Coracoacromial ligament integrity influences scapular spine strain after reverse shoulder arthroplasty

The purpose of this biomechanical study was to examine the effect of coracoacromial (CA) ligament state (intact vs. released) and arm position on acromial and scapular spine strain following reverse total shoulder arthroplasty (rTSA).

Eight cadaveric shoulders were implanted with a custom rTSA system and tested using an in-vitro shoulder simulator. The specimens were cycled through static range of motion in both abduction and forward elevation; first with the CA ligament in the "intact" state followed by the "released" state. Scapular spine strain was quantified via 4 strain gauges placed along anatomic locations on the acromion and scapular spine.

Increases in strain were observed across all 4 strain gauge locations upon release of the CA ligament in both 0° of abduction and forward elevation. Increases in the mean strain were observed to be as great as 14% in abduction and 31% in forward elevation. The increases in strain at 0° elevation approached but did not reach statistical significance (P ≥ .072). At 90° of abduction and forward elevation, no increases in the mean strain were observed. The greatest strain was consistently observed with the arm positioned in 0° of forward elevation (P < .001).

CA ligament release in the setting of rTSA resulted in increased scapular spine and acromial strain with the arm adducted, although these increases in strain were not statistically significant. Caution should be taken intraoperatively as the release of the CA ligament may alter scapular spine and acromion stresses from deltoid loading, which may increase the risk for postoperative scapular spine fracture.