Skeletal adaptations in hemiplegic patients.

Abstract

Osteoporosis is commonly defined as generalized skeletal disorder, characterized by changes of microarchitecture. The critical role in pathogenesis is attributed to hormonal processes. Our findings of tibial bone changes in hemiplegic resp. hemiparetic patients are not compatible with this view. The adaptations are neither generalized nor are they restricted to the microarchitecture, i.e. trabecular bone. Rather, changes are found in trabecular bone in the epiphysis as well as in cortical bone in the diaphysis. Nor can they adequately be explained by hormones, because they represent an individually different distribution of local changes. The usually cited characteristics of osteoporosis – generalized disorder of predominantly microarchitecture, determined by hormones – are thus called into question by our findings. The bone adaptations found in the limbs of chronically hemiplegic resp. hemiparetic patients can be explained by the feed back principles of the muscle-bone-unit, in which bone strength is controlled by the muscle forces that act upon the bone. An overwhelming amount of data collected from athletes, hemiparetic and paraparetic individuals, as well as animal demonstrate that the reduction of muscle forces acting on bone lead to reduction of bone strength. Based on our ongoing study of post stroke patients it is our hypothesis that a comparison of the paretic limb with the contralateral side gives insight into the structural bone adaptations which follow reduced muscle forces. We make the assumption that the muscle forces acting habitually on the paretic limb are considerably less than on the opposite side. This reduction of forces reduces the strain on bones. This reduction of forces reduces the strain on bones. This leads to loss of bone mass and bone strength. We are testing this assumption in two parallel trials. Sports with clearly different forces acting on different limbs can, similarly to hemiparesis and paraparesis, be seen as experiments of nature explaining the relationship between habitual forces and bones. These data support Wolff’s law and the Utah paradigm 1-8 . Post stroke patients have a two- to fourfold risk of hip fractures 9-13 , presumably resulting from a combination of increased fall frequency and reduced bone strength (66% of fractures occur on the stroke-affected side 12 ). However, the published bone data of post stroke patients are inconsistent, and often measured by DEXA, therefore lacking any information about architecture. Data collected by DEXA-technique only give information about mass changes and not about changes of structure.