Acknowledgment
We acknowledge James S. Starman, Mario Ferretti, Timo Järvelä, and Anthony Buoncristian, the authors of the previous edition, for their great contribution to this chapter.
Introduction
Anterior cruciate ligament (ACL) reconstruction is among the most common procedures performed in orthopaedics, with over 130,000 reconstructions performed annually in the United States alone.
The ACL has therefore been extensively studied, and outcomes of ACL surgery have received considerable attention. Research on the ligament began with a search to determine its proper structure and function so as to best determine tunnel positions, graft choices, and fixation methods. In recent years, the focus has expanded to include postoperative rehabilitation protocols, changes in knee kinematics, and neuromuscular changes observed after surgery.
The ACL is composed of two bundles, termed the anteromedial (AM) and posterolateral (PL) bundles based off their relative insertion sites on the tibia. Each bundle possesses unique functional qualities that contribute to knee stability. When the ACL is injured, most often both bundles are involved. The primary goals after ACL reconstruction surgery are to restore knee stability and function, and prevent long-term joint degeneration.
Traditional ACL reconstruction focused on single-bundle reconstruction of the AM bundle. Using this technique, outcomes were satisfactory, with quoted success rates ranging from 69% to 95%.
However, defining “success” after ACL reconstruction is difficult, as continued instability, inability to return to play, and early onset of posttraumatic osteoarthritis are frequent complications reported throughout the literature.
A prospective study on ACL reconstructed patients 7 years after surgery revealed degenerative radiographic changes in 95% of patients, and only 47% were able to return to their previous activity level following ACL reconstruction.
Several other studies have noted degenerative changes after ACL reconstruction, regardless of the chosen graft type.
While trauma at the time of injury plays a role in inciting the degenerative cascade, surgical factors inevitably play a role as well. New studies are emerging in attempts to answer these questions.
In order to understand the injured ACL, one must first understand the anatomy and biomechanics of the normal ACL and its complex role in stabilization of the knee. Improved awareness of the anatomy and biomechanical properties of the normal ACL will lead to improvements in reconstruction techniques and outcomes. This chapter describes the normal anatomy and biomechanical contributions of the two bundles of the ACL.
Anterior Cruciate Ligament Anatomy
Historical Descriptions
The earliest known descriptions of the human ACL were made around 3000
bc
and written on an Egyptian papyrus scroll. During the Roman era, Claudius Galen of Pergamon (199–129
bc
) described the ligaments of the knee and termed the ACL as “ligamenta genu cruciate.”
In 1543 Andreas Vesalius completed the first known formal anatomic study of the human ACL in his book
De Humani Corporis Fabrica Libris Septum
.
For about 400 years, the ACL was considered to be a single homogeneous structure. Two bundles of the ACL were described for the first time in 1938 by Palmer.
Despite other subsequent descriptions of the two-bundle anatomy by Girgis et al. (1975), the discovery did not become well known for many decades.
Each author described an AM bundle and a PL bundle based on their relative tibial insertion sites; the same nomenclature is used today. Since then, it has become widely accepted that the ACL is composed of two bundles.
The AM and PL bundles can be appreciated arthroscopically, particularly with the knee held in 90–120 degrees of flexion (
). Cadaveric dissection will also reveal the two bundles of the ACL (
). Although there is a considerable amount of individual variability with respect to the relative sizes of the AM and PL bundles depending on the type of study (i.e., fetal, arthroscopic, or cadaveric), all individuals with an intact ACL have both bundles.
Arthroscopic view of anteromedial (AM) and posterolateral (PL) bundles. Right knee, 110 degrees flexion.
MFC,
Medial femoral condyle.
Two distinct bundles of anterior cruciate ligament present in cadaveric specimen. Right knee, 110 degrees flexion.
AM,
Anteromedial;
LFC,
lateral femoral condyle;
PL,
posterolateral.
