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

Abstract

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

In Brief

Spinal manipulative thrusts were delivered to L₄ and L₆ before and after placement of left L_5–6 and L_6–7 or L_4–5, L_5–6, and L_6–7 facet screws in a cat. Facet fixation significantly increased lumbar spinal stiffness and decreased L₆ muscle spindle response. L₆ spindle response during L₄ manipulation was 60% to 80% of L₆ manipulation.