METABOLIC EFFECTS OF ANGULATION, COMPRESSION AND REDUCED MOBILITY ON ANNULUS FIBROSIS IN A MODEL OF ALTERED MECHANICAL ENVIRONMENT IN SCOLIOSIS.

STUDY DESIGN: Comparison of disc tissue from rat tails in six groups having 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 due in part to altered biomechanical environment.

METHODS: Tail discs of 5-week-old immature Sprague-Dawley rats were subjected to an altered mechanical environment using an external apparatus applying permutations of loading and deformity for 5 weeks. Four groups of rats (A) 15 degrees Angulation, (B) Angulation with 0.1 MPa Compression, (C) 0.1 MPa Compression, and (R) Reduced mobility, together with a sham and a control group were studied. Disc height changes and matrix composition (water, DNA, GAG and HA content) were measured after 5 weeks, and proline and sulphate incorporation and mRNA expression were measured at 5 days and 5 weeks.

RESULTS: After 5 weeks, disc space was significantly narrowed relative to internal controls in all four intervention groups. Water content and cellularity (DNA 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 GAG content in compressed tissue (in Groups B and C), as expected, and compression resulted in a decrease in hyaluronic acid size. Slightly increased incorporation of tritiated-proline into the concave side of angulated discs and compressed discs was observed. 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.