What Is the ACL?
With a full name of "anterior cruciate ligament," the ACL is one of the four main ligaments of the knee. Like all ligaments, it is a tough band of tissue that connects two bones in an effort to stabilize and coordinate motion at a joint.
Specifically, the ACL connects the bottom, flat end of the femur—or thigh bone—to the top, flat end of the tibia, or shin bone. By doing so, it prevents the tibia from slipping forward relative to the femur, thus helping keep the upper and lower legs in one line.
Additionally, the ACL helps the tibia resist internal rotation—or being twisted toward the body's midline.
Without an intact ACL, walking becomes extremely difficult after a tear—not to mention sprinting and cutting. The knee feels unstable, and motions that cause internal rotation of the tibia—such as changing directions or planting—can cause the knee to collapse under a player's own weight.
What Causes ACL Injury?
As mentioned, a ligament's job is to resist certain types of movements. However, if an outside hit overwhelms a ligament's ability to deter one of those movements, an injury occurs.
In the case of the ACL, any motion or hit that forces the lower leg to sharply twist inward or move forward can cause an ACL rupture. A common example is during the motion of planting on one foot to twist, turn and change directions.
More famous than non-contact ACL injuries—though less frequent—are those involving contact. A particularly memorable example is the hit that
ended Minnesota Vikings running back
Adrian Peterson's 2011 season.
As seen in the video, Peterson sustained a hit to the back-outside portion of his knee just as he planted his leg. In such a scenario, the planted leg cannot give way, and the knee takes the full force of the blow, twisting inward and moving forward in the process. If the hit is strong enough—as it was in this case—the ACL cannot keep up and tears.
How Do Doctors Diagnose an ACL Tear?
When an ACL tears, players often report hearing or feeling a "pop" in the knee. Severe pain immediately follows, and significant knee swelling is not far behind.
A number of physical exam maneuvers can also suggest an ACL injury. For example, the "anterior drawer" test—where an examiner pulls forward on the tibia—is quick and simple to perform. If an examiner can move the tibia significantly forward relative to the femur—one of the motions an intact ACL prevents—an injury is more likely.
All of that said, doctors usually employ magnetic resonance imaging (MRI) to confirm a tear. Imaging is one of the most accurate ways to diagnose the injury.
Why Does an ACL Tear Usually Result in Season-Ending Surgery?
In general, ligaments do not heal well on their own—at least relative to other tissues in the body. For instance, the body heals a cut on the skin much quicker than a tear in a knee ligament.
Even worse, completely torn ACLs generally do not heal at all. In fact, reconstructive surgery actually involves replacing the ligament altogether. To do so, surgeons remove a piece of muscle tendon from elsewhere in the patient's body—called a "graft"—and use it to serve as a new ACL.
Usually, the graft comes from a patient's patellar tendon—the part of the knee a doctor hits with a reflex hammer. It can also be taken from the hamstring.
Following surgery, an athlete will work with physical therapists to slowly regain strength and range of motion as the body cements the new ACL into place. Through a fascinating process called ligamentization, the healing process transforms the graft into tissue more closely resembling a native ligament.
The entire rehab process usually takes between nine to 12 months or more in most athletes.
Did Bryan Bulaga Really Practice Through a Torn ACL?
Believe it or not, according to McGinn, he probably
did. It wasn't until after practice that Bulaga reported "a problem in one of his knees."
Weird, right?
Actually, maybe not.
If Bulaga suffered a partially torn ACL—rather than a completely torn one—it is possible his knee retained enough stability to carry him through the remainder of the Packers scrimmage before swelling up and making the injury known.
Why Aren't There More Cases Like Bulaga Discovering His Injury After the Fact?
There exist at least two possible answers to this question.
First, the ACL is the weakest of the four knee ligaments.
According to Medscape, the tensile strength of an average ACL is 2160 newtons—about half the strength of the medial collateral ligament (MCL), for example.
While players frequently suffer partial tears of the MCL, the relative weakness of the ACL allows for much less leeway during an injury-inducing event. In other words, the ACL is more likely to be injured in an all-or-nothing fashion.
The second possibility is that cases like Bulaga's actually do occur more frequently than the public knows, but the public only hears of a "torn ACL" because the endpoint is the same: surgery.
What's the reason for the common endpoint?
If a player suffers a partial tear, and the tear is significant enough to produce knee instability, reconstructive surgery is often necessary regardless of how much of the ligament is torn. After all, when it comes to elite-level athletes, any instability is too much.
Why Do Players Get a Second Opinion?
In cases where an athlete has a known ACL injury but feels his or her knee remains satisfactorily stable, it is theoretically reasonable to forgo reconstructive surgery in favor of conservative treatment. Presumably, Bulaga wanted to make sure such an option was entirely off the table before going under the knife and resigning himself to the aforementioned nine-to-12-month rehab.
Can Athletes Really Play with a Partially Torn ACL? Should They?
The answer to the first question is a definite yes. The answer to the second question is not as clear.
As long as a knee is stable and strong enough to meet the demands of the NFL—while remaining an effective player, of course—athletes can play on a partial tear. The other muscles and ligaments in the knee can support the joint, and they are all likely in prime shape in NFL players.
Nevertheless, attempting to play carries risks.
First, injured ligaments are weaker ligaments, and as such, an injured ACL is less able to resist the forward motion or internal rotation that likely caused the injury in the first place. Protective braces may provide some additional support, but nothing can replace a native, healthy ligament.
Additionally, the four main ligaments of the knee work in tandem with the muscles of the leg to support the athlete. Playing on an injured ACL, however, effectively removes part of its protection from the equation. As a result, the other three ligaments and muscles must work harder to pick up the slack, placing them at higher risk of injury in their own right.
It is possible this exact scenario transpired just last year, as many speculate
Robert Griffin III's possibly still-ailing lateral collateral ligament (LCL) contributed to the eventual
ACL tear and complete LCL tear that he sustained in the playoffs.
), Nic Batum ( 
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