It happened on the B3081 near Verwood - he had to be cut free by firefighters.
Shop Worker Threatened With Knife
Detectives in Poole are investigating an armed robbery in the Branksome area of Poole.
A man entered M & J Stores on Winston Avenue and demanded money by threatening a member of staff at around 8.45pm on Tuesday 5 March 2013.
The man demanded money from the till before threatening a member of staff with a knife. He left the scene with a quantity of money.
Detective Constable Jonathan Sainsbury, of Bournemouth and Poole CID, said: "This was a very distressing incident for the member of staff. I am releasing a CCTV image of a man who may be able to help us with our investigation.
"I would urge the man pictured and anyone who recognises him to contact Dorset Police, in confidence, as soon as possible."
Anyone with information should call Dorset Police in confidence on the 101 number.
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