Postoperative acromial fracture is a complication unique to reverse total shoulder arthroplasty (RSA).
It is not seen with anatomic total shoulder arthroplasty (link) or with cuff tear arthropathy arthroplasty (link).
As with other fatigue fractures, post-reverse acromial fractures are likely to result from a change in the loading of the acromion. In reverse total shoulder, the deltoid origin on the acromion is subjected to increased and repetitive loading after surgery in comparison to the preoperative state. This situation is similar in many ways to the girl who develops a tibial stress fracture after taking up cross country running.
Acromial stress fratures are not infrequent, occurring in as many as 1 out of 10 reverse total shoulders (10%). When displaced, they can cause devastating loss of function because of slackening of the deltoid and abutment of the displaced acromial fragment against the proximal humerus. Open reduction and internal fixation of these fractures is frequently unsuccessful.
The authors of Radiographic and Anatomic Variations on Postoperative Acromion Fractures Following Inlay and lateralized Reverse Shoulder Arthroplasty point out that there is limited knowledge regarding the etiology of these fractures and methods for their prevention.
They sought to identify the association of the acromioclavicular (AC) joint and relative humeral and glenoid positioning on the occurrence of acromial fractures at a minimum of two years after RSA in a retrospective case-controlled study of primary RSAs. All patients were treated by an expert, experienced surgeon using the DJO Global RSA which has a lateralized center of rotation range of 2-10 mm) and an inlay humeral prosthesis with a 135-degree neck-shaft angle. Soft tissue balancing was achieved through the use of polyethylene humeral shells of neutral, 4 mm, and 8 mm with standard and semiconstrained options.
Patients with a postoperative acromion fracture (n=47) were matched in a 3:1 ratio based on gender, indication, and age to those without a fracture (n=141).
The two groups were compared with respect to critical shoulder angle (CSA), acromion-humeral interval (AHI), global lateralization, delta angle, preoperative glenoid height, the level of inlay or onlay of the humeral stem, the morphology, width, and CT appearance of the acromioclavicular joint before surgery.
None of these attributes were significantly correlated with the occurrence of acromial fractures.
Comment: As suggested by the authors, risk factors such as osteoporosis, inflammatory arthropathy, prior surgery, female sex, history of dislocations, rotator cuff pathology, and steroid use may be more important than the variables that can be measured on radiographs. Still other factors, such as the rapidity of return to activities may merit additional study. Until means of avoiding these fractures are identified, it is important to inform patients of this potential complication of reverse total shoulder arthroplasty and, when applicable, to compare the risks of RSA to those of other types of shoulder arthroplasty.
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