Introduction The knee is comprised of 2 joints tibiofemoral joint patellofemoral joint Patellofemoral Articulation Function transmits tensile forces generated by the quadriceps to the patellar tendon increases lever arm of the extensor mechanism patellectomy decreases extension force by 30% Biomechanics patellofemoral joint reaction force up to 7x body weight with squatting 2-3x body weight when descending stairs Motion "sliding" articulation patella moves 7cm caudally during full flexion maximum contact between femur and patella is at 45 degrees of flexion Stability passive restraints to lateral subluxation medial patellofemoral ligament primary passive restraint to lateral translation in 20 degrees of flexion 60% of total restraining force medial patellomeniscal ligament 13% of total restraining force medial retinaculum 10% of total restraining force dynamic restraint quadriceps muscles Q angle definition line drawn from the anterior superior iliac spine --> middle of patella --> tibial tuberosity normal Q angle in extension males 13 degrees females 18 degrees in flexion 8 degrees pathology Tibiofemoral Articulation Function transmission of body weight from femur to tibia Biomechanics tibiofemoral joint reaction force 3x body weight with walking 4x body weight with climbing Motion in sagittal plane range of motion 3 degrees of hyperextension to 155 degrees of flexion thigh-calf contact is usually the limiting factor to full flexion normal gait requires ROM from 0 to 70 degrees Rotation instant center of rotation definition point at which the joint surfaces are in direct contact relevance posterior rollback as the knee flexes, the instant center of rotation on the femur moves posteriorly allows for increased knee flexion by avoiding impingement "screw home" mechanism definition tibial externally rotates 5 degrees in the last 15 degrees of extension cause medial tibial plateau articular surface is longer than lateral tibial plateau relevance "locks" knee decreasing the work performed by the quadriceps while standing Stability varus stress lateral collateral ligament valgus stress superficial portion of medial collateral ligament anterior translation anterior cruciate ligament attachments origin semicircular area on the posteromedial aspect of lateral femoral condyle insertion just anterior to and between the intercondylar eminences of the tibia components anteromedial bundle tight in flexion posterolateral bundle tight in extension function primary static restraint to anterior translation also plays a roll in axial rotation posterior translation posterior cruciate ligament attachments origin anterolateral medial femoral condyle insertion tibial sulcus below articular surface components anterolateral tight in flexion posteromedial tight in extension function primary static restraint to posterior translation external rotation posterolateral corner is the primary stabilizer of external tibial rotation