical Technology; and has received research or institutional support from Seminars in Arthroplasty, Aesculap/B.Braun, AxioMed, Biomet, Cervitech, DePuy, Encore Medical, Finsbury, Link Orthopaedics, Wright Medical Technology, Amedica, OrthoDevelopment, Theken, Maxx Orthopaedics, and Smith & Nephew. Dr. Lemons or a member of his immediate family has received research or institutional support from Biomet, DePuy, Smith & Nephew, Stryker, Wright, and Zimmer, and serves as a consultant to or is an employee of Biomet. Dr. Oral or a member of her immediate family has received research or institutional support from Zimmer and Biomet.
The manufacture of polyethylene components for joint arthroplasty has changed over the past few decades. New sterilization techniques and new forms of ultra-high–molecular-weight polyethylene (UHMWPE) have improved the material’s wear resistance. Radiation sterilization and component storage in an oxygen-free environment reduce wear, as do higher levels of radiation exposure for intentional cross-linking of the material. Free radicals generated by irradiation lead to in vivo oxidation, which can be addressed by annealing or melting the UHMWPE after irradiation. Elevated cross-linking with annealing or melting has markedly enhanced the clinical wear performance of acetabular components in total hip arthroplasty (THA). The advent of newer, so-called second-generation cross-linked polyethylenes holds the promise of providing surgeons with wear-resistant materials with improved mechanical properties.
Improvements in polyethylene are not the only design solution to the wear problem. Alternative bearing materials have seen resurgence, particularly in THA. Choices include ceramic-on-UHMWPE, ceramic-on-ceramic, and metal-on-metal bearings. In general, the harder the surface, the greater the wear resistance. Thus, ceramic-on-ceramic and metal-on-metal bearings offer the potential for higher abrasion resistance than UHMWPE bearings. Clinical experience with hard-on-hard bearings has shown them to be very wear-resistant except under exceptional conditions (eg, third-body inclusions). Unexpected complications can arise in hard-on-hard bearings from separation of the femoral head from the acetabular liner during activity, leading to damage caused by impingement between the femoral neck/stem and the acetabular liner or bone, and edge loading as the head reseats under load. Audible squeaks have also been reported with hard-on-hard bearings, although factors affecting their occurrence and severity are poorly understood. Short-term adverse results (eg, hypersensitivity, radiologic loosening) associated with wear debris have been reported in metal-on-metal THA cases, even though laboratory predictions suggest very low wear rates.
UHMWPE remains of strong interest, therefore, as a bearing material, particularly with the clinical introduction of elevated cross-linked forms. The post-processing of these materials to reduce or eliminate free radicals affects properties. For example, remelting leads to a reduction in crystal size, which leads to a reduction in mechanical properties. In contrast, annealing below the melt temperature better preserves crystal structure and properties but reduces free radicals less effectively, leaving the material susceptible to oxidation. Perhaps the greatest concern is that highly cross-linked polyethylenes with lower crystallinities offer significantly reduced fatigue crack propagation resistance and easier crack inception. In vivo fractures of highly cross-linked THA acetabular liners have been reported. Retrieval analysis of such failures suggests fatigue fractures under impingement conditions.
Wear reduction is only one aspect of joint arthroplasty design and must be considered within the context of other goals, such as function and fixation. A wealth of information exists regarding design factors that influence wear of metal-on–conventional UHMWPE bearings, and much of it is applicable to newer bearing surfaces as well. In metal-on-polyethylene THAs, for example, the effect of component thickness depends on whether the head diameter or the inner diameter of the shell is being changed. When the shell diameter is fixed and the head size increased, stresses and therefore linear polyethylene wear are reduced, although volumetric wear increases. By contrast, for hard-on-hard bearings, appropriate surface clearance is a crucial consideration. Appropriate clearances for fluid film lubrication can be determined from engineering analysis. For typical conditions in THA, maintaining lubrication requires highly polished surfaces and very small deviations in spherical concentricity between the bearing surfaces. Unfortunately, this idealized behavior can be severely compromised under conditions of component malalignment or with the occurrence of even a small amount of component geometric distortion brought about by acetabular reaming.
New designs require careful preclinical evaluation for wear. Comparing wear among designs is difficult, however, because only limited preclinical data are generally available, and often very different testing methods are used from one design to another. Wear is typically measured under conditions simulating level walking and thus does not reveal wear modes in "heavier duty" activities. Nonetheless, general conclusions have emerged on design influences. For example, in fixed-bearing total knee arthroplasties (TKAs), lower conformity between bearing surfaces allows larger, more variable tibiofemoral displacements and higher contact stresses, leading to elevated wear. Damage mechanisms for knee components require further elucidation with regard to the use of enhanced polymers as well as changing design conformities. Many TKA designs now provide high flexion while maintaining sufficient tibial contact areas posteriorly to resist polyethylene wear. Tibial post damage in posterior stabilized designs can be reduced by providing for rotation while maintaining sufficient contact areas with the femoral component in both flexion and extension. In contrast to TKA and THA wear, little is known about in vivo UHMWPE degradation or the contribution of wear debris to biologically mediated failure mechanisms in artificial disk replacements. Retrieval studies provide evidence that wear and degradation occur in vivo and that implant subsidence, malpositioning, or migration may result in rim damage, plastic deformation, and component fracture. Extensive and realistic preclinical laboratory wear testing is imperative for prototype disk arthroplasties.
Such testing has long been a mainstay in THA design. Hip joint simulator tests conducted under loads and motions that mimic level walking have accurately predicted clinical wear performance for a variety of bearing materials, most recently highly cross-linked polyethylenes. Such tests can be conducted under more adverse conditions, simulating stair climbing and jogging. Tests have also been conducted to simulate microseparation of the femoral head from the socket. Retrieval analyses suggest that microseparation occurs frequently with hard-on-hard bearings and may be a cause of squeaking in vivo.
Knee wear simulation is more challenging because of the increased degrees of freedom in the knee joint and because of the need for reduced articular conformity between bearing surfaces as a means of providing adequate function for the patient. Nonetheless, knee wear simulation has been used successfully to rank the performance of bearing materials under benign conditions. Improved wear performance has been demonstrated with highly cross-linked and compression-molded polyethylene tibial components and with zirconia and oxidized-zirconia femoral components. As with hip simulation, attempts have been made to simulate aggressive in vivo conditions, such as malaligned components or deliberate roughening of the bearing surfaces. However, these more severe wear protocols have yet to be universally accepted or standardized.
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