An implant assembly and its method of use, used in computer-aided orthognathic surgery for properly repositioning a mobile bone segment separated by osteotomy from a malformed mandible, maxilla or chin, the implant assembly comprising fixation bone plates, at least one removable alignment plate for proper pre-osteotomy placement of the implant assembly onto the mandible, maxilla or chin, at least one removable post-osteotomy alignment plate for proper positioning of the mobile bone segment relative to the mandible, maxilla or chin, and a removable cutting guide for guiding the osteotomy saw during separation of the mobile bone segment from the mandible, maxilla or chin.