An inmate at HMP Bristol sank his teeth into a prison officer when he was restrained, a jury has heard.
Lee Sage is accused of being agitated and abusive after a prison visit in June last year.
And when officers tried to restrain him in his cell, he bit a chuck out of officer Paul Harsant's right forearm, Bristol Crown Court was told.
The 33-year-old, of Kings Weston Avenue in Lawrence Weston, denies unlawful wounding.
Mr Harsant told the jury he was working on D-Wing when Sage, who he said is known to abuse the drug Spice, was returning to his cell.
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He said Sage was "quite argumentative" with senior officer (SO) Darren Liddington before the inmate slammed the door of his cell shut.
Mr Harsant told the court: "Mr Sage was being verbally aggressive to the senior officer.
"It was just about what he would do to him on the out."
Mr Liddington returned to the cell and opened the door to try and calm Sage down, the court heard.
Mr Harsant continued: "Senior officer Liddington asked Mr Sage what his problem was and why he was being aggressive towards him."
With that there was a bang, Mr Harsant said, and the incident escalated into SO Liddington restraining Sage.
"I had my arm a foot away from his face.
"I reached to get his arm and then he bit me.
"It was very forceful, it was one, clean bite.
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"I screamed he had bit me," Mr Harsant told the court.
As other officers assisted the officer, he went to the prison's health care unit before being taken for treatment at Southmead Hospital.
He said the circular wound he suffered was too big to be stitched and was just left to heal.
Mr Harsant told the court he had lost some movement to his fingers, was on daily pain relief and was off work for two and a half weeks before returning to four weeks' restricted duty.
SO Liddington told the court he heard Sage verbally abusing a female prison officer and was aware he was upset after receiving bad news in a prison visit.
The officer said he decided to go to the cell.
He accepted Sage could be "challenging" but denied going in the cell to show him who was boss.
Mr Liddington said: "It would be very poor practice to leave someone in an agitated state."
He told the court that in the restraint that followed, he didn't think he managed to turn on his body worn camera.
The case continues.
