Reduced loading due to spinal-cord injury at birth results in “slender” bones: a case study
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INTRODUCTION: The present case study compared bone density, bone geometry and muscle cross-sectional area (CSA) in a male who sustained spinal-cord injury (SCI) at birth (from here called SCI-B) with two matched controls without SCI, and also with four individuals with SCI of similar level and injury completeness but sustained at age 15 or greater. METHODS: All subjects with SCI were at least 3 years post-injury and had experienced motor incomplete lesions at the cervical level. Computed tomography was used to measure volumetric bone density, indices of bone strength [CSA and maximum, minimum and polar area moments of inertia (I (max), I (min), I (pol))] and muscle CSA at the tibia (66% of tibia length, measured proximally from the distal end). RESULTS: Lower leg muscle CSA of SCI-B was 63+/-6% of values in non-SCI controls, and 72+/-12% of values in other males with SCI. In SCI-B, bone CSA was roughly half (52+/-4%) that of non-SCI controls and 73+/-16% of bone CSA values in other males with SCI. The magnitudes of the area moment of inertia variables (I (max), I (min), and I (pol)) in SCI-B were approximately 25% of control values. Further, the moment of inertia variables in SCI-B were 27-54% of values obtained in other males with SCI, indicating that experiencing SCI in the early stages of life has a remarkable impact on bone shape. Interestingly, tibia bone density did not appear to be affected; the average difference in bone density between SCI-B and non-SCI controls was -1.2+/-0.7%. The bone densities of other males with SCI were 4-19% lower than in SCI-B. CONCLUSIONS: Muscle atrophy and bone loss are commonly reported consequences of SCI. This case reveals that important changes in bone geometry occur after SCI, and that mechanical loading during growth plays a vital role in the development of bone size and shape.
