Intervertebral disc changes with angulation, compression and reduced mobility simulating altered mechanical environment in scoliosis

The intervertebral discs become wedged and narrowed in scoliosis, and this may result from altered biomechanical environment. The effects of four permutations of disc compression, angulation and reduced mobility were studied to identify possible causes of progressive disc deformity in scoliosis. The purpose of this study was to document morphological and biomechanical changes in four different models of altered mechanical environment in intervertebral discs of growing rats and in a sham and control groups.

External rings were attached by percutaneous pins transfixing adjacent caudal vertebrae of 5-week-old Sprague–Dawley rats. Four experimental Groups of animals underwent permutations of the imposed mechanical conditions (A) 15° disc angulation, (B) angulation with 0.1 MPa compression, (C) 0.1 MPa compression and (R) reduced mobility (N = 20 per group), and they were compared with a sham group (N = 12) and control group (N = 8) (total of 6 groups of animals). The altered mechanical conditions were applied for 5 weeks. Intervertebral disc space was measured from micro-CT images at weeks 1 and 5. Post euthanasia, lateral bending stiffness of experimental and within-animal control discs was measured in a mechanical testing jig and collagen crimp was measured from histological sections.

After 5 weeks, micro-CT images showed disc space loss averaging 35, 53, 56 and 35% of the adjacent disc values in the four intervention groups. Lateral bending stiffness was 4.2 times that of within-animal controls in Group B and 2.3 times in Group R. The minimum stiffness occurred at an angle close to the in vivo value, indicating that angulated discs had adapted to the imposed deformity, this is also supported by measurements of collagen crimping at concave and convex sides of the disc annuli.

Loss of disc space was present in all of the instrumented discs. Thus, reduced mobility, that was common to all interventions, may be a major source of the observed disc changes and may be a factor in disc deformity in scoliosis. Clinically, it is possible that rigid bracing for control of scoliosis progression may have secondary harmful effects by reducing spinal mobility.