Your favorite player just went down with a knee injury. You're reading all this stuff about ACL, MCL, Scoping, Surgery. What does it all mean?
First, let's agree on the basics. The human knee is not one of the engineering wonders of the world. It was "designed" (let's just take that word at whatever face value you prefer) for a 4-legged creature then pressed into service for a roughly 100 pound upright ape. 350 pound linemen are dramatically exceeding design limits and they're going to wind up having knee problems, it's really that simple. Add in cleats that don't let your foot slide naturally to protect the knee, and cut blocks and pile ups, and it's a wonder any of these guys ever walk again.
You have two leg bones that join together, and you want a hinge. The hinge you're looking for should be quite solid side to side, reasonably frictionless, and swing in one dimension only. The knee is made to accomplish these goals. There's this very slippery almost teflon-like stuff, Cartilage, that covers the ends of the bones, and there's some lubricating fluid called synovial fluid that makes everything slide very easily. There are a couple pieces of wedge shaped cartilage called the Meniscus that help the knee hinge in one direction only.
There are four ligaments, tough string-like pieces that hold the knee together. These ligaments can be damaged by large forces. The damage is almost always a tear. A small tear is called a "sprain" or grade I injury. These heal themselves in time, requiring only that you work to keep the swelling down with anti- inflammitories like advil (ibuprofen) or cortisone. A medium sized tear is also called a sprain or grade II injury. These tears will usually require a knee brace to aid with knee stability while the joint heals. If the ligament is torn completely in two it's called grade III and usually surgery is required, along with a long recuperation.
There's a ligament on each side of the knee called the collateral ligaments. The one to the outside of your body is called the lateral collateral ligament (LCL). The LCL holds the outside of your knee together, keeping your leg from collapsing outwards. The LCL is rarely damaged in football.
The ligament to the inside of your body is called the medial collateral ligament (MCL). The MCL holds the inside of your knee together, and keeps your leg from collapsing inwards. This ligament can be damaged by a blow to the outside of the knee pushing inwards. MCL injuries are moderately common in football. Usually the player can continue playing, perhaps with a brace. The MCL is not critical to knee stability in most football situations.
There are two more ligaments that cross each other, so they are called cruciate ligaments. The posterior cruciate ligament (PCL) keeps the knee from hyper-extending. Grade III injuries to this ligament are not common in football. Typically a hyper-extended knee will result in a grade I or II injury to the PCL which can be healed with time, anti-inflammitories, and a knee brace. PCL injuries are more common in auto accidents due to a large impact on the bent knee with the dash board.
Right in the middle of the knee there's a ligament that holds the whole joint together, the anterior cruciate ligament (ACL). This is the big one. If you tear this completely, your knee is completely destabilized and you can't trust it at all. A grade III injury to the ACL requires highly invasive surgery where they will open up the entire knee and re-attach the ACL to the bone, or more often these days take a section of your patella tendon and substitute that for the ACL. Then your leg is immobilized several weeks, crutches for several more weeks, and recuperating for a long, long time.
During the couple months of not using your leg, your quadriceps muscle gets weaker. To strengthen it again you have to exercise it hard, but you have a reconstructed knee which is healing and a cut up patella tendon which can easily get irritated and inflamed. The joint will typically be healed and rehabilitated enough to get medical clearance to play in about a year. However, it's well known in the NFL that it takes another complete year before the player trusts his knee again. A torn ACL is a large setback for a lineman, and can be career ending for a running back. Without a solid ACL, you cannot pivot or cut. ACL injuries were the end of Terrell Davis' career.
Some random facts about ACL injuries in the NFL:
In the front of your knee you have a kneecap called the Patella. This is a floating bone wrapped in ligaments and tendons. Ligaments hold bones together, tendons hold muscles to bones. The kneecap connects your quadriceps, the large muscle on top of your thigh, to your shin bone below your knee. The purpose of the knee cap is to let this tendon slide easily over the front of the knee, and to push the tendon out further from the center of the knee to increase leverage. When working properly, your kneecap increases the force from your quadriceps about 30% compared to no kneecap. The kneecap can become irritated or the tendon can become torn or sprained. This is very serious for people who make their living bending their knees. When climbing stairs, for example, the kneecap holds about twice your body weight. When descending stairs, your kneecap holds about seven times your body weight.
If the patella tendon becomes chronically irritated it is intensely painful and gets in the way of playing football. Also, the pain can be an indicator that your patella tendon is about to tear. If this tendon tears, you aren't walking anywhere anytime soon. Packers center Mike Flannigan had an irritated patella tendon in '04 and wound up out of football for the year when he had surgery to attempt to clean up the cartilage and lessen the irritation. He was told if he didn't have the surgery, there was a good chance his tendon would rupture in a game and he would not walk again for several months.