Dental caries burden across age groups in Kerman province, Iran (1992–2025): a systematic review and meta-analysis

Abstract

Background

Kerman is Iran's largest province, with a population exceeding three million and documented socioeconomic disparities in oral health outcomes. This systematic review and meta-analysis aimed to characterize temporal trends in dmft/DMFT indices across six age groups in Kerman province over nearly three decades.

Methods

This study was conducted in accordance with the PRISMA 2020 guidelines. English and Persian databases, including PubMed, Scopus, Embase, Web of Science, and SID, were searched up to April 2026. This search was supplemented by a manual review of dental master's theses from Kerman Dental School and national oral health reports. Cross-sectional studies reporting dmft/DMFT indices in the general healthy population of Kerman province were included. Two reviewers independently performed study selection, data extraction, and quality assessment using the Newcastle–Ottawa Scale. Pooled estimates and 95% confidence intervals (CI) were calculated using a random-effects model. Heterogeneity was assessed using I² and Q statistics. Subgroup analyses by time period (prior to 2015 versus 2015 to present) and univariable meta-regression with four pre-specified covariates were performed to explore sources of heterogeneity. The certainty of evidence was assessed using the GRADE framework.

Results

A total of 49 studies (comprising 26 peer-reviewed articles, 19 theses, and 4 national reports) were included. The pooled dmft was 3.55 (95% CI: 3.26–3.84) for children aged 3–6 years, 5.12 (95% CI: 4.74–5.50) for 6-year-olds, and 4.09 (95% CI: 2.73–5.46) for 9-year-olds. Pooled DMFT values were 2.04 (95% CI: 1.70–2.39) for 12-year-olds and 2.61 (95% CI: 1.38–3.84) for the 12–15-year age group. In adults aged 35–44 years, the pooled DMFT was 13.80 (95% CI: 10.80–16.78). Meta-regression and subgroup analysis revealed a significant increasing temporal trend in the 3–6-year group (+ 0.21 units per year; p < 0.001), while geographic location significantly predicted caries levels in 6-year-olds (p < 0.001). Heterogeneity was very high across all age groups (I² > 94%, p < 0.001). The certainty of evidence was rated as very low for all age groups using the GRADE framework.

Conclusion

The dental caries burden in Kerman province remains high across all age groups, with a significant worsening trend in early childhood caries. Due to the very low certainty of current evidence, a standardized provincial oral health survey is needed. Future strategies should prioritize expanded water fluoridation and targeted community-based preschool interventions.

Supplementary Information

The online version contains supplementary material available at https://doi.org/10.1186/s12903-026-08889-0.

Introduction

Dental caries is among the most prevalent chronic conditions worldwide, affecting people irrespective of ethnicity, age, gender, or socioeconomic status. It imposes a substantial economic burden and significantly impacts overall health and quality of life. It is also linked to systemic diseases such as cardiovascular disease, stroke, and respiratory disease due to the chronic infections it may cause [1]. According to the World Health Organization (WHO) report in 2022, oral diseases affect close to 3.5 billion people worldwide [2]. Furthermore, an estimated 2 billion people have caries of permanent teeth and 514 million children are affected by caries of primary teeth [2]. The prevalence of caries of primary teeth in Iranian children aged 1–9 years was reported to be 46.7% in 2022 [3]. Also, the prevalence of untreated caries of permanent teeth among Iranians aged 5 years and older was 33.6% [3].

Effective management of dental caries depends largely on its risk assessment, making it crucial to map out its prevalence in any given population [4, 5]. Therefore, accurate and up-to-date research on oral health status is essential for planning and projecting dental health services. Epidemiologic surveys often evaluate dental caries using the WHO criteria with dmft and DMFT indices, which measure the number of decayed, missing and filled teeth [6]. Numerous studies in Iran have reported a high prevalence of dental caries among different age groups [7]. A recent study by Shoaee et al. [8] found that the overall mean DMFT across all ages in Iran increased by nearly 58% from 1990 to 2017. Another study from the PERSIAN cohort reported an average DMFT index of 18.0 across 14 provinces in Iran, with significant variation between provinces [9].

Kerman province, the largest province in southeastern Iran with over 3.4 million residents, is characterized by significant socioeconomic inequalities that have contributed to a disparate dental caries burden [9]. Over the last three decades, the region has undergone substantial transitions in socioeconomic status, dietary patterns, and public health frameworks, most notably the 2015 national Oral Health Reform Plan (OHRP). These evolving contextual factors have likely had varying effects on the caries burden across different age groups. Elucidating these temporal trends is essential for developing evidence-based public health interventions. Therefore, this systematic review and meta-analysis aimed to characterize temporal trends of DMFT/dmft indices in Kerman province from 1992 to 2026, synthesizing evidence from both peer-reviewed and grey literature.

Methods and Materials

Study protocol and registration

This study was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines (Supplementary file 1). The study protocol was retrospectively registered on the Open Science Framework (OSF) (Registration: https://doi.org/10.17605/OSF.IO/MPBQY).

Sources of information and search strategy

An expert (A.SS.) searched PubMed, Scopus, and Embase in English using the following keywords (Table 1): "Oral Health", "Dental Caries", "Root Caries", "DMF Index", "Prevalence", "Epidemiology", and "Incidence", in combination with keywords associated with "Kerman Province". The Scientific Information Database (SID) Persian database was also searched for relevant studies. A manual search was conducted for master's theses from Kerman Dental School, available through the Kerman University repository (http://vlib.kmu.ac.ir/). Each thesis underwent formal academic evaluation: (1) the research proposal was reviewed and approved by at least three experts in restorative dentistry and/or dental public health, and (2) the completed thesis was defended before a committee of at least three dental specialists. The findings of national surveys from 1999, 2004, 2012, and 2016 were also included. The last search was conducted in April 2026.

Table 1.

