|Year : 2018 | Volume
| Issue : 1 | Page : 18-22
Assessment of dental caries and oral hygiene status among twins
Priya Subramaniam1, KL Girish Babu2, Bharath Vardhana1
1 Department of Pedodontics and Preventive Dentistry, The Oxford Dental College and Hospital, Bengaluru, India
2 Department of Dentistry, Hassan Institute of Medical Sciences, Hassan, Karnataka, India
|Date of Web Publication||30-Mar-2018|
Department of Pedodontics and Preventive Dentistry, The Oxford Dental College and Hospital, Hosur Road, Bangalore-68, Karnataka
Source of Support: None, Conflict of Interest: None
The study of twins provides a unique opportunity to evaluate the genetic and nongenetic contributions to a variance of multitude of traits. Twin research has revealed the impact of a strong genetic component to the variance of many dental traits. The present study was conducted to assess the dental caries and oral hygiene status in monozygotic (MZ) and dizygotic (DZ) twins. A random sample of 129 MZ and DZ twin pairs aged 6-15 years were included in this study. The zygosity of twin pairs was initially recorded by facial appearance and later determined by dermatoglyphics. Dental caries and oral hygiene status were recorded in these twin pairs and the data were subjected to statistical analysis. No significant difference was observed in the mean decayed, extracted due to caries and filled teeth (deft) in primary dentition and decayed, missing and filled (DMFT) in permanent dentition scores and oral hygiene status in MZ and DZ twin pairs. The similarity observed in oral health status reflects the effects of a shared environment in MZ and DZ twin pairs.
Keywords: Dental caries, dermatoglyphics, facial appearance, oral hygiene index-simplified, zygosity
|How to cite this article:|
Subramaniam P, Girish Babu K L, Vardhana B. Assessment of dental caries and oral hygiene status among twins. J Forensic Sci Med 2018;4:18-22
| Introduction|| |
Twin studies are typically used to detect genetic variance of traits or conditions that are multifactorial.  Twins can be dizygotic (DZ) (nonidentical) or monozygotic (MZ) (identical). DZ twins develop from a separate ovum and a separate spermatozoon with each embryo having its own placenta and amniotic sacs and independent genetic constitutions. These twins therefore need not be of the same sex nor resemble each other.  The MZ twins develop from a single fertilized ovum. This type of twins results from the splitting of the zygote at various stages of development. Although the arrangement of the embryo on the uterine wall resembles that of DZ type, the two can be recognized as partners of an MZ pair by their strong resemblance in blood groups, fingerprints, sex, and external appearances such as eye color and hair color. 
Genetics is said to have a role in phenotypic expression of traits. A reasonably accurate means of distinguishing between MZ and DZ twin pairs is by comparisons of physical features, for example, body build, facial appearance, eye color, and ear form. However, this does not always lead to correct determination of zygosity.  Of the various techniques which have been employed for zygosity determination, dermatoglyphics is among the oldest and is a noninvasive method.
MZ and DZ twins provide an effective means for establishing detectable genetic variability and studying genetic environment interactions in dental development. Data on oral conditions of twins in India are lacking. Therefore, this study was undertaken to assess and compare both dental caries and oral hygiene status in MZ and DZ twin pairs.
| Materials and Methods|| |
The present study was conducted on 129 twin pairs, including MZ and DZ, aged 6-15 years in Bangalore city, India. The city was divided into four zones, and schools were selected from all these zones. A total of 120 schools were visited. After obtaining consent from the schools, preliminary identification of twins was done from the records of the school office admission register. On subsequent visits to the school, oral examination of the twin pairs was carried out.
The study protocol was approved by the Institutional Human Ethics Committee. The nature of study was explained to both school authorities and parents before conducting the study. Permission and written consent were obtained from the school authorities and parents for examination of the twin children.
Initially, 142 pairs of twins were examined. The following children were excluded from the study: (1) Children with handicapping conditions including medically compromised individuals. (2) Children on long-term medication. (3) Children in whom first permanent molars had not completely occluded. (4) Children undergoing orthodontic treatment. (5) Children with primary dentition only. Thus, a total of 129 pairs of twins were included which comprised 71 pairs of MZ and 58 pairs of DZ twins.
