A head fixation assembly includes a head fixation frame, a plurality of upper head fixation members, a plurality of lower head fixation members, and at least one drive mechanism. The head fixation frame includes a pair of upwardly extending spaced-apart arms defining a free space therebetween. At least one upper head fixation member extends from each of the respective arms of the head fixation frame, with each upper head fixation member adjustable relative to the head fixation frame and adapted to engage a patient's head within the free space of the head fixation frame. The lower head fixation members extend from the head fixation frame between the pair of arms, with each member adjustable relative to the head fixation frame and adapted to engage an underside of the patient's head within the free space of the head fixation frame. The at least one drive mechanism is in communication with the lower head fixation members, and is externally accessible so as to allow a user to be able to directly or indirectly advance and/or retract the lower head fixation members while the patient's head resides in the free space of the head fixation frame.