Tibial component loosening continues to be the most common mode of TKA failure. A debate persists on the dependence of mobilisation of this component on the equilibrium between mechanical load transfer and counterbalancing bone resistance. The aim of the present work is to study the in-vivo kinematics of TKA and to relate it with the degree of posterior slope with which the tibial component was implanted for two prosthesis designs with congruent polyethylene insert.

Twenty-three patients with osteoarthritis of the knee had TKA using a cemented prosthesis (OPTETRAK, Exactech). A cruciate retaining (CR, 10 knees) or a posterior stabilized (PS, 13 knees) implant was randomly assigned at operation. Standard pre- and post-operative antero-posterior and lateral roentgenograms of the knee were taken. Fluoroscopic analysis was performed after at least 18 and 7 months after surgery for the CR and the PS group, respectively. Patients performed stair ascending, chair rising-sitting and step up-down motor tasks. Articular contacts were assumed as the two points on the medial and lateral femoral prosthetic condyles closest to the tibial component base-plate. The spine-cam distance was calculated as the minimum distance between corresponding surfaces.

Only small differences in the position of the contacts over knee flexion angles were found among the motor tasks and between the two TKA designs. An overall posterior location of the tibio-femoral contact points was found at the medial and lateral compartments over all motor tasks, a little more pronounced for the PS patients. Statistically significant correlation over the three motor tasks analysed was found between posterior position of the tibio-femoral medial contact in maximum knee flexion and the post-operative tibial posterior slope. This is true for the PS and for the aggregated groups. Although no statistically significant, a general trend is observed of higher degree of flexion at which the cam contacts the spine as the post-operative posterior slopes increases: a 35 higher knee flexion angle for a tibial component implanted with a 5 of posterior slope. Generally, even when the correlations were statistically significant the correlation coefficients were always lower than 0.4.

The present work reports combined measurements of post-operative posterior slope and full in-vivo relative motion of the components in both CR and PS TKAs. General trends were found between posterior slope of the tibial component and positions of the tibio-femoral contacts, but a statistically significant correlation was found only for the tibio-femoral medial contact in maximum knee flexion in the PS and in the aggregated. General trends were found between posterior slope of the tibial component and degree of flexion at which the cam starts to be in contact with the spine. The nearly standard antero-posterior translation of the tibio-femoral contacts can be bigger in flatter polyethylene inserts.

Aims:Understanding total knee replacement mechanics and their influence on patient mobility requires accurate analysis of both operated joint accurate kinematics and full body kinematics and kinetics. The main aim of this study is to perform these two analyses conjointly, as never been reported previously. An innovative graphic-based interface is also pursued aimed at supporting quantitative functional assessment of these patients during the execution of daily living motor tasks in a single synchronized view.

Methods: Three-dimensional fluoroscopic and gait analysis were carried out on eleven patients with PCL-retaining mobile bearing (Interax ISA, Stryker / How-medica / Ostetonics) and on ten posterior stabilized fixed bearing (Optetrak PS, Exactech) knee prostheses. Patients performed three trials of stair ascent twice on the same day: first in the radiology department for fluoroscopy acquisition and later in the Movement Analysis Laboratory, utilizing an identical staircase. Three-dimensional fluoroscopic analysis entails reconstruction of absolute and relative positions and orientations of the two metal components in space by analyzing series of fluoroscopic images of the operated knee and utilizing knowledge of the 3D cad models of these components. Conventional stereophotogrammetry and dynamometry were used to calculate kinematics and kinetics of the trunk, pelvis and of the major joints of the lower limb. An advanced computer-based interface was developed (MULTIMOD, EU-funded project: IST-2000-28377) to show together a) original video of the patient tasks, b) 3D graphical representation of bony segment motion, c) original fluoroscopic images, d) 3D reconstruction of prosthesis component relative motion, and e) graphical transverse plane representation of the contact areas at the base-plate of the replaced knee. All these were registered in space and synchronized in time.

Results: No significant statistical differences on clinical data were found between the two patient populations. Observations at the interface allowed distinct identification of the most critical phases of the task and of the most common compensatory mechanisms utilized by these patients. Statistically significant correlation was found between knee flexion at foot strike and the position of the mid-condylar contact points, and between maximum knee adduction moment and corresponding lateral trunk tilt.

Conclusions: A more complete and powerful assessment of the functional performances of different TKR designs is obtained by combining gait and fluoroscopic in-vivo analyses, which provide correlated and synergic quantitative information.