In unicondylar knee prostheses, the relative angle and congruency of the femoral against the tibial component is not mechanically constrained and may vary with the surgical implantation technique. The contact area between both components was measured with increasing varus (0-20 degrees) and flexion angles (-20 degrees to 90 degrees) in five prosthesis models in the laboratory. The contact area varied with the relative position of the components and was critically reduced up to 70% at a varus range between >5 degrees and <25 degrees. The importance of relative malpositioning of the femoral and tibial components may be underestimated and reduces the contact area of unicondylar prostheses decisively, independent from the limb axis. This increases local pressure and may thus importantly contribute to increased wear and early loosening.