Biomechanics of the ACL

The ACL resists anterior tibial translation and rotational loads. With a ruptured ACL, the anterior translation of the tibia relative to the femur can be four times greater than in normal knees. It also restrains internal rotation significantly and plays a minor role in controlling the external rotation and varus-valgus angulation. Moreover, the ACL prevents medio-lateral translation of the tibia. In an ACL-defident knee, this poor translation can lead to tears in the medial meniscus and hypertrophy of the tibial spine and notch, and it increases contact loading of the medial compartment of the knee.  

When an ACL is ruptured, the axis of rotation shifts more medially and the tibial rotation causes a coupled anterior tibial translation, magnifying the movements of the tibial plateau. The primary insult is thus to the lateral compartment, mainly the posterior aspect, and injury to the medial compartment occurs secondarily. The lateral compartment is most frequently injured mainly because it can sublux more easily. 

In the absence of the ACL, muscles around the knee produce compensatory mechanisms, resulting in a net posterior force and flexor and external rotation moment on the tibia in an attempt to maintain normal kinematics. In patients with an ACL-defident knee, Williams et al. also showed that voluntary muscle control that was diminished in the preoperative evaluation, mostly the quadriceps and lateral head of the gastrocnemius, improved significantly following surgery.  

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Bryant JT and Cooke TDV. A biomechanical function of the ACL prevention of medial translation of the tibia. In Feagin JA (ed.). The Crucial Ligaments. Diagnosis and Treatment of Ligamentous Injuries About the Knee. Livingstone Edinburgh, 1988. 

For ACL reconstruction, some have created femoral tuimels in the direct attachment of the midsubstance fibers [2, 12], whereas others have recommended that they should include as much as the whole area including the attachment of the fanlike extensimi fibers [13, 14]. This discrepancy can occur due to our lack of knowledge on the transmission of the load carried by the ACL to the femoral attachment, fri some biomechanical studies in which the ACL was separated into 2 fiber bundles [15] or 3 fiber bundles [16], however, those did not use recent anatomic knowledge of the ACL attachment. The purpose of the second study was to clarify the load-bearing functions of the fibers of the femoral anterior cruciate ligament (ACL) attachment in the resistance of tibial anterior drawer and rotation. 

Shino et al. [33] introduced the triple-bundle ACL reconstraction technique in which the three bundles of the ACL are individually reconstructed with hamstring tendon autografts. This technique may have much greater potential to closely mimic not only the morphology but also the biomechanical behavior of the native ACL [34-36]. Clinically, immediate postoperative stability was better in the triplebundle procedure than in the double-bundle reconstruction [37]. In addition, Suzuki et al. [38] reported that laxity match pretension at 15° knee flexion became smaller in the triple-bundle reconstruction than in double-bundle reconstruction. The enlarged contact area between the graft and the tunnel wall in the three tibial tunnels might have contributed to this improved biomechanical performance. 

The primary motion of the knee joint is flexion-extension rotation around an axis passing through the medial and lateral femoral condyles. The three-dimensional motions of the knee other than flexion-extension rotation are constrained by ligaments, menisci, and articular surface configuration. The biomechanical functions of the ACL are mainly to resist anterior tibial translation, and secondly to resist internal and valgus tibial rotation, or combined motions. 

A different biomechanical study was performed to compare the anatomical DB reconstruction performed with the arthroscopic transtibial procedure, which has been clinically used, with the conventional SB reconstraction procedure using eight fresh-frozen cadaveric knees [11], These two procedures were reported in the previous clinical smdy [10]. The same measurement system and loading conditions as the above-described study [16] were used in this experiment. The test regimen was repeated with the knee in three further states (1) after arthroscopic transection of the ACL, (2) after arthroscopically assisted anatomic DB ACL reconstruction, and (3) after arthroscopically assisted SB ACL reconstruction. The bone tuiuiels were filled with polyester resin paste. 

It is critical to understand functional anatomy and biomechanics of the AM and PL bundles of the ACL in order to understand the theory of the anatomic double-bimdle ACL reconstruction. It has been well known that the mid-substance fibers of the AM and PL bundles have different functions The AM bundle mid-substance is stretched in the full extension position, relaxed at 20-60 ° of knee flexion, and again stretched in a flexion position of more than 90° [9]. The PL bundle mid-substance is stretched in the full extension position, whereas it becomes slack in a flexion position [9]. In response to an anterior tibial load, the magnitude of the in situ force in the PL bundle mid-substance was larger than that in the AM bundle mid-substance at knee flexion angles between 0 ° and 45 ° [10]. Under a combined rotatory load, the PL bundle mid-substance is as important as the AM bundle mid-substance, especially when the knee is in the near extension position [11]. 

We present a technique that combines the biological advantage of maximal preservation of the ACL remnant and the biomechanical advantage of performing anatomical ACL reconstmction. 

Avulsion fracture of the ACL occurs during sports activities and traffic accidents. Generally, it is seen more often in children than in adults, most likely because the ACL attachment site on the bone is immature and biomechanically weak in children [2]. Avulsion fracture of the ACL often occurs in children between ages 6 and 17 years [3-5]. Previous reports have suggested that the mechanism of injury is direct force, with hyperextension of the knee and injury patterns similar to ACL tears [6-9]. 

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