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St. Mary's Center for Orthopaedics

ACL (Anterior Cruciate Ligament) Injuries

What is the Anterior Cruciate Ligament (ACL)?

The ACL (anterior cruciate ligament) may be the most important ligament in the knee. Ligaments are strong non-elastic fibers that connect our bones together. The ACL, which is located in the center of the knee, connects the thighbone (femur) to the shinbone (tibia). The main function of the ACL is to stop the forward movement of the shinbone on the thighbone.

The ACL is one of the most commonly injured ligaments in the knee.  The ACL, along with the posterior cruciate ligament (PCL), medial collateral ligament (MCL), and menisci (cartilages), helps keep the knee stable and helps protect the knee from shifting, rotating, and hyper-extending during running, jumping, or landing.

What is an Anterior Cruciate Ligament (ACL) Injury?

The anterior cruciate ligament can be injured in several ways:

  • Changing direction rapidly
  • Stopping suddenly
  • Slowing down while running
  • Landing from a jump incorrectly
  • Direct contact or collision, such as a football tackle

The ACL can tear during strong twisting motions of the knee.  The ACL can also tear if the knee is hyper-extended or bent backwards.  People frequently tear the ACL while pivoting, landing awkwardly from a jump, changing directions suddenly, or abruptly slowing down from running.  ACL tears occur most frequently in young athletes.  Football, basketball, skiing, and soccer are sports associated with the highest injury rates.

It is common for additional injuries to result when an ACL tear occurs. Surrounding structures, such as the meniscus, cartilage, and ligaments can be injured as well.  Some people may also experience bruised or broken bones. Injuries range from mild, such as a small tear, to severe, such as when the ligament tears completely or when the ligament and part of the bone separate from the rest of the bone.

Without treatment, the injured ACL is less able to control knee movement, and the bones are more likely to rub against each other. This is called chronic ACL deficiency.

The three grades of ACL injury range from mild to severe:

  • Grade I – Trauma to the ligament is relatively minor. Some of the fibers are stretched. This is considered a “sprain”.
  • Grade II – Trauma to the ligament is more severe. Some of the fibers are torn. This is called a “partial tear”.
  • Grade III – This is the most severe ACL injury. The fibers of the ligament are completely torn. It is referred to as a “complete tear”.

Partial tears of the anterior cruciate ligament are rare; most ACL injuries are complete or near complete tears.

What are the Symptoms of an ACL Injury?

When you injure your anterior cruciate ligament, you might hear a “popping” noise and you may feel your knee give out from under you. Other typical symptoms include:

  • Pain on the outside and back of the knee.
  • The knee swelling within the first few hours of the injury. This may be a sign of bleeding inside the knee joint. Swelling that occurs suddenly is usually a sign of a serious knee injury.
  • Limited knee movement because of pain or swelling or both.
  • The knee feeling unstable, buckling, or giving out.

After an acute injury, you will probably have to stop whatever you are doing because of the pain, but you may be able to walk.

How is an ACL Injury Diagnosed?

An individual should seek medical care if they have a knee injury with a pop, swelling within 24 hours, restricted knee motion, or instability in the knee. Inability to run, jump, pivot, or change directions should also warrant medical evaluation.  The physician will use a combination of the following to make a diagnosis:

  • Medical history – You will be asked how you injured your knee, about your symptoms at the time of injury, whether you have had any other knee injuries, and general questions about your health.
  • Knee examination – check for stability, strength, range of movement, swelling, and tenderness. Tests for stability include a Lachman test and a pivot shift test. The Lachman test compares the degree of looseness (laxity) in your knees.
  • X-rays – usually done for any knee injury if there is pain, swelling, or you cannot put your weight on the leg. Although an ACL injury cannot be directly diagnosed by an X-ray, an X-ray can show broken bones, bone fragments in the knee, if the ACL is torn from the bone (avulsion), or if blood is present in the knee (effusion).
  • MRI – can show damage to ligaments, tendons, muscles, and knee cartilage.
  • Arthroscopy – Using arthroscopy, the surgeon is able to look at the inside of the knee.

What are the Treatment Options for ACL Injuries?

Initially following an injury, your knee will be treated with rest, ice, compression, and elevation.  You should rest your knee by not placing weight on it.  You may use crutches to help you walk.  Applying ice packs to your knee can help reduce pain and swelling.  You should apply ice immediately after injuring your knee.  Your physician will provide you with a continued icing schedule.  Your physician may provide over-the-counter or prescription pain medication.  In some cases, a knee brace may be recommended to immobilize and support the knee.  A knee immobilizer is used for only a short period of time. Elevating your knee at a level above your heart helps to reduce swelling.

