At a glance

  • Noncontact ACL injuries are six to eight times more common in women than in men.
  • Acute hemarthrosis—an extravasation of blood into a joint or synovial cavity—is a textbook sign of acute ACL injury.
  • The physical exam consists of inspection, palpation, and assessment of knee-joint function.
  • Clinicians must determine which patients can attempt a return to strenuous activities without surgical intervention.

The anterior cruciate ligament (ACL) is one of the most commonly injured structures of the knee.1 An estimated 100,000 ACL ruptures are recorded each year in the United States, 60% of which require some type of surgical intervention.1 Total costs associated with ACL diagnosis and management exceeds $1 billion in North America alone.2

ACL rupture is an injury that affects more than professional athletes. It is frequent among active people in the general population as well. Physical activity is a normal part of everyday life for most individuals. Whether someone is a high-school football player, an avid runner, or a father playing catch with his son, physical activities fulfill competitive and social desires of people from all walks of life.

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Sports at any level are accompanied by numerous risk factors that can result in injury. Many injuries are minor and only require short-term rehabilitation, but others — especially ACL ruptures — can be season- or career-ending. This results in an unfortunate situation for the athlete: Such positive effects of athletic activity as social interaction and health benefits can be destroyed if an injury of considerable magnitude occurs and an athlete cannot continue to participate.2 A swift, successful return to activity is the initial priority.3 Therefore, early surgical stabilization of the ACL is recommended for athletes wishing to return to a pre-injury level of ability.

Today, the gold-standard treatment for correcting an ACL rupture is surgical reconstruction using autograft or allograft tendons to effectively reproduce knee stability.4 In short, an orthopedic surgeon removes the injured person’s hamstring tendons or uses donated tendons to build a new ACL. After reconstruction and rehabilitation, the athlete should regain knee stability and be able to compete at a pre-injury level.


The ACL is an important structure for forward stabilization of the knee. An ACL rupture causes impaired function and destroys the knee’s normal mechanics.5 Approximately 70% of ACL injuries arise in noncontact situations.6 Noncontact ACL injuries are six to eight times more common in women than in men.7 In the United States, the incidence is rising as a result of increasing ACL injuries among women aged 15 to 19 years.6

Anatomic, environmental, hormonal, and biomechanical factors increase an athlete’s risk for noncontact injuries.6 Anatomic risks factors primarily contribute to noncontact injuries in women. Because women have smaller intercondylar notches in the knee joint, there is an increased likelihood of ACL impingement or rupture. Typically, men have wider U-shaped intercondylar notches, reducing the likelihood of injury resulting from an anatomic or structural issue.6 Women also tend to have larger pelvises, which lead to larger Q angles. To form the Q angle, draw a line from the anterior superior iliac spine (ASIS) of the pelvis to the center of the patella; then draw another line from the center of the patella to the tibial tuberosity. Larger Q angles are believed to result in greater amounts of stress and strain on the knees and a higher susceptibility to knee injury.6

Faulty equipment and inadequate shoe-to-surface interactions equally increase the risk of injury in both male and female athletes. Special shoes and braces are used to provide better joint alignment and knee support to eliminate environmental factors of this nature.7

Female sex hormones have recently been studied as contributors to ACL injuries. Estrogen and progesterone are thought to affect the composition and mechanics of the ACL, thus heightening a woman’s chance of injury.

Finally, biomechanical factors are unique to each athlete. Degrees of muscle strength, control, and athletic ability affect each individual differently.7 Usually, ACL tears occur when large loads or forces cannot be dissipated safely through the knee. Normally, the bones and soft tissues around the knee will safely disperse the applied forces through the knee and gradually decrease the forceful momentum. If loads or forces are too strong, the body’s neuromuscular system becomes challenged, ultimately resulting in injury.8 For example, if an athlete lands with his or her lower body in a stiff position, higher ground reaction forces are transferred through the knee, placing unbearable strain on the joint and increasing chance of injury. Providers should encourage athletes to make soft landings with large amounts of knee-joint flexion to lower risks and prevent ACL injuries.9

Common athletic situations known to produce noncontact ACL injuries include events involving quick changes in direction, decelerating with cutting maneuvers, and pivoting movements or jump landings with full- or nearly full-knee extension.6 Contact injuries are responsible for approximately 30% of ACL injuries and are caused by collisions with other players or objects on the field.6

