(1) Background: Modular megaprosthetic reconstruction using a proximal humerus replacement has emerged as a commonly chosen approach after bone
tumor resection. However, the long-term risk for
revision surgery is relatively high. One factor that might be associated with mechanical failures is periprosthetic
osteolysis around the stem, also known as stress shielding. The frequency, potential risk factors, and the effect on implant survival are unknown. (2) Methods: A retrospective single-center study of 65 patients with
sarcoma who underwent resection of the proximal humerus and subsequent reconstruction with a modular
endoprosthesis. Stress shielding was defined as the development of
bone resorption around the
prosthesis stem beginning at the
bone/prosthesis interface. The extent of stress shielding was measured with a new method quantifying
bone resorption in relation to the intramedullary stem length. All patients had a minimum follow-up of 12 months with conventional radiographs available and the median follow-up amounted to 36 months. (3) Results: Stress shielding was observed in 92% of patients (60/65). The median longitudinal extent of stress shielding amounted to 14% at last follow-up. Fifteen percent (10/65) showed
bone resorption of greater than 50%. The median time to the first radiographic signs of stress shielding was 6 months (IQR 3-9). Patients who underwent
chemotherapy (43/65) showed a greater extent of stress shielding compared to those without
chemotherapy. Three percent (2/65) of patients were revised for aseptic loosening, and one patient had a
periprosthetic fracture (1/65, 1.5%). All these cases had >20% extent of stress shielding (23-57%). (4) Conclusions: Stress shielding of the proximal humerus after shoulder reconstruction with modular megaprosthesis is common. It occurs within the first year of follow-up and might be self-limiting in many patients; however, about one third of patients shows progression beyond the first year. Still, mechanical complications were rare, but stress shielding might be clinically relevant in individual cases. The extent of stress shielding was increased in patients who underwent perioperative
chemotherapy. Stress shielding can be quantified with an easy method using the stem length as a reference.