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The ACL is the anterior cruciate ligament of the knee. It is attached to the bottom of the femur and runs diagonally forwards to insert into the tibia. This diagonal direction means it is possible for the ACL to control and restrict the forward movement of the shin relative to the femur. There is a second cruciate ligament, the PCL, or posterior cruciate ligament which is similar to the ACL but runs diagonally backwards from the femur to the tibia and has the opposite function, of restricting and controlling the backwards movement of the tibia relative to the femur.
Ligaments are thick, strong tissue made of collagen. Their main role is restricting unwanted movements at a joint to allow for efficient movement in the desired planes. They also have a secondary role which is proprioception, and they can feedback to the brain information about the movement and position of the knee. This information is valuable as the brain can then send signals to the muscles of the body to correct positions to keep up balance and upright, as well as moving efficiently to prevent injury.
When a ligament is injured both of these functions can become compromised. An injury can be a stretch or tear of the ligament. The extent of stretch or the percentage of fibres torn will influence the extent of the functional compromise. Ligament injuries are diagnosed as grade 1-3 depending on the laxity and percentage of fibres torn.
The ACL is one of the most commonly injured ligaments of the knee. Injuries of the ACL often also involve injury to another structure in the knee, a tear of the meniscus or medial collateral ligament are the most common.
An ACL tear is often referred to as a pivot shift injury due to the mechanism that causes this injury. The ACL is most stressed and vulnerable to injury with sudden changes of speed or direction especially slowing down or suddenly stopping, landing awkwardly on your knee, pivoting on your knee when your foot is fixed is in a fixed position, or a direct impact to the side of the knee like when players collide in a tackle.
When an ACL tear occurs many people report a popping or crack sensation, and other symptoms such as significant swelling that occurs within several hours. The swelling will make it difficult to fully bend or straighten the knee. It will often be painful to weight bear on the injured leg and not possible to continue playing sport. In some cases, a feeling of instability is also noticed. With a mild injury, pain may be much lower, but the feeling of instability or that the leg might give out will be felt. In some instances swelling can be so significant it can distort the feeling of instability.
The feeling of instability can vary from feeling very vulnerable with any movement to more subtle and only felt with movements that stress the injured ACL, such as jumping and landing, rapid deceleration, quick changes of direction or pivoting on the knee.
As time passes the later symptoms will be noticed. Bruising can often take several days to appear and can be significant in some injuries.
See a medical professional if you suspect you have injured your ACL, such as a sports doctor or physical therapist. They will be able to examine your knee and recommend any additional diagnostics such as MRI or X-Ray that can confirm the diagnosis. Radiology such as MRI, is a very helpful to specifically diagnose the extent of the injury and if other structures have been injured concurrently. Ligament tears are classified as grade 1-3 detailed below:
The ACL is stretched but not torn, so there will be no bruising
Typically there is low pain and slight swelling
The knee will not feel unstable unless the ACL is stressed specifically
On assessment the ACL will feel firm with no increase in laxity
The ACL is torn and there can be bleeding linked to this damage resulting in bruising
Pain may still be low or moderate but there can be more swelling
Function will be impaired so the knee will feel unstable
On assessment the ACL will have more laxity, but will still have a firm end feel
There can be pain with assessment and stress of the ACL
The ligament is significantly torn or completely ruptured, there will be bleeding so bruising will be come evident following the injury
Pain will still only be moderate in most people, but swelling can be extensive
Function will be severely impaired with the knee feeling very unstable
On assessment the ACL will have significant laxity and no end feel
This is not medical advice. We recommend a consultation with a medical professional such as James McCormack. He offers Online Physiotherapy Appointments for £45.