Why Do Some Patients Experience Scapular Spine Fracture After Reverse Shoulder Arthroplasty?
With the increasing prevalence of reverse shoulder arthroplasty (RSA) comes the need to understand why certain complications occur and what can be done to mitigate them.
One such complication is postoperative scapular spine fracture. Previous studies have found the rate of scapular spine fracture after RSA to be from 0.8% to 11%, with differences in the reported rates attributable to variations in the implants used. [1-4]. Because these fractures are relatively rare,  their causes are not well understood, although researchers have found some association between scapular spine fracture and osteoporosis,  acromial thickness,  inflammatory arthritis, and glenoid lateralization  in post-RSA patients.
A multicenter study recently published online by the Journal of Shoulder and Elbow Surgery suggests that the type of stem used may be another cause: The risk of scapular spine fracture was 2.5 times higher with the use of an onlay humeral stem compared with the use of an inlay stem. Increased postoperative distalization, which was 10 mm higher with an onlay stem, was also a significant risk factor for scapular spine fracture, whereas increased lateralization was not.
For this study, 3 fellowship-trained shoulder surgeons practicing at 3 different institutions pooled data on outcomes of RSA procedures they had performed between July 2015 and July 2018. Patients were included if they had undergone primary RSA for rotator cuff arthropathy, primary osteoarthritis, or failed rotator cuff repair and had been followed for a minimum or 1 year or had a documented scapular spine fracture prior to 1 year after RSA.
Of the 676 patients who met the inclusion criteria, 426 were available for follow-up. The choice of implant had been left up to the surgeon, resulting in the use of 2 implants with an inlay humeral stem (n=342) and 1 implant with an onlay humeral stem (n=84).
Grashey (true glenohumeral anteroposterior view), scapular Y, and axillary radiographs were evaluated for fractures, which were classified as Levy type I, II, or III,  and charts were reviewed for documentation of a fracture. The radiographs were also used to measure distalization, humeral offset, and lateralization.
The researchers used patient-reported American Shoulder and Elbow Surgeons (ASES), VAS pain, and Single Assessment Numeric Evaluation (SANE) scores to evaluate function, and range of motion was measured by the surgeons before surgery and at the last follow-up visit. They also recorded patient satisfaction and return to activity at the last visit.
Postoperative radiographs showed scapular spine fracture in 26 patients (6.1%): 12 Levy type I, 9 Levy type II, and 5 Levy type III (Figure 1). The acromiohumeral distance was higher in the fracture group compared with the non-fracture group, 37.5 mm vs 33.7 mm (P=0.042), while lateralization and center of rotation offset were similar in the 2 groups.
Figure 1. Radiographs showing the 3 different types of fractures: Levy I (A), Levy II (B), and Levy III (C).
Of the 26 postoperative scapular spine fractures, 11.9% occurred in patients with an onlay humeral stem and 4.7% in patients with an inlay humeral stem (P=0.043). The postoperative acromiohumeral distance, humeral lateralization, and center of rotation were higher in patients with an onlay stem than in patients with an inlay stem:
- Acromiohumeral distance: 41.9 mm for onlay vs 31.5 mm for inlay (P< 0.001)
- Humeral lateralization: 57.0 mm for onlay vs 53.1 mm for inlay (P< 0.001)
- Center of rotation offset: 23.2 mm for onlay vs 10.2 mm for inlay (P< 0.001)
Clinically, patient-reported outcome scores and range of motion improved significantly from preoperative to postoperative in the cohort as a whole (P< 0.001). As might be expected, patients without a scapular spine fracture had better forward flexion and were more likely to return to activity than patients with a scapular spine fracture. Forward flexion was 135° for non-fracture patients and 120° for fracture patients (P=0.009), while 92.1% of non-fracture patients returned to activity compared with 71.4% for fracture patients (P<0.001).
No statistically significant differences were seen between groups with regard to ASES, VAS, and SANE scores; patient satisfaction; or other measures of range of motion (external rotation with the arm at the side and internal rotation).
“The aim of this study was to compare the effect of lateralization and distalization on scapular spine fracture following RSA,” the authors said. “The major findings are that distalization was higher in the scapular spine fracture group and that the incidence of scapular spine fracture was 2.5 times higher with an onlay stem compared to an inlay stem.
“These findings support our hypothesis [that postoperative distalization would be associated with an increased risk of scapular spine fracture but that increasing lateralization would not] and may have important implications for both prosthetic design and component placement.”
Georges Haidamous G, Lädermann A, Frankle MA, Gorman II RA, Denard PJ. The risk of postoperative scapular spine fracture following reverse shoulder arthroplasty is increased with an onlay humeral stem. 9 June 2020. Journal of Shoulder and Elbow Surgery. Published online ahead of print.
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