Around this time last year, fans noticed a dramatic change to Zac Efron's appearance, particularly in his jawline, sparking speculation that he'd secretly undergone plastic surgery. But the actor insists it was just an old injury, not vanity, that caused his dramatic new look.

Efron explained in a new cover story for Men's Health that his noticeably different facial structure while promoting Bill Nye's “Earthy Day Musical” PSA on Facebook Watch in April of last year was due to a scary injury he sustained almost a decade ago. The accident, which he says happened in November 2013, capped off a year and a half journey of recovering from several mishaps, including tearing his ACL, dislocating his shoulder, breaking his wrist, and throwing out his back. The actor says that he was running through his house one day while wearing socks when he slipped and smacked his face on the corner of a granite fountain shattering his jaw. He was knocked unconscious and when he came to again his chin bone “was hanging off his face.”

To address this serious incident, Efron began regularly going to physical therapy to rehabilitate his jaw. But last year, while traveling around Australia where he taped season 2 of his Netflix series Down to Earth, he had to take time off from doing his usual facial exercises. This also happened to be right around the time he filmed that clip for Nye's special. He said that since the muscles in the face all work together “like a symphony,” his masseter muscles, which are the ones used for chewing, began to overcompensate for the other injured parts. “The masseters just grew,” he explained. “They just got really, really big.” But Efron confessed he had no idea the social media firestorm he'd set off with his new appearance until he got a call from his mom who was also curious to know if her son had secretly gotten plastic surgery.

But the High School Musical star says that type of speculation doesn't bother him in the slightest and that being in Hollywood has given him a particularly thick skin on the topic. He concluded, “If I valued what other people thought of me to the extent that they may think I do, I definitely wouldn’t be able to do this work.”