Work Question - Surgery but not what they think.

This is actually pretty simple. Just tell them that this surgery will relieve pressure on your knee. Problem solved. 

This is sort of what I would say. If someone asks, you say "the surgery I am having is not on my knee directly, but is designed to help that issue along with a couple others." 

It is correct to say that you are not required to tell people exactly why you are having surgery. But people are not completely stupid, and will notice you losing weight. I was at our local garage yesterday having some minor work done on the car. This is as close as one can get to a purely "guy" environment. There is not a lot of emotional, touchy feely discussion of stuff, or even mention. But even the mechanics, all of whom I know, were commenting on my weight loss.

Surgery or other means were not discussed, but the observation was made. The one mechanic who usually works on my truck was doing the job, and he is (was) fairly portly himself. He is losing weight too. The whole conversation was, "Bill, you look like you're slimming down there!"

"I am, George. You look like you are too."

"Yup. I'm working on it."

So yeah, people are going to notice. Don't paint yourself into a corner ahead of time which will destroy your credibility later on. 

Not sure what laws are in Canada, but in the US we are not obligated to be specific.

Now, that being said - I highly recommend you being open about it and here's why:
You will be losing weight at an alarming rate, and people will start asking what's going on or whats your secret.  If you tell them anything false and it comes out (it always does), that will ruin your credibility professionally as well as personally.  You will be known as someone willing to conceal facts or outright lie.  Whether it's their business or not, that's the stigma left behind.

You may have nay-sayers, but you may just be surprised with the level of support.