A sensing device 205 for detecting displacement of a cannula 206 relative to a body in which it is inserted, includes a sensor element 209 having an attachment portion 211 for engaging with the cannula 206 and an insertion portion 212； a sensor unit 210 having a receptacle 213 configured to receive the insertion portion 212 of the sensor element 209 and electronic sensing means 214 configured to detect displacement of the insertion portion 212 relative to the receptacle 213.