Clinical presentation

Patients most often describe a popping or tearing sensation immediately followed by severe pain. Common complaints include, “My knee buckled under,” or “My knee suddenly gave out.”10 Either of these scenarios should raise the question of possible ACL derangement. About 70% to 80% of patients with ACL ruptures complain of knee instability during daily activities or with specific high-level activities.11 Symptoms occur while walking, but patients usually report an onset of symptoms after pivoting movements or abrupt changes in direction. Immediate joint dysfunction with an inability to ambulate is the most common symptom associated with ACL rupture.12

Patients describe immediate swelling of the knee after the initial mechanism of injury. Swelling is observed with ligament injury because of hemorrhaging that occurs when a ligament is torn or ruptured. Acute hemarthrosis — an extravasation of blood into a joint or synovial cavity—is a textbook sign of acute ACL injury. Pain, edema, and stiffness of the injured knee joint are consistent with hemarthosis. The sensitivity level of the patient and/or the experience level of the examiner may hinder an adequate physical examination.12 Earlier recognition of an ACL rupture leads to greater patient satisfaction11 and provides more optimal surgical treatment and rehabilitation.

Primary-care workup

A complete history and physical examination will diagnose approximately 90% of all ligament injuries.13 Before examining the patient, document all past knee injuries to improve accuracy of the exam. Patients with previous ACL ruptures are more likely to incur subsequent tears.14 Because joint position can determine which anatomical structures are most likely to be injured, discerning the mechanism of injury is extremely important during the interview. A detailed summary of the accident — to include the position of the knee, direction of external forces, and description of the joint after trauma — is essential.13

Initial workup for a ruptured ACL requires a thorough physical examination. Focusing on the healthy knee aids in maneuvering the patient into a comfortable position and establishes patient-provider trust. The primary goal of the exam is to create a relaxing environment that allows easier movement of the patient. Properly examining the healthy knee first provides a necessary control for comparison during evaluation of the injured knee.15

The physical exam consists of inspection, palpation, and assessment of knee-joint function. Inspecting the knees for asymmetry will determine the presence of effusion or hemarthrosis. Three functionality tests should be performed to determine the stability of the joint and ACL. These tests evaluate joint integrity and any degree of separation by applying stress in certain directions and assessing the endpoints. Intact ligaments produce an abrupt, firm end-feel, whereas sprained or torn ligaments have soft, indistinct endpoints.15

The best functionality test for assessing the stability of the ACL is the Lachman test (Figure 1). With the patient lying in the supine position, flex the knee 20° to 30° while the heel rests on the end of the exam table. Grasp the femur with the nondominant hand to prevent movement of the upper leg. Then, grasp the lower leg at the proximal tibia and apply a forward tug. This movement should produce a firm endpoint. If the endpoint is not firm or there is increased anterior translation of the tibia, the Lachman test is positive.15

The second functionality test to be performed is the anterior drawer test (Figure 2). With the patient lying the supine position, place the knee in 90° of flexion without rotation. Place both hands on the proximal tibia, and pull the upper part of the calf forward. An anterior drawer test is positive when the tibia moves anteriorly without an abrupt, hard endpoint.15

Finally, the pivot shift test (Figure 3) assesses anterior subluxation of the lateral tibial plateau when the lower leg is stabilized in near full extension. With increasing flexion, a palpable springlike reduction should be observed. A positive pivot shift test usually produces a thud or jerk around 10° to 20° of flexion. During a positive exam, the force created by the examiner will cause the knee joint to slip, giving a positive visual for identifying rotational knee instability.15

Diagnostic criteria

There are four criteria that definitively diagnose an ACL rupture. Determination is made after establishing (1) popping during injury, (2) knee instability, (3) effusion or hemarthosis, and (4) a positive Lachman test.10 The physical examination functionality tests for assessing ACL derangement are specific and moderately sensitive. Therefore, the physical examination is remarkably accurate in the hands of a trained practitioner. If the examination is negative, the likelihood that the patient has a ligament injury is exceptionally low. Patients that do not meet criteria for an ACL rupture should be managed with conservative and supportive treatment. In light of the fact that 1% to 2% of ligament pathologies are missed during physical examination, careful follow-up and re-evaluation is necessary if symptoms do not improve.12