A review of the tape—and some anatomy—shows why in cases like Gronkowski's, an MRI may confirm what doctors and medical staff already know. The only remaining question: What other damage also exists?
UPDATE: Dec. 9, 2013, 10:10 am ET
The injury toll officially includes a torn ACL and MCL, according to Pro Football Talk:
--End of update--
When a joint moves in an abnormal fashion, it may stretch and injure surrounding tissues—such as ligaments. Ligaments are tough bands of tissue that connect one bone to another and serve to stabilize and coordinate motion at a joint.
In the knee, the anterior cruciate ligament—or ACL—prevents the lower leg from rotating inward or moving forward with respect to the thigh. The medial collateral ligament (MCL) keeps the knee from buckling inward.
The other two main ligaments of the knee, the posterior cruciate ligament (PCL) and the lateral collateral ligament (LCL), carry similar, analogous functions. The PCL prevents the lower leg from moving backward, and the LCL protects the knee from collapsing outward.
In this case, Gronkowski's injury mechanism mainly threatened his ACL and MCL.
A video of the injury, courtesy of NFL.com, shows exactly what happened.
As seen, Gronkowski received a low hit to the outside of his right knee, causing it to buckle inward.
When a knee buckles inward due contact from the outside, the lower leg may overrotate—the exact motion the ACL attempts to limit. If a hit is strong enough, it overwhelms the ACL, and it tears.
Additionally, the inward motion stretches—and possibly tears—the MCL.
Frequently, a meniscus injury accompanies a concurrent ACL and MCL injury—the third leg of the so-called "unhappy triad" injury combination. The meniscus is the cushioning cartilage between the femur—the thigh bone—and the tibia or shin bone.
An MRI will clarify whether or not Gronkowski suffered an injury to all three structures or some other combination altogether.
However, according to Jason Cole, the diagnosis of a torn ACL seemed almost definite long before Sunday came to a close—and well before his MRI:
How could doctors know without imaging?
Simple. Physical exam tests designed to test the ACL—such as the anterior drawer test, the Lachman test and the pivot-shift test—likely suggested a tear immediately after the injury occurred.
For instance, the anterior drawer test assesses the ability of the tibia to move forward relative to the femur. If an examiner can continue to pull the tibia forward without a firm endpoint, the ACL is likely no longer intact. On the other hand, a healthy ACL limits such motion.
Once doctors more firmly establish Gronkowski's diagnosis using an MRI, surgery will soon follow. That said, the true extent of the damage within the tight end's knee will only become apparent in the operating room.
Until then, the Patriots medical team will surely keep an eye out for any signs of nerve or artery damage, as severe knee injuries can sometimes compromise the popliteal artery—the main blood supply of the lower leg.
According to Jim Hoban of the Boston Globe, medical personnel transported Gronkowski to Massachusetts General Hospital following the injury. There, doctors likely monitored for decreased sensation in the leg or foot, increasing pain or other signs of blood vessel compromise.
Weakness or numbness in the limb—if any—could also suggest nerve damage.
Unfortunately, a surgical knee reconstruction comes with a lengthy recovery time. Recent isolated ACL tears in the NFL are carrying a very unscientifically determined average of 10 months.
Additional injury to the MCL and the meniscus may or may not lengthen the rehab process, though it can certainly complicate it.
On the other hand, damage to a third or a fourth ligament, blood vessel injury or nerve dysfunction make for an entirely different story. Fortunately, no reports of these complications yet exist.
Regardless of what further tests show, it appears the oft-injured tight end faces a long road to recovery. Over the weeks and the months following his all-but-certain surgery, he will need to progress through a long, progressive series of range-of-motion, strength and agility exercises.
Regrettably, a return for the start of the 2014 season is by no means a guarantee—even in the case of an isolated ACL tear—and the Patriots may need to plan on starting the season without him for the second year in a row.
Given how lost the Gronkowski-less Patriots looked early this year, the injury represents a brutal loss, and everyone in the NFL surely hopes for nothing but a speedy and full recovery.
That said, while Tom Brady's squad usually seems to find a way to overcome adversity, this latest development may be the toughest obstacle yet.
Dr. Dave Siebert is a resident physician at the University of Washington who plans to pursue fellowship training in Primary Care (non-operative) Sports Medicine. He is a member of the PWFA.