Search terms and keywords used in the database search

Dental Caries	AND	Epidemiology/Prevalence Measures	AND	Geographical Location (Kerman Province)
Dental Caries"[Mesh] OR dental decay OR carious OR carious lesion* OR dental white spot* OR "Root Caries"[Mesh] OR root caries OR DMF Index[Mesh] OR DMFT OR DMFS OR dmft OR dmfs OR Caries Free Index OR SiC Index OR ECC OR “Early Childhood Caries” OR baby bottle syndrome		Prevalence"[Mesh] OR "Incidence"[Mesh] OR "Epidemiology"[Mesh] OR epidemiology[subheading] OR endemics OR outbreaks OR occurrence* OR morbidity OR frequency		Kerman Province OR Kerman OR Anbarabad OR Baft OR Bam OR Bardsir OR Ekhtiar Abad OR Faryab OR Ghale-e-Ganj OR Jiroft OR Joopar OR Kahnooj OR Koohbanan OR Mahan OR Manojan OR Rabor OR Rafsanjan OR Ravar OR Roodbar OR Shahr-e-Babak OR Sirjan OR Zarand

Eligibility criteria

Studies were included if they: (a) had a cross-sectional design; (b) reported DMFT or dmft indices in a general healthy population residing in Kerman province, of any sex or age group; and (c) were published or defended up to April 2026.

Studies were excluded if they: (a) did not report DMFT or dmft indices; (b) were conducted exclusively in populations with systemic diseases, such as diabetes mellitus, cardiovascular disease, renal failure, or other chronic conditions; (c) were conducted exclusively in pregnant women; (d) were conducted exclusively in smokers or tobacco users; (e) had low data quality with incomplete or inconsistent data, as determined by a Newcastle–Ottawa Scale (NOS) score below 4; or (f) addressed unrelated topics such as genetic studies, laboratory diagnostic tests, animal experimental studies, letters to the editor, case reports, or case series.

Two authors (S.Z. and S.RP.) independently conducted the screening process, with disagreements resolved through consultation with the senior author (A.S.). Full texts of the included studies were obtained, and data extraction was performed independently by two reviewers (S.Z. and M.H.). Relevant information, including authorship details, year and place of data collection, sample size, and DMFT/dmft indices, was recorded. Where available, the individual components of the indices (D/d, M/m, and F/f) were also extracted. Corresponding authors were contacted via email to obtain missing or unclear primary data and to ensure the completeness and accuracy of the analysis.

Quality assessment

The methodological quality of the included studies was evaluated using a modified version of NOS specifically adapted for cross-sectional designs [10]. This assessment tool comprises three primary sections and seven items, with a maximum possible score of 10 points. Following a pre-specified threshold, only studies scoring 4 or above were included in this systematic review, resulting in the exclusion of two studies that failed to meet this quality criterion. The assessment was conducted independently by two reviewers (S.Z. and S.RP.). Any discrepancies in scoring were resolved through formal discussion and consensus between the researchers.

Data analysis

A meta-analysis was conducted using Comprehensive Meta-Analysis (CMA) Version 3 software. A random-effects model using the method-of-moments (MM) estimator was applied to all analyses. The DMFT/dmft indices and their 95% confidence intervals (95% CI) were calculated for each age group separately. For the adult population, quantitative synthesis was restricted to the WHO-recommended 35–44-year index age group. Studies reporting only mean age without specifying the age range of participants, or those reporting broad unstratified adult age ranges incompatible with WHO index age groups, were excluded from quantitative synthesis and retained in a narrative synthesis. Studies lacking standard deviation data that could not be obtained from authors or imputed from available statistics were excluded from the quantitative synthesis and included in the narrative synthesis only.

Between-study heterogeneity was assessed using the Q statistic, the I² statistic, and Tau². I² values of 25%, 50%, and 75% were considered to represent low, moderate, and high heterogeneity, respectively. To address the methodological concern regarding temporal heterogeneity inherent in synthesizing three decades of data, subgroup analyses were performed by dividing studies into two evidence-based time periods: prior to 2015 and 2015 to present. This cut-point was selected based on the implementation of OHRP at the beginning of 2015 [11]. The statistical significance of subgroup differences was assessed using the Q-between statistic.

To further explore sources of heterogeneity univariable meta-regression was performed using four pre-specified categorical covariates: time period (prior to 2015 vs. 2015 to present), geographic location (Kerman city vs. other cities or province-wide), publication status (published peer-reviewed articles and official national reports vs. unpublished grey literature) and calendar year as a continuous covariate. Each covariate was examined independently, consistent with the recommended minimum ratio of ten studies per covariate. For age groups with fewer than ten studies, meta-regression was not performed. The proportion of between-study variance explained by each covariate was quantified using the R² analogue.

Publication bias was assessed separately for each age group with sufficient studies (k ≥ 10) using funnel plot asymmetry and Egger's regression test. Sensitivity analysis was conducted using the one-study-removed approach. All statistical tests were two-sided, and a p-value of less than 0.05 was considered statistically significant.

Level of evidence and certainty of outcomes

The certainty of evidence for each pooled estimate was assessed using the Grading of Recommendations Assessment, Development and Evaluation (GRADE) framework using GRADEpro GDT software (McMaster University, Ontario, Canada; gradepro.org). The overall certainty of evidence was classified as high, moderate, low, or very low for each age group.

Results

Study selection

The flowchart in Fig. 1 illustrates the search strategy and study selection process. Duplicate studies, as well as studies focusing on populations outside of Kerman Province or oral health variables other than DMFT/dmft, were excluded. In addition, 1,504 master's theses from Kerman Dental School were screened from 1992–2026. A total of 49 documents were included in the quantitative and qualitative analysis, including 26 peer-reviewed articles, 3 peer-reviewed national reports, 19 theses, and 1 non-peer-reviewed national report. Among these, 38 documents (11 theses, 1 non-peer-reviewed national report, 2 peer-reviewed national reports, and 24 articles) were included in the quantitative analysis.

Fig. 1.

PRISMA flow diagram of the study selection process

Study characteristics and pooled dmft/DMFT

• 3–6-year-olds

  Seven studies examining dental caries in children aged 3–6 years were included (Table 2), with a total sample size of 4,575 children [12–18]. Studies were conducted between 2008 and 2018. Three studies were conducted in Kerman city and four in other cities of the province. Four studies were theses [12–15] and three were peer-reviewed manuscripts [16–18]. The overall pooled dmft was 3.55 (95% CI: 3.26–3.84) (Fig. 2). Subgroup analysis by time period revealed a pooled dmft of 3.14 (95% CI: 2.79–3.51) for studies conducted prior to 2015 and 4.25 (95% CI: 3.78–4.73) for studies from 2015 onward, a statistically significant difference (Q-between = 13.28, df = 1, p < 0.001).

