The anterior cruciate ligament (ACL) is one of four main ligaments that help provide stability to the knee joint. It is deep in the center of the knee and serves as an important restraint to excess front-to-back and rotational motion of the knee.

Lee H. Diehl, MD, a sports medicine specialist at Duke Orthopaedics and head team physician at Elon University, discusses causes, symptoms, and treatment options for anterior cruciate ligament (ACL) injuries.

How Does Injury to the ACL Occur?

Lee Diehl, MDAnterior cruciate ligament (ACL) injuries are very common, especially in sports.

The ACL can be injured during contact with another player, in collision sports, or in non-contact situations such as planting and twisting or landing awkwardly from a jump.

The incidence of ACL injury in our society continues to increase. Women and young athletes are increasingly at risk due to an increased amount of team sports participation and year-round play.

What Does an ACL Injury Feel Like? 

Athletes often describe that they felt or heard a "pop" or "explosion" in their knee. Some say that they felt the bones shift or move in a way they shouldn't.

The knee may feel unstable or loose immediately after the injury, but often motion in the knee is close to normal.

Overnight or over the next few hours, the knee often swells significantly and becomes sore. Being in tune with their bodies, many athletes sense that they've sustained a serious injury.

What Are the Symptoms of ACL Problems After Injury?

People who have ACL tears often fall into three groups. A small group will develop a very loose knee which shifts or gives out easily with activities of daily living such as turning suddenly or walking down stairs or on uneven ground.

Another small, very fortunate group is at the other extreme. These individuals can do virtually anything, including playing sports, seemingly without trouble. We really don’t know why some people fall into one of these two groups, but we suspect that a number of factors may contribute towards making some knees very dependent on the ACL and others not so.

The majority of people with a torn ACL fall between these two extremes. They can tolerate daily activities and forward-motion sports such as running, swimming, and cycling without much difficulty. However, they may have episodes of "giving way" when they engage in sports requiring jumping, pivoting, cutting, or sudden changes of direction.

How Are ACL Tears Diagnosed?

The diagnosis of a torn ACL can usually be made by a careful history and physical exam.

Occasionally, serial examinations or other special studies such as magnetic resonance imaging (MRI) will be useful to confirm the diagnosis and evaluate for other injuries inside the knee.

How Are ACL Injuries Treated?

After the diagnosis of an ACL tear is made, patients have two main choices -- have the ligament surgically reconstructed or live without an ACL.

In either case, the goal is to prevent or limit episodes where the knee gives out. Not only are these episodes painful and inconvenient, they may cause serious damage to the meniscus cartilage and joint surface cartilage inside the knee.

Damage to these structures has been shown to increase the rate of wear-and-tear arthritis.

Non-operative treatment usually involves physical therapy to help reduce swelling, regain motion, and rebuild strength in the surrounding muscles. Some people will wear a brace during at-risk activities, but most people tend to modify their activity level to avoid situations where their knee shifts.

What Is an ACL Reconstruction and What Are Typical Outcomes?

To prevent recurring "giving way" episodes, many people choose to have an ACL reconstruction. Reconstruction or rebuilding of the ACL involves an operation to remove the old, torn ACL tissue and then replace it with a new ligament graft.

There are many ways to reconstruct the ACL, different sources of graft, different devices to fix the graft, and different ways to put the graft in place. We have not yet proven that there is one best way to do it, but the goal is to create a restraint that functions as close to the original as possible.

After acute ACL injury, most surgeons will wait several weeks or more to allow the knee to calm down before performing a reconstruction. After reconstruction, the graft must heal to the bone inside the knee, and the body must remodel it to become a new ACL.

If all goes well -- which it does 90-95 percent of the time -- after a nine to 12-month rehabilitation period, a person can expect to have a knee that moves normally, does not swell, hurt, or give way with activity -- including competitive sports.

Can I Prevent an ACL Injury?

It is unclear if there is any way to truly prevent ACL tears. However, there are exercise programs directed toward modification of movement patterns that may help reduce risk, especially in young female athletes.

How Do I Choose a Treatment That Is Right for Me?

You should base your choice upon your own life or sports priorities and the degree of instability you’re experiencing in your knee.

After an ACL tear, even with a reconstruction that works well, the knee is never truly normal again. If you wish to play high-risk sports at a competitive level, or are having frequent “giving way” episodes with activities of daily living, then surgery is usually the better option for you.

If you are not having instability and are a recreational athlete willing to moderate your sports participation, you will likely be satisfied with the results of non-surgical treatment.

Lee Diehl, MD, is a sports medicine expert at Duke Orthopaedics and is the head team physician at Elon University.