I am very excited that Dr. Brence invited me to join him here at forwardthinkingpt.com. He has a great blog with lots of wonderful content. My first few posts are going to be recycled from my bog, which I am shutting down.
I recently read an article by Muth et al which looked at the effects of thoracic spine thrust manipulation on patients with signs of rotator cuff pathology. This study utilizes the concept of regional interdependence which has gained popularity over the last several years. This theory suggests that pain and dysfunction in one area of the body can be treated with interventions to adjacent regions. This concept is not new, however more recent studies have fueled its increased popularity by providing support for it. For example, treatment of the hip is thought to have an impact on knee and low back pain, and treatment aimed at the thoracic spine as been shown to have effects on shoulder and neck pain. These effects are often explained based on biomechanics however, the exact mechanism is still unclear. It is possible that some of these effects can be explained by changes in biomechanics, although there are a few reasons why I remain skeptical about this explanation . First, many of these studies use manual therapy and exercise as an intervention. It is assumed that these interventions have an impact on the biomechanics of a joint however this hypothesis remains unproven. The often immediate effects of manual therapy may suggest that its actions go further than just local changes to a joint. Although possible, I would not expect small changes in biomechanics at one joint to immediately result in reduced pain and improved function at a distant joint. It seems more plausible that a change in the nervous system (increased pain threshold etc.) would be more likely to result in this immediate effect. If we consider exercises to either strengthen or stretch a muscle it may theoretically change biomechanics, however there are few examples of this in the literature. The other major reason I am skeptical of the biomechanical explanation for regional interdependence is the consistent failure of studies to find a strong connection between biomechanical factors and patient symptoms. Even in cases where a correlation between mechanics and symptoms has been documented (for example shoulder pain and scapular dyskinesia) it is often unclear if the changes in biomechanics are the cause or a result of the problem.
The study by Muth et al built off other studies (here and here) which found a reduction in shoulder pain after thoracic spine manipulation (TSM). The goal of this study was to try and determine a mechanism for the reduction in shoulder pain after TSM. The authors took 30 subjects with shoulder pain and put them through a battery of tests including: scapular kinematic testing (using electromagnetic tracking), EMG (infraspinatus, upper/middle/lower trap, serratus), pain rating (during empty can test, Hawkins-Kennedy test, Neer test and loaded elevation), peak shoulder elevation force and a few shoulder function questionnaires.
As with previous studies they found a reduction in pain and an improvement in function after the TSM. This reduction in pain was not associated with changes in biomechanics. The only statistical significant change in scapular motion was a reduction in upward rotation after the TSM. Based on current thinking we would expect a reduction in upward rotation to decrease the subacromial space, result in more pressure on the cuff and cause increased pain. At this point you are probably thinking “but what about thoracic motion”. Good thought but no, that did not change either.
The results of this study calls into question the biomechanical explanation for the improvements found after TSM. I would not suggest that regional interdependence is never a result of biomechanical changes, however we should be careful not to assume the mechanism is biomechanical. It is important for us to consider the mechanism behind regional interdependence because often our understanding of mechanism dictates how we use the intervention. For example, TSM may not be used in a patient with shoulder pain who is found to have a normal or hypermobile thoracic spine. We assume that TSM is only effective in patients with a stiff thoracic spine because we believe the mechanism of action is increased thoracic extension (or motion in general). If the patient has good extension with no stiffness there would be no reason to perform a TSM. If we understand that the mechanism behind TSM may not be mechanical, we would avoid using only mechanical findings to dictate whether we performed a TSM or not. At this point, all we know is that the presence of shoulder pain is an indication that TSM may be helpful.
Another important issue to address about the Muth et al study (and many other studies looking at manual therapy) is that there was no sham treatment group. The absence of a sham treatment group makes it impossible to determine whether the benefits from TSM were due to a specific effect or a result of non-specific effects such as placebo or patient expectation.
To sum things up, TSM seems to provide short term improvements in pain and function for patients with shoulder pain. The mechanism behind these improvements does not appear to be biomechanical. We need more studies looking into the mechanism behind this, and other examples of regional interdependence, so we can more accurately match interventions to patients.