Maxillary First Premolars’ Internal Morphology: A Systematic Review and Meta-Analysis

Abstract

Objectives: This systematic review analyzed the root canal morphology and configuration (RCC) of maxillary first premolars (Mx1Ps) and sex-specific differences based on existing literature. Methods: Registered in PROSPERO (CRD42023394460) and following PRISMA guidelines, systematic searches were conducted in five databases (Cochrane, Embase, LILACS, Scopus, MEDLINE via PubMed) using predefined MeSH terms. Additional studies were identified through cross-referencing. Studies on Mx1P RCCs were included, assessed using the AQUA tool. Data extraction focused on RCC prevalence, sex differences, root variations, and examination methods. Results: Of 865 studies, 86 were included, analyzing 31,325 teeth. The most common RCCs were 2-2-2/2 (IV, frequencies between 0.6–80.5%) and 1-1-1/1 (I, 1.1–72.0%). Mx1Ps primarily had two roots (7.1–96.2%) or one root (3.8–93.2%), with three-rooted variants being rare (0.4–6.5%). Males more frequently exhibited two- or three-rooted Mx1Ps with RCCs like 2-2-2/2 (IV; OR = 1.39 [1.22, 1.58]), and 1-1-3/3 (VIII; OR = 2.22 [1.59, 3.11]). Females showed higher frequencies of RCCs like 1-1-1/1 (I; OR = 0.71 [0.53, 0.96]), 2-2-1/1 (II; OR = 0.66 [0.57, 0.77]), 1-2-1/1 (III; OR = 0.70 [0.59, 0.83]), and 1-1-2/2 (V; OR = 0.81 [0.70, 0.95]). Conclusions: Mx1Ps predominantly have two roots and a 2-2-2/2 (IV) RCC. CBCT was the most used method, followed by staining and clearing. Clinicians should consider sex-specific and morphological variations.

1. Introduction

The primary goal of clinical endodontic treatment is to thoroughly eliminate bacteria, prepare the root canal system through mechanical shaping, chemical cleaning, and subsequent obturation with filling material. However, untreated root canals often lead to treatment failure, primarily because they are not identified by the clinician [1]. Insufficient knowledge of root canal anatomy remains one of the key reasons for endodontic failure [2]. To minimize the risk of missing canals and errors during access cavity preparation, cleaning, shaping, or obturation, a comprehensive understanding of root canal morphology is essential [3], especially regarding sex-specific and morphological differences in the maxillary first premolars, which are important for an accurate diagnosis and successful root canal treatment. Over the years, various methods have been employed to study the root canal anatomy of maxillary first premolars. Earlier techniques included radiography [2,4,5,6,7,8], grinding [9], staining and clearing [2,10,11,12,13,14,15,16,17,18,19,20,21,22,23], microscopy [9,11,12,13,14,18,20,23], or cross-sectional analysis [2,5,8,22]. Additionally, one study used polyester casting resin combined with red dye for anatomical evaluation [24]. In recent decades, cone-beam computed tomography (CBCT) [25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56] has provided more detailed insights. While CBCT enables retrospective in vivo anatomical investigations [57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86], micro-CT remains restricted to ex vivo studies [87,88]. The results of these studies are frequently reported using different classification systems, including Vertucci’s system [1] and that of Weine et al. [89]. For more complex root canal systems, classification methods such as those proposed by Sert et al. [13], Briseño-Marroquín et al. [3] or Ahmed et al. [90] have been utilized. Briseño-Marroquín et al. [3], for example, describe the root canal system in four digits: the first three digits represent the morphology of the coronal, middle, and apical thirds of the root, while the last digit indicates the number of main apical foramina. The maxillary first premolar is often characterized by two roots and a predominant root canal configuration (RCC) of 2-2-2/2 (Type IV according to Vertucci and Type III according to Weine). However, significant variations in the number of roots and RCCs have been reported in the literature [2].

The aim of this systematic review was to analyze the internal root canal morphology of Mx1Ps and to investigate sex-related differences using meta-analysis.

2. Materials and Methods

This review was registered in the International Prospective Register of Systematic Reviews (PROSPERO) system of the National Institute of Health Research of the Centre for Reviews and Dissemination of the University of York (United Kingdom) (CRD42023394460). This systematic review follows the PRISMA guidelines (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) (Supplementary Materials File S1) [84,91]. This systematic review was structured according to the PICO framework: studies that examined the root canal morphology and configuration of the first premolars of the human maxilla (population) using various methods such as irrigation, X-rays, CBCT, or micro-CT (intervention) without a specific comparison (comparison) were analyzed in terms of the frequency and characteristics of root canal configurations, including the number of roots, physiological foramina, accessory canals, anastomoses, and dimensions of physiological foramina (outcome). A literature search was conducted in five electronic databases (MEDLINE via PubMed, Embase, Scopus, LILACS, and Cochrane Database) on the RCC of the maxillary first premolar. The search was conducted until September 2025 using the following Medical Subject Heading (MeSH) terms and search terms (search string): (root canal configuration OR root canal system OR root canal morphology) AND (morphology OR anatomy) AND (maxillary first premolar OR maxillary premolars). The identical search string was used without the application terms or filters across all databases (MEDLINE via PubMed, Embase, Scopus, LILACS, and the Cochrane database). Further relevant studies were added to the list through cross-referencing and handsearching. Thus, further papers were looked for manually by screening reference lists or citations of articles already found. Duplicates were removed by using EndNote version 20.6 (Clarivate, Philadelphia, PA, USA) and comparing the articles retained from the databases. The remaining articles were screened for relevance based on title and abstract by two independent reviewers (D.S.U., A.L.W.). Articles that were irrelevant to the topic were excluded. The remaining articles that met the inclusion criteria were viewed in full text and were examined again by the same two independent reviewers (D.S.U., A.L.W.).