• 6-year-olds

  Fifteen studies examined dental caries in 6-year-old children ‬‬‬‬‬‬‬‬‬‬‬[14, 18–31] (Table 3). Three studies were not included in the quantitative analysis due to their failure to report the standard deviation ‬‬‬‬‬‬‬‬‬‬‬[19–21]. Studies were conducted between 1992 and 2018. Six studies were conducted in Kerman city and six in other cities or at the provincial level. Seven were peer-reviewed articles [18, 26–31], three were national reports [23–25], and two were theses [14, 22]. Individual study dmft values ranged from 2.31 to 6.82. The overall pooled dmft was 5.12 (95% CI: 4.74–5.50) (Fig. 3). Subgroup analysis by time period showed a pooled dmft of 4.78 (95% CI: 4.13–5.43) for studies prior to 2015 and 5.30 (95% CI: 4.83–5.76) for studies from 2015 onward; this difference was not statistically significant.

• 9-year-olds

  Four studies examining dental caries in 9-year-old children were included (Table 4) [19, 23, 32, 33]. These were conducted between 1999 and 2018. Individual study dmft values ranged from 3.34 to 7.00. The overall pooled dmft was 4.09 (95% CI: 2.73–5.46) (Fig. 4). Subgroup analysis was not performed due to the limited number of studies available.

• 12-year-olds

  Fifteen studies examining dental caries in 12-year-old children were identified (Table 5) [15, 20, 24, 25, 31, 32, 34–42]. One study was not included in the quantitative analysis due to reporting the DMFT of the first permanent molar [34]. Four other studies were not included in the quantitative analysis due to a failure to report the standard deviation [20, 31, 39, 40].

  Included studies were conducted between 1998 and 2018. Three studies were from Kerman city. Five were peer-reviewed articles [35–38, 43], two were peer-reviewed national reports [24, 25], and three were theses [32, 41, 42].

  Individual DMFT values ranged from 1.31 to 3.56. The overall pooled DMFT was 2.04 (95% CI: 1.70–2.39) (Fig. 5). Subgroup analysis revealed a pooled DMFT of 2.26 (95% CI: 1.83–2.69) prior to 2015 and 1.64 (95% CI: 1.06–2.23) from 2015 onward, though this difference was not statistically significant (Q-between = 2.76, p = 0.096).

• 12–15-year-olds

  As indicated in Table 6, nine studies examining dental caries in adolescents aged 12–15 years were identified [24, 41, 44–50]. Among these, one study was not included in the quantitative analysis due to a failure to report the standard deviation [41]. The study by Dadalahi et al. [50] was also excluded from the quantitative synthesis because the study population included adolescents aged 13–18 years, and age-specific data for the 13–15 year-old subgroup were not available.

  The included studies in this meta-analysis were conducted between 1997 and 2017. Three studies were from Kerman city. Five were peer-reviewed articles [44–47, 49], one was a peer-reviewed national report [24], and one was a thesis [48]. Individual DMFT values ranged from 0.91 to 4.74. The overall pooled DMFT was 2.61 (95% CI: 1.38–3.84) (Fig. 6). Subgroup analysis based on publication period was not conducted, as only one study was identified from 2015 onward.

• Adults

  Eleven studies reported the DMFT index in adults (Table 7) [20, 24, 51–59]. Eight of these studies reported DMFT data for the 35–44-year WHO index age group [24, 51–56, 59]; however, one study was not included in the quantitative analysis due to a failure to report the standard deviation [20].

  Four studies were conducted in Kerman city, two in Rafsanjan, and one reported province-wide data. Six were published peer-reviewed articles or national reports, and one was unpublished grey literature [59]. Studies were conducted between 2006 and 2025. Individual DMFT values ranged from 9.37 to 19.88. The overall pooled DMFT was 13.80 (95% CI: 10.80–16.78) (Fig. 7). Subgroup analysis was not performed due to the limited number of studies available for this age group.

Table 2.

The dmft index in 3- to 6-year-olds in Kerman province

	Authors	Data collection date	Region	Sample size (Male%)	dmft	dt	mt	ft	NOS
Published	Bafti et al. [18]	2009–2010	Kerman	1482	2.58 ± 3.13	2.02 ± 2.66	0.14 ± 0.63	0.42 ± 1.16	10
	Sadeghi et al. [16]	2008–2010	Rafsanjan	204 (57.35)	2.4 ± 3.3	-	-	-	10
	Sajadi et al. [17]	2016	Kerman	857 (46.7)	3.70 ± 2.00	2.27 ± 1.46	0.87 ± 0.55	0.88 ± 1.14	8
Unpublished	Ghahri et al. [13]	2010–2011	Bam	120 (62.5)	3.78 ± 2.38	3.24 ± 2.25	0.31 ± 0.57	0.21 ± 0.76	9
			Jiroft	120 (43.33)	2.9 ± 2.92	2.57 ± 2.68	0.16 ± 0.58	017 ± 0.54
	Borjali et al. [12]	2010–2011	Shahr-e Babak	122 (53.3)	2.91 ± 2.91	2.38 ± 2.66	0.27 ± 0.62	0.26 ± 2.91	9
			Bardsir	121 (54.5)	3.12 ± 3.07	2.7 ± 0.91	0.14 ± 0.41	0.27 ± 1.11
	Sajjadi et al. [15]	2010–2011	Baft	120 (75)	2.85 ± 2.72	2.67 ± 2.65	0.26 ± 0.55	0.27 ± 0.56	9
			Rabor	120 (29.2)	3.78 ± 2.38	3.34 ± 2.35	0.39 ± 0.54	0.24 ± 0.74
	Pourahmdian Nadiki et al. [14]	2013	Kerman	204 (60.3)	3.81 ± 3.07	3.24 ± 2.75	0.06 ± 0.48	0.52 ± 1.34	8
		2014		324 (40.1)	3.37 ± 3.50	2.73 ± 3.26	0.11 ± 0.51	0.56 ± 1.27
		2015		170 (29.4)	4.05 ± 3.54	3.41 ± 3.27	0.15 ± 0.68	0.54 ± 1.21
		2016		265 (24.2)	4.79 ± 3.84	4.17 ± 3.69	0.13 ± 0.54	0.49 ± 1.15
		2017		285 (36.5)	4.28 ± 3.43	3.27 ± 3.05	0.33 ± 0.92	0.7 ± 1.27
		2018		265 (32.8)	4.58 ± 4.40	3.97 ± 4.21	0.11 ± 0.71	0.51 ± 1.22

Fig. 2.

