The anterior cruciate ligament (ACL) is the most commonly injured ligament of the knee, and it is most frequently injured during an athletic activity. Sports are becoming an increasingly important part of day-to-day life in the United States, increasing the number of ACL injuries. This injury has received a great deal of attention from orthopedic surgeons over the past 15 years, and very successful operations have been developed to reconstruct the torn anterior cruciate ligament.
The ACL controls how far forward the tibia moves in relation to the femur. If the tibia moves too far, the ACL can rupture. The ACL is also the first ligament that becomes tight when the knee is straight. If the knee is forced past this point, or hyperextended, the ACL can also be torn.
This tearing of the ligament results in a loud pop and a feeling of instability in the knee. The ACL may not be the only ligament injured when the knee is twisted violently, such as in a clipping injury in football. It is not uncommon to see both the medial collateral ligament (MCL) and the ACL injured.
The major cause of injury to the ACL is sports related. Numerous types of sports-related activities have been associated with ACL tears. Those sports requiring the foot to be planted and the body to change direction rapidly (such as basketball) carry a high incidence of injury. Football is frequently the source of an ACL tear because it combines the activity of planting the foot and rapidly changing direction with the threat of bodily contact. Downhill skiing is another frequent source of injury, especially since the introduction of ski boots that extend higher up the calf. These boots move the forces caused by a fall to the knee rather than the ankle or lower leg. The ACL injury usually occurs when the knee is forcefully twisted or hyperextended. Many patients recall hearing a loud pop when the ligament tears and feeling the knee give out.
There has been a dramatic increase in the number of females who suffer ACL tears. This is in part due to the rise in women’s athletics, but studies have shown that female athletes are more likely to suffer this injury then their male counterparts. It is uncertain why this is the case. Initially, it was thought that females were at higher risk because of differences in training intensity. But more evidence suggests that there may be a difference in the anatomy of the female knee, or the female ligament may not be as strong due to the effects of the female hormone estrogen. These factors may lead to a higher risk of ACL injury for the female athlete.
The symptoms following a tear of the ACL vary in different people. Usually, swelling of the knee occurs within a short time following the injury. This is due to bleeding into the knee joint from torn blood vessels in the damaged ligament.
The instability caused by the torn ligament leads to a feeling of insecurity and weakness of the knee, especially when trying to change direction on the knee. The knee may feel like it wants to bend too far backwards.
The pain and swelling from the initial injury will usually be gone after two to four weeks, but the instability remains. The symptom of instability and the inability for the patient to trust the knee for support is what requires treatment.
Also important in making decisions on how to treat the knee is the growing realization by orthopedic surgeons that long-term instability leads to early arthritis of the knee. Many orthopedic surgeons feel that by treating the instability and performing a reconstruction of the ligament, the risk of developing wear and tear arthritis in the knee can be reduced.
The history and physical examination is probably the most important tool in diagnosing a ruptured or deficient ACL. In an acute injury, the swelling is a good indicator. Any intense swelling that occurs within two hours of a knee injury usually means blood in the joint, or a hemarthrosis. If the swelling occurs the next day, the fluid is probably from the inflammatory response. Placing a needle in the swollen joint and draining as much fluid as possible gives relief from the swelling and provides useful information to your doctor. If blood is found when draining the knee, there is a 70 percent chance it came from a torn ACL. X-rays of the knee may also be ordered on the initial examination to rule out a fracture. Ligaments and tendons do not show up on x-rays. However, bleeding into the joint also occurs when a fracture through the knee joint is present, or when portions of the joint surface are chipped off.
The most accurate of the noninvasive tests for the knee is the MRI scan. The MRI (magnetic resonance imaging) machine uses magnetic waves rather than x- rays, to show the soft tissues of the body. With this machine, we are able to “slice” through the area we are interested in and see the anatomy and injuries very clearly. This test does not require any needles or special dye and is painless.
