Mandibular incisors
Mandibular incisors differ in size, with the central incisor being smaller than the lateral incisor. The roots of the mandibular incisors are so small and slender that they cannot be used as abutment teeth and should not have crowns placed on them. The size and morphology of these teeth are such that preparation of the tooth may damage the pulp.
The average dimensions of the mandibular central incisor are (Figs 4-24 and 4-25):
- Crown width (mesiodistal): 5.2 mm
- Crown depth (labiolingual): 6.0 mm
- Crown length: 9.0 mm
- Total apicocoronal length: 21.0 mm
The average dimensions of the mandibular lateral incisor are (Figs 4-26 and 4-27):
- Crown width (mesiodistal): 6.2 mm
- Crown depth (labiolingual): 6.5 mm
- Crown length: 9.5 mm
- Total apicocoronal length: 23.0 mm
In the case of the mandibular incisors, it is difficult to distinguish between left and right because only the lateral incisor has a trace of an angle characteristic. The horizontal curvature characteristic is insufficient on both teeth, but during manual fabrication, it can be constructed in the form of a more pronounced mesial marginal ridge. One possible distinguishing feature is the abrasion of the incisal margin: While the central incisor only has one wear facet, the lateral incisor has two wear facets because it has two antagonists. In addition, the incisal margin of some mandibular incisors is twisted in a distal direction in relation to the base of the crown.
The mandibular incisors need to exhibit a statically optimum chisel shape in order to perform their function because even the sturdy mandibular lateral incisor is generally smaller than the maxillary lateral incisor. This is also why the lingual surfaces are slightly concave.The prominent curvatures and convexities of the individual surfaces are poorly developed in comparison with those of the maxillary teeth.
The incisal margins have minimal mamelons shortly after eruption, similar to the maxillary teeth. These are soon abraded so that the edges are straight and level. This makes it even more difficult to distinguish between right and left. The contact points lie immediately incisally because of the abrasion of the incisal margins.
The vestibular (labial) surfaces of both teeth are almost smooth. There are traces of vertical grooves or cervical grooves.The triangular chisel shape of the vestibular surfaces and the straight incisal margin mean that the transition from incisal margin to approximal surface follows an acutely angled course mesially as well as distally. An angle characteristic is only partially present on the lateral incisor.The cervix is narrow and tapers sharply. The distal approximal margin is slightly drawn in compared with the mesial edge.
The lingual surfaces have the same basic shape but are slightly narrower. They are rather concave with a trace of marginal and medial ridges. The dental tubercle is less pronounced. The labiolin-gual diameter at the base of the mandibular incisor crown is greater than the width at the cutting edge.
The approximal surfaces are virtually the same size mesially and distally. They are extremely narrow in the incisal area and wider cervically, in keeping with the slender chisel shape. The slight vertical curvature of the labial surfaces and the slight depression in the lingual surfaces over the tubercle can be clearly seen. The cervical lines bend in an incisal direction.
The incisal views show the larger labiolingual diameter in contrast to the mesiodistal crown width. The basic incisal shape is oval without any appreciable curvature characteristics. The contact surfaces may be slightly depressed. On the lateral incisor, the cutting edge is twisted in relation to the base of the crown, so that the distal incisal edge lies posterior to the mesial one.
