Commentary & Perspective

Commentary & Perspective on
"Backside Wear of Polyethylene Tibial Inserts: Mechanism and Magnitude of Material Loss"
by Michael A. Conditt, PhD, et al.

Commentary & Perspective by
Gerard A. Engh, MD, and Matthew B. Collier, MS*,
Anderson Orthopaedic Research Institute, Alexandria, Virginia
E-mail for M. B. Collier: collier@aori.org

This is the third publication by these investigators on the subject of backside wear in modular tibial components^1,2. Here, Conditt et al. describe a method for quantifying the volume of material lost from the distal surface of polyethylene tibial inserts and apply this approach to retrieved implants of the Anatomic Modular Knee (DePuy, Warsaw, Indiana) design. The findings in this study strongly support our clinical and laboratory observations of many modular tibial component designs that feature a grit-blasted or bead-blasted, satin proximal surface^3,4.

The "out of round" presentation of screw hole impressions, first reported by Wasielewski et al.⁵ with other designs, strongly suggests the presence of modular insert motion. We have previously quantified insert motion at the backside interface with a variety of unimplanted and retrieved implants³. Conditt et al. demonstrate that coupling these magnitudes of motion with a relatively rough surface that was not designed to serve as an articulation can sometimes generate large volumes of particulate debris at the backside interface.

The Anatomic Modular Knee modular tibial component was redesigned in 1992 from a titanium baseplate with a grit-blasted, satin proximal surface finish to a cobalt-chromium baseplate with a polished, mirror-like surface finish. We have noted a marked reduction in the prevalence of osteolysis with this redesign and suggest that there is a corresponding decrease in the volume of backside wear debris because of polishing the proximal surface of the baseplate. The implants examined by Conditt et al. had been assembled onto the original titanium baseplate that had four screw-holes. We would anticipate that the methodology employed by Conditt et al. could subsequently be used to demonstrate a dramatic reduction in backside volumetric wear when the varieties of the polyethylene insert examined here were implanted onto a tibial baseplate of identical geometry having a polished proximal surface.

The importance of even a small amount of backside wear must be appreciated. Since backside wear involves nearly the entire undersurface of the component, rather large magnitudes of debris can be generated with relatively small losses in thickness. Backside wear is difficult to identify at revision surgery. The surgeon should carefully inspect the modular side for wear during revision surgery and formulate the optimal intraoperative strategy for maximizing the long-term durability of the reconstruction. Sterilizing polyethylene inserts by means other than gamma-irradiation-in-air should offer substantial benefits in terms of fatigue wear resistance on the articular side but may afford only limited benefits in terms of reducing adhesive and abrasive wear on the backside. In theory, delaying the development of articular surface fatigue wear would slow the progressive tibiofemoral articular constraint that accompanies such wear and thereby limit the stress transferred to the backside interface.

Backside wear remains a concern even when modern polyethylene components are affixed to a blasted (roughened) metal baseplate surface. Importantly, other factors, such as the initial stability and durability of modular tibial component locking mechanisms, malalignment between the femoral and tibial components (in the sagittal and transverse planes), and knee malalignment (in the coronal plane), can also contribute to backside wear and osteolysis and merit additional study.

*The authors did not receive grants or outside funding in support of their research or preparation of this manuscript. G.A. Engh received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity (DePuy, a Johnson and Johnson Company). In addition, a commercial entity (INOVA Health Care Services) paid or directed, or agreed to pay or direct, benefits to a research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.

References

1. Conditt MA, Stein JA, Noble PC. Factors affecting the severity of backside wear of modular tibial inserts. J Bone Joint Surg Am. 2004;86:305-11.
2. Conditt MA, Ismaily SK, Alexander JW, Noble PC. Backside wear of modular ultra-high molecular weight polyethylene tibial inserts. J Bone Joint Surg Am. 2004;86:1031-7.
3. Engh GA, Lounici S, Rao AR, Collier MB. In vivo deterioration of tibial baseplate locking mechanisms in contemporary modular total knee components. J Bone Joint Surg Am. 2001;83:1660-5.
4. Rao AR, Engh GA, Collier MB, Lounici S. Tibial interface wear in retrieved total knee components and correlations with modular insert motion. J Bone Joint Surg Am. 2002;84:1849-55.
5. Wasielewski RC, Galante JO, Leighty RM, Natarajan RN, Rosenberg AG. Wear patterns on retrieved polyethylene tibial inserts and their relationship to technical considerations during total knee arthroplasty. Clin Orthop. 1994;299:31-43.

Copyright © 2005 by the The Journal of Bone and Joint Surgery, Inc.