Key words:
Dental occlusion - mandible - maxilla - molar tooth - permanent dentition
INTRODUCTION
According to Angle’s definition, a correct molar relationship exists when the upper
first molar’s mesiobuccal cusp occludes into the buccal groove of the lower first
molar, and the teeth are arranged on a smoothly curving line of occlusion.[1] If this relationship occurs, a normal occlusion is present. The concept of angle
was supplemented by the “Six Keys to Normal Occlusion” by Andrews.[2] In his description, the first key, molar inter-arch relationship, is not very different
from Angle`s definition except that Andrews stated the distal marginal ridge of the
upper first molar must be occluded with mesial marginal ridge of lower second molar
[Figure 1].
Figure 1: Molar relationships with an excellent Class I occlusion described by Andrews
In 1998, the American Board of Orthodontics (ABO) introduced the objective grading
system (OGS) or cast/radiographic evaluation, a standard method to evaluate finished
cases considering eight criteria (alignment, marginal ridges, buccolingual inclination,
occlusal relationships, occlusal contacts, overjet, interproximal contacts, and root
angulation) in dental casts and panoramic radiographs.[3] The first molar relationship and cusp to fossa relationship are an important goal
of the OGS.
The purpose of the present study was to determine whether the sizes of the first molars
allow clinicians to achieve the first goal of an ideal clinical outcome on the cast
models were presented to the ABO.
MATERIALS AND METHODS
The research material consisted of 78 set diagnostic cast models that had been presented
at an exit examination at the University of Alabama Birmingham. In addition, these
cases had been examined and passed the clinical component of the ABO. Final casts
were obtained from the records of patients who have normal appearing teeth. No large
restorations or fixed prosthodontic replacement were present. Class I canine and premolar
relationships were also present. A boley gauge was used to measure the length from
the mesiobuccal cusp to the distobuccal cusp of the maxillary first molar (mbdbmax)
and the length from the mesiobuccal groove of the mandibular first molar to the occlusal
embrasure (mboemand) between the mandibular first and second molars. These two measurements
were taken on both sides of each set of dental casts for a total of four measurements
per set [Table 1].
Table 1:
Table 1: Distance of reference points (mm)
|
Measurements
|
n
|
Mean
|
SD
|
P
|
|
⋆Statistically significant for P<0.05. SD: Standard deviation
|
|
mbdbmax right (a)
|
78
|
5.12
|
0.68
|
0.004⋆
|
|
mboemand right (a1)
|
78
|
6.41
|
0.55
|
|
|
mbdbmax left (b)
|
78
|
5.22
|
0.66
|
0.003⋆
|
|
mboemand left (b1)
|
78
|
6.52
|
0.54
|
|
|
a/a1
|
78
|
80.3
|
10.7
|
0.93
|
|
b/b1
|
78
|
80.4
|
10.2
|
|
Statistical analysis of data was performed by means of SPSS v. 19 software (SPSS Inc.,
Chicago, IL, USA). A paired sample t-test revealed significant differences in tooth sizes between the reference points
on maxillary and mandibular first molars on both sides. The level of significance
was P < 0.05.
RESULTS
The results are presented in [Table 1]. The maxillary measurements ranged from 3.6 to 6.9 mm with an average of 5.2 mm.
The mandibular measurements ranged from 5.0 to 8.0 mm with an average of 6.5 mm. On
average, the mesiodistal length measured on maxillary first molars was about 80% of
that of their mandibular counterparts. Only 5 of the 78 sets of dental casts evaluated
had equal maxillary and mandibular measurements on one side (either left or right),
and none of them had equal measurements on both sides [Figure 2].
Figure 2: Molar relationships with a Class I occlusion case was presented and passed in the
American Board of Orthodontics
DISCUSSION AND CLINICALIMPLICATION
DISCUSSION AND CLINICALIMPLICATION
Tooth size discrepancies play an important role in precise planning and achieving
the best possible outcome.[4]
[5] To achieve the ideal Class I molar relationship that was described by Andrews, the
length from the mesiobuccal cusp to the distobuccal cusp of the maxillary first molar,
and the length from the mesiobuccal groove of the mandibular first molar to the occlusal
embrasure between the mandibular first and second molars should be equal. Unfortunately,
It is not possible every time, even if we have excellent Class I relationships in
canine and premolar areas. This presents a significant problem.
In this study, the following three recommendations are suggested because only 6% of
the sample would achieve Andrew’s goal.
-
If there is no difference, every effort should be used to finish the case with excellent
Class I occlusion. Anchorage management is critical
-
If the distance between the mandibular first molar reference points is a little bigger
than distance between the maxillary reference points, the clinician can biomechanically
create a little mesiobuccal rotational movement to maxillary first molar. This will
allow the maxillary first molar to occupy more space
-
If the distance between the mandibular first molar reference points is much more bigger
than distance between the maxillary reference points, a carefully clinical judgement
has to be made. Instead of finishing the occlusion with the mesiobuccal cusp of the
maxillary molar in the mesiobuccal cusp of the mandibular molar, it is suggested the
distobuccal cusp of the maxillary molar be finished in the embrasure between the mandibular
first and second molar [Figure 3]. This position will allow the maxillary second molars and second bicuspids to be
finished in an ideal cusp to fossa position.
Figure 3: Clinical tip to finish the case for first molar position
CONCLUSION
Tooth-size discrepancy must be taken into consideration when planning orthodontic
care, and clinicians have to understand that tooth size discrepancies do exist in
patients and that these discrepancies make the completion of a perfect case challenging.
In this article, clinical suggestions have been made to better finish orthodontic
cases.
Financial support and sponsorship
Nil.
