[Observations of normal and ACL-deficient knee joints after stress MRI]

The Lachman test is the most reliable clinical test for diagnosing rupture of the anterior cruciate ligament (ACL). Previous X-ray studies have presented a "radiologic Lachman test". Recently anterior tibial translation was demonstrated using open access MRI. Two methods were developed to transfer a similar technique to a more widely available closed MRI.

Using closed MRI we investigated 22 knees in 21 patients with pure rupture of the ACL. Anteriorly and posteriorly directed shear forces were applied to the tibiofemoral joint at 20 degrees flexion either by positioning a 9-kg load on the distal femur (method 1) or performing a semi-manual Lachman test with a custom-made orthosis (method 2).

Both methods produced relative anterior tibial translation in both compartments of the normal and ACL-deficient knee which could be measured on sagittal images. They were greater laterally than medially and in injured than in uninjured knees. However, instability of the medial compartment predicted clinical and symptomatic instability as translation was posterior to positions achieved in normal knees during the active and passive flexion arc.

A Lachman sign can be produced in a closed magnet with different methods and findings can be used for more precise information regarding kinematics and degree of instability and could be helpful if surgical treatment is necessary.