A pro forma was used to record date of birth, gender, demographic details, oral findings, and also for recording of fingerprints. The children were seated upright on a chair and were examined in adequate natural daylight so as to receive maximum illumination. Examination of the child was done by only one examiner to avoid interexaminer variability. The examination was performed using disposable sterile mouth mirror and probe. Recording of data was done by a single trained assistant throughout the study.
The WHO criteria  was followed to record dental caries (ie decayed, extracted due to caries and filled teeth (deft) in primary dentition and decayed, missing and filled teeth (DMFT) in permanent dentition). The oral hygiene index simplified-modified (OHIS-M) index was proposed by Miglani et al.  and OHI-S by Green and Vermilion  was used to record the oral hygiene status of the children with mixed and permanent dentition, respectively.
Determination of zygosity
Facial photographs of twins were taken and the zygosity of twins, i.e., either MZ or DZ based on facial appearance was noted down in the pro forma.
Zygosity of the twins was also determined using dermatoglyphics.  The impressions of the distal phalanges and palm impressions of the right and the left hands were recorded using printer's ink by the ink method.  A small amount of ink was placed on the glass slab. A rubber roller/eraser was then used for an even spread of the ink on the glass slab followed by recording of the distal phalanges and palm impressions onto a standard size paper placed on a firm surface. The inked finger or the palm was pressed. 
The ridge count was made of the distal phalanges of all ten fingers of both hands for each child of the twin pair. The ridges, also called epidermal ridges, are more or less parallel lines within a limited area of the ridged skin. The basic dermatoglyphic landmarks found on the fingerprint patterns are the triradii and core. A triradius is formed by the confluence of three ridge systems. Ideally, the triradial point is the meeting point of three ridges that form angles of approximately 120Ί with one another. The triradial point forms the terminus of the line along which ridges are counted. Another important landmark employed in ridge counting is the core which forms the approximate center of the pattern [Figure 1].
The ridge counting was done along a straight line connecting the triradial point to the point of core. The ridges containing the point of core and the triradial point were both excluded from the count. Every ridge crossing the line was counted, including a ridge that was seen to terminate just after crossing the line. However, a ridge terminating just before touching the line was not counted. If the ridge showed bifurcation before or on meeting the line, two ridges were counted [Figure 2].
Total finger ridge count: to establish zygosity, total ridge count of all the fingers of both hands of the twin pair was compared. The comparisons were made as described by Nylander  for a twin pair (A and B). Homolateral comparison: the sum of all the ridge counts of the distal phalanges on the right hand of twin A was compared with sum of the ridge count on the right hand of twin B, and the difference calculated. Similarly, the total ridge count of the left hand of twin A was compared with that of the left hand of twin B. The mean difference was then calculated. Bilateral comparison: the total ridge count of right hand of twin A was compared with that of his left hand, and the same was done for twin B. The average difference in ridge count between the two pairs of hands compared was determined. Heterolateral comparison: comparison of total ridge count was made between the right hand of twin A and left hand of twin B and also between left hand of twin A and right hand of twin B. The mean difference in total ridge count compared was found and was tabulated.
The mean of the difference of the homolateral and the heterolateral was then compared with the bilateral difference.
From the above data, the mean difference of homolateral and heterolateral comparisons was (Homolateral + Heterolateral) = x; this value should not be more than the differences in the bilateral comparisons = y; therefore, if x < y, based on this evidence, the twins were considered to be MZ,  or if the difference was greater, it was DZ. 
Descriptive statistical analysis was carried out in the present study. Significance was assessed at 5% level of significance. Student's t-test (two-tailed, independent) was used to find the significance of study parameters on continuous scale between two groups. Intergroup analysis on metric parameters and Chi-square/Fisher exact test was used to find the significance of study parameters on categorical scale between two or more groups. Kappa statistic for agreement: interrater agreement statistic (Kappa) to evaluate the agreement between two classifications on ordinal or nominal scale. Agreement is quantified by the Kappa (k) or Weighted Kappa (kw) statistic. SPSS software 19.0 (IBM Corp. Released 2010. IBM SPSS Statistics for Windows, Version 19.0. Armonk, NY: IBM Corp.) were used for the analysis of the data.
