The Prognathous Mandible: Its Investigation

T contours of the face are largely dependent on the proportionate si/.c and shape of the individual bones forming the skuli. Thus a concave profile could be caused by an anterior position of the fron­ tal region of the skull; a small maxillary complex; a large m andible; a forward position of the m andible; or a combina­ tion of these skeletal abnormalities. The d if fe re n tia l diagnosis be tw een maxillary deficiency and m andibular over­ growth and certain aspects of the investi­ gations prior to surgical treatm ent of m andibular overgrowths will be discussed. The first phase of the investigation of a patient with a concave profile is inspec­ tion. If the patient with a prognathous m an­ dible is tall and has well developed mid­ facial structures the prognathism can usually be attributed to an overgrowth of the mandible. On the other hand a lack of growth of the maxilla is usually respon­ sible for the seemingly prognathous m an­ dible seen in the small patient with the concave profile. The intra-oral examination and dental history of the patient usually provides valuable information. A part from such complicating factors as early extractions, habits, abnorm al tooth material, incorrect incline plane relationship, altered axial inclinations and pathological processes, the arch form is indicative of the location of the skeletal dysplasia. Correct align­ ment of all the teeth in a well rounded maxillary arch is a sign that maxillary growth has been adequate and that the prognathism is thus mainly due to m andi­ bular overgrowth. U nder these circum­ stances the teeth in the lower arch arc usually easily accommodated — often with spacing (Fig. 1). The movements of an enlarged m andi­ ble may also aid in the diagnosis. The re­ striction of protrusive movement, often to less than 10 mm., and the predominance of a hinge movement in the temporo­ m andibular jo in t during opening and closing arc more often associated with overgrowth of the mandible than with a maxillary deficiency. Cephalometric analysis is not only a valuable aid in diagnosis, but is essential for planning the surgical treatm ent. The cephalometric analysis of a patient with a m andibular prognathism not only yields information about dental pattern but the skeletal appraisal is perhaps one of the most reliable methods of assessing the proportional growth of the maxillary complex and the mandible. The Downs analysis of prognathism caused by a m andibular dysplasia shows the following features: 1. The facial plane angle is high or w ith­ in normal limits. 2. Angle of convexity is usually lower than normal. 3. A-B plane is usually more positive than normal. 4. M andibular plane angle is usually higher than normal. 5. Y axis is often within the normal range. Even though the diagnosis of an exces­ sive antero-posterior growth of the m an­ dible can usually be made on the Downs analysis only, the presence of the so-called button on the chin and high gonial angles will lend further support to the diagnosis. The main differences between Class I l l ’s (Angle) caused by m andibular over­ growth and by maxillary deficiency are demonstrated in Fig. 2. After clinical and cephalometric exami­