Paper 93: In Vivo Hyperflexion Kinematics After PS TKA in Asian Patients

Abstract

Achieving high flexion after total knee arthroplasty (TKA) is important for kneeling in commonly encountered activities of daily living such as prayer, meditation and gardening. However, little is known about the tibiofemoral articular contact and function of the cam-post mechanism of the posterior stabilizing (PS) TKA during kneeling. Therefore, the objective of this study was to investigate the biomechanics of the knee after PS TKA with a novel dual fluoroscopic imaging technique to help elucidate whether patients should pursue kneeling activities after reconstruction [1]. Sixteen South Korean female patients (22 knees) after PS TKA (LPS-Flex, Zimmer Inc) were randomly recruited from the practice of the same surgeon (SEP). Using the Knee Society scoring system, the patients had clinically successful results. The average International Knee Score was 99 ± 2, and the average Functional Knee Score was 96 ± 6. The patients kneeled under weight-bearing conditions from initial to maximum flexion while being imaged simultaneously by two fluoroscopes (BV Pulsera, Philips Medical) placed orthogonally to each other. The acquired images and 3D CAD models were then used to create a virtual fluoroscopic system. The in-vivo pose of the components was determined by manipulating the TKA models until the component projections matched the component outlines on the fluoroscopic images. Contact was determined by locating the intersection between the surfaces of the femoral component and polyethylene tibial insert. A Wilcoxon Rank test was performed to detect changes in kinematics and articular contact. Patients flexed, on average, from 107.3° to 128.0° during the kneeling activity. Changes in kinematics from initial to maximum kneeling included 1.0 mm of proximal, 0.9 mm of medial and 7.6 mm of posterior translation and 1.7° of varus rotation (p < 0.04). A change in internal tibial rotation was not observed. Articular contact moved posteriorly by 5.9 mm and 6.4 mm in the medial and lateral compartments, respectively. Contact also moved medially by 3.2 mm and 5.8 mm in the medial and lateral compartments. A decrease in articular contact was observed in both condyles, and lateral lift-off was observed to increase with flexion (p = 0.0001). Over 80% of the patients demonstrated cam/post engagement, which always occurred in the distal portion of the post. During the kneeling activity, patients were observed to have kinematics similar to that reported in the literature. Normal articular contact was also observed in this patient cohort. Furthermore, the knee joint was constrained during the weight-bearing activity such that femoral subluxation and dislocation did not occur. As the patients flexed, a high incidence of posterior cam/post engagement occurred, and all instances were detected in the distal portion of the polyethylene post. Excessive stress and loading have been previously noted as possible etiologies of tibial post fracture. From a mechanical point of view, distal tibial post contact found in this study occurred in a favorable location relative to bending stress, thereby not increasing the risk of tibial fracture and revision surgery. Given clinically successful results, patients who were able to comfortably kneel without pain did not demonstrate detrimental biomechanics during the activity. The articular contact observed in the tibiofemoral and cam/post compartments further suggest that kneeling may be performed by patients after PS TKA who feel comfortable doing so and are free of pain.