Forest plot of the pooled dmft index in children aged 3–6 years

Table 3.

The dmft/DMFT index in 6-year-olds in Kerman province

Document	Authors	Data collection date	Region	Sample size (Male (%))	dmft/DMFT	dt/DT	mt/MT	ft/FT	NOS
Published	Meimandi et al. [31]	1992	Kerman	130 (49.2)	5.14 ± 4.14/-	3.1	0.35	0.047	7
	Mohammadi et al. [30]	2013	Kerman	222 (45.5)	4.87 ± 4.14/-	-	-	-	10
	Bafti et al. [18]	2009–2010	Kerman	774	2.98 ± 3.25/-	-	-	-
	Torabi et al. [29]	2013	Kerman	200 (100)	5.25 ± 3.27/-	-	-	-	9
	Sadeghi et al. [26]	2015	Rafsanjan	2197 (51)	6.27 ± 3.76/-	5.55 ± 3.70	0.31 ± 0.82	0.41 ± 1.10	7
	Kamiab et al. [27]	2018	Rafsanjan	2031	6.37 ± 3.40/-	4.49 ± 3.07	1.16 ± 1.77	0.72 ± 1.45	10
	Eskandarizadeh et al. [28]	2011	Kerman	300 (50)	4.63 ± 2.41/-	3.53 ± 2.17	0.55 ± 0.9	0.55 ± 1.05	9
	Oral health situation of Iranian children by Samadzadeh et al. [23]	1999	Kerman Province	300	5.69 ± 0.48/0.25 ± 0.08	87.8%/100%	9.10%/0%	2.30%/0%	7
	Oral health situation of Iranian children published by Bayat-Movahed et al. [19]	2004	Kerman Province	300	4.833/-	4.52	0.287	0.027	9
	Oral health status and healthcare system in IR Iran, published by Khoshnevisan et al. [24]	2012	Kerman Province	247	6.65 ± 0.30/0.4 ± 0.07	6.01/0.4	0.41/0.0	0.21/0.0	8
	National Oral Health survey 2016 published by Ghasemianpour et al. [25]	2016	Kerman Province	480 (50.2)	6.26 ± 0.2/-	5.7	0.42	0.15	7
Unpublished	Akhavan et al. [21]	1997	Kerman	1500 (54.4)	-/4.9	-	-	-	8
	Ruholamini et al. [22]	1996	Sirjan	500 (50.8)	6.82 ± 4.32/-	-	-	-	9
	Ziaaddini et al. [20]	2004–2005	Bam	687	4.35/-	3.37	0.86	0.12	8
	Pourahmdian Nadiki et al. [14]	2013	Kerman	140	3.44 ± 3.01/-	0.09 ± 0.57	0.6 ± 1.52	4.12 ± 3.34	9
		2014		161	2.31 ± 2.84/-	0.12 ± 0.52	0.7 ± 1.4	3.07 ± 3.1
		2015		106	3.61 ± 3.51/-	0.23 ± 0.84	0.58 ± 1.25	4.41 ± 3.82
		2016		181	3.9 ± 3.47/-	0.15 ± 0.6	0.59 ± 1.25	4.64 ± 3.71
		2017		253	3.17 ± 2.99/-	0.34 ± 0.91	0.73 ± 1.31	4.23 ± 3.4
		2018		162	5.27 ± 4.46/-	0.13 ± 0.66	0.68 ± 1.41	6.08 ± 4.45

Fig. 3.

Forest plot of the pooled dmft index in 6-year-old children

Table 4.

The DMFT/dmft index in 9-year-olds in Kerman province

No	Authors	Data collection date	Region	Sample size (Male)	DMFT/dmft	DT/dt	MT/mt	FT/ft	NOS
Published	Oral health situation of Iranian children by Samadzadeh et al. [23]	1999	Kerman Province	300	1.3 ± 0.17/3.61 ± 0.3	96.4%/82.40%	0.50%/14.30%	3.10%/2.50%	7
	Oral health situation of Iranian children published by Bayat-Movahed et al. [19]	2004	Kerman Province	300	0.32/2.85	0.31/2.40	0.00/0.39	0.00/0.05	9
Unpublished	Nabavizadeh et al. [33]	2011	Kerman	296 (46.6)	-/7 ± 1.48	-	-	-	8
	Mousavi et al. [32]	2013	Kerman	141	0.71 ± 1.1/3.34 ± 2.5	-	-	-	8
		2014		72	0.96 ± 1.35/3.47 ± 2.96	-	-	-
		2015		49	1 ± 1.74/3.61 ± 3.28	-	-	-
		2016		103	0.52 ± 1.03/4.26 ± 3.07	-	-	-
		2017		95	0.56 ± 1.13/3.36 ± 2.25	-	-	-
		2018		54	0.93 ± 1.24/4.07 ± 2.87	-	-	-

Fig. 4.

Forest plot of the pooled dmft index in 9-year-old children

Table 5.

