The gonial angle: A survey

Abstract

A survey of the literature reveals diverse interpretations of the term “gonial angle.” The term has been used in referring to the posterior inferior angle region of the mandible (Brodie), as well as to the angle formed by a tangent to the lower border of the mandible and a tangent touching the posterior border of the ramus at two points, one at the condyle and one at the angle region (Broca). Although the latter is more properly an indication for the relationship of the condyle to the mandibular body than a measurement of the gonial angle itself, it is the one commonly recorded and also the one considered in this survey. 1. 1. During evolution the gonial angle has decreased from an almost straight line in early reptiles to almost a right angle in anthropoids. It has become obtuse again in the transition from anthropoid to man. These changes in the gonial angle are associated with altered function of the jaws and masticatory muscles. as well as with the changes in posture and in the differential rates of growth of the components of the head. 2. 2. The gonial angle in man may vary from 100 degrees to 148 degrees. Its mean angle is highest in Caucasians, nearly as high in Chinese, Eskimos, and Negroes, and lowest in early Caucasians, Australian, and American Indians. 3. 3. In all racial groups the mean angle in females is 3 degrees to 5 degrees higher than in males. 4. 4. The large variation in the size of the gonial angle makes this trait useless for racial or sexual classification of mandibles. 5. 5. In well-muscled individuals the mandible is heavier and the gonial angle smaller than in individuals with less muscle mass. 6. 6. Conflicting results are reported when the size of the gonial angle is related to the cephalic index. According to Kieffer, individuals with short and broad faces (chamaeprosopic) have a smaller angle than individuals with a long and narrow face (leptoprosopic). 7. 7. A mandible with a broad and solid ramus has a smaller gonial angle than a mandible with a slender ramus. 8. 8. Cross-sectional studies indicate an increase in the size of the gonial angle from early embryonic stages to the time of birth and a continuous decrease from birth to old age. The decrease is greatest before 6 years of age; thereafter until adult life the rate of decrease is about one-half as great; and from maturity to old age it is only a few degrees. A complete loss of teeth may reverse the usual age changes and the gonial angle becomes more obtuse again. 9. 9. A longitudinal study (3 months to 8 years) by Brodie indicates that the contour of the mandible in the region of the angle remains the same during this period. 10. 10. The size of the gonial angle is associated with the proportion between facial height and ramus height. With a relatively greater facial height the angle is more obtuse (for example, open-bite); conversely, with a relatively smaller facial height it is more acute (for example, deep overbite). 11. 11. The size of the gonial angle is rather independent of variations in facial development seen in the sagittal plane.