Harizanova et al.
Journal of Physiological Anthropology(2023) 42:15
https://doi.org/10.1186/s40101-023-00332-5

Journal of
Physiological Anthropology

Open Access

ORIGINAL ARTICLE

Few dental indices in modern Bulgarian
population from southern Bulgaria
Zdravka Harizanova1* , Atanas Baltadjiev1, Ferihan Popova1 and Marieta Peycheva2

Abstract
Aim The aim of the present study is to evaluate the reduction of the distal teeth towards the medial ones in one
functional dental group in southern Bulgarian population.
Materials and methods The study included 232 Bulgarians aged 20–40 years. Mesiodistal dimensions of the teeth
were measured by Dentistry Sliding Vernier Caliper and analyzed with SPSS 23.0. Four interdental indices were calculated: inter-incisive, premolar, upper, and lower molar indices.
Results We found a decrease in the percentage ratio of the lateral to the central incisors of people from the Bulgarian
population compared to those dating from the Eneolithic period on the territory of Bulgaria. Furthermore, we founda reduction in the percentage ratio of the upper and lower second molars compared to the first ones. The biggest
reduction in the percentage ratio (more than 6%) was found in the lower second premolars compared to the firstones, which is characteristic for southern Bulgarians.
Conclusion There was a dental reduction in all the distal members compared to the medial ones participating
in one morphological dental group. As a result, we think that interdental indices can be used for explaining historical, cultural, and biological macro and microevolutionary processes and thus for understanding the origin, formation,
contacts, and migration pathways of the different populations leading to ethnic variation of humanity. Therefore, they
can be a reliable source of information in physiological anthropology.
Keywords Physiological anthropology, Odontometry, Bulgarian population, Evolution, Dental reduction
Introduction
In anthropology, the ratios, between dimensions of theteeth from one morphological and functional group,
are of great interest because they can assist in studying
evolutionary trends in anthropological processes. The
human masticatory system, which consists of superior
jaw, inferior jaw, teeth, temporomandibular joint, and*Correspondence:
Zdravka Harizanova
Zdravka.Harizanova@mu-plovdiv.bg
1
Department of Anatomy, Histology and Embryology, Facultyof Medicine, Medical University of Plovdiv, 15A Vassil Aprilov Blvd.,
Plovdiv 4002, Bulgaria
2
Department of Neurology, Faculty of Medicine, Medical Universityof Plovdiv, 15A Vassil Aprilov Blvd., Plovdiv 4002, Bulgaria

masticatory muscles, is functionally related not only tonutrition but also to articulated speech. For this reason,
it has undergone enormous development in the historyof mankind. The factors that led to this are environmental factors: change in the type of food, its preparation,use of fire, and different tools and change in climate andlifestyle, as well as genetic factors associated with various
mutations, in relation to the upright posture, the increasein brain size, and the formation of language and speech.
There are many theories that are based on the change
in the diet as the main factor in the reduction of the masticatory complex in humans [12, 15, 20]. With the discovery of the fact that the development of dental germs andtheir differentiation into different functional groups (incisors, canines, premolars, molars) is under the control of© The Author(s) 2023. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which
permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to theoriginal author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images orother third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line
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Harizanova et al. Journal of Physiological Anthropology(2023) 42:15

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signal molecules that are stimulated by different genes,
two theories are formed:

lower molars in mastication, the length of the lower jaw
is shortened.
Cziko [6] points out that the evolution of the maxillofacial system is also related to the development of thebrain, which occurs as a result of verbal communication
between individuals. According to Aiello and Dean [2],another reason for that is the upright posture, since thesmaller size of the oral cavity supports the center of gravity of the human skull and therefore facilitates balance.
Stedman et al. [19] also support this theory, stating thatthe encephalization of the human species lead to a gracilization of the maxillofacial region and hence a reduction
in tooth sizes.
The aim of this study was to determine certain valuesof some interdental indices in South Bulgarians and toassess the degree of reduction of the distal tooth towards
the medial one in one morphological dental group.

