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Analysis of Temporal Wear Patterns for Porous-Coated Acetabular Components: Separating True Wear from
So-Called Bedding-InChristi J. Sychterz, MSE, C. Anderson Engh, Jr., MD, Anthony Yang, Charles A. Engh
Background: Standard radiographic assessment of head penetration into a polyethylene liner does not enable clinicians to distinguish between the two processes that cause head movement: true wear (the removal of polyethylene particles) and bedding-in (other factors, such as creep and settling-in of the liner, that contribute to head movement). By analyzing radiographs taken over time, researchers can separate true wear from the bedding-in process. The purpose of this study was to compare the wear performance of the initial modular acetabular cup designs from three different manufacturers (first-generation components) to the performance of one manufacturer's second modular cup design (second-generation component).
Methods: A two-dimensional computerized radiographic method was used to analyze 1300 annual radiographs from 315 hips followed from 3.0 to 10.5 years. Temporal head penetration for the three different first-generation cup groups was compared to that of the second-generation cup group. Multiple linear regression analysis was used to model penetration versus time data as a line for each group. The slope of each regression line indicated the true wear rate for that group, and the intercept of the regression line indicated the amount of bedding-in.
Results: Although design modifications of the second-generation component (thicker polyethylene, improved locking mechanism) decreased average head penetration, the second-generation component's true wear rate was not lower than the true wear rates of the first-generation components. Rather, the decreased head penetration of second-generation component resulted from decreased bedding-in of the liner.
Conclusions: These findings and analysis technique are clinically relevant to surgeons evaluating polyethylene wear radiographically. First, head penetration in the early postoperative years might not be due entirely to abrasive wear of the polyethylene liner, but, rather, to a change in head position resulting from the bedding-in process. Including bedding-in in wear calculations artificially inflates wear rates and, therefore, can misrepresent the potential risk for wear-related complications. This is especially true with different modular cup designs in which conformity and tolerances between the polyethylene liner and metal shell can vary greatly. Second, analysis of head penetration at multiple time intervals can be used to uncouple true polyethylene wear from the bedding-in process. Such an analysis more accurately determines true wear rates of different modular cup designs and, therefore, more accurately assesses potential wear-related complications.