The DMFT/dmft index in 12-year-olds in Kerman province

	Authors	Data collection date	Region	Sample size (Male)	DMFT/dmft	DT	MT	FT	NOS
Published	Sadeghi [34]	2007	Rafsanjan	563 (54.54)	1.90 ± 1.60 (First permanent molar)	1.64 ± 1.54	0.01 ± 0.16	0.25 ± 0.78	8
	Sadeghi and Bagherian [35]	2007	Rafsanjan	353 (50.99)	2.46 ± 2	75.7%	6.70%	17.60%	8
	Sadr et al. [36]	2011	Zarand	350 (100)	2.03 ± 2.34	-	-	-	7
	Moshrefian et al. [37]	2009	Kerman	706 (47.45)	1.31 ± 1.57	1.08 ± 1.42	0.04 ± 0.22	0.19 ± 0.68	8
	Sajjadi et al. [43]	2008–2009	Sirjan	700 (100)	3.56 ± 2.34	2.22 ± 1.72	1.09 ± 1.23	0.25 ± 0.66	9
	Shakerian et al. [38]	2016	Rafsanjan	460 (50)	2.66 ± 1.40	1.33 ± 0.92	0.43 ± 0.58	0.90 ± 0.78	7
	Oral health status and healthcare system in IR Iran, published by Khoshnevisan et al. [24]	2012	Kerman Province	250	2.92 ± 0.24	2.47	0.11	0.32	8
	National Oral Health survey 2016 published by Ghasemianpour et al. [25]	2016	Kerman Province	463 (48.59)	1.49 ± 0.11	1.26	0.06	0.17	7
Unpublished	Ranjbar et al. [76]	1992	Kerman	119	4.5	0.98	0.74	0.04	7
	Raisi et al. [39]	1997–1998	Shahr-e Babak	340 (47.05)	1.75				7
	Fathizadeh et al. [40]	1997–1998	Baft	411 (52.55)	1.29	-	-	-	9
	Eftekhari Kanzarki et al. [41]	1998–1999	Jiroft	300 (100)	1.59 ± 1.88	-	-	-	8
	Ziaaddini et al. [20]	2004–2005	Bam	679	1.57	1.28	0.11	0.17	8
	Parval et al. [42]	2010–2011	Kerman	300 (50)	3.39 ± 2.60	2.65	0.16	0.43	9
	Mousavi et al. [32]	2013	Kerman	100	1.54 ± 1.52/0.8 ± 1.4	-	-	-	8
		2014		68	1.35 ± 1.57/0.53 ± 1.15	-	-	-
		2015		70	1.36 ± 1.58/0.81 ± 1.66	-	-	-
		2016		39	1.45 ± 1.77/1.05 ± 1.49	-	-	-
		2017		73	1.26 ± 1.65/1.17 ± 0.63	-	-	-
		2018		54	1.59 ± 1.91/0.69 ± 1.34	-	-	-

Fig. 5.

Forest plot of the pooled DMFT index in 12-year-old children

Table 6.

The DMFT index in 12–15-year-olds in Kerman province

	Authors	Data collection date	Region	Age (years)	Sample size (Male)	DMFT	D	M	F	NOS
Published	Sadeghi et al. [45]	2008	Rafsanjan	12–15	747 (47.25)	2.83 ± 2.2	2.03 ± 2.1	0.16 ± 0.5	0.64 ± 1.3	9
	Bakhtiar et al. [46]	2012	Kerman	11–13	400 (46.75)	1.76 ± 2.4	-	-	-	10
	Ramezani et al. [47]	2013	Rafsanjan	12–15	210 (52.9)	2.75 ± 2.07	2.6 ± 2.1	0.7 ± 0.26	0.45 ± 1.1	9
			Zarand	12–15	171 (56.7)	3.24 ± 2.10	2.4 ± 1.8	0.01 ± 0.01	0.48 ± 1
	Torabi-Parizi et al. [44]	2012	Kerman	15	300 (21.66)	1.76 ± 1.83	1.32 ± 1.64	0.44 ± 1.09	0.01 ± 0.15	9
	Hasheminejad et al. [49]	2017	Kerman	12–15	600 (50)	4.0 ± 3.0	3.6	1.1	0.3	10
	Oral health status and healthcare system in IR Iran, published by Khoshnevisan et al. [24]	2012	Kerman Province	15	250	4.74 ± 0.39	3.97	0.37	0.36	8
Unpublished	Amiri Jazeh et al. [48]	1997	Mahan	12–15	500	1.52 ± 1.89	-	-	-	7
			Sarasiyab Farsangi	12–15	300	0.91 ± 1.3	-	-	-
	Eftekhari Kanzarki et al. [41]	1998–1999	Kerman	15	71	3.6	1.28	0.09	0.09	7
	Dadalahi et al. [50]	2024	Sirjan	13–18	200	2.51 ± 1.54	1.58 ± 1.16	0.23 ± 0.49	0.71 ± 0.86	9

Fig. 6.

Forest plot of the pooled DMFT index in children aged 12–15 years

Table 7.

The DMFT index of adults in Kerman province

	Authors	Data collection date	Region	Age	Sample size (Male (%))	DMFT	DT	MT	FT	NOS
Published	Torabi et al. [51]	2006	Kerman	35–44	154 (39)	10.88 ± 6.47	4.51 ± 3.56	4.3 ± 4.53	2.99 ± 8.45	8
	Navabi et al. [52]	2016	Kerman	18–65	419 (50.9)	6.14 ± 4.35	-	-	-	10
	Karimi Afshar et al. [53]	2016	Kerman	35–44	401 (51.5)	13.3 ± 4.00	5.4 ± 2.3	4.3 ± 2.8	3.5 ± 1.7	9
	Nekouei et al. [54]	2014–2018	Kerman	18–24	494	4.17 ± 0.16	-	-	-	9
				25–34	1072	8.07 ± 0.15	-	-	-
				35–44	1207	10.45 ± 0.18	-	-	-
				45–54	1132	13.04 ± 0.22	-	-	-
				55–64	1106	14.7 ± 0.27	-	-	-
				65–74	507	16.67 ± 0.52	-	-	-
	Kamalabadi et al. [55]	2019	Rafsanjan	25–34	130	13.28 ± 7.62	-	-	-	10
				35–44	143	19.88 ± 4.94	-	-	-
				45–55	77	23.81 ± 4.28	-	-	-
	Sardari et al. [56]	2015	Rafsanjan	35–44	2982	16.44 ± 7.38	3.84 ± 3.80	8.18 ± 7.10	4.34 ± 4.78	9
				45–54	2671	21.30 ± 7.88	3.40 ± 3.72	14.23 ± 10.09	3.68 ± 4.88
				55–64	2421	25.46 ± 7.52	2.22 ± 3.22	20.94 ± 10.66	2.31 ± 4.15
				> 65	607	28.66 ± 6.03	1.40 ± 2.92	26.06 ± 9.39	1.21 ± 3.10
	Oral health status and healthcare system in IR Iran, published by Khoshnevisan et al. [24]	2012	Kerman Province	35–44	275	16.24 ± 0.78	5.9	8.59	1.74	8
				65–74	244	27.73 ± 0.46	2.09	25.28	0.35	8
Unpublished	Ziaaddini et al. [20]	2005	Bam	35–44	905	9.94	4.89	4.64	1.14	8
	Saberi et al. [57]	2015	Bam	18 ≤	400 (33.5)	9.65 ± 6.15	4.25 ± 2.84	4.53 ± 5.05	3.47 ± 3.34	8
	Attaollahi et al. [58]	2021	Kerman	18 ≤	130 (31.5)	11.45 ± 4.93	3.83 ± 1.16	2.99 ± 3.12	4.66 ± 3.26	9
	Salari et al. [59]	2025	Kerman	18–24	84	4.02 ± 3.45	2.23 ± 2.03	0.26 ± 0.64	1.59 ± 2.23	10
				25–34	123	7.34 ± 4.50	2.95 ± 2.35	0.73 ± 1.31	3.69 ± 3.39
				35–44	45	9.37 ± 2.33	2.33 ± 2.34	1.58 ± 1.84	5.47 ± 4.47

