Aims: To study knee kinematics using a dynamic computer model of 2 cadaver knees obtained by bone morphing. Patellar kinematics was simultaneously investigated using transosseous pins. Method: Ligamentous injuries were done (dicision of the anterior cruciate ligament (ACL) and of the popliteus). While maintaining the foot in slight external rotation, femoral rotation was measured in relation to knee flexion in the intact and injured knee. Results: The screw home rotation between −5 and +5° was comprised between 8 and 10°. From 10° on, the lateral condyle roll-back induced 30° of femoral external rotation. Femoral rotation could be blocked by externally rotating the tibia. On the screen, the rollback of the lateral condyle and the lift-off of the medial condyle at the end of the flexion appeared clearly. The patella rotated about its longitudinal axis. Moreover, it made a lateral translation. At deep knee flexion, it contacted mainly the lateral condyle. Dicision of the ACL decreased the screw home rotation to 3° and the femoral external rotation to 20°. At 110° and over, femoral rotation ceased, while both condyles rolled anteriorly. Dicision of the popliteus reduced the lateral condyle roll-back by 50%. Conclusion: 2 types of movement can be described: the end of rotation depending of the ACL; the external rotation of the femur depending on the popliteus.

Purpose: Dynamic MRI studies have confirmed the posterior displacement of the lateral condyle during flexion of the knee.

Material and methods: We used bone morphing navigation equipment to study knee kinetics in cadaver specimens and determine the effect of ligament injury. Patellar movement was controlled with pins. Femur movement over the tibia was measured by the navigation system during knee flexion.

Results: We noted external rotation of the femur during knee flexion when the tibia was maintained with the foot in the walking angle (15° with the flexion/extension plane), the femur being free to rotate. This rotation was nevertheless suppressed by constraining external rotation of the tibia. Unlocking rotation between −5° and +5° was measured between 8 and 10°. It was prolonged by greater rotation, increasing regularly with flexion to reach about 30° at 130° flexion. The computer display of the kinetics of the bony components demonstrated the posterior displacement of the lateral condyle. From 130° flexion, there was an elevation of the medial condyle which lost contact with the medial tibial plateau.

Posterior displacement of the lateral condyle was confirmed by rotation of the patella in its longitudinal axis. The patella appeared to make a lateral translation movement to come in front of the lateral condyle at near complete flexion. It pulled the vastus medialis as is suggested by its movement around the anteroposterior axis.

Section of the anterior cruciate ligament had little effect on the observed kinetics. It limited the unlocking rotation which did not exceed 3°. At about 110° the femoral rotation reached a plateau for about twenty degrees. The computer display illustrated the movement of the condyles showing that the movement of both was influenced by the posterior thigh soft tissue. Section of the popliteal muscle clearly lessened external rotation of the cadaveric knee.

Discussion: Two types of movement can be described: – external rotation of the tibia at the end of extension due to the influence of the anterior cruciate ligament; – facultative external rotation of the femur during flexion under the control of the popliteal muscle. This is expressed during single leg stooping and could protect the patella from excessive pressure by progressively displacing the lateral condyle.