An introducer assembly is provided with a sheath and a positioning device. The positioning device is able to be configured between released and locking conditions, wherein in the released condition the device can be slid along the outer sheath of the introducer. In the locked condition, the position device is locked to the sheath. The position device is used to fix the length of sheath which can be inserted into a patient and thereby to set the position of the distal end the sheath and thereby the position at which a medical device would be deployed in the patient. The positioning device includes a coil spring with a plurality of coil turns able to apply an even compressive force to the sheath in all radial directions in order to minimize the chance of collapse of the sheath as a result of compressive force.
