Radiologist-Integrated Finite Element Framework for Acromial Stress in Reverse Shoulder Arthroplasty

Background and Rationale. Acromial stress fracture is a recognised complication following reverse shoulder arthroplasty (RSA), with a clinically relevant incidence reported in prospective cohorts. The mechanical link between RSA-induced load redistribution and acromial fracture localisation remains incompletely characterised, and no validated preoperative tool currently exists to stratify acromial fracture risk on a patient-specific basis. This study addresses that gap by integrating musculoskeletal radiology grading with subject-specific finite element (FE) modelling within a single prospective pipeline.

Study Design. A prospective single-centre observational cohort of consecutive elective RSA candidates will be enrolled. Each participant will undergo high-resolution preoperative imaging consisting of computed tomography (voxel ≤1 mm) for cortical bone reconstruction and 1.5T or 3T magnetic resonance imaging for soft-tissue and rotator cuff assessment.

Radiologist Grading and Cuff Deficiency Index. Rotator cuff integrity will be graded by a senior musculoskeletal radiologist using the Goutallier classification for fatty infiltration. From these grades, a continuous Cuff Deficiency Index (CDI) will be derived as a quantitative descriptor of cuff functional capacity. The CDI will be used as a direct biomechanical input to scale the deltoid force estimates applied to the FE model.

Finite Element Pipeline. Patient-specific finite element models will be constructed from the segmented imaging data. Cortical bone will be assigned heterogeneous Hounsfield-mapped material properties, deltoid muscle anatomy will be reconstructed from MRI, and physiological as well as RSA-specific loading scenarios will be simulated using FEBio. Mesh convergence and boundary-condition sensitivity will be verified for each subject.

Outcome Assessment. The primary outcome is the regional 95th-percentile von Mises stress within acromial regions of interest corresponding to Levy zones I-III, evaluated at six months post-surgery. Stress-based and morphometric parameters will be correlated with postoperative radiographic outcomes assessed at the same six-month follow-up.

Expected Contribution. By coupling radiologist-graded clinical input with patient-specific FE modelling, this study aims to establish the feasibility of a prospective framework for preoperative acromial stress assessment in RSA candidates.

Inclusion criteria

• Adult patients (60-80 years) with cuff tear arthropathy (Hamada grade 3 or above, Walch glenoid B/C) scheduled for primary unilateral reverse shoulder arthroplasty
• Availability of preoperative high-resolution CT (voxel 1 mm or less) and 1.5T/3T MRI
• Written informed consent for prospective use of anonymised data

Exclusion criteria

• Revision RSA or history of prior shoulder prosthetic complications
• Uncorrectable imaging artefacts
• Post-RSA follow-up shorter than 6 months