Context: During human evolution, the morphology of mandibular molar occlusal surface has changed from pattern "y" to pattern "+". Six types of occlusal patterns were classified as: +4, 4-y,+5, 5-y,+6 and 6-y.
Aims: To determine the prevalence of these six types of mandibular second molars in Iranian adolescents.
Settings and Design: This descriptive investigation was undertaken in the high schools of Isfahan city, Iran. The students were selected by cluster sampling method, and then they were screened and only those with erupted mandibular second molars bilaterally were selected.
Materials and Methods: A total of 794 cases were randomly selected and the number of cusps and groove pattern of mandibular second molar were examined intra-orally and by studying dental casts.
Statistical Analysis Used: Descriptive statistics and chi-square test were used for data analysis. Results: The most frequent occlusal configuration was the "+4" form (76.9%). A total of 683 cases (86%) were found to have four-cusp form, 104 cases (13.1%) were five-cusp form and 7 cases (0.9%) were six-cusp form.
Conclusion: The most frequent occlusal configuration was the "+4" form; thus, there is a high evolutionary trend in Iranian mandibular second molars.
How to cite this article: Mosharraf R, Ebadian B, Ali Z, Najme A, Niloofar S, Leila K. Occlusal morphology of mandibular second molars in Iranian adolescents. Indian J Dent Res 2010;21:16-9
How to cite this URL: Mosharraf R, Ebadian B, Ali Z, Najme A, Niloofar S, Leila K. Occlusal morphology of mandibular second molars in Iranian adolescents. Indian J Dent Res [serial online] 2010 [cited 2011 Dec 18];21:16-9. Available from: http://www.ijdr.in/text.asp?2010/21/1/16/62802
In most dental anatomy textbooks, the permanent mandibular second molar is often described as having a simple morphological design, that consists of four cusps, placed on a square occlusal surface and a cruciform (+) groove pattern., However, variations in size, cusp number and groove pattern have been observed in mandibular molars of different populations.
In anthropological studies, morphological categories used to describe these variations in occlusal surfaces of the mandibular molars are based on a topology developed by Gregory and Hellman and Hellman: "5 - y," "4 - y", "+5" and "+4". The criterion for determining whether a pattern is a "y" or a "+" is contact of the metaconid with the hypoconid. If contact occurs, the pattern resembles a "y"; if no contact occurs, the pattern resembles a "+". The occurrence of the "y" or "+" fissure pattern is independent of the number of cusps. Thus, groove pattern and cusp number are considered separately because their evolutionary changes are not well correlated phenotypically. It is assumed that this trait (fissure pattern) is polygenic and its expression is determined by combinations of alleles at two or more loci. Basically, the five-cusp mandibular second molar is far more characteristic of Mongoloid and Negroid populations than Caucasoid.,,,, It is therefore not uncommon to attempt to differentiate different ethnic populations by their different morphological features. ,,,,,
It is not known whether ethnicity influences dental morphology. However, it is observed that there are different degrees of expression and frequency in variation of teeth in dentitions of different populations. The final tooth form represents the sum total of its genetic endowment and long-term environmental influences.
Dental anthropology is the study of the origin and the variations of the human dentition. It is a useful tool to identify geographic or racial affinities. Dentoanthropologic structures useful for identification purposes include cusp size, number and location of cusps, occlusal pattern, root configuration, number and arrangement of teeth, and individual tooth measurements.,, Few dental anthropological studies have investigated the associations between these dental features and crown traits in humans using quantitative methods.
The present descriptive study was undertaken to investigate the prevalence of six types of mandibular second molars in Iranian adolescents.
Materials and Methods
This descriptive investigation was undertaken in the high schools of Isfahan city, Iran. The students were selected by cluster sampling method, and then they were screened and only those with erupted mandibular second molars bilaterally were selected. Direct intraoral examination was done carefully and morphological details of the crown, namely, the number of cusps and groove patterns of teeth and gender of the subjects were recorded on prepared forms. Data were excluded from the investigation in cases in which the teeth were restored, worn or heavily broken down. After that, hydrocolloid impressions were taken and dental casts were immediately poured. Alginate (Alginoplast; Heraeus Kulzer, Hanau, Germany) was the impression material used and the casts were made of dental stone type III (Moldano; Heraeus Kulzer, Hanau, Germany). The study was repeated on the dental casts and the resultant data were compared with clinical data, for each subject. If any difference was found, the examination of that subject was repeated; hence, the final decision was made on the basis of clinical data. A cusp was considered as a pronounced elevation on the occlusal surface of a tooth terminating in a conical, rounded or flat surface.
