RESUMEN
INTRODUCTION: The proximal humerus is a common location for primary and non-primary tumors. Reconstruction of the proximal humerus is commonly performed with an endoprosthesis with low rates of structural failure. The incidence and risk factors for stress shielding are under reported. METHODS: Thirty-nine (19 male, 20 female) patients underwent resection of the proximal humerus and reconstruction with a cemented modular endoprosthesis between 2000 and 2018. The mean resection length was 12 ± 4 cm and was most commonly performed for metastatic disease (n = 26, 67%). RESULTS: Stress shielding was observed in 9 (23%) patients at a mean of 29 (6-132) months postoperatively. Patients with stress shielding were noted to have shorter intramedullary stem length (87 vs. 107 mm, p < .001), longer extramedullary implant length (16 vs. 14 cm, p = .01) and a higher extramedullary implant to stem length ratio (2.1 vs. 1.1, p < .001). The incidence of stress shielding was higher (p = .003) in patients reconstructed with 75 mm stem (n = 6, 67%) lengths. CONCLUSION: Stress shielding of the humerus was associated with the use of shorter stems and long extramedullary implants. The long-term ramifications of stress shielding on implant stability, complications at the time of revision surgery, and overall patient outcomes remain unknown.