J Am Acad Orthop Surg, Vol 16, No suppl_1, July 2008, S107-S110.
© 2008 the American Academy of Orthopaedic Surgeons
How have new designs and new types of joint replacement influenced wear behavior?
Steven M. Kurtz, PhD and
Peter S. Walker, PhD
Dr. Kurtz is Corporate Vice President, Exponent, Inc, and Research Professor, Drexel University, Philadelphia, PA. Dr. Walker is Professor of Orthopaedic Surgery (Research), New York University, Hospital for Joint Diseases, New York, NY.
*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. Kurtz or a member of his immediate family has received research or institutional support from the National Institutes of Health, Stryker, Zimmer, DePuy Spine, Synthes, and Medtronic. Dr. Walker or a member of his immediate family has received research or institutional support from New York University–Hospital for Joint Diseases and Zimmer.
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Abstract
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As the principles of joint arthroplasty become increasingly refined and more widely established, new designs are being developed that require careful evaluation for their propensity to generate wear debris in vivo. In the past several years, new designs intended to improve clinical performance have emerged in both total knee replacement and total spinal disk replacement. Advances in these types of implants have the potential for major clinical impact in the coming decade, due to the large number of patients seeking treatment of knee arthritis as well as back pain, neck pain, and radiculopathy.
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Knee Replacement Designs
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Knee replacements available today can be classified under the main categories of unicompartmental and total knee (Figure 1). In the former, the primary subdivision is into fixed-bearing and mobile-bearing designs, both of which are used with retention of the cruciate ligaments. The fixed-bearing designs have surfaces of low conformity, while in mobile-bearing designs, both bearing pairs are nominally fully conforming. For the total knee designs, five types of fixed bearings exist. When one or both cruciate ligaments are retained, the bearing surfaces are usually of low conformity, relying on the cruciates for stability. However, when the cruciate ligaments are resected, some means of providing anteroposterior (AP) and rotary stability is required—either a cam-post mechanism (posterior-stabilized designs) or, more recently, by the shapes of the bearing surfaces themselves (surface-guided designs). In the case of mobile-bearing knees, 10 types of bearings exist because the motion at the platform can be either rotation about a single axis, whether central or medial, or rotation with some translation. The femoral-tibial bearing surfaces are either nominally fully conforming or partially conforming when the manufacturers use the same femoral component as is used for the fixed-bearing design in the same system.
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Figure 1 A classification for total knee replacement designs. ACL = anterior cruciate ligament, AP = anteroposterior, PCL = posterior cruciate ligament
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A systematic comparison of the wear (ie, removal of material and damage caused by deformation) between these different types is difficult because of the limited data available in the literature, the different testing methods employed, and the extensive volume of data required for validity. Measuring wear in only a walking cycle, as in the current ISO standard, does not reveal possible wear modes in other activities, even when these activities occur relatively infrequently. Hence, "heavy-duty" versions of the ISO standard, which include activities with higher forces and flexion angles, are often used. Nevertheless, the ISO standard method will provide baseline data for the particular configuration and materials used, and represents the most frequent activity, level gait. A survey of simulator studies, retrieval studies, and fluoroscopic motion studies reveals several key findings relating to wear as a function of geometric factors.
In unicompartmental total knee designs, wear of the mobile-bearing type has been extremely low, even lower than in total hip arthroplasties. In fixed-bearing designs, deformation in the area of most frequent contact has usually occurred, but this does not appear to have had any adverse wear effect in polyethylenes able to withstand delamination. However, in cases of instability, contact near the anterior or posterior margins of the tibial component has sometimes led to severe wear and further instability.
In fixed-bearing designs, lower conformity has shown larger and more variable femoral-tibial displacements that, in combination with higher contact stresses, can lead to slightly elevated wear. For higher conformity, the relative femoral-tibial positions have been more restricted and predictable. This fact, together with lower contact stresses, has tended to reduce wear. However, these factors must be balanced against the requirement of appropriate AP and rotary laxity for functional reasons.1
Given that many patients reach flexion angles 
130°, largely as a result of advances in surgical technique, the bearing geometry needs to preserve similar contact areas at high flexion angles as those that are experienced in lower flexion angles. This performance can be achieved by increased "wraparound" of the superior-posterior femoral condyles. Without such a design modification, digging in of the edges of the femoral condyles occurs. Many designs now provide the capability of achieving high flexion while maintaining sufficient areas of contact.
Femoral-tibial contact areas should not reduce significantly in different femoral-tibial positions, especially in combinations of rotations and displacements. In some designs, digging in of the plastic has occurred at the edges of the bearing areas on rotation, causing elevated wear and deformation. This is significant because the optimal rotational position of the tibial component for tracking in all activities is difficult to predict at the time of implantation of the components. Improved testing methods are now available, making it possible to avoid this situation at the design stage.
