Stress shielding influences shoulder function after reverse shoulder arthroplasty using a short stem at minimum 2 years follow-up and can be predicted using a preoperative planning software: A retrospective cohort study

Background: The aim of the study was to assess if 3D CT-scan-based planification of distal filling ratio (DFR) is accurate when compared to postoperative DFR calculated on plain X-rays. A secondary aim was to investigate if an association between clinical outcomes and the degree of stress shielding (SS) exists. Methods: Fifty patients with short-stem reverse shoulder arthroplasties, preoperative CT scans, and a minimum 24-month radiographic follow-up were included in the study. At 24 months follow-up, all patients were evaluated by assessing the postoperative filling ratios, the 3D DFRs, the simple shoulder test, Constant-Murley score, and visual analogue scale through a prospectively data collection and statistically analyzed. Results: A correlation between the 3D DFR and SS onset (rs 0.54; P < 0.001), and between postoperative DFR and SS (rs 0.71; P < 0.001), was found. The 3D DFR and the postoperative DFR were correlated (rs 0.89; P < 0.0001). SS negatively affects the postoperative range of motion, decreasing the forward elevation of the shoulder. Conclusion: The calculation of DFR based on 3D CT planning is a good predictor of humeral SS after short-stem RTAs. The presence of SS decreases clinical outcomes by lowering the anterior elevation of the shoulder. Level of evidence: Case series.