Rotator Cuff and Deltoid Involvement Are Associated with Shoulder Instability After Proximal Humerus Megaprosthetic Reconstruction.

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.