Antonio Cromartie's Timeline to Recovery After Suffering Hyperextended Knee

The New York Jets faithful received a scare Thursday night when ESPN's Rich Cimini tweeted defensive back Antonio Cromartie suffered a non-contact knee injury earlier in the day during practice.

Cimini later reported Cromartie underwent an MRI to determine the extent of the damage, one that thankfully showed "only a hyperextension."

Still, the term "hyperextension" represents an injury mechanism rather than a precise diagnosis, and underlying damage may still exist. To understand exactly what happened—as well as what may follow—let's take a look at that injury mechanism in further detail.

Generally speaking, the knee moves via two main motions—flexion and extension. Flexion describes bending at the knee, while extension refers to knee straightening.

The knee can also bend inward and outward to a slight degree, though such motions are largely prevented by the medial collateral ligament (MCL) and lateral collateral ligament (LCL), respectively.

Normally, human knees can only extend to the point where the lower leg lines up with the upper leg—a zero-degree angle.

When an outside forces pushes the lower leg even further so that the knee bends in the opposite direction—past zero degrees—hyperextension occurs.

Depending on the degree of hyperextension, a wide range of ligament damage can result—from none at all to a true knee blowout.

Additionally, just about any of the four main knee ligaments can suffer such damage—such as the MCL, the LCL, the anterior cruciate ligament (ACL), the posterior cruciate ligament (PCL) or, theoretically, any combination of the four.

The magnitude and exact direction of the hyperextension determines the ultimate injury constellation.

With that said, hyperextensions place the ACL at a particularly high risk, as they often cause the tibia, or shin bone, to move forward relative to the femur, or thigh bone—the exact type of motion the ACL usually prevents.

Severe hyperextensions can also threaten the PCL.

When a hyperextension stretches a ligament, it can partially tear—a grade-two sprain—or rupture altogether, a grade-three sprain. Grade-one sprains involve over-stretches with microscopic tearing but no obvious tear on MRI.

In football, hyperextensions classically occur when a tackler hits a runner's knee from the front, such as in the case of Washington Redskins quarterback Robert Griffin III:

The above play resulted in RGIII's first of two knee injuries during the 2012 season—a grade-one LCL sprain. Presumably, the slight outward-directed force of the hyperextension overtaxed the LCL, though this thinking represents only theoretical musings after watching the above video.

On the other hand—as in Cromartie's case—athletes can also sustain non-contact knee hyperextensions.

If a player leaps up and lands on the heel of a straightened leg, the knee can hyperextend—especially if the athlete comes down in such a way that his or her weight is too far in front of the knee.

Should such a scenario happen to the extreme, the ACL can tear—likely the reason for the immediate concern throughout the Jets organization.

Fortunately, no reports of serious damage to Cromartie's knee yet exists, and it appears the MRI demonstrated no significant tears.

In other words, he likely sustained only low-grade damage to his knee.

With that being said, playing on damaged tissue—even minimally so—carries risks. Specifically, sprained ligaments are weaker ligaments, and they are therefore less able to resist the damaging movements that caused the injury in the first place.

For that reason, the Jets may elect to rest Cromartie for the next game or two—a typical time frame for a low-grade LCL sprain, for example.

Then again, the possibility of the presence of more extensive damage always exists. No imaging is perfect—especially if the knee is swollen at the time—so reports of more serious damage may surface.

Unless that happens, however, it's safe to remain cautiously optimistic for a relatively quick return to action.

Dr. Dave Siebert is a resident physician at the University of Washington. Find more of his work at the Under the Knife blog.

Follow @DaveMSiebert