The Anterior cruciate ligament (ACL) is one of several ligaments in the knee.  Like all ligaments it functions to keep the knee stable during activities.  Injury to the ACL is a surprisingly common occurrence.  In the United States alone the reported rate of ligament rupture approaches 1 injury for each 3,000 people.  ACL reconstruction is also common with approximately 300,000 performed each year. A better understanding of mechanism, physical exam findings, and more accurate imaging has led to increased rates of diagnosis and improved treatment options.

Mechanism

ACL injury generally occurs when the knee pivots (twists) awkwardly while the foot remains planted.  Although we often associate this injury with direct trauma while playing a sport, it can just as commonly occur as a misstep on uneven terrain or a slip on a wet floor. Those ACL injuries that do happen during sports occur most frequently in noncontact athletic situations where there is not a collision or direct blow to the knee.  In these situations the knee buckles during a sudden change in direction and therefore essentially any athletic endeavor can be associated with an ACL injury.  Soccer and basketball are among the highest non-contact incident sports.

Another misconception is that the injury occurs mostly to young men.  Actually females have a much higher rate of injury compared to their male counterparts. Some studies have shown injury rates 4 to 6 times higher in young ladies.  There are many reasons for this discrepancy, but new data demonstrates that supervised injury prevention programs can reduce this rate.  When ACL tears occur the patients often describe a twisting or hyperextension of the knee that elicits an audible “pop”. This is generally followed by progressive swelling, pain, and sometimes difficulty walking.

Physical exam

Performing a proper physical exam is a key component to diagnosing ligament injuries.  When there is an ACL injury, the knee translates (slides) abnormally.  One exam finding used to illicit this difference is the Lachman’s test.  With the knee slightly bent, the examiner gently slides the tibia (shin) forward relative to the knee.  If the ACL is torn, the tibia will slide forward further than normal because the ACL is no longer able to prevent this motion.  A more accurate finding on examination is the pivot shift.  This test challenges both the sliding and rotation that the ACL is designed to counteract.  A positive pivot shift finding is the hallmark of ACL rupture.  During a thorough physical exam the physician will compare the injured knee to the uninjured knee.  This can be useful to understand the baseline motion for each patient.  It is especially important to compare the injured knee to the uninjured knee in adolescents who often have a slightly more lax (looser feeling) knee compared to adults.  Magnetic Resonance Imaging (MRI) confirms the diagnosis when there is a high degree of suspicion and ensures there is no other associated damage. Meniscus tear, surface cartilage injury, and other ligament injuries commonly occur with ACL rupture.

Indications

Without the anterior cruciate ligament the knee has difficulty resisting sudden changes in motion/direction and pivots (twisting and sliding) abnormally. When an ACL deficient knee pivots, meniscus cartilage (cushion) and surface cartilage are jeopardized. Many studies have shown that repeated pivoting increases the rate of both surface damage and meniscus tears. For active individuals this sliding prevents them from participating in their athletic endeavors, but it also means that even moderately active patients can further damage their knee during routine activities.  The older, more sedentary patient that does not subject themselves to these repetitive stresses may elect to undergo conservative care emphasizing hamstring and quadriceps strengthening to try to minimize this risk.

Surgical Techniques

The goal of ACL reconstruction is to replace the torn ligament with a tissue that mimics the size and orientation of the original ligament. The techniques used for reconstruction have evolved significantly.  Originally the procedure was performed with a long open incision.  This became two smaller incisions and is now often performed with an arthroscope (tiny camera) and a single very small incision.  The source of the new ligament material (graft) is an important discussion between and physician and patient.  Graft can be harvested from the patient (autograft) utilizing the central portion of the patellar tendon, part of the hamstring tendon complex, or occasionally a part of the quadriceps tendon. Allograft (donor) tissues are also available if the patient requires additional sources due to age, repeat injury, or preference.

The procedure is generally performed on an outpatient basis so a hospital stay is not required.  An appropriately supervised physical therapy regimen emphasizing early range of motion begins shortly afterward.  As motion and strength return, the therapy progressively provides new challenges that help ready the knee for returning to pre-injury performance.  ACL injuries are unique to each individual and return to specific activities is monitored by all members of the treatment team. 

Orthopaedic Associates is a proud provider of cutting edge technology for ligament injuries.  All ligament surgeons are members of the American Academy of Sports Medicine (AOSSM).  The AOSSM is the leading sports injury and reconstruction society in orthopaedic surgery.  Our physicians continuously update their knowledge with specialty training courses and some even teach these courses to other surgeons.  Orthopaedic Associates is also beginning studies to monitor our outcomes after ligament reconstruction utilizing information gathered from our new electronic medical record system.  This new tool will further help us provide the best opportunity for our patients and families to have an excellent outcome. 

 

Author: Douglas Lowery, M.D.

February 23, 2012