In Warhammer Horus Heresy, the Primarch Angron had surgical implants in his head known as the Butchers Nails. These implants were forced upon him in his youth.
In the second Horus Heresy book, 'Galaxy in Flames' it is said "Although the technology to have them taken out existed, he had never wanted them removed."
I doubt that's the exact quote and if someone has the quote I would much appreciate it. But you get the gist. They could remove them if Angron wanted.
However, later on we are given evidence to the contrary.
In the Audio Drama entitled, 'Butcher's Nails' Lorgar says to Angron,
"They cannot be removed. Not without killing you. I had no intention of trying. if it is possible for us to die, you will do so with those wretched things inside your skull."`
Also in the Lexicanum it says that, because the Butcher's Nails were hazardous to psykers, most of them tried to have them removed. But, they all died.
It seems they know what they were signing up for as Angron offered them a chance to be honourably executed by him instead, but almost all decided to risk having them removed.
So can they, or can't they be removed? Did the technology used to exist but was lost early in the Heresy? Or was Loken just wrong? Or was Lorgar wrong and Angron lied to his Psykers?
In regards to Warhammer lore I believe the saying goes, "Everything is Canon. But, not everything is true."
So what is the truth of this matter? Does anyone have any other sources that support the for or against?