Abstract
Objective:
The lateral knee contains several ligamentous structures that contribute to varus and rotational stability, including the fibular collateral ligament (FCL), anterolateral ligament (ALL), and iliotibial band (ITB) fibers extending from Gerdy's tubercle to the proximal and distal Kaplan fibers. A detailed understanding of the flexion-dependent behavior of these structures is essential for guiding graft fixation during reconstructive surgery. This study aimed to characterize the native length changes of the FCL, ALL, and ITB fibers across knee flexion using a cadaveric model.
Methods:
Fellowship-trained orthopedic surgeons dissected fresh-frozen cadaveric knees, preserving the tendon and ligament attachments of the lateral knee (Figure 1A). Bony landmarks on the femur, tibia, and fibula were marked with pins, including the footprint centers of the FCL (n=10), ALL (n=7), gastrocnemius tendon, popliteus tendon, iliotibial band tibial insertion (Gerdy's tubercle), and anterior biceps femoris (Figure 1B). Using a MicroScribe system (MicroScribe; Solution Technologies), 3D coordinates of each landmark were recorded at 0°, 15°, 30°, 45°, 60°, and 90° of flexion (Figure 1C). We calculated the ALL and FCL lengths as the 3D distance between footprint centers. ITB fiber lengths from Gerdy's tubercle to the distal Kaplan fibers (n = 10) and proximal Kaplan fibers (n = 9) were analyzed independently. We used paired t-tests to compare ligament lengths between full extension (0°) and deep flexion (90°). We also measured distances from ligament attachment sites to surrounding landmarks.
Results:
The FCL femoral attachment is generally more anterior and distal compared to the ALL, as the FCL is farther from key proximal landmarks like the Kaplan fiber ridges (proximal ridge p = 0.041, distal ridge p = 0.089) and closer to distal landmarks such as the popliteus sulcus (p = 0.005) and tibial plateau (p=0.003) (Table 1A). The FCL inserts distally onto the fibular head, while the ALL inserts more anteriorly and proximally onto the tibia, often in closer proximity to Gerdy's tubercle (p = 0.001) and the tibial plateau (p = 0.0005) (Table 1B). The FCL and ALL demonstrated statistically significant shortening from 0° to 90° of knee flexion (FCL: 7.59 mm, p = 0.004; ALL: 6.44 mm, p = 0.020), supporting their dynamic length-dependent function (Figure 2C). In contrast, length changes from Gerdy's tubercle to the distal Kaplan fibers (0.78 mm, p = 0.853) and proximal Kaplan fibers (3.99 mm, p = 0.506) were not statistically significant, indicating minimal length variation across the arc of flexion (Figure 2C).
Conclusions:
The FCL and ALL have anatomically distinct femoral and tibial attachment sites with different spatial relationships to surgical landmarks, reinforcing the importance of individualized tunnel placement to avoid convergence. The FCL and ALL shorten with knee flexion and should be fixated at angles that preserve native length to avoid graft over-constraint. Fixing the FCL in near-full extension and the ALL before 60° of flexion may better reproduce native biomechanics. In contrast, the ITB fibers from Gerdy's tubercle to the Kaplan fibers remain relatively isometric throughout flexion and may not require precise tensioning during reconstruction. These findings offer nuanced insight into lateral knee anatomy and inform surgical decision-making for anatomic graft placement.