More recently, Norwood and Cross (1979)
and Amis and Dawkins (1991)
have described a third ACL bundle termed the intermediate bundle. Although the two-bundle description may simplify the true anatomy and histology of the ACL, many studies have been based on this functional division, and this has been accepted as a reasonable way to understand the ligament. The intermediate bundle is most similar to the AM bundle in its anatomic and biomechanical properties. For the purposes of this chapter, it will be considered as part of the AM bundle.
Anterior Cruciate Ligament Development
ACL formation has been observed in fetal development as early as 8 weeks, corresponding to O’Rahilly stages 20 and 21.
A leading hypothesis suggests that the ACL originates as a ventral condensation of the fetal blastoma and gradually migrates posteriorly with the formation of the intercondylar space.
The menisci are derived from the same blastoma condensation as the ACL, a finding that supports the hypothesis that these structures function in concert. Another proposed mechanism of fetal ACL formation is that it forms from a confluence between ligamentous collagen fibers and fibers of the periosteum.
Following the initial formation of the ligament, no major organizational or compositional changes are observed throughout the remainder of fetal development.
The two distinct bundles of the ACL begin to develop at 16 weeks of gestation (
).
The fetal ACL has similar characteristics as the adult ligament, but differs in that the bundles are more parallel and the femoral origins are broader.
Histologically, the fetal ACL is also more cellular and vascular.
The two bundles are separated by a septum, similar to the adult ligament (
).
Sixteen-week fetus demonstrating two bundles of the anterior cruciate ligament with the knee in extension
(A)
sagittal view with medial femoral condyle removed and flexion
(B)
frontal view.
AM,
Anteromedial;
LFC,
lateral femoral condyle;
PL,
posterolateral.
Arthroscopic probe placed through the septum separating the anteromedial (AM) and posterolateral (PL) bundles. Right knee, 110 degrees flexion.
MFC,
Medial femoral condyle.
Histology of the Anterior Cruciate Ligament
The ACL is an intra-articular but extrasynovial structure, as two synovial layers envelop it.
The ACL is a structure composed of numerous fascicles of dense connective tissue that connect the distal femur and the proximal tibia. Histological studies have demonstrated that a septum of vascularized connective tissue separates the AM and PL bundles (
). In addition, it has been shown that the histological properties of the ligament are variable at different stages in ACL development. At the time of fetal development, the ACL is hypercellular, with circular, oval, and fusiform-shaped cells. Later, in the adult, histology reveals the ACL to have a relatively hypocellular pattern with predominantly fibroblast cells with spindle-shaped nuclei.
In addition to fibroblast cells, the ACL contains chondrocyte-like cells at higher concentrations around the tibial insertion, where “physiologic impingement” occurs.
Fetal anterior cruciate ligament histology, sagittal cut.
Arrows
indicate the septum of vascularized connective tissue dividing the anteromedial (AM) and posterolateral (PL) bundles.
Anatomy of the Anterior Cruciate Ligament
The ACL originates on the medial surface of the lateral femoral condyle (LFC), runs an oblique course within the knee joint from lateral and posterior to medial and anterior, and inserts into a broad area of the central tibial plateau. It is formed predominantly of Type I Collagen and secondarily of Type III Collagen.
Proximally, the ACL receives its blood supply from the middle genicular artery, which feeds a synovial plexus around the ligament.
Distally, the medial and lateral inferior genicular arteries supply the plexus but have a lesser contribution.
Immunohistochemical analysis has demonstrated watershed areas exist around the insertion sites and along the anterior aspect of the distal third of the ligament.
About 1% of the ACL’s area consists of neural tissue supplied by branches of the tibial nerve.
The neural tissue serves many functions within the ACL. The perivascular neural elements surround the vascular plexus and function in vasomotor control. Other nerve fibers function to transmit slow pain impulses.
Surrounding the synovium are slowly adapting and rapidly adapting mechanoreceptors. The slowly adapting mechanoreceptors relay information about motion, position, and rotation of the joint, while the rapidly adapting mechanoreceptors detect changes in tension within the ligament.
After ACL rupture, studies have demonstrated that the residual mechanoreceptors within the torn stumps may still function in proprioception.
Further research is required to determine the extent of residual function.
