An arrangement for mounting a stent graft prosthesis (1) onto a deployment device (2) with a retention arrangement (3) which includes retention at a number of points of the circumference of the proximal end of the stent graft prosthesis (1). The arrangement provides a greater circumferential distance (13) between two adjacent retention points (9) then other of the points. When the deployment device is deployed in a curved lumen such as the thoracic arch it is oriented so that the greater circumferential distance (13) is on the inner side of the curve.