Acromial bony adaptations in rotator cuff tear arthropathy facilitates acromial stress fracture following reverse total shoulder arthroplasty

Background

Acromial stress fractures (ASFs) after reverse total shoulder arthroplasty (RSA) can have a potentially devastating impact on shoulder function. They are often difficult to effectively treat. Multiple studies have shown a higher incidence of ASF after RSA in patients with cuff tear arthropathy (CTA). This study introduces and explores a new hypothesis. Our hypothesis is that 1) patients with CTA experience a preoperative pathologic superiorly directed force on their acromion, and 2) this leads to acromial bending moments on the acromion that are very different from patients with glenohumeral osteoarthritis (GHOA) and after RSA, and finally 3) these pathologic loads may result in abnormal bony remodeling and adaptations in CTA, which may then predispose patients to ASF after RSA.

Methods

A finite element analysis model was developed to compare three loading conditions on the acromion: preoperative CTA, preoperative GHOA, and postoperative-RSA. Regions of the highest tensile and compressive stresses were identified and compared between groups.

Results

The finite element analysis model presented shows that patients with a preoperative diagnosis of CTA experience a stress distribution reversal after RSA, whereas GHOA patients do not. The results support that in CTA, the humerus produces pathologic strains and torques on the acromion. Over time, the acromion may slowly remodel in response, resulting in bony adaptations. Abrupt reversal of stresses and strains in CTA after RSA may lead to ASFs, as the acromion has adapted to a different stress pattern.

Conclusion

This study introduces one potential contributing factor for the higher rates of ASF after RSA in patients with a preoperative diagnosis of CTA; understanding this phenomenon is the first step to preventing it. Once the forces seen in CTA are abruptly reversed after RSA, the biological race is on between the formation of an acromial stress fracture and the body’s ability to again remodel the acromion to accommodate its new loading state. Additional clinical studies are needed to further investigate this new theory.