It's a very complex procedure (presumably they don't just make you lie down under a guillotine) with a vast array of physical, mental and emotional implications for the patient (use your imagination). Therefore it takes a serious set of circumstances to even consider this. We're talking a malignant tumor below the waist that can't be stopped any other way, or other conditions such as pressure ulcers or catastrophic infections of bones or bone marrow.
It also takes a bunch of doctors to pull it off, because there is no one doctor who is a specialist in slicing dudes in half in a way that actually improves their quality of life.
"Quality of what, now?"
Being short isn't one of those things you think of as curable at all. But, if you finally get fed up of not being able to reach the last box of cereal on the top shelf and decide to hop into your Mini Cooper and demand that your doctor add inches to your height, what do you think he'd recommend? Growth hormones? Taller shoes?
Would you think he was just messing with you if he nodded and said, "You know, why don't we just stretch those leg bones? Of course, we'll have to break them first ..."
"And then after you've paid your medical bills, we'll break them again to do the actual surgery."
It's called distraction osteogenesis, because any truly descriptive name would sound like a medieval torture technique. The theory is that, while the cells in your bones won't grow to make them longer (after a certain age, anyway), they will still grow when they need to heal a break. So, just force the issue.
You're only a few hundred hours of agony away from reaching the top of the bookshelf.
First they go in and break the bone. Then they attach a "distractor" (a brace thing) to keep both broken ends in place, with a little gap in between. In the following days or weeks, regenerated tissue fills the empty space, which then heals into perfectly normal, functioning bone.
While it's normally done to correct birth defects (like malformed jawbones and such), as you can imagine, such a procedure has captured the minds of many short people imagining themselves with longer legs. It's becoming a popular process, especially overseas, where the idea of gaining a couple inches has made distraction osteogenesis a new trend in cosmetic surgery. We also wouldn't be surprised if some college basketball recruiter wasn't at this moment telling some teenage power forward, "You know, you could probably get a scholarship if you were only a couple of inches taller. Now, I know a guy ..."
"He made one leg longer than the other, and now he runs circles around his opponents."
For more medical insanity, check out 8 Terrifying Instruments Old-Time Doctors Used on Your Junk and The 6 Most Terrifying Medical Malpractice Cases Ever.