Fig. 7.

Forest plot of the pooled DMFT index in adults (35–44 years)

Heterogeneity

Substantial between-study heterogeneity was observed across all age groups (Figs. 2–7). Overall, I² values ranged from 94.18% in the 3–6-year group to 99.97% in the 35–44-year adult group. Subgroup analysis by time period significantly reduced heterogeneity only in the 3–6-year age group (Q-between = 13.28, p < 0.001).

Meta-regression

The findings of univariable meta-regression are provided in Table 8. Regression plots for calendar year treated as a continuous covariate are provided in Supplementary file 2. In the 3–6-year age group, the time period was a statistically significant predictor of dmft (coefficient = − 1.10, 95% CI: − 1.72 to − 0.50, p < 0.001, R² analog = 0.42), with studies conducted from 2015 onward reporting significantly higher dmft values compared to those prior to 2015. Geographic location was also a significant predictor in this age group (coefficient = − 0.76, 95% CI: − 1.51 to − 0.007, p = 0.047), with studies conducted outside Kerman city reporting lower dmft values; however, this covariate explained negligible between-study variance (R² analog = 0.00). Meta-regression using calendar year as a continuous covariate identified a statistically significant positive temporal trend in this age group (slope = + 0.21, 95% CI: 0.13–0.30, p < 0.001, R² analog = 0.66). Residual heterogeneity remained substantial after accounting for all covariates (I² = 81.19–94.18%).

Table 8.

Univariable meta-regression analysis of factors associated with DMFT/dmft indices across age groups in Kerman province, Iran

Age	Covariate	Coefficient	95% CI	p-value	R2 analog
3–6	Time (category)	− 1.10	− 1.72 to − 0.50	0.0004*	0.42
	Region	− 0.76	− 1.51 to − 0.007	0.047*	0.00
	Time (continuous)	0.21	0.13 to 0.30	< 0.001*	0.66
6	Time (category)	− 0.46	− 1.39 to 0.46	0.238	0.00
	Region	+ 2.20	1.79 to 2.62	< 0.001*	0.56
	Study type	− 1.14	− 1.70 to − 0.58	0.0001*	0.15
	Time (continuous)	−0.03	−0.08 to 0.01	0.134	0.00
12	Time period	+ 0.61	− 0.08 to 1.32	0.087	0.50
	Region	+ 0.75	− 0.22 to 1.73	0.132	0.00
	Study type	− 0.67	− 1.64 to 0.29	0.171	0.00
	Time (continuous)	− 0.03	− 0.13 to 0.05	0.417	0.15

^*p-values less than 0.05 indicate a statistically significant regression coefficient according to the Z-test

In the 6-year age group, univariable meta-regression identified geographic location as the strongest predictor of dmft (coefficient = + 2.20, 95% CI: 1.79–2.62, p < 0.001, R² analog = 0.56), indicating that studies conducted outside Kerman city reported substantially higher dmft values. Publication status was also a significant predictor (coefficient = − 1.14, 95% CI: − 1.70 to − 0.58, p < 0.001, R² analog = 0.15), with theses reporting significantly lower dmft values compared to published studies. Time period and calendar year as a continuous covariate were not significant predictors in this age group. Residual heterogeneity remained very high across all models (I² = 99.37%).

In the 12-year age group, none of the covariates examined was statistically significant predictors of DMFT. The time period showed a borderline non-significant association (coefficient = + 0.61, 95% CI: − 0.08 to 1.32, p = 0.087, R² analog = 0.50), with studies conducted prior to 2015 reporting higher DMFT values.

Sensitivity analysis

Sensitivity analysis was performed using the one-study-removed approach. Maximum deviations from the original estimates upon sequential study removal were 0.09 dmft units (3–6 years), 0.13 units (6 years), 0.49 units (9 years), 0.14 units (12 years), 0.27 units (12–15 years), and 1.01 units in the 35–44-year adult group.

In the 9-year age group, the removal of Nabavizadeh et al. [33] produced the largest downward shift, yielding a leave-one-out pooled estimate of 3.61. In the 12–15-year age group, the removal of Khoshnevissan et al. [24] produced the largest shift, reducing the pooled estimate from 2.61 to 2.34, suggesting a moderate influence of this study on the overall result. In the 35–44-year age group, the removal of Kamalabadi et al. [55] produced the largest deviation from 13.80 to 12.79. No single study exerted a disproportionate influence in the remaining age groups.

Publication bias

Publication bias was assessed for age groups with sufficient studies (Supplementary file 3). Egger's test indicated statistically significant funnel plot asymmetry in the 6-year age group (intercept = − 8.08, 95% CI: − 14.56 to − 1.60, p = 0.018), while no significant asymmetry was detected in the 3–6-year or 12-year age groups.

Certainty of evidence

The certainty of evidence was assessed using the GRADE framework for all age groups (Table 9). Evidence was rated as very low certainty across all age groups, primarily due to very serious inconsistency (I² > 94% in all groups).

Table 9.

