Mechanical evaluation of mobile bearing knees.

To evaluate the mechanical behavior of a new mobile bearing knee prosthesis (MBK, Zimmer, Swindon, United Kingdom), two experimental studies were performed. In the first study, by superimposing a transparent template of the MBK on lateral radiographs taken in 15 normal cadaveric knees at 15 degrees intervals from 0 degree to 120 degrees of flexion, the displacement of the mobile bearing surface in respect to the tibial plateau was measured. By increasing the knee flexion, the mobile bearing surface moved progressively backward from an average value of 1.4 mm at 0 degree to an average value of 7 mm at 120 degrees of flexion. In the second study, by using pressure-sensitive Fujifilm, the contact pressure distribution across the upper and lower bearing surfaces of closely conforming mobile bearing knees was analyzed to determine if small differences in the relative radii of curvature resulted in pressure changes distribution. Five metal cylinders of different radii of curvature were used to reproduce the femoral components. For the femoral plastic bearing surface, the contact pressure distribution was uniform when there was an interference of 0.1 to 0.2 mm. For the plastic tibial plate bearing surface, uniform pressure was achieved for radii that were equal. The results of the first study indicate that the MBK prosthesis, retaining the posterior cruciate ligament and having no preferred axis of movement on its own, can follow the pattern of movement that the retained ligaments dictate. The second study confirms that between closely conforming surfaces, the pressure distribution is uniform, and small differences in the relative radii of curvature affect the pressure distribution.