Following his successful surgery for the removal of the kidney stones that caused him to leave the stage on Sunday night (March 10) in Sydney, Australia, Vince Neil is expected to rejoin Motley Crue tonight in Brisbane as scheduled. But, considering Nikki Sixx said that Neil never cancels a show, it got us wondering about how painful kidney stones must be.

Fortunately, CNN was thinking the same thing, and interviewed Dr. Dianne Sacco of Massachusetts General Hospital's Kidney Stone Program about the condition, whose proper medical term is "hydronephrosis." She said that kidney stones "block the flow of urine in the kidney, and it causes backup. And it's an excruciating pain. A lot of people do describe it as worse than childbirth."

Sacco provided an analogy. "If you take a pipe and you clog it off and somehow you're still getting fluid into the other end ... if it's a pipe that can expand, it starts expanding," she continued. "The backup is like that. It causes a great deal of pain because you're expanding your system. You don't have any pop-off valve. Once it starts expanding, it's expanding unnaturally."

Many stones are small enough -- less than 5mm -- to pass by themselves, and the pain can often be managed with common medication. However, if they don't pass and wind up growing too large, as it was in Neil's case, surgery is required. Sacco also added that kidney stones are often associated with obesity and diabetes, although genetics can also be a factor.