Comparison of posterior occlusion between patients with anterior open bite and scissor deep bite

Objective A minor alteration in the posterior occlusal height elicits a large transformation in the anterior vertical dimension. Thus, the present study was performed to determine whether a posterior cusp-to-cusp relation that increases the posterior vertical dimension contributes to an anterior open bite. Methods Study casts were examined from orthodontic patients aged 10 to 27 years, 21 with an open bite and 28 with a scissor deep bite. A logistic regression model was used to analyze the contribution of various factors to these two anterior occlusal patterns. The dental arch width and number of worn cusps were compared between the two groups. Results Patients with an open bite had a significantly higher incidence of a posterior buccal–lingual cusp-to-cusp relation, wider mandibular arch in the molar region, and larger numbers of worn maxillary buccal cusps and mandibular lingual cusps than patients with a scissor deep bite. Conclusions A posterior buccal–lingual cusp-to-cusp relation is associated with a larger anterior vertical dimension, such as that in patients with an open bite.

Mandibular and dental characteristics of Late Triassic mammaliaform Haramiyavia and their ramifications for basal mammal evolution

As one of the earliest-known mammaliaforms, Haramiyavia clemmenseni from the Rhaetic (Late Triassic) of East Greenland has held an important place in understanding the timing of the earliest radiation of the group. Reanalysis of the type specimen using high-resolution computed tomography (CT) has revealed new details, such as the presence of the dentary condyle of the mammalian jaw hinge and the postdentary trough for mandibular attachment of the middle ear—a transitional condition of the predecessors to crown Mammalia. Our tests of competing phylogenetic hypotheses with these new data show that Late Triassic haramiyids are a separate clade from multituberculate mammals and are excluded from the Mammalia. Consequently, hypotheses of a Late Triassic diversification of the Mammalia that depend on multituberculate affinities of haramiyidans are rejected. Scanning electron microscopy study of tooth-wear facets and kinematic functional simulation of occlusion with virtual 3D models from CT scans confirm that Haramiyavia had a major orthal occlusion with the tallest lingual cusp of the lower molars occluding into the lingual embrasure of the upper molars, followed by a short palinal movement along the cusp rows alternating between upper and lower molars. This movement differs from the minimal orthal but extensive palinal occlusal movement of multituberculate mammals, which previously were regarded as relatives of haramiyidans. The disparity of tooth morphology and the diversity of dental functions of haramiyids and their contemporary mammaliaforms suggest that dietary diversification is a major factor in the earliest mammaliaform evolution.