Background: Alendronate is a pyrophosphate analogue
of bisphosphonate that has been shown to inhibit osteoclastic bone
resorption. Bone formation and remodeling are necessary to establish
initial fixation of uncemented implants, especially those coated with
a bioactive surface such as hydroxyapatite. Because the process
of bone-remodeling that culminates in new-bone formation is thought
to be initiated by osteoclastic bone resorption, it is appropriate
to test the influence of osteoclast-inhibiting medications on bone
apposition to hydroxyapatite-coated implants.
Methods: Twelve dogs underwent staged bilateral
total hip arthroplasty, with twenty weeks between the first and
second operations, with use of a titanium-alloy femoral stem that
had a proximal macrotextured surface and a plasma-sprayed hydroxyapatite coating.
Six of the dogs received oral alendronate therapy from the time
of the surgery until they were killed; the other six dogs were untreated
controls. The animals were killed four weeks after the second operation.
Sections from matched implant sites (proximal, middle, and distal)
were histologically analyzed. The linear extent of bone apposition,
the linear extent and the thickness of the hydroxyapatite coating,
and the total amount of cortical and trabecular bone were measured with
the use of an interactive image analysis system.
Results: There were no significant differences in
radiographic or histologic findings between the two groups at either
four or twenty-four weeks. Although the extent of the hydroxyapatite coating
decreased significantly with time in both groups (p < 0.01),
we identified no significant influence of alendronate on the extent
of bone apposition, the extent or thickness of the hydroxyapatite
coating, or the cortical or trabecular bone area around the implants.
Conclusions: Many patients who are receiving alendronate
for osteoporosis or other disorders may also be candidates for cementless
total joint arthroplasty. Although bone formation is generally thought
to be initiated by and coupled with bone resorption, our results
suggest that alendronate has no discernible effect on the initial
fixation of or the short-term bone-remodeling around hydroxyapatite-coated
femoral total joint implants.