Review Question - QID 219840

The patient had a reverse total shoulder arthroplasty with an onlay humeral component performed for rotator cuff tear arthropathy, which was complicated by an acromial stress fracture. This complication occurs because of the deltoid tensioning that allows the implant to compensate for a non-functional rotator cuff, and is more common with an onlay humeral component (Answer choice 3).

Rotator cuff tear arthropathy represents a specific pattern of shoulder degenerative joint disease that results from a rotator cuff tear leading to abnormal glenohumeral wear and subsequent superior migration of the humeral head. Treatment for minimally symptomatic patients involves activity modification, subacromial steroid injections, and physical therapy; however, with progressive pain and deterioration of shoulder function, reverse total shoulder arthroplasty (RSA) is often indicated. By design, RSA moves the center of rotation inferiorly and medial, which improves the mechanical advantage of the deltoid muscle and allows it to act on a longer fulcrum to provide functional elevation of the arm. Given this increased tensioning of the deltoid muscle, increased stresses are placed across its origins on the scapular spine and acromion, predisposing to post-operative stress fractures. Though not extremely common, they occur with an incidence of ~4-5% at a mean of 9 months postoperatively, with risk factors being osteoporosis, female sex, and a more medialized pre-op center of rotation given functional lateralization post-operatively relative to this initial medialization. Treatment with conservative management leads to union in ~40-50% of cases, with operative management often indicated for nonunion or those with persistent pain.

Zmistowski et al. reviewed the incidence and predictors of acromial stress fracture following reverse total shoulder arthroplasty (RTSA). The authors reviewed 958 RTSAs with at least 3 months follow-up and found 40 acromial stress fractures (ASFs = 4.2%) and 61 acromial stress reactions (6.4%). They concluded that acromion-related symptoms are common following RTSA, with female patients and those with increased preoperative center-of-rotation medialization having an increased incidence of ASFs. They highlight that although this study establishes which patients are at risk of ASFs, methods for the prevention of ASFs in these patients remain unclear.

Lau et al. performed a systematic review of acromial fractures after reverse total shoulder arthroplasty. The authors included 25 papers reporting a total of 208 fractures, indicating an overall incidence of 5%, with stress fractures being more common than post-traumatic ones. Of these fractures, 24 underwent osteosynthesis, with nine requiring revision arthroplasty surgeries. They concluded that acromial stress fracture after reverse total shoulder arthroplasty occurs relatively commonly but is poorly reported in the literature, and it is unclear whether immobilization, fixation, or revision arthroplasty is the best treatment, although fixation may offer a better outcome.

Ecklund et al. provide a review of rotator cuff tear arthropathy. The authors note that, traditionally, surgical management of rotator cuff tear arthropathy has been disappointing because of the development of complications long-term and poor patient satisfaction with functional outcomes; however, recent studies report promising experiences with reverse ball-and-socket arthroplasty. They conclude that the ability of this implant to maintain the humeral socket centrally on the glenosphere may restore normal glenohumeral kinematics in the face of the compromised primary and secondary stabilizers of the glenohumeral joint.

Figure A is a pre-operative coronal CT image demonstrating the sequelae of rotator cuff tear arthropathy with superior humeral head migration and acromial acetabularization. Figures B-E represent sequential coronal, sagittal, and axial post-operative CT images of a reverse total shoulder arthroplasty with an acromial process fracture. Illustration A demonstrates this fracture with a label.

Incorrect Answers:
Answer 1: Osteoporosis and female sex represent risk factors for post-operative acromial stress fractures after RSA, but bisphosphonate use is not the mechanism by which these fractures occur.
Answer 2: Most modern RSA designs, though still medialized, have more lateralization than the originally described Grammont RSA, with options to build out or increase lateralization, which increases tensioning of the deltoid and subsequent risk for scapular spine and acromial process stress fractures.
Answer 4: Though increased bioburden of C. acnes can result in chronic, indolent post-operative infections after RSA, this is not the mechanism by which acromial process stress fractures occur.
Answer 5: Superior tilt of the glenosphere predisposes to scapular notching, not scapular spine/acromial process fractures.