If there is a question about what is causing the knee problem, arthroscopy may be used to make the definitive diagnosis. Arthroscopy is an operation where a small fiber optic TV camera is placed into the knee joint, allowing the orthopedic surgeon to look at the structures inside the knee joint directly. The vast majority of ACL tears are diagnosed without resorting to surgery, and arthroscopy is usually reserved to treat the problems identified by other means.
Initial treatment for ACL injury includes crutches and rest until the swelling resolves. The knee joint may be aspirated to remove the blood in the joint. The word “aspiration” means to remove fluid from the body. The knee is aspirated by inserting a needle into the joint and drawing out the blood.
Once the initial pain and swelling begins to resolve, physical therapy will probably be initiated to regain as much of the normal range of motion as possible. One of the problems with a torn ACL is that small proprioceptive nerve endings in the ligament are torn as well. These nerves are there to give the brain information about where the body is in 3D space. These nerves are what make it possible for you to touch your nose with your eyes closed.
The joints rely on these nerves to fine tune the muscles’ actions to allow the joint to function properly. A good physical therapy program will help retrain these nerves and strengthen other muscles that will take over some of the functions of stabilizing the knee joint from the loss of the ACL.
To help replace the stability of the knee, an ACL brace may be suggested. These braces are fairly effective at preventing the knee from giving way during strenuous activity. Most of these braces must be fitted by a certified orthotist, a physical therapist, or physician. They are not the type you can buy at the drugstore. Most orthopaedists will recommend wearing a brace for at least one year after a reconstruction. So even if surgery is performed, a brace is a good investment.
If the symptoms of instability are not controlled by a brace and rehabilitation program, then surgery may be suggested. Most surgeons now favor reconstruction of the ACL using a piece of tendon or ligament to replace the torn ACL. Today, this surgery is most often done using the arthroscope. Incisions are usually required around the knee, but the joint itself is not opened. The arthroscope is used to perform the work needed on the inside of the knee joint. Most patients can expect at least one night in the hospital, although more and more surgeries are being done on an outpatient basis.
In the typical surgical reconstruction, the torn ends of the ACL must first be removed. Once this has been done, the type of graft that will be used is determined. One of the most common tendons used for the graft material is the patellar tendon. This tendon connects the knee cap (patella) to the lower leg bone (tibia). Another very common graft combines two of the hamstring muscle tendons that attach to the tibia just below the knee joint — the gacilis tendon and the semitendinosis tendon. Studies have shown that these two tendons can be removed without affecting the strength of the leg. There are other hamstring muscles that can take over the function of the two tendons that are removed.
If it is used for graft material, about one third of the patellar tendon is removed, with a plug of bone at either end.
The bone plugs are rounded and smoothed. Holes are drilled in each bone plug to hold sutures that will pull the graft into place. The next procedure prepares the knee to receive the graft.
The intracondylar notch is enlarged so that there is no rubbing on the graft. This process is referred to as a notchplasty. Once this is done, holes are drilled in the tibia and the femur to place the graft.
These holes are placed so that the graft will run between the tibia and femur in the same direction as the original ACL. The graft is then pulled into position using sutures placed through the drill holes. Screws are used to hold the bone plugs in the drill holes.
Other types of materials are also used to replace the torn ACL. In some cases, an allograft is used. An allograft is tissue that comes from someone else. This tissue is harvested from tissue and organ donors at the time of death and sent to a tissue bank. There the tissue is checked for any type of infection, sterilized, and stored in a freezer. When needed, the tissue is ordered by the physician and used to replace the torn ACL. The advantage of using allograft is that the surgeon does not have to disturb or remove any of the normal tissue from the knee to use as a graft. The operation also usually takes less time because the graft does not need to be harvested from the knee.
After surgery, a physical therapist will be contacted to begin a rehabilitation program. Some type of rehabilitation will likely be required for six months after surgery to ensure the best result from the ACL reconstruction. Most patients see the physical therapist about three times a week the first six weeks following surgery. Following the initial period, a home program may be initiated and monitored by the therapist.