Causal mechanisms of a scapular stabilization intervention for patients with subacromial pain syndrome: a secondary analysis of a randomized controlled trial

Abstract

Background Causal mediation analysis is one way to bridge this gap by exploring the causal pathways of a given intervention. The aim of this study was to assess whether scapular motion, position, and periscapular muscle strength are mediators for pain and shoulder disability outcomes following a scapular stabilization intervention for patients with subacromial pain syndrome. Methods Sixty patients were randomized into two groups: scapular stabilization or periscapular strengthening exercises. The intervention consisted of three sessions per week for 8 weeks. The primary outcome measures were pain and disability and the following outcome measures were considered as potential mediators: scapular motion, scapular position, periscapular muscle strength, age, duration of symptoms, and side of the complaint. A model-based inference approach with bootstrap simulations was used to estimate the average causal mediation effect, average direct effect, and the average total effect from the data of a randomized clinical trial that evaluated the effect of adding scapular stabilization exercises to a scapulothoracic strengthening program in people with subacromial pain syndrome. Results The results demonstrated that none of the putative mediators were influenced by the intervention. However, muscle strength of serratus anterior, upper, middle, and lower trapezius muscles was associated with shoulder disability. Conclusion Scapular kinematic and periscapular muscle strength did not mediate the effect of scapular stabilization exercises on shoulder pain or disability scores in subjects with subacromial pain syndrome. Muscle strength of serratus anterior, upper, middle and lower trapezius were associated with shoulder disability scores at 8-weeks follow-up.

Physiotherapists nearly always prescribe exercise for rotator cuff‐related shoulder pain; but why? A cross‐sectional international survey of physiotherapists

“…Muscle strength is also a popular causal explanation proposed in clinical research (Powell et al., 2022), indicating agreement between clinicians and clinical researchers. Just how relevant improvements in shoulder strength are in the clinical outcomes of people with RCRSP is currently uncertain (Hotta et al., 2022; Powell & Lewis, 2021) but evidence from knee osteoarthritis research suggests it is a plausible mediator of positive clinical outcomes (Hall et al., 2018). Interestingly, an increase in acromiohumeral distance was the least proposed neuromuscular mechanism in this study, which accurately reflects the evidence base (Park et al., 2020).…”

“…Most physiotherapists in this study do not believe scapular dyskinesis or upper body postural abnormalities represent important treatment targets. This belief is backed up by high quality evidence, which suggests that while scapular focused exercise is effective at reducing shoulder pain and improving function, it does not seem to do so via a change in scapular motion (Hotta et al., 2022; Reijneveld et al., 2017). A recent secondary mediation analysis of a randomised controlled trial (Hotta et al., 2022) reported scapular motion and position did not mediate the effect of scapular focused exercise on shoulder pain and disability in a cohort of people with RCRSP.…”

“…This belief is backed up by high quality evidence, which suggests that while scapular focused exercise is effective at reducing shoulder pain and improving function, it does not seem to do so via a change in scapular motion (Hotta et al., 2022; Reijneveld et al., 2017). A recent secondary mediation analysis of a randomised controlled trial (Hotta et al., 2022) reported scapular motion and position did not mediate the effect of scapular focused exercise on shoulder pain and disability in a cohort of people with RCRSP. Moreover, the relationship between static posture and shoulder pain and function are currently tenuous (Barrett et al., 2016).…”

Is exercise therapy the right treatment for rotator cuff‐related shoulder pain? Uncertainties, theory, and practice

“…Mediation analysis is an empirical method of ascertaining how a treatment causes an outcome (Lee et al., 2021); for example, how does resistance exercise reduce shoulder pain? Mediation analyses investigating the causal mechanisms of exercise for shoulder pain are scarce and only two could be identified at the time of writing (Gutierrez Espinoza et al., 2023; Hotta et al., 2022), of which, both warrant discussion. Hotta et al.…”

Exercise for rotator cuff tendinopathy: Proposed mechanisms of recovery

“…Most exercise intervention studies find that participants improve in clinical outcomes (i.e., pain and function) without changes in kinematics [130][131][132][133][134][135][136][137][138] (See Supplemental Table 4). Hotta et al 139 performed a causal mediation analysis and determined that changes in scapular motion did not mediate the improvements in pain or disability with resisted exercise with RC tendinopathy.…”