An ulnar prosthesis assembly comprising a head formed with a curved surface extending between opposed ends and a stem. One of the ends of the head has an attachment bore defined therein. The attachment bore generally has a first diameter and includes an attachment groove thereabout with a second diameter greater than the first diameter to define at least one attachment shoulder. The stem has a stem body extending between a proximal end configured for implantation in a target bone and a distal end configured to be received in the attachment bore. The distal end has a locking flange extending radially therefrom. The locking flange defines a locking shoulder having a diameter greater than the first diameter such that upon receipt in the attachment bore, the locking shoulder engages the attachment shoulder.