Treatment for ACL tears is very individualized.  Many factors need to be considered, such as your activity level, severity of injury, and degree of knee instability.  Treatments may include physical therapy, surgery, or a combination of both.  The most likely candidates for non-surgical treatments have partial ACL tears without knee instability, complete tears without knee instability, sedentary lifestyles or are willing to give up high-demand sports, or are children whose knees are still developing.

Physical therapy and rehabilitation can help restore knee functioning for some individuals.  Your physical therapist will help you strengthen your knee.  Special emphasis is placed on exercising the quadriceps muscles on the front of the thigh and the hamstring muscles on the back of the thigh.  Eventually, you will learn exercises to improve your balance and coordination.  You may need to wear a knee brace during activities.  Your therapists will educate you on how to prevent further injury.

What type of other treatment you have depends on:

  • How much of your ACL is torn (whether it is a grade I, II, or III injury).
  • When the injury occurred and how stable your knee is.
  • Whether other parts of the knee are injured. If they are, it will be harder for the strong parts of your knee to compensate and protect the injured parts.
  • Whether you had other knee problems before, such as injuries that caused long-term (chronic) ACL deficiency, or osteoarthritis.
  • How active you are.
  • Your age and overall health.
  • Your willingness and ability to complete a long and rigorous rehab.

Surgical Treatment

Surgical treatment is most frequently recommended for individuals with ACL tears accompanied with other injuries.  The most likely candidates for surgical treatment are active individuals in sports or jobs with heavy manual work that requires pivoting or pushing off with the knee.  Surgery is also recommended for people with unstable knees or injuries combined with damage to the menisci, articular cartilage, joint capsule, or ligaments.

Prior to surgery, most people participate in physical therapy. Swelling can make the knee stiff.  Immobility can cause the muscles and ligaments to shorten.  Your physical therapist will help you stretch your knee to regain full movement.  If your collateral ligaments are involved, you may need to wear a brace to allow them to heal prior to your surgery.  These steps will help you prepare for a successful recovery after your surgery.

Surgery to rebuild an anterior cruciate ligament is done with an arthroscope using small incisions. Arthroscopic surgery is less invasive. The benefits of less invasive techniques include less pain from surgery, less time spent in the hospital, and quicker recovery times.

Unless ACL reconstruction is treatment for a combined ligament injury, it is usually not done right away. This delay gives the inflammation a chance to resolve, and allows a return of motion before surgery. Performing an ACL reconstruction too early greatly increases the risk of arthrofibrosis, or scar forming in the joint, which would risk a loss of knee motion.

The goal of ACL repair is to reconstruct your knee joint to restore its function and stability, and prevent further injury.  During surgery, your surgeon will replace your damaged ACL with a healthy tendon, called a graft.  There are several options for acquiring grafts.  They may be taken from an area near your knee or from a cadaver donor.

During ACL surgery, the surgeon may repair other injured parts of the knee as well:

  • Patellar Tendon Autograft uses the middle third of the patellar tendon and bone plugs from the shin and kneecap.  This type of reconstruction is most often recommended for high-demand athletes and individuals that do not have to perform a lot of kneeling activities.  This grafting procedure has been considered the “gold standard” for ACL repair.
  • Hamstring Tendon Autograft uses one or two tendons from the hamstring muscles at the inner side of the knee.   The Hamstring Tendon Autograft is most appropriate for lighter-weight individuals with a small patella bone and a history of pain.  This method can be associated with a faster recovery.
  • Quadriceps Tendon Autograft uses the middle third of the quadricep tendon and a bone plug from the upper end of the kneecap.  The quadricep graft is large.  It is most appropriate for taller and heavier individuals.  It is also used for individuals with prior failed ACL reconstructions.  Because it is a large graft, this method uses a larger incision.

Allografts are tendon grafts taken from cadaver donors.  Allografts are most appropriate for older individuals that are moderately active or those with a history of pain. It is also used for individuals with prior failed ACL reconstructions, those attempting to return to sports more quickly, and those that need more than one ligament reconstructed.  Because the graft is not taken from the individual, this method is associated with less pain, smaller incisions, and a shorter surgery time.

