Introduction

Anterior cruciate ligament (ACL) reconstruction is the sixth most common procedure performed in orthopaedics, and it is estimated that between 75,000 and 100,000 ACL repair procedures are performed annually in the United States alone.^1,² The ACL has therefore been intensively studied, and outcomes of ACL surgery have received considerable attention. This has included research on surgical technique factors such as tunnel position, graft choices, and fixation methods, as well as postoperative rehabilitation protocols.

Traditional single-bundle ACL reconstruction has focused on reconstruction of one portion of the ACL, the anteromedial (AM) bundle, and although outcomes are generally good, with success rates between 69% and 95%, there remains room for improvement.^3,⁴ A prospective study of a cohort of 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.⁵ However, it should be noted that some studies of long-term follow-up have more encouraging results. Jarvela et al demonstrated tibiofemoral degenerative changes in only 18% of patients at 7 years follow-up post ACL reconstruction with bone–patella–bone grafts.⁶ In addition, Roe et al reported on a cohort of patients reconstructed with bone–patellar tendon–bone grafts and found an incidence of 45% with degenerative radiographic changes at 7 years follow-up, as well as an incidence of 14% with degenerative changes in a group with hamstring grafts.⁷

A thorough review of the anatomy and biomechanics of the normal ACL reveals key points regarding its complex role in stabilization of the knee joint. Improved awareness of the anatomy and biomechanical properties of the normal ACL may lead to improvements in techniques for ACL reconstruction and an associated improvement in outcomes over traditional results. This chapter describes the normal anatomy of the two bundles of the ACL and reviews the biomechanical contributions of each bundle.

Anterior Cruciate Ligament Anatomy

Historical Descriptions

One of the earliest known descriptions of the human ACL was made around 3000 BC, written on an Egyptian papyrus scroll. During the Roman era, the earliest description of the ligament using its modern name was made by Claudius Galen of Pergamon (199–129 BC), who described the “ligamenta genu cruciate.” In 1543 the first known formal anatomical study of the human ACL was completed by Andreas Vesalius in his book De Humani Corporis Fabrica Libris Septum.

Two bundles of the ACL were described for the first time in 1938 by Palmer et al, followed by Abbott et al in 1944 and Girgis et al in 1975.⁸^–¹⁰ Each author described an AM bundle and a posterolateral (PL) bundle, named for the relative location of the tibial insertion sites of each bundle. More recently, in 1979 and again in 1991, Norwood et al and Amis et al, respectively, described a third bundle of the ACL anatomy, the intermediate (IM) bundle.^11,¹² Although it may be said that a two-bundle description of the ACL anatomy is an oversimplification of the complete anatomy, many studies have been based on this functional division, and it has been accepted as a reasonable way to understand the anatomy and biomechanics of the ligament. The IM bundle is most similar to the AM bundle in both anatomical and biomechanical considerations, and for the purposes of this chapter it is therefore considered as part of the AM bundle.

Anatomy of the Anteromedial and Posterolateral Bundles

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 is present that separates the AM and PL bundles (Fig. 1-1). 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 ACL development, the ACL is observed to be hypercellular with circular, oval, and fusiform-shaped cells. Later, in the adult ACL, the histology reveals a relatively hypocellular pattern with predominantly fibroblast cells with spindle-shaped nuclei.^13,¹⁴

Fig. 1-1 Fetal anterior cruciate ligament, sagittal cut. Arrows indicate the septum of vascularized connective tissue dividing the anteromedial (AM) and posterolateral (PL) bundles.

The ligament finds its origin 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. The cross-sectional area of the ligament varies significantly throughout its course from approximately 44 mm² at the midsubstance to more than three times as much at both its origin and insertion.^10,^15,¹⁶ The total length of the ligament is approximately 31 to 38 mm and varies by as much as 10% throughout a normal range of motion.¹⁷

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.^18,¹⁹ A leading hypothesis holds that the ACL originates as a ventral condensation of the fetal blastoma and gradually migrates posteriorly with the formation of the intercondylar space.²⁰ The menisci are derived from the same blastoma condensation as the ACL, a finding that is consistent with the hypothesis that these structures function in concert.²¹ Another proposed mechanism of fetal ACL formation is from a confluence between ligamentous collagen fibers and fibers of the periosteum.²² Following the initial formation of the ligament, no major organizational or compositional changes are observed throughout the remainder of fetal development.¹⁹

Two distinct bundles of the ACL are present at 16 weeks of gestation (Fig. 1-2). In arthroscopy, the AM and PL bundles can also be appreciated, particularly with the knee held in 90 to 120 degrees of flexion (Fig. 1-3). Finally, cadaveric dissection also reveals two anatomical bundles of the ACL (Fig. 1-4). In summary, there is a considerable amount of interindividual variability with respect to the relative sizes of the AM and PL bundles, as seen in fetal, arthroscopic, and cadaveric studies; however, all individuals with an intact ACL have both bundles of ligament.

Fig. 1-2 16-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.

Fig. 1-3 Arthroscopic view of anteromedial (AM) and posterolateral (PL) bundles in 14-year-old female. Left knee, 110 degrees flexion. LFC, Lateral femoral condyle.

Fig. 1-4 Two distinct bundles of ACL present in cadaveric specimen. Left knee, 90 degrees flexion. AM, Anteromedial; LFC, lateral femoral condyle; PL, posterolateral.

Insertion Site Anatomy

Anatomical studies have characterized the individual contributions of both the AM and PL bundles to the overall ACL architecture. Odensten and Gillquist described the femoral origin of the ACL as an ovoid area measuring 18 mm in length and 11 mm in width.²³ Within this area, the AM bundle occupies a position located on the proximal portion of the medial wall of the LFC, and the PL bundle occupies a more distal position near the anterior articular cartilage surface of the LFC (Fig. 1-5, A). Harner et al studied the digitized origin and insertion of the AM and PL bundles in five cadavers and concluded that each bundle occupies approximately 50% of the total femoral origin, with cross-sectional areas of 47 ± 13 mm² and 49 ± 13 mm² for AM and PL, respectively. ¹⁶