Predictive factors influencing internal rotation following reverse total shoulder arthroplasty

The Influence of the Joint Volume on the Prevention of Impingement and Elbow-at-Side Rotations: Could the 36 mm Sphere with an Inferior Offset of 2 mm Be the New Gold Standard?

Background: Reverse shoulder arthroplasty (RSA) improves shoulder function in cases of glenohumeral osteoarthritis and rotator cuff arthropathy. The design of the glenosphere influences mobility and scapular impingement. This study evaluates the impact of joint volume on the range of motion (RoM) and identifies design modifications to enhance mobility while reducing the impingement risk. Methods: Thirty-four cadaveric shoulders were implanted with the Aequalis Reversed II® prosthesis in seven configurations: four with 36 mm spheres (centered, 2 mm eccentric, and lateralized by 5 mm and 7 mm) and three with 42 mm spheres (centered, and lateralized by 7 mm and 10 mm). The joint volumes (inferior, anteroinferior, and posteroinferior) were measured via 3D CT scans. The RoM in adduction and elbow-at-side rotations (IR1 and ER1) was recorded. A statistical analysis identified threshold joint volumes correlating with improved mobility. Results: Larger joint volumes correlated with enhanced mobility. The 42 mm spheres demonstrated better adduction and ER1 compared to those of the 36 mm spheres (p < 0.0001). An inferior volume > 5000 mm3 and anteroinferior/posteroinferior volumes >2500 mm3 were thresholds for significant mobility improvement. Lateralization (≥7 mm) or inferior eccentricity (2 mm) improved the mobility with the 36 mm spheres, with the 36 + 2 configuration offering a practical balance for smaller patients. Conclusions: Increased joint volume enhances mobility, particularly in adduction and elbow-at-side rotations. A sphere with a 2 mm inferior offset or a 42 sphere with 7 mm lateralization optimizes the RoM while minimizing impingement risks. Patient-specific considerations, including anatomy and soft tissue tension, remain essential for optimal prosthesis selection.

Lateralising reverse shoulder arthroplasty using bony increased offset (BIO-RSA) or increasing glenoid component diameter: comparison of clinical, radiographic and patient reported outcomes in a matched cohort

BACKGROUND

This study aims to compare the range of motion (ROM) of reverse shoulder arthroplasty lateralised by bony increased offset (BIO-RSA) using a standard 38-mm (mm) component to regular reverse shoulder arthroplasty (RSA) lateralised by using a 42-mm glenoid component. The secondary aims are to compare patient-reported and radiographic outcomes between the two groups.

MATERIALS AND METHODS

All patients with a BIO-RSA and size 38 glenosphere were retrospectively identified and matched to patients with a regular RSA and size 42 glenosphere. Matched patients were invited for a follow-up visit. ROM was assessed as well as radiographic outcomes (lateralisation, distalisation, inferior overhang, scapular notching, heterotopic bone formation, radiolucency, stress shielding, bone graft healing and viability and complications) and patient-reported outcomes (subjective shoulder value, Constant score, American Shoulder and Elbow Surgeons, activities of daily living which require internal rotation, activities of daily living which require external rotation and a visual analogue scale for pain). Outcomes were compared between the two groups.

RESULTS

In total, 38 BIO-RSAs with a size 38 glenosphere were matched to 38 regular RSAs with a size 42 glenosphere. Of the 76 matched patients, 74 could be contacted and 70 (95%) were included. At the final follow-up, there were no differences between the two groups in ROM, patient-reported outcomes or radiographic outcomes (p > 0.485).

CONCLUSIONS

Using a larger glenosphere is a feasible alternative to BIO-RSA for lateralising RSA, providing comparable ROM, patient-reported and radiographic results, while potentially decreasing costs, operative time and complication rates.

LEVEL OF EVIDENCE III