Spinous process hypertrophy associated with implanted devices in the external link model

Recent development of a chiropractic subluxation mimic, the external link model, uses titanium implants on lumbar vertebrae in the rat. The objective of this study was to evaluate potential correlations in the model between linking history, bone resorption, exudate formation, and experimentally induced intervertebral hypomobility.

Serial lateral radiographs of 73 male Sprague Dawley rats with implanted devices were reviewed. A baseline radiograph was obtained after a 6-week surgical recovery period, and a second radiograph was exposed after an 8-week hypomobility induction period. Spinous hypertrophy at the implant sites (L4, L5, and L6) was measured on the radiographs with a vernier caliper. Bone resorption and exudate build-up were assessed and compared with intervertebral hypomobility data previously collected. Data trends were described using cross-tabulated counts, analysis of variance, and regression analysis.

Cross-tabulation suggested differences between hypomobility-induced rats and control rats. However, correlation analysis showed no predictive role for spinous hypertrophy relative to intervertebral mobility. Similarly, exudate level did not predict spinous hypertrophy. However, implant presence and vertebral level had a significant interaction, with moderate and severe hypertrophy occurring more frequently at L4 and L6 in hypomobility-induced rats. Age did not materially influence spinous hypertrophy.

Mechanical stresses produced at the implant bone interface in rats with induced hypomobility contribute to spinous hypertrophy beyond that simply due to the presence of the implants. However, spinous hypertrophy does not contribute significantly to intervertebral hypomobility in the external link model.