Amelia Wnorowski
NYU Medical School
December 2006
The Anatomy of the ACL
The anterior cruciate ligament, or ACL, is the anterior of two criss-crossing ligaments inside the fibrous joint capsule of the knee that join the femur and the tibia, helping to create the hinged knee joint. The ACL has a close relationship with the other ligament internal to the fibrous joint capsule of the knee, the posterior cruciate ligament, or PCL. Both of these ligaments are intraarticular, meaning they extend from one bone to another within a joint. However, they are also extrasynovial, meaning they lie outside of the synovial sheath of the joint, which extends from the posterior intercondylar area to fold around them entirely1. Both the ACL and PCL lie within the joint capsule more posteriorly than anteriorly2.
The ACL is made up of collagen that is organized into fibers and bundles, called fascicles, on the microscopic scale. It is largely made up of Type I collagen, with some Type III and Type VI, as well as elastin that enables the ligament to be somewhat elastic in nature3. It attaches to both the femur proximally and the tibia distally. To form connections to these bones, the collagen fibers of the ACL must integrate with the fibers of each bone. This interdigitation results in a transitional zone made up of both plain and mineralized fibrocartilage and prevents stress concentration by creating a gradual shift from tendon to bone1.
The ACL attaches to the femur proximally on the lateral femoral condyle within the intercondylar notch. There is a fossa on this condyle’s posteromedial surface for this attachment1. This fossa is located anterior to the intercondylar notch’s posterior aspect3. Distally, there is another fossa for the attachment of the ACL on the tibia. This fossa is located anterolateral to the anterior tibial spine1, between the lateral and medial tibial spines3. This attachment is larger and stronger than the ACL’s attachment to the femur and there are some fibers at this attachment that blend with the anterior and posterior attachments of the lateral meniscus of the knee1. The attachment sites to the femur and tibia have been shown to be greater than the area of the ligament in the middle. In addition to the interdigitation of collagen and bone fibers, this expansion of fibers for attachment on the bone also helps reduce the amount of stress at the attachment sites4.
In general, the ACL is oriented in an oblique direction from the femur to its attachment on the tibia. It moves anteriorly, medially and distally while also spiraling laterally. Within the joint, the ACL lies anterior to the PCL, which also has an oblique course posteriorly, laterally, and distally. Together, the ACL and PCL create a cross1. There is also a connection between the ACL and PCL, called the intercruciate ligament3.
The fascicle bundles of collagen fibers of the ACL are arranged into two groups depending on their attachment to the tibia and femur as well as their different tension patterns during joint movement4. These two groups of fibers are the anteromedial band and the posterolateral band. On the femur, the two bands are separated into proximal and distal components, respectively. On the tibial attachment, the anteromedial band is attached more anterior, while the posterolateral band is attached more posterior4. In reality, the bundles are a continuum, with an intermediate area as well3. Moreover, the names of the two bundles are derived from their attachments to the tibia and femur and not their overall relationship with each other, especially not through the entire range of motion of the knee. When extended, the two bundles are parallel to on another, but when the knee if flexed, they take on a more twisted relationship, with the posterolateral bundle crossing the anteromedial one5.
The main blood supply to the ACL is through the ligamentous branches of the middle, medial inferior, and lateral inferior genicular arteries. The middle genicular artery supplies the synovial sheath that surrounds the ACL. It forms an anastamosis of vessels with branches from the lateral and medial inferior genicular arteries, which supply the knee’s fat pad. This plexus of vessels then sends ligamentous branches to the ACL1.
The nerve supply to the ACL comes from branches of the tibial nerve. These tibial nerve branches are primarily vasomotor, but also may have some functions of relaying proprioceptive information1. In addition, it has been suggested that the intercruciate ligament may play some role in proprioception as well3.
Function of the ACL
The anterior cruciate ligament, along with the three other main ligaments of the knee, functions as a stabilizer of the knee joint. Generally speaking, the role of the ACL is to prevent the tibia from moving forward off the femur during knee extension2. Therefore, the ACL is the chief stabilizer specifically during extensional strain on the knee. The ACL works together with the PCL, which keeps the tibia from moving too far backwards during flexion, to attach the tibia and femur and to keep the tibia in the correct position during motion2. In addition to providing support to the knee joint during extension, the ACL plays a very important role in providing rotational support as well6.
