The morphological association between microcracks and osteocyte lacunae in human cortical bone.

We studied the spatial relationship between the osteocyte lacunar-canalicular network and microdamage accumulation in bone matrix. Rib sections from 9 white women aged 50-60 were stained with basic fuchsin and examined using bright-field and fluorescence microscopy. The results showed that the numerical and length density of cracks were 5-fold higher in interstitial bone than in osteons (P<0.001). Osteocyte lacunar density was 17% lower in interstitial bone than in osteonal bone (P<0.001). In addition, the osteocyte lacunae in interstitial bone were significantly fewer (by 16%) in the area adjacent to microdamage as compared with the area remote from microdamage (P<0.001). The proportion of fields with lacunar density less than 728/mm2, the cut-off point calculated from ROC analysis, was 30% in osteonal bone, 55% in interstitial bone remote from microcracks and 83% adjacent to microcracks. The mean values of lacunar density in these bones were 10%, 22% and 27% lower than the cut-off point, respectively. The likelihood of microdamage was 3.8 times higher in bone with osteocyte lacunar density <728/mm2. About 73% of the crack profiles were spatially associated, at least partly, with bone fragments in which osteocyte lacunae were absent. We conclude that microdamage and osteocyte deficiency occur in the same bone regions; there is likely a causal relationship between them but we are unable to say which comes first.