The anterior cruciate ligament (ACL) is one of the most commonly injured ligaments of the knee. Injuries occur predominantly in a young and sports-active population. Many patients are left with significant disability following injury to the ACL. The injury leads to alteration in the mechanics of the knee. This mechanical deficit can lead to an increased risk of meniscal injury. The incidence of osteoarthritis rises sharply when the meniscus is injured.
Understanding and preventing associated meniscal pathology is the key to management of this condition. Treatment aims to protect the meniscus by modifying activity levels or reconstructing the ACL. This article endeavors to explain the complex nature of the ligament and its injuries and aid the reader in making informed management decisions.
Bonnet first discussed ACL injury in the medical literature in 1845. Further discussion was made by Segond in 1879. Stark made the first reports in the English literature in 1850. In 1900, Battle described surgical intervention when he attempted repair of the ACL. Subsequent operative descriptions include those by Groves and Jones in 1913.
In 1917, Groves described a reconstructive procedure. He used the iliotibial band (ITB) as the graft. In 1918, Smith detailed combined intra-articular and extra-articular procedures. Zur Verth first used the patellar tendon graft in 1933. Campbell used this same donor tissue in 1936. In 1966, Bruckner reported using the patellar tendon as a free graft. Since that time, numerous entries have been made in the literature describing natural history, operative reports, and surgical series.
Future research in the field of ACL surgery will attempt to address a number of features, including the following:
Kinematics - True kinematics of the knee have not yet been restored, and research continues in an attempt to accomplish this; more complex three-dimensional (3D) substitutes that accurately match the original ligament always are being considered
Fixation - New types of graft fixation are being tested regularly; in particular, anchoring devices that incorporate into the tissues themselves seem to be a good option 
Synthetic substitutes - Newer synthetics may emerge to replace those that have been tried in the past; results would have to be substantially better than those of previous attempts
Other developments - Morphogenic proteins will be used to aid graft incorporation, vascularization, and fixation; a number of different chemicals and delivery systems presently are in experimental use [4, 5] ; gene therapy also may emerge as an adjunct, and if genetic predisposition can be identified, modification may be useful to alter tissue types to create more resilient ligaments and other soft tissues
Problems of abnormal kinematics and early osteoarthritis continue, despite excellent surgery. Which of the emerging technologies will allow continued gains to be made remains to be seen. Until many of these problems are addressed, physicians can only continue to inform patients and allow them to choose the best management course.
For patient education resources, see Knee Injury and Knee Pain.
The anatomy of the ACL is very complex. The ligament is intra-articular but extrasynovial. It is described as being composed of the following three main bundles:
The ACL really functions as a continuum, with a portion being tight through all ranges of knee flexion.
The ligament courses obliquely, running from the tibia anteriorly and medially to the femur posteriorly, superiorly, and laterally. The broad tibial footprint lies at a point one third to one half the distance between the medial and lateral tibial spines, 5-7 mm anterior to the posterior cruciate ligament (PCL).
On the femoral side, the attachment lies on the medial aspect of the lateral femoral condyle, just anterior to the posterior aspect of the intercondylar notch. An intercruciate ligament joins the ACL to the PCL. This intercruciate ligament may have some role in proprioception and coupling of the two ligaments.
The microstructure of the ligament is composed of collagen fiber bundles, grouped into fascicles. Type I collagen is the predominant collagen type, accounting for more than 90% of total collagen. Types III and VI also are found. Elastin is found in significant amounts and provides some of the elastic properties of the ligament.
The major blood supply for the ACL comes from the synovium and fat pads. The middle geniculate and terminal branches of the inferior medial and lateral geniculate vessels are the vessels involved.
Sensory receptors and nerve fibers have been identified in the ligament and associated feeding blood vessels. This suggests some sensory role and possible proprioceptive function.
The ACL acts as the primary restraint to anterior tibial translation and guides the screw-home mechanism associated with knee extension. It acts secondarily to prevent varus and valgus, particularly in the extended knee. Injury leads to abnormal kinematics of the knee. Subluxation episodes occur, creating abnormal shear forces on the meniscus and articular cartilage. Subsequent meniscal injury therefore is increased significantly.
The authors have found a significant increase in this meniscal pathology when ACL reconstruction is delayed. Associated with this meniscal pathology is an increased incidence of osteoarthritis. A series conducted by the authors demonstrated a 15% incidence of ACL tears in patients undergoing total knee replacement (TKR). This incidence is at least three times the incidence of ACL tears found in the general population.
Approximately 70% of ACL injuries occur through noncontact mechanisms. Patients experience giving way of the knee when attempting to rapidly change direction. This involves deceleration, coupled with a cutting, pivoting, or sidestepping maneuver. The remainder of cases tend to occur through direct contact and often are associated with other ligament injury.
The incidence of ACL injury in the United States is estimated to approach 1 case per 3000 individuals. In the United States alone, more than100,000 injuries occur from snow skiing each year. The estimated cost of management is on the order of 2 billion dollars annually, which is a significant problem. The authors' series demonstrated an incidence of 1.5% of the population of New South Wales, Australia, with males affected twice as often as females.
Females are at higher risk of ACL injury when sports participation numbers are taken into account. This higher risk is believed to be related to both intrinsic factors (increased Q angle, decreased notch width, increased joint laxity, hormonal influences) and extrinsic factors (less muscle strength, different muscle activation patterns, altered cutting and landing patterns). Further investigation is required to fully identify which of these factors are the most important and determine whether any alterations to ACL injury patterns can be made as a result of intervention.
The natural history of ACL injury is an interesting topic. Meniscal preservation and activity levels are the key points to consider. Multiple studies demonstrate an increased incidence of osteoarthritis following meniscectomy. Nonoperative management with adequate rehabilitation can be undertaken in those patients with sedentary lifestyles. Castelyn et al have demonstrated this finding.  In 228 low-demand patients, long-term risk of requiring meniscal surgery or ACL reconstruction was low.
Patients that remain active should undergo reconstruction. Daniel states that more than 50 hours of high-level sports per year is significant. Incidence of meniscal injury increases with time, and therefore, there is an increased incidence of osteoarthritis. The authors' series of over 1000 cases confirms these findings; meniscal injury is time-dependent in those with ACL injury.
ACL reconstruction decreases the incidence of meniscal injury that requires meniscectomy. Normal kinematics are not restored. Osteoarthritis levels are thus reduced by the meniscal preservation associated with the reconstruction; however, these levels of osteoarthritis are not eliminated. Bone bruising may also play some role in the ongoing incidence of osteoarthritis following reconstruction.
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