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ACL 101

The anterior cruciate ligament (or “ACL”) is one of four primary ligaments in the knee. The other knee ligaments are the posterior cruciate ligament, the lateral collateral ligament, and the medial collateral ligament. The ACL and the posterior cruciate ligament form an “X” shaped bundle of ligaments that connect the femur to the tibia, and enable the standard range of motion of the knee.



While a knee joint can handle a small amount of torsional or “twisting” force, it is intended to function properly as a hinged joint. Significant twisting or rotating forces can put too much stress on the knee ligaments, and cause them to stretch, strain or even tear completely through. The ACL is important because it helps keep the knee’s range of motion in a safe and sustainable range. Without the ACL and the other ligaments that comprise the knee, the joint wouldn’t stop flexing when it reached a fully straight (or just past straight), and the ability of the leg to move the body would be seriously compromised.

ACL Injuries

Frequency and incidence of ACL injuries

While exact statistics are hard to determine, it’s estimated that somewhere between 100,000 and 200,000 ACL injuries occur each year in the United States, and that a significant portion of those injuries will result in ACL reconstructive surgery. Most ACL injuries happen while the individual is participating in a sport or some other fitness related activity. A vast majority of those injuries (approximately 75%) occur by the athlete landing or moving improperly, or trying to turn or “cut” too quickly, but without any direct contact from another player or athlete. Surprisingly, female athletes are at a significantly greater risk of ACL injury as compared to male athletes. In fact, for athletes in the same sport and roughly the same age, female athletes injure their ACLs at a rate that’s roughly 5 times greater than that of male athletes. The exact reason for the different injury rates among the genders is unclear, but it’s likely that there are a number of factors contributing to the difference. There are structural differences between male and female hips, which means that the thigh bones are generally oriented at different angles in relation to the knee. In addition, many of the most popular female sports – soccer, gymnastics and basketball – involves some of the most potentially damaging forces on the knee.

Types of ACL Injuries

ACL injuries range from relatively mild, as with a small tear in the ligament, to very severe. The most significant injuries to the ACL involve complete tears to the ligament, or a detachment of the ligament from the bone. Sometimes these detachments can also involve a piece of bone breaking off where the ligament attaches to the bone. ACL injuries are generally classified as follows:

“Grade I” ACL injuries are minor injuries that involve very small tears to or stretches of the ACL. Grade I ACL injuries will likely impact an athlete’s ability to perform in training or competition, but a non-athlete might not suffer much of a change to their quality of life.

“Grade II” ACL injuries are moderate injuries in which the ACL is partially torn. The knee can be somewhat unstable and can “give way” periodically when the injured individual attempts to stand or walk.

“Grade III” ACL injuries are those in which the ACL is completely torn. The knee will feel very unstable with a Grade III ACL injury. With a Grade III ACL injury it will be very difficult to engage in any sports activity.

The most common ACL injury is a Grade III injury, with over 75% of all diagnosed ACL injuries being at that level. This may be due to the fact that some individuals who suffer from Grade I injuries (as well as some Grade II injuries) do not seek medical assistance and are therefore not properly diagnosed.

Causes of ACL Injuries

Even though the ACL is a strong fibrous tissue, it’s still susceptible to injury. While it’s common for ACL injuries to occur to athletes during training or competition, non-athletes can also injure their ACLs in the course of an inactive lifestyle. An individual who stumbles or trips and quickly puts their leg in front of their body in an effort to regain their balance, for example, is at a risk of damaging their ACL if the force into the knee is just a bit too strong. In a sporting context the vast majority of all ACL injuries usually fall into one of three categories. First, a sudden change of direction that translates into the knee joint can tear the ACL. Any sport that involves quick stops or pivots, such as basketball, soccer, gymnastics and football, put the knee at risk. Second, any sport that involves hits or any direct contact to the knee or lower leg can put the ACL under potentially damaging stress. Football and soccer are examples of sports where this cause of ACL injury is somewhat common. Finally, hyperextending the knee beyond its normal range can put enough force on the ACL to cause it to tear. Sometimes the cause of the hyperextension is landing poorly after a jump, so these types of ACL injuries are sometimes found in basketball, skiing and gymnastics. In fact, a recent study by a major health care provider found that the sports of football, baseball, soccer, skiing and basketball account for over three-fourths of all sports-related injuries. A significant contributing factor that leads to a higher incidence of ACL injury is the use of cleated shoes during athletic training or competition. Shoes with cleats can cause an athlete’s foot to be fixed for too long while other forces act upon the knee (therefore allowing those forces to continue acting upon the knee), whereas an uncleated foot would leave the ground and not direct the forces into the knee joint. Finally, some individuals may simply have a predisposition to ACL injuries, given the makeup of their particular knee structure, as well as any preexisting weaknesses in any of the underlying tissues.

