Theory of small vertebral motions: an analytical model compared to data

Develop an analytical theory describing the dynamics of small impulses applied to vertebrae, such as in chiropractic adjustments or spinal manipulative therapy.

Data were compared with damped harmonic oscillator models of vertebrae.

Evidence accumulates that chiropractic adjustments are effective in addressing a variety of health problems. However, the biomechanics characterizing spinal manipulation is largely unknown. Recently, relative separations of the L2-L3 vertebrae subsequent to activator adjusting instrument thrusts were measured in vivo at 2048 Hz.

Nine-parameter models for axial, shear and angular motions were fit to sets of 350 data points. They required frequencies of 5, 8 and 11 Hz, corresponding to well-known spinal resonances. The fits are excellent, with 0.75 < corrected R(2) < 0.96. Stiffnesses are calculated to be less than 10% of elastic zone values. Viscosities of the paravertebral medium are predicted to be between 6 and 30 poise, comparable to synovial fluids.

This model ties together vertebral responses to small impulses with spinal resonances and other spinal/vertebral characteristics. Stiffness dominates damping resistance except in angular motions. Coupling appears unimportant in adjacent vertebral responses to small impulses. Further research is needed to clarify this issue.

This study indicates how both force (determining amplitude) and thrust speed or duration (determining frequencies excited) may enter in terms of optimizing the efficacy of chiropractic adjustments. If stimulation of specific spinal frequencies, say as central nervous input, were most essential, then many chiropractic thrusts could be clinically similar. This may explain how over 90 chiropractic techniques can co-exist.