View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by RERO DOC Digital Library Knee Surg Sports Traumatol Arthrosc (2006) 14: 204–213 KNEE DOI 10.1007/s00167-005-0679-9 V. B. Duthon Anatomy of the anterior cruciate ligament C. Barea S. Abrassart J. H. Fasel D. Fritschy J. Me´ne´trey (PLB). They are not isometric with the Received: 15 February 2005 J. Me´ne´trey (&) Accepted: 2 April 2005 Unite´d’Orthope´die et de Traumatologie du main change being lengthening of the Published online: 19 October 2005 Sport, Service de Chirurgie Orthope´dique et AMB and shortening of the PLB Ó Springer-Verlag 2005 de Traumatologie de l’Appareil Moteur, during flexion. The ACL has a Hoˆ pitaux Universitaires de Gene` ve, microstructure of collagen bundles of 24, rue Micheli-du-Crest, 1211 Geneva 14, Switzerland multiple types (mostly type I) and a E-mail: [email protected] matrix made of a network of proteins, Tel.: +41-22-37279211 glycoproteins, elastic systems, and Fax: +41-22-3727927 glycosaminoglycans with multiple functional interactions. The complex Abstract The anterior cruciate ultrastructural organization and ligament (ACL) is a band of dense abundant elastic system of the ACL connective tissue which courses from allow it to withstand multiaxial stres- the femur to the tibia. The ACL is a ses and varying tensile strains. The V. B. Duthon Æ S. Abrassart Æ J. H. Fasel key structure in the knee joint, as it ACL is innervated by posterior artic- Division of Anatomy, resists anterior tibial translation and ular branches of the tibial nerve and is Department of Morphology, University of Geneva, Geneva, Switzerland rotational loads. When the knee is vascularized by branches of the middle extended, the ACL has a mean length genicular artery. C. Barea Æ D. Fritschy Æ J. Me´ne´trey of 32 mm and a width of 7–12 mm. Sports Medicine Unit, Department of Orthopaedic Surgery, There are two components of the Keywords Anterior cruciate University Hospital of Geneva, Geneva, ACL, the anteromedial bundle ligament Æ Anteromedial bundle Æ Switzerland (AMB) and the posterolateral bundle Posterolateral bundle Introduction the complex anatomy, function, and biomechanics of the ACL is critical to elucidate the mechanisms of injury, The anterior cruciate ligament (ACL) is a key structure understand the fate of chronic ACL deficiency, and to in the knee joint, as it resists anterior tibial translation improve surgical reconstruction. The purpose of this and rotational loads [15, 37, 46]. It is one of the most paper is to present a systematic review of original re- frequently injured structures during high impact or search studies dealing with the macroanatomy and sporting activities [45]. The ACL does not heal when microanatomy of the ACL. torn, and surgical reconstruction is the standard treat- ment in the field of sports medicine [9]. Such recon- Embryology struction aims at restoring the kinematics and stability of the injured knee, to prevent future degenerative The ACL appears as a mesenchymal condensation in the changes [27]. Therefore, an adequate understanding of blastoma at 6.5 weeks of gestation, well before joint 205 cavitation [18]. It is surrounded by a mesentery-like fibers fan out as they approach their tibial attachment fold of synovium that originates from the posterior [12]. They attach to a fossa located anterior and lateral capsular apparatus of the knee joint. Thus, while the to the medial tibial spine. This fossa is a wide, depressed ACL is located intraarticularly, it remains extra-synovial area approximately 11 mm wide (range, 8–12 mm) and throughout its course [18]. 17 mm (range, 14–21 mm) in the antero-posterior direction [7, 21, 22, 44]. Near its attachment, the ACL sends a variable amount of fibers anteriorly beneath the Macroanatomy transverse intermeniscal ligament, and some extensions may blend with both the attachment of the anterior or The ACL is a band-like structure of dense connective posterior horn of the lateral meniscus. The tibial tissues. Its femoral attachment displays a shape com- attachment is somewhat wider and stronger than the parable to a vertically disposed semicircle [21]. The bony femoral attachment [7, 21, 22]. attachment is located at the posterior part of the inner Functionally, Girgis et al. divided the ACL into two surface of the lateral femoral condyle and not, as parts, the anteromedial bundle (AMB) and the pos- sometimes presumed, at the roof of the intercondylar terolateral bundle (PLB) [21], while other authors have notch. The ACL is lateral to the midline and occupies separated the ACL in three functional bundles (AMB, the superior 66% of the lateral aspect of the notch on an intermediate band, and PLB) [6, 26]. However, the two- anterior view of the flexed knee joint. The size of the bundle model has been generally accepted as the best bony attachment can vary from 11 to 24 mm across [7, representation to understand ACL function (Fig. 2). 