Paraspinal Muscle Spindle Response to Intervertebral Fixation and Segmental Thrust Level During Spinal Manipulation in an Animal Model.

STUDY DESIGN

In vivo cat model study.

OBJECTIVE

To determine whether intervertebral facet joint fixation and segmental thrust level alter paraspinal muscle spindle activity during simulated spinal manipulation.

SUMMARY OF BACKGROUND DATA

Intervertebral motion is commonly assessed by manual therapy practitioners during clinical evaluation and treatment. Mechanoreceptor activity elicited during spinal manipulation has been theorized as a potential mechanism of its efficacy. The degree to which intervertebral fixation and segmental thrust level alter paraspinal muscle spindle activity during high velocity low amplitude spinal manipulation (HVLA-SM) is unclear.

METHODS

Intervertebral fixation was created by inserting facet screws through the left L(5-6) and L(6-7) and left L(4-5), L(5-6), and L(6-7) facet joints of a cat spine. Changes in the mean instantaneous frequency of L6 muscle spindle discharge were determined during 5 HVLA-SM thrust durations (0-control, 75, 100, 150, 250 ms) delivered at the L4 or L6 spinous process in each of the 3 conditions within the same preparation: laminectomy-only (surgical control; n = 23), L(5-6) and L(6-7) fixations (n = 20), and L(4-5), L(5-6), and L(6-7) fixations (n = 7). Comparisons were made between thrust levels, thrust durations, and spinal joint conditions using a linear mixed model.

RESULTS

Insertion of facet screws compared with laminectomy-only significantly increased (P < 0.001) lumbar spinal stiffness during L6 HVLA-SM. Compared with laminectomy-only, both the 2 facet screw (100 ms; P < 0.05) and 3 screw conditions [75 ms and 100 ms (P < 0.001), 150 ms (P < 0.005), and 250 ms (P < 0.05)] significantly decreased L6 spindle response during the L6 HVLA-SM. HVLA-SM-delivered 2 segments rostral to the level of muscle spindle input significantly decreases spindle response compared with HVLA-SM-delivered at-level; however, nontarget HVLA-SM still elicits 60% to 80% of at-level muscle spindle response.

CONCLUSION

Intervertebral fixation decreases paraspinal muscle spindle response during L6 HVLA-SM in a cat model. Whereas HVLA-SM target accuracy maximizes spindle response, nontarget HVLA-SM still elicits substantial levels of muscle spindle activity.

LEVEL OF EVIDENCE

N/A.