Acromion and scapular spine fractures in reverse total shoulder arthroplasty: an extended finite element model study

Reverse total shoulder arthroplasty is widely used to restore shoulder function and alleviate pain. In approximately 1%-4% of patients, acromion and scapular spine fractures (ASF) occur and present a clinical challenge for orthopedic surgeons. While impact trauma, impingement, and excessive muscle forces are proposed causes, the relationship between these loading conditions and ASF types remains unclear, hindering targeted prevention strategies. The objective of this study is to identify the biomechanical relationship between loading conditions and ASF types, with the goal of informing targeted preventive strategies to reduce fracture incidence.

An extended finite element model was developed using computed tomography-derived shoulder geometry from virtual surgeries to simulate ASF. Bone material properties were based on computed tomography Hounsfield units. Loading scenarios, including impact trauma, impingement, and excessive deltoid force, were informed by existing literature. Simulations were performed in Abaqus, with mesh independence testing ensuring result accuracy. Fracture locations were analyzed according to the modified Levy ASF classification.

For impact traumas, the analysis identified that fractures occurred primarily at the initial contact point of the acromion, predominantly in the lateral region. Specifically, forward and sideway impact traumas typically resulted in modified Levy Type I ASF, while backward impact traumas could produce modified Levy Type IIA ASF. Lateral impingement typically led to lateral fractures, specifically modified Levy Type I ASF. The effects of deltoid force varied with the type of shoulder activity; excessive movements during shoulder abduction and forward flexion could cause modified Levy Type IIA ASF. In contrast, hyperextension of the shoulder could shift the fracture location to the medial region, leading to modified Levy Type III ASF.

Impact trauma, impingement, and excessive deltoid force can all induce ASF, with different loadings leading to different fracture types. Impact trauma and impingement typically cause lateral acromial fractures, while excessive deltoid force may result in medial scapular spine fractures, which are more difficult to manage. This study directs future research towards developing more effective preventive approaches to reduce the incidence of ASF that needs postoperative interventions. Additionally, the complexity of muscle loading, influenced by shoulder movements and activation patterns, highlights the need for further investigation into the relationship between excessive deltoid loading and medial scapular spine fractures.