44]. From its femoral attachment, the ACL runs ante- The fascicles of the AMB originate at the most riorly, medially, and distally to the tibia. Its length anterior and proximal aspect of the femoral attachment ranges from 22 to 41 mm (mean, 32 mm) and its width and insert at the anteromedial aspect of the tibial from 7 to 12 mm [6] (Fig. 1). attachment [6]. Conversely, the fascicles of the PLB The cross-sectional shape of the ACL is ‘‘irregular’’ originate at the postero-distal aspect of the femoral and not circular, elliptical or any other simple geometric attachment and insert at the posterolateral aspect of the form. This shape changes with the angle of flexion, tibial attachment [6]. A larger number of fascicles make but is generally larger in the anterior–posterior direc- up the PLB as compared to the AMB [50]. With the knee tion. The cross-sectional area increases from the femur in extension the fascicles of the ACL run in a fairly to the tibia, as follows: 34 mm2 proximally, 33 mm2 parallel fashion when viewed sagitally. During flexion, mid-proximally, 35 mm2 at mid-substance level, 38 mm2 there is a slight lateral rotation of the ligament as a mid-distally, and 42 mm2 distally [23] (Fig. 1). The ACL whole around its longitudinal axis, and the AMB begins Fig. 1 Front view of a left knee showing the ACL in the femoral intercondylar notch. The mean length is 32 mm (range, 22–41 mm) (left picture) and the mean width is 10 mm (range, 7–12 mm). The cross- sectional area varies in size and shape from the femur to the tibia (right picture) 206 Fig. 2 The AMB and the PLB are not isometric; the AMB tightens during knee flexion while the PLB becomes slack. The AMB spirals around the rest of the ligament so that the two bundles are no longer par- allel at 110° of flexion to spiral around the rest of the ligament. This relative movement of one bundle upon the other is due to the Microanatomy orientation of the bony attachments of the ACL [7, 21] (Fig. 2b). In full extension there is a significant differ- Microscopically, we can distinguish three zones within ence in length between the AMB (34 mm) and the PLB the ACL: (22.5 mm) [26]. The two bundles are not isometric in 1. The proximal part, which is less solid, is highly cel- flexion/extension, but experience different patterns of lular, rich in round and ovoid cells, containing some length changes during passive knee flexion. Hollis et al. fusiform fibroblasts, collagen type II and glycopro- [26] showed that the AMB lengthens and tightens in teins such as fibronectin and laminin (Fig. 4a). flexion, while the PLB shortens and becomes slack. 2. The middle part, containing fusiform and spindle- According to the same authors, the AMB increases by shaped fibroblasts (Fig. 5a), is a high density of 1.9 mm (5%) at 30° of knee flexion, and by 4 mm (12%) collagen fibers, a special zone of cartilage and fibro- at 90°. Conversely, the PLB decreases by 3.2 mm (14%) cartilage (especially in the anterior part where the when the knee is passively flexed from 0 to 30°, and by ligament faces the anterior rim of the intercondylar 7.1 mm (32%) at 90° of knee flexion [26]. As compared notch), and elastic, and oxytalan fibers. The oxytalan to full extension, the posterior fibers become slack in fibers withstand modest multidirectional stresses, flexion, and thus leave the anteromedial fibers as the while elastic fibers absorb recurrent maximal stress restraint to anterior tibial load. The two bundles are no [51]. The fusiform and spindle-shaped fibroblasts are longer parallel because the AMB spirals around the rest prominent in this middle part, which is also named of the ligament. Amis and Dawkins [6] showed that the fusiform zone, and is located in the middle part beyond 90° of knee flexion the AMB continues to and the proximal one-quarter of the ligament. The lengthen and surprisingly the PLB tightens as one ap- fusiform cell zone is characterized by a high number proaches full flexion (Fig. 3). In contrast to Hollis et al. of longitudinally oriented cells with fusiform-shaped [26], they found that the AMB initially shortens until 30° nuclei, longitudinal blood vessels, and high crimp of flexion, and then gradually elongates until it reaches length. These fibroblasts show features close to the maximal length at 120°. Internal rotation lengthens the medial collateral ligament (MCL) and dermal ACL a little more than does external rotation, most fibroblasts [38]. The cytoplasm of the cells in this noticeably at 30° of flexion.