The pop star underwent an operation on the joint to correct a long-running problem in January, 2014 and was still recovering from the surgery when One Direction began a world tour in April (14).

Horan has now revealed he has only just been able to fully bend the knee 18 months on by sharing a close-up picture of his foot touching his bottom in a post on

He writes, "First time I've been able to touch my a**e with my heel since I got the operation. Sounds so dumb, but it's a big day in my head haahah (sic)!"