Scapular Morphology: Survival of the Fittest

Three-dimensional (3D) anatomical studies of the normal shoulder have gained increasing attention over the past decade, in part to better understand shape variations that may contribute to pathomechanisms1,2. Shape variations of the osseous anatomy of the scapula have been associated with several shoulder pathologies. A more horizontal acromion has been associated with primary osteoarthritis and posterior shoulder instability3,4. It has been postulated that this shape variation allows the humeral head to escape posteriorly due to the absence of an osseous buttress effect from the acromion. Conversely, a steeper, more angulated acromion combined with a coracoid process that is oriented superiorly and medially has been associated with anterior instability5. This finding suggests that, in healthy shoulders, the conjoined tendon may provide a native sling effect that keeps the humeral head in place. Werthel et al. performed a 3D morphologic study of 369 healthy scapulae with use of previously described measurements, which they assessed in an automated manner. Pathological thresholds identified in previous studies were used to determine the proportion of healthy scapulae exceeding the threshold values for shape variation, thereby identifying patients considered potentially at risk for future shoulder problems—in other words, those with scapulae that could be called “out of normal shape.” Overall, a relatively low variability in healthy scapular morphology was reported. However, when the study cohort was stratified by age, older patients (≥60 years of age) had a higher percentage of scapulae that were “in normal shape,” whereas younger patients (<60 years of age) demonstrated more “out of shape” features. This finding prompted the authors to hypothesize that “specific scapular variations may predispose individuals to progressive degenerative shoulder conditions, causing those individuals to transition out of the healthy cohort over time.” According to the authors, “it is conceivable that individuals who remain healthy at 60 years of age and beyond possess a ‘healthy’ anatomy that does not predispose them to any pathological conditions.” This theory might remind the reader of a form of natural selection occurring over time, whereby scapulae that are “in shape” are more likely to remain free from pathology. In other words, “in shape” scapulae may be considered “the fittest” to withstand the test of time. Should we identify “out of shape” scapulae early, and could preventive measures render them “more fit to survive”? Such questions might arise from the evolutionary analogy one might draw after reading this study. While the concept of natural selection appears logical, caution is warranted when interpreting the causal conclusions drawn by the authors. As the authors themselves note, “while such anatomical variations may be associated with the development of shoulder disorders, their potential causal role remains hypothetical and must be interpreted with caution.” Although the study included a large data set, patient selection was driven primarily by the availability of images of sufficient quality to be compatible with the software that was used, rather than by a strict cross-sectional sampling of the population. A longitudinal observation, which is needed to strengthen the proposed theory that recalls natural selection, is also missing. Does the scapula “get in shape,” and does it “stay in shape”? Does shoulder morphology evolve over time (for example, through Wolff’s law), and might such changes contribute to shoulder pathology? These remain hypothetical and unresolved questions. Nevertheless, Werthel et al. provide an extensive reference for normal 3D scapular morphology, offering a valuable foundation for future research. Without a doubt, this article is an interesting read that gives food for thought on how scapular anatomy may evolve and influence the incidence of pathology.