Purpose of the study: The Alpina unicompartimen-tal knee prosthesis (Biomet) is an anatomic prosthesis inserted with a femoral cut, a tibial base plate made of titanium and a flat modular polyethylene insert. Cemented and non-cemented versions are available with hydroxyapatite ceramic (HAC) coating.
Material and methods: We retrospectively reviewed the radiological outcome at more than five years (fie to eight years follow-up) in a consecutive series of patients who had an Alpina HAC prosthesis with tibial fixation completed with a titanium screw. Clinical and radiological findings were recorded. Radiologically: pre- and postoperative angles, implant position, reliability of the instrument set. At last follow-up, we studied the presence of lucent lines, polyethylene wear easily measured on the flat insert, and bone remodeling around the tibial fixation screw.
Results: At last follow-up (5 to 8 years), three patients were lost to follow-up and three had died. Radiological outcome was thus assessed for 44 implants (41 medial and 3 lateral). One patient required an early total knee prosthesis (diagnostic error). Three patients underwent revision at 5–7 years (for rupture of the polyethylene insert in two very active patients, and for significant polyethylene wear in the third). Two had a revision procedure for a partial knee prosthesis and the third for a total knee prosthesis. For the 40 other patients, the following observations were made: partial lucent line along the tibial polyethylene plate with no functional impact (n=1), polyethylene wear visible but measuring less than 1 mm (n=12), remodeling around the tibial fixation screw probably corresponding to a granuloma but not threatening the implant (n=10).
Discussion: The clinical results of partial knee prostheses are well known. Mid-term radiographic results of non-cemented unicompartmental prostheses with a flat tibial plateau producing a minimal contact surface has shown: good reliability of the instrument set, excellent bony integration of the HAC-coated implants, but measurable polyethylene wear on more than one quarter of the prostheses, with two ruptures of the polyethylene insert at 5 and 6 years.
Conclusion: A study currently under way will examine finished pieces on a simulator to study wear and failure as a function of several parameters: polyethylene thickness, lateral restraint with a metal rim, presence of a basal stem fixing the polyethylene on the metal base, importance of the femur-polyethylene surface contact.
