The knee joint is a hinge joint that forms the junction between the upper leg (femur) and the lower leg (tibia and fibula) with the kneecap (patella) sitting at the front. It is a large weight bearing joint which requires a number of strong rope like ligaments to stabilise the joint and to keep the bones in place during movement.
There are two types of ligament found in the knee joint. The collateral ligaments are located on both sides of the knee. These prevent the bones moving from side to side over each other. The cruciate ligaments are found within the middle of the knee joint. The cruciate ligaments cross over each other in an “X” formation. They prevent the bones sliding backwards and forwards over each other or rotating sideways.
The anterior cruciate ligament (ACL) is one of the cruciate ligaments located in the middle of the knee and it prevents the lower leg (tibia) from sliding too far forwards or twisting abnormally during movement.
Injury to the anterior cruciate ligament may range in severity. A grading system is used to guide how severe an injury is and what treatment may be required. Grade 1injuries reflect mild stretching or spraining of the ligament, however the ligament is still able to perform its normal function. Grade 2 injuires are moderate stretching of the ligament that results loosening of the ligament or partial tearing. Grade 3 injuries are complete tearing of the ligament and can result in instability of the knee joint.
The most common cause of damage to anterior cruciate ligaments is sporting injuries. Sports or activities that involve the following motions are most likely to cause injury to the anterior cruciate ligament:
- Suddenly stopping
- Slowing down whilst running
- Rapid change in direction or sidestepping
- Twisting or turning
- Direct contact such as a during a tackle or a collision with another player
A popping sound is often heard when ACL injuries occur. This is often followed by the knee giving way and feeling unstable to walk on. Other symptoms include:
- Swelling – the knee will begin to swell within 24 hours of the injury
- Limited range of movement
- Localised tenderness
- Discomfort on weight bearing
Your doctor will ask you questions about the injury and your symptoms.
Your doctor will do a clinical examination of your knee to assess the structures in your knee joint.
Your doctor may refer you for an X-ray or MRI (magnetic resonance imaging) scan to assess your knee. The diagnosis of an ACL injury can generally be made after taking a history of the injury and examining the knee alone. These investigations, however, can help to determine if other injuries of the bones (fracture), menisci or other ligaments have occurred.
Immediate treatment of an injury to the ACL includes:
- Apply ice – ice packs should be applied to the knee in 20-minute intervals. Ice should not be applied directly to the skin.
- Compression – apply a supportive bandage to help with the swelling
- Elevate the affected leg
- A brace may be required to immobilise the knee and crutches may be needed to limit weight bearing
An ACL tear will not heal itself. Surgical treatment may not be required where the knee is stable and your activity level is low. In this instance, physiotherapy and bracing are effective non-surgical options. Returning to sporting activities or a physically active lifestyle will generally require surgical treatment.
Complete tearing of the ACL requires ACL reconstruction surgery (link to ACL reconstruction surgery page). ACL reconstruction is generally performed through a minimally invasive surgical procedure called arthroscopy. The procedure involves removing the torn segments of ligament and replacing them with graft tissue. The graft is usually made from a tendon around the knee joint. During the procedure, other structures in the knee that were damaged in the injury can also be treated.
Following an ACL reconstruction, it is important to follow your surgeon’s post-operative instructions. It is usually six months or more before you will be able to return to sport and other highly physical activities.