Plastic posts in the cam-post posterior-stabilized designs have shown various forms of wear and damage.2 Some posts are designed only for symmetric flexion without rotation, such that in rotation, corner loading has occurred, leading to high contact stresses and eventually to a "bow-tie" wear appearance. In the long-term, this has resulted in fracture of the post in rare cases. Cam-posts were designed to achieve posterior femoral displacement in high flexion. However, anterior contact on the post can occur in extension because of the variable relative flexion angle of the femoral and tibial components on the bones in extension and because of anterior shear forces on the tibia. Post damage can now be reduced by designs that provide for rotation while maintaining sufficient contact areas and that reduce the overall bending stresses on the post.
Most total knee designs use lateral and medial bearing surfaces with the same geometry and thus provide equal constraint on each plateau. Recently, however, surface-guided bearings have attracted interest. These designs use asymmetric geometries intended to be in synchrony with the average neutral path of motion of the healthy intact knee, namely, a tendency to pivot on the medial side, with the main posterior translation being on the lateral side. This proposed design approach has several advantages, such as stability on the medial side, and compatibility with ligament lengths and patellofemoral tracking. In general, such designs have a more conforming medial side, which could reduce wear because of low contact stresses.
Backside wear remains a concern in any fixed-bearing design. Even when the polyethylene tibial insert is tightly fixed in the metal tray, displacement will occur at the polyethylene-metal interface because of deformations of the polyethylene under cyclic weight bearing. The nature of backside wear is not fully understood. How much lubricant is accessible to the insert-tray interface? Do particles readily escape from the interface into the surrounding joint? Are the particles smaller than from the main bearing surfaces? In general, polished tray surfaces and rigid-insert locking mechanisms that can withstand repetitive loading provide the greatest resistance to backside wear. All-polyethylene components and components in which the polyethylene is directly molded onto the tray eliminate backside wear.
The wear of polyethylene patellar components depends on the contact stresses between the component and the femoral trochlea. When the patella is not resurfaced as part of the total knee arthroplasty, the profile of the trochlea of the femoral component should match the anatomy of the natural patellar surface. In increased flexion, when the forces are highest, the notch for retention of the posterior cruciate ligament or for a cam in a posterior stabilized design should be smoothly blended with the trochlea surface and must be narrow enough to avoid contact between the small edge and the patella.
Any new design must be tested for wear, not only by using a standard ISO test,3 but also with a heavy-duty wear test.4 Additional tests are required that target possible wear and failure modes based on variations of surgical technique5 or on specific design features. For example, in mobile-bearing designs, in which some form of track is used to guide motion, repetitive AP and rotary forces may cause wear and deformation of the tracks. Suitable tests for such a design include cyclic AP tests and cyclic loading at the extreme anterior and posterior points of the plastic component. In general, total knee designs that avoid areas of high stress, even in extremes of motion, are likely to show the least wear and deformation in long-term testing.
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Total Disk Replacement
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In the 1960s, total disk arthroplasty was conceived as an alternative treatment to fusion in an effort to avert adjacent-segment degeneration. Total disk replacements are primarily intended for patients in whom the anulus and nucleus are painful and no longer functioning biomechanically. The objective of total disk replacement is to restore pain-free motion and load-carrying capacity of the cervical or lumbar functional spinal unit. The knowledge base in disk arthroplasty is expanding rapidly; however, most of the published literature on this topic has thus far focused on surgical technique and clinical results. Few studies document the wear behavior of disk replacements.
In recent years, interest in disk replacement has increased for both the cervical and lumbar spine. Many artificial disk designs are in clinical use in Europe and are currently under consideration by the US Food and Drug Administration (FDA) for use in the United States. To date, the two designs that have received FDA approval for the lumbar spine feature a polyethylene component between two cobalt-chromium end plates. For the cervical spine, one design using stainless steel–on–stainless steel articulation has recently received FDA approval. Other designs in development include cobalt-chromium–on–cobalt-chromium. In addition, bearing combinations are being considered for total disk replacement that have no precedence as orthopaedic bearing surfaces in hip or knee replacement, such as titanium-on-polyurethane, polyetheretherketone (PEEK)-on-PEEK, polymer composites, and metal matrix composites. Because of their placement directly adjacent to the spinal cord, the consequences of excessive wear and gross mechanical failure of spinal disk arthroplasties may be substantially more serious than for hip or knee joint arthroplasties. Additionally, revision surgery in the lumbar spine can be much more problematic.
