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