Introduction: It is well known that spinal manipulative therapy (SMT) transfers forces to different spinal tissues. Previous studies have shown that the intervertebral disc experiences the greatest forces during SMT and that the distribution of SMT forces among spinal tissues change as a function of the applied SMT parameters. However, a more comprehensive description of SMT force distribution contextualizing the forces experienced by spinal structures relative to the ones applied and experienced by the whole functional spinal unit is still needed to understand the underlying mechanisms of this common conservative therapy. Therefore, this study aimed to describe the percentage force distribution between spinal tissues relative to the applied SMT forces and total force experienced by the functional unit.

Methods: A secondary analysis of forces experienced by spinal structures during a posterior-to-anterior SMT application was conducted. Thirty-five fresh porcine cadavers were exposed to a simulated 300N SMT thrust to the skin overlying the left L3/L4 facet joint via servo-controlled linear motor actuator. Vertebral kinematics were tracked optically using indwelling bone pins. The functional spinal unit was then removed and mounted on a parallel robotic platform equipped with a 6-axis load cell. The kinematics of the spine during SMT were replayed by the robotic platform. By using serial dissection, peak (maximun force during thrust) and mean (average force during preload and thrust) forces induced by the simulated SMT experienced by spinal structures in all three axes of motion were recorded by the load cell. Forces experienced by spinal structures were analyzed descriptively and the resultant force magnitude was calculated.

Results: During SMT, the intact functional spinal unit experienced a median peak resultant force magnitude of 36.4N (IQR: 14.1N) and a mean resultant force magnitude of 25.4N (IQR: 11.9N). Peak resultant magnitude experienced by the spinal segment corresponded to 12.1% of the total force that was applied during SMT thrust (300N). The resultant force magnitude experienced by the intact functional spinal unit was then considered to be 100%. Relative to this, the supra and interspinous ligaments experienced 0.3% of the peak forces and 0.5% of the mean forces. Facet joints and ligamentum flavum experienced 0.7% of the peak forces and 3% of the mean forces, while intervertebral disc and longitudinal ligaments experienced 99% of the peak and 96.5% of the mean forces.

Discussion: In this animal model, a small percentage of the forces applied during a posterior-to-anterior SMT reach spinal structures in the lumbar spine. Most SMT forces (over 96%) are experienced by the intervertebral disc. This study provides a novel perspective on SMT force distribution within spinal tissues.