Page 6 - GP Fall 2020

The Anterior Midline Critical Zone: Esthetic Shape Determinant

By Arthur R. Volker DDS, MSEd, FAGD, FACD and Katherine Mayorga, BS

“Symmetry is what we see at a glance; Elements of the Anterior Midline Criti- IV restoration was completed on the MILF
based on the fact that there is no reason for cal Zone surface of tooth #9. During the clinical ex-
any difference.” ecution, the author (Volker) deemed the in-
- Blaise Pascal 1. Horizontal and Vertical Cant cisal embrasures to be symmetrical. When
The vertical and horizontal positioning of the post-operative photo was taken, it was
Introduction the maxillary central incisors are the start- observed that these embrasures were not
There are several criteria that need be met ing point for smile design. They are the symmetrical. This most likely occurred due
for a successful esthetic outcome. While first teeth addressed when setting complete to the creation of a parallax. A parallax is a
this criterion can be complex, it can be dentures and other restorations. Once po- visual bias that occurs due to the observer’s
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distilled into four major categories: pro- sitioned, each subsequently placed tooth position relative to that of the object.
portionality, shape, color and texture. This will follow along this axis. If the initial
article seeks to discuss the anterior midline positioning of the maxillary centrals is If positioned as on a clock, the clinician,
critical zone as a means to assess and hone incorrect, and remaining teeth are set to who is right-handed, was working at the
the symmetry of restorations for maxillary this position, the smile is irreparably er- 10:00 position, with the patient at the 6:00
anterior teeth, thereby aiding in smile de- roneous, and must be completely redone position and assistant at the 2:00 (Figure
sign. since an undesired cant will result. Figure 4). If the angulation of the restored in-
1 demonstrates a clinical example of fixed
Seen throughout nature and adapted to
dentistry by Levin, golden proportions act
as a scaffolding for smile design. It estab-
1
lishes the width ratio of 0.6 (canine) : 1.0
(lateral) : 1.6 (central), and highlights cen-
tral dominance with the maxillary central
incisors possessing the widest mesio-distal
dimension with canines having the nar-
rowest dimensions. It should be noted that Figure 4 - Typical patient, assistant, and
a successful smile need not follow golden Figure 1 - Demonstration of restorations dentist alignment for a right-handed operator.
the were crafted with improper vertical and
proportionality exactly. While golden horizontal cant.
2,3
proportionality is variable, there are some cisal embrasure of #9 is followed, it points
elements of smile design that are im- restorations that were fabricated with an to the 10:00 position (Figure 5). This is
mutable. improper anterior midline critical zone. what the authors deem a “right-hand posi-
Although the proportions of the restoration tional bias”. For a left-handed clinician, a
Regarding shape, it is imperative that the are appropriate, there is an improper hori- “left-handed positional bias” would point
maxillary central incisors be mirror images zontal and vertical cant present. to the 2:00 position if the clinician is seat-
of each other. They must exhibit bilateral
symmetry. If symmetry is not achieved, 2. Incisal Embrasure
the smile can have visual tension and can Figures 2 and 3 are clinical examples of
look non-esthetic. It is the opinion of the the effect that an improper viewing per-
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authors that the closer to the midline, the spective can have on the outcome of an
more important that the teeth be bilaterally anterior restoration. In this case, a class
symmetrical. As a comparison, the central
incisors should be twins while the lateral Figure 5 - Orientation of incisal embrasure
incisors and canines should be siblings, aligning to the “10:00” position.
meaning they do not have to be mirror im-
ages, but should look similar. ed at that position. It is also important that
golden proportions should also be calculat-
To further the study of bilateral symmetry, ed from a direct frontal view. If calculated
there are definable criteria that must be Figure 2 - Pre-operative situation. from a non-centered view, a parallax will
considered, and when necessary, altered to result.
achieve a successful outcome. This crite-
rion is found at the midline of the smile, To prevent a parallax, the clinician should
specifically at the mesial facial surfaces seat the patient upright, and assume a direct
between the central incisors, named the an- frontal view, essentially face to face. It has
terior midline critical zone. often been taught that working at the 12:00
position is optimal, and that is often the
Figure 3 - Post-operative situation with case. Note however, that although the face-
asymmetric incisal embrasure.
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