UW Health Sports Medicine 

Sports Medicine Corner: Why are ACL Injuries So Bad?

ON WISCONSIN The structure of the knee, as depicted in Gray's Anatomy
The structure of the knee, as depicted in Gray's Anatomy

May 12, 2010

Turf toe? What in the world is turf toe? University of Wisconsin graduate assitant athletic trainer Ryan Dean answers questions many fans have about common sports injuries in a new series on UWBadgers.com, The Sports Medicine Corner. In part one of the series, Dean examines the dreaded ACL injury.

-- In the world of athletics, no phrase or term can strike more fear in an athlete, coach or fan than three simple letters: A.C.L.  Anyone that follows college or professional sports is almost certain to have heard that terminology being thrown around.  

While many fans don't know much, if anything, about the ACL, one thing that seems to be common knowledge: that an ACL injury is bad news.

But what is an ACL, and why is injuring it such a bad thing?  Why does an ACL injury knock your favorite player out for a season when a sprained ankle would only sideline them for a few weeks?  And how can some players continue to play on an injured ACL while some head straight for the operating table?

What is it? 
The Anterior Cruciate Ligament, or ACL, is one of the four major ligaments of the knee joint.  

Where is it? 
The ACL is located on the inside of the knee joint, with one end attaching to the back side of the femur (thigh bone) with the other end attaching to the front of the tibia (lower leg/shin bone).  

Why is it important?  The purpose of the ACL is to keep the lower leg from sliding in a forward direction (the PCL keeps it from sliding backwards). Most importantly, the ACL aids in the stability of the knee joint, preventing it from shifting during quick side-to-side motion such as shuffling, sidestepping, or cutting.  

Without an ACL, the knee is very unstable (and weak) during lateral movement. This is why some players that depend on quick, rapid side movements can't play with an injured ACL, while others who move mainly forward and backward with less lateral motion have a better chance to wear a brace and continue play.

Who's at risk? 
Athletes involved in contact sports are at a high risk of suffering an ACL injury.  Also, any sport that involves high-velocity movement (such as start-and-stop running and cutting) can pose a risk.  

Some research has suggested that female athletes are at higher risk than their male counterparts.  This is attributed to several reasons, including differing hormone levels, different jumping and landing mechanics, and anatomical differences which can lead to an increased angle at the knee joint that yields sharper forces at the knee than in males athletes.

How is it injured?  The common mechanisms for an ACL injury are a direct blow to the leg or an abrupt twisting of the leg. When a blow to the outside of the leg forces the knee inwards, the force generated at the knee joint causes the inside of the knee to "open" (like a hinge), which can damage the ACL as well as the MCL (medial collateral ligament) and the meniscus (knee cartilage).  

Non-contact mechanisms exist where the knee endures a twisting motion that can cause the ACL to be damaged by stretching the ligament past its limit leading to tearing or rupture.  Further causes of injury may be hyperextension, a blow to the back of the knee, or the use of improper mechanics when running, jumping, etc.

Then what?  When an ACL injury is suspected, an athlete may undergo an MRI scan to confirm.  The MRI will determine if the ACL is intact, partially torn or completely torn. If significantly or completely torn, the athlete may have to undergo orthopedic surgery to repair the ligament.  

The surgery consists of taking a "graft" from the patellar tendon (the band right below the kneecap), the hamstrings, or a cadaver ligament and replacing the damaged ACL with this graft.  

How long?  Following surgery, it's time for the rehab. Recovery from this type of injury can vary anywhere from three months up to a year.  While a three-month recovery is rare, the typical time out of action is around 6-9 months. The length of the recovery depends on a long list of factors such as type of graft used, the sport played, leg strength and range of knee motion.

Rehab  The athlete will start rehab 1-3 days after surgery. The rehab starts with simple actions such as increasing range of motion (the day after surgery, the athlete can barely bend the knee) and muscle strengthening (the day after, they will be physically unable to lift their leg).  Gradually, the patient will work up to weight-bearing exercises and, finally, sport-specific activities (like cutting in football or jumping in basketball).

Who Had It?
  Lots of professional athletes have incurred ACL injuries including golfer Tiger Woods, former Wisconsin and current Green Bay Packers offensive lineman Mark Tauscher, as well as Tauscher's teammate, Al Harris.

The National Football Post followed Harris' rehab in a four-part video series: Part 1 | Part 2 | Part 3 | Part 4

For more on ACL injuries and treatment: McNicholas Knee Clinic

Ryan Dean
UW Sports Medicine

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