The function of the ACL can be further understood by studying the individual functions of the two bundles: the anteromedial and posterolateral. This division of function can be seen by the stress put on each bundle during different ranges of knee motion. The posterolateral and anteromedial bundles of the ACL alternate tightening during extension and flexion of the knee, respectively1. In addition, experiments on rotational stress on each of the bundles have shown that both the anteromedial and posterolateral bundles play a role in rotational stabilization at different degrees of flexion and extension5. Moreover, the posterolateral bundle has been observed to tighten during actual rotation of the knee, suggesting a significant role in stabilization during rotational motion5. While the role of each of the bundles individually may be complicated and not completely clear, it is understood that both bundles play integral roles stabilizing the knee joint throughout its range of motion as well as when it is under rotational stress.
The Problem of the Anterior Cruciate Ligament
The problem of the anterior cruciate ligament is a relevant one for many reasons. First, the ACL is commonly injured among sports players, at the workplace, and even in everyday small accidents. In the United States, the incidence of ACL injury is about 1 in 3,0003. There are more than 250,000 ACL injuries seen by doctors a year; 150,000 of which result in operations7, costing around two billion dollars a year3. Females are injured with a higher frequency than males due to may factors including slightly different and disadvantageous joint anatomy, hormonal factors, and less muscle mass3.
Second, while these injuries are common, there is little to no potential for natural healing. The most common tears in adults are right through the middle of the ligament, called midsubstance tears, or more rarely, stripping the ligament off the femur, called avulsion tears8. In the case of these injuries, there is little hope for rejoining ligament pieces or for reattachment to the bone. Additionally, if left untreated, there is a natural history in the ACL-deficient knee of meniscus and cartilage deterioration, which can eventually lead to osteoarthritis3. Therefore, even among non-active individuals, an ACL injury can be extremely debilitating.
Why Surgery?
The decision of whether or not to perform surgery on a patient with an ACL injury is largely dependent upon the activity level of the patient. For many sedentary patients, the risks and cost of surgery are not worth the repair of the knee. There are non-surgical options for such patients that minimize the negative effects of the natural history of an ACL-deficient knee, such as medication and physical therapy3. Bracing and rehabilitation of the leg muscles also may help prevent further injury8. In addition, even relatively inactive patients must be willing to modify their lifestyle and activity levels to help reduce the wear on the ACL-deficient knee. However, in an active patient who does not wish to modify his or her lifestyle, such as an athlete, surgery is often considered a much better option.
Surgical reconstruction of the ACL has many benefits. It typically leads to increased stability of the knee joint, reduces the risk of late arthritis, and reduces the risk of meniscal tears8. Most importantly, it allows the patient to maintain his or her quality of life and activity level. In addition, the ongoing development of ACL surgical procedures has led to a relatively quick procedure (about sixty minutes) with a low morbidity rate. It can be done with small incisions and is considered an outpatient surgery8.
The Most Recent Surgical Procedure: Double Bundle Double Tunnel
The surgical approach to anterior cruciate ligament repair has evolved over time. Currently, there is a lot of debate over what procedure is most appropriate. Many different approaches have been used to reconstruct the ACL over time, all of which have had advantages and disadvantages.
Most recently, surgeons and researchers have been studying and debating the efficacy of a relatively new procedure, the double bundle double tunnel procedure. Supporters of this surgery argue that it is the best option available today as it reconstructs the anatomy of the original ligament most accurately. This precise reconstruction of the original anatomy of the ACL may have important implications for the function of the newly reconstructed ligament and, as a result, the overall success of the surgery.
The double bundle double tunnel procedure uses tendon autografts from local sources, normally the semitendinosous and gracillis muscles, to create two bundles that represent the two bands of the natural ACL8. Two tunnels each are then drilled through both the femur and tibia for the attachment of each of the grafts using what is called the transtibial technique9. Many studies have been done to look at the sizes of the individual anteromedial and posterolateral bands as well as their exact insertions on each bone. This data helps determine the best sized grafts to use as well as the best placement for the tunnels4.
