Analysis of temporal wear patterns of porous-coated acetabular components:Distinguishing between true wear and so-called bedding-in

Background Standard radiographic assessment of penetration by the femoral h ead into a polyethylene liner does not enable clinicians to distinguish bet ween the two, processes that cause movement of the head: true wear (the rem oval of polyethylene particles) and so-called bedding-in (other factors, su ch as creep and settling-in of the liner), By analyzing radiographs made ov er time, researchers can distinguish true wear from the bedding-in process. The purpose of the current study was to compare the wear performance of th e initial modular acetabular cup design (so-called first-generation compone nts) of three different manufacturers with that of a so-called second-gener ation component made by one of the manufacturers, Methods: A two-dimensional computerized radiographic method was used to ana lyze 1300 radiographs of 315 hips that,were followed for 3.0 to 10.5 years. Temporal penetration by the head in the three groups of first-generation c ups was compared with penetration in the group of second-generation cups. M ultiple linear regression analysis was used to model penetration-versus-tim e data as a line for each group. The slope of each regression line indicate d the true rate of wear, and the intercept of the regression line indicated the amount of bedding-in. Results: Modifications in the design of the second-generation components, i ncluding thicker polyethylene and an improved locking mechanism, led to a d ecrease in the mean penetration by the head; however, the second-generation component did not have a lower true rate of wear than two of the first-gen eration components. Rather, the decreased penetration by the head into the second-generation component resulted from decreased bedding-in of the liner . Conclusions: These findings and this technique of analysis are clinically r elevant to surgeons who evaluate polyethylene wear radiographically. First, penetration by the head in the early postoperative years might not be due entirely to abrasive wear of the polyethylene liner but, rather, to a chang e in the position of the head resulting from the bedding-in process. The in clusion of bedding-in in calculations of wear artificially inflates the rat e of wear and may result in a misrepresentation of the potential risk of we ar-related complications. This is especially true with regard to comparison s of different designs of modular cups, in which conformity and tolerances between the polyethylene liner and the metal shell can vary greatly. Second , analysis of penetration by the head at multiple time-intervals can be use d to distinguish true polyethylene wear from the bedding-in process. Such a n analysis allows more accurate determination of the true rates of wear of different designs of modular cups and, therefore, of potential wear-related complications.