Analysis of knee movement with low-field MR equipment--a normal volunteer study

This study was performed to make a normal standard by analyzing normal knee movement in detail. An open low-field unit was used for 23 healthy knee joints. With three-dimensional Fourier transformation (3DFT) gradient echo sequence, 50 sagittal slices of 4.5 mm in thickness were obtained at four flexion angles: 0, 30, 60, and 90 degrees (lateral position). Although the tension ratio of the anterior and posterior cruciate ligaments (ACL, PCL) increased during knee flexion, the change in the tension ratio was significantly different between the ACL and PCL. The femur-ACL angle and femur-PCL angle were parallel with the knee flexion angle, but the tibia-ACL angle and tibia PCL angle changed complexly. The lateral and medial condyles rolled and slid during knee flexion, and the medial side moved more than the lateral side, consistent with rotation of the lower thigh. The difference in backward movement distance on the tibia between the two condyles was significantly larger in females than in males. This might explain the dominance of knee osteoarthritis in women. Although the lateral position is not completely physiological, we could show initial cinematic data of up to 90 degrees of knee flexion using open-type MRI, which is impossible with high- and middle-field machines.