Influence of Radiographic Parameters on Reduction of the Critical Shoulder Angle With Arthroscopic Lateral Acromioplasty—A Mathematical Model

Objectives To develop a mathematical model for the preoperative planning of arthroscopic lateral acromioplasty (ALA) and to evaluate the role of radiographic parameters with regards to the critical shoulder angle (CSA). Methods Anteroposterior (AP) radiographs of patients who underwent rotator cuff surgery were screened to identify true AP radiographs. Radiographs were assessed for (1) native CSA, (2) CSA after simulated resection of a spur if present, (3) amount of ALA necessary to achieve a CSA of 34°, (4) CSA after 5-mm ALA, (5) lateral acromion angle, (6) acromion index, and (7) sclerosis of the greater tuberosity. Results A total of 1191 radiographs were screened. Of the 124 patients included, the native CSA was large (≥35°) in 56 patients (45%). In 30 patients (24%), a subacromial spur was detected and resection reduced the CSA by a median of 2°. Spur resection alone reduced the CSA to ≤34° in 19 patients (15.3%). Mean amount of ALA to achieve a CSA of 34° was 3.9 ± 1.8 mm, and this value strongly correlated with the CSA before ALA (R = 0.88, P R e q u i r e d A L A i n m m = − 39.120 + 1.165 ∗ C S A n a t i v e The multiple R2 for this model was 0.777. Mean reduction of CSA by 5-mm ALA was 3.8 ± 0.8° and 75% of large CSAs were reduced to a CSA of 30-34°. The acromion index had no significant independent influence on the model (P = .427), whereas lateral acromion angle was an independently significant predictor of required ALA to achieve a CSA of 34° (P = .019). Sclerosis of the greater tuberosity was significantly associated with a CSA of 35° or greater (P = .003). Conclusions The amount of ALA needed to reduce a large CSA to 34° correlates with the CSA before ALA and can preoperatively be planned with the use of a simple equation. Level of Evidence Level III; cross-sectional design; epidemiology study.