Structure and Function

The cruciate (meaning crossed) ligaments are the main stabilisers of the knee joint [13]_.

Their anatomy and function in humans is shown to be comparable to a number of animal species such as dogs [55, 56]_{, sheep} [57]_{, goats} [58]_{, pigs} [59]_{, and rabbits} [60]_{. This}

similarity across species is invaluable as it permits animal models to be utilised in the study of knee joint disease in humans, as well as facilitating the crossover of research from veterinary and medical disciplines.

The knee joint contains two cruciate ligaments that course between the tibia and the femur (Figure 1.9). They are known as the anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) in humans [13, 61]_{, and the cranial cruciate ligament}

(CCL) and caudal cruciate ligament (CaCL) in animal species such as the dog [55, 56, 62]_.

In this discussion it should be assumed that comments relating the ACL and PCL in humans also pertain to the CCL and CaCL in animals unless otherwise stated.

Both cruciate ligaments are intra-articular but extra-synovial structures as they are covered by a mesentery-like fold of synovium that is thought to protect the ligaments from the synovial fluid [13, 63, 64]_{. The PCL is the strongest ligament of the knee joint with}

approximately twice the strength of the ACL, and rupture of this ligament is less common [65, 66]_{. The superior strength of the PCL most likely stems from its larger cross-}

sectional area and its extensive femoral attachment [66, 67]_{. The main function of the PCL}

is to prevent the posterior dislocation of the tibia in relation to the femur [65]_{, also}

Figure 1.9: Anterior view of the knee joint in slight flexion showing the location of the cruciate ligaments (red text). The quadriceps tendon has been cut and reflected distally to permit visualisation of the cruciates. (Image from Marieb, 2003).

IMAGE REMOVED

DUE TO COPYRIGHT

referred to as tibial posterior tibial draw [66]_{. The PCL is made up of two functional fibre}

bundles known as the anterolateral (AL), and the posteromedial (PM), which become taught at different angles of knee flexion [67]_{. The AL bundle attaches to the femur at the}

roof of the femoral intercondylar notch, while the PM bundle attaches to the femoral intercondylar notch on the medial side. Attachment of both bundles to the tibia occurs at the posterior intercondylar fossa [55, 66]_.

Ligaments are attached to bones via direct or indirect insertions (or entheses), with direct insertions being the most common. With indirect insertions, the ligament passes obliquely along the surface of the bone. They tend to cover more bone surface than direct insertions and their boundaries are difficult to define [11, 68]_{. Direct insertions are}

where the ligament seems to pass directly into the bone cortex. These regions are more easily defined and possess four morphologically distinct layers, namely ligament, uncalcified fibrocartilage, calcified fibrocartilage and bone. There is a gradual transition through these layers at direct insertions. The cruciate ligaments of the knee attach to the bone via direct insertion [13, 68, 69]_{. Ligaments transfer force to and from the}

skeleton, and distribute the loads applied to them dynamically in order to execute movement patterns. Stress is concentrated in the region where they attach to bone, and these areas may be vulnerable to acute or overuse injuries [70]_.

The ACL is the most significant ligament of interest in this study. It is the smaller of the two cruciate ligaments and is much more prone to injury. In both humans and dogs, ACL injury is well reported to be one of the most common causes of pelvic limb pain and joint instability [64, 71]_{. The ACL crosses the knee joint from the medial aspect of the}

lateral femoral condyle to the anterior aspect of the middle portion of the tibial plateau (also referred to as the intercondylar area) [72, 73]_{. Its main functions are the resistance of}

anterior translation of the tibia with respect to the femur (anterior draw) and to limit hyperextension of the knee joint. Secondary functions include resistance of internal rotation of the tibia during flexion of the knee, and resistance of valgus rotation of the tibia. It also plays a minor role in preventing medial displacement of the tibia along with the collateral ligaments [64]_.

Like the posterior ligament, the ACL is separated into two functional bundles, known as the anteromedial (AM) and the posterolateral (PL) based on the anatomic positions of the fascicular attachments of the tibial insertions. Observations have shown the fibres of the AM bundle originate most anteriorly on the femoral side and insert anteriorly and medially at the tibial point of attachment. Therefore, as the name suggests, the fibres of the PL bundle course from the posterior part of the femoral attachment to the posterior and lateral aspect of the tibial attachment site. Furthermore, the two bundles spiral 90 degrees laterally between their attachments [74]_{. The AM and PL bundles are thought to}

have reciprocal functions, as the AM bundle is taut during flexion of the knee, and the PL bundle is taut in extension [75-77]_{. From a functional perspective, the AM bundle is}

the primary restraint against anterior tibial translation, while the larger PL bundle serves to stabilise the knee against rotary loads when the knee is near full extension [71, 78]_.

Within these bundles are fascicles of wavy or undulating collagen fibres with a complex ultra-structural organisation and abundant elastic system, making it different to most other ligaments and tendons [72, 79]_{. The ACL is primarily comprised of type I collagen}

fibrils, orientated parallel to the longitudinal axis of the ligament, and which are chiefly responsible for the ligament’s tensile strength [13]_{. Type II collagen, although not}

typically found in ligament, is present in the ACL within the fibrocartilaginous regions of its tibial and femoral sites of attachment [64]_{. Type III collagen is found throughout}

the ACL in the loose connective tissue dividing the type I collagen bundles, and in higher concentrations near the ligament’s attachment sites [13, 64]_{. Collagen types IV and}

VI may also be found in ACL in low concentrations. Like other ligaments, the ACL contains GAGs and glyco-conjugates such as laminin, tenascin and fibronectin, which function to attract key elements in normal, healing and growing tissues. Finally, ACL contains elastic components which protect the ligament from damage during times of extreme tension throughout normal joint motion [73]_.