1. “Space theory” was proposed by Butler [5] and developed by Dahlberg [7]. Butler believed that in eachdental arch, there is a concentration of morphological genes, the result of which is that each tooth
resembles its neighbor. This concentration is highest when the first tooth of each morphological groupis developing, so the medial tooth of each group is
the most stable and is termed the key tooth. At thesame time, the difference in it leads to variations in
tooth morphology. With moving away from the key
tooth, this concentration decreases for the more distally positioned tooth. As an inference to this theory,
Dahlberg [7] believed that more variations are shownin the size of the distal teeth of each group, i.e., thevariations in lateral incisors are greater than these inmedial ones, the second premolars are greater than
the first, and in the second upper and lower molars,
they are larger than the first.
van Valen [21] developed Butler’s theory by suggestingthat variation in tooth size within a group is due to a prepattern that is responsible for the development of tooth
germs, and that evolves along different gradients leading
to variation in distal teeth.
2. The second theory — “clone theory,” was proposed by
Osborn [16]. According to this theory, there are threedifferent patterns: for incisors, for canines, and formolars that form the human dentition. Development
starts from these patterns, and then different gradients start to act, which explains the different variations in the same group. As progressing to the moredistal tooth, the primary pattern changes according
to the remaining available space.
Some authors such as Schwartz and Tattersall [18]believe that the reasons for the reduction of the dental
dimensions of the distal teeth may be in the formation of
the human chin as a result of the shortening of the lower
jaw and the dental arch.
Other researchers such as Macho and Berner [13]believe that the reduction in the size of the second molars
may be due to the formation of a helicoidal occlusal plane
as well as the curve of Spee, which are considered to be afunctional adaptation to the masticatory function uniqueto humans. An important factor according to Osborn[17] is also the evolution in the position of the temporomandibular joint as well as the mandibular ramus. In
modern man, these anatomical structures move forward,
which is important for the occlusion of the upper andMaterials and methods
The study samples consisted of 232 individuals (121males and 111 females) of Bulgarian origin living insouthern Bulgaria aged 20–40 years. Patients were
included based on the following criteria: Bulgarian ethnicity, the presence of complete set of fully erupted andperiodontally healthy teeth, the presence of non-carious
and non-worn teeth, no dental history of any crownrestorations or bridges, and normal occlusion. Patientswith orthognathic surgery or trauma, history or clinicalevidence of cleft palate, history or clinical features suggestive of endocranial disorders, metabolic disorders,developmental disorders, and history of prolonged illness
were excluded.
Mesiodistal measurements of the teeth were obtainedby Dentistry Sliding Vernier Caliper, Ridge Mapping
Caliper type A and type B. We used the method of direct
anthropological technique, modified by Prof. Yordanov
et al. [22]. The maximum mesiodistal dimension wasassessed as being the maximum distance between themesial and distal proximal surfaces of the tooth crown,
usually in the upper or middle third of coronal height. It
is also termed the dental width.
We calculated the following interdental indices:• Inter-incisive index as ratio of the mesiodistal dimensions of maxillary lateral incisor to the mesiodistal
dimensions of the maxillary central incisor (MD12/
MD11) × 100• Index of the premolars as ratio of the mesiodistal
dimensions of second lower premolar to the same of
the first one, PI = (MD45/MD44) × 100• Index of the upper molars as ratio of the mesiodistal
dimension of the second upper molar to the same of
the first one, UMI = (MD17/MD16) × 100

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• Index of lower molars as ratio of the mesiodistal
dimension of the second lower molar to the same of
the first one, LMI = (MD47/MD46) × 100

incisor to the central one, without statistical significance (p > 0.05) (Table 1).
2. There was no statistically significant difference
between mesiodistal dimensions of first lower premolars, but we found such in second lower premolars(p < 0.001). The index of premolars found on the territory of Bulgaria dating from the Eneolithic period
is 104.62%. The same in people from southern Bulgarian population studied in the current research is
98.06%. The decrease in this index is statistically significant (p < 0.001), which shows a strong reductionof the percentage ratio of the second lower premolar
compared to the first one (Table 2).
3. We also found statistically significant differences
between mesiodistal dimension of upper first andsecond molars (p < 0.001). The index of the upper
molars found on the territory of Bulgaria dating fromthe Eneolithic period is 96.88%. The same in people
from southern Bulgarian population studied in thecurrent research is 95.94%. There is a decrease in thisindex, which shows a tendency in reducing the percentage ratio of the second upper molar compared to
the first one, without statistical significance (p > 0.05)(Table 3).
4. We did not find statistically significant difference
between mesiodistal dimensions of inferior firstand second molars (p > 0.05). The index of the lower
molars, found on the territory of Bulgaria datingfrom the Eneolithic period, is 96.19%. The same inpeople from southern Bulgarian population studied
in the current research is 95.22%. There is a decreasein this index, which shows a tendency in reducing

The measurements were analyzed with SPSS 23.0 using
Student’s t-test. The level of statistical significance wasset at p < 0.05. The degree of significance was considered weak (p < 0.05), moderate (0.01 > p > 0.001), or high(p < 0.001).
An ethical approval was taken for this study by the ethics committee in Medical University-Plovdiv. Informed
consents were taken from all patients involved in the
study. All methods were performed in accordance withthe relevant guidelines and regulations.

