Home | Hip Research | Knee Research Characterization of Long-term Femoral Head Penetration Rates: Association With and Prediction of Osteolysis James E. Dowd, MD Christi J. Sychterz, MS Anthony M. Young, BS Charles A. Engh, MD Background: The present study examined the relationship between long-term head penetration patterns and osteolysis using ten-year follow-up of a well-controlled patient population. The purposes of this study were to characterize the linearity of long-term head penetration patterns over time, to describe the relationship between ten-year true wear rates and osteolysis, and to determine whether the occurrence of osteolysis at ten years could be predicted by penetration data prior to five years. Materials and Methods: We examined minimum ten-year temporal head penetration patterns of forty-eight primary total hip arthroplasties implanted with an Arthropor acetabular cup (Joint Medical Products) and a thirty-two-millimeter cobalt-chrome femoral head (DePuy). Using a computer-assisted radiographic technique, we evaluated two-dimensional head penetration on serial annual radiographs. Linear regression analysis modeled penetration versus time data as a line for each patient. The slope of the regression line indicated the true wear rate for each patient. For a sub-group of thirty-two hips having three annual radiographs prior to five years, we compared early head penetration patterns to the later occurrence of osteolysis. Results: For all forty-eight hips, the true wear rate averaged 0.18 millimeters per year (0.01 to 0.44 millimeter per year) and temporal head penetration patterns tended to be linear (mean r2 = 0.91 ± 0.16). Osteolysis at ten years was strongly associated with increasing true wear rates (p<0.001). None of the nine hips with a true wear rate less than 0.1 millimeter per year developed osteolysis. However, nine of twenty-one hips (forty-three percent) with rates between 0.1 and 0.2 millimeter per year developed osteolysis, as did eight of ten hips (eighty percent) with rates between 0.2 and 0.3 millimeter per year, and all eight of the hips (100 percent) with rates greater than 0.3 millimeter per year. Evaluation of early true wear rates as a predictor of late osteolysis showed a similar relationship. Discussion: This study demonstrates that true wear rates tend to be constant, and that increased true wear is significantly associated with osteolysis at ten-year follow-up. A similar relationship also existed at early follow-up, indicating that early true wear rates (determined from serial radiographs) might enable orthopaedists to predict if patients are at risk of developing osteolysis. Clinical Relevance: Based on these findings, the authors use temporal head penetration data in their own practice to evaluate polyethylene inserts in asymptomatic patients, to estimate time to component wear-through, and to adjust the frequency of follow-up evaluations for monitoring the development of osteolytic lesions in at-risk patients.