From ATLANTA Medicine, Vol. 86, No. 1
“A-C-L”is the three-letter word that no athlete wants to hear. The anterior cruciate ligament is critical to stability of the knee and is the primary restraint to anterior tibial translation. Every year, professional athletes such as Tiger Woods, Tom Brady and Lindsey Vonn sustain season-ending ACL ruptures. These high-profile athletes are among the 400,000 patients who undergo ACL reconstructions annually in the U.S.
Various factors influence the incidence of ACL tears. Risk of ACL injury varies depending upon the sport, with a higher incidence in sports such as football, soccer and basketball. Several studies have reported a higher rate of football-related ACL injuries on artificial surfaces compared to grass. ACL injuries occur up to 10 times more frequently in games than practices. In recent years, ACL tears have been on the rise in pediatric patients due to increased participation in competitive sports at younger ages.
Female athletes have up to a three to eight times higher rate of ACL tears compared to males in high-risk sports such as soccer and basketball. The higher risk of ACL tears in the female athlete has been attributed to differences in neuromuscular control, lower extremity alignment and ligament size. Further, females tend to land from a jump with more of a valgus moment, which places the ACL at risk for rupture. Some studies have suggested an association between risk of ACL tears and the menstrual cycle.
The classic presentation for an ACL rupture involves a sudden deceleration, a “pop,” immediate swelling and inability to continue play. Approximately two-thirds of ACL injuries are non-contact pivoting injuries.
A physical exam is sufficient to diagnose most ACL tears. An effusion is typically present, and an aspiration in the acute setting will typically reveal a hemarthrosis. The most sensitive physical exam maneuver to diagnose an ACL tear is the Lachman test. Additionally, the anterior drawer and pivot shift can help to confirm the diagnosis.
It is imperative to assess for associated ligamentous injury such as tears to the medial collateral ligament (MCL), posterior cruciate ligament (PCL), lateral collateral ligament (LCL) and posterolateral corner (PLC). Evaluation of axial alignment (varus or valgus deformity) is critical, as malalignment can predispose to surgical failure.Plain radiographs in the setting of an ACL tear assess for preexisting degenerative changes as well as associated fracture or dislocation. A Segond fracture represents an avulsion fracture of the lateral aspect of the tibial plateau and is often associated with an ACL tear. A tibial spine avulsion, which is typically seen in the pediatric population, represents an ACL avulsion and requires more urgent surgical intervention.
Magnetic resonance imaging (MRI) is often ordered to confirm the diagnosis and evaluate for concomitant intra-articular pathology. Classic bone bruises associated with an ACL tear are present at the mid aspect of the lateral femoral condyle and posterior aspect of the lateral tibial plateau. These contusions represent the bony injury that occurs during the so-called pivot shift.
The differential diagnosis for ACL injury includes patellar instability, meniscus tear, chondral injury or injury to other knee ligaments. An MRI is helpful to assess for these conditions. Meniscus tears are present in approximately half of all ACL ruptures. Associated ligament pathology can influence timing and approach to ACL treatment.
The natural history of the ACL-deficient knee is one of recurrent instability with subsequent injury to the menisci and articular cartilage. This can result in accelerated arthritis in the knee.
ACL injuries can be treated with or without surgery. Nonoperative treatment is an option in low-demand or sedentary patients who are willing to modify their activities and avoid cutting, pivoting and jumping activities. Patients who choose not to have surgery are treated with rehabilitation and functional bracing.
The recommended surgical management of ACL tears is reconstruction. Simple repair is no longer advised. ACL reconstruction consists of replacing the ruptured ACL with graft tissue, which will then become a new ACL through a process called ligamentization. ACL reconstruction is indicated in active patients who are experiencing functional instability. Skeletally immature patients are increasingly being considered candidates for surgery, as compliance with nonoperative treatment in this age group is typically poor.
Grafts that are available to surgeons can be divided into autografts harvested from the patient or allografts obtained from cadaveric donors. Autograft reconstruction is often considered in young patients who engage in high-risk sports. The two most common autografts include bone-patellar tendon-bone (BTB) and hamstring.
Bone-patellar tendon-bone autograft has historically been considered the “gold standard.” Bone-to-bone healing results in less laxity on stability testing and faster incorporation compared to soft tissue grafts. However, there is significant morbidity to BTB autografts, such as kneeling pain and patella fracture.
Hamstring autograft is a desirable graft for several reasons. Quadruple hamstring grafts have very strong tensile load compared to the native ACL. There is typically less kneeling pain and better cosmesis compared to BTB. In addition, hamstring autograft is indicated in the skeletally immature patient secondary to less risk of physeal bar formation and angular deformity.
There are several disadvantages of hamstring autografts. Hamstring grafts are entirely soft tissue and do take longer to incorporate than BTB grafts. Hamstring weakness can also result, but postoperative hamstring strength is typically 90 percent of normal.
Allografts have become a popular choice for reconstruction secondary to technical ease and absence of graft harvest morbidity. In recent years, allografts have come under scrutiny secondary to a higher failure rate, particularly in young active patients.
The reasons for the higher failure rate are likely multifactorial. Sterilization with gamma irradiation weakens the biomechanical properties of the graft. Although allografts take longer to incorporate compared to autografts, athletes treated with allograft feel less pain and may return to sport prematurely. This can predispose to graft failure. Another disadvantage of allografts is the small potential risk of disease transmission. Allografts may be suitable for the older recreational athlete who needs to return to sedentary work more rapidly.
Several concomitant injuries may require treatment with the ruptured ACL. MCL tears often occur at the same time as ACL tears. These typically can be treated nonsurgically with a period of relative immobilization prior to ACL reconstruction. For meniscus tears, partial meniscectomy versus repair are options. Meniscus repair at the time of ACL reconstruction has a higher healing rate compared to isolated meniscus repair and should be attempted for tears that are amenable to repair.
Most surgeons release athletes to full unrestricted sports 6 to 12 months after ACL surgery, depending on graft selection and surgeon preference.
ACL surgical failure has historically been less than 5 percent. Failure can result from inappropriate graft selection, inaccurate graft placement, inadequate graft fixation or tensioning, reinjury, overly aggressive rehabilitation or a premature return to sport.
Failure to address associated pathology can compromise ACL surgical outcome. Excessive malalignment or associated ligament injury, such as a posterolateral corner injury, that was not addressed at the time of the initial surgery can result in graft failure. Concomitant meniscus tears or chondral injuries can also lead to symptoms postoperatively.
Although ACL reconstruction is generally considered a successful operation, only two-thirds of NFL players undergoing ACL reconstruction will return to their previous level of play. Furthermore, while repetitive instability from an ACL deficiency can lead to degenerative changes in the knee joint, studies have yet to show that ACL reconstruction prevents the development of arthritis.
Neuromuscular training programs in female athletes have been shown to prevent ACL injuries. These programs have done so by teaching proper technique for jump landing and emphasizing core and lower extremity strengthening.
Functional bracing may provide a proprioceptive benefit to athletes postoperatively. However, there is no evidence that functional bracing after ACL reconstruction will prevent reinjury, except in the setting of downhill skiing.
The treatment of ACL tears has evolved substantially over the last few decades. Fortunately, abundant research has been dedicated to improving outcome and a return to function after this devastating injury.
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