Results
1. We found statistically significant difference with highdegree of significance (p < 0.001) between mesiodistal dimensions of central incisors in South Bulgarian population and remnants from the Eneolithic
period. Furthermore, we detected statistically significant difference with moderate degree of significance (p < 0.01) in mesiodistal dimensions of lateralincisors between the two populations. The interincisive index in the teeth found on the territory of
Bulgaria dating from the Eneolithic period is 81.25%.
The same in people from southern Bulgarian population studied in the current research is 80.44%. Thereis a decrease in this index, which shows a tendency
in reducing the percentage ratio of the lateral upper

Table 1 Comparison of the inter-incisor index in teeth found on Bulgarian territory during the Eneolithic period and in modern
southern Bulgarian population
Parameters

Eneolithic period

Current study

P

N

Mean

SD

SE

Parameters

N

Mean

SD

SE

I11MD

98

8.0

0.65

0.07

I11MD

232

8.44

0.66

0.11

< 0.001

I12MD

98

6.5

0.75

0.08

I12MD

232

6.76

0.71

0.11

< 0.01

Incisive index

98

81.25

8.20

0.83

Incisive index

232

80.44

8.44

0.91

> 0.05

Table 2 Comparison of the index of premolars found on Bulgarian territory during the Eneolithic period and in modern southern
Bulgarian population
Parameters

Eneolithic periodN

Mean

Current study
SD

SE

Parameters

P
N

Mean

SD

SE

I44MD

98

6.5

0.64

0.07

I44MD

232

6.46

0.68

0.11

> 0.05

I45MD

98

6.8

0.71

0.07

I45MD

232

6.33

0.78

0.13

< 0.001

Premolar index

98

104.626.59

0.67

Premolar index

232

98.06

6.78

0.73

< 0.001

Harizanova et al. Journal of Physiological Anthropology(2023) 42:15

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Table 3 Comparison of the index of the upper molars, found on Bulgarian territory during the Eneolithic period and in modern
southern Bulgarian population
Parameters

Eneolithic period

Current study

P

N

Mean

SD

SE

Parameters

N

Mean

SD

SE

I16MD

98

9.60

0.70

0.07

I16MD

232

10.32

0.74

0.10

< 0.001

I17MD

98

9.30

1.09

0.11

I17MD

232

9.77

1.13

0.08

< 0.001

Upper molar index

98

96.88

5.19

0.52

Upper molar index

232

95.94

5.42

0.58

> 0.05the percentage ratio of the second lower molar compared to the first one, without statistical significance(p > 0.05) (Table 4).

Discussion
Eneolithic period is a transitional period between the
Neolithic period and the Bronze Age, during which theearliest metallic (copper) artifacts appeared; hence, it isalso known as the Copper Age (4800–2000 BC). Artifacts
from this period have been found throughout the Balkan
Peninsula. This is the historical period when agricultural tribes began their migration from Western Asia to
Europe. The Balkan Peninsula became the most important route for resettlement. After them, many different
tribes settled on the territory of modern Bulgaria, such as
Thracians, Greeks, Roman colonists, Slavs, proto-Bulgarians, and Turks. Some of them belong to the Indo-European language group, but others belong to the Turkish
language group. The mentioned populations have beensuccessfully assimilated. The consequences were a verylarge genetic diversification of the modern Bulgarian
population. Regardless of the genetic diversity, it should
be noted that all the listed people belong to the species
Homo sapiens. The situation is similar throughout theEuropean continent. There are no pure genetically ethnicities. In order to achieve greater scientific precision,
we excluded patients with non-Bulgarian origins fromthe study.
Information about the Eneolithic population was takenfrom the studies of Prof. Yordanov [22], who researchedand described odontometric dimensions of archeologicaldiscoveries (skulls) found on the territory of Bulgaria.
In our study, we calculate the interdental indices ofobjects from southern Bulgarian population: the interincisive index, which shows the percentage of the upper
lateral incisor to the upper central one; the premolar
index, which shows the percentage of the lower second
premolar to the lower first premolar; and the step indices of the molars, which show the percentage of theupper and lower second molars to the respective upperand lower first molars. Then, we calculate these indicesfor the teeth found on the territory of Bulgaria from the
Eneolithic period [22]. When comparing them, we founda decrease in the values of these indices in modern man,
which confirms the theories of reducing dental size.
According to our study, the size of the distal membersof each morphological tooth group (incisors, premolars,
and molars) is reduced compared to that of their medial
neighbors. The greatest reduction is in the percentageof the second lower premolar to the first. We detected atendency in the reduction of the percentage of the lateral
upper incisor to the central upper incisor. These findings are confirmed by Abadjiev [1]. A possible reason
for this is the simpler morphology of the lateral upper
incisor, which makes it more susceptible to mutations
and other factors. Reduction of the inter-incisive index,
i.e., reduction of the percentage of the lateral incisor tothe central one, has been reported by other researchers such as Brook et al. [4] and Gungor and Turkkahraman [9]. Endo et al. [8], who studied these ratios in theJapanese, confirmed that the percentage of the lateral
upper incisor with the central one exhibited the greatest

