Understanding Control and Mechanisms of Shoulder Instability in FSHD

Overview

The aim of this study is to identify factors for shoulder instability in people with Facioscapulohumeral dystrophy (FSHD). FSHD is a non-life limiting condition with symptoms presenting in the second decade of life (Evangelista et al., 2016). Between 2500 to 3000 people are diagnosed with FSHD in the UK and it is the third most common dystrophy. The overall prevalence is 1: 20,000 and on average 52 people are newly diagnosed with FSHD each year (Emery, 1991; Padberg et al., 1995; UK, 2020) As the disease progresses, patients lose the ability to adequately control muscles around the shoulder girdle, possibly contributing to the development of shoulder instability i.e. partial or complete dislocation of the shoulder joint (Bergsma, Cup, Geurts, \& De Groot, 2015; Bergsma, Cup, Janssen, Geurts, \& de Groot, 2017; Mul et al., 2016). Loss of control around the shoulder is also thought to contribute to pain and a reduced capacity to perform tasks above shoulder height. Additionally, the development of fatigue and chronic pain further limit patient's abilities and engagement with rehabilitation. If we better understand the mechanisms associated with instability, we can better target physiotherapy interventions to improve rehabilitation. If we identify specific patterns of activity associated with instability, these could be addressed through personalised and improved exercise prescription and rehabilitation. Additionally, we may identify causes of instability for which physiotherapy or exercise programmes may not be appropriate, therefore ensuring patients are referred to the correct service in a timely manner, improving patient outcomes and allocating resources more appropriately.

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.