The issue of metal spines arises in debates about the medical technology of the Galactic Empire. Near the end of RotJ, when Darth Vader decides to save his son from Emperor Palpatine, the Sith lightning hits Vader and causes a brief "x-ray" effect (a special effect that is not exclusive to Star Wars, although it's more common in humorous cartoons). During this time, we can see parts of his skeleton, including his skull and upper vertebrae. His spine clearly has artificial implants to treat injuries he received earlier in life.
Darkstar -- in his never ending jihad to downplay Star Wars technology -- once tried to claim that such metal spines might actually be natural. "Of course, given that we have nothing to compare it to, it may simply represent the natural order of the dominant humanoids of the SW galaxy."