Neither critical shoulder angle nor acromion index were related with specific pathology 20 years later

Purpose The critical shoulder angle (CSA) and the acromion index (AI) are measurements of acromial shape reported as predictors of degenerative rotator cuff tears (RCT) and glenohumeral osteoarthritis (GH OA). Whether they are the cause or effect of shoulder pathologies is uncertain since pre-morbid radiographs most often are lacking. The main aim of this study was to investigate if CSA or AI were related to the development of RCT or GH OA after 20 years. A secondary aim was to investigate if the CSA and AI had changed over time. Methods In the hospital archive, 273 preoperative plain shoulder radiographs were found of patients scheduled for elective surgery other than cuff repair and arthroplasty. Forty-five images fulfilled the strict criteria published by Suter and Henninger (2015) and were used to measure CSA and AI with two independent assessors. No patient had any sign of OA in the index radiographs or any information in the medical records indicating RCT. After a median of 20 (16–22) years, 30 of these patients were radiologically re-examined with bilateral true frontal views and ultrasound of the rotator cuff. There were 19 men (20 study shoulders) and 11 females (12 study shoulders). Results Mean age at follow-up was 56 (32–78) years. There was no correlation between CSA ( r = 0.02) (n.s) or AI ( r = − 0.13) (n.s) in the primary radiographs and OA at follow-up. Nor was any correlation found between index CSA ( r = 0.12) (n.s) or AI ( r = − 0.13) (n.s) and RCT at follow-up. Mean difference in CSA was − 1.7 (− 10–3) degrees and mean AI difference was − 0.04 (− 0.13–0.09) between the first and the second radiographs, 20 years later. Bilaterally, mean CSA was 32 and AI 0.61 at follow-up. Conclusion In this study, no correlation between the CSA, AI and development of OA or RCT could be found. The mean CSA and AI decreased over a 20-year period but the difference was very small. No difference was found between the study shoulders and the contralaterals. These findings question previously reported etiological associations between scapular anatomy and the development of OA or RCT and thereby the use of these calculations as the basis of treatment. Level of evidence III.