Should we be looking for the PST ‘black swan’?
Just thinking about this recent JOSPT article on the posterior shoulder that I wrote with Professor John Borstad: Hall and Borstad (2018)
Firstly, a huge thank you to Professor Borstad for being so generous with his vast knowledge base on the subject; it was a real pleasure to work with John on this paper. This post contains my thoughts only and relates only to the non-throwing populations I am researching/treating. This piece is not about the effectiveness or efficacy of treating PST or even its role as a possible physical impairment in shoulder dysfunction, but instead a discussion of the scientific thinking and methodology that has been used to study it.
What we know so far …
There are many laboratory studies that look at the kinematic effect of experimentally tightening the posterior capsule, and the effects this may have on sub-acromial contact forces, glenohumeral joint compression and/or potential shear forces. These studies provide knowledge relating to the possible ways in which PST can cause pain and shoulder disability. They help us develop theories about what is going on in the shoulder in the presence of PST, but they do not provide evidence to support the treatment of PST in clinical practice.
Evidence and knowledge to support the treatment of PST in clinical practice must come from a process of scientific enquiry. The scientific method is used to answer ‘cause and effect’ type questions, for example, ‘Will my patients get better faster if I treat their PST?’ or ‘Will my patients improve more if I treat their posterior shoulder?’
There are some randomized controlled clinical trials that have investigated the cause and effect questions of this type (Yang et al (2012), Cools et al (2011), Manske et al (2010)), but the design of these studies can provide only limited knowledge. To explain; Yang et al. (2012) found that a manual treatment for PST was superior to a sham treatment. So we think treating the posterior shoulder is better than a sham treatment, but we don’t know if it is better than gold standard treatment. Based on recent systematic reviews the gold standard treatment for patients with Rotator Cuff Related Shoulder Pain (RCRSP) seems to be a resisted exercise program: “A structured exercise program is unequivocally the main intervention for SAPS [RCRSP]” (Lewis 2016). So how does treatment for PST compare with this treatment? We don’t know.
We know, that when Tyler et al (2010) treated the posterior shoulder and delivered a multidirectional rehabilitation program, patients tended to get better and their PST resolved. But we don’t know if their recovery was due to the exercise, the posterior shoulder treatment or both?
Cools et al (2011) and Manske et al (2010) have compared two different types of treatment of the posterior shoulder. Both authors found that both types of treatment for PST had positive effects, suggesting that we should be treating PST with manual therapy and/or stretches, right? But what we don’t know is ‘would we be getting similar (or greater???) benefit from doing something else? The design of these studies cannot answer this question.
Moreover, many of the interventional studies have investigated the impact of treating PST in asymptomatic participants. It’s difficult to extrapolate these findings to symptomatic populations, particularly as we don’t really know what causes PST. If PST is generated in part by processes related to the pain experience, which is possible, then interventional studies on asymptomatic participants become even less relevant to our clinical practice.
Why changing our questions could lead to better answers …
Karl Popper promoted the concept of falsification in the method of scientific enquiry. According to Karl, if we want to test the theory that all swans are white, finding another white swan is not helpful. Finding more white swans cannot confirm the theory that all swans are white, no matter how many we find. Finding one black swan, however, will falsify the theory. Thus, as we cannot find all the swans in the universe (no matter how hard we try!) the theory can only be disproved - by finding a single black swan.
In relation to the evidence base around PST, is seems as though we are continuing to find white swans, by comparing one treatment for PST with another and finding benefit. We are, however, failing to falsify the theory. I am not saying that treating PST doesn’t have a place in clinical practice: current evidence, however limited, suggests it does. I am, however, advocating the integration of a process of falsification in the design of clinical trials to test the theory.
Consider, for example, these trials described as the ‘most significant’ 15 physiotherapy trials as voted for by PEDro users.
Many of these trials used a design allowing falsification of the theory in question. As a result, they had an impact of reducing the use of ineffective interventions such as McKenzie method for acute LBP, traction for NSLBP or diclofenac in acute LBP, enabling patients to access more effective interventions and allowing researchers to focus on investigating and developing more effective treatment options.
And what it means for PST …
So, back to PST; what if we designed a clinical trial where patients with PST were randomized into two groups. Both groups will receive a ‘gold standard treatment’ of structured exercise. On top of that, one group will receive an intervention for PST and the other a sham intervention for PST. Then we would be able to determine if there is an additional benefit of treating PST. We would also be able to see if the range of movement of the posterior shoulder (internal rotation, low flexion and horizontal adduction) resolved without direct treatment. So if patients in the sham group got better through a structured exercise program and their posterior shoulder tightness spontaneously resolved this might help us understand more about the mechanism of generation of PST. It would give us every chance of finding the elusive black swan. And if we still can’t find it …….well, then we might just be on to something!?