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B. Jenny Kiratli, PhD; Mutsuko Yamada; Amy Smith; Robert M. Marcus, MD; Sara Arnaud, PhD; Marjolein C.H. van der Meulen, PhD
Reduction in bone mass of the lower extremity is well documented in individuals with paralysis resulting from spinal cord injury (SCI). The consequent osteopenia leads to elevated fracture risk with fractures occurring more commonly in the femoral shaft and supracondylar regions than in the hip. In this study, we investigated the relationship between body mass and femoral geometry and structure in adults with normal habitual mechanical loading patterns and those with severely reduced loading following SCI.
The subjects were 78 ambulatory men (aged 20-72 years) and 113 men with complete paralysis from SCI of more than 4 years duration (aged 21-73 years). Subregional analysis of whole body DXA scans was performed to determine mid-femoral bone mineral content and diameter. Using a hollow circular model, we calculated cortical thickness, cross-sectional moment of inertia, and section modulus. For all femoral parameters, the mean values were compared between the ambulatory and SCI subjects. Associations between the femoral parameters and body mass were examined in both populations and compared between the two groups.
All measured bone variables were significantly lower in SCI compared with ambulatory subjects: -29% for bone mineral content, -33% for cortical thickness, -23% cross-sectional moment of inertia, and -22% for section modulus while body mass was not significantly different. Further, the associations between body mass and bone properties were notably different; r2 values were higher for ambulatory than SCI subjects in regressions of body mass on bone mineral content, cross-sectional moment of inertia, and section modulus (Fig. 1). No association was seen between body mass and cortical thickness for either group.
Figure 1. Regression analyses of bone mineral density (BMD) and section modulus (Z) on body mass for ambulatory persons and individuals with spinal cord injury (SCI).
The greatest difference between groups is in the femoral cortex, consistent with reduced bone mass via endosteal expansion. The relatively lesser difference in geometric and structural properties implies that there is less effect on mechanical integrity than would be expected from bone mass results alone. The reduced association in SCI subjects between body mass and bone properties is not unexpected. Although mean body mass differs little between ambulatory and SCI individuals, the association between body mass and in vivo skeletal loading is no longer present, as mechanical influences are removed except for transfer activities. The residual association is probably attributable to the strength of this influence during growth. These results highlight the importance of examining geometry and structure in conjunction with bone mass.
Republished from the 1996 Rehabilitation R&D Center Progress Report. For current information about this project, contact: B. Jenny Kiratli.