Table 4 Comparison of the index of the lower molars, found on Bulgarian territory during the Eneolithic period and in modern
southern Bulgarian population
Parameters

Eneolithic periodN

Mean

Current study
SD

SE

Parameters

P
N

Mean

SD

SE

I46MD

98

10.50

0.72

0.07

I46MD

232

10.56

0.70

0.12

> 0.05

I47MD

98

10.10

0.51

0.05

I47MD

232

10.03

0.50

0.08

> 0.05Lower molar index

98

96.19

5.20

0.53

Lower molar index

232

95.22

5.13

0.56

> 0.05

Harizanova et al. Journal of Physiological Anthropology(2023) 42:15

reduction. In our study, however, the largest reduction
is in the percentage of the lower second premolar to
the first. Similar results were obtained by Norihisa et al.
[14]. Furthermore, we find a tendency of reduction in theindices of the upper and lower molars, which shows adecrease in the percentage of the second upper and lower
molars compared to the first upper and lower molars,
respectively. We believe the main reason is the population migration as well as human evolution. For example,
canines in prehistoric man are longer than other teeth,and the second molar is the largest tooth in humanmouth. In modern man, this has changed as the size ofthe canines has decreased, and their level has reached theocclusal surface of other teeth. The mesiodistal dimension of the second upper molars also decreases significantly, so that in modern man the first upper molar has
the largest mesiodistal dimension. These processes havebeen described by Haile-Selassie [10], as well as by Braceet al. [3]. Some authors, such as Schwartz and Tattersall[18], believe that the reasons for this may be the formation of the human chin, which is formed as a result of
shortening of the lower jaw and dental arch. This contradicts the research of Ichim et al. [11], according to which
the formation of the chin may be due to the reduction of
perioral muscles, as well as that of the tongue.

Conclusion
The biggest reduction with statistically significant difference in the percentage ratio (more than 6%) was found in
the lower second premolars compared to the first ones,
which is characteristic for southern Bulgarians.
We found a tendency in reducing the percentage ratioof the lateral towards the central incisors of people fromsouthern Bulgarian population compared to those datingfrom the Eneolithic period on the territory of Bulgaria.
We also detected a tendency in reducing the percentage
ratio in the upper and lower second molars compared to
the first ones.
As a result, we believe that interdental indices canbe used for explaining historical, cultural, and biological macro and microevolutionary processes and thus
for understanding the origin, formation, contacts, andmigration pathways of the different populations leading to ethnic variation of humanity. Therefore, they
can be reliable source of information in physiologicalanthropology.
Acknowledgements
Not applicable.
Authors’ contributions
Dr. H and Assoc. Prof. B performed the research and wrote the manuscript,
and Assoc. Prof. P and Dr. P worked on the tables and revised the English
translation.

Page 5 of 6

Funding
This research did not receive any specific grant from any funding agency inthe public, commercial, or not-for-profit sector.
Availability of data and materials
The paper is containing original research and has not been submitted/
published earlier in any journal and is not being considered for publicationelsewhere.

Declarations
Ethics approval and consent to participate
An ethical approval was taken for this study by the ethics committee in
Medical University-Plovdiv. Informed consents were taken from all patients
involved in the study.
Consent for publication
The authors have seen and approved the manuscript.
Competing interests
The authors declare that they have no competing interests.
Received: 4 February 2023 Accepted: 14 July 2023

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