Computational analysis of compressive joint stability and acromial stress associated with varied rotator cuff...

Controversy exists regarding the optimal reverse shoulder arthroplasty (rTSA) implant configuration to maximize range of motion and joint stability while minimizing risk of acromial or scapular spine fracture. The purpose of this study was to use finite element analysis (FEA) to evaluate changes in joint compressive stability and acromial stress with varied rotator cuff integrity, glenoid component lateralization, and humeral distalization after rTSA.

FEA was conducted to evaluate the mechanical influence of varied rotator cuff integrity after rTSA. The FEA model incorporated the deltoid, subscapularis, infraspinatus, and teres minor tendon and muscle geometries, implanted with the commercially available Stryker Tornier Perform rTSA system. Simulations were performed with 0, 3, and 6 mm of glenoid lateralization. Humeral-sided tensioning included both inlay and onlay geometries. A total of 30 model configurations were analyzed. Glenohumeral joint stability, evaluated using joint contact compressive force, was compared between a baseline rTSA configuration (0 mm glenoid lateralization, inlay humeral stem) with intact cuff and after progressive removal of cuff geometries. Contact forces and acromial stresses were compared for each rotator cuff configuration and with varying levels of glenoid lateralization and humeral distalization.

Glenohumeral contact force decreased with progressive cuff removal. Compared to the intact rotator cuff state, contact force decreased 59% when the subscapularis was absent, 11% when the infraspinatus was absent, and 67% when both subscapularis and infraspinatus were absent. Contact force increased with progressive levels of glenoid lateralization. Six mm lateralization with an inlay stem resulted in a marginal 0.2%-1.8% increase in the proportion of acromial and scapular spine cortical bone regions exceeding the yield stress threshold, although the gains in contact forces were roughly 2-fold with intact subscapularis. Addition of an onlay humeral stem, although resulting in larger improvements in joint compressive stability, had considerable 23.0%-30.4% increase in acromial and scapular spine cortical bone regions exceeding the yield stress threshold compared to inlay humeral configurations.

These results provide quantifiable evidence of the role that rotator cuff integrity plays in joint stability after rTSA. With a compromised rotator cuff, joint compressive stability could be recovered by increasing glenoid lateralization and humeral distalization. However, the improvements in joint stability resulted in corresponding increases in acromial stress. Computational modeling that incorporates anatomical morphology, bone quality, and demographic variables may assist surgeons in selecting optimal implant configurations to enhance joint stability and minimize the risk of acromial stress fractures.