Jeremiah Adebolajo, the brother of Woolwich murder suspect Michael Adebolajo, said his brother told him he has five officers dealing with him at a time and he is never in contact with any other prisoner.
He claimed the altercation last Wednesday in Belmarsh began when his brother was given an instruction by one of the guards.
Mr Adebolajo told the BBC: "They decided to twist his arm back... the arm that had been shot... He manoeuvred out of that so they smashed his head against a window."
He added: "I wonder why it took five prison officers to restrain one man and how in the course of that restraint his teeth were able to be knocked out if they were following procedures."
Five prison officers were suspended while the incident is investigated. All "strenuously deny" any wrongdoing.