Symptoms of ACL Injuries

When an individual injures their anterior cruciate ligament, the injury will often manifest itself by exhibiting some common symptoms, regardless of the exact cause of the injury. The symptoms of an ACL injury that are most often seen include the following: The most common symptom of a significant ACL injury is for the injured individual to feel (or even hear) a “pop” coming from the knee. After the injury the knee may also “click” or “snap” each time the joint flexes or extends. At the time of injury, there will be a sharp and sudden pain that can be felt on the back or outside of the knee area. The pain might also be localized directly inside or underneath the knee itself. The pain likely will not be felt in any of the muscle tissue that surrounds the knee. Immediately after the injury, the knee will quickly swell, and will lose much of its mobility. Although painful, the swelling won’t necessarily prevent the injured individual from walking or limping on the damaged knee. In fact, some people may be able to walk around for days or weeks before getting an expert diagnosis. Unfortunately, trying to “tough it out” could wind up damaging other parts of the knee structure while the injured ACL is not addressed. After the injury the knee will feel unstable, as if it will collapse or buckle if it is fully weighted or tested. The intensity of one or more of these symptoms, however, is not a reliable indicator of the severity of the underlying ACL injury

Determining the scope an ACL injury

The symptoms listed above are only indicators of a potential ACL injury. The most prudent approach to determining whether there has been an ACL injury is to consult with a qualified doctor or sports medicine professional as soon as practicable after the injury occurs. It is possible that very minor and insignificant ACL injuries can heal themselves. This is particularly the case for children and teenagers who are not actively involved in competitive sports. But a significant ACL tear won’t repair itself. An anterior cruciate ligament that’s torn simply isn’t capable of healing itself the same way that an injured muscle can. Furthermore, when the injury to the ACL occurred there might have been damage to other parts of the knee structure (other ligaments, meniscus or other tissues) as well. Getting a professional diagnosis is therefore important. It can be very difficult, if not impossible, for an injured individual to determine whether their soreness or pain is due to ACL damage or some other reason. The extent of an ACL injury can generally be accurately diagnosed with a physical examination. Such an examination will generally involve a doctor or physical therapist placing their hands on the knee of the injured individual and feeling how the various components of the knee respond to flexion and extension within the joint. Sometimes, however, use of magnetic resonance imaging (MRI) is necessary to perform a complete and accurate diagnosis. For example, an ACL injury can cause the knee to become swollen and painful, which could make an accurate manual examination virtually impossible. Another advantage of an MRI is that the resulting imagery may help to uncover other injuries in the knee.

ACL Surgery and Reconstruction


General Info

Not every ACL injury will require surgery. Relatively minor ACL injuries might not require surgery to heal on their own. Furthermore, some individuals with ACL tears might decide to forego surgical repair even if that will mean reduced function and mobility in the knee. These are generally older patients with already low levels of activity, and who do not wish to take any of the risks of surgery (even the relatively low risks of ACL surgery), or who do not want to work through a rehabilitation program. However, individuals who are going to require their knees to bear a higher level of stress will require ACL replacement surgery if they have complete or significant ACL tears. Typically these individuals will be athletes, or will at least be used to leading very active lifestyles. In addition, even non-athletes may elect to go forward with ACL replacement surgery because even with relatively low activity levels, a damaged ACL may lead to meniscus damage within the knee or even osteoarthritis or other degenerative diseases within the knee. The primary factors in determining whether ACL reconstructive surgery is the desired course of action will include: the desire to return to the same activity levels as before the injury (including whether those activities include demanding sports such as soccer or basketball), any injuries that occurred at the same time as the ACL injury, the existing range of motion or mobility in the joint, and what the patient expects from the surgery. It’s generally common practice for surgery to be delayed for several weeks after the ACL injury, in order to allow the swelling of the knee to reduce, and to have some time to increase the range of motion of the knee joint before surgery.

ACL surgery options

With advances in medical technology over the past few decades, new surgery options have developed for ACL injuries. There are now three common options for ACL reconstructive surgery. These three surgeries are the “autograft” technique, the “allograft” technique and a hamstring graft. In the past there have been other techniques for ACL replacement (such as grafts using synthetic materials), but these have fallen out of favor in recent years. Each surgical technique has its own advantages and disadvantages. An injured athlete should weigh the pros and cons of each procedure in conjunction with a qualified medical professional before making a decision on which type of reconstructive surgery to receive. In general, the long term success rate of the various ACL reconstructive surgeries is between 80-95%.

Autograft surgery

Perhaps the most popular technique for ACL reconstructive surgery uses the “autograft” method. With the autograft technique the surgeon takes part of the patient’s own body and uses that as the ACL replacement. In order to reconstruct the ACL with this method, the surgeon removes approximately 1/3 of the patellar tendon and uses that tissue to fashion a replacement ACL. When the section of patellar tendon is removed, small pieces of bone where the tendon attaches to the patella and the tibia are removed as well. Because the patellar tendon is structurally very similar to the anterior cruciate ligament, the body has a greater chance of assimilating it without problems, and the replacement tissue will be able to handle functions that the ACL previously handled. On the downside, since autograft surgery requires small portions of the bones be removed from the patella and tibia, those bones are slightly weakened and therefore at a somewhat higher risk for damage or infection in the future.

