Analysis of scapular morphology as a key factor in unstable painful shoulder: an magnetic resonance imaging-based...

Unstable painful shoulder (UPS) is a recently recognized condition characterized by pain as the chief symptom, with no awareness of dislocation or subluxation but presenting imaging or arthroscopic findings indicative of anterior shoulder instability (ASI). However, the factors contributing to symptoms differentiating UPS from ASI remain unclear. The purpose of this study was to investigate and compare morphological factors on magnetic resonance imaging between pain-predominant (UPS) and apprehension-dominant (ASI) instability presentations.

From our institutional database of 5,400 patients presenting with shoulder complaints between January 2013 and December 2023, we extracted 2,278 patients who underwent shoulder magnetic resonance imaging and identified active athletes matching strict eligibility criteria. The study included 74 patients diagnosed with UPS, 81 with only one dislocation episode because of ASI, and 36 controls with no shoulder pathology. The UPS and ASI groups were limited to collision sports, and each contained 57 patients matched for sex, height, and age. Fourteen parameters related to glenoid, acromial, and coracoid morphology were compared among the 3 groups. Logistic regression and receiver operating characteristic curve analyses were performed to identify predictors of UPS.

The UPS group had a significantly greater glenoid width and depth than the ASI and control groups (P < .001). The UPS group also had a significantly higher posterior acromion height and significantly less anterior and posterior acromial coverage (P < .05). Logistic regression analysis identified glenoid depth as the strongest independent predictor of UPS (odds ratio, 9.73; 95% confidence interval, 3.11-30.47). A cutoff value of 1.85 mm for glenoid depth distinguished UPS from ASI with an area under the receiver operating characteristic curve of 0.790.

Glenoid depth was identified as the strongest independent parameter distinguishing pain-predominant (UPS) from apprehension-dominant (ASI) instability. UPS was characterized by a stable scapular morphology, featuring a deeper and wider glenoid and a more anteriorly covering acromion. These subtle native structural variations characterize distinct phenotypes within the shoulder instability spectrum. While pain-predominant instability is a recognized clinical phenotype, understanding this characteristic scapular morphology may help clinicians to anticipate the presence of occult instability lesions, prevent delays in clinical diagnosis, and avoid misdiagnosis as alternative shoulder pathologies due to the lack of subjective dislocation history.