In vivo biomechanics assessment of a CR total knee prosthesis during sit to stand: coupling dynamic RSA and FE analysis

Introduction Total knee arthroplasty (TKA) is the gold standard treatment for patients with primary osteoporosis, as it can relief pain and restore joint function. However, there is stillroom for improvement as far as prosthesis behavior are concerned. In this study, an innovative technique, combining RSA dynamic and patient-specific finite element models, was applied to analyze a group of 15 patients who underwent total knee arthroplasty with a cemented PCL highly congruent MB TKA during the execution of a sit to stand from chair. The aim of our study was to evaluate the position and translation of the femoral polyethylene insert contact points. Materials and Methods A cohort of 15 patients, underwent cemented CR highly congruent MB TKA (Gemini, Waldemar LINK GmbH & Co. KG, Barkhausenweg 10, 22339 Hamburg, Germany) with patella resurfacing for primary OA, was evaluated at minimum 9-month follow-up using Model-based dynamic RSA in weight-bearing conditions and during the execution of a sit to stand from chair. The in vivo kinematic data obtained from RSA Dynamic was used as an input for a patient specific finite element models of the implants to calculate condylar contact points between the femoral component and polyethylene insert. The contact points were determined by the FE software as the centroid of the pressure distribution between the femoral condyles and the polyethylene insert. Results The AP translation showed that the femoral component, during the sit to stand, performed an anterior translation during the last 40° of flexion (range 4 mm). Concerning the IE rotation, femoral component performed an internal rotation (range 8.18°) during the whole task. Interpretation and Conclusions This study evaluated the knee kinematics in terms of articular surface contacts. The anterior translation showed from 40° to 0° may represents a paradoxical anterior translation.