Allograft surgery

The “allograft” technique (also known as an “allogenic transplant”) uses replacement tissue from a cadaver donor. This ACL replacement technique is favored by some surgeons and some patients because having the replacement tissue come from outside of the body means that there is no need to remove any ligaments or other tissues from the donor’s own body. This means that all other structures of the donor can retain 100% of their original strength. On the downside, tissue that comes from a cadaver generally does not have the same strength as live tissue that comes from the donor’s own body. While this might not make a difference for many individuals undergoing ACL replacement surgery, a small reduction in strength might be important enough to a competitive athlete for him or her to reject this surgical technique as a possibility. Furthermore, because the replacement tissue for an allogenic transplant comes from outside the patient’s body, there is a chance (albeit only a slight chance) that the patient will contract an infection.

Hamstring tendon

The third main surgical ACL replacement technique is for a portion of the injured individual’s hamstring tendons to be removed and combined into structure that is similar to the torn ACL. Patients who receive this type of procedure generally indicate that it is less painful then either of the other popular ACL replacement techniques. However, using this technique generally results in a longer recovery and rehabilitation time.

Rehabilitating a damaged ACL

What to expect

ACL reconstructive surgery only repairs the physical structure within the knee joint. After surgery, the function of the knee will need to be restored through a sometimes lengthy rehabilitation process. For minor ACL injuries, and for individuals who choose not to undergo surgery, the rehabilitation period will be significantly shorter.

Rehabilitation expectations and timeline.

It is important that an injured individual have realistic expectations about the rehabilitation process, in terms of both how long it will take, and how much effort will be required. While the nature and scope of rehabilitation will depend how significant the ACL injury is, and (assuming that an ACL replacement was necessary) the type of replacement procedure that was performed. With an ACL replacement, for example, rehabilitation can take up to nine months or a year, and can at times be uncomfortable for the individual. Ideally, rehabilitation should begin prior to any ACL surgery. During the preoperative period, it’s important to restore a full range of motion to the damaged knee. Making sure that the muscles which support the knee are able to work throughout the full range of motion is necessary for success in post-surgical rehabilitation. During the first two to three weeks following ACL replacement surgery, the goal will again be to establish a full range of motion throughout the knee. The ideal range of motion will be where the joint flexes up to 90°, and can extend fully (straight leg). Furthermore, steps will be taken during those initial weeks to reduce any swelling persists around the site of the surgery. The next two to three weeks of rehabilitation will focus on maintaining full (straight leg) extension, as well as increasing the flexion within the joint to well beyond 90°. During this phase exercise machinery such as stationary bicycles and stair climbers are often used. Rehabilitation may also involve walking (but not running) on a treadmill. These activities will greatly benefit the ability of the replacement ACL to return to a normal range of motion. Beginning approximately two to three months after surgery, the patient will begin to return to their former sports and athletic activities. But it may take six to nine months (or more) before the patient is able to approach their pre-surgery levels of activities. During this stage of rehabilitation, the best path back to former performance levels will involve continued work with their surgeon and a physical therapist. Expert input is necessary because the knee joint might feel completely healed, even though it’s not yet fully back to the pre-surgery strength, and athletes (particularly patients) will likely face a temptation to go back to their former activity levels too quickly.

Rehabilitation exercises.

The initial exercises for rehabilitation will focus on the very small range of motion that will be possible in the first days after surgery. Exercises that work toward the goal of full leg extension will play a heavy role. In fact, most of the exercises in the initial stages of rehabilitation will be focusing primarily on getting the leg to full extension. ACL replacement surgery can result in a buildup of scar tissue around the area of the surgery, so there will also be a protocol or rehabilitation exercises that seek to appropriately break up that scar tissue. Because ACL injuries can affect the injured individual’s proprioceptive abilities, rehabilitation exercises will also focus on regaining the natural and instinctive balance within the leg. It’s impossible (and dangerous) to have a rehabilitation exercise plan prepared just after an ACL surgery and expect to stick to that plan without modification. It may be possible that the pace of rehabilitation goes more slowly (or more quickly) than originally anticipated, so involvement of doctors and qualified sports professionals is necessary to make any adjustments.

Nonsurgical treatments.

Not every injury to the ACL will require surgery. This is generally due either to the injury itself being minor, or the injured individual opting out of surgical repair. For example, some individuals who lead sedentary lifestyles will often not see much of a decline in their quality of life after an ACL tear. Furthermore, older individuals often do not want to go through the stresses of a surgical procedure, even one as relatively non-invasive as an ACL replacement. Knee braces are sometimes an option for these patients. While medical studies have concluded that knee braces can actually restrict the proper functioning of the knee and leg, and are not a good long term solution to a compromised knee joint, many patients favor their use.