Five police officers are being investigated after a man lost the end of three of his fingers while being held in a cell at Colchester police station.
The Independent Office for Police Conduct has opened a gross misconduct investigation, following the incident in May 2015.
The prisoner had got his foot stuck in a toilet - and severed his fingers while officers were trying to release him.
The IOPC has announced proceedings will begin against an inspector, three police constables and a temporary police sergeant.
IOPC regional director Sarah Green said:
"We concluded that five officers have a case to answer for gross misconduct and Essex agreed. It will now be for an independent panel to consider the evidence and make its judgment."
The case will begin on 7 May.