A device comprises an arm cover to be positioned under the arm before being closed thereover, a shoulder strap to be positioned under the shoulder and adapted to wrap the shoulder when attached, over the shoulder, to the closed arm cover and a longitudinal strap to be fixed to the table opposite to the head. The longitudinal strap can apply tension on the shoulder strap when shortened. A method for installing a device on an immobilised patient comprising positioning an arm cover under the arm and a shoulder strap connected to the arm cover under the shoulder, closing the arm cover over the arm and attaching the shoulder strap, over the shoulder, to the closed arm cover, fixing an end of a longitudinal strap, connected to the arm cover, to the table opposite to the head and pulling on the longitudinal strap to apply tension on the shoulder strap.