PS3: !!SPOILERTASTIC!! Metal Gear Solid 4: Sooo...why's Raiden a cyborg ninja?

So i finished mgs4 earlier today and besides wanting to immediately start a new game (which i didn't because i needed sleep) there was one thing that was never explained: why did Raiden become a cybernetic ninja? Last time i saw him he was standing in a crowd of people in new york and now everything but his head, heart and spine are cybernetic prosthetics? WTF!? when did this happen and why? was he just bored with not being able to sword fight with his foot/teeth? was there a terrible accident where he sat in a hot tub full of acid? no one ever even asks: "So, what's with the crazy new transparent lower lip/oddly Gekko-like feet/white blood/ability to withstand swords through the abdomen? Something crazy happen?" You can't just leave that stuff out, man! And at the end he's had it all replaced with an organic looking body, only clearly attached in the exact same shaped patches as his previous ninja body! ...well...uhhhh...what?