It is clear that bone mineral mass is markedly reduced in the lower limbs following extreme skeletal unloading, such as occurs after spinal cord injury (SCI). However, the degree of deterioration in geometric structure and strength has received limited attention.
The purpose of the present study was to assess bone structure and strength in the mid-femur of men with complete SCI (7.7 6.0 years) and healthy able-bodied controls (CON; n = 8).
Axial images of the femur were collected on a GE 1.5 T magnetic resonance imager using a Tl weighted sequence. Images were analyzed using X-Vessel. Bone mineral was assessed using dual-energy X-ray absorptiometry (Delphi A; Hologic, Inc.).
There were no differences in age (34.1 ± 9.3y vs 32.5 ± 8.7 y), height (179.2 ± 8.5 cm vs 177.4 ± 7.3 cm), or weight (79.6 ± 21.5 kg vs 78.7 ± 20.7 kg), between SCI and CON, respectively. The volume of the total mid-femur was not different in the two groups and the width in the mediolateral and anteroposterior directions was only slightly smaller in SCI (3 and 6%; P < 0.05). The volume and width of the medullary cavity were 54.1 % and 21–23 % larger in SCI, however (P < 0.05). A large effect size (partial eta squared = 0.25; P = 0.059) suggested a larger endosteal volume in SCI than CON (44 %). In contrast, cortical volume was 23 % lower and the lateral, medial, anterior and posterior aspects of the cortical wall were 22 to 39 % thinner in SCI (P < 0.05). The structural compromise was accompanied by lower calculated cross-sectional moment of inertia, polar section modulus and bone strength index (15 to 35 %; P < 0.05).
The degree of structural difference was similar in the most proximal and most distal portions of the mid-femur (P > 0.05). The findings suggest that after SCI the mid-femur deteriorates from the inside out with the greatest degree of deterioration in the posterior aspect of the bone. The result is a subsequent decrease in bone strength, consistent with the remarkably high incidence of fracture in the femur after injury. Supported by NIH (HD40323).