Definition

The anterior cruciate ligament (ACL) is an intra-articular structure essential for the normal function of the knee. It is commonly injured during activities that involve complex movements, such as cutting and pivoting. It is estimated that 1 in 3000 individuals sustains an ACL injury each year in the United States, corresponding to an overall injury rate of approximately 100,000 injuries annually [1]. The injury usually results from a sudden deceleration during high-velocity movements in which a forceful contraction of the quadriceps muscle is required. Other mechanisms of injury are valgus stress, hyperextension, and external rotation, as in landing from a jump, and severe internal rotation of the knee with varus or hyperextension [1]. Approximately 70% of the acute ACL injuries are sports related and affect women more than men, particularly in sports such as basketball and soccer [2]. Non–sports-related injuries might include a patient slipping on ice or falling from considerable height and landing with the knee in hyperextension and valgus. In the last two decades, there has also been an increase in the incidence and appropriate diagnosis of ACL injuries in children associated with more participation in high-demand contact and noncontact sports, increased awareness of the injury, and better imaging techniques [3]. Risk factors associated with ACL injuries can be classified as anatomic, hormonal, environmental, biomechanical, and neuromuscular. Some of the modifiable risk factors include proprioception, core strength, decreased hamstring strength (relative to quadriceps strength), conditioning, footwear, playing surface, weather conditions, training techniques, and biomechanical variations in landing from a jump or cutting. Nonmodifiable factors include gender, reduced femoral intercondylar notch size, increased slope of the tibial plateau, knee hyperextension, physiologic rotatory laxity, small ACL size, and familial predisposition [1,4–6].

The ACL may be partially or completely torn. It also may be injured in combination with other structures, most commonly tears of the medial collateral ligament and medial meniscus.

The ACL is a collagenous structure approximately 38 mm in length and 10 mm in width. The ligament arises from a wide base in the tibia anterolateral to the anterior tibial spine. It then traverses the knee in a posterolateral direction, attaching in a broad fan-like fashion at the posterolateral corner of the intercondylar notch of the femur. According to Fu and collaborators, it is organized in two major bundles named after their insertion sites on the tibia [7]. The anteromedial bundle, which tightens in flexion and is the longer of the two, controls anterior translation of the tibia on the femur. The posterolateral bundle, which tightens in extension and internal rotation, controls rotation [8–10].

Biomechanical studies, with use of cadaver specimens, have evaluated the forces that affect the ACL [11]. These forces are highest in the last 30 degrees of extension, in hyperextension, and under other load conditions, including anterior tibial translation, internal rotation, and varus. The ACL is a static stabilizer of the knee with a primary function of resisting hyperextension and anterior tibial translation in flexion and providing rotatory control. It is also a secondary restraint to valgus and varus forces in all degrees of flexion.

Symptoms

Individuals usually present with pain, immediate swelling, and limited range of motion. They may give a history of hearing a “pop.” In an acute injury, the individual will have severe pain and difficulty with walking. In a chronic injury, a patient may have a history of recurrent episodes of knee instability associated with swelling and limited motion. Patients may describe locking or a “giving way” phenomenon. They may also give a history of a remote injury to the knee that was not rehabilitated.

Physical Examination

The physical examination has been found to be sensitive and specific in the diagnosis of ACL tears and correlates with arthroscopically documented knee injuries [1]. The clinician should observe the knee for asymmetry, palpate for areas of tenderness, measure active and passive range of motion, and document muscle atrophy. The apprehension test to rule out patellar instability, valgus and varus testing with the knee in full extension and 30 degrees of flexion to evaluate the collateral ligaments, and joint line palpation as well as the McMurray test to evaluate the meniscus are all important tests to look for injury of associated structures.

The key physical examination maneuver for evaluating the integrity of the ACL in the patient with an acute injury is the Lachman test, in which an anterior force is applied to the tibia with the knee in 30 degrees of flexion while the clinician tries to reproduce anterior migration of the tibia on the femur (Fig. 63.1). Another important test in the acute setting is the lateral pivot shift maneuver, in which the examiner attempts to reproduce anterolateral instability by internally rotating the leg, applying a valgus stress to the knee as it is flexed, and feeling for anterior migration of the tibia on the femur (Fig. 63.2). After injury, the Lachman test is most important for acute diagnosis, whereas the lateral pivot shift test has shown a better correlation to future sports participation and functional stability [1,12]. In the patient who is able to flex the knee to 90 degrees, particularly in chronic or recurrent injury, the anterior drawer test, in which an anterior force is applied to the tibia, should be performed (Fig. 63.3). In acute injury, this test may provide a false-negative result because the secondary stabilizers may reduce anterior tibial displacement with the knee in 90 degrees of flexion [13]. It is important to complete the examination by performing the posterior drawer test, which evaluates the posterior cruciate ligament; a torn posterior cruciate ligament with posterior tibial subluxation may give a false-positive result of the anterior drawer maneuver as the tibia is reduced. In general, findings should be normal on the neurologic examination, including muscle strength, sensation, and reflexes; however, there may be some associated weakness (particularly of the knee extensors) due to pain inhibition or disuse.

Functional Limitations

Limitations include reduced knee motion, muscle weakness, and pain that interferes with activities involving pivoting and jumping. Recurrent episodes of instability may limit participation in strenuous sports, such as basketball, soccer, tennis, and volleyball [14,15]. These episodes of the knee’s giving way may result in increased ligamentous laxity, leading to limitations with activities of daily living, such as going down stairs and changing directions while walking.

Diagnostic Studies

Diagnostic studies include plain radiographs to rule out intra-articular fractures (tibial spine avulsion, Segond fracture), loose bodies, and arthritic changes. These include the standing anteroposterior view, lateral view, tunnel view, standing posteroanterior 45-degree flexion view, and Merchant view of the patella. Magnetic resonance imaging may be indicated in the acute setting to evaluate associated pathologic changes, such as bone bruises, meniscal tears, and other ligamentous injuries, and to aid in treatment planning of combined injuries. In the pediatric and adolescent athlete, magnetic resonance imaging may also give information about physeal injuries that may otherwise go unnoticed.

Immediately after injury, the management of an ACL tear includes relative rest, ice, compression, elevation, and analgesic or anti-inflammatory medication. Many patients will initially benefit from use of a knee immobilizer and crutches. If the knee is very swollen and painful with limited motion that restricts participation in treatment, arthrocentesis may be performed. It is important to establish an accurate diagnosis and the presence of associated injuries as these may necessitate prompt surgery. These include chondral or osteochondral fractures, meniscal tears, and other injured capsular structures. In general, in the absence of associated injuries, the acute management can be conservative with early protected rehabilitation.

Treatment of ACL injuries depends on a number of factors, including the patient’s age, level of activity, presence of associated injuries, and importance of returning to athletic activities that involve acceleration and deceleration and cutting moves. Surgery is the only definitive treatment of complete ACL injuries, but it is generally not necessary for older individuals who do not complain of knee instability with recreational activities or work.

In general, younger patients and those with a high activity level should be considered for ACL reconstruction. Surgical referral is not necessary in the immediate postinjury period but should be facilitated as soon as it is clear that an individual desires surgery as a definite treatment measure. When associated injuries are present, especially if these cause mechanical symptoms, or in the case of the elite competitive athlete, surgical treatment should be considered as soon as the initial inflammatory phase has passed (Fig. 63.4