Anterior Cruciate Ligament Injury Flashcards
Practice Essentials
Anterior cruciate ligament (ACL) injuries are most often a result of low-velocity, noncontact, deceleration injuries and contact injuries with a rotational component. Contact sports also may produce injury to the ACL secondary to twisting, valgus stress, or hyperextension all directly related to contact or collision. The image below depicts a ruptured ACL.
Signs and symptoms
i. Symptoms of an acute ACL injury may include the following:
ii. Feeling or hearing a “pop” sound in the knee
iii. Pain and inability to continue activity
iv. Swelling and instability of the knee
v. Development of a large hemarthrosis
Diagnosis
Most ACL injuries may be diagnosed through a careful history emphasizing mechanism of injury coupled with a good physical examination.
Signs and Symptoms
and
Diagnosis
Signs and symptoms
Symptoms of an acute ACL injury may include the following:
i. Feeling or hearing a “pop” sound in the knee
ii. Pain and inability to continue activity
iii. Swelling and instability of the knee
iv. Development of a large hemarthrosis
Diagnosis
Most ACL injuries may be diagnosed through a careful history emphasizing mechanism of injury coupled with a good physical examination.
Diagnosis of ACL injuries
Most ACL injuries may be diagnosed through a careful history emphasizing mechanism of injury coupled with a good physical examination.
- Lachman test
The Lachman test is the most sensitive test for acute ACL rupture. It is performed with the knee in 30° of flexion, with the patient lying supine. The amount of displacement (in mm) and the quality of endpoint are assessed (eg, firm, marginal, soft).
Asymmetry in side-to-side laxity or a soft endpoint is indicative of an ACL tear. Although difficult to measure, a side-to-side difference of greater than 3 mm is considered abnormal.
- Pivot shift test
The pivot shift test is performed by extending an ACL-deficient knee, which results in a small amount of anterior translation of the tibia in relation to the femur. During flexion, the translation reduces, resulting in the “shifting or pivoting” of the tibia into its proper alignment on the femur. It is performed with the leg extended and the foot in internal rotation, and a valgus stress is applied to the tibia. - Anterior drawer test
The anterior drawer test is performed with the knee at 90° flexion, with the patient lying supine. There is an attempt to displace the tibia forward from the femur. If there is more than 6 mm of tibial displacement, an ACL tear is suggested. This test is not very sensitive and has been found to be positive in only 77% of patients with complete ACL rupture. - MRI
MRI has a sensitivity of 90-98% for ACL tears. MRI also may identify bone bruising, which is present in approximately 90% of ACL injuries.
An MRI allows the physician to confirm an ACL tear, but it should not be used as a substitute for a good history and physical examination.
Management
Nonoperative treatment:
Nonoperative treatment may be considered in elderly patients or in less active athletes who may not be participating in any pivoting type of sports (eg, running, cycling). Arthroscopy may also be considered for persons who are poor candidates for reconstruction but have a mechanical block to range of motion.
Surgical intervention:
Generally, the recommendation is that surgical intervention be delayed at least 3 weeks following injury to prevent the complication of arthrofibrosis. The methods of surgical repair may be categorized into 3 groups: primary repair, extra-articular repair, and intra-articular repair.
Primary repair is not recommended except for bony avulsions, which are mostly seen in adolescents. Because the ACL is intra-articular, the ligamentous ends are subjected to synovial fluid, which does not support ligamentous healing.
Extra-articular repair generally involves a tenodesis of the iliotibial tract. This may prevent a pivot shift but has not been shown to decrease anterior tibial translation.
Intra-articular reconstruction of the ACL has become the criterion standard for treating ACL tears.
Background to ACL injuries
Anterior cruciate ligament (ACL) injuries are most often a result of low-velocity, noncontact, deceleration injuries and contact injuries with a rotational component.
Contact sports also may produce injury to the ACL secondary to twisting, valgus stress, or hyperextension, all directly related to contact or collision.
When matched for activities, a greater prevalence for ACL injury is found in females compared with males. Approximately 50% of patients with ACL injuries also have meniscal tears. In acute ACL injuries, the lateral meniscus is more commonly torn; in chronic ACL tears, the medial meniscus is more commonly torn.
The importance of the ACL has been emphasized in athletes who require stability in running, cutting, and kicking. The ACL-deficient knee has also been linked to an increased rate of degenerative changes and meniscal injuries. For these reasons, approximately 60,000-75,000 ACL reconstructions are performed annually in the United States.
For restoration of activity and stability, the expected long-term success rate of ACL reconstruction is between 75-95%. The current failure rate is 8%, which may be attributed to recurrent instability, graft failure, or arthrofibrosis.
