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.