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Injury Breakdown: Adrian Peterson And The Unhappy Triad

Running back Adrian Peterson #28 of the Minnesota Vikings is helped off the field after being injured in the third quarter against the Washington Redskins at FedEx Field on December 24, 2011 in Landover, Maryland. (Photo by Patrick Smith/Getty Images)

Running back Adrian Peterson #28 of the Minnesota Vikings is helped off the field after being injured in the third quarter against the Washington Redskins at FedEx Field on December 24, 2011 in Landover, Maryland. (Photo by Patrick Smith/Getty Images)

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By Abby Sims
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Adrian Peterson, 26-year-old Minnesota Vikings’ running back, went down last Saturday with what is sometimes called an “Unhappy Triad”. The “unhappy” part is obvious. The “triad” is, because the injury represents a triple threat, with tears of the anterior cruciate ligament (ACL), medial collateral ligament (MCL), and medial (innermost) meniscus of the knee. Surgery is a given in these cases to reconstruct the ACL and either repair the meniscus or remove the torn portion. MCL tears are often treated non-operatively, with surgery reserved for only certain circumstances, as when a portion of the bone is pulled away (an avulsion).

The ACL is the primary stabilizer of the knee, while the MCL protects the medial (inner) compartment of the knee.  Both ligaments attach to the femur (of the thigh) and the tibia (of the lower leg). The medial and lateral (outer) menisci are the pieces of fibrocartilage that lie between these two bones.  They serve to create a better fit at the joint, and act as secondary stabilizers as well as shock absorbers.

These three structures are oftentimes injured together because the nature of the trauma forces the joint into a position that stresses the ligaments beyond their capacity to check the motion and simply tears these stabilizing structures.  According to Wheeless’ Orthopaedics, the mechanism of injury most often involves internal rotation of the femur on the tibia with the knee in flexion (bent).  The femur tends to position the medial meniscus toward the back of the center of knee joint, catching the posterior portion between the femur and tibia. When the joint is suddenly extended, the meniscus tears along its length.

The medial meniscus is affected more often than the lateral both because of the way in which the knee is generally stressed and because it is attached to the deep fibers of the MCL.  Thus, stress to the medial collateral also pulls on the medial meniscus.  The lateral meniscus is not anchored to the lateral collateral on the outer side of the knee.

Peterson will have a challenging rehab ahead of him, but he is young and motivated. Willis McGahee did even more damage to his knee in the Fiesta Bowl in 2002 and continues to be a very productive running back, even in spite of a string of new injuries.  Don’t count Peterson out.  If he’s not back for the start of the season, he will likely join the Vikings midway.

Follow Abby on Twitter @abcsims