Bone density adjacent to press-fit acetabular components - A prospective analysis with quantitative computed tomography

Background: The status of periprosthetic bone stock is an important concern when revision total hip arthroplasty is undertaken. Remodeling of peripros thetic femoral bone after total hip arthroplasty has been studied extensive ly, and the phenomenon of femoral stress-shielding has been well characteri zed. Finite element analysis and computer-simulated remodeling theory have predicted that retroacetabular bone-mineral density decreases after total h ip arthroplasty; however, remodeling of periprosthetic pelvic bone in this setting has yet to be well defined. This study was conducted to evaluate th e short-term natural history of periacetabular bone-mineral density followi ng primary total hip arthroplasty. Methods: Periacetabular bone-mineral density was studied prospectively in a group of twenty-six patients who underwent primary hybrid total hip arthro plasty for the treatment of advanced osteoarthritis. Density within the cen tral part of the ilium (directly cephalad to a press-fit acetabular compone nt) was assessed with serial quantitative computed tomography. Baseline den sity was measured within the first five days following the total hip arthro plasty. Ipsilateral density measurements were repeated at an average of 1.2 8 years postoperatively. Density values at corresponding levels of the cont ralateral ilium were obtained at both time-points in all patients to serve as internal controls. Results: Bone-mineral density decreased significantly (p less than or equal to 0.001) between the two time-points on the side of the operation. The me an absolute magnitude of the interval density reduction (75 mg/cc) was grea test immediately adjacent to the implant (p < 0.001), but it was also signi ficantly reduced (by 35 mg/cc) at a distance of 10 mm cephalad to the impla nt (p = 0.001). Relative declines in mean density ranged from 33% to 20% of the baseline values. No focal bone resorption (osteolysis) was detected at the time of this short-term follow-up study. With the numbers available, n o significant interval alteration in bone-mineral density was found on the untreated (internal control) side (p <greater than or equal to> 0.07). Conclusions: We suggest that the observed decline in bone-mineral density r epresents a remodeling response to an altered stress pattern within the pel vis that was induced by the presence of the acetabular implant. This findin g corroborates the predictions of finite element analysis and computer-simu lated remodeling theory. It remains to be seen whether this trend of atroph y of retroacetabular bone stock will continue with longer follow-up or will ultimately affect the long-term stability of press-fit acetabular componen ts.