The exact surgical procedure is more complex than what is needed for the purposes of this paper and many variations on the procedure exist. However, what is important for this discussion is the ultimate goal of the double bundle double tunnel procedure, which is to restore both bundles of the ACL, both the anteromedial and the posterolateral. Previously, most ACL reconstructions used only one bundle. In doing so, the surgeon would be forced to choose which anatomic bundle, and thus which function, he or she wished to restore. As a result, some of the function of the ACL to stabilize the knee during different ranges of extension, flexion and rotation would be compromised in the post-op patient compared to the normal ACL8. In most cases using other procedures, the bundle that is restored is the anteromedial bundle, and thus rotational stability and other load stability related to the posterolateral bundle would not be regained10.
The double bundle double tunnel surgery is still being studied by many surgeons and researchers. It has been shown to more accurately reconstruct the actual anatomy of the ACL, but further studies must be done to show the effects on the new function of the reconstructed knee11. It is the hope of proponents of this procedure that by reproducing the original anatomy of the two bundles more closely, the original, more complete function of the ACL during both extension and rotation will be restored and thus there will be greater post-op success in patients with ACL reconstruction.
The double bundle double tunnel procedure is a perfect example of how knowledge of anatomy can improve clinical procedures. Because the anatomy of the ACL is so closely tied to its function, the greater the understanding we have of the anatomy of this ligament, the better the surgical outcome will be after reconstruction. Therefore, the anatomy of the ACL, the ACL’s attachments to the femur and tibia, and its bundle components have important implications for its function as well as for surgical practice.
References
1. Feagin J. The Crucial Ligaments: Diagnosis and Treatment of Ligamentous Injuries About the Knee. 2nd ed. New York: Churchill Livingstone, 1994.
2. Gray H. Gray’s Anatomy, Descriptive and Surgical. 15th ed. New York: Bounty Books, 1997.
3. Maguire J, Cross MJ. Anterior Cruciate Ligament Pathology. eMedicine from WebMD 2004.
4. Harner CD, Baek GH, Vogrin TM, Carlin GJ, Kashiwaguchi S, Woo SLY. Quantitative Analysis of Human Cruciate Ligament Insertions. Arthroscopy: The Journal of Arthroscopic and Related Surgery 1999;15:741-9.
5. Buoncristiani AM, Tjoumakaris FP, Starman JS, Ferretti M, Fu FH. Anatomic Double-Bundle Anterior Cruciate Ligament Reconstruction. Arthroscopy: The Journal of Arthroscopic and Related Surgery 2006;22:1000-6.
6. Bellier G, Christel P, Colombet P, Dijan P, Franceschi JP, Sbihi A. Double Stranded Hamstring Graft for Anterior Cruciate Ligament Reconstruction. Arthroscopy: The Journal of Arthroscopic and Related Surgery 2004;20:890-4.
7. Cooper J. Fiber-based Tissue-engineered Scaffold for Ligament Replacement: Design Consideration and in vitro Evaluation. Biomaterials 2005;26:1523-33.
8. Wnorowski D. Orthopaedic Surgeon. Personal Interview. December 9, 2006.
9. Harner CD. Double Bundle or Double Trouble? Arthroscopy: The Journal of Arthroscopic and Related Surgery 1999;15:741-9. 2004;20:1013-4.
10. Kim SJ, Jung KA, Song DH. Arthroscopic Double Bundle Anterior Cruciate Ligament Reconstruction Using Autogenous Quadriceps Tendon. The Journal of Arthroscopic and Related Surgery 2006;22:797.
11. Yasuda K, Kondo E, Ichiyama H, Kitamura N, Tanabe Y, Tohyama H, Minami A. Anatomic Reconstruction of the Anteromedial and Posterolateral Bundles of the Anterior Cruciate Ligament Using Hamstring Tendon Grafts. Arthroscopy: The Journal of Arthroscopic and Related Surgery 2004;20:1015-25.
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