Age / Sex : 75 / F

Chief complaint :

• s/p reverse total shoulder arthroplasty, right (op. date: 2018-08-27)


• abnormal findings on post-op. f/u images (2019-02-26)

Discussion

Answer:  acromion stress fracture (acromion fracture, type 2, in reverse total shoulder arthroplasty)

Findings: post-operative acromion stress fracture involving the anterior acromion posterior to the acromioclavicular joint

Differential Diagnosis: anterior dislocation, scapular notching…

Discussion: 

The reverse total shoulder arthroplasty, which reverses the ball and socket of the shoulder joint, was designed by Grammont et al in 1987 as a solution for pseudoparalysis and rotator cuff arthropathy. This design allows for the restoration of deltoid tension as the humeral prosthetic component depresses and medializes the center of rotation of the shoulder joint and lengthens its moment arm. When the deltoid contracts, the arm levers along the glenosphere, thereby facilitating abduction and elevation of the arm without an intact rotator cuff. An intact deltoid is therefore required for proper function of a reverse total shoulder arthroplasty.

Anterior dislocation is the most common early complication of reverse total shoulder arthroplasty, occurring in nearly 20% of patients, often without preceding trauma. A reverse arthroplasty dislocates anteriorly and superiorly along the axis of the deltoid muscle because of unopposed deltoid contraction. This is in contradistinction to anterior dislocation of the native shoulder, in which the humerus dislocates anteriorly and inferiorly.

Scapular notching is an exceedingly common complication of reverse total shoulder arthroplasty, occurring in 50%–96% of patients. On radiographs, bone resorption of the inferior scapular border is seen. This finding is likely related to medialization of the shoulder’s center of rotation, leading to repetitive contact between the humeral cup and the inferior scapular border during adduction. The severity of scapular notching can be described by using the four grades of the Sirveaux classification: grade 1, limited to pillar; grade 2, contacts lower glenosphere screw; grade 3, extends over the lower glenosphere screw; and grade 4, extends under the glenosphere base plate. Grade 1 and grade 2 scapular notching often stabilize and require no further treatment. Grade 3 and grade 4 scapular notching may undermine or loosen the glenosphere, sometimes requiring additional surgery.

Scapular spine and acromion fractures are complications unique to reverse total shoulder arthroplasty, with a reported prevalence of 5%–6.9%. These acromion fractures are divided into three types. Type 1 fractures involve the anterior acromion near or including the coracoacromial ligament footprint, tend to occur at the time of initial surgery, and often heal without intervention. Type 2 fractures involve the anterior acromion just posterior to the acromioclavicular joint. These fractures are thought to represent stress fractures of the acromion from restored abduction and elevation of the upper extremity exerting stress upon the acromion, which is often stiffened by adjacent acromioclavicular joint osteoarthritis. The radiologist should scrutinize the acromion in patients with a reverse arthroplasty for early signs of stress injury, including periosteal reaction and increased sclerosis. If radiographically occult, CT and bone scintigraphy may be helpful for diagnosis, especially in patients with focal pain. If untreated, type 2 fractures are at risk for displacement . Type 3 fractures involve the posterior acromion or scapular spine and are thought to arise from increased stress from the superior glenosphere screw.

References:

Shoulder arthroplasty, from indications to complications: what the radiologist needs to know. RadioGraphics 2016; 36:192-208

Classification of postoperative acromial fractures following reverse shoulder arthroplasty. J Bone Joint Surg Am 2013; 95:e104