Many ACL reconstructions are performed as outpatient procedures.  You can be anesthetized for surgery or receive a nerve block to numb your knee and leg area.  After you have received your anesthesia and your leg is relaxed, your doctor will examine your knee by performing similar tests that were done in your clinical examination.  This provides your doctor with more information about your knee and helps to formulate the surgical plan.

What Are there Risks in ACL Reconstruction Surgery?

ACL reconstruction surgery is generally safe. Complications that may arise from surgery or during rehabilitation (rehab) and recovery include:

  • Problems related to the surgery itself. These are uncommon but may include:
  • Numbness in the surgical scar area.
  • Infection in the surgical incisions.
  • Damage to structures, nerves, or blood vessels around and in the knee.
  • Blood clots in the leg.
  • The usual risks of anesthesia.
  • Problems with the graft tendon (loosening, stretching, reinjury, or scar tissue). The screws that attach the graft to the leg bones may cause problems and require removal.
  • Limited range of motion, usually at the extremes. For example, you may not be able to completely straighten or bend your leg as far as the other leg. This is uncommon, and sometimes another surgery or manipulation under anesthesia can help
  • Grating of the kneecap (crepitus) as it moves against the lower end of the thighbone (femur), which may develop in people who did not have it before surgery. This may be painful and may limit your athletic performance.
  • Pain, when kneeling, at the site where the tendon graft was taken from the patellar tendon or at the site on the lower leg bone (tibia) where a hamstring or patellar tendon graft is attached.
  • Repeat injury to the graft (just like the original ligament). Repeat surgery is more complicated and less successful than the first surgery.

Recovery from ACL Surgery

  • Medication – You will most likely go home on the same day of your surgery.  You will receive pain medication to make you feel as comfortable as possible.  In some cases, ice is applied to the knee throughout the day to help to reduce pain and swelling.  Your surgeon may prescribe blood thinning medication and special support stockings.  You should keep your leg elevated and move or pump your foot and ankle.
  • Continuous Passive Motion (CPM) Machine – In some cases, surgeons prescribe compression boots.  Compression boots are inflatable leg coverings that are attached to a machine.  They work to gently squeeze your legs to aid blood circulation.  A Continuous Passive Motion (CPM) Machine will move your leg in a cycling motion while you are in bed.  The CMP machine is helpful to improve circulation, decrease swelling, and restore movement in your knee.
  • Physical Therapy – Walking and knee movements are very important to your recovery.   Exercising will begin immediately after your surgery.  You will begin physical therapy soon after your surgery. Your first goals will include straightening your knee and strengthening your quadriceps muscles. You will also learn ways to exercise to further strengthen your quadriceps and hamstring muscles and regain balance and coordination.  It can take up to four to six months to restore proprioception and coordinated leg movements.
  • Occupational Therapy – The occupational therapist can show you ways to dress and bathe within your movement restrictions.  Your therapists can also recommend durable medical equipment for your home, such as a raised toilet seat or a shower chair.  The equipment may make it easier for you to take care of yourself as you heal and help to prevent further injury.

The success of your surgery will depend, in part, on how well you follow your home care instructions and participate in exercise during the weeks following your ACL reconstruction.

Recovery times differ depending on the severity of your injury, the type of procedure that you had, and your health at the time of your injury.  Your surgeon will let you know what to expect.  Generally, you should be able to resume some of your regular activities in one to three weeks after your procedure and progress to full activity in about six months.  Overall, you should notice a steady improvement in your strength and endurance over the next six to twelve months.  The majority of people are able to resume functional activities after ACL reconstruction.


The best way to prevent anterior cruciate ligament (ACL) injuries is to stretch and strengthen the leg muscles, especially the front and back muscles of the thigh (quadriceps and hamstrings).

You may help prevent ACL injuries if you:

  • Avoid wearing shoes with cleats in contact sports.
  • Avoid wearing high-heeled shoes.
  • Avoid sports or other activities that involve lots of twisting and contact.

If you have already had an ACL injury, you can avoid another one by:

  • Strengthening the injured knee through rehabilitation (rehab) exercises.
  • Changing your activities and/or sports techniques to avoid motions that might stress the injured knee.
  • Changing your lifestyle to avoid sports that have a high risk of injuring your knee further, such as skiing, football, soccer, or basketball.
  • Wearing a knee brace during high-risk activities. But braces should be used only if rehab is also being done. Wearing a brace alone may be of little benefit and may give you a false sense of security.