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Anterior Cruciate Ligament (ACL) Tears

What is the Anterior Cruciate Ligament (ACL)?

The ACL is a ligament situated in the middle of the knee that prevents forward motion of the tibia (shin bone) on the femur (thigh bone).  It is the primary anterior knee stabilizer.  It is closely related in location to the Posterior Cruciate Ligament (PCL), which prevents excessive backward motion of the tibia on the femur.  The ACL can be seen in the picture below, where the knee is seen from the posterior (back) side.  In this view, the PCL is in the foreground, while the ACL is partially hidden.

How is the ACL injured?

The ACL can be injured through external contact against the knee, or through non-contact mechanisms, such as with an awkward landing.  It is estimated that there are about 75,000 ACL injuries each year in the United States, with 50% of these injuries occurring in active 15 to 25 year old individuals, and 70% of all ACL injuries occurring during some form of sports participation.  A classic example of contact that can injure the ACL is a “clipping” injury, where the outer part of the knee is struck, and the knee forced inward.  This inward, or “valgus” force, puts strain on the ACL and when the force is great enough, can cause a tear.  Non-contact injuries usually occur when an athlete lands awkwardly while coming down from a jump.  Often these athletes land with the knee relatively straight rather than flexed (bent), and the impact forces of landing can force the knee into both valgus and rotation, straining the ACL and increasing the risk of tear.  Often the quadriceps (anterior thigh) muscles become involved in the injury, by contracting excessively in relation to the hamstring muscles, and causing a sudden forward thrust of the tibia on the femur when the ACL is already tightened into an at-risk position.

What are the long and short-term effects from an ACL injury?

Studies seem to indicate that people who have torn their ACL automatically have a higher chance of developing knee arthritis than people who have never injured this ligament, regardless of whether surgical repair takes place.  This is because the joint surface itself is often injured at the same time that the ACL is torn, as the bones in the knee are violently thrust against one another.  If injury to the meniscal cartilage in the knee occurs as well, the long-term risks increase.

In the short-term, the disability that comes with an ACL-deficient knee can vary, but often makes sports participation difficult.  Sports that involve lots of sudden stops from a forward-directed run, cutting maneuvers, or jumping and landing become difficult to participate in without experiencing an ongoing sense of “shifting” or “giving way” in the knee, whereby the portion of the leg below the knee feels as if it is moving independently of the portion of the leg above the knee.  This extra joint motion is often accompanied by pain or aching with or after activity, intermittent swelling, and a higher ongoing chance for further knee injury, such as meniscal tears.

What are the risk factors for ACL injuries?

•    Sports participation itself is a risk, with the incidence of ACL injury varying by type of sport (higher in football than golf).

•    Female athletes as a group have a much higher risk for ACL injury than their male peers.  For instance, female soccer players have an approximately four-fold greater risk than male soccer players.

•    High friction of shoe to playing surface (more grip of the shoe on the field or court) probably is a risk factor.

•    Hormonal factors associated with the female menstrual cycle may play a role, but research has not clearly defined this as of yet.

•    As stated above, awkward landings or sudden stops put an athlete at risk for ACL injury.  Females as a group may have muscle weakness patterns in the hip/thigh and leg biomechanics that lead to different landing patterns that put them at higher risk relative to their male peers.

What about “partial” ACL injuries?

The ACL is like a rope that is anchored into two bones that at times are moving away from each other at high speeds.  For the most part, this “rope” needs to be intact in order to reliably withstand the forces that it regularly meets during sports participation.  Just as mountain climbing with a rope that is fraying is not optimal, neither is sports participation with an ACL that is partially torn.  Studies that have attempted to quantify how much of the ACL can be torn and still allow tolerable sports participations seem to indicate that less then 20% of the width of the structure can be lost before replacement becomes the best option.  Of course, our abilities to estimate ACL injury to this level of detail, even with MRI, are limited.  Therefore, athletes with partial injuries that wish to attempt return into sports need to be watched carefully, strengthened optimally, and have risks fully explained to them before further participation is allowed.

What is involved with the surgery to “fix” a torn ACL?

Torn ACL’s cannot be repaired, since the ligament never regains it’s normal strength again, even if stitched back together.  Instead, a new ligament must be placed into the area where the old one used to be.  This new ligament can be obtained in one of two ways, either through use of a cadaver graft, or through the use of the injured patient’s own tendon tissue as a substitute.  Typically, the patellar tendon or a portion of the hamstring tendons are used for this.  Using the body’s own tissue removes the risk of transmission of infection, which remains a very small, but real risk of cadaver grafting.  The advantage to use of cadaver grafting is a smoother post-operative rehab course, as less damage to tissues has occurred to the injured patient in the process of treatment.

Surgery to replace the ACL is very effective at restoring stability to the knee for sports participation.  It probably reduces the ongoing risks for development of knee arthritis by reducing the risk for meniscal tears away from the injury.  It does not remove the higher risk of knee arthritis completely.

Older athletes who sustain an ACL injury are usually good candidates for repair, but should be assessed on an individual basis, with attention paid to the baseline condition of the knee prior to the injury, the overall activity level of the patient, and the type of activity that individual participates in.  For instance, a person who primarily runs for exercise may tolerate an ACL-deficient knee quite well as long as adequate muscle strength is maintained in the legs and core.  Bracing may or may not be of benefit in some of these patients to minimize the sense of functional instability that sports participation brings out.

Can ACL injuries be prevented?

Much research has been done in the last 15 years trying to answer this question with respect to non-contact injuries.  Much of the direction of this research has been towards development of pre-season training programs that attempt to minimize an athlete’s individual risk for ACL injury by improving leg strength and endurance, overall balance, and improvement of landing techniques.  Generally, these programs have shown positive results, with lower incidences of injury in program participants than in matched controls.  These programs generally average six week in duration, with some maintenance exercise as the season progresses.

Physicians who are knowledgeable in sport-related biomechanics (such as sports medicine physicians) also can screen athletes on an individual basis before the season starts, in order to identify individuals whose leg biomechanics or strength imbalances might put them at-risk for injury.  These individuals can then be targeted for specific intervention as described above.

If you are interested in finding out more about ACL prevention programs, please contact us via our website.


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