The natural history of an untreated PCL injury is the eventual development of degenerative changes in the medial and patellofemoral compartments of the knee. However, the surgical treatment of PCL injuries has not significantly improved outcomes, as joint arthrosis has been reported in 20 to 60% of patients after operative management. Altered knee kinematics has been assumed to cause long term joint degeneration. However, the biomechanical role of the PCL in vivo is unclear. There is little data on the PCL function during in vivo activities. Advancements in surgical treatment of PCL deficiency require an accurate knowledge of in vivo PCL function in the intact knee. The broad aim of this project is to determine the effect of the PCL on knee function during in vivo knee flexion using a novel imaging technique. In vivo kinematics of normal knees will be obtained from healthy volunteers during passive and weight bearing flexion, using orthogonal fluoroscopic and MRI scans. The relative positions of the PCL attachment areas on the tibia and femur will then be determined. The elongation and twist of the PCL during in vivo knee motion will be accurately quantified. To quantify the effect of PCL injury on knee kinematics in vivo, patients with unilateral PCL rupture will be investigated using the same image technique. Kinematics of both injured and intact contralateral knees will be determined. Further, this technique will also allow us to determine the effect of the PCL on tibiofemoral and patellofemoral joint contact biomechanics, including the measurement of changes in the articular cartilage-cartilage contact areas and locations in vivo caused by PCL rupture. The data will provide surgeons with an accurate in vivo objective when treating PCL injuries. The proposed investigation will thereby have a direct impact on the clinical management of PCL deficiency. In the long term, we will develop an optimized surgical technique for the treatment of PCL injuries aimed at restoring normal in vivo knee function. The innovative usage of orthogonal fluoroscopic imaging and MRI technology in this investigation will be an invaluable tool for the development of optimal treatments of combined PCL and posterolateral structure injuries. Our image technique will be further applicable to other soft tissue injuries such as cartilage defects and soft tissue defects in the shoulder, hip, wrist, ankle and spine.