No Strength Differences Despite Greater Posterior Rotator Cuff Intramuscular Fat in Patients With Eccentric Glenohumeral Osteoarthritis

Abstract Background When 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? Methods From 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 posterior-to-anterior RC fat percent (infraspinatus + teres minor fat percent/subscapularis fat percent) were computed and compared between the OA groups. Results There was no relative external rotation weakness in patients with eccentric deformities (Y component of 3D strength balance, mean ± SD: -4.7% ± 5.1%) compared with patients with concentric deformities (-0.05% ± 4.5%, mean difference -4.7% [95% CI -7.5% to -1.9%]; p = 0.05). However, there was more variability in 3D strength balance in the eccentric group (95% CI volume, %3: 893) compared with the concentric group (95% CI volume, %3: 579). In patients with eccentric compared with concentric deformities, there was no difference in median (IQR) resting pain (1.0 [3.0] versus 2.0 [2.3], mean rank difference 4.5 [95% CI -6.6 to 16]; p = 0.61) or torque-dependent pain (0.70 [3.0] versus 0.58 [1.5], mean rank difference 2.6 [95% CI -8.8 to 14]; p = 0.86). In the subset of 18 of 33 patients with OA who underwent MRI, seven patients with eccentric deformities demonstrated a higher posterior-to-anterior RC fat percent ratio than the 11 patients with concentric deformities (1.2 [0.8] versus 0.70 [0.3], mean rank difference 6.4 [95% CI 1.4 to 11.5]; p = 0.01). Conclusion Patients with eccentric deformities demonstrated higher variability in strength compared with patients with concentric deformities. This increased variability suggests patients with potential subtypes of eccentric wear patterns (posterior-superior, posterior-central, and posterior-inferior) may compensate differently for underlying anatomic changes by adopting unique kinematic or muscle activation patterns. Clinical Relevance Our findings highlight the importance of careful clinical evaluation of patients presenting with eccentric deformities because some may exhibit potentially detrimental strength deficits. Recognition of such strength deficits may allow for targeted rehabilitation. Future work should explore the relationship between strength in patients with specific subtypes of eccentric wear patterns and potential forms of kinematic or muscular compensation to determine whether these factors play a role in TSA failures in patients with eccentric deformities.