Prevalence of trabecular microcallus formation in the vertebral body and the femoral neck

Trabecular microcallus formation (TMF) has been-described previously in the human vertebra and femur, but the difference in TMF prevalence at these two sites has not been studied and the role of TMF remains controversial. In this study, the 4th lumbar vertebra (L4) and right proximal femur were removed from 27 male and 23 female cadavers. A 2 cm cube cut from the center of L4 and a 1 cm-thick slice cut from the femoral neck were cleaned, defatted, and dried. The apparent density of the L4 cubes was determined as dry weight/bulk bone volume. Using a dissecting microscope at low magnification (4-60x), TMF were identified and counted in both the vertebral and femoral samples. A 8 mm diameter core was then cut from the center of the L4 cubes in the vertical direction, and selected histomorphometric parameters of the core were evaluated with an X-ray microcomputed tomography system (mu-CT). There was a significantly greater prevalence of TMF in vertebral cubes (82%) than in the femoral slices (11%) (P < 0.001). TMF prevalence did not differ significantly between males and females, but the mean number of TMF in the vertebra was significantly (P < 0.05) greater in females (15.0/vertebra) than in males (7.7/vertebra). In the vertebra, the majority of the observed TMF were in vertical trabeculae. Subjects over 60 years old had a higher TMF prevalence than those under 60 years old (P < 0.01). TMF numbers increased with decreasing apparent density (P < 0.05), whereas no significant correlations were found between TMF and bone volume (BV/TV), trabecular number (Tb.N), or trabecular thickness (Tb.Th) as assessed by mu-CT. In two fractured vertebra, very few TMFs (2 and 4, respectively) were observed. These results demonstrated that the occurrence of TMF is strongly related to the anatomical site, probably due to differences in the applied loads and the trabecular structure between sites. The results were consistent with the hypothesis that TMF is a mechanism acting to maintain bone strength, but further studies are needed to clarify this important issue.