Anatomical Knowledge for Modeling

(PHOTOGRAPH COURTESY OF STANISLAV GERANIN, POLTAVA, UKRAINE.)

A direct bonded composite restoration must blend into the tooth structure in terms of morphology and color. Just as no two teeth are identical, one model should never be the same as another. It is essential to study dental anatomy to know how teeth are made. This allows a faithful reproduction to be constructed that fulfills two fundamental objectives: blending in with remaining healthy tooth tissue and ensuring proper function during chewing movements.

This chapter describes the anatomical principles behind modeling, paving the way for relating the model to the residual tooth anatomy through interpolation of missing parts.

Anatomical Elements

The clinical anatomy of posterior teeth is characterized by certain well-defined elements explained in the following definitions and in Figs 2-1 to 2-5.

FIG 2-1 Main anatomical elements of the occlusal surface of a molar: cusp ridge (A), supplemental groove (B), central fossa (C), central developmental groove (D), marginal ridge (E), and the occlusal perimeter is A and E together.

FIG 2-2 Ridge slope (F) and cusp crest (G).

FIG 2-3 Cusp tip (blue), cusp slope (orange) cusp ridge (red), and cusp crest (purple).

FIG 2-4 Transverse ridge (H).

FIG 2-5 Oblique ridge (I).

  • Fossa (see Fig 2-1): Round, triangular, or four-sided depression in the crown. It is designated according to its position (eg, central, mesial, distal, marginal).
  • Pit: Deepest point of the fossa.
  • Groove (see Fig 2-1): Elongated linear depression. Differentiated into developmental and supplemental grooves.
  • Triangular ridge (see Fig 2-2): Defined by a cusp crest formed by two slopes. Each slope ends in a groove (primary or secondary).
  • Cusp (see Fig 2-3): Union between the triangular ridge occlusally (yellow) and the outer contour of the tooth (green), delimited by the cusp tip, cusp slope, mesial and distal cusp ridges (which, together with the marginal ridge, form a portion of the occlusal perimeter), and the cusp crest.
  • Transverse ridge (see Fig 2-4): Set of two opposing cusp crests that run perpendicular to the central developmental groove.
  • Oblique ridge (see Fig 2-5): Typical of maxillary molars, a set of cusp crests that run at an angle to the central developmental groove.

Fossae form where grooves meet one another: If there are three, the fossa will be triangular; if there are four, the fossa will be four-sided (Fig 2-6). The distinguishing traits of maxillary molars are always present and identify them as maxillary molars (Fig 2-7). Figure 2-8 shows two maxillary left first molars; they can easily be identified as maxillary molars even though they differ completely from one another in terms of cusp morphology, mesial marginal ridge type, and number of cusps. The characteristic traits include the presence of a central fossa, buccal groove, central developmental (mesiodistal) groove, distopalatal groove, and oblique ridge in particular positions. The characteristic shape, position, and dynamic function of these all-important anatomical elements must be understood. During occlusal reconstruction, the residual anatomical details are analyzed in order to extrapolate the missing shapes and achieve an anatomical restoration that works mechanically and esthetically. This chapter covers the distinguishing elements of each tooth type that should be considered during restoration.

FIG 2-6 Four-sided and triangular fossae (red).

FIG 2-7 Main and accessory anatomical elements of a maxillary left first molar.

FIG 2-8 (a and b) Two maxillary left first molars that appear completely different from each another.

Maxillary Premolars

Although the maxillary premolars are very similar to one another (Fig 2-9), they possess characteristics that help distinguish and identify them so that they can be drawn and modeled:

FIG 2-9 Maxillary first (left) and second premolars.

  • The first premolar looks squarer and more hexagonal than the second.
  • The concavity on the mesial marginal ridge of the first premolar is almost always absent on the second premolar.
  • The central developmental groove is longer on the first premolar than on the second.
  • The surface anatomy of the second premolar is more pronounced and complex.

Maxillary first premolar

The maxillary first premolar is bicuspid (Fig 2-10). The buccal cusp predominates over the palatal cusp, being slightly larger and higher.

FIG 2-10 (a and b) Clinical photograph and illustration of the occlusal surface of a maxillary first premolar. M, mesial.

One interesting feature, particularly for reconstructive purposes, is an interradicular concavity on the mesial side. This continues along the mesial wall and very often along the occlusal surface, causing a break in the marginal ridge. The central developmental groove runs along the premolar mesiodistally and is longer on the first premolar compared with the second premolar. Also in comparison with the second molar, the first premolar also displays a more uniform occlusal anatomy, featuring fewer secondary grooves, and the palatal cusp tip is often positioned more mesially (Fig 2-11). Figure 2-12 shows anatomical references whose specificities and variants should be considered when modeling. Figures 2-13 and 2-14 provide additional views of the occlusal surfaces of maxillary premolars, highlighting the variations that occur naturally.

FIG 2-11 Maxillary left first (left) and second premolars.

FIG 2-12 Maxillary right first premolar. (a) The central developmental groove is centered mesiodistally but located slightly palatally. The buccal cusp is slightly larger than the palatal cusp. In subtractive techniques, this will be the first groove to be defined; in additive techniques, the cusps will be defined as the result of moving masses closer together. M, mesial. (b) The central developmental groove extends in a buccal direction both mesially and distally, separating the buccal ridges from the palatal cusp as well as from the mesial and distal marginal ridges. Two small supplemental grooves sometimes extend from the primary groove toward the palatal cusp. (c) The mesial interradicular concavity can extend to the mesial marginal ridge, creating a depression or break in the ridge. (d) Sometimes this break in the marginal ridge continues into the central developmental groove. (e) The triangular ridges of the cusps are not usually accentuated. When they are, the associated supplemental grooves must be exaggerated accordingly.

FIG 2-13 Maxillary right first (right) and second premolars.

FIG 2-14 Maxillary left first (left) and second premolars.

Maxillary second premolar

The maxillary second premolar is very similar to the first (Fig 2-15). However, the central developmental groove is shorter, and there are many more supplemental grooves extending from it than in the first premolar. This tooth is much more symmetric than the first premolar and significantly more rounded. Figure 2-16

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May 25, 2021 | Posted by in General Dentistry | Comments Off on Anatomical Knowledge for Modeling
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