Anterior cruciate ligament rupture.

Professional and amateur athletes alike dread the diagnosis of an acute anterior cruciate ligament (ACL) tear. From Willis MaGahee's highly visible injury, to the recreational basketball player at your local gym, ACL tears temporarily halt athletic activities for many across the world. An estimated 100,000 acute ACL ruptures will occur in the United States this year alone. Seventy percent of these injuries will occur during some form of athletic competition. Recent literature has documented a higher rate of injury among female athletes. Multiple studies have documented the lack of effectiveness of functional bracing to prevent ACL tears, and the functionally unacceptable outcomes of nonsurgical treatment.

Biomechanics

The anterior cruciate ligament is the primary restraint to anterior translation of the tibia relative to the femur. It also serves as a secondary restraint to tibial rotation and varus/valgus stress. It has an average cross-sectional area of 44 m[m.sup.2], and an ultimate tensile load of approximately 2100 N. Its stiffness has been measured at over 240 N/mm.

Presentation

The vast majority of injuries to the ACL can de diagnosed by taking a thorough history. Most ACL tears occur by a low-energy, noncontact mechanism involving a sudden deceleration with a rotational maneuver, such as cutting. Most patients report hearing or feeling a "pop" or "snap." They have the inability to bear full weight without assistance and will frequently be nonambulatory, needing to be transported off the field. They describe an immediate effusion (within the first few hours), and have significant pain around the knee joint. If their ACL tear is old, they will describe recurrent sensations of instability, or their knee "giving out." These episodes usually occur in provocative positions, though they may describe them with activities of daily living. It is important to determine the presence of any antecedent symptoms or any history of prior knee surgeries.

Physical Examination

The importance of a thorough physical examination cannot be overemphasized. Combined with an appropriate history, the diagnosis of ligament injuries about the knee can be accurately made. On-the-field examinations can be helpful before the swelling and muscular spasm hinder pathologic findings.

Prior to examining the injured knee, the astute physician will assess the patient's overall ligamentous laxity. Thumb-to-forearm sign, elbow hyperextension, and other findings will help in this area. For comparison, the contralateral knee will then be examined. Range of motion, ligamentous stability, and overall lower extremity alignment should be assessed. Examination of the injured knee should be thorough and as gentle as possible. Significant guarding of the knee by the patient will mask pathologic laxity. Inspection of the knee for effusion, abrasions, range of motion, and limb alignment should be performed. A mechanical block to full extension can indicate a displaced bucket-handle tear of the medial meniscus, or a painful ACL stump displaced anteriorly. Tenderness to palpation should be sought, especially in the medial and lateral joint line. Any tenderness about the patella should be noted, for many patients will perceive a patellar dislocation as their knee "giving out." A competent extensor mechanism can be evaluated by asking the patient to perform a straight leg raise. Varus and valgus stress can evaluate the competence of the lateral and medial collateral ligaments, respectively.

Evaluating the ACL requires complete relaxation of the patient's quadriceps. The patient should be supine with neck and upper extremities in a comfortable position. The Lachman test (Fig. 1) is the most sensitive for ACL disruption, and should be graded in comparison to the contralateral extremity. The amount of tibial translation, as well as the firmness of the endpoint, should be noted. The anterior drawer test (Fig. 2) is also useful and requires relaxation. A positive pivot shift (Fig. 3) is pathognomonic for ACL deficiency. In the acute setting, the physician will usually get one attempt at this maneuver, for a positive result will probably result in significant guarding. The evaluation is completed with the assessment of distal neurovascular status.

Imaging

Associated bony injuries, such as the Segond fracture (lateral capsular avulsion), can be evaluated with plain radiographs and should be obtained initially. In children, the presence of a tibial spine fracture should be excluded. Magnetic resonance imaging (MRI) is helpful, though not required, in the preoperative setting. MRI is 95% accurate in identifying ACL tears and is useful in visualizing meniscal injuries and chondral damage.

Lateral meniscal tears are more common in the acute ACL tear, whereas medial meniscal tears are seen in the chronic setting. Approximately 20% of acute ACL disruptions are associated with tears of the medial collateral ligament. The presence of loose bodies and osteochondral defects should be noted. These findings should confirm those elicited during the physical examination.

Treatment

Initial treatment is aimed at pain relief, edema control, and prevention of further injury. The inflammatory phase of healing usually lasts three weeks and surgical intervention during this period results in a substantial incidence of arthrofibrosis. Crutchassisted ambulation with touch-down weight bearing is appropriate with cold compresses and activity modification. A long leg range of motion brace locked in extension may make the patient more comfortable initially, but early knee motion should be instituted. Supervised protocols are available and can be directed by devoted sports physical therapists. Nonsurgical treatment can be continued if the patient does not desire to return to high-impact sports activities and has no sensations of instability with activities of daily living.

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Orthopaedic referral will be necessary if the patient desires a return to competitive athletics or has recurrent instability. The goals of treatment will be a restoration of functional stability, and the prevention of further injury. Nonsurgical treatment results in unacceptable outcomes in the majority of patients. Current surgical techniques of reconstruction of the ACL result in 85 to 90% "good" and "excellent" results, with return to desired activities within 4 to 6 months.

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Table. Anterior cruciate ligament anatomy

Intraarticular, extrasynovial
Femoral attachment
  Posteromedial aspect of the intercondylar notch of the lateral femoral
  condyle.
Tibial attachment
  1.5 cm posterior to the anterior border of the articular surface,
  just medial to the attachment of the anterior horn of the lateral
  meniscus.
Blood supply
  Middle geniculate artery (branch of the popliteal artery)
Neural innervation
  Posterior articular nerve (branch of the tibial nerve)
Two discrete bundles, anteromedial and posterolateral
Fibers externally rotate 90 degrees

Accepted May 21, 2004.

Recommended Reading

Griffin LY, Agel J, Albohm MJ. Noncontact anterior cruciate ligament injuries: risk factors and prevention strategies. JAAOS 2000;8(3):141-150.

Leaphart SM, Ferris CM, Fu FH. Risk factors associated with noncontact anterior cruciate ligament injuries in female athletes. AAOS Instructional Course Lecture 2002;51:307-310.

Fu FH, Bennett CH, Latterman C, et al. Current trends in anterior cruciate ligament reconstruction. Part I. Biology and biomechanics of reconstruction. Am J Sports Med 1999;27:821-830.

David Koon, MD, and Frank Bassett, MD

From the Department of Orthopaedic Surgery, University of South Carolina School of Medicine, Columbia, SC., and the Division of Orthopaedic Surgery, Duke University Medical Center, Durham, NC.

Reprint request to David Koon, MD, Two Medical Park, Suite 404, Columbia, SC 29203.