Detectives traced a suspected thief after he left his severed finger at the crime scene.
The digit was found at a South Shields building site after workers reported the theft of a motorbike on Tuesday.
Officers then took the finger to South Tyneside Hospital, where they found a man waiting in the accident and emergency department without a finger.
It is believed the alleged thief ripped his finger off after becoming trapped under a crashed motorbike.
'Pool of blood'
Insp Simon Charlton said: "I am sure it must be very painful for an offender to have ripped a finger from his hand, but it hasn't half left the police a good clue to go on."
The man has been transferred to the University of North Durham Hospital for treatment to reattach the finger.
A Northumbria Police spokesman said: "He is still undergoing surgery and he will be questioned by us in due course."
The finger was found in a pool of blood at the Persimmon homes development in Boldon Lane in the West Harton area.