The femoral and tibial insertion sites demonstrate a four-layered structure. The layers include ligamentous, fibrocartilaginous, mineralized cartilage, and a subchondral bone plate.
This gradual transition prevents stress concentration at the insertion sites by dissipating the forces throughout the four layers.
The total intra-articular length of the ligament is approximately 22–41 mm (average: 32 mm) and varies by as much as 10% throughout a normal range of motion.
The cross-sectional area of the ligament varies significantly throughout its course, from approximately 44 mm
2
at the midsubstance to more than 3 times as much at both its origin and insertion.
While the overall cross-sectional area is constant during knee range of motion, the cross-sectional shape changes. At the tibial insertion, the ligament fans out into a “foot” region, allowing it to avoid impingement in the intercondylar notch during extension.
This area demonstrates adaptive changes in which chondrocyte-like cells are mixed with the typical tenocytes in response to the “physiologic impingement.”
After reconstruction, this “foot” region is unable to be restored, which can have implications for graft impingement and susceptibility for failure.
It is important to note that the bony anatomy is important as well. While the size of the ACL varies among people, the bony anatomy does as well, but not in a 1:1 proportion. Thus patients can have a relatively large ACL within a very small intercondylar space. As such, a small notch width index (ratio of the notch width to epicondylar width) has been linked to increased likelihood of ACL rupture.
Anatomy of Anteromedial and Posterolateral Bundles
Anatomic studies have characterized the individual contributions of both the AM and PL bundles to the overall ACL architecture. The AM bundle is approximately 38 mm in length.
The PL bundle has been less well studied. Kummer
determined an average PL length of 17.8 mm based on 50 cadaver dissections. Despite differences in length, the AM and PL bundles have a similar diameter. The narrowest aggregate cross-sectional area has been reported to be 36 mm
2
for females and 44 mm
2
for males.
As the ligament approaches the insertion sites, it fans out. Harner et al.
showed the footprints of the ACL are about 3.5 times larger than the midsubstance. This fact must be recognized during reconstruction.
Odensten and Gillquist
described the femoral origin of the ACL as an ovoid area at an average of 18 mm in length and 11 mm in width. Within this area, the AM bundle occupies a position located on the proximal portion of the medial wall of the LFC, while the PL bundle occupies a more distal position near the anterior articular cartilage surface of the LFC (
). The lateral bifurcate ridge separates the origins of the two bundles.
Harner et al.
studied the origin and insertion of the AM and PL bundles using a laser micrometer system and concluded that each bundle occupies approximately 50% of the total femoral origin, with cross-sectional footprints of 47 ± 13 mm
2
and 49 ± 13 mm
2
for the AM and PL bundle, respectively. Mochizuki et al.
contradicted these results and found the AM bundle origin was 1.5 times larger than the origin of the PL bundle, although a less sensitive methodology was used.
A,
Femoral insertion sites of anteromedial (AM) and posterolateral (PL) bundles (right knee, medial femoral condyle removed).
B,
Tibial insertion sites of AM and PL bundles (right knee tibial plateau, menisci removed).
Lat Men,
Lateral meniscus;
MM,
medial meniscus.
On the tibia, the insertions of the AM and PL bundles are located between the medial and lateral tibial spines over a broad area. The full ACL insertion has been described as an area measuring 9–13 mm (average 11 mm) in diameter medial-lateral and 14–20 mm (average 17 mm) in diameter anterior-posterior; not only does the size vary among individuals, but also the shape of the footprint differs as well.
Within this area, the AM bundle insertion can be found in an anterior and medial position, whereas the PL bundle insertion is located more posteriorly and laterally (see
). Posteriorly, fibers of the PL bundle are in close approximation to the posterior root of the lateral meniscus
and, in some individuals, may attach to the meniscus itself (
). Similarly, the AM bundle may have attachments to the anterior horn of the lateral meniscus. The overall size of the tibial insertion is approximately 120% of the femoral origin. However, as is the case with the femoral origin, the two bundles share approximately equal tibial insertion site areas: the AM bundle occupies 56 ± 21 mm
2
, and the PL bundle occupies 53 ± 21 mm
2
.
