US Patent 9451993 Bi-radial pop-on cervical bone anchor

A cervical polyaxial bone anchor includes a longitudinal connecting member receiver structure and a cooperating shank. The shank has a body and an integral head with first and second spherical surfaces of different radii and a plurality of spaced curvate apertures for receiving portions of the longitudinal connecting member to result in a greater angle of inclination of the shank with respect to the receiver. The receiver has an upper channel and a lower cavity that further includes an expansion area and an operational seat for a split retainer ring that captures and fixes the shank head within the receiver.