Scapular Ring Preservation: Coracoacromial Ligament Transection Increases Scapular Spine Strains Following Reverse Total Shoulder Arthroplasty.

BACKGROUND

Scapular fractures following reverse total shoulder arthroplasty (RSA) are devastating complications with substantial functional implications. The role of the coracoacromial ligament (CAL), which is often transected during surgical exposure for RSA, is not fully known. We hypothesized that the CAL contributes to the structural integrity of the "scapular ring" and that the transection of this ligament during RSA alters the scapular strain patterns.

METHODS

RSA was performed on 8 cadaveric specimens without evidence of a prior surgical procedure in the shoulder. Strain rosettes were fixed onto the acromial body (at the location of Levy type-II fractures) and the scapular spine (Levy type III). With use of a shoulder simulator, strains were recorded at 0°, 30°, and 60° glenohumeral abductions before and after CAL transection. The deltoid and glenohumeral joints were functionally loaded (middle deltoid = 150 N, posterior deltoid = 75 N, and joint compression = 300 N). Maximum principal strains were calculated from each rosette at each abduction angle. A repeated-measures analysis of variance with post hoc analysis was performed to compare the maximum principal strain at each abduction angle.

RESULTS

With the CAL intact, there was no significant difference between strain experienced by the acromion and scapular spine at 0°, 30°, and 60° of glenohumeral abduction. CAL transection generated significantly increased strain in the scapular spine at all abduction angles compared with an intact CAL. The maximum scapular spine strain observed was increased 19.7% at 0° of abduction following CAL transection (1,216 ± 300.0 microstrain; p = 0.011). Following CAL transection, acromial strains paradoxically decreased at all abduction angles (p < 0.05 for all). The smallest strains were observed at 60° of glenohumeral abduction at the acromion following CAL transection (296 ± 121.3 microstrain; p = 0.048).

CONCLUSIONS

The CAL is an important structure that completes the "scapular ring" and therefore serves to help distribute strain in a more normalized fashion. Transection of the CAL substantially alters strain patterns, resulting in increased strain at the scapular spine following RSA.

CLINICAL RELEVANCE

CAL preservation is a modifiable risk factor that may reduce the risk of bone microdamage and thus the occurrence of fatigue/stress fractures in the scapular spine following RSA.