A total of 794 cases (15-17 years old) were selected and the number of cusps and groove pattern of mandibular second molars were examined; 6 types of occlusal fissure pattern (4 - y, +4, 5 - y, +5, 6 - y and +6) [Figure 1] were recorded. Descriptive statistics and chi-square test were used for data analysis.
Of the 794 persons examined, 405 (51%) were males and 389 (49%) were females. In this population, we examined 1588 teeth (794 x 2), and the four-cusp form was the most frequent occlusal configuration (86%) [Table 1]. The predominant occlusal pattern was groove form with a "+" shape (87.6%) [Table 2]. The most frequent occlusal surface configuration was the "+4" form (76.9%), and the "6 - y" form was seen only in one case [Table 3]. Most cases (567/794; 71.4%) were bilateral "+4" form [Table 4]. The rates of each type of occlusal patterns between males and females had no significant differences in "5 - y", "+4", "6 - y" and "+6" groups. However, "+5" form had significantly higher rate in males (P Left = 0.001/P Right = 0.020). Chi-square analysis also revealed significantly higher rate in females in the "4 - y" form group (P < 0.001).
Using the Pearson correlation test, significant correlations were observed between left and right occlusal pattern shapes (P < 0.01).
Few dental anthropological studies have investigated the associations between these dental features and crown traits in humans using quantitative methods.
In this type of studies, some researchers used intraoral examination,,, some studied dental casts, ,,,, and some used both methods., Intraoral examination has the advantages of accurate recording, proper identification of teeth, and follow-up of patients when needed. It ensures racial and sexual identification. A sample of extracted teeth would be less than ideal., For more precise results, we used both methods (intra-oral and cast examinations) in the present study.
Hellman classified the mandibular molars based on the occlusal pattern and the number of cusps. According to him, the basic pattern is the "y-5" type, with five cusps and a Y-shaped occlusal configuration. Loh stated that the distobuccal cusp (hypoconulid) is the most variable and in the evolutionary advanced type, it disappears and therefore leads to a four-cusp form. He observed a relatively high incidence of five-cusp second molars in the Singaporean population. Observations on the teeth of the Chinese from mainland China were made by Montelius, who reported a high incidence (56%) of five-cusp forms in this tooth. In 1997, Guo et al. by observations in new population of china, stated that the rate of "+4" in the second mandibular molars is the highest, while the rate of "y - 5" is the lowest. They stated that this fact may be due to gradual evolution in the morphology of mandibular molar occlusal grooves that have changed from pattern "y" to pattern "+".
In 1985, Hasund and Bang revealed that in Alaskan Eskimo dentition, the predominant pattern of the lower first molar was "y - 5," while for the second molar the dominating patterns were "+5" and "+4," and in the lower third molar, "+5" was found in the majority of cases. Devoto and Perrotto stated that the "+" groove pattern appeared on second molar more than other two molars.
In this study, the most frequent occlusal configuration was four-cusp form (86%) and the predominant groove pattern was "+" shape (87.6%). The most frequent occlusal surface configuration was the "+4" form (76.9%), and most cases (71.4%) were bilateral "+4" form. This high percentage of groove pattern with "+" shape and low percentage of primitive "Y" pattern in our study show a high evolutionary trend persisting in Iranian second molars.
Some of the studies suggested a sex predilection in men to have the "y - 5" or Dryopithecus pattern.,, In this investigation, no significant difference was observed between males and females in "5 - y," "+4," "6 - y" and "+6" groups. However, "+5" form had a significantly higher rate in males and "4 - y" form had a significantly higher rate in females. In study by Guo et al., the rates of each type of groove between males and females showed no significant difference.
In this study, Pearson correlation test revealed there were significant correlations between the left and right occlusal pattern shapes. This would suggest an inherent genetic factor rather than a casual occurrence.
The study of dental morphological characteristics and odontometry is important in anthropological research as it can provide information on the phylogenetic relationship between species, as well as variations and diversities within a population. Furthermore, knowing common variations in dental anatomy and morphology about each individual tooth can help in performing some dental treatments such as restorative, endodontic and orthodontic treatments., Therefore, the results of this anatomical study can be used in both anthropological researches and clinical aspects of dental sciences.
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