Understanding Control and Mechanisms of Shoulder Instability in Patients With Facioscapulohumeral Muscular Dystrophy (FSHD)

Shoulder instability in Facioscapulohumeral dystrophy (FSHD) is a significant problem, with over 80% of patients reporting that it affects their ability to perform activities of daily living (Faux-Nightingale , 2021). The underlying mechanisms of shoulder instability in FSHD are not well understood. It is thought that instability at the shoulder subsequently contributes to the development of shoulder pain and progressive loss of function, particularly during tasks performed above head height. Despite a large proportion of patients reporting shoulder instability that affects function, only 50% report engaging in some form of upper limb rehabilitation (Faux-Nightingale , 2021).

Both surgical and nonsurgical interventions are based on a current understanding of the associated mechanisms that may include muscle wasting, weakness, changes to the structure of the muscle tissue or inappropriate muscle coordination (Bergsma et al., 2014). As there is no cure for FSHD, rehabilitation is fundamental in the management of the condition. Overall, rehabilitation strategies are aimed at maintaining existing levels of function, avoiding complications associated with progression of the disease and targeting mechanisms associated with the development of instability. e.g. exercises to improve co-ordination of the shoulder muscles if the source of instability is dysfunctional muscle control.

Rehabilitation in patients with FSHD is complex and it is therefore important that rehabilitation is appropriately targeted. There is limited evidence to support the effectiveness of existing rehabilitation strategies in FSHD. In order for rehabilitation to be appropriately allocated disease mechanisms should be understood. Existing mechanisms of shoulder instability in FSHD are not well understood and may explain why more than 50% of patients are not engaging in any form of upper limb rehabilitation.

Shoulder stability results from complex mechanisms comprising of finely balanced forces in ligaments, muscles and joint surfaces (Ameln, Chadwick, Blana, & Murgia, 2019). Currently, we are unable to capture this complexity to quantify instability during dynamic upper limb tasks performed during clinical assessment and rehabilitation (Marchi, Blana, & Chadwick, 2014). Biomechanical or mathematical modelling of this complex structure can help to understand the mechanisms associated with instability and predict outcomes for surgical and non-surgical interventions (Arnold, Liu, Ounpuu, Swartz, & Delp, 2006; Delp et al., 2007; Laracca, Stewart, Postans, & Roberts, 2014). Loading on internal structures that cannot be measured can also be estimated by this approach.

This project is therefore a fundamental step, in the development of biomechanical models which can ultimately be used to further our understanding of the shoulder, specifically behaviour of the articulating bony surfaces and muscle forces. In this application we hope to identify mechanisms for shoulder instability which may help better inform rehabilitation and surgical decision making in the management of FSHD.