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Biomechanics of ACL

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- Anatomy of the ACL:
- Biomechanics:
    - ultimate tensile load: 2160 ± 157 N
    - stiffness: 242 ± 28 N/mm;
    - passive knee extension produces forces along ACL only during last 10 degrees of knee extension;
    - hyper-extension:
            - the posterolateral bundle of the ACL is tight in extension;
            - at 5 degrees of hyperextension, anterior cruciate ligament forces range between 50 and 240 newtons;
            - hyperextension of the knee develops much higher forces in ACL than in the PCL;
    - flexion:
            - the anteromedial bundle of the ACL is tight in flexion;
            - during isometric quadriceps contraction, ACL strain at 30 deg of knee flexion are significantly higher than at 90 deg where ligament remain unstrained
                     with isometric quadriceps activity;
            - active extension of knee between the limits of 50 and 110 degrees does not strain the anterior cruciate;
            - at 90 deg of knee flexion:
                     - ACL accounts for approx 85% of resistance to anteior drawer test
    - ref: Tensile properties of the human femur-anterior cruciate ligament-tibia complex. The effects of specimen age and orientation

- Sectioning of ACL:
    - in unsectioned ACLs in neutral rotation, application of 100 newtons of anterior force produces:
            - 2-5 mm of anterior translation at full extension;
            - 5-8 mm of translation at 30 deg of flexion;
            - as flexion angle increases further, anterior translation decreases;
    - sectioning of ACL results in increased laxity at all flexion angles;
            - 20-30 deg of flexion:
            - maximum anterior translation occurs w/ 100-newton anterior force, 7-9 mm of increased translation is seen;
    - clinically, combined ACL and MCL tears result in large increases in anterior translation;
    - following sectioning of the ACL: anterior restraint derives from:
            - iliotibial band: 24%
            - mid medial capsule: 22%
            - mid lateral capsule: 20%
            - MCL:16%
            - LCL: 12%

- Functional Role:
    - ACL is the predominant restraint to anterior tibial displacement;
    - ligament accepts 75 % of anterior force at full extension & approx 85 % at 30 and 90 degrees of flexion;
            - deep MCL is a major secondary restraint to anterior translation;
    - role in gait: (gait menu and role of knee in locomotion)
            - ACL is taut in full knee extension, and tends to externally rotate tibia;
            - tension in ACL is least at 40 to 50 deg of knee flexion;
            - as knee moves from flexion to extension, shorter, more highly curved lateral condyle exhausts its articular surface & is checked by ACL;
            - larger and less curved medial condyle continues its forward roll and skids backward, assisted by tightening of PCL;
            - towards full extension there is lateral rotation of tibia & joint is "screwed home;" 
            - consequences of ACL deficient knee
                    - absence of the normal internal rotation of the femur during the terminal swing phase

- Isometry:
    - anterior cruciate ligament does not remain an isometric, or constant length, structure as the knee is flexed and extended;
    - ligament increases in strain magnitude as the lower leg is passively extended, with the femur in a horizontal plane;
    - reconstruction of the ACL should not strive to achieve an isometric placement of the graft, but rather reproduce strain behavior of the normal ACL

Biomechanics of intra-articular and extra-articular reconstruction of the anterior cruciate ligament.

Biomechanics of the knee-extension exercise. Effect of cutting the anterior cruciate ligament.

Functional properties of knee ligaments and alterations induced by immobilization: a correlative biomechanical and histological study in primates.

Biomechanical analysis of human ligament grafts used in knee-ligament repairs and reconstructions.

The role of the posterolateral and cruciate ligaments in the stability of the human knee. A biomechanical study.

Effect of intra-articular corticosteroids on ligament properties: a biomechanical and histological study in rhesus knees.

Direct in vitro measurement of forces in the cruciate ligaments. Part II: The effect of section of the posterolateral structures.

Direct in vitro measurement of forces in the cruciate ligaments. Part I: The effect of multiplane loading in the intact knee.

Biomechanics of intra-articular and extra-articular reconstruction of the anterior cruciate ligament.

Biomechanical consequences of replacement of the anterior cruciate ligament with a patellar ligament allograft. Part I: insertion of the graft and anterior-posterior testing.

In Vivo Elongation of the Anterior Cruciate Ligament and Posterior Cruciate Ligament During Knee Flexion

Anatomy of the Anterior Cruciate Ligament with Regard to Its Two Bundles.

The strength of the anterior cruciate ligament in humans and Rhesus monkeys

Effects of joint load on the stiffness and laxity of ligament-deficient knees. An in vitro study of the anterior cruciate and medial collateral ligaments

Coactivation of the hamstrings and quadriceps during extension of the knee

The physiology of mechanoreceptors in the anterior cruciate ligament. An experimental study in decerebrate-spinalised animals.