Biomechanical analysis of an induced joint dysfunction (subluxation-mimic) in the thoracic spine of rabbits

The purpose of the present experiment was to evaluate functional biomechanics of the lesioned area in rabbits subjected to surgically placed vertebral lesions which may mimic a subluxation. It was hypothesized that wedging a bar between three adjacent vertebrae would lead to visual and palpatory alterations and also to a decrease in range of motion (ROM) at the affected segments.

A controlled laboratory experiment was performed to measure directly ROM and to evaluate the functional and anatomic relationships of vertebrally lesioned rabbits.

Twelve healthy adult New Zealand rabbits of either sex, weighing 3-5 kg, were maintained in the PCC animal facilities under normal husbandry conditions. Six operated controls and six experimentally lesioned rabbits were available for analysis.

Rabbits were anesthetized and a vertebral lesion surgically placed at various locations along the thoracic spine. A stainless steel bar was implanted to produce a putative partial fixation and misalignment of three adjacent segments. The middle spinous process was forced contralaterally and slightly rotated relative to the adjacent two. At intervals ranging from a few weeks to several months postsurgery, the animals were evaluated for spinal ROM and also by visual and palpatory means for spinal misalignment.

X rays, using rubber foam molds to standardize positioning, were taken pre- and postsurgery to measure ROM. Statistical analysis was made to compare ROM between the lesioned and adjacent areas of experimental and control animals.

All but one of the experimental animals, but none of the controls, showed some reduced ROM on palpation at autopsy. Radiographic measures, however, showed that the ROM in the experimental animals as a group was not significantly reduced compared to control values, nor was there a significant ROM decrease compared to nonlesioned motor units within the experimental group. In some individual animals, however, there was a significant decrease in spinal ROM.

This study failed to demonstrate any effect of the vertebral lesion on spinal ROM as measured radiographically, although by palpation the lesioned area did appear to be partially fixated in most animals. This surprising result may perhaps be explained by the large variability in measurements, the small number of subjects used or by inadequacies of X-ray positioning. Further work on biomechanical effects of vertebral lesions is clearly required.