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

Abstract

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...

External Rotation Strength After TSA in Osteoarthritic Shoulders with Eccentric Deformity Is Not Impacted by Posterior Rotator Cuff Deficiency

“…The findings of the current study support this theory, as patients who had undergone TSA had relatively weaker internal rotation compared with control participants. While relative internal rotation weakness has been shown to exist in patients before TSA compared with control participants (range of mean difference, 2.9% to 7.6% of SM) 19 , the degree of weakness was greater after TSA (range, 10.9% to 13.6% of SM). Relative subscapularis weakness after TSA may offset underlying external rotator weakness.…”

“…Furthermore, 1D dynamometers cannot discern the contribution of rotation to functional combined motions (e.g., adduction with rotation). Prior work has demonstrated posterior RC muscle deficiency in patients with eccentric deformity before TSA 7 , 19 , which is theorized to persist and contribute to TSA failures in this subgroup. However, postoperative strength has not been compared according to preoperative deformity types, to our knowledge, so it remains unknown whether patients with eccentric deformity exhibit relative external rotation weakness after TSA.…”

“…Furthermore, 1D dynamometers cannot discern the contribution of rotation to functional combined motions (e.g., adduction with rotation). Prior work has demonstrated posterior RC muscle deficiency in patients with eccentric deformity before TSA 7,19 , which is theorized to persist and contribute to TSA failures in this subgroup.…”

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

“…Previous studies evaluating imbalance in horizontal torque relied mainly on imaging [2,6]. The current study [4] relies on a clinical evaluation of torque production in a single posture while a patient performs isometric motions in different directions, where the goal is to determine whether there is a correlation with the onset of pain. Considerable uncertainty persists regarding the origin of pain in patients with osteoarthritis; it is unclear how joint deformity, cuff inflammation, synovitis, and subchondral bone contribute to it.…”

“…In this issue of Clinical Orthopaedics and Related Research ® , Coats-Thomas et al [4] attempted to clarify the question of whether there are differences in muscle strength regarding internal and external rotation of the rotator cuff and any relation to wear status, concentric or eccentric, of the glenoid in patients with shoulder osteoarthritis. The authors concluded that eccentric shoulders do not demonstrate external rotation weakness compared with concentric shoulders; The opinions expressed are those of the writer, and do not reflect the opinion or policy of CORR ® or The Association of Bone and Joint Surgeons ® .…”