The ACL is an intra-articular, extra-synovial ligament of nonparallel collagen fibers that arise from the posterior aspect of the lateral femoral condyle and insert on the tibial plateau between the medial and lateral tibial spines²,⁷,⁸ (FIG 1).

To fully understand the principles of ACL reconstruction, it is important to understand the complex anatomy of the ACL, which is composed of two major bundles, the anteromedial (AM) and posterolateral (PL) bundles, named according to their relative insertion sites on the tibia.¹²,¹⁵

At the femoral insertion site, the lateral intercondylar ridge (“residents ridge”) represents the upper (anterior) limit of the ACL, whereas the lateral bifurcate ridge divides the AM and PL bundle insertion sites and is perpendicular to the intercondylar ridge.⁹,¹⁰

The AM and PL bundle are synergistic according to knee flexion angle and control anterior and rotational stability of the knee. The AM bundle remains constant through knee flexion but attains peak tension between 45 and 60 degrees of knee flexion. The PL bundle is tightest in extension and loosens with knee flexion, thereby allowing rotation.⁶,¹¹

The majority of ACL ruptures, approximately 70%, occur via a noncontact injury mechanism.

Females are approximately seven times more likely to suffer an ACL injury than males.¹⁷

The natural history of an ACL rupture is multifactorial and depends on patient age, activity level, concomitant injury, and degree of functional instability.

The majority of athletic patients suffering an ACL rupture will be unable to perform cutting and/or pivoting activities without surgical reconstruction of the ACL.

A small percentage of carefully identified patients may be able to undergo successful nonoperative rehabilitation. However, the patient should be aware that repeated episodes of instability is likely to induce secondary damage to the knee in the form of meniscal and/or cartilage pathology.

In the long-term, patients are likely to develop some form of osteoarthritis after an ACL rupture regardless of conservative or surgical treatment.

A noncontact valgus pivoting injury followed by an effusion of the knee is highly suspicious of an ACL rupture. If an ACL rupture has occurred, patients are typically unable to return to the same athletic competition.

The physical examination and methods for examination of the ACL are covered in Chapter 49.

Radiographs should include the following views:

Magnetic resonance imaging (MRI) should be ordered to confirm a suspected ACL tear and evaluate potential rupture pattern, including partial ACL tears. MRI is also used to evaluate potential associated injuries of the chondral surfaces, menisci, and other ligamentous structures.

The sagittal MRI can be used to measure the length of the tibial insertion site of the ACL in the AP dimension as well
as the inclination angle and length of the ACL. These measurements can be used in individualized ACL reconstruction and are discussed in the following text. Moreover, the thickness of both the quadriceps and patellar tendons can be used to provide the surgeon with an approximation for graft size prior to harvest.

Computed tomography scan should be ordered in revision cases to assess for factors such as tunnel widening. The images can also be reconstructed three-dimensionally for an accurate assessment of primary tunnel placement for preoperative planning of revision surgery.

An instrumented laxity test (eg, KT-1000 arthrometer) to assess anteroposterior knee laxity can be performed to determine absolute translation and side-to-side translation difference.

Contusion

Meniscal tear

Osteochondral injury

Patellar dislocation, which may mimic the initial presentation of an ACL injury

Collateral ligament injury (eg, medial collateral ligament, lateral collateral ligament)

Multiple ligament injury

Potential nonoperative treatment candidates and rehabilitation protocol are detailed in Chapter 49.