Zac Efron Tore His ACL "Shredding The Gnar"

Zac Efron shared an Instagram post explaining that he tore his anterior cruciate ligament while "shredding the gnar," which we can extrapolate means skiing. "But all is good," Efron wrote in the Instagram caption. "I opted for surgery so I can come back stronger than ever."

The anterior cruciate ligament, aka the "ACL," is the band of fibrous tissue that’s found inside your knee joint, and it's responsible for providing rotational stability, and keeping the shin bone from sliding in front of the femur, according to the American Academy of Orthopedic Surgeons (AAOS). When someone tears their ACL, it means that the ligament snaps into two pieces, rendering the knee joint unstable.

Most often, an ACL tear is caused by a sudden twisting motion in the knee, when someone lands or steps, according to MedlinePlus. Usually, you can hear a popping noise when an ACL tears, and your knee will give out from underneath you, according to the AAOS. Within 24 hours, the knee will swell and you'll experience extreme pain and tenderness, especially along the joint line of the knee. In other words: you can't miss it.

Skiing is, in some ways, a recipe for this kind of injury. If someone wearing skis falls, their foot will stay planted in the boot, bindings, and ski, which can lead to twisting and ultimately a tear. Interestingly, about 25% of people who tear their ACL while skiing can be treated without surgery, simply because the way the tear occurs isn't as severe as other activities, like basketball, according to a Hospital for Special Surgery study. A minor ACL tear can be repaired with a brace and physical therapy.

If someone opts for surgery to repair the ACL, that can be a six-to-12-month process, which includes physical therapy and rehabilitation, according to MedlinePlus. To repair an ACL, you can't just sew it back together; surgeons have to reconstruct the ACL using a graft that's cut from the patellar tendon, or from a cadaver (yep, a dead person), according to the AAOS.

While Efron is just one guy who's experienced this brutal type of injury, it's especially important for women to understand what an ACL tear entails. Young women are eight times more likely to tear their ACL compared to men, because women's knees tend to be aligned slightly different than men's, according to MedlinePlus. Other experts say that the hormone estrogen can affect women's ligaments and make them predisposed to tearing.

So, if you find yourself "shredding the gnar" like Efron, try to stay safe and challenge the status quo.