In contrast to hip and knee wear, little is known about in vivo degradation or the contribution of wear debris to biologically mediated failure mechanisms of artificial disks. Ongoing retrieval studies6 provide evidence that clinically relevant wear and polyethylene degradation may occur in vivo with artificial disks (Figure 2). Implant subsidence, malpositioning, or migration may result in rim damage, plastic deformation, and component fracture. Chronic inflammatory reactions and wear debris in tissues have also been observed surrounding failed artificial disks.7 However, the clinical significance of wear in the spine remains poorly understood. Although it appears that wear particles may result in a local inflammatory reaction, few cases of osteolysis around artificial disks have been reported.7
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Figure 2 Consistent with a pattern associated with in vivo wear in total hip replacements, the penetration of cobalt alloy end plates into the ultra-high–molecular-weight polyethylene dome of retrieved artificial disks increases with implantation time (n = 21, Spearman’s rho = 0.48, P = 0.03). (Adapted with permission from Kurtz SM, van Ooij A, Ross R, et al: Polyethylene wear and rim fracture in total disc arthroplasty. Spine J 2007;7:12-21.)
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Extensive preclinical laboratory wear testing is imperative for prototype disk arthroplasties.8 Until recently, retrieval data for disk replacements have not been widely available. Consequently, wear testing for total disk replacements remains in its infancy. Validation of current wear-testing protocols has not been demonstrated for any artificial disk design in the peer-reviewed literature. In addition to reliable and clinically relevant wear-test development, retrieval studies are necessary to better understand the long-term in vivo wear and damage mechanisms of disk replacements, as well as to evaluate the clinical significance of wear debris in the spine.
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Future Directions for Research
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Unanswered questions remain related to wear behavior, in vivo stability, and wear testing for new total knee replacement designs and artificial disks. Reliable clinical methods of radiographic wear measurement for knees and total disk replacements are needed. Retrieval analysis and, in the case of metal-on-metal disk replacement, metal ion studies represent current state-of-the-art techniques with an established track record for evaluating alumina, zirconia, cobalt alloy, and ultra-high–molecular-weight polyethylene components. Research is needed to establish the long-term (>10 years in vivo) wear behavior in the spine for these conventional biomaterials in comparison with hip and knee replacements. Both basic and clinical research are required into the biologic response to wear particles in the spine.
Currently, new polymeric biomaterials are being introduced clinically to the spine with limited understanding of their long-term biostability and biocompatibility. Experience with polymeric materials in hip and knee replacements suggests that polymers that are implanted in the spine may be susceptible to in vivo degradation. Research is needed to investigate the long-term in vivo stability of polymeric biomaterials in the spine.
In general, holistic testing regimens need to be further refined and validated for preclinical testing; validation will likely involve integration of computational models, which could also increase the efficiency of testing. Studies of forces and kinematics in vivo can be used to upgrade input data for such testing and modeling. Testing regimes should recognize the design features that provide functional advantages while also being consistent with the tolerance of newer materials for wear and strength during adverse loading conditions, such as impingement. Studies are also necessary for the development and validation of in vitro tests to screen new candidate biomaterials for disk replacement.
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Figures
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References
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- Haider H, Walker PS: Measurements of constraint of total knee replacement. J Biomech 2005; 38:341-348. [ISI][Medline]
- Hanson GR, Suggs JF, Kwon YM, Freiberg A, Li G: In vivo anterior tibial post contact after posterior stabilizing total knee arthroplasty. J Orthop Res 2007; 25:1447-1453. [ISI][Medline]
- Walker PS, Blunn GW, Perry JP, et al: Methodology for long term wear testing of total knee replacements. Clin Orthop Relat Res 2000; 372:290-301.[Medline]
- DesJardins JD, Walker PS, Haider H, Perry JP: The use of a force-controlled dynamic knee simulator to quantify the mechanical performance of total knee replacement designs during functional activity. J Biomech 2000; 33:1231-1242. [ISI][Medline]
- Haider H, Walker P, DesJardins J, Blunn G: Effects of patient and surgical alignment variables on kinematics in TKR simulation under force-control. Journal of ASTM International 2006;3:3-14.
- Kurtz SM, van Ooij A, Ross R, et al: Polyethylene wear and rim fracture in total disc arthroplasty. Spine J 2007; 7:12-21. [Medline]
- van Ooij A, Kurtz SM, Stessels F, Noten H, van Rhijn L: Polyethylene wear debris and long-term clinical failure of the Charite disc prosthesis: A study of 4 patients. Spine 2007; 32:223-229. [ISI][Medline]
- Kurtz S, Siskey R, Ciccarelli L, van Ooij A, Peloza J, Villarraga M: Retrieval analysis of total disc replacements: Implications for standardized wear testing. Journal of ASTM International 2006; 3:1-12. [Medline]
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Abstract
Knee Replacement Designs
