Fig. 2

The procedure of the precise acromioplasty. a Simulating virtual acromioplasty on computer. The pre-operative CSA (critical shoulder angle) is 36°(green line), the post-operative target CSA is set to be 30°(red line), and the green regions is planned to be resected. b Measurement of the resected bone border (green regions) by software. A, anterolateral corner; B, anteromedial resection margin; C, posterolateral resection margin; distance from A to B is about 10 mm, distance from A to C is about 22 mm. c Marking acromion and determining the resection margin on the patient’s skin. d View from the posterior portal: The spinal needles mark the points A, B and C, the line BC is the resection line. # undersurface of the acromion. e View from the lateral portal: After the precise resection of the acromion. * delta muscle fibers; ## resection surface of the acromion. f Measurement of post-operative CSA (29.4°) on standard anteroposterior radiography of the shoulder