The aim of this study was on the one hand to compare the fixation of a posterior stabilised prosthesis (PS) and a PCL retaining mobile bearing design (Interax, How-medica Osteonics, Rutherfort, USA) and on the other hand to measure the mobility of the mobile bearing. All measurements were carried out by means of Roentgen Stereophotogrammetric Analysis (RSA-CMS, Medis, Netherlands).

Thirty-three patients with a total of forty-two consecutive primary cemented TKA were included in a prospective, randomised study at Leiden University Medical Centre. There were no significant differences among the two groups with regard to age (66. 5 ± 12. 1 years). The pre-operative Knee Society Score did not differ significantly between the two groups. At the one-year follow-up the PS group showed a significantly lower function score (p=0. 04) compared to the mobile bearing group.

At the one-year follow-up evaluation, the micromotion of the PS-components and the mobile bearing components were not significantly different. The PS-tibial components subsided −0. 003 ± 0. 192 mm and the mobile bearing knee tibial components subsided 0. 057 ± 0. 082 mm. The PS tibial component showed a higher variability in the migration results indicating a number of PS with rather large micromotion.

For three patients, the in vivo motion of the mobile bearing with respect to the metal backing was assessed at 30, 60 and 90 degrees of passive flexion. Two of the mobile bearings moved posteriorly (2. 4 and 2. 9 mm) at respectively 60 and 90 degrees of flexion and showed a medial-lateral translation of 0. 03 and 3. 5 mm. One mobile bearing moved only 0. 4 mm posteriorly at 90 degrees of flexion but showed a lateral-medial translation of 3. 1 mm.

The wide range of kinematic patterns of mobile bearings during flexion that is observed in fluoroscopic studies is also observed in this study. A possible positive effect of mobile bearing movement may be found in the smaller variability of the micromotion of the mobile-bearing knees compared to the PS knees. The assumption was that shear forces in tibial bones implanted with a mobile bearing prosthesis would be better dissipated from the prosthesis-bone interface resulting in less micromotion. The kinematics of an additional number of mobile bearing knees -already included in the micro-motion study- will have to be assessed in order to determine the relation between mobility and micromotion.

Mobile bearing, Posterior Stabilised, RSA, Kinematics