Mechanisms of cortical bone loss from the metacarpal following digital amputation.

Abstract

Immobilization bone loss, whether due to whole body immobilization or local causes, is associated with an initial rapid phase of trabecular bone loss, but the long-term effects of immobilization on cortical bone are not well described. We have studied metacarpal morphometry in 16 men who had undergone partial or complete traumatic digital amputations 4-71 years earlier. Noninvolved metacarpals from the affected and unaffected hands were used as controls. Cortical bone width was significantly reduced in the metacarpals proximal to the amputated digits (P = 0.001). In the 7 subjects who suffered amputation before the age of 19, the cortical bone deficit was primarily due to a reduction in the total width of the medullary shaft (P = 0.007), whereas medullary width was not changed. In these subjects the metacarpal was also significantly reduced in length, by a mean 2.9 mm (P = 0.35). In the 9 subjects who had their amputation after the age of 19, both a reduction in total width and an increase in medullary width (P = 0.017) accounted for the cortical bone deficit. The deficit in total width was related to the time since amputation (P = 0.008) and could be accounted for by loss of the normal age-related increase in total width (0.01 mm/year). We conclude that in this model of immobilization osteoporosis, the metacarpal proximal to the amputated digit demonstrates cortical osteopenia. An inhibition of the periosteal bone formation, which is responsible for the normal age-related gain in total width, is an important factor in this bone loss, which thus differs from normal age-related bone loss (which is endosteal) and from bone loss following acute immobilization (which is from trabecular sites). The pattern of cortical bone loss differs according to the age at amputation.