In Vivo Anterolateral Ligament Length Change in the Healthy Knee During Functional Activities-A Combined Magnetic Resonance and Dual Fluoroscopic Imaging Analysis

Purpose: To measure the in vivo anterolateral ligament (ALL) length change in healthy knees during step-up and sit-to-stand motions. Methods: Eighteen healthy knees were imaged using magnetic resonance and dual fluoroscopic imaging techniques during a step-up and sit-to-stand motion. The ALL length change was measured using the shortest three-dimensional wrapping path, with its femoral attachment located slightly anterior-distal (ALL-Claes) or posteriorproximal (ALL-Kennedy) to the fibular collateral ligament attachment. The ALL length measured from the extended knee position of the non-weight-bearing magnetic resonance scan was used as a reference to normalize the length change. Results: During the step-up motion (approximately 55 degrees flexion to full extension), both the ALL-Claes and ALL-Kennedy showed a significant decrease in length of 21.2% (95% confidence interval 18.0-24.4, P<.001) and 24.3% (20.6-28.1, P<.001), respectively. During the sit-to-stand motion (approximately 90 degrees flexion to full extension), both the ALL-Claes and ALL-Kennedy showed a consistent, significant decrease in length of 35.2% (28.8-42.2, P<.001) and 39.2% (32.4-46.0, P<.001), respectively. From approximately 90 degrees to 70 degrees of flexion, a decrease in length of approximately 6% was seen; 70 degrees of flexion to full extension resulted in an approximately 30% decrease in length. Conclusions: The ALL was found to be a nonisometric structure during the step-up and sit-to-stand motion. The length of the ALL was approximately 35% longer at approximately 90 degrees of knee flexion when compared with full extension and showed decreasing length at lower flexion angles. Similar ALL length change patterns were found with its femoral attachment located slightly anterior-distal or posterior-proximal to the fibular collateral ligament attachment.