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The anterior cruciate ligament (ACL) is one of the knee joint’s four major ligaments, which work together to coordinate function and promote stability. A ligament connects one bone to another. Running from the back of the femur (thighbone) to the front of the tibia (the larger long bone of the calf), the ACL prevents forward movement of the tibia and provides rotational stability to the knee.

ACL injuries commonly occur during exercise or sports, most often during a noncontact event, such as a quick change of direction in soccer. A smaller percentage of ACL injuries result from direct contact with another player or object. Several studies have shown that females have a higher incidence of this injury than males, especially in certain sports. It’s been proposed that this is because of differences in physical conditioning, muscular strength, neuromuscular control and bone alignment, as well as women’s looser ligaments and the effects of estrogen on ligament properties.

The classic ACL injury occurs when decelerating, stopping suddenly, twisting or jumping. People often hear or feel a pop at the moment of injury and sometimes experience a brief hyperextension of the knee joint. Immediately afterward, some people are able to continue their activity, but most are not. A few hours later, the knee will be considerably swollen.

ACL Injury Signs and Symptoms
Patients with ACL tears often:
Hear or feel a pop at the moment of injury
Experience immediate swelling
Feel as though their knee will give out or is unstable, especially when turning, pivoting, sidestepping and going downstairs
Experience lingering instability of the knee

ACL Injury Diagnosis

To detect an ACL injury, doctors examine the knee for range of motion, swelling, stability and ligament integrity, and assess lower body strength. X-rays may be used to check for injuries to bone, and an MRI scan can help with assessing the injury’s severity and detecting associated problems, such as sprains of other knee ligaments, meniscus tears or articular cartilage damage.

ACL Injury Treatments

Most ACL tears are treated with reconstructive surgery, but depending on the patient’s age, degree of injury, activity level and recovery goals, a nonsurgical approach is sometimes chosen. Patients choosing between these options will benefit from a collaborative decision-making process with their doctors and physical therapists.

For patients who aren’t very physically active or don’t participate in sports that involve quick changes in direction, a physical therapy treatment plan (instead of ACL reconstruction surgery) is a reasonable option, as the injury may not interfere with their daily activities. The regimen takes six to 10 months and can restore knee stability to a point where it’s close to the pre-injury state. Using a hinged knee brace (ACL functional brace) can help prevent episodes of instability during this period and support a safe return to low-stress sporting activities.

Unfortunately, many people who decide against surgery may experience a secondary injury to the knee due to repetitive instability episodes and altered biomechanics. Patients who have had an ACL tear that do not have ACL reconstruction surgery may be at a higher risk of further injury to the meniscus and cartilage. Rarely, some people with an ACL tear are able to participate in sports without symptoms of instability. This variability is related to the severity of the original knee injury, as well as to the specific activities an individual usually engages in.

Surgical treatment

The goal of surgical treatment is to restore stability to the knee. After a complete ACL tear, most patients can’t participate in sports that involve pivoting or quick direction changes, and some experience instability even during everyday activities such as walking. Athletes can generally return to unrestricted activity and competitive sports 9 to 12 months after surgery, having completed an appropriate physical therapy program.

After the ACL injury and before any surgical treatment, patients may work with a physical therapist to restore motion and strength at the knee. Patients who have a stiff, swollen knee lacking full range of motion at the time of ACL surgery may have significant problems regaining motion after surgery due to scar tissue formation. It usually takes three or more weeks from the time of injury to achieve full range of motion. Injuries to other knee ligaments that occur along with the ACL injury – especially injury to the medial collateral ligament – may need to be braced and allowed to heal prior to ACL surgery.

The surgery to treat ACL injuries is called an arthroscopic ACL reconstruction. An arthroscope is an endoscope for use in joints, and in this minimally invasive procedure, the surgeon uses it to see inside the knee and pass surgical instruments through very small incisions. The procedure typically takes less than two hours but may be longer if the patient also has cartilage or meniscus injuries that require repair. Patients are usually under general anesthesia (completely asleep) and are given a nerve block (an injection that interrupts pain signals in the area and lessens post-op pain).

ACL tears aren’t usually repaired by simply sewing the ligament’s torn ends back together, because these repairs have been shown to fail over time. Therefore, the torn ACL is replaced or “reconstructed” using a graft (living tissue that will foster new growth) from one of the following sources:

  • Autograft (the patient’s own tissues)
    • Patellar tendon
    • Hamstring tendon
    • Quadriceps tendon
  • Allograft (cadaver tissues)
    • Patellar tendon
    • Achilles tendon
    • Hamstring tendon
    • Tibialis tendon

Patients undergoing an autograft ACL reconstruction will have an additional incision, made to remove tissue from one of their own tendons. After surgery, patients return home the same day and use crutches initially and a knee brace for up to six weeks. Stitches are removed one to two weeks after surgery, and a physical therapy program is initiated around this time. Participation in physical therapy is key to ensuring a full recovery that includes getting back to sports and other physical activities.


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