Understanding the influence of acromial morphology on the pathogenesis of rotator cuff injury is important for developing effective treatment and management strategies for this condition. The causal relationship between the variation of lateral acromial extension and the risk of a full-thickness rotator cuff tear was first investigated by Nyffeler et al.7 who originally defined the AI. They found that a larger lateral extension of the acromion provides a higher ascending force than a smaller lateral extension of the acromion during abduction of the shoulder. Banas et al.8 revealed that the LAA may be useful for evaluating patients thought to have or to be at risk for rotator cuff tear. They reported an association between the angle of lateral tilt of the acromion and the prevalence of subacromial tear. Moor et al.9 developed a radiological parameter, the CSA, which takes into account both the tilt of the glenoid in the frontal plane and the AI. They showed that the CSA was correlated with wear of the rotator cuff tendons and the articular cartilage of the glenohumeral joint but eliminated the influence of degenerative changes in the humerus. In a second study, Moor et al.10 evaluated the predictive power of the AI, LAA, and CSA to determine the presence of degenerative rotator cuff tear. They concluded that the CSA is the most valuable measure to distinguish between patients with intact rotator cuff tendons and those with torn rotator cuff tendons. Notably, we found that the surface of the glenoid cavity is the baseline for measurement of the AI, LAA, and CSA. Therefore, we hypothesized that there is a relationship between these parameters that are relevant in the diagnosis of rotator cuff tears.
To the best of our knowledge, the current study is the first to comprehensively analyze the association between multiple morphological parameters of the acromion, the AI, LAA, and CSA, with rotator cuff tear. In contrast to other studies, we measured three radiologic parameters of acromial morphology in each healthy participant or patient using the same true standardized anteroposterior radiograph. Our findings confirmed the results of Nyffeler et al., Banas et al., and Moor et al. In addition, we demonstrated two other phenomena. First, we found an inverse relationship between the AI and LAA; the lateral extension of the acromion increased as the inclination of acromion increased. Second, we found a positive relationship between the AI and CSA; the lateral extension of the acromion increased as the angle between the line connecting the lowest point of the glenoid plane and the inferolateral point of the acromion and a line delineating the glenoid plane increased. We call this phenomenon “The Acromion Rule” [Figure 5]. These findings imply that the lateral extension of the acromion, the upward tilt of the glenoid fossa, and the downward tilt of the acromial cortex represent relevant risk factors for rotator cuff tear.
We sought to understand if “The Acromion Rule” is congenital or acquired. There is an ongoing debate about whether the shape of the acromion is congenital or acquired. In 2001, based on macroscopic, radiographic, and microscopic examination of the acromion process, Nirav et al. hypothesized that changes in acromion morphology are a secondary degenerative phenomenon rather than congenital.9 We developed “The Acromion Rule” according to Nyffeler et al., and based on imaging studies, we speculate that “The Acromion Rule” is acquired rather than congenital. The middle deltoid muscle originates on the acromion; therefore, during active abduction, the deltoid muscle provides ascending and compressive forces, which acts on the acromion [Figure 6]. If collagen, fibrocartilage, and bone grow along the reverse plane of the compressive force component, the AI will increase. If collagen, fibrocartilage, and bone grow along the reverse plane of the ascending force, the AI will decrease. At present, the clinical study of many scholars has found that patients with rotator cuff tear are significantly higher than healthy ones.1–8 Due to the mechanical factors of the acromion, the large AI variation, and the morphological variation of the acromion became the cause of rotator cuff tear.
There were several limitations associated with this study. First, we did not account for a possible association between the shape of the acromion and the presence of rotator cuff tear in our study participants. Second, we did not account for a possible association between the acromiohumeral interval and the presence of rotator cuff tear. Third, our study was performed in a Chinese population, and the findings may not be generalizable to populations in other regions. Fourth, the age difference between the patients with rotator cuff tears and the healthy participants may have affected the results. Finally, this study does not take into account the intrinsic pathology of rotator cuff tear and the correlation between rotator cuff tear and factors such as diabetes, genetics, and smoking.