"Even today, I still can't open my hand wide enough to use my fingers normally on the keyboard," he wrote. "The impact that my injury and subsequent surgeries had on my career was immense, but more importantly, the fallout proved to be devastating to some of the closest relationships in my life."
He claims the accident occurred in 2001, while taking part in a "MSNBC Investigates" parody skit, about a group of kids impersonating The Golden Girls.
He was supposed to lean back in a chair and topple over for laughs; but when he did he claims he smashed his head off the stage.
He claimed he had raised concerns about the safety of the stunt, but the props department never replaced the chair with something safer.
After the incident he claimed he told creator Lorne Michaels, and was told he would be "looked after"; but said NBC only paid for two of his five required surgeries.
However NBC told variety it had no record of the injury; extensive interviews of Kattan's castmates and crew at the time carried out by the publication found no-one else that remembered it. Kattan also had no record of NBC paying for surgeries.
"NBC had stopped paying my medical costs after the second surgery," Kattan claimed. "The 'SNL' family I was part of had stopped taking care of me, and soon I wasn't able to pay for everything myself."
"But I never really fought for myself or demanded anything. I never thought about the potential legal ramifications of what had happened to me on the set and what was happening now. I had been brought up to be responsible for myself. I wasn't about to sue anybody."
"I never wanted to be that person: spending my life debilitated and fighting a network. I wanted to hide everything, pretending I was okay and in good enough shape to be go out in public and be social."
"Had I known how everything would end up I would have been better off saying something about it, as opposed to being quiet because I thought it would get in the way of work," he said.
"It's a different day and age where people if they get any injury or harassment or anything, it's a good time to say anything about it. This is not too long ago but it was more of a faux pas to say anything, especially if it has to do with your showbiz family."
Kattan, whose father Kip King was a Groundlings founding member, left "SNL" in 2003 after seven years.
He also starred in the 1998 film A Night At The Roxbury, a film based on his and Will Ferrell's characters in the famous SNL sketch -- characters whose trademark move was to bob their heads vigorously.
In 2014 he was arrested for DUI, which he attributed to pain medication following his fourth surgery.
"As a physical comedian, I had always been worried about waking up with a whole different body one day," he wrote in the memoir, which is released on May 7. "That fear became my reality. After those forty-five seconds on the 'SNL' stage in May of 2001, my body would never, ever be the same."