An intramedullary pin for treating a proximal fracture of the humerus, the pin having at least one distal and at least one proximal part which can be moved in relation to one another in order to modify the length of the implant and which have co-operating guide surfaces. The proximal and the distal part have co-operating stops for limiting the axial relative movement, the proximal part and distal part being capable of relatively free movement within the delimitations defined by the stops and each being provided with at least one transverse bore for receiving and/or fastening fixing means. The implant is also provided with a mechanism for locking the relative movement of the two parts about the implant axis.