5-Jahres-Erkenntnisse durch die Multizentrische Beobachtungsstudie mit dem zementfreien POLARSTEM Hüftschaft

As hip replacement is no longer just a concept for old patients, with more young and active patients recently undergoing treatment, the development of new prosthesis designs is essential. The rationale for developing the cementless POLARSTEM was the combination of a self-blocking principle with improved metaphyseal load transfer. The stem is made of a titanium alloy (Ti6Al4V) with a porous titanium plasma spray coating for increased surface roughness. This achieves a very good primary stability and osteointegration. Thus, the purpose of the present study was the long-term evaluation of the cementless POLARSTEM over a defined period of ten years. Data from Australia and Wales already showed a revision rate of 0.15-2.9% of cases over a period of 3-5 years. However, no prospective studies have been published considering patients' self-assessment of the clinical outcome. This study evaluates the clinical and radiological findings 5 years following implantation of the cementless POLARSTEM. Clinical outcome was assessed using the Harris Hip Score (HHS) and Western Ontario and McMaster Osteoarthritis Index (WOMAC). The radiological evaluation was performed using standard radiographs in an anteroposterior and lateral view at each follow-up. Between March 2009 and February 2010, 72 patients with 75 treated hips were included in the study at our institution. As a multicenter study, 75 additional patients were enrolled in the Cantonal Hospital Aarau (Switzerland) and the University Hospital Bochum, resulting in a total subject population of 225 patients. The patient population of this study consists of 31 men (41.33%) and 44 women (58.66%) with a mean age of 65.4 years (± 8.2) at the time of surgery. Body mass index (BMI) averaged 29.51 kg/m² (± 4.82). Primary coxarthrosis (76%) was the predominant indication for implantation of the total hip endoprosthesis. When compared to the preoperative status, the HHS and WOMAC showed a significant improvement 3 months, 1 year, 3 years and 5 years following implantation of the cementless POLARSTEM (p <0.001, respectively). Patients having a BMI of less than 30 kg/m² achieved a significantly better result at the 5-year follow-up compared to those with a BMI of more than 30 kg/m² (p < 0.05). In addition, patients with an age of less than 65 years had a significantly higher HHS compared to those with an age of over 65 years (p = 0.039). Compared to the preoperative examinations, degree of range of motion of the hip joint showed a significant increase in all different directions 5 years postoperatively (p <0.001, respectively). Radiographic evaluation showed an unchanged position of the implanted stem and socket components without migration, tilting or subsidence at terminal follow-up. Characteristic radiological changes of the acetabular or stem component including osteolysis, atrophy or hypertrophy of the bone substance as well as radiolucent lines were not observed. The survivorship of the implanted POLARSTEM was 98.7% after a period of 5 years with stem revision due to any reason. Comparable studies reported similarly good results in terms of postoperative HHS and WOMAC with a follow-up period of 5-10 years. The negative influence of high BMI or age on clinical outcome is still controversially discussed in the literature. The survival rate of 98.7% at 5-year follow-up is consistent with other studies and well in line with current requirements. Recent data from the Australian Endoprosthesis Registry showed the cementless POLARSTEM to have a revision rate of 0.3-2.9% after 3-5 years. The only revision of our patient population was caused by an infection 6 weeks postoperatively. Thus, stem design may not be the decisive factor in this case. Due to the convincing clinical and radiographic results, the safety and efficacy of the cementless POLARSTEM can be confirmed at least for a medium-term period of 5 years. However, it remains to be seen how the POLARSTEM will present in the long-term after a period of 10 or 15 years.