Background: Glenohumeral instability is one of the most frequent and clinically impactful complications following proximal humerus resection and reconstruction with a megaprosthesis, especially in patients treated for bone tumors or complex fractures. Its incidence, risk factors, and influence on functional recovery remain variably reported in the literature. Methods: A systematic review was conducted according to PRISMA guidelines, searching PubMed, Scopus, and Google Scholar up to April 2025. Studies reporting on postoperative instability, dislocation, functional outcomes (MSTS, DASH), and related complications were included. Two independent reviewers performed data extraction and quality assessment. A pooled analysis was performed using random-effects models. Results: A total of 17 studies including 387 patients were analyzed. The pooled incidence of glenohumeral instability was 32%, with a revision surgery rate of 10% due to instability. The most common reconstruction technique was modular megaprosthesis (47%), followed by allograft–prosthesis composites (APCs) and reverse total shoulder arthroplasty (RSA). Functional outcomes were reported in 12 studies using the Musculoskeletal Tumor Society (MSTS) score, with a weighted mean of 22.3 ± 3.8 (74.3% ± 12.7%). Disabilities of the Arm, Shoulder, and Hand (DASH) scores, reported in 3 studies, showed worse outcomes in unstable shoulders (mean 61.4 ± 5.2 vs. 26.6 ± 4.1). Soft tissue reconstruction, particularly involving the rotator cuff and deltoid, significantly influenced postoperative stability and function. Conclusions: Glenohumeral instability after proximal humerus megaprosthesis is a common and disabling complication that adversely affects functional outcomes and revision rates. Optimizing soft tissue management and prosthetic design is essential to improve joint stability and long-term results.

Background: Reconstruction with megaprostheses is an important limb-salvage option in cases of bone tumors and metastases, as it allows extensive bone resection while preserving the limb. However, postoperative shoulder instability remains a frequent complication. Although implant instability is one of the leading causes of treatment failure, the factors contributing to it are still not clearly defined. This study analyzes the correlation between resection length, implant type, and involvement of the deltoid and rotator cuff in relation to postoperative instability and the risk of postoperative dislocation. Methods: Twenty-five patients treated at our institution between 2014 and 2025 who underwent proximal humerus replacement with a megaprosthesis were included. Patients were divided into two groups: those with postoperative dislocation (Group A) and those without (Group B). Functional outcomes were assessed using the MSTS and DASH scores. The mean follow-up was 20.38 ± 11.87 months. Results: Of the twenty-five patients included, six (24%) experienced instability and subsequent dislocation (Group A). Rotator cuff invasion was present in 100% of Group A and 36.8% of Group B (p < 0.001), while deltoid invasion was present in 83.3% of Group A and 21% of Group B (p = 0.001). The mean resection length was 14.4 ± 2.2 cm in Group A and 13.7 ± 3.7 cm in Group B (p = 0.636). No significant differences were found between the groups in DASH and MSTS scores. Conclusions: In this cohort, rotator cuff and deltoid involvement were significantly associated with postoperative shoulder instability following proximal humerus megaprosthetic reconstruction. These findings suggest that careful preoperative assessment of rotator cuff and deltoid involvement may help identify patients at higher risk of postoperative instability and optimize surgical planning. Future studies should focus on preoperative planning and the development of a risk score to identify patients at higher risk of instability, potentially allowing for additional procedures such as latissimus dorsi flap reconstruction.