Anterior Cruciate Ligament

Anatomy

Two functional bundles of the anterior cruciate ligament (ACL), the anteromedial (AM) and posterolateral (PL), were first described in 1836 ( Fig. 6.1 ). On the femoral insertion site, two osseous landmarks can be identified. The lateral intercondylar ridge (i.e., ‘resident’s ridge’) is the anterior border of the ACL and runs from proximal to distal. The lateral bifurcate ridge is located between the AM and PL bundles. For the tibial insertion site, different shapes are described in the literature (e.g., elliptical, triangular or C shaped). ^, There is a close relationship between the anterior horn of the lateral meniscus and the ACL, with more than 50% overlap of both insertions on the tibial plateau.

The intraarticular length of the ACL varies from 22 to 41 mm, with a mean of 32 mm. ^, Moreover, a large variation in length of the femoral and tibial insertions sites has been demonstrated. In 137 patients the arthroscopically measured insertion site length varied between 12 to 22 mm. The ACL midsubstance cross-sectional area (CSA) was reported as 46.9 ± 18.3 mm. ^, In terms of graft choice and graft size it is important to know that the tibial insertion site (123.5 ± 12.5 mm ² ) is typically larger than the femoral insertion site (60.1 ± 16.9 mm ² ).

Biomechanics

For successful ACL reconstruction it is crucial to understand the biomechanical function of the ACL and the properties of the native ACL. Structural properties describe the complex containing different tissues (e.g., femoral bone–native ACL–tibial bone) and mechanical properties define the properties of an individual tissue (e.g., ACL without insertion site). The structural properties of the femur–ACL–tibia complex (FATC) are significantly affected by age, with an ultimate load to failure in younger specimens (22 to 35 years) of 2160 ± 157 N and a linear stiffness of 242 ± 28 N/mm. For comparison, the ultimate load of a patellar tendon (PT) graft is 1784 ± 580 N and the stiffness is 210 ± 65 N/mm. The quadruple hamstring graft has an ultimate load of 2422 ± 538 N and a stiffness of 238 ± 71 N/mm. The quadriceps tendon (QT) graft has an ultimate load of 2186 ± 759 N graft and a stiffness of 466 ± 133 N/mm.

In the fully extended knee, the PL bundle is tight and the AM bundle is moderately lax. In contrast, with increasing knee flexion angle, the femoral insertion is oriented more horizontally, making the AM bundle tighter and the PL bundle more lax. As a result, the distribution of load changes within the ACL based on the knee flexion angle. Whereas the amount of in situ forces experienced by the PL bundle is significantly affected by the knee flexion angle, the forces in the AM bundle remain relatively constant.

The ACL is the primary stabiliser for anterior tibial translation (ATT) with regard to the femur and is a secondary restraint to internal tibial rotation. Transection of the AM bundle results in increased ATT, especially at higher knee flexion angles (60 degrees and 90 degrees), and isolated transection of the PL bundle increases ATT at 30 degrees of knee flexion. Moreover, an isolated transection of the PL bundle also leads to increased ATT in response to a combined valgus and internal torque, which is effectively a simulated pivot shift test.

In addition to the ACL, the anterolateral complex (ALC), consisting of the superficial and deep iliotibial band (ITB), capsulo-osseous layer of the ITB, and anterolateral capsule, contributes to rotatory knee stability, especially with increasing knee flexion. Compared with the ITB, the anterolateral capsule is substantially weaker and less stiff. Furthermore, the anterolateral capsule demonstrates a nonuniform strain distribution when subjected to different loading conditions, with a maximum principal strain ranging from 22% to 52% that is not aligned in the direction of a proposed ligament. The anterolateral capsule of the knee behaves like a sheet of fibrous tissue rather than a discreet ligament.

Diagnosis

The typical injury mechanism for the ACL involves a noncontact twisting movement, sometimes associated with a ‘pop’ or ‘tear’ sensation. Afterwards almost every patient presents with an effusion. For the clinical diagnosis of an ACL injury, several physical examination manoeuvres are described. For the anterior drawer test, the patient is positioned supine with the knee flexed to 90 degrees. The examiner translates the tibia forward with respect to the femur. An excessive anterior translation of the tibia indicates a positive test. For the Lachman test (sensitivity 0.87; specificity 0.97), the patient is again in a supine position with the knee in 15 degrees of flexion. The examiner places one hand behind the tibia with the thumb at the tibial tuberosity and the thigh is held with the other hand. The tibia is then translated anteriorly, and the amount of anterior translation, as well as the quality of the endpoint, is evaluated. The pivot shift test (sensitivity 0.49; specificity 0.98) is a dynamic test of rotatory knee stability that produces a subluxation and successive reduction, felt as a glide or clunk, of the lateral tibial plateau. ^, Video-based imaging analysis technologies were demonstrated as reliable options to assess the translation of the lateral plateau or tibial acceleration and to quantify the pivot shift test. ^,

The next step in the diagnostic algorithm, after the physical examination, is plain radiography to rule out a fracture or dislocation. If there is a concern for ACL injury, magnetic resonance imaging (MRI) is recommended to evaluate the ACL and concomitant meniscal, articular cartilage and collateral ligament pathological conditions ( Fig. 6.2 ). Based on the clinical examination and the radiological findings, the optimal treatment is chosen.