What Does the Anterior Cruciate Ligament Do?

The knee joint has 4 main ligaments stabilizing it: (1) the anterior cruciate ligament (ACL), (2) the posterior cruciate ligament (PCL),  (3) the fibular collateral or lateral collateral ligament (LCL), and (4) the medial collateral ligament (MCL). The ACL and PCL are intraarticular or within the knee joint capsule where they are bathed in joint fluid. The LCL and MCL are extraarticular or outside the knee joint where there is better blood supply. The ACL prevents the tibia from anterior translation relative to the femur as well as providing some rotational control. The PCL prevents the tibia bone from posterior translation relative to the femur. The MCL prevents the knee from falling into a valgus or knock-kneed stance. The LCL prevents the knee from falling into a varus or bowlegged stance.

What Causes an ACL Rupture?

An anterior cruciate ligament rupture is a complete tear of the ACL ligament. An ACL rupture results from a severe injury to the knee. ACL’s usually tear in the mid-substance or mid portion of the ligament. However, the ACL can also tear by peeling away from where it normally attaches to the femur. In other cases, the ACL may prove stronger than the bone and pull away a small island of bone onto which it attaches with the ligament itself remaining intact.

ACL ruptures occur due to excessive energy loads placed upon ligament. This commonly occurs in several ways:

• Poor landing mechanics
• Sudden redirection of motion
• High energy pivoting or lateral maneuver
• An immediate stop while running
• Collision with another person or object while in a compromised knee position

Females are more prone to ACL ruptures than their male counterparts. This may be due to differences in their alignment resulting from the angle of their hips, landing mechanics, hormonal factors, and other anatomical and biological differences.

How Does a Torn ACL Occur?

ACL injury often occur as a result of sporting activities and other high-energy mechanisms. This event creates a stretch of the ligament that exceeds its tensile strength causing a tear. Along with factors mentioned above, an ACL tear can occur with:

• Inadequate core body strength
• Lower extremity musculature imbalances
• Inadequate hip muscle strength
• Use of incorrect or worn-out equipment
• Sub-standard footwear
• Uneven or artificial playing surfaces

What are the Different Grade ACL Tears?

The amount of laxity of an injured ACL is graded from mild to severe on a scale of 0 to 3. A grade 1 injury is a sprain of the ligament with stretching and micro tearing.

Usually in these cases, the knee maintains enough stability such that the injury can be treated without surgery. With grade 2 laxity, there is partial tearing of the ligament and moderate instability. In a grade 3 tear, there is complete disruption of the ligament. Grade 3 tears usually result in the loss of knee stability that necessitates surgical management with an ACL repair or reconstruction. Due to the nature of the tensile strength of the ACL, it is more common for patients to sustain a grade 3, full thickness rupture.

How to Test for an ACL Rupture?

In the clinic, Dr. Cunningham and his team will obtain a complete history of how the injury occurred. Often, patients report a significant injury to the knee and report hearing or feeling a “pop” in the knee. There is immediate pain and swelling. The knee then feels unstable when trying to weight bear on it. The knee feels tight and it is difficult to fully straighten and bend the knee.

After a history is obtained, Dr. Cunningham performs a gentle physical exam of the knee. The knee is assessed for swelling. The knee is gently palpated to see if there are any tender areas. Range of motion is checked. All the ligaments of the knee are then gently stressed to determine if they are intact.

A Lachman’s test is performed to gently stress the ACL to determine its integrity. During this test, the patient will be lying down. The thigh (femur) will be gently stabilized by the examiner and the lower leg (tibia) will be gently translated forward. This motion is normally prevented by an intact ACL. The degree of translation compared to the contralateral side (given that the contralateral side has an intact ACL ligament) indicates the severity or grade of the injury.

• Grade I – mild. 1 – 5 mm of increased anterior tibial translation
• Grade II – moderate. 5 – 10 mm of increased anterior tibial translation
• Grade III – severe. More than 10 mm of increased anterior tibial translation

The accuracy of this test can be compromised in the setting of an acute injury where there is a lot of knee swelling and muscle guarding. During the test, it is best if the patient can be as relaxed as possible as muscle guarding can produce a false negative.

After a physical exam is performed, xrays are obtained to be sure that there is no fracture, arthritis, malalignment, or other problems within the knee. Treating a displaced fracture will take precedence over treating an ACL tear as the bone has to be uncompromised to do ACL surgery.

Finally, if an ACL tear is suspected, an MRI will be ordered. An MRI shows all the ligaments, cartilage, menisci and tendons about the knee as well as showing the bone.

Does an ACL Tear Always Require Surgery?

An complete anterior cruciate ligament tear usually requires surgical treatment in young, active patients. In older patients who do not participate in cutting and pivoting sports, surgery is not typically required as these patients do not experience symptomatic instability of their knee with their normal activities. One does not need an intact ACL to walk or ride a bike. However, if one wishes to resume skiing or playing soccer or tennis, then most patients with an ACL tear will feel their knee give way on a regular basis if they were to participate in these sports with a torn ACL. Knee braces are not effective in sufficiently stabilizing the knee after the ACL has been torn in most people, as one will still feel the knee shift out of place even while wearing a knee brace.

Prior to ACL surgery, most patients are sent to at least a few visits of physical therapy to help reduce swelling and restore their normal gait and knee range of motion. Once patients have achieved a knee that is minimally swollen, has full knee extension (straightening) and nearly full knee flexion (bend) and they are off crutches and walking with a normal gait, then ACL surgery can be undertaken. This will involve either an ACL reconstruction or an ACL repair. Dr. Cunningham is an ACL doctor at Vail-Summit Orthopedics & Neurosurgery who specializes in the treatment of ACL injuries.