Abstract
INTRODUCTION
The lifetime of UHMWPE implants may be limited by wear and oxidative degeneration. Wear produced particles are in general biologically active, and may induce osteolysis. As threshold of PE wear rate below which osteolysis is rarely observed is postulated to be less tahn 0.1 mm per year. Moreover, PE delamination and breakage are consequences of the embrittlement of the PE due to oxidation. Both demonstrate, that improving the clinical behaviour of UHMWPE means reduction of wear particles. The first can be achieved by cross-linking the second by the anti-oxidative stabiliser vitamin E. The highly cross-linked PE vitamys ® used for the isoelastic monobloc cup RM Pressfit (Mathys AG Bettlach, Bettlach, Switzerland) is mixed with 0.1% of synthetic vitamin E and is the first and only highly cross-linked PE used in total hip replacement that meets all requirements for the best grade UHMWPE in yield strength, ultimate tensile strength and elongation at break.
METHODS AND MATERIAL
With the first implantation of RM Pressfit vitamys® a prospective multicentre study was started. So far 256 cases in 7 clinics from Europe and New Zealand are included. This report presents the first clinical experiences of one Swiss clinic from the multicentre study. Prospective data collection includes Harris Hip score (HHS), patient satisfaction and radiographic analysis. Clinical and radiographic follow-up is done after 6 weeks, 6, 12 and 24 months, and thereafter for long-term results. Standardized documentation of surgery and postoperative course is performed.
RESULTS
81 patients were included in the study. Mean age at implantation is 69 years. The indication was primary OA in 57% and secondary OA in 36%, the others were treated either for necrosis, fracture or hip dysplasia. 68% of the patients received a 36mm femoral head, 31% a 32mm and 1% a 28mm. Intraoperatively two minor complications occurred, one femur and one trochanter fissure. No complications occurred during implantation of the cup. Most of the cups (59%) were implanted with an inclination of 40°-50°, 30% with 35°-40°. There were no signs of early loosening and a good Harris Hip Score was achieved.
DISCUSSION
The principals of this monobloc cup with its isoelastic property and the thin titanium coating is a proven concept. Ihle et al. (JBJS 2008) reported 91% survival rate with revision for aseptic loosening as endpoint after 20 years. They found an increase of cup revisions after 14 to 16 years after implantation due to osteolysis probably due to PE wear. At short term, we haven't encountered any problems related to the implantation of this cup. So far one patient in the entire multicentre study needed a stem revision after periprosthetic femur fracture. To prove the durability of this novel material a follow-up of 14 years and more will be required.
