MECHANISMS OF CORTICAL BONE LOSS FROM THE METACARPAL FOLLOWING DIGITAL AMPUTATION

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 bon e are not well described. We have studied metacarpal morphometry in 16 men who had undergone partial or complete traumatic digital amputatio ns 4-71 years earlier. Noninvolved metacarpals from the affected and u naffected hands were used as controls. Cortical bone width was signifi cantly 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 widt h was not changed. In these subjects the metacarpal was also significa ntly reduced in length, by a mean 2.9 mm (P = 0.035). In the 9 subject s who had their amputation after the age of 19, both a reduction in to tal 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 lo ss of the normal age-related increase in total width (0.01 mm/year). W e conclude that in this model of immobilization osteoporosis, the meta carpal proximal to the amputated digit demonstrates cortical osteopeni a. An inhibition of the periosteal bone formation, which is responsibl e for the normal age-related gain in total width, is an important fact or in this bone loss, which thus differs from normal age-related bone loss (which is endosteal) and from bone loss following acute immobiliz ation (which is from trabecular sites). The pattern of cortical bone l oss differs according to the age at amputation.