What is the ACL and how is it injured?

The anterior cruciate ligament, or ACL, is the central pivot mechanism of the knee. The ACL is responsible for keeping the knee stable when it is twisted or pushed from the front. The most common ways an ACL is injured are during a hyperextension (where the knee goes beyond straight) or when there is rotation at the same time as an impact. When the ACL breaks, your bones will be forced together and bruise, and the blood vessels in your knee usually bleed. The combination of these injuries leads to a painful, stiff and swollen knee.

The ACL is usually injured when you’re on the field or the court and you’re playing a sport that requires stopping, starting or cutting, like soccer or basketball. Your knee wobbles, you land awkwardly after a jump, or you get knocked off your feet and your ACL ruptures. The common symptom is feeling a pop in your knee followed immediately by swelling. You’re unable to stay in the game and you go to the sidelines for medical attention.

How is an ACL injury treated?

The first medical attention you receive after an evaluation is icing to help reduce the swelling and the pain. You’ll also get a knee brace to protect the knee and crutches to use because you’re typically not able to put a lot of weight on the leg. If you’re out of town when you get injured, you’ll want to get the brace and crutches where you are but you will almost want to come home before you have any surgery done. The knee is not ready for surgery in the first couple of days after the injury. There’s a great deal of recovery the knee has to go through before surgery.

At your appointment with your orthopedic surgeon, you’ll have an x-ray to make sure there are no additional injuries to your leg and discuss next steps, including therapy and MRIs.

Before you have an ACL surgery you’ll do physical therapy to try to get the motion in your knee to come back.

The MRI will confirm that the ACL is torn and will show associated injuries, such as meniscus damage or cartilage damage, that will affect how urgent surgery is. Someone who has meniscus damage, for example, needs surgery sooner than someone without meniscus damage, who can afford to let their knee heal more first.

The MRI also allows patients and their families to have time to come to terms with the injury. Many times for young athletes in high school and college, the ACL tear may be the first time that they have had a major problem in their lives and faced the possibility of surgery. Many patients have strong emotions. We’ll talk with the patients about the sports they play and their future plans, such as a high schooler planning to play in college, to factor into the timing of surgery and rehabilitation and the choice of ACL graft to use.

How easily the patient gets their motion back in their knee is also a consideration. Having good motion in the knee before surgery helps predict how much motion will be in the knee after surgery. Patients usually have their motion back within about three weeks, but occasionally we’ll see a patient whose motion doesn’t come back for several weeks despite physical therapy. Assuming no other issues such as a problem with their meniscus, we’ll delay ACL surgery on that patient until they have gotten the motion back so they can have a better recovery after surgery.

Graft choice

A graft is a piece of tissue from one part of the body put into another part of the body. To repair the ACL, there are several choices of graft from different tendons that can be used.

The two main types of grafts are autografts, which come from your own body, and allografts, which come from cadaver tissue. Autografts can come from several different places in your body, but the long-term success of all of them is similar.

While allografts are less stressful on your body at the time of surgery, they don’t heal as well overall. Allografts are more likely to fail in younger patients.

We recommend that you use a graft your surgeon is comfortable with. Each physician has their own biases about the best graft choice, and each choice has its pros and cons.

Who needs ACL surgery? When should it occur?

Not every person who has an ACL tear needs surgery. Most young people need surgery and will get surgery. As you age the need for surgery is less but does not disappear.

I have had a patient who was knocked over by a wave at the beach and tore her ACL. She didn’t play sports, but three months later she was having trouble vacuuming. When she turned a corner and pivoted with her vacuum, her knee would give out. She’s an example of someone who is not playing sports but still needs surgery to stabilize the knee and protect it from future injury.

A very recent study in the American Journal of Sports Medicine showed that people who had their ACLs repaired within a few months of the injury had better long-term success compared with people who waited over a year to have their surgery or people who did not get their ACL repaired.


Is it possible for the ACL to reattach itself when there has been a complete tear? And if there is a complete tear and no surgery, what activities will no longer be possible?

The ACL has almost no healing potential because it is in the middle of the knee without other soft tissues around it. When the ACL is ruptured, the blood supply is severed and there’s no way for nutrition and healing elements to get to the ACL anymore. It may heal but it won’t heal into the right position – the end of the ACL may attach itself to another part of the knee, for example.

Without surgery, you can do activities that don’t require twisting on your leg. For example, running, biking, swimming, or working out in the gym would probably be OK. However, if you start accelerating, twisting, turning, pivoting, or otherwise moving from side to side, you would have issues without an ACL. In that case, you would be at risk for further injury. With surgery, all sports are possible again.

I didn’t get my ACL repaired 20 years ago when it was injured, and I was told I’m at higher risk of arthritis because of it. Can I help prevent the arthritis now? Can my ACL be repaired?

Wearing a brace to support and protect your ACL can help. A good quad and hamstring exercise program to strengthen the muscles in the thigh and across the knee would also help. Activity modification may also be an option.

If your knee is unstable, you can have ACL surgery to repair it now. It can be disconcerting and dangerous for patients to suddenly have their knee give out on them. If you don’t have any ongoing issues in your knee, however, then it will not necessarily be helped by the ACL reconstruction and it may not be at high risk for arthritis. A surgeon can look at your x-rays and MRIs and talk to you about your best options.

What exercises would you recommend to strengthen muscles to support the ACL?

The muscle group that’s most dynamically responsible for stabilizing the knee and reproducing with the ACL does is the hamstring group. Many people will strengthen their quads in their workouts, but the hamstrings is at least as important and should be built up equally. Some example exercises include hamstring curls, plank exercises, and hip exercises. You may also want to consider bracing if your knee is feeling unstable.