I'm wondering if there's even a need to concoct a backstory to the surgery. I think you might be overthinking this a bit, tbh. I don't plan to tell anyone at work why I am going to be away, and should anyone ask I'll take Ayden's approach and just say "surgery" because most people don't ask the details. Like when I was at the hospital yesterday I met a woman and I asked if she worked there, and she said "no, I'm a patient." I wasn't about to ask her what she was there for because obviously something like that is a sensitive topic.
I figure if someone kept pressing I'd honestly just tell them "sorry, but this is something I'm not comfortable talking about. Thank you for being concerned about me, but I am OK."