The video below shows what happened. Please be aware that it contains footage some may find disturbing.
As can be seen, Houston Texans rookie D.J. Swearinger hit Keller's right knee from the outside at a high speed, causing it to bend inward awkwardly. According to ESPN's Adam Schefter, Keller sustained a dislocated right knee with associated tears to the ACL, PCL and MCL on the play.
The outpouring of support began immediately, and fans, media and the entire football community were left hoping for a full recovery.
Unfortunately, that recovery will likely prove long and complicated. A look at the basics of the injury explains why.
Ordinarily, the femur—or thigh bone—lines up lengthwise with the tibia, or shin bone. While tissues like the skin and quadriceps muscles help keep the upper leg connected to the lower leg, the four main ligaments of the knee help keep them in line. Those four ligaments are the anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), medial collateral ligament (MCL) and lateral collateral ligament (LCL).
By connecting the femur to the tibia, the ACL, PCL, MCL and LCL work in tandem to stabilize and coordinate the complex motions of the knee.
For instance, the MCL connects the inside of the femur to the inside of the tibia, preventing the knee from collapsing inward. Conversely, the LCL connects the outside of the femur to the outside of the fibula—another bone of the lower leg—preventing the knee from buckling outward.
Though it also has other functions, the ACL deters the tibia from moving forward in relation to the femur. On the other hand, the PCL keeps it from moving backward.
For a better mental image, imagine two rectangular blocks of wood attached end to end using four pieces of tape—one on each side. Though not entirely anatomically accurate, the two pieces of wood represent the femur and the tibia, and the four pieces of tape represent the four knee ligaments.
Now, imagine cutting each piece of tape one by one. With each cut, the two blocks of wood become less and less stably attached.
In Keller's case, the hit from Swearinger immediately tore three of his four ligaments—or snapped three of the four pieces of tape, so to speak.
The nature and direction of the hit—called the "injury mechanism"—directly coincide with the injuries doctors eventually diagnosed.
Specifically, the hit came from the outside, thereby forcing Keller's knee to move inside. The inside motion lengthened the distance between the respective attachment points of the ACL, PCL and MCL, causing them to tear.
On the other hand, the hit seems to have spared the LCL, as the inward-directed motion of the knee likely shortened—rather than lengthened—the distance between the two attachment points of the knee's outer ligament.
Since Swearinger's hit immediately tore three ligaments, Keller's knee lost the vast majority of its support network, allowing the blow to also move the tibia out of alignment with the femur—a "dislocation." Though exact medical details aren't available to the public, speculation suggests the tackle forced Keller's tibia to move backward or inward relative to the femur.
As if three ligament injuries aren't enough, knee dislocations also frequently come with vascular injuries, nerve injuries, bone fractures or all three. Such complications occur because the injury drastically distorts the normal anatomy of the knee—so much so that structures running within it can suffer the consequences.
For example, knee dislocations can tear or compress the popliteal artery—the main blood supply of the lower leg—as it runs within the joint. If the artery is sufficiently compromised, the lower leg can die within a few to several hours due to lack of blood supply, and any signs of low blood flow following a dislocation necessitate emergency vascular surgery to save the limb.
Complicating nerve injuries are also possible. Specifically, the peroneal nerve—responsible for controlling the muscles in the front of the lower leg—can stretch or tear during a knee dislocation. Without an intact peroneal nerve, the foot cannot bend upward, and the toes drag.
While nothing yet suggests Keller sustained popliteal artery damage, a Miami Herald report from after the injury claims doctors fear a nerve injury.
If Keller did indeed injure a nerve—such as the peroneal nerve—his rehabilitation becomes all the more complex.
In order to regain the strength he will undoubtedly lose during his prolonged recovery, Keller must slowly, steadily and systematically target individual muscle groups for restrengthening. Without an intact peroneal nerve telling one of those muscle groups what to do—the muscles in the front of the lower leg—restrengthening them becomes extremely difficult, if not impossible.
In the worst-case scenario, a completely destroyed nerve no longer allows them to function whatsoever.
The severity of a nerve injury depends on how much it overstretched or how much compression is present. Delayed treatment can also lead to permanent nerve dysfunction.
Additionally, while some nerve injuries are reversible with immediate treatment—such as decompressing the area around the nerve—more extensive ones are not.
Unfortunately, the successful treatment of serious peroneal nerve injury—even with surgery—is difficult to attain, and as a result, any damage can pose a very real threat to a player's career.
What does that mean for Keller?
In short, it's too early to tell.
It's true that San Francisco 49ers running back Marcus Lattimore is well on his way to a full recovery from the same constellation of ligament injuries and dislocation as Keller's. However, it would seem Lattimore did not suffer a concurrent nerve injury to go along with his dislocation, as his recovery is simply too rapid and too impressive.
In the coming weeks and months, Keller will work slowly to regain his range of motion, knee strength and joint stability, but the downstream consequences of a peroneal nerve injury—if it exists—may not make themselves known for some time.
If the native nerve is still intact, it needs to rehab just like any other injured tissue before the permanence of any damage becomes apparent. If it isn't, a surgically repaired or replaced nerve must do the same following an operation.
The result? An agonizing, tormenting waiting game as well as a rehab timeline that is impossible to predict.
Hopefully, Keller did not sustain any permanent nerve damage and will make a full recovery, as no one likes to see a career cut short by injury. Unfortunately, the battle is certainly an uphill one.
Dr. Dave is a resident physician at the University of Washington with plans to pursue fellowship training in Primary Care (non-operative) Sports Medicine. Except where otherwise cited, medical information discussed above is based on his own knowledge.