Importance of the medial meniscus in the anterior cruciate ligament-deficient knee

The incidence of meniscal tears in the chronically anterior cruciate ligame nt-deficient knee is increased, particularly in the medial meniscus because it performs an important function in limiting knee motion. We evaluated th e role of the medial meniscus in stabilizing the anterior cruciate ligament -deficient knee and hypothesized that the resultant force in the meniscus i s significantly elevated in the anterior cruciate ligament-deficient knee. To test this hypothesis, we employed a robotic/universal force-moment senso r testing system to determine the increase in the resultant force in the hu man medial meniscus in response to an anterior tibial load following transe ction of the anterior cruciate ligament. We also measured changes in the ki nematics of the knee in multiple degrees of freedom following medial menisc ectomy in the anterior cruciate ligament-deficient knee. In response to a 1 34-N anterior tibial load, the resultant force in the medial meniscus of th e anterior cruciate ligament-deficient knee increased significantly compare d with that in the meniscus of the intact knee; it increased by a minimum o f 10.1 N (52%) at full knee extension to a maximum of 50.2 N (197%) at 60 d egrees of flexion. Medial meniscectomy in the anterior cruciate ligament-de ficient knee also caused a significant increase in anterior tibial translat ion in response to the anterior tibial load, ranging from an increase of 2. 2 mm at full knee extension to 5.8 mm at 60 degrees of flexion. Conversely, coupled internal tibial rotation in response to the load decreased signifi cantly, ranging from a decrease of 2.5 degrees at 15 degrees of knee flexio n to 4.7 degrees at 60 degrees of flexion. Our data confirm the hypothesis that the resultant force in the medial meniscus is significantly greater in the anterior cruciate ligament-deficient knee than in the intact knee when the knee is subjected to anterior tibial loads. This indicates that the de mand on the medial meniscus in resisting anterior tibial loads is increased in the anterior cruciate ligament-deficient knee compared with in the inta ct knee, suggesting a mechanism for the increased incidence of medial menis cal tears observed in chronically anterior cruciate ligament-deficient pati ents. The large changes in kinematics due to medial meniscectomy in the ant erior cruciate ligament-deficient knee confirm the important role of the me dial meniscus in controlling knee stability. These findings suggest that th e reduction of resultant force in the meniscus may be a further motive for reconstructing the anterior cruciate ligament, with the goal of preserving meniscal integrity.