RADIOGRAPHIC EVALUATION OF PENETRATION BY THE FEMORAL-HEAD INTO THE POLYETHYLENE LINER OVER TIME

We examined the pattern of temporal penetration by thirty-two-millimet er-diameter femoral heads into polyethylene liners in a group of 105 h ips (103 patients) in which an Arthropor metal-hacked cop had been imp lanted, Each patient was evaluated radiographically and clinically at a minimum of four different postoperative intervals, The initial evalu ation was performed a mean of 2.9 weeks (range, one to fifteen weeks) postoperatively, and the latest evaluation was performed a mean of 7.9 years (range, five to ten years) postoperatively. Two-dimensional wea r - that is, penetration by the femoral head into the ultra-high molec ular weight polyethylene liner - was determined from anteroposterior r adiographs of the pelvis with a computer analysis system that calculat ed the change in the position of the center of the head relative to th e center of the cup. Three new findings are reported. First, there was a large difference (mean, 1.1 millimeters) between the center of the head and that of the cup as measured on the initial postoperative radi ographs. This difference underscores the need for researchers to consi der the initial displacement of the head when measuring and reporting polyethylene wear, Second, although there was wide variation in respon ses among individuals, temporal examination of the data revealed a tre nd toward a decreasing rate of penetration with time, Moreover, the ra te of penetration appeared to reach a steady-state value after the six th postoperative year and remained nearly constant until the ninth pos toperative year, Third, by comparing the subsets of patients who had t he greatest and the least initial penetration by the head, we found th at penetration behavior, although remarkably different between the gro ups in the first three years postoperatively became similar with time, CLINICAL RELEVANCE: When making decisions regarding individual patien ts or hip systems that demonstrate penetration by the femoral head int o the polyethylene liner, clinicians should consider the patterns of p enetration over time, Measurements of the amount acid rate of penetrat ion that are based solely on the most recent radiograph do not represe nt the full clinical picture, We advocate more frequent radiographic f ollow-up and, when available, analysis of serial radiographs for patie nts who have excessive penetration by the femoral head into the acetab ular liner.