| Results|| |
Of 129 pairs of twins, there were 71 pairs of MZ twins and 58 pairs of DZ twins. The age-wise distribution of twin pairs is given in [Table 1]. Oral hygiene was good in about 90% of both MZ and DZ twin pairs. Twin A and twin B of MZ twins showed similar oral hygiene status, with no significant difference between them. A similar finding was observed in DZ twin pairs [Table 2]. In both MZ and DZ twin pairs, the deft scores were higher than the DMFT scores. There was no statistically significant difference in deft and DMFT scores between twins in MZ and DZ twin pairs [Table 3]. The concordance between zygosity as determined by facial appearance and dermatoglyphics showed 77.5% concordance for MZ twin pairs and 82.8% concordance for DZ twin pairs (degree of agreement was moderate, k =0.597) [Table 4].
|Table 2: Comparison of oral hygiene index - simplified in monozygotic and dizygotic twin pairs |
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|Table 3: Comparison of deft and DMFT in monozygotic and dizygotic twin pairs |
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|Table 4: Determination of zygosity in monozygotic and dizygotic twin pairs |
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| Discussion|| |
The classical twin approach of comparing identical (monozygous) twins and nonidentical twins (dizygous) for differentiating the effects of nature and nurture reflects environmental factors whereas differences between DZ pairs are both due to genetic and environmental factors. ,
In the present study, twins were initially segregated as MZ (identical) and DZ (nonidentical) on the basis of general facial appearance. This method of zygosity recording is easier, noninvasive, and requires little cooperation from the twin pairs. Comparison of facial appearance is a reasonably accurate means of distinguishing between MZ and DZ twin pairs. ,, However, there can be errors;  hence, zygosity was further confirmed with the help of dermatoglyphics.
Dermatoglyphics is one of the oldest methods, and there are marked differences in the variability of dermatoglyphic patterns within MZ and DZ twin pairs. , Although zygosity determination by DNA identification analysis is the most accurate, it involves drawing blood. Studies have found dermatoglyphics quite useful in zygosity determination. ,,,
Fingerprints show patterns such as loops, whorls, and arches. The hand comparison of ridge count is stated to be efficient in determining zygosity.  Dermal ridges are formed during the 3 rd or 4 th months of fetal life and are thus largely determined by heredity , and remain unchanged throughout life. The differences in the intrauterine environment of the MZ twin pair are responsible for the differences in ridge configurations in as much the same way that the prints of the right hand of an individual show differences from those of the left hand. This forms the basis for the analysis of fingerprints by hand comparison.
Moderate degree of agreement was obtained between zygosity established by facial appearance and by hand comparison of total ridge count. While MZ twin pairs showed 77.5% concordance, the DZ twin pairs showed 82.8% concordance. This was in accordance with Nylander who reported 82% concordance in MZ twin pairs and 89% concordance in DZ twin pairs. 
Studies have reported a marked genetic component to dentate status and dental caries experience, including that of tooth size, dental malalignment, occlusion, and tooth morphology. , Genetic factors are said to play an appreciable part in determining an individual's resistance to dental caries.  Further, an increased contribution of environmental factors to the occurrence of dental caries in twins has been stated. ,
In earlier investigations , on twins, similar environmental aspects represented a confounding variable in the separation of full contribution of inheritance to the incidence of dental caries. To understand the role of inheritance and incidence of dental caries, a landmark study on twins reared apart; Boraas et al.  concluded that there is a marked genetic component to dentate status and dental caries experience.
Dental caries was observed to be similar in both MZ and DZ twin pairs. Primary dentition caries was seen to be higher. This may be due to larger number of twin pairs included in the 6-11 age groups and probably be due to their similar dietary pattern and oral hygiene habits. Some studies have suggested a stronger environmental contribution to caries experience , whereas others have found a strong genetic contribution. , It has been suggested that at an early age, the genetic contribution to caries susceptibility is significant, and as individuals age, environmental factors account for more of the variance in dental caries traits.  Similarly, the oral hygiene status did not differ between the twin pairs, irrespective of zygosity (monozygosity or dizygosity). This is due to similar oral hygiene practice followed by the twin pair of children at home.
| Conclusion|| |
- The dental caries and oral hygiene status were similar among the twins
- Dermatoglyphics can be used as a noninvasive method for determination of zygosity.
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Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2]
[Table 1], [Table 2], [Table 3], [Table 4]