There are several conditions that cause varying levels of anterior cruciate ligament (ACL) instability. These include: sprains, avulsion fractures, ACL deficiencies, and complex ligament injuries. More than half of all types of ACL injuries occur with associated damage to another part of the knee – most commonly, another ligament, articular cartilage (bone lining), or meniscus (cushion pad). Types of ACL injuries can be diagnosed by a thorough examination, X-rays, and magnetic resonance imaging (MRI).

ACL Sprains

A sprain occurs when the fibers or threads of the ligament are stretched, partially torn, or in severe cases, completely ruptured. ACL sprains are classified by a grade as follows:

Grade I ACL Sprain

Grade I sprains are considered the most mild of the different types of ACL injuries. While patients will experience symptoms, they can usually be treated without surgery. This type of ACL sprain occurs when the fibers of the ligament are stretched, but a tear does not exist. For the most part, the knee will remain stable. Symptoms of a Grade I sprain include tenderness, swelling, moderate knee pain, and some limit to mobility. A period of rest, along with anti-inflammatory medications, crutches, and ice can usually remedy this type of injury.

Grade II ACL Sprain

A Grade II sprain refers to an ACL injury where the fibers of the ligament are partially torn. This particular injury occurs occasionally, but in most cases the tears will be complete. The same symptoms apply as a Grade I sprain, however they tend to be more severe. The difference is in the instability, because some fibers of the ligament are actually torn, and the joint may feel unstable or as if it will give out during activity. In other cases, the joint will feel stable, so not all Grade II sprains will require surgery. Reconstruction of a partially torn ACL will depend on the patient’s age, activity level, and sports or fitness goals, and will primarily depend on the patient’s symptoms. In cases of instability, partial, or complete, ACL reconstruction is typically recommended.

Grade III ACL Sprain

Grade III sprains are the most common types of ACL injuries among athletes. This occurs when the fibers of the ligament are completely torn. This is often referred to as a rupture, meaning the ligament has completely torn apart into two sections. Symptoms of a Grade III sprain are frequently more severe. Swelling and tenderness may be immediate, knee pain can be severe, and stiffness may result. Rarely, a complete ACL tear can occur without pain, swelling, or stiffness Grade III sprains almost always leave the knee unstable. For this particular grade of injury, reconstructive surgery is usually recommended, once full extension is achieved and the swelling decreases.

ACL Avulsion Fracture

ACL avulsion fracture is more rare than an ACL sprain, and occurs when the ACL tears by breaking a piece of bone off from where the ligament attaches to the thigh or leg. In most cases, it’s the tibia (leg bone) that is affected—meaning, the ACL rips by breaking (fracturing) a piece of bone from where it attaches to the shinbone. This is usually the result of excessive overuse and muscular contraction during sports. Direct trauma through a blunt force or hit can also cause this type of injury. ACL avulsion fractures are more common in children than in adults, but can occur in adults, and are quite common in skiers.

ACL Deficient Knee

Knees that do not have an anterior cruciate ligament, are in most cases unstable, and instability results in unwanted, ongoing symptoms for patients. The most prominent symptom is knee buckling, which will be felt during running and cutting activities, as well as walking down stairs, and sometimes during other everyday activities. In other cases, there is a sensation that the knee will buckle if the patient is not careful and attentive. While patients can live with ACL deficiency by decreasing activity, this can lead to meniscus tearing, cartilage damage, and eventually, the onset and progression of osteoarthritis. These additional knee injuries are due to buckling, the sensation of buckling, or even as a result of micro-instability. Therefore, surgery is usually recommended for younger patients, active patients, or patients who live in areas or have occupations which require climbing or walking on uneven ground.

Complex and Multi-Ligament Knee Injuries

Sometimes, in conjunction with an ACL injury, other damage may exist within the knee. The ACL may become injured along with meniscus damage (which occurs in the vast majority of cases), or with damage to cartilage. Damage may also occur to the ACL plus another ligament. Multi-ligament damage usually occurs when a traumatic event is severe. If it is suspected that the ACL is injured along with another knee structure, a thorough exam, followed by an X-ray and an MRI is required. When multiple knee ligament injuries occur, sometimes, a knee dislocation or fracture can also be present. In order to correct the overlapping injuries, your physician will need to perform a multi-ligament reconstruction surgery to repair damaged ligaments, as well as use a graft to replace ligaments and tendons that cannot be saved.