Effects of Instrument-Assisted Spinal Manipulation on Lumbar Muscle Spindle Response in an Animal Model

Abstract

Purpose: High velocity low amplitude spinal manipulation (HVLA-SM) has been clinically shown to increase mechanical pressure pain thresholds via unknown physiologic mechanisms. Evidence points to the involvement of central nervous system pain processing and modulation centers. The thalamus is a key pain processing center. The objective of this study was to determine if HVLA-SM thrust duration alters trunk mechanical activation thresholds of nociceptive specific (NS) lateral thalamic neurons. Methods: Extracellular recordings were obtained from 18 NS lateral thalamic neurons in 9 normal anesthesized Wistar rats. Activation thresholds to electronic von Frey anesthesiometer (rigid tip) mechanical trunk stimuli in three directions (dorsal-ventral, 45°caudal, 45°cranial) were determined prior to and immediately following the dorsal to ventral delivery at the L5 vertebra of 3 HVLA-SM thrust durations (time-control-0, 100, 400ms). Mean changes in mechanical trunk activation thresholds were compared using a mixed model ANOVA and one-sample t-tests. Results: Using the mixed model ANOVA analysis, there were no differences of changes in NS lateral thalamic mechanical activation thresholds between the timed-control and both the 100 or 400ms HVLA-SM thrust durations. Due to the variability experienced, changes in mechanical trunk activation thresholds were subsequently analyzed with parametric one-sample t-tests to determine whether there were significant change differences in the 100 and 400ms mean thrust durations when compared to the observed value for the timed-control (0ms). Changes in mechanical trunk activation thresholds increased significantly (t = 2.2, df = 17, p < .05) only following the 400ms thrust duration compared to the 0ms thrust duration in the 45°caudal testing direction. Conclusion: This study suggests that HVLA-SM thrust duration may increase mechanical activation thresholds of NS lateral thalamic neurons when tested in the 45°caudal direction. Due to variability, these results need to be extended in a larger study.Purpose: To determine how instrument-assisted spinal manipulation affects muscle spindle discharge following thrusts delivered at different magnitudes. Methods: Muscle spindle activity from the L6 spinal root was recorded in 7 anesthetized cats prior to and after spinal manipulative thrusts were delivered to the L7 vertebra using 2 clinical instruments (the PulStarÔ and Activator VÔ). Both instruments were used in 3 cats. Typically, 3 thrusts at each of 3 peak magnitudes were delivered with 5 minute intervals between thrusts. Spindle activity was determined in the absence of any contact during 2 secs prior to and immediately following the manipulative thrust. Mean frequency (MF), mean instantaneous frequency (MIF), and time until first action potential (AP) following the thrust were determined. Results: Change from baseline discharge during 2 sec after thrust ({ n = 21, { n = 18) Change from baseline discharge during 2 sec after thrust (*n = 6, 6n = 9, :n = 8) PulStarÔ Activator VÔ 5 lbs{ 10 ...