Anatomy and Biomechanics of the Anterior Cruciate Ligament

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 ( Fig. 1.1 ). Cadaveric dissection will also reveal the two bundles of the ACL ( Fig. 1.2 ). 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.

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 ( Fig. 1.3 ). 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 ( Fig. 1.4 ).

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 ( Fig. 1.5 ). 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.