Anatomical determinants of dysfunction inform the evolution of the human shoulder

We previously demonstrated that fossil hominins support a scenario in which the lateralized shoulder joint configuration found in modern humans evolved from the more cranial organization seen in African apes. This trajectory is consistent with selection against overhead functions (e.g., climbing) and for more human-specific behaviors (e.g., tool use). Human populations retain remarkable variability in the skeletal shape and configuration of the shoulder, which may reflect both functional tradeoffs and weak costs associated with more primitive shape configurations. We predicted these configurations experience higher shear forces during lateralized behaviors leading to a higher lifetime incidence of rotator cuff injury (RCI). To test the second of these predictions, we compared computed tomography (CT) data of the scapula from individuals diagnosed with a rotator cuff tear and age-matched controls (n=48). To characterize shape, we collected three-dimensional landmark data, performed Procrustes superimposition to remove the effect of orientation and scale, a Canonical Variates Analysis (CVA) to identify shape differences that maximize between group variation, and ANOVA to test for significance of mean shape differences. RCI and control populations exhibit a significant difference in mean shape (F=1.77, df=77, p\u3c0.0001), the former having a more cranial orientation of the glenoid and scapular spine, a reduced supraspinatus attachment area, and a smaller attachment area and altered orientation of the teres major. These results are consistent with a direct role of selection in the hypothesized LCA-hominin evolutionary trajectory and help explain the observed variability of scapular shape in modern human populations