ACL Anatomy and Function
The anterior cruciate ligament (ACL) is a dense connective tissue band extending from the femur to the tibia, measuring approximately 32 mm in length and 7-12 mm in width when the knee is extended, serving as the primary restraint against anterior tibial translation and rotational loads. 1
Structural Organization
Two-Bundle Architecture
The ACL consists of two distinct functional bundles that work synergistically to stabilize the knee 1, 2:
- Anteromedial Bundle (AMB): Located proximal and anterior in the femoral ACL origin (high and deep in the notch at 90° flexion), positioned at approximately 10:30 on the clock face in the frontal plane 2
- Posterolateral Bundle (PLB): Originates distal and posterior in the femoral ACL origin (shallow and low at 90° flexion), positioned at approximately 9:30 on the clock face 2
Attachment Sites
Femoral Origin: The ACL originates from the posteromedial aspect of the lateral femoral condyle, with the two bundles having distinct but overlapping origins 2
Tibial Insertion: The ligament fans out at its tibial attachment to form the "foot region," with the AMB inserting anteriorly and the PLB inserting posteriorly within the tibial footprint 2
Biomechanical Function
Dynamic Behavior During Knee Motion
The two bundles exhibit non-isometric behavior that varies with knee position 1, 2:
- In Extension: The PLB is tight while the AMB is moderately lax 2
- In Flexion: The femoral attachment assumes a more horizontal orientation, causing the AMB to tighten and the PLB to relax 2
Primary Stabilizing Functions
Anterior Translation Control: The AMB serves as the primary restraint against anterior tibial translation throughout the range of motion 2
Rotational Stability: The PLB primarily stabilizes the knee near full extension, particularly against rotatory loads 2
Overall Joint Stability: The ACL resists multiaxial stresses and varying tensile strains through its complex ultrastructural organization 1
Microstructural Composition
The ACL has a sophisticated internal architecture 1:
- Collagen Framework: Predominantly type I collagen bundles arranged in a complex pattern
- Matrix Components: Network of proteins, glycoproteins, elastic systems, and glycosaminoglycans with multiple functional interactions
- Elastic System: Abundant elastic fibers allow the ligament to withstand multiaxial stresses and varying tensile strains
Neurovascular Supply
Innervation: Posterior articular branches of the tibial nerve provide sensory feedback for proprioception 1
Vascularization: Branches of the middle genicular artery supply blood to the ligament 1
Clinical Significance
Impact on Knee Function
ACL injuries have notable effects on knee function, particularly for activities involving cutting, pivoting, and landing, though some patients experience instability even with mundane tasks 3
Risk of Secondary Injury
Without adequate treatment, ACL-deficient knees face increased risk 3:
- Persistent or recurrent instability
- Additional meniscal injury
- Progressive cartilage damage
- Increased risk of contralateral ACL tear
Time-Sensitive Considerations
The risk for additional cartilage and meniscus damage begins rising within 3 months of ACL rupture, making timely treatment decisions critical for joint preservation 4