Is It All in the Genes?

Commentary The etiology of rotator cuff disease is multifactorial; both mechanical and biologic mechanisms are undoubtedly at play. Following the classic work by Neer in 1972, the focus became the acromion as the culprit in generating shoulder pain1. The work of Bigliani et al. introduced the relationship between acromial morphology and rotator cuff tears2. Those authors presented an association between a hooked or type-3 acromion and the presence of rotator cuff disease. Subsequently, a couple of generations of shoulder surgeons interpreted a cause-and-effect relationship in this association. The prospect that anatomic scapular variations may be an etiologic factor has remained a source of intrigue. It is now believed that rotator cuff contact with the acromion is a normal phenomenon and likely prevents anterosuperior migration with shoulder elevation. Developmental spurs, on the other hand, are not pushing into the rotator cuff but represent calcification of the coracoacromial ligament secondary to traction forces3. Well-performed surgical trials have failed to show a clinical benefit to performing an acromioplasty4. The complex interaction between genetic and environmental factors is a common theme throughout the medical literature. Typically, the study of monozygotic (MZ) “identical” and dizygotic (DZ) “fraternal” twins allows the researcher an opportunity to sort out the relative impact of genetically inherited traits. This is far from a pure science, as identical or MZ twins rarely share identical life experiences and environmental exposures, but it is considered the “gold standard” when studying the human species. When discussing rotator cuff disease, environmental factors such as occupational and recreational loads, smoking history, nutrition, systemic diseases, and medications must all be considered. When looking at genetic factors, it has been proposed that we may all have age-related predetermined cellular repair mechanisms leading to collagen breakdown, but our genetically determined anatomy may alter the biomechanical forces that our shoulder sees as well. The authors have studied the acromiohumeral distance (AHD) as measured on magnetic resonance images (MRIs) in a cohort of twenty-nine selected pairs of elderly twins from the Italian Twin Register. In addition, they studied the role of occupational factors by categorizing each subject as either being a laborer, an administrative support worker, or a professional or manager. They looked at fifteen sets of MZ and fourteen sets of DZ twins who were selected on the basis of reasonable inclusion criteria. The authors used previously published statistical analyses to look at heritability and shared and unique environmental influences relating to the AHD. The assessment of a single static, supine measurement of AHD from an MRI is likely not ideal for truly assessing osseous impingement. Previous studies have looked at the acromial index, acromial slope, and lateral acromial angle, which might have been more meaningful. Banas et al. noted that a lateral acromial angle of >70° had a high correlation with rotator cuff tears5. MacGillivray et al. noted a type-B acromion that angled >10° down from the lateral aspect of the clavicle was predictive of rotator cuff disease6. In addition, Gumina et al. did not assess the rotator cuff (thickness, volume, and presence of pathology) at the same time, which might have told us whether small rotator cuffs exist in small outlets. As one may expect, MZ twins showed a higher correlation in the measured AHD than did DZ twins; however, the actual differences in the mean AHD between groups were <1 mm. They calculated that heritability accounted for 82% of the differences and unique and shared environmental factors accounted for only 9% each. Interestingly they did not find any association between self-reported occupation and the measured AHD, although they did not specifically power the study to evaluate this. Genetics has become increasingly important in understanding many aspects of health. It is now possible for us to send a saliva sample in the mail and receive an analysis of our genome and our predisposition to hundreds of diseases and conditions; perhaps we can add rotator cuff disease to that list in the years to come? It would appear that, to some degree, shoulder morphology is inherited; this study fails to answer whether rotator cuff disease is as well. It would be interesting to know whether we inherit the cellular traits involved in impaired collagen synthesis and repair, cellular apoptosis, and subsequent rotator cuff tears. Would we want this information when advising our offspring about career pathways and recreational pursuits? So much remains unanswered, but Gumina et al. should be congratulated on heading down this path of discovery.

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