PURPOSE OF THE STUDY The purpose of the study was to evaluate tibio-femoral rotation during a simulated squat and to investigate the relationship between the rotational position of the femur in full extension and the amount of external rotation of the femur on the tibia during flexion. MATERIAL AND METHODS Part 1: MRIs of volunteers Data on healthy knees of 10 volunteers were obtained using 2D MRI measurements. The foot and the ankle were fixed to prevent rotation and adduction/abduction movements. Sagittal MRIs of the knees have been performed in 4 positions of flexion. The amount of longitudinal rotation in each position of flexion was calculated. Part 2: Mathematical model experiment a) The model of the femur has been positioned in the 3D coordinate system in full extension and at 12.8° of internal rotation and then flexed to 90° without longitudinal rotation. The distance between the centre of the femoral head and the sagittal plane passing through the centre of the knee was then measured. b) Subsequently, the femur was flexed and rotation allowed to retain femoral head within the sagittal plane. The amount of femoral rotation was then calculated. RESULTS Part 1: In full extension the femur was on average in 12.8° of IR relative to the tibia. By 90° flexion femur rotated on average 12.2° externally. Part 2: a)From full extension to 90° flexion the femoral head moved 93.1 mm laterally from the sagittal plane. b)Between full extension and 90° flexion the femur rotated 12.8° externally, a degree which corresponds to the amount of initial internal rotation of the femur in full extension. DISCUSSION The most important finding of the presented in vivo study lies in the fact that in normal knees with tibia rotationally fixed flexion is always coupled with femoral external rotation in order to keep the femoral head in the acetabulum. This rotation is obligatory. CONCLUSIONS We have demonstrated that if the tibia is rotationally fixed, the knee flexion is possible only when accompanied by femoral external rotation to keep the femoral head in the acetabulum. A mathematical description of the experiment has been proposed, the results of which confirm the stated premise. This finding can be explained by initial internal rotation of the femur in full extension of the knee and is allowed by the shape of articulating bones and tension of soft tissues Key words: knee, terminal extension, knee rotation, knee movement, MRI, hip joint.

1 ortopedická klinika 1 lékařské fakulty Univerzity Karlovy a Fakultní nemocnice v Motole Praha

Fakulta jaderná a fyzikálně inženýrská České vysoké učení technické Praha

Ortopedická klinika 1 lékařské fakulty Univerzity Karlovy a Nemocnice Na Bulovce Praha

Radiodiagnostická klinika 1 lékařské fakulty Univerzity Karlovy a Všeobecné fakultní nemocnice Praha

Rotational laxity of the knee joint - in vivo MRI study