The subacromial index: a novel radiological parameter associated with rotator cuff tears

Previous studies have demonstrated associations between radiological parameters such as the acromial index (AI), critical shoulder angle, and lateral acromial angle and rotator cuff tears (RCTs). The aim of this study was to evaluate the relationship between RCTs and a novel measurement method termed the subacromial index (SAI), which has not been previously described in the literature, and to compare this parameter with established radiological measurements.

This retrospective study evaluated 132 patients who underwent shoulder magnetic resonance imaging between January 2017 and March 2021. The study population was categorized into 2 groups according to the presence or absence of RCTs. Demographic variables and radiological parameters, including age, sex, AI, critical shoulder angle, lateral acromial angle, SAI, and subacromiohumeral distance (SHD), were assessed. Receiver-operating characteristic curve analysis was used to determine cutoff values, and multivariable as well as stratified logistic regression analyses were performed to identify factors associated with RCTs.

Patients with RCTs were older and more frequently female. In the RCT group, AI and critical shoulder angle values were higher, whereas SAI and SHD values were significantly lower. Receiver-operating characteristic analysis demonstrated that older age, female sex, increased AI and critical shoulder angle, and decreased SAI and SHD were associated with RCTs. In the multivariable analysis including demographic variables, age and AI were identified as the strongest factors associated with RCTs. In the stratified analysis excluding demographic variables, the SAI remained significantly associated with RCTs.

The SAI, which simultaneously reflects acromial length and inclination, is significantly associated with RCTs. Although age and AI remain the strongest factors overall, the SAI provides additional anatomical information regarding RCT risk independent of demographic factors.