Characterization of long-term femoral-head-penetration rates. Association with and prediction of osteolysis.

BACKGROUND

We examined the relationship between long-term femoral-head-penetration patterns and osteolysis in a ten-year follow-up study 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 obtained prior to five years.

METHODS

Temporal femoral-head-penetration patterns were examined at a minimum of ten years after forty-eight primary total hip arthroplasties. The arthroplasties were performed with the use of an Arthropor acetabular cup (Joint Medical Products) and a thirty-two-millimeter-diameter cobalt-chromium 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. In a subgroup of thirty-four hips for which three annual radiographs had been made less than five years after the arthroplasty, we compared early head-penetration patterns with the later occurrence of osteolysis.

RESULTS

For all forty-eight hips, the true wear rate averaged 0.18 millimeter per year (range, 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). Osteolysis did not develop in any of the nine hips with a true wear rate of less than 0.1 millimeter per year. However, osteolysis developed in nine (43 percent) of twenty-one hips with a rate between 0.1 and less than 0.2 millimeter per year, in eight of ten hips with a rate between 0.2 and 0.3 millimeter per year, and in all eight hips with a rate of greater than 0.3 millimeter per year. Evaluation of early true wear rates as a predictor of late osteolysis showed a similar relationship.

CONCLUSIONS

This study demonstrates that true wear rates tend to be constant and that increased true wear is significantly associated with osteolysis at ten years after the operation. A similar relationship was also found at the early follow-up interval, indicating that early true wear rates (determined from serial radiographs) might enable orthopaedists to predict if patients are at risk for the development of osteolysis.

CLINICAL RELEVANCE

On the basis of these findings, we use temporal femoral-head-penetration data in our practice to evaluate polyethylene inserts in asymptomatic patients, to estimate the 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.