The anterior cruciate ligament is located centrally in the knee. It connects the femur(thighbone) to the tibia(tibia). More specifically, the anterior cruciate ligament runs obliquely down the inside of the lateral femoral condyle and attaches to the upper part of the tibia, just before the eminentia intercondylaris. 

The anterior cruciate ligament is described as an intra-articular structure, meaning that the anterior cruciate ligament lies within the knee joint. A special feature of the anterior cruciate ligament is that it has its own synovial membrane. This membrane provides nutrient blood flow specific to the anterior cruciate ligament. The anterior cruciate ligament is mainly composed of type I collagen (90%), which is strong and tensile and also provides structure to tissues such as bones and tendons. Besides type I, approximately 10% of the anterior cruciate ligament consists of type III collagen. This type of collagen is much thinner and more flexible, essential for tissues that require elasticity such as blood vessels. This composition makes the anterior cruciate ligament ideally suited for providing strength and some degree of flexibility in the knee joint. 

The structure and distribution of connective tissue cells in the anterior cruciate ligament vary within the structure. The anterior cruciate ligament transitions from a rigid ligamentous structure in the central region to a more cartilaginous structure, to mineralised cartilage, then to the bone at both its attachment to the femur and tibia. This gradual transition allows the anterior cruciate ligament to properly distribute forces at its around the attachments to the bone.

Length is one of the most commonly described features of the anterior cruciate ligament. In the past, X-rays were widely used to map the length of the anterior cruciate ligament. Nowadays, MRI has become the standard for measuring length. Several anatomical studies of the anterior cruciate ligament have shown that the length ranges from 27 to 38 mm. This difference in length also depends on the position of the knee. On average, the width of the anterior cruciate ligament is 10 to 12 mm. The anterior cruciate ligament is also described as two separate bundles. The anteromedial and posterolateral bundles range from 6 to 7 mm and from 5 to 6 mm in width. 

Initially, the anterior cruciate ligament was described as a single structure. In 1938, Ivar Palmer did the first description of the anterior cruciate ligament consisting of two separate bundles. These two bundles are described as the anteromedial (AM) and posterolateral (PL) bundles. They are usually but not always divided by a sheath between the bundles. Both bundles are named after the attachment points from the tibia (shin bone). 

Although the presence of the anteromedial and posterolateral bundles of the ACL is widely accepted, recent research findings suggest that the anterior cruciate ligament has a threefold structure. In these studies, the bundles are defined as anteromedial, intermediate and posterolateral. Different studies describe the structure of the anterior cruciate ligament differently. Where sometimes a single bundle is described, other literature mentions an additional bundle MRI scans. At the same time, many studies cannot clearly find this extra bundle when it is studied. These findings underline the possible variation and the existing uncertainties regarding the exact anatomy of the anterior cruciate ligament. In addition, it is possible that these variations are influenced by age and gender. The anterior cruciate ligament, like so many other structures in our body, is subject to ageing. One possibility is that change in fibre structure causes the anterior cruciate ligament to look different over the years. This is especially observed in the posterolateral bundle, where fatty degeneration is often observed in older individuals.Also, the anterior cruciate ligament tends to be thinner and less dense in shape in later life.

In recent years, the anterior cruciate ligament has also been described as more of a bow or hourglass to describe its shape. In the literature, this is indicated by "ribbon like" The narrowest part of the anterior cruciate ligament, also called the isthmus, is located equidistant from the tibial and femoral attachment points. The cross-sectional area of the isthmus is smallest when the knee is fully extended and increases when the knee is flexed. This part of the anterior cruciate ligament is less than half the size of the attachment areas on the femur and tibia. When the knee is flexed, from a front view we see a more twisted shape. When the knee extends, we see that the fibres become more parallel longitudinally. The transverse section of the anterior cruciate ligament varies along its length. In the centre, we see an average cross-sectional area of 44mm2.

The footprints indicate the specific places where the anterior cruciate ligament attaches to the femur (thigh bone) and tibia (shin bone). The size and shape of these attachment areas vary from person to person. From various studies based on anatomical, cadaveric and radiological examinations, more and more is known about the shape and location of these footprints. This knowledge is of great importance for refining surgical techniques and improving outcomes after anterior cruciate ligament reconstruction.

Femoral footprint

On the femur, the lateral intercondylar ridge and the bifurcate ridge can be used to identify the precise attachment points for both bundles on the lateral femoral condyle. The cross-sectional area of the femoral footprint ranges from 60 to 130 mm². In extension of the knee, we see that the anteromedial bundle is located more proximally (top) and the posterolateral bundle is located more distally (bottom). 

Two types of attachments have been described at the femoral attachment of the anterior cruciate ligament. A distinction is made between a direct and indirect type. The direct attachment of the anterior cruciate ligament is a strong and dense collagen tissue that attaches directly to the bone surface of the femoral condyle. These fibres form a strong connection to the bone, providing high tensile strength and stability during dynamic loading.

The indirect form is a less tightly organised system of collagen fibres. These fibres form a membrane that extends more towards the femoral articular cartilage. This structure helps to absorb and distribute forces over a larger surface area, contributing to the overall resilience and flexibility of the joint. The indirect joint allows forces acting on the knee to be better distributed. 

We see that the femoral footprint of the anterior cruciate ligament changes shape over the course of our years of life. In young people, the area is larger and shaped more like a semicircle, while in older individuals, the shape becomes smaller and flatter, similar to a ribbon. 

