The Anterior cruciate ligament (ACL) is the primary knee ligament providing knee joint stability. It is also the most commonly injured ligament sustained during pivoting knee injuries. An ACL injury occurs most frequently during a non- contact sporting injury.
ACL – Function
The primary role of the ACL is to prevent the tibia (‘Shin bone’) from sliding forward underneath the femur (‘Thigh bone’). The ACL is able to do this owing to its insertion into both the tibia and the femur.
Secondary roles include providing ‘sensory’ feedback to the joint, and prevention of some twisting movements of the knee joint. While the ACL may have little role to play during normal walking activities, it is essential in controlling rotational forces during pivoting, side stepping, and landing from a jump.
ACL injury – Mechanism
The majority of ACL tears occur as a ‘non-contact’ injury as part of a pivoting motion, or landing awkwardly from a jump. It is common for patients to hear an audible ‘snap’ which is followed by immediate pain and falling to the ground. The knee joint swells very quickly and the player is no longer able to finish the game, and often has to get carried from the field. While the above scenario represents the most common presentation, an ACL injury can also occur during a collision with another player, or in more high energy injuries such as motor vehicle accidents.
ACL Injury – Making the diagnosis
It is often possible to make the diagnosis of an ACL injury on the history alone. In the chronic setting, patients will present with a loss of confidence in the knee, and the feeling that it is going to ‘collapse’ underneath them. A thorough physical examination will reveal ‘laxity’ of the ligament, indicated by the ability of the tibia to be ‘subluxed’ forward beyond what is considered normal. It is very important to consider other structures that may be injured at the same time, including collateral ligament injuries (Add hyperlink here), or a meniscus tear. An MRI scan is typically arranged to confirm the diagnosis and assess for these additional injuries.
ACL Injury – Management
The goal of managing an ACL injury is to return the patient to their desired level of activity. In the majority of cases this will require an ACL reconstruction to return stability to the knee joint. If patients are content with returning to activities that don’t require pivoting, jumping, or side to side movement, then conservative management may be appropriate.
Nonoperative management of an ACL tear involves several phases. In the acute setting, the aim is to reduce the swelling inside the joint and restore normal knee joint motion. Progression with a individualised program aims to return muscle strength and ‘proprioception’ (sensory feedback) to the joint. A gradual return to exercise program is initiated with a focus on more controlled activities.
Surgical management of an ACL injury
Patients wishing to return to full function and partake in contact sports, or sports that require side to side movement, will be offered an ACL reconstruction. Once torn, the native ACL is not capable of healing itself, and requires a ‘reconstruction’. This is performed arthroscopically. An ACL reconstruction involves inserting a ‘graft’ in the place of the torn ACL. The two most frequently used grafts are:
One of the hamstring tendons (There are three in each leg!) is used to reconstruct the native ACL.
The central 10mm of the patella tendon with accompanying bone plugs is used to reconstruct the native ACL.