Metabolic Effects of Angulation, Compression, and Reduced Mobility on Annulus Fibrosis in a Model of Altered Mechanical Environment in Scoliosis.

ABSTRACT

STUDY DESIGN: Comparison of disc tissue from rat tails in 6 groups with different mechanical conditions imposed. OBJECTIVES: To identify disc annulus changes associated with the supposed altered biomechanical environment in a spine with scoliosis deformity using an immature rat model that produces disc narrowing and wedging. BACKGROUND: Intervertebral discs become wedged and narrowed in a scoliosis curve, probably partly because of an altered biomechanical environment. METHODS: We subjected tail discs of 5-week-old immature Sprague-Dawley rats to an altered mechanical environment using an external apparatus applying permutations of loading and deformity for 5 weeks. Together with a sham and a control group, we studied 4 groups of rats A) 15° angulation, B) angulation with 0.1 MPa compression, C) 0.1 MPa compression, and R) reduced mobility. We measured disc height changes and matrix composition (water, deoxyribonucleic acid, glycosaminoglycan, and hyaluronic acid content) after 5 weeks, and proline and sulphate incorporation and messenger ribonucleic acid expression at 5 days and 5 weeks. RESULTS: After 5 weeks, disc space was significantly narrowed relative to internal controls in all 4 intervention groups. Water content and cellularity (deoxyribonucleic acid content) were not different at interventional levels relative to internal controls and not different between the concave and convex sides of the angulated discs. There was increased glycosaminoglycan content in compressed tissue (in Groups B and C), as expected, and compression resulted in a decrease in hyaluronic acid size. We observed slightly increased incorporation of tritiated proline into the concave side of angulated discs and compressed discs. Asymmetries of gene expression in Groups A and B and some group-wise differences did not identify consistent patterns associating the discs' responses to mechanical alterations. CONCLUSIONS: Intervertebral discs in this model underwent substantial narrowing after 5 weeks, with minimal alteration in tissue composition and minimal evidence of metabolic changes.