A bearing positioner kit for use with a bearing, the bearing having a convex surface and an oppositely facing bearing surface configured to bear against a first bone of a joint or prosthetic replacement for a portion of a first bone and a prosthetic component configured for attachment to a second bone of the joint which prosthetic component includes a bone facing surface configured to face the second bone and a concave surface facing away from the bone facing surface and defining a cavity, the kit comprising a bearing positioner and a fixation screw for fixing the bearing positioner to the prosthetic component, and the bearing positioner comprising: (a) an external convex surface having a shape configured to be received in the cavity in multiple orientations and to bear and be secured against the concave surface of the prosthetic component when in any orientation selected from the multiple orientations, (b) an internal concave surface defining a bearing-receiving cavity configured to receive a portion of the convex surface of the bearing therein and configured to orient the bearing surface of the bearing to bear against the first bone of the joint or the prosthetic replacement for the first bone when the external convex surface of the bearing positioner is secured in an orientation selected from the multiple orientations, and (c) a rim surface extending between and coupling the convex and concave surfaces, characterised in that the bearing positioner has an elongate slot at its apex for receiving the fixation screw so that the shaft of the screw extends through the slot allowing the screw to be rotated relative to the bearing positioner, and so that the shaft of the screw can slide in the slot, allowing the bearing positioner to adopt multiple angular orientations and multiple rotational orientations relative to the prosthetic component when fastened in the cavity of the prosthetic component by means of the fixation screw. (a) an external convex surface having a shape configured to be received in the cavity in multiple orientations and to bear and be secured against the concave surface of the prosthetic component when in any orientation selected from the multiple orientations, (b) an internal concave surface defining a bearing-receiving cavity configured to receive a portion of the convex surface of the bearing therein and configured to orient the bearing surface of the bearing to bear against the first bone of the joint or the prosthetic replacement for the first bone when the external convex surface of the bearing positioner is secured in an orientation selected from the multiple orientations, and (c) a rim surface extending between and coupling the convex and concave surfaces, characterised in that the bearing positioner has an elongate slot at its apex for receiving the fixation screw so that the shaft of the screw extends through the slot allowing the screw to be rotated relative to the bearing positioner, and so that the shaft of the screw can slide in the slot, allowing the bearing positioner to adopt multiple angular orientations and multiple rotational orientations relative to the prosthetic component when fastened in the cavity of the prosthetic component by means of the fixation screw.