Did they ever mention why Yirim need to undergo surgery and what was it about??

I think it had something to do with either his spine or legs and it being like a rehabilitation surgery. I could be wrong, though.

I'm not sure but I think it's because of his pheromone being unstable. Because of it, he's in pain and suffers from a sudden heat or sumth. After the surgery he doesn't have to worry about a sudden flare up and can perform well when he's competing.

It's a surgery for his hormone imbalance that caused him to quit skating in the first place, ch 20 if you want to re-read. Though I wished they explained what kind of surgery is done for that.