The current prospective single-center study is designed to examine an approach that utilizes both high-resolution CT and MRI scans, coupled with customized finite element modelling to analyze the stress placed on the acromion of patients who have undergone RSA surgery. The degree of rotator cuff deficiency will be assessed using Goutallier classification and quantified as Cuff Deficiency Index (CDI), which directly provides biomechanical information for calculating deltoid forces used in finite element analysis. Von Mises stresses will then be analyzed in anatomically defined acromial zones (Levy zones I-III).

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

Consecutive elective RSA candidates at a single orthopaedic centre