The mechanisms of manual therapy in the treatment of musculoskeletal pain: a comprehensive model

Abstract

Prior studies suggest manual therapy (MT) as effective in the treatment of musculoskeletal pain; however, the mechanisms through which MT exerts its effects are not established. In this paper we present a comprehensive model to direct future studies in MT. This model provides visualization of potential individual mechanisms of MT that the current literature suggests as pertinent and provides a framework for the consideration of the potential interaction between these individual mechanisms. Specifically, this model suggests that a mechanical force from MT initiates a cascade of neurophysiological responses from the peripheral and central nervous system which are then responsible for the clinical outcomes. This model provides clear direction so that future studies may provide appropriate methodology to account for multiple potential pertinent mechanisms.

Figure 1. Comprehensive model of the mechanisms of manual therapy

Figure Key: The model suggests a transient, mechanical stimulus to the tissue produces a chain of neurophysiological effects. Solid arrows denote a direct mediating effect. Broken arrows denote an associative relationship which may include: = an association between a construct and its measure Bold boxes indicate the measurement of a construct ACC = anterior cingular cortex; PAG = periaqueductal gray; RVM = rostral ventromedial medulla

Figure 2. Pathway for a spinal cord mediated effect of manual therapy from George et al (2006)

Figure Key: Proposed model pathway of study by George et al (2006) suggesting a spinal cord mediating effect of manual therapy. Bold arrows indicate suggested mechanism. Note mediating effect is suggested to be through the spinal cord due to measurement of the associated relationship of temporal summation. Also note, the design of this study neglects to consider potential supraspinal mediated effects.

Figure 3. Pathway considering both a spinal cord and supraspinal mediated effect from Bialosky et al (2008)

Figure Key: Proposed model pathway of study by Bialosky et al (2008) which considers both a spinal cord and supraspinal mediating effect of MT. Bold arrows indicate suggested mechanism. Note mediating effect is suggested to be through both the spinal cord due to measurement of the associated relationship of temporal summation and through a supraspinal mechanism due to measurement of the associated relationship of expectation.

Figure 4. Comprehensive model for the mechanisms of manual therapy illustrating similar neurophysiological activity in response to non- specific effects such as placebo and expectation

A limitation of the current mechanistic literature in MT is the failure to adequately account for non- specific effects such as placebo and expectation. Italicized references are examples of studies from the placebo and expectation literature which have reported similar neurophysiological effects as have been associated with MT. These similarities emphasize the potential for non- specific effects to play a significant role in the mechanisms behind MT and the need to specifically address these factors in future studies.