Review Question - QID 219911

Reverse Shoulder Arthroplasty Complications Acromial or scapular spine fractures QID: 219911 (SBQ24SE.44)

QID 219911 (Type "219911" in App Search)

A 73-year-old woman undergoes right reverse total shoulder arthroplasty with an onlay humeral implant component for an irreparable rotator cuff tear and advanced glenohumeral osteoarthritis after failing nonoperative management. She returns to the clinic for her 1-year follow-up, reporting a 2-week history of acute onset, severe right shoulder pain without any inciting incidents, trauma, or falls. On examination, she has tenderness over the scapular spine, with shoulder abduction limited due to pain. Plain radiographs are obtained and confirmed on computed tomography of the right shoulder (Figure A). What factor would have most effectively decreased this patient’s risk of developing this complication?

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Increasing the neck-shaft angle

20%

106/526

Use of a non-constrained liner

37/526

Decreasing the thickness of the polyethylene liner

25%

129/526

Avoiding the inferior glenoid baseplate screw

13%

70/526

Use of an inlay humeral implant design

33%

176/526

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This 73-year-old female developed a scapular spine fracture approximately one-year status post reverse total shoulder arthroplasty likely due to increased stress on the scapular spine as a result of increased humeral lateralization due to the use of an onlay humeral component. The use of an inlay humeral component places less stress on the scapular spine and would have reduced the risk of fracture.

These fractures can result from traumatic injury or excessive stress placed on the scapular spine during daily shoulder use. The incidence of scapular spine fractures has decreased with improvements in implant design and surgical techniques. Risk factors include older age, female gender, osteoporosis, inflammatory arthritis, revision shoulder arthroplasty, rotator cuff arthropathy, massive rotator cuff tears, and prior ipsilateral shoulder surgery. Implant design and positioning significantly impact arthroplasty biomechanics. Onlay humeral components, which increase humeral lateralization, have been linked to a higher incidence of scapular spine fractures than inlay components. Additional risk factors include increased lateralization or superior placement of the glenoid component and the use of a superior screw in the glenoid baseplate.

Paszicsnyek et al. (2022) conducted a systematic review of acromial and scapular spine biomechanics after RTSA. They reviewed six biomechanical studies and found that humeral and glenoid lateralization, as well as superior glenoid placement, increase strain on the scapular spine. The authors concluded that these factors significantly elevate the risk of scapular spine fractures.

Haidamous et al. (2020) performed a multicenter retrospective review of 342 RTSA patients with at least one year of follow-up. They found a scapular spine fracture rate of 11.9% in the onlay group compared to 4.7% in the inlay group (p<0.05). Despite similar lateralization in both groups, the fracture group had poorer functional outcomes. The authors concluded that the distalization associated with onlay stems increased fracture risk by 2.5 times compared to the inlay group.

In a 2018 retrospective review, Ascione et al. studied 485 RTSA patients to determine the incidence of scapular spine fractures. They found an incidence of 4.1%, with an average time to diagnosis of 8.6 months. Increased rates of fractures were observed in patients with an onlay humeral component. The authors concluded an association between onlay components and scapular spine fractures but could not identify clear risk factors.

Moverman et al. (2024) conducted a multicenter retrospective study with 6,320 RTSA patients as part of the ASES Complications of RSA Multicenter Research Group. They identified a scapular spine fracture rate of 0.9% and an acromial stress fracture rate of 2.8%. Increased glenoid lateral offset and a larger lateral shoulder angle were identified as risk factors for scapular spine fractures. The authors concluded that patient-related factors such as poor bone density, rotator cuff pathology, and increased glenoid and global lateralization are strong predictors of fracture.

Figure A is the corresponding CT right shoulder axial cuts demonstrating the patient's minimally displaced scapular spine fracture.

Illustration A depicts an inlay humeral component, with the humeral tray seated within the metaphysis, and an onlay humeral component, with the humeral tray seated at the level of the humeral neck cut on the metaphysis.

Incorrect Answers:
Answer 1: Increasing the neck-shaft angle has been implicated in increasing the risk of scapular spine fracture, as this creates a larger moment arm for the deltoid resulting in increased stress on the scapular spine.
Answers 2 and 3: Neither the use of a constrained liner nor decreasing the thickness of the liner has been shown to decrease the risk of scapular spine fracture
Answer 4: The superior glenoid baseplate screw has been implicated as a potential cause of scapular spine fractures as the fractures often originate at the tip of the superior screw, while the inferior screw does not increase this risk.