Femoro-tibial and menisco-tibial translation patterns in patients with unilateral anterior cruciate ligament deficiency--a potential cause of secondary meniscal tears.

OBJECTIVE: To analyze menisco-tibial and femoro-tibial translation patterns in healthy and ACL-deficient knees in different knee flexion angles under muscle activity.
METHODS: The ACL-deficient and contralateral healthy knees of 10 patients were examined with an open MRI system at 30 degrees and 90 degrees of knee flexion, under isometric contraction of the extensors or flexor muscle groups. Translations between the tibia, the femoral condyles and the menisci were analyzed by three-dimensional image postprocessing.
RESULTS: Posterior translation of the femur and menisci relative to the tibia occurred during knee flexion (30-90 degrees) in all knees. In ACL-deficient knees, posterior translation of the medial femoral condyle (+1.3 +/- 3.8 mm) was significantly larger than in healthy knee (-0.9 +/- 2.9 mm; p<0.05), while the translation pattern of the menisci was similar (med. meniscus 0.6 +/- 2.3 mm vs. 0.6 +/- 2.7 mm). Under isometric contraction of the extensors (relative to the flexor muscle group), an increased posterior position of the femur and menisci was observed at 30 degrees knee flexion, but not at 90 degrees. This applied to ACL-deficient and healthy knees.
CONCLUSIONS: This study shows a significant increase of translation of the medial femoral condyle in ACL-deficient knees, whereas menisco-tibial translation remains almost unchanged. This difference in translation patterns indicates that the posterior horn of the medial meniscus might encounter shear, potentially explaining the high rate of secondary medial meniscal tears in patients with ACL-deficiency.