What really happens during the Lachman test? A dynamic MRI analysis of tibiofemoral motion.

BACKGROUND

The Lachman test is the most reliable clinical test for diagnosing rupture of the anterior cruciate ligament (ACL). It is generally believed that the lateral side contributes more than the medial side to anterior tibial translation, but this has never been convincingly proven. The MRI study presented in this article addresses this issue.

HYPOTHESIS

The lateral side of the ACL-deficient knee contributes more than the medial side to anterior tibial translation.

STUDY DESIGN

Prospective cohort study.

METHODS

The Lachman test and the radiologic Lachman test were performed on 10 patients with isolated rupture of the ACL while the knee was scanned dynamically using open-access magnetic resonance imaging. The amount of movement in the midmedial and midlateral compartments of both the contralateral normal knee and the ACL-deficient knee was measured.

RESULTS

In both normal and ACL-deficient knees, the lateral compartment contributes more than the medial to anterior translation of the tibia. Rupture of the ACL leads to increased laxity in both medial and lateral compartments with a statistically significant greater contribution from the lateral side.

CONCLUSION

Rupture of the ACL leads to increased anterior tibial translation coupled with tibial internal rotation.