Certainty of evidence assessment using the GRADE approach for pooled DMFT/dmft estimates across age groups in Kerman province, Iran

Variable	№ of studies	Certainty assessment						Effect			Certainty	Importance
		Study design	Risk of bias	Inconsistency	Indirectness	Imprecision	Other considerations	№ of events	№ of individuals	Mean (95% CI)
dmft 3–6 years olds	7 (15 values)	non-randomised studies	not serious	very seriousa	not serious	seriousb	none	-	4779	3.55 (3.26–3.84)	⨁◯◯◯ Very lowa,b	CRITICAL
dmft 6 years olds	12 (17 values)	non-randomised studies	seriousc	very seriousa	not serious	not serious	publication bias strongly suspectedd	-	8384	5.13 (4.74 to 5.50)	⨁◯◯◯ Very lowa,c,d	CRITICAL
dmft 9 years olds	4 (8 values)	non-randomised studies	not serious	very seriousa	seriouse	very seriousf	none	-	1110	4.09 (2.73–5.45)	⨁◯◯◯ Very lowa,e,f	CRITICAL
DMFT 12 years olds	10 (15 values)	non-randomised studies	seriousg	very seriousa	not serious	seriousb	none	-	7463	2.04 (1.70–2.39)	⨁◯◯◯ Very lowa,b,g	CRITICAL
DMFT 12–15 years olds	7 (9 values)	non-randomised studies	serioush	very seriousa	not serious	very seriousf	none	-	3478	2.61 (1.38–3.84)	⨁◯◯◯ Very lowa,f,h	CRITICAL
DMFT 35–44 years olds	7 (7 values)	non-randomised studies	not serious	very seriousa	not serious	very seriousb	none	-	5207	13.79 (10.80–16.78)	⨁◯◯◯ Very lowa,b	CRITICAL

a. I² > 75%, indicating high between-study heterogeneity; b. The 95% prediction interval was wide; c. NOS scores ranged from 7–10, with several studies scoring 7; d. Egger's regression test was statistically significant (p < 0.05), indicating funnel plot asymmetry; e. Only 2 included studies were peer-reviewed and published; the remainder were unpublished reports from a single research group; f. The 95% prediction interval included biologically implausible negative values, reflecting substantial uncertainty in the pooled estimate; g. NOS scores ranged from 7–9, with several studies scoring 7–8; h. NOS scores ranged from 7–10, with several studies scoring 7-8

Discussion

The present systematic review synthesized nearly three decades of dental caries data from Kerman province across six age groups. Overall, the findings reveal a divergent pattern: a significant worsening trend in early childhood caries contrasted with a directional decline among 12-year-olds, alongside persistently high caries burdens and notable geographic disparities, particularly among 6-year-old children.

The most notable finding concerns children aged 3–6 years, where dmft increased significantly by approximately 0.21 units per calendar year (p < 0.001), with studies from 2015 onward reporting dmft values approximately 1.10 units higher than those from prior to 2015 (p < 0.001). The pooled dmft of 3.55 substantially exceeds the WHO target of zero caries in primary teeth. This finding aligns with Iran's national trend, where the decline in deciduous caries incidence between 1990 and 2021 (average annual percent change (AAPC) = − 0.08%) was substantially slower than the global (− 0.25%) and the Middle East and North Africa (MENA) (− 0.57%), indicating limited effectiveness of existing preventive measures in this age group [60]. According to Sistani et al. [61], the mean dmft among 3–6-year-old children in Babol, Northern Iran, increased significantly from 2007–2010 to 2011–2015. Javadzadeh et al. [62] reported that the prevalence of early childhood caries among 1–5-year-old children in four Iranian provinces was 53.2%, with a mean dmft of 2.7 ± 0.3, ranging from 1.13 in toddlers to 5.29 in preschoolers. A systematic review by Shoaee et al. [63] reported that the dmft of Iranian children increased from 2.22 to 3.01 between 1990 and 2017. Data from developing countries further confirm that the prevalence and severity of dental caries among 3–6-year-olds show an increasing trend [64–66]. In contrast, Capurro et al. [67] reported a declining trend in untreated caries among 2–12-year-old children in the United States, attributed in part to dental insurance coverage and an overall higher quality of life.

In 6-year-old children, geographic location was the dominant predictor of dmft, explaining 56% of between-study variance (coefficient = + 2.20, p < 0.001). However, time period was not a significant predictor. Similarly, Ghasemianpour et al. [25] reported that rural residency was positively associated with dmft in 6-year-old children in Iran. This finding has important policy implications: the burden of caries at age 6 in Kerman province appears to be primarily a geographic phenomenon, driven by disparities in access to preventive services, water fluoridation, and oral health literacy between urban and peripheral areas. The pooled dmft of 5.12 substantially exceeds the national estimate and is among the highest reported for this age group in Iran [63]. Shoaee et al. [63] also reported an increase from 4.34 to 5.06 between 1990 and 2017 among 5–9-year-old children in Iran. The high dmft in 6-year-olds likely reflects the accumulation of untreated early childhood caries from the 3–6-year group, reinforcing the need for earlier interventions.

Among 12-year-old children, a non-significant declining trend was observed, with the pooled DMFT of 2.04, aligning with the WHO target of DMFT < 3. Studies from 2015 onward reported lower DMFT values than those conducted before 2015 (1.64 vs. 2.26); while this difference did not reach statistical significance and causal attribution cannot be established from the available data, the directional pattern is consistent with a possible reduction of caries severity in this age group following OHRP implementation. Notably, the pooled DMFT of 2.04 remains higher than the latest national estimate of 1.84 among Iranian 12-year-old children [25] and higher than estimates from more affluent provinces such as Tehran [68], reflecting the persistent oral health disadvantage of Kerman province.

