Tibiales Malalignment bei Mobile-Bearing-Prothesen

Total knee replacement requires exact implantation in order to
avoid long term failure. Longitudinal in vivo studies of malaligned
prostheses are problematic for ethical reasons. To assess the
critical angle of tibial (varus / valgus) malalignment of knee
prostheses in vitro, a simulator investigation was performed.
Mobile-bearing- prostheses were implanted in a simulator with 1°-30
valgus- and varus malalignment as well as in neutral position. More
extreme malalignment caused unexceptional forces in the simulator.
After each 500,000 cycles (ca. 0,5 years of walking) the replicas
of the polyethylene inlay were examined and the extent of the
tribocontact-areas was measured until 1,500,000 total cycles.
Finally the original polyethylene was examined with a scanning
electron microscope. The polyethylene showed only abrasive wear
without any fatigue effect. Tribocontact-areas of 900-1,500mm2 were
observed under malalignment of mobile-bearing prostheses according
to the producer’s reference. With 1-30 of malalignment
tribocontact-areas were located at untypical polyethylene-zones,
but still showing abrasive wear only. Increasing malalignment due
to a lift off of the femoral part of the prostheses with a strong
torsional strength of the polyethylene made an extremer simulation
impossible. 3 degree of Malalignment could be suggested as maximum
in-vivo tolerability, perfect alignment with ideal implantation
should be aimed at. Soft tissue is less considered within this
simulator-study.