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Surface Stress: A Factor Influencing Ultra High Molecular Weight Polyethylene Durability in Mobile-bearing Knee Design

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Abstract

The continuing global interest in the use of total and unicompartmental mobile-bearing knee designs is manifest by an appreciation of their clinical performance. Like their fixed plateau counterparts, mobile-bearing knees are influenced by patient and surgical variables as well as design, material, and manufacturing choices. This article is a focused description of tibiofemoral surface stress distributions, as a predictor of in vivo material durability for three contemporary designs at positions encountered during daily activity.

The increased use of mobile-bearing knee designs throughout Europe and the Pacific Rim is an indication of their clinical utility despite limited use in the United States (Table 1). Issues of regulatory restriction, the clinical success of fixed plateau designs, suspected technical difficulty, a relatively small number of seminal reports from major teaching centers, and cost are factors that have impeded use in the United States.

Mobile-bearing Knee Design

Mobile-bearing knees represent an optimization of the two extremes of fixed plateau knee design; they offer significant increases in the articulation conformity, distributing the contact stresses that develop, decreasing the prospect of ultra high molecular weight polyethylene (UHMWPE) material damage while through insert mobility, minimizing constraint forces transferred to fixation interfaces (Figure 1).¹

However, the construct of dual surface motion, which encourages distal sliding, seems almost intuitive of greater wear volumes. Simulator studies conducted almost 30 years ago demonstrated that the UHMWPE wear loss volume for the low contact stress meniscal-bearing knee design corresponded favorably with contemporary, fixed plateau wear volumes.² Resolution of this seeming conundrum lies in the fact that determinants of abrasive wear may be characterized by both sliding distance and surface stress distribution, which is described by Rose and Goldfarb³ (Figure 2). A nonlinear relationship is described between increasing surface stress and resulting UHMWPE wear volume.

Contemporary use of finite element analyses enables a visualization of contact areas and surface stress distributions. This article presents these data for two total and one unicompartmental mobile bearing knee designs: the Dual Bearing Knee (Finsbury Orthopaedics Ltd, Leatherhead, United Kingdom), the e.motion (Aesculap, AG & Co KG, Tuttlingen, Germany), and the Oxford unicompartmental Phase 3 (Biomet Ltd, South Wales, UK). The first two systems are not currently available for clinical use in the United States.

Materials and Methods

Finite element analyses. A three-dimensional, finite element model was created for each total knee design by measuring the articular surfaces of implantable quality parts using a laser profilometer. Maximum joint loads and the angle of knee flexion at which they occur were determined through a meta-analysis of the literature for three activities of daily living: heel strike during walking gait (0°),^4-9 stair ascent (60°),^4,10-12 and rising from a chair (90°).¹³ For the Oxford unicompartmental Phase 3 design, the positions were simulated with the medial compartment receiving 60% of these stresses (Table 2).¹⁴

The loads were applied, and the virtual components were allowed to settle into their preferred alignments without friction or consideration of soft-tissue constraints. To aid in comparison, all polymer inserts were characterized by the same gamma irradiated, nonlinear material of 10-mm thickness maintained at 37° C.¹⁵ Contact areas and stresses on the polymer inserts were calculated, and their magnitudes and locations were then photorealistically imaged.

Results

Figures 3 through 5 present the surface stresses from a superior posterior view of the left knee for three contemporary, mobile-bearing knee designs.^16-18 These images give an indication of the areas where surface abrasion caused by contact with the femoral component can occur, illustrating that the higher the surface stresses, the greater the propensity for material damage.

Discussion

When comparing the surface stress distributions in Figures 3A, 4A, and 5A to those of a fixed plateau knee design at the heel strike position of walking gait (Figure 6), it is apparent that the tibiofemoral articulating interfaces have been optimized, decreasing the potential for abrasive UHMWPE material damage, a result attributed to the mobility of the tibial insert.

A concern about designs is the potential loss of conformity as the knee flexes beyond heel strike, which is substantially influenced by the J curve of the femoral component. For these three designs, significant conformity is appreciated at 0° during the stance phase of walking gait, a frequently encountered, high-cycle loading activity. With further flexion (Figures 3B and C; 4B and C), the Dual Bearing Knee and e-motion contact areas decrease with concomitant increases in surface stresses, increasing their potential for abrasive damage.

By contrast, this does not occurr in the Oxford unicompartmental Phase 3 design where the J curve gives way to a constant radius of curvature (Figure 5B and C). This also describes differences in design philosophy between contemporary unicompartmental and total mobile-bearing knee systems.

Conclusion

This commentary describes the role of articulation conformity in modulating surface stress distributions across mobile-bearing knee plateaus, contributing to the durability of the UHMWPE component. In 2004, the US Food and Drug Administration (FDA) approved the Oxford meniscal unicompartmental knee Phase 3 system for distribution in the United States where it is realizing clinical utility in situations of medial compartment degenerative arthritis, concomitant with functioning and retained cruciate ligaments. Parenthetically, in June 2004 the USFDA Orthopedic Advisory Panel recommended the reclassification of total mobile-bearing knee systems for general use. This reclassification petition is currently under review at the USFDA.

References

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Authors

Mr Morra, Ms Heim, and Dr Greenwald are from Orthopedic Research Laboratories, Lutheran Hospital, a Cleveland Clinic hospital, Cleveland, Ohio.

Mr Morra, Ms Heim, and Dr Greenwald have no financial interests in the materials mentioned herein.