[Anterior Cruciate Ligament Tears - Influence on Terminal Extension].

Area of Science:

• Orthopedics
• Biomechanics
• Radiology

Background:

• The role of the anterior cruciate ligament (ACL) in knee terminal extension and rotation is debated.
• Previous studies have not fully elucidated the kinematics of terminal extension in living knees with ACL tears.

Purpose of the Study:

• To compare terminal extension in normal and ACL-deficient knees.
• To determine the specific role of the ACL in knee rotation and stability during terminal extension.

Main Methods:

• Magnetic Resonance Imaging (MRI) was used to examine 10 ACL-deficient knees and 10 normal contralateral knees.
• Knees were assessed in passive hyperextension and at 20° flexion with and without a posterior femoral load.
• Femoral condyle movements relative to the tibia were calculated to analyze knee kinematics.

Main Results:

• ACL-deficient knees exhibited posterior femoral subluxation under load and in hyperextension, indicating antero-posterior instability.
• In ACL tears, the lateral femoral condyle showed posterior movement (external rotation) from hyperextension to 20° flexion.
• The axis of longitudinal rotation shifted centrally in ACL tears, causing abnormal medial condyle movement, unlike the medial axis in normal knees.

Conclusions:

• The ACL is crucial for preventing anterior tibial subluxation in hyperextension.
• The ACL does not actively cause rotation during terminal extension.
• The ACL dictates the axis of longitudinal rotation during terminal extension, influencing normal knee kinematics.