Treatment options must be tailored to a patient’s preoperative level of activity. The following activity levels are based on the International Knee Documentation Committee:
Level I includes jumping, pivoting, and hard cutting
Level II is heavy manual work or side-to-side sports
Level III encompasses light manual work and noncutting sports (eg, running and cycling)
Level IV is sedentary activity without sports
Nonsurgical treatment may be considered for patients who participate in level III or IV activities; all others should be considered as candidates for surgery. In addition, consider surgical consultation on any young athlete due to potential complications from recurrent instability.
Functional Anatomy ACL
The knee joint develops as a cleft between mesenchymal rudiments of the femur and the tibia. This occurs around the eighth week of fetal development. The cruciate ligaments appear as condensations of vascular synovial mesenchyme at the same time.
By 14weeks’ gestation, the ACL and posterior cruciate ligament have divided; both have a functional blood supply, which is mainly derived from the middle geniculate artery. The inferomedial and lateral genicular arteries also provide blood supply through the fat pad.
The ACL is composed of densely organized, fibrous collagenous connective tissue that attaches the femur to the tibia. The ACL is composed of 2 groups, the anteromedial and the posterolateral bands. During flexion, the anterior band is taut, while the posterior band is loose; during extension, the posterolateral band is tight, while the anterior band is loose.
The ACL attaches to bone through a transitional zone of fibrocartilage and mineralized cartilage. On the femur, the ACL is attached to a fossa on the posteromedial edge of the lateral femoral condyle. The tibial insertion is located in a fossa that is anterior and lateral to the anterior tibial spine. The tibial attachment is noted to be somewhat wider and stronger than the femoral attachment.
The ACL is intracapsular and extrasynovial. It courses anteriorly, medially, and distally as it runs from the femur to the tibia.
The ACL receives nerve fibers from the posterior branch of the posterior tibial nerve. The main function is believed to be proprioception, providing the afferent arc for postural changes during motion and ligament deformation.
Sport-Specific Biomechanics
The ACL is the primary (85%) restraint to limit anterior translation of the tibia. The greatest restraint is in full extension.
The ACL also serves as a secondary restraint to tibial rotation and varus/valgus angulation at full extension. Since the relationship between the tibia and femur provides little bony stability, the ligamentous structures must provide stability. When the ACL is injured, a combination of anterior translation and rotation occurs.
The average tensile strength for the ACL is 2160 N. This is slightly less than the strength of the posterior cruciate ligament and approximately half as strong as the medial collateral ligament (MCL).
History of ACL injury
Most ACL injuries may be diagnosed through a careful history emphasizing mechanism of injury coupled with a good physical examination. Remember that a previous ligamentous injury may be the cause of instability. When discussing the history, be sure to document mechanism of injury for this episode and any previous episodes.
- Noncontact injury
An audible pop often accompanies this injury, which often occurs while changing direction, cutting, or landing from a jump (usually a hyperextension/pivot combination).
Within a few hours, a large hemarthrosis develops.
Patients usually are unable to return to play, secondary to pain, swelling, and instability or giving way of the knee.
- Contact and high-energy traumatic injuries
These injuries often are associated with other ligamentous and meniscal injuries.
The classic “terrible triad” (ACL, MCL, and medial meniscus tears) involves a valgus stress to the knee with resultant acute injury to the ACL and MCL; however, the medial meniscus tear is now thought to occur later, as a result of chronic ACL deficiency.
Inspection and Palpation Exam for ACL Tear
Immediately after the acute injury, the physical examination may be very limited due to apprehension and guarding by the patient.
The examiner should begin with inspection, looking for any gross effusion or bony abnormality. An immediate effusion indicates significant intra-articular trauma. According to Noyes et al, in the absence of bony trauma, an immediate effusion is believed to have a 72% correlation with an ACL injury of some degree.
Assess the patient’s range of motion (ROM), especially looking for lack of complete extension, secondary to a possible bucket-handle meniscus tear or associated loose fragment.
Palpation of bony structures may suggest an associated tibial plateau fracture.
i. Palpation of the joint lines to evaluate a possible associated meniscus tear. Palpation over the collateral ligaments to suggest any possible injury (sprain) of these structures.
Up to 50% of ACL ruptures have associated meniscal injuries; acute injuries are likely to have associated injuries of the MCL and meniscus.
Physical Tests for ACL tear
-
The Lachman test, as shown in the image below, is the most sensitive test for acute ACL rupture. Since the Lachman test must be performed when the patient is relaxed, it is often better to conduct this test prior to manipulating the painful knee.
i. The knee is placed in a position of 20-30° of flexion.
ii. The femur is stabilized with a nondominant hand, and an anteriorly directed force is applied to the proximal calf.
