The Anterior Cruciate Ligament (ACL) and Sports Injuries
We have all seen it before. An athlete is running and attempts to change directions and suddenly falls to the ground. They instinctively grab their knee. What could have possibly caused an athlete who has made that same maneuver many times to fall so suddenly? Before we answer the question, lets take a closer look at how our knees and the Anterior Cruciate Ligament are made and how they function.
We often think of a knee as a simple hinge joint. One that straightens and bends or extends and flexes. The reality is much more complicated than that. The knee has three planes of motion with six different directions! Beyond flexion and extension there is also side to side motion as well as rotation. Each step involves varying degrees of each of these motions. Ligaments are the structures that attach bone to bone and provide stability to the knee while still allowing the freedom of movement needed to walk, jump, skip, or run. There are four ligaments in our knee, the medial and lateral collateral ligaments on the inside and outside, and the anterior and posterior cruciate ligaments located in the middle of the knee. The medial and lateral collateral ligaments keep the knee from bending too far inside or outside. The anterior and posterior cruciate ligaments keep the knee from sliding too far forward or backward. The combination of all four ligaments is what keeps our bones appropriately aligned and our knee stable.
Functions of the ACL
Now let’s go back to our athlete. One of the most common causes of sudden instability when changing directions is a tear of the anterior cruciate ligament (ACL). This happens because the ACL has two primary functions. We have already talked about the first function, to keep the knee from sliding too far forward, the medical term for this is anterior translation. The other function of the ACL is to keep the knee stable during rotation. The ACL and PCL are called “cruciate” ligaments because they cross over one another. This orientation gives significant stability to rotation in the knee. When an athlete plants their foot to change direction, the ACL is at maximum tension preventing both rotation and anterior translation. When the force from the athlete’s pivot is too much, the ligament fails. This causes sudden instability in the knee which can damage other structures in addition to the ACL.
One of the earliest symptoms of an ACL tear is significant swelling inside the knee joint. Because the ligament is located inside the knee, the tear causes bleeding inside the joint resulting in large amounts of swelling. It is often very difficult to walk immediately following an ACL injury. Patients with torn ACLs will often say their knee feels tight and it may be difficult to bend or straighten their knee.
Treatment Options for an Anterior Cruciate Ligament Injury
In the days following an Anterior Cruciate Ligament tear, the swelling will often subside, and motion will return. Many people can walk without needing crutches or a brace. Patients will often describe a sensation of feeling like their knee is unstable when they walk. Unfortunately, a torn ACL will not heal on its own. Our knee joint is filled with lubricating fluid that allows the joint to move smoothly. This is great for motion but limits the ability of structures inside the knee to heal. Because of the limited healing ability, the most typical treatment recommendation is an ACL reconstruction, or “rebuilding”, of the ligament, commonly performed using arthroscopic techniques.
There are two primary ways to rebuild the ligament. For younger patients, the preference is typically to use the patient’s own tissue to reconstruct the ACL. The most common tissue types used are the patellar tendon underneath the kneecap, hamstring tendon from the back of the thigh, or quadriceps tendon from the front of the thigh. Each tissue, or graft, has its own strengths and weaknesses and all have been used with great success. Surgeon and patient preference is often the deciding factor on which graft is best. Another commonly used tissue is called an allograft. This comes from a donor and is sterilized prior to the procedure to limit risk of infection or rejection.
Surgery typically takes 1-2 hours depending on the type of graft chosen and if any additional procedures need to be done at the same time as the reconstruction. Treating an ACL tear without surgery is rare. The lack of stability and increased motion in the knee following ACL injury is thought to increase risk of arthritis and additional damage in the future.
ACL Recovery Time
Postoperative ACL rehabilitation focuses on early range of motion and appropriate strengthening exercises to increase function while protecting the graft. Full graft incorporation is thought to occur 6 months following surgery although some studies show that the graft continues to evolve even up to 1 year following surgery. Objective criteria are typically used to guide return to sport protocols such as return of normal motion, strength the same as the other leg, and an appropriate time interval.
Good news for our athlete, Anterior Cruciate Ligament tears are commonly treated successfully, and athletes often return to the same type of sports activities. While the road to recovery may take several months, we can often expect to see that athlete competing at a high-level by the next season.
Developed by Dr. Craig Yager, Board Certified Sports Medicine Surgeon