Reduced loading due to spinal-cord injury at birth results in "slender" bones: a case study

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.

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).

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.

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.