The amount of displacement (in mm) and the quality of endpoint are assessed (eg, firm, marginal, soft).
iii. Asymmetry in side-to-side laxity or a soft endpoint is indicative of an ACL tear. Although difficult to measure, a side-to-side difference of greater than 3 mm is considered abnormal.
Other ligamentous tests are less reliable especially for primary care providers who may not have as much experience in using these maneuvers.
- The pivot shift test, as shown in the image below, is performed by extending an ACL-deficient knee, which results in a small amount of anterior translation of the tibia in relation to the femur. During flexion, the translation reduces, resulting in the “shifting or pivoting” of the tibia into its proper alignment on the femur.
i. The pivot shift test is performed with the leg extended, the foot in internal rotation, and a valgus stress is applied to the tibia.
ii. Flexion causes a reduction of the anteriorly subluxed tibia at approximately 20-30°. - The anterior drawer test may be influenced by hamstring spasm in the acutely injured knee; thus, this test is considered the least reliable.
i. This test is performed with the patient supine and the knee flexed to 90°. The examiner can sit on the patient’s foot and grasp around the patient’s calf with both hands.
ii. An anterior force is applied, and tibial excursion is compared to the unaffected knee
Causes of ACL tears
High-risk sports
Based on a study performed by Kaiser Permanente, football, baseball, soccer, skiing, and basketball account for up to 78% of sports-related injuries.
Hewson et al found a 100-fold increase in the incidence of ACL injury in college football players when compared to the general population.
- Gender
Female athletes are more susceptible to ACL injuries. Studies have shown a 2-fold increase in collegiate soccer players and a 4-fold increase in basketball compared with their male counterparts.
Differences may be due to experience, differences in training, different strength-to-weight ratios, limb alignment, joint laxity, muscle recruitment patterns, and notch index, but further studies to document a definitive cause are ongoing. One study has determined that ACL laxity does not vary with the menstrual cycle, thus dismissing this possible etiology.
- Femoral notch stenosis
Femoral notch stenosis is the ratio of the femoral notch width to the width of the femoral condyles. A notch width index of less than 0.2 is defined as notch stenosis. Individuals with notch stenosis have a higher risk of noncontact ACL injuries. - Footwear
Cleats, which have a predominant grip on the periphery, have a higher coefficient of friction on artificial turf and may result in a higher incidence of ACL injuries.
MRI for ACL tears
MRI has a sensitivity of 90-98% for ACL tears. MRI also may identify bone bruising, which is present in approximately 90% of ACL injuries. An MRI allows the physician to confirm an ACL tear, but it should not be used as a substitute for a good history and physical examination. According to the Dutch Orthopaedic Association clinical guidelines for the treatment of ACL injury, MRI has no additional value when physical examination has shown anterior-posterior or rotational instability of the knee. However, MRI is reliable for establishing other intraarticular lesions.
Plain radiographs for ACL tears
Radiographic findings are usually negative. Anteroposterior, lateral, merchant, sunrise, and notch views may be used by the physician to diagnose certain radiographic findings that are associated with ACL ruptures. Oblique radiographs may be helpful to exclude tibial plateau fractures.
ACL Tear Treatment
Physical Therapy:
Before any treatment, encourage strengthening of the quadriceps and hamstrings, as well as ROM exercises. Performance of ROM helps reduce the amount of effusion and helps the patient regain motion and strength.
Surgical Intervention
When deciding whether to perform reconstructive surgery, the physician should consider the following factors:
1) Preinjury activity level
2) Desire to return to high-demand sports (eg, basketball, football, soccer)
3) Associated injuries
4) Abnormal laxity
5) Patient’s expectations
Generally, the recommendation is that surgical intervention be delayed at least 3 weeks following injury to prevent the complication of arthrofibrosis. However, the results of one study noted that increased time to surgery (6-12 mo and >12 mo) is strongly associated with a higher risk of medial meniscus injury and decreased repair rate. While females experienced a lower risk of cartilage injury, increasing age and increasing time to surgery (>12 mo) in male patients realized a greater risk.
The methods of surgical repair may be categorized into 3 groups, primary repair, extra-articular repair, and intra-articular repair.
1) Primary repair is not recommended except for bony avulsions, which are mostly seen in adolescents. Because the ACL is intra-articular, the ligamentous ends are subjected to synovial fluid, which does not support ligamentous healing.
2) Extra-articular repair generally involves a tenodesis of the iliotibial tract. This may prevent a pivot shift but has not been shown to decrease anterior tibial translation.
3) Intra-articular reconstruction of the ACL has become the criterion standard for treating ACL tears.
