Dynamic MRI assessment of normal knee kinematics

Background: Knee motion is complex and has been studied intensively. The concept of an instant center of rotation postulates that the axis around which the tibia flexes and extends moves with knee flexion and extension. This concept has been contradicted by the concept of fixed axes around which the tibia and patella flex and extend during knee motion. Advanced imaging technologies help to further characterize knee motion and facilitate localization of the axis around which the tibia flexes and extends. Using dynamic magnetic resonance imaging (MRI), the purpose of this study was 1) to establish a setup for dynamic MRI knee measurements, 2) to construct a cadaver knee model and 3) to generate a mathematical algorithm to facilitate a 3- dimensional characterization of knee kinematics and calculate a defined and fixed axis around which the tibia flexes and extends. Methods: MR images were obtained using a 1.5T Magnetom Avanto MRI Scanner (Siemens Healthcare GmbH, Erlangen). A u-shaped 16 channel RF (radiofrequency) coil array with 160 mm inner diameter and a length of 180 mm was used and covered the cadaver knee on three sides. The fourth side of the knee is open to allow knee motion. A pneumatic movement device was specifically designed to generate reproducible and repetitive knee flexion and extension. The MRI sequence was synchronized with the frequency of the motion cycle. Ten cadaver knees were thawed, dissected and prepped with contrast media filled spheres. They were then fixed in the movement device and were scanned with dynamic MRI. The center of rotation was calculated using circular interpolation and the error (F) was calculated comparing the measured value at any time point with an ideal position based on the calculated flexion-extension axis (=nonlinear curve optimization). Results: Knee movement was analyzed within a motion arch of 90°. All knee specimens had intact ligamentous structures, no meniscal pathology, and no osteoarthritic changes of the cartilage surface. A fixed axis around which the tibia flexes and extends was localized in all ten specimens and the accuracy of the axis was calculated. Conclusions: This study presents a novel technique of using dynamic MRI to visualize knee kinematics ex vivo and confirms the presence of a fixed axis around the tibia flexes and extends. The passive movement device and the mathematical algorithm generates an accurate system to evaluate knee motion that will be of further assistance in characterization of physiologic biomechanics and in detection of pathological kinematics of the knee joint in vivo.