From the lingual aspect, the gross outline of the maxillary first molar is the reverse of that from the buccal aspect (see Figures 11-5, 11-6, 11-13, and 11-14). Photographs or drawings show this only approximately, because all teeth have breadth and thickness; consequently, the perspective of two dimensions plus the human element, which enters into the technique of posing specimens and making drawings and photographs, are bound to result in some error in graphic interpretation.
The variation between the outline of the mesial surface and that of the distal surface is apparent. Because of the roundness of the distolingual cusp, the smooth curvature of the distal outline of the crown becoming confluent with the curvature of the cusp creates an arc that is almost a semicircle. The line that describes the lingual developmental groove is also confluent with the outline of the distolingual cusp, progressing mesially and cervically and ending at a point at the approximate center of the lingual surface of the crown. A shallow depression in the surface extends from the terminus of the lingual groove to the center of the lingual surface of the lingual root at the cervical line and then continues in an apical direction on the lingual root, fading out at the middle third of the root.
Figure 11-10 Maxillary right first molar, distal aspect. (Grid = 1 sq mm.)
Figure 11-11 Maxillary molar primary cusp triangle. The distolingual lobe, represented by shaded areas, becomes progressively smaller on maxillary molars, starting with the first molar, which presents the greatest development of the lobe. The plain areas, roughly triangular in outline, represent the max’llary molar primary cusp triangles.
FIGURE 11-12 Maxillary right first molar, occlusal aspect. (Grid = 1 sq mm.)
The lingual cusps are the only ones to be seen from the lingual aspect. The mesiolingual cusp is much larger, and before occlusal wear, it is always the longest cusp the tooth possesses. Its mesiodistal width is about three fifths of the mesiodistal crown diameter, with the distolingual cusp making up the remaining two fifths. The angle formed by the mesial outline of the crown and the mesial slope of the mesiolingual cusp is almost 90 degrees. An obtuse angle describes the junction of the mesial and distal slopes of this cusp.
The distolingual cusp is so spheroidal and smooth that it is difficult to describe any angulation on the mesial and distal slopes.
The lingual developmental groove starts approximately in the center of the lingual surface mesiodistally, curves sharply to the distal as it crosses between the cusps, and continues on to the occlusal surface.
The fifth cusp appears attached to the mesiolingual surface of the mesiolingual cusp. It is outlined occlusally by an irregular developmental groove, which may be described as starting in a depression of the mesiolingual line angle of the crown, extending occlusally toward the point of the mesiolingual cusp, then making an obtuse-angle turn toward the terminus of the lingual groove and fading out near the lingual groove terminus. If the fifth cusp is well developed, its cusp angle will be sharper and less obtuse than that of the mesiolingual cusp. The cusp ridge of the fifth cusp is approximately 2 mm cervical to the cusp ridge of the mesiolingual cusp (see Figure 11-5).
All three of the roots are visible from the lingual aspect, with the large lingual root making up most of the foreground. The lingual portion of the root trunk is continuous with the entire cervical portion of the crown lingually. The lingual root is conical, terminating in a bluntly rounded apex.
All of the mesial outline of the mesiobuccal root and part of its apex may be seen from this angle.
The distal outline of the distobuccal root is seen above its middle third, including all of its apical outline.
From the mesial aspect, the increased buccolingual dimensions may be observed, as well as the outlines of the cervical curvatures of the crown at the cervical third buccally and lingually, the difference in dimensions between the crown at its greatest measurement, and the distance between the cusp tips in a buccolingual direction (see Figures 11-7, 11-8, 11-13, 11-14, and 11-16).
Starting at the cervical line buccally, the outline of the crown makes a short arc buccally to its crest of curvature within the cervical third of the crown. The extent of this curvature is about 0.5 mm (see Figure 11-13). The line of the buccal surface then describes a shallow concavity immediately occlusal to the crest of curvature.The outline then becomes slightly convex as it progresses downward and inward to circumscribe the mesiobuccal cusp, ending at the tip of the cusp well within projected outlines of the root base.
If the tooth is posed so that the line of vision is at right angles to the mesial contact area, the only cusps in sight are the mesiobuccal, the mesiolingual, and the fifth cusps. The mesiobuccal root hides the distobuccal root.
The lingual outline of the crown curves outward and lingually approximately to the same extent as on the buccal side. The level of the crest of curvature is near the middle third of the crown rather than a point within the cervical third, as it is buccally.
If the fifth cusp is well developed, the lingual outline dips inward to reveal it. If it is undeveloped, the lingual outline continues from the crest of curvature as a smoothly curved arc to the tip of the mesiolingual cusp. The point of the cusp is more clearly centered within projected outlines of the root base than the tip of the mesiobuccal cusp. The mesiolingual cusp is on a line with the long axis of the lingual root.
The mesial marginal ridge, which is confluent with the mesiobuccal and mesiolingual cusp ridges, is irregular, the outline curving cervically about one fifth the crown length and centering its curvature below the center of the crown buccolingually.
Figure 11-13 Maxillary right first molar. Graph outlines of five aspects are shown. (Grid = 1 sq mm.)
The cervical line of the crown is irregular, curving occlu-sally, but as a rule not more than 1 mm at any one point. If a definite curvature is present, it reaches its maximum immediately above the contact area.
The mesial contact area is apical to the marginal ridge but closer to it than the cervical line, approximately at the junction of the middle and occlusal thirds of the crown (see Figure 11-8). It is also somewhat buccal to the center of the crown buccolingually. A shallow concavity is usually found just above the contact area on the mesial surface of the maxillary first molar. This concavity may be continued to the mesial surface of the root trunk at its cervical third.
The mesiobuccal root is broad and flattened on its mesial surface; this flattened surface often exhibits smooth flutings for part of its length. The width of this root near the crown from the buccal surface to the point of bifurcation on the root trunk is approximately two thirds of the crown measurement buccolingually at the cervical line. The buccal outline of the root extends upward and outward from the crown, ending at the blunt apex. The greatest projection on this root is usually buccal to the greatest projection of the crown. The lingual outline of the root is relatively straight from the bluntly rounded apex down to the bifurcation with the lingual root.
The level of the bifurcation is a little closer to the cervical line than is found between the roots buccally. A smooth depression congruent with the bifurcation extends occlusally and lingually almost to the cervical line directly above the mesiolingual line angle of the crown.
Figure 11-14 Maxillary right first molar.
Figure 11-15 Maxillary first molar, buccal aspect. Ten typical specimens are shown.
Figure 11-16 Maxillary first molar, mesial aspect. Ten typical specimens are shown.
The lingual root is longer than the mesial root but is narrower from this aspect. It is banana-shaped, extending lingually with its convex outline to the lingual and its concave outline to the buccal. At its middle and apical thirds, it is outside of the confines of the greatest crown projection. Although its apex is rounded, the root appears more pointed toward the end than the mesiobuccal root.