Tibial footprint

The exact location of the attachment site of the anterior cruciate ligament is 15 mm in front of the posterior cruciate ligament. The area of the tibial footprint is larger than that of the femoral footprint. The cross-sectional area of the tibial footprint varies between 100 and 160 mm² 

The centre of the insertion site of the anterior cruciate ligament on the tibia lies 15 mm in front of the posterior cruciate ligament and covers two-fifths of the width of the distance between the spinae medial to lateral.Three main forms of the tibial footprint have been described: oval-shaped, triangular and C-shaped, which are wider at the posterior and located in the intercondylar region at the anterior. Other shapes such as boot-shaped, L-shaped, J-shaped and double C-shaped have also been reported. However, the most common shape is the elliptical insertion shape, followed by the triangular and C-shaped.

The tibial footprint is obliquely oriented. It is wider than the femoral footprint and is located in a clearly marked area at the anterior horn of the lateral meniscus. The anteromedial bundle is positioned on the anteromedial side of the tibial insertion, the medial boundary being the anteromedial edge of the articular surface of the medial tibial condyle. It is believed that the posterolateral bundle is positioned on the posterolateral side of the intercondylar region, its lateral boundary being the medial border of the articular surface of the lateral tibial condyle.

The width of the tibial anterior cruciate ligament attachment site averages 12.6 mm (ranging from 7.7 to 16.3 mm) with an average thickness of 3.3 mm (ranging from 2.5 to 3.9 mm). The tibial ACL attachment site also shows variations between younger and older individuals.

The anterior cruciate ligament is supplied with blood by the arteria media genus, This is a branch of the popliteal artery. This vessel also provides the blood supply for the synovial membrane at the bottom of the knee. The artery enters the joint through the back of the knee capsule, where it supplies the synovial membrane and the anterior cruciate ligament. 

The anterior cruciate ligament is innervated by the tibial nerve, which passes through the posterior capsule and runs close to the synovial membrane of the anterior cruciate ligament. This nerve also contains mechanoreceptors that contribute to the proprioceptive capabilities of the anterior cruciate ligament. Remarkably, there are very few pain receptors in the anterior cruciate ligament itself, which helps explain why little pain is often felt in an isolated rupture of this ligament. Complaints such as pain usually arise from swelling (haemarthrosis) or additional damage that may occur during the time of injury.

A synovial membrane is a thin membrane of connective tissue that lines the inside of a joint, excluding the articular cartilage. It produces synovial fluid that lubricates the joint and delivers nutrients to structures such as cartilage. The anterior cruciate ligament lies within the joint capsule but is surrounded by its own synovial membrane, separating the anterior cruciate ligament from the rest of the knee joint, so to speak. This synovial membrane forms a protective layer around the ligament and provides nutrition through both small blood vessels and by diffusing nutrients through the synovial fluid itself. This is important because the anterior cruciate ligament itself has limited blood flow. The membrane also has a protective function; this sheath reduces friction with surrounding structures.

When injury occurs, however, this membrane poses a disadvantage. When the anterior cruciate ligament is damaged, in almost all cases the synovial membrane is also damaged. Whereby healing is not possible. This membrane makes the anterior cruciate ligament unique compared to other ligaments. But also creates vulnerability. The anterior cruciate ligament is not entirely unique in this regard. The posterior cruciate ligament, like the anterior cruciate ligament, is surrounded by its own synovial membrane and thus also lies intracapsularly but extrasynovially in the knee joint.

The posterior cruciate ligament is distinguished from the anterior cruciate ligament by its natural repair ability, This is due to some specific anatomical and biological properties. In a cruciate ligament rupture, there must be a blood clot form that could act as a bridge, in which new collagen tissue could then grow. Inside a joint, this is difficult because the clot comes into contact with synovial fluid. Synovial fluid (synovial fluid) actually has (fibrinolytic) enzymes that can break down the clot before healing can occur. In an anterior cruciate ligament rupture, a stable connection of the ends occurs very rarely if ever. 

The posterior cruciate ligament, on the other hand, benefits from its anatomical position. The posterior cruciate ligament is also intracapsular, but is located more towards the back of the knee and is largely surrounded by synovial membrane except at the back​. On that backside, towards the back wall of the capsule, the blood vessels enter that can feed the posterior cruciate ligament. This means that in a posterior cruciate ligament rupture, blood can more easily organise itself locally to form a clot. The synovial membrane, as it were, creates an enclosed environment that allows some healing. We see that an acute posterior cruciate ligament tear often refills (partially) with scar tissue on MRI, while an anterior cruciate ligament tear almost always remains "open"

Mechanically, the posterior cruciate ligament experiences less stress compared to the anterior cruciate ligament, allowing more connection of the torn ligament. The use of a brace also plays an important role here. This helps keep the posterior cruciate ligament in an optimal position for healing by pushing the tibia forward, supporting natural healing and improving conditions for recovery.

This combination of improved blood supply, protective positioning within the joint, and mechanical support means that the posterior cruciate ligament can often recover naturally without surgery, all things that unfortunately do not apply to the anterior cruciate ligament. For the latter, reconstruction is often required to restore the stability of the knee joint, depending, for example, on the desire to return to sports level. This is because of insufficient natural healing capabilities.

The anatomy of the anterior cruciate ligament is complex, but it is precisely this complexity that makes it quite a fascinating structure for us. From its unique location within the knee joint to its own synovial sheath and specific attachment points - every detail plays a role in the stability and function of the knee.

Understanding this anatomy is important not only for doctors and surgeons, but also for anyone working with those with the anterior cruciate ligament. Even if you are dealing with this injury yourself, understanding more about your situation is always enlightening. The better we understand the anterior cruciate ligament, the more targeted we can treat injuries and hopefully prevent them in the future.

In short, the anterior cruciate ligament is small but powerful - and deserves all the attention it gets