Is Global Scapular Morphology, and Particularly Posterior Acromial Morphology, Symmetrical in a Healthy Cohort?

INTRODUCTION: Certain scapular morphologies, particularly those involving the posterior acromion, have recently been associated with posterior instability and primary glenohumeral osteoarthritis. In particular, a shorter, higher, and flatter posterior acromion appears to represent an at-risk phenotype. These extreme morphologies are uncommon, and their prevalence appears to decrease in older healthy cohorts. However, bilateral symmetry of overall scapular morphology in healthy individuals remains poorly understood. The aim of this study was to assess scapular morphologic symmetry in subjects without detectable structural abnormality on bilateral CT and determine whether it varied with age.

MATERIALS AND METHODS: A total of 304 patients (608 shoulders) were included and divided into 2 age groups: <60 years (204 patients, 408 shoulders) and ≥60 years (100 patients, 200 shoulders). For each subject, a reference shoulder was defined as the shoulder showing the lowest cumulative standardized absolute deviation from previously established older reference means across 15 morphologic parameters. The contralateral shoulder was then compared with the reference shoulder. Bilateral symmetry was analyzed in the overall cohort and after age stratification. Morphologic differences between age groups were then assessed. Additional analyses were performed to determine whether asymmetry was concentrated in the most morphologically extreme subjects.

RESULTS: In the overall cohort, 8 measurements showed significant asymmetry between the reference shoulder and the contralateral shoulder. After age stratification, 9 measurements were significantly asymmetric in subjects <60 years, whereas no significant asymmetry was observed in subjects ≥60 years. However, the mean magnitude of these side-to-side differences in the younger cohort was generally small. Extreme-value analysis showed a significant reduction in 6 parameters in the older cohort. When differences between age groups, bilateral asymmetry, and extreme-value analyses were integrated, posterior acromial coverage was the only parameter that fulfilled all 3 criteria. Additional subgroup analyses showed that morphologically extreme younger subjects were not more asymmetric than the remaining younger subjects, but instead frequently exhibited bilateral morphologic deviation in the same direction.

CONCLUSION: Overall bilateral scapular morphology appears largely symmetrical in subjects without detectable structural abnormality on CT. Although statistically significant side-to-side differences were observed in younger subjects, their mean magnitude was generally small whereas significant asymmetry was absent in subjects ≥60 years. Importantly, morphologically extreme younger subjects were not more asymmetric than the remaining younger subjects but instead showed bilateral deviation from the older reference morphology. These findings suggest that extreme scapular morphology may represent a bilateral constitutional phenotype rather than an asymmetric morphologic pattern.