Despite the progress that has been made in recent years in the field of hip joint endoprostheses, the problem of aseptic and septic loosening of endoprosthetic elements remains one of the unresolved issues. Therefore, the research on solutions to minimize the risk of periprosthetic inflammations and aseptic loosening of endoprostheses is still ongoing. The subject of the study was a stem of the Mittelmeier’s endoprosthesis removed due to its aseptic loosening after 16 years of implantation. The structural analysis was carried out using a light and scanning microscope and the chemical composition of the implant material was examined. The analysis of the chemical composition compared to the original composition of the stem revealed that direct contact of the implant with body fluids and living tissue affects the quantitative change of individual elements. Scanning electron microscopy revealed numerous pitting, cracks and losses caused by wear and corrosion on the metal stem. The research carried out showed that not only does the implant inserted in the bone affect the surrounding tissue, but there are also changes within the implanted prosthesis. The direct cause of the implant loosening was osteolysis in the space between the implant and the bone, associated with the accumulation of elements of corrosion and erosion wear. Based on the collected results from the Trauma and Orthopaedic Surgery Department of the Czerniakowski Hospital in Warsaw, a statistical analysis of primary and revision endoprostheses and complications was performed, with particular emphasis on the loosening of the endoprostheses elements that required revision surgeries.