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BackgroundWhen nonoperative measures do not alleviate the symptoms of glenohumeral osteoarthritis (OA), patients with advanced OA primarily are treated with anatomic total shoulder arthroplasty (TSA). It is unknown why TSAs performed in patients with eccentric (asymmetric glenoid wear) compared with concentric (symmetric glenoid wear) deformities exhibit higher failure rates, despite surgical advances. Persistent disruption of the posterior-to-anterior rotator cuff (RC) force couple resulting from posterior RC intramuscular degeneration in patients with eccentric deformities could impair external rotation strength and may contribute to eventual TSA failure. Pain and intramuscular fat within the RC muscles may impact external rotation strength measures and are important to consider.Questions/purposes(1) Is there relative shoulder external rotation weakness in patients with eccentric compared with concentric deformities? (2) Is there higher resting or torque-dependent pain in patients with eccentric compared with concentric deformities? (3) Do patients with eccentric deformities have higher posterior-to-anterior RC intramuscular fat percent ratios than patients with concentric deformities?MethodsFrom February 2020 to November 2021, 65% (52 of 80) of patients with OA met study eligibility criteria. Of these, 63% (33 of 52) of patients enrolled and provided informed consent. From a convenience sample of 21 older adults with no history of shoulder pain, 20 met eligibility criteria as control participants. Of the convenience sample, 18 patients enrolled and provided informed consent. In total for this prospective, cross-sectional study, across patients with OA and control participants, 50% (51 of 101) of participants were enrolled and allocated into the eccentric (n = 16), concentric (n = 17), and control groups (n = 18). A 3-degree-of-freedom load cell was used to sensitively quantify strength in all three dimensions surrounding the shoulder. Participants performed maximal isometric contractions in 26 1-, 2-, and 3-degree-of-freedom direction combinations involving adduction/abduction, internal/external rotation, and/or flexion/extension. To test for relative external rotation weakness, we quantified relative strength in opposing directions (three-dimensional [3D] strength balance) along the X (+adduction/-abduction), Y (+internal/-external rotation), and Z (+flexion/-extension) axes and compared across the three groups. Patients with OA rated their shoulder pain (numerical rating 0-10) before testing at rest (resting pain; response to “How bad is your pain today?”) and with each maximal contraction (torque-dependent pain; numerical rating 0-10). Resting and torque-dependent pain were compared between patients with eccentric and concentric deformities to determine if pain was higher in the eccentric group. The RC cross-sectional areas and intramuscular fat percentages were quantified on Dixon-sequence MRIs by a single observer who performed manual segmentation using previously validated methods. Ratios of poste...

Muscle activation helps stabilize the glenohumeral joint and prevent dislocations, which are more common at the shoulder than at any other human joint. Feedforward control of shoulder muscles is important for protecting the glenohumeral joint from harm caused by anticipated external perturbations. However, dislocations are frequently caused by unexpected perturbations for which feedback control is essential. Stretch-evoked reflexes elicited by translations of the glenohumeral joint may therefore be an important mechanism for maintaining joint integrity, yet little is known about them. Specifically, reflexes elicited by glenohumeral translations have only been studied under passive conditions, and there have been no investigations of how responses are coordinated across the functional groupings of muscles found at the shoulder. Our objective was to characterize stretch-evoked reflexes elicited by translations of the glenohumeral joint while shoulder muscles are active. We aimed to determine how these responses differ between the rotator cuff muscles, which are essential for maintaining glenohumeral stability, and the primary shoulder movers, which are essential for the large mobility of this joint. We evoked reflexes using anterior and posterior translations of the humeral head while participants produced voluntary isometric torque in six directions spanning the three rotational degrees-of-freedom about the shoulder. Electromyograms were used to measure the stretch-evoked reflexes elicited in nine shoulder muscles. We found that reflex amplitudes were larger in the rotator cuff muscles than in the primary shoulder movers, in part due to increased background activation during torque generation but more so due to an increased scaling of reflex responses with background activation. The reflexes we observed likely arose from the diversity of proprioceptors within the muscles and in the passive structures surrounding the shoulder. The large reflexes observed in the rotator cuff muscles suggest that feedback control of the rotator cuff augments the feedforward control that serves to compress the humeral head into the glenoid. This coordination may serve to stabilize the shoulder rapidly when preparing for and responding to unexpected disturbances.