An apparatus and modeling method of the present invention includes the successive steps of generating cartographic data representative of points belonging to a glenoid surface; distinguishing from among the cartographic data a first group of cartographic data corresponding to a first part of the glenoid surface, the first surface part being situated farthest down in the vertical direction in relation to the scapula; calculating from the first group of cartographic data a first ellipsoid portion that coincides substantially with the first surface part; and obtaining a theoretical glenoid surface from the first ellipsoid portion. By virtue of the theoretical glenoid surface obtained by this method, it is possible to assist the surgeon in optimizing the position of implantation of a glenoid component and to produce a glenoid component “made to measure” for the scapula that is to be fitted with a prosthesis.