he variations with time in function and morphology at the resorptive site in normal iliac crest trabecular bone were reconstructed from tetracycline double-labeled iliac crest bone biopsies from 20 normal individuals.
Resorption depths below osteoclasts, mononuclear cells, and preosteoblast-like cells were measured by counting the number of lamellae of known thickness eroded. Mean resorption depth below osteoclasts was 19.0 (±4.9) μm. Lacunae containing mononuclear cells were deeper (P < 0.0001), with a mean resorption depth of 49.1 (±10.2) μm. The deepest lacunae were lacunae containing preosteoblast-like cells, with a mean resorption depth of 62.6 (±12.5) μm. This depth was significantly deeper (P < 0.001) than the resorption depth found below mononuclear cells.
Median total resorption period was 48 days (31–68; 95% confidence interval). Median osteoclastic function period was 8 days (6–12), median mononuclear cell function period was 34 days (24–48), and median period where preosteoblast-like cells were present in lacunae before matrix synthesis started was 9 days (6–13). Distribution curves describing the occurrence of the three cell types in relation to resorption depth showed that osteoclasts occupied the more superior parts of the resorption lacunae, with mononuclear cells and preosteoblast-like cells situated in the deeper parts. The distribution curves support the hypothesis that osteoclasts precede mononuclear cells, which again precede preosteoblast-like cells. Based on this hypothesis, curves showing the variation in resorption depth with time were constructed in duplicate. Resorption rates were calculated for three periods. An initial osteoclastic resorption rate showed a median value of 3.8 μm/day (2.4–6.0), a mixed osteoclastic-mononuclear rate was found to be 1.3 μm/day (1.2–1.6), and the terminal mononuclear resorption rate was 0.7 μm/day (0.3–1.2). Median total resorption rate was 1.4 μm/day (1.2–1.7). No significant differences between the distributions of completed wall thickness and preosteoblast-like cell resorption depths could be demonstrated.