Posterolateral (PL) bundle tibial insertion is located just anterior to the posterior root of the lateral meniscus
(Lat men)
. Right knee, arthroscopic view.
Crossing Pattern
Based on their anatomical positions, the AM and PL bundles change alignment as the knee moves from extension to flexion. The femoral insertion sites are oriented vertically when the knee is in full extension, and the two bundles of the ACL are parallel in orientation (
). As the knee moves into 90 degrees of flexion, the AM bundle insertion site on the femur rotates posteriorly and inferiorly, in contrast to the femoral insertion of the PL bundle, which rotates anteriorly and superiorly; the femoral footprints become horizontally oriented (
). This change in the femoral footprint orientation causes the two bundles to twist around each other and become crossed. As the knee is flexed, the PL bundle can be seen anterior to the AM bundle at its femoral insertion (
).
Crossing pattern of anteromedial (AM) and posterolateral (PL) bundles. With the knee in extension, the AM and PL bundles are parallel
(A)
left knee, medial femoral condyle removed, and the insertion sites are oriented vertically
(B)
.
Crossing pattern of anteromedial (AM) and posterolateral (PL) bundles. With the knee in flexion, the AM and PL bundles are crossed
(A)
left knee, medial femoral condyle removed, and the insertion sites are oriented horizontally
(B)
.
Arthroscopic view and computer model of anteromedial (AM) and posterolateral (PL) bundle crossing pattern in extension
(top)
and flexion
(bottom)
. The PL bundle is obscured in extension but becomes visible in flexion as it moves anteriorly on the femoral side.
LFC,
Lateral femoral condyle.
Tensioning Pattern
The change in alignment of the AM and PL femoral insertion sites allows the ACL bundles to twist around themselves as the knee moves through a complete range of motion. This crossing pattern, along with the differences in the length of each bundle, has implications for the tensioning pattern of the overall ligament and each individual bundle. In a study by Gabriel et al.,
forces were measured in each bundle during an anterior tibial load of 134N over several flexion angles, as well as for a combined rotatory load of 10 Nm valgus and 5 Nm internal tibial torque. They showed that the PL bundle is tightest in extension (in situ force of 67 ± 30N) and becomes relaxed as the knee is flexed. When flexed and not on stretch, the PL bundle will shorten by 1.5–7.1 mm.
Conversely, the AM bundle is more relaxed in extension, and reaches a maximum tension as the knee approaches 60 degrees of flexion (in situ forces of 90 ± 17N).
Compared with extension, at 90 degrees of flexion, the AM bundle will stretch by about 3.3 mm.
Several studies have corroborated these findings.
This tensioning pattern can be observed grossly in cadaveric and arthroscopic views of the ACL (
). The PL bundle is also observed to tighten during internal and external rotation by about 2.7 mm.
In summary, the ACL consists of two distinct bundles, the AM and PL bundles, and these bundles contribute synergistically to the dynamic stability of the knee.
Cadaveric view of anterior cruciate ligament in 110 degrees knee flexion. Note how the posterolateral (PL) bundle appears lax while the anteromedial bundle (AM) appears taut.
LFC,
Lateral femoral condyle.
Anatomic Considerations in Anterior Cruciate Ligament Surgery
The goal of ACL surgery is to reconstruct the ligament to have the same properties as prior to injury. While this may not be 100% feasible, certain principles must be employed to optimize outcomes. In order to quantify the femoral and tibial footprint locations, several methods have been developed using radiographic or anatomic landmarks. These include the quadrant method based off lateral x-rays, the clock description based off arthroscopic views, and the cylindrical coordinate system. Studies have tried to reference the ACL off other structures (i.e., the tibial insertion is 7–10.4 mm anterior to the anterior edge of the PCL in the flexed knee).
Special guides have also been created to help create the femoral tunnel. Because all of these techniques attempt to generalize the footprints of the ACL, which vary person to person, they all have their limitations.
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