Introduction: Combinations of angulation (A), compression (C) and angulation and compression (B) reduced mobility (I) have been shown to induce intervertebral disc narrowing and increased bending stiffness in tails of growing rats (IRSSD 2008). Reduced mobility was probably present in all interventions, and in adjacent discs.
Objective: To determine in vivo mechanisms responsible for these changes.
Methods: Disc tissue from discs subjected to each of the above interventions for 5 weeks provide measurements of matrix protein content (DMMB assay for GAG and hydroxyproline assay), synthesis (via incorporation of radiolabelled proline and sulphate tracers) and gene expression of matrix proteins, their degradative enzymes and inhibitors (via real-time RT-PCR). In addition, tracer incorporation and gene expression were measured in animals euthanized after 5 days. Values were compared between groups, and against within-animal controls.
Results: Tissue composition (water content, and cellularity via DNA Hoechst assay) was not significantly altered at any of the intervention discs. Compression resulted in increased GAG content, but angulation did not result in asymmetrical content. Synthesis rates (tracer incorporation) were higher at 5 days than 5 weeks. After 5 days, compression was associated with greater incorporation of both proline and sulphate. Gene expression studies showed matrix degradation indicative or degeneration and/or remodeling at 5 days in all groups. After 5 weeks, gene expression and tracer incorporation measurements indicated declining levels of tissue remodeling.
Conclusion and Significance: Measurements of disc composition and metabolism indicated relatively small changes relative to large reduction in disc space and mechanical flexibility, but compression was associated with increased GAG synthesis, and increased turnover of collagen. Changes associated with reduced mobility were not evident in measurements of disc metabolism. Disc wedging and structural changes in human scoliosis may result from asymmetrical compression, as well as reduced mobility.