Polyethylene debris can cause patient osteolysis, patient pain and discomfort, and implant revision. Previous fluoroscopic studies have determined the incidence of femoral head separation from the acetabular cup, but clinical significance of this phenomenon has not been established. It has been hypothesized that hip separation may lead to polyethylene wear, while others hypothesize that hip separation may be occurring due to wear. Therefore, the purpose of the study is to conduct an in vivo kinematic analysis to determine if there is a correlation between-femoral head separation and wear and to utilize a mathematical modeling to determine the clinical significance of these variables.

Twenty subjects were strategically selected to participate in this study. Ten subjects were determined to have at least1.0 mm of polyethylene wear, while ten subjects had less than 0.1 mm of polyethylene wear. All 20 patients were asked to perform gait on a treadmill while under fluoroscopic surveillance. The incidence of femoral head separation was determined for each subject. Then, a three-dimensional mathematical model of the hip joint was used to determine bearing surface conditions for each subject.

Fifty-five percent of the subjects evaluated demonstrated femoral head separation. Subjects deemed to have greater than 1.0 mm of wear experienced less separation, on average and overall magnitude than subjects without wear. In this study, only 10% of the subjects tested-demonstrated no wear and no separation. The derived force profiles in this study were greater for both groups, compared with the non-implanted hips, previously evaluated. The forces in the hip joint ranged from 2.0 to 3.0times body weight.

Although it was expected that subjects having more wear would have greater magnitudes of femoral head separation, the opposite was true. Further kinetic analysis determined that the subjects having wear also experienced greater force profiles through gait. Therefore, it is assumed that the subjects having wear may have been-implanted with a tighter socket, thus leading to greater shear forces.