Impact of Overload and Overuse Following Subscapularis Tears: Mechanisms of Shoulder Joint Damage

Rotator cuff tears affect millions of individuals each year in the United States, impacting veterans and civilians alike. Tears dramatically alter joint mechanics leading to significant damage to articular cartilage and adjacent (intact) tendons within the shoulder. This damage leads to loss of function, pain, tear progression, and even tendon re-tear following repair. Therefore, identifying mechanisms of joint damage after tendon tear is an active area of research. We have previously discovered how joint damage occurs following supraspinatus and infraspinatus tears using an in vivo rat rotator cuff tear model. These results altered clinical practice by clearly demonstrating that loss of the anterior-posterior force couple leads to permanent detrimental changes to the shoulder joint. While subscapularis tendon tears were once thought to be uncommon, increased use of arthroscopy and improved imaging have led to a dramatic increase in subscapularis tear diagnosis, specifically in ~ 50% of patients undergoing rotator cuff repair. Furthermore, it has been suggested that unrecognized subscapularis tears may be a source of chronic dysfunction and pain. However, no studies have examined the critical question of how subscapularis tendon tears affect the shoulder joint, and as a result, a clear best clinical approach has not been identified for this common and debilitating condition. Therefore, the clinical objective of this work is to specifically determine the extent and mechanisms of shoulder joint damage, functional alteration, and pain that result from rotator cuff tears involving the subscapularis. We hypothesize that loss of the anterior-posterior force couple after subscapularis tear leads to detrimental joint damage, loss of function, and pain, and that, due to the critical role of the subscapularis, these effects are significantly increased when compared to a supraspinatus tear alone. Additionally, we hypothesize that these detrimental changes are exacerbated by joint overuse after tendon detachment. Our well-characterized, controlled in vivo pre-clinical model system is ideal for the posed questions. The hypotheses will be evaluated with a series of tendon detachments followed by comprehensive combination of mechanical property measurements, tissue composition measures, and longitudinal in vivo functional evaluation of joint kinematics, shoulder range of motion, and grip strength of the affected limb, defining how in vivo changes are reflected in ex vivo measurements. Experiments will be repeated with the addition of a treadmill overuse protocol post-injury to identify the compounding effects of overuse after a tendon tear. The proposed research will identify how subscapularis tears affect joint health and function, the consequences of an undiagnosed tear, and the role of shoulder overuse after subscapularis tear in intensifying these issues, while also providing important insights into chronic shoulder pain and loss of function. Results from these studies will be used to appropriately design targeted randomized controlled trials. Subsequently, results from one or more prospective clinical trials will have a clear and direct impact on clinical care. Results from this pre-clinical study will provide critical information that will ultimately guide treatment strategies for subscapularis tears in order to reduce pain, improve function, and preserve long-term shoulder joint health.