The pooled DMFT of 13.80 for the WHO index age group of 35–44 years reflects both a high caries burden and profound uncertainty, as indicated by the wide prediction interval (2.73–24.86) and the small number of available studies (k = 7). The marked geographic variation in this group, with studies from Rafsanjan reporting substantially higher DMFT values than those from Kerman city, is consistent with the disparities identified in younger age groups and likely reflects differences in access to dental care, water fluoride levels, and socioeconomic conditions between the provincial capital and peripheral cities. The pooled DMFT in this age group exceeded the national estimate of 10.3 reported by Shoaee et al. [8] in 2017. According to Najafi et al. [9], based on the PERSIAN cohort, the mean DMFT of adults older than 35 years in Kerman was 19.4, and socioeconomic-related inequality in DMFT was among the largest in provinces including Kerman, Ardabil, Yazd, East Azerbaijan, and Fars. The higher DMFT reported by the PERSIAN cohort compared to the pooled estimate of the present review is expected and methodologically explicable, since the PERSIAN cohort included adults across a broad age range extending well beyond 44 years, whereas the quantitative synthesis in the present review was restricted to the WHO index age group of 35–44 years.

Iran's OHRP, launched in 2015 as part of the national Health Reform Plan, introduced free preventive dental services — including fluoride therapy, fissure sealants, and restorative treatments — for children under 14 years through school-based delivery [11]. The directionally declining DMFT trend observed among 12-year-olds in the present study is consistent with a modest beneficial effect of this program on school-age populations, and aligns with findings from Shiraz province where DMFT decreased following OHRP implementation [11]. In contrast, evidence from Gilan province suggested no significant impact on DMFT [69], indicating that program effectiveness may vary considerably by region, likely reflecting differences in implementation fidelity, resource allocation, and local infrastructure. However, dmft among children aged 3–6 years increased significantly over the study period. This divergence between age groups — a non-significant decline in 12-year-olds alongside a statistically significant worsening in 3–6-year-olds — suggests that the school-based delivery framework of the OHRP is structurally misaligned with the needs of the preschool population. Furthermore, it suggests that the program's reach does not extend to the settings, caregivers, and age groups where the caries burden is most severe and most rapidly increasing.

While demographic shifts and reductions in sugar intake have contributed to a significant decrease in the global burden of oral conditions since 1990 [60], these gains have not materialized equally in Iran, where implementation of evidence-based preventive strategies remains hindered by treatment-oriented policy frameworks, inadequate insurance coverage, lack of specialized workforce training, and systemic weaknesses in service monitoring and distribution [70]. Within Kerman, these challenges are further intensified by a low human development index, significant geographic dispersion, and acute financial constraints, disproportionately impacting low-income and rural cohorts [9, 71, 72]. Addressing these disparities requires sustainable public financing, strategic purchasing of cost-effective services, and expansion of insurance coverage for preventive care [73].

Three priority interventions are recommended. First, community-based programs targeting caregivers of children under three years — including dietary counseling, facilitated early dental visits, and oral health literacy promotion — are needed to address the structural preschool gap in the OHRP framework, given the significant worsening trend in dmft among 3–6-year-olds. Particular emphasis should be placed on improving dietary quality, as evidence from Iranian preschool populations indicates that healthier dietary patterns are independently associated with reduced caries risk [74]. Second, targeted resource allocation to peripheral cities is warranted, with priority given to water fluoridation expansion, as provincial fluoride concentrations (mean 0.35 ± 0.22 mg/L) fall substantially below WHO-recommended levels [47], consistent with the documented inverse association between water fluoride and DMFT in Iranian adults [75]. Third, the extremely high adult caries burden identified in this review underscores the need for a paradigm shift toward prevention-based dentistry, accompanied by expansion of dental insurance to cover preventive services [70].

The present study has several limitations. A primary constraint is the inclusion of dental master's theses, which are classified as grey literature. Incorporating these theses was essential due to the limited peer-reviewed data available for Kerman province. In addition, substantial between-study heterogeneity persisted across all age groups. While meta-regression identified time period and geographic location as significant predictors in the 3–6-year and 6-year groups, respectively, residual heterogeneity remained very high across all models, suggesting that unmeasured factors — including differences in examiner calibration, sampling strategies, gender, diet, oral health habits, socioeconomic status, urban–rural population composition, and local fluoride levels — account for a substantial proportion of between-study variance. Under these conditions, the pooled estimates should be interpreted as descriptive summaries of the available provincial evidence rather than precise population parameters.

Moreover, the pooled estimates for the 6-year age group warrant cautious interpretation due to statistically significant Egger's test results. However, under conditions of very high heterogeneity, funnel plot asymmetry may reflect true between-study variance rather than publication bias. Another limitation was that the majority of included studies did not consistently report the decayed, missing, and filled components of the DMFT/dmft index separately, precluding a quantitative assessment of treatment uptake and unmet restorative needs across age groups.

Additionally, the 9-year and 12–15-year age groups could not be meaningfully contextualized within the broader literature, as neither constitutes a WHO index age group; comparable external data for these specific cohorts are also scarce. The synthesis of adult data was restricted to the WHO 35–44-year index group because of incompatible age ranges across other adult studies. Also, meta-regression and Egger's test for publication bias were not performed for the 9-year, 12–15-year, and adult age groups due to an insufficient number of studies (k < 10). Ultimately, the overall very low GRADE certainty across all age groups underscores the urgent need for a standardized provincial oral health survey. Future research should prioritize the adoption of WHO Basic Methods, implementation of systematic examiner calibration, and consistent socioeconomic data collection.

Conclusion

Dental caries remains a major public health challenge across all age groups in Kerman. The most critical finding was a statistically significant worsening trend in early childhood caries, contrasted with a non-significant decline in 12-year-olds. Furthermore, geographic location was a dominant source of heterogeneity among 6-year-olds, emphasizing urban-peripheral disparities. The high adult caries burden, coupled with very low certainty of evidence across all groups, reflects a critical lack of standardized data. These findings support the need for community-based preventive programs targeting preschool children and more equitable allocation of dental resources across the province.

Authors’ contributions

A.S. conceptualized, designed, and supervised the study, and reviewed the manuscript. S.Z., S.RP., and M.H. acquired data and wrote the original draft. M.H. and A.SS. conducted data analysis. A.SS. reviewed the manuscript. All authors read and approved the final manuscript.

Funding

The present study was supported by the Deputy of Research of Kerman University of Medical Sciences.

Data availability

The datasets used and/or analysed during the current study will be available from the corresponding author on reasonable request.

Declarations

Ethics approval and consent to participate

This study was approved by the ethics committee of Kerman University of Medical Sciences (code: IR.KMU.REC.1400.441).

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.