Anti-resorptive agents reduce the size of resorption cavities: a three-dimensional dynamic bone histomorphometry study.

Alterations in resorption cavities and bone remodeling events during anti-resorptive treatment are believed to contribute to reductions in fracture risk. Here, we examine changes in the size of individual remodeling events associated with treatment with a selective estrogen receptor modulator (raloxifene) or a bisphosphonate (risedronate). Adult female rats (6months of age) were submitted to ovariectomy (n=17) or sham surgery (SHAM, n=5). One month after surgery, the ovariectomized animals were separated into three groups: untreated (OVX, n=5), raloxifene treated (OVX+Ral, n=6) and risedronate treated (OVX+Ris, n=6). At 10months of age, the lumbar vertebrae were submitted to three-dimensional dynamic bone histomorphometry to examine the size (depth, breadth and volume) of individual resorption cavities and formation events. Maximum resorption cavity depth did not differ between the SHAM (23.66±1.87μm, mean±SD) and OVX (22.88±3.69μm) groups but was smaller in the OVX+Ral (14.96±2.30μm) and OVX+Ris (14.94±2.70μm) groups (p<0.01). Anti-resorptive treatment was associated with reductions in the surface area of resorption cavities and the volume occupied by each resorption cavity (p<0.01 each). The surface area and volume of individual formation events (double-labeled events) in the OVX+Ris group were reduced as compared to other groups (p<0.02). Raloxifene treated animals showed similar amounts of bone remodeling (ES/BS and dLS/BS) compared to sham-operated controls but smaller cavity size (depth, breadth and volume). The current study shows that anti-resorptive agents influence the size of resorption cavities and individual remodeling events and that the effect of anti-resorptives on individual remodeling events may not always be directly related to the degree of suppression of bone remodeling.