The opening bout of the June 8th UFC on Fuel 10 prelims featured a vicious injury to Anthony Smith. Antonio Braga Neto secured a quick knee bar in the opening round, causing Smith to tap and immediately grasp his left knee. There was an obvious injury, as shown in this gif of the submission, courtesy of Bloody Elbow's Zombie Prophet.
The injury occurs at the 3:11 point in the round. As Smith is rolled onto his back, Neto pulls his knee into full extension (possibly hyperextension) combined with a severe valgus stress. At the same time, the lower leg appears to be extremely externally rotated ie, rotated outward (see photo below). Given Smith's quick reaction of pain and grabbing his knee, I expect that he ruptured the medial collateral ligament (MCL).
Smith underwent surgery to repair the knee on June 12. He tweeted a picture apparently taken during the surgery. (Warning: Graphic!)
Grade I or II (incomplete) tears of the MCL are not typically treated by surgery. Grade III tears are even treated conservatively with good success, given enough time. However, as Dr. Gelber notes in this article about Dan Henderson's MCL tear, surgery is needed when a piece of bone pulls off with the ligament or the ligament gets trapped inside the knee joint. It is also interesting to note that additional ligaments (namely, the ACL) can be injured along with the MCL during the same traumatic event. The medial meniscus can also be injured, as the deep fibers of the MCL have an attachment to the medical meniscus.
At this point, the full extent of the Smith's knee injury is not known. Let's hope he has an uncomplicated recovery from this severe knee injury.