Study Design: Controlled laboratory study, with measurements taken before and after a standardized clinical intervention. Objectives: To determine if thoracic manipulation alters the posteroanterior (PA) spinal stiffness of the thoracic spine, and the factors associated with any potential changes in stiffness. Background: Spinal manipulation is commonly used to treat thoracic pain and dysfunction. Therapists use manual assessment of PA spinal stiffness to determine the appropriateness and effectiveness of treatment, with potential changes in spinal stiffness possibly contributing to symptomatic improvement following manipulation. Methods: Thoracic PA spinal stiffness was measured at 5 vertebral levels (manipulated level and 2 levels above and below), in 24 asymptomatic subjects, before and after manipulation. Five cycles of standardized mechanical PA force were applied to the spinous process while recording resistance to movement and concurrent displacement, with stiffness defined as the slope of the linear portion of the force-displacement curve. A 2-way repeated-measures analysis of variance determined differences between premanipulation and postmanipulation among multiple spinal levels. Linear regression determined the relationship between stiffness magnitude and its change following manipulation. Generalized linear mixed models were used to determine if subject age, gender, spinal level, premanipulation stiffness, or manipulative thrust parameters were associated with postmanipulation stiffness. Results: Thoracic spine PA stiffness differed between spinal levels (F₄,₉₂= 21.1, P<.001) but was not significantly different following manipulation. The mean change in spinal stiffness correlated with stiffness magnitude at the manipulated spinal level only but not other levels (Pearson r, –0.65; P<.001). Greater postmanipulation stiffness was associated with being male (regression coefficient, 1.16; 95% CI: 0.52, 1.79; P<.001) and with higher premanipulation stiffness (regression coefficient, 0.63; 95% CI: 0.49, 0.77; P<.001). Manipulation force parameters were not associated with postmanipulation stiffness. Conclusion: In asymptomatic individuals, thoracic PA spinal stiffness is not significantly different when measured before and after thrust manipulation, but any potential mechanical effects appear associated with the manipulated spinal level rather than other levels.
Journal of Orthopaedic & Sports Physical Therapy Vol. 40, Issue 11, p. 685-693