Molar occlusion in the ape-man (Australopithecus)

Abstract

T he common type of molar occlusion in man is normal occlusion; that is, when upper and lower molars are in maximum contact (centric occlusion), the mesiobuccal groove of the lower first molar receives the apex of the mcsiobuccal cusp of the upper first molar (Fig. 1). According to Baumc,l normal occlusion may develop by one of three mechanisms. The first permanent molars may erupt into normal occlusion directly. This occurs when the distal surface of the lower second deciduous molar is mesial to the plane of the upper second deciduous molar, a relationship referred to as a mesial terminal plane. Should a straight terminal plane obtain (the distal surfaces of both upper and lower second deciduous molars lie in the same vertical plane), then the first permanent molars, upon eruption, arc not engaged in normal occlusion; rather, they meet each other in a cusp-tip-to-cusp-tip relation. Subsequent development of normal occlusion depends upon the presence or absence in the deciduous dentition of a gap between the lower canine and the first molar. If this deciduous postcanine diastema is present, the lower first permanent molar soon drifts mesially into normal occlusion. If the diastema is absent, the cusp-tip-to-cusp-tip relationship is maintained until the deciduous molars are lost. Their loss creates the so-called leeway space, which is the difference measured mesiodistally between the larger deciduous molars and their smaller premolar (bicuspid) successors2, 3 With the loss of the deciduous molars and the gain of additional arch space, the lower first permanent molar may drift mesially into normal occlusion. Today, most children acquire normal occlusion indirectly by early and late rnesial driftI% 3 Baume’ believed that direct eruption was t,he “primitive pattern,” and Meyers,” also thought that direct eruption probably was ‘(more in keeping with nature’s plan.” Because of these views on the phylogeny of occlusion and