In recent years, indications for implanting mega-implants were established in managing major bone defects linked to revision arthroplasty due to loosening, periprosthetic fractures, re-implantation following periprosthetic joint infection, non-union following fractures as well as complex intraarticular primary fractures. This study was conducted to discuss and analyze the strategy of diagnosis and management of complications following the use of mega-implants in treating primary and periprosthetic fractures of the lower extremities. This is a monocentric retrospective study. Patients aged ≥ 18 years who underwent implantation of a megaendoprosthesis due to periprosthetic or primary fractures of the lower extremity between January 2010 and February 2023 were identified from the authors' hospital information system. We identified 96 patients with equal numbers of fractures (71 periprosthetic fractures and 25 primary fractures). 90 cases out of 96 were investigated in this study. The drop-out rate was 6.25% (six cases). The average follow-up period was 22 months (1 to 8 years) with a minimum follow-up of 1 year. The diagnosis of complications was provided on the basis of subjective symptoms, clinical signs, radiological findings and laboratory investigations such as C-reactive protein, leucocyte count and the microbiological findings. The indications for implantations of modular mega-implants of the lower extremities were periprosthetic fractures (65 cases/72.22%) and primary fractures (25 cases/27.78%). Pathological fractures due to malignancy were encountered in 23 cases (25.56%), in one case due to primary tumor (1.11%) and 22 cases due to metastatic lesions (24.44%). Two cases (2.22%) presented with primary intraarticular fractures with severe osteoporosis and primary arthrosis. In all cases with malignancy staging was performed. Regarding localization, proximal femur replacement was encountered in 60 cases (66.67%), followed by distal femur replacement (28 cases/31.11%) and total femur replacement (2 cases/2.22%). The overall complication rate was 23.33% (21 complications in 21 patients). The most common complication was dislocation which was encountered in nine cases (10%), all following proximal femoral replacement (9 cases out of 60, making 15% of cases with proximal femoral replacement). The second most common complication was infection (six cases, 6.67%), followed by four aseptic loosenings (4.44%), further intraoperative periprosthetic fracture in one case (1.11%) and a broken implant in one case (1.11%). We noticed no cases with wear and tear of the polyethylene components and no cases of disconnections of the modular components. Mega-endoprostheses enable versatile management options in the treatment of primary and periprosthetic fractures of the lower extremities. The rate of complications such as loosening, implant failure, dislocation and infection are within an acceptable range in this preliminary analysis. However, implantation of mega-endoprostheses must be strictly indicated due the limited salvage options following surgery.