2.1. Inclusion and Exclusion Criteria

There were no language restrictions. Case reports, reviews, and papers addressing other morphological research questions than RCC were excluded. Articles in which the teeth were not clearly identified as maxillary first premolars or in which certain information, such as the total number of teeth examined or the population studied, was missing, were also excluded. Randomized controlled trials, comparative studies, evaluation and validation studies, and cross-sectional studies of any population and of all age groups were included.

2.2. Data Collection and Synthesis

The authors of the papers were identified, along with the date of publication, the population studied, and the number of teeth, the methods used in the studies for analysis, and the data obtained regarding the number of roots and the RCC. The information was summarized in tables. The classifications of Vertucci [1], Weine et al. [89] and Briseño-Marroquín et al. [3] were used to present these results. In recent years, the system proposed by Ahmed et al. [90] has been increasingly utilized in the literature. In this paper, RCC was not presented using this system, as it is difficult to correlate older classification systems, such as the one proposed by Vertucci [1], which has been used in a great number of the articles included in this review. The Review Manager software (RevMan version 5.4 software, Cochrane Collaboration, Copenhagen, Denmark, 2014) was used for statistical analysis and meta-analyses. The odds ratio was calculated to determine the effect size. The I2 statistic was determined to quantify the degree of variation due to heterogeneity instead of chance [92]. Depending on the heterogeneity (I2 < 35% for low heterogeneity, fixed effects and I2 > 35% for high heterogeneity, random effects), a meta-analysis was carried out. The primary measures of effect were the odds ratio and 95% confidence intervals (95% CI), comparing different RCC geographic regions and patients’ sex for studies with dual outcome data. Using a p-value of 0.05 or less indicates statistical significance.

2.3. Quality Assessment and Risk of Bias

We assessed the quality of the studies using the quality assessment tools provided by the National Heart, Lung and Blood Institute (https://www.nhlbi.nih.gov/health-topics/study-quality-assessment-tools, last accessed on 23 June 2025) (Supplementary Materials File S2). We used the Anatomical Quality Assessment (AQUA) tool to assess the risk of bias in the studies [93] (Supplementary Materials File S3). The AQUA tool uses five areas with a total of twenty questions to determine whether a study is at risk of error. The five areas are as follows: (1) aim and characteristics of the teeth examined, (2) study design, (3) description of the research methods used, (4) illustration of anatomy and (5) reporting of the results. Each question is answered with “yes” or “no”. Depending on the answers, the work can ultimately be classified as “at high risk of bias” or “at low risk of bias”. If all questions for a section are answered with “yes”, then the risk of bias is “low”. The quality of the studies included was examined by two independent reviewers (D.S.U., A.L.W.). In the event of a disagreement, a third reviewer (T.G.W.) was consulted.

3. Results

The literature search in the five databases (MEDLINE via PubMed, Embase, Scopus, LILACS and Cochrane Database) yielded a total of 865 articles. After the duplicates were removed, 540 articles remained, which were checked for inclusion and exclusion criteria based on the title and abstract. In this process, 403 articles were excluded because they did not fit the topic. The remaining 137 articles that fulfilled the inclusion criteria were reviewed in full text. After analyzing the full-text articles and handsearching, three additional articles were added. Thus, a total of 86 articles were included in this systematic review (Figure 1).

Table S1 (Supplementary Materials File S4) summarizes all the results and details of the articles listed, such as authors, date of publication, population studied, and research method used.

The RCC and the number of roots were presented in Table S2 (Supplementary Materials File S5) based on the classification systems of Vertucci [1], Weine et al. [89], and Briseño-Marroquìn et al. [3].

The total number of teeth examined was 31,325. The articles examined the differences in root canal morphology of the maxillary first premolar in different populations and between males and females. Some studies also compared the maxillary right first premolar [14] to the maxillary left first premolar [24]. Two studies compared root canal morphology in the same population within different age groups [36,74]. The most used examination method was CBCT imaging, followed by staining and clearing of the canals. Analysis of RCC using a microscope, radiographs, cross sections, micro-CT and grinding of teeth was less frequently performed. More recent studies favor CBCT imaging. Only two of the included studies used micro-CT to visualize root canal morphology [87,88].

3.1. Number of Roots

The results show that most maxillary first premolars have two roots, with a frequency between 7.1 and 96.2% [2,5,6,8,10,12,14,16,17,18,27,28,31,32,34,38,39,40,42,43,45,46,48,49,52,54,55,56,57,60,61,62,63,65,66,68,69,73,74,75,77,79,81,83,86]. The first premolars in the upper jaw with one root showed a proportion between 3.8 and 93.2% [15,19,20,21,23,25,30,33,35,38,41,44,47,50,51,59,64,67,71,72,78,80,82,84,85]. Three roots were observed with a much lower frequency between 0.3 and 11.7% [2,5,8,10,12,14,15,17,18,19,20,21,25,27,28,30,31,32,33,34,35,37,38,40,41,42,43,44,45,46,47,48,49,51,52,54,56,57,60,61,63,64,65,66,69,70,71,73,74,75,78,79,80,81,83,84,86].

3.2. Root Canal Configuration (RCC)

The most frequently observed RCC was type 2-2-2/2 according to Briseño-Marroquín (type III according to Weine and type IV according to Vertucci), with a frequency of 0.6–80.5% [2,4,5,6,7,8,9,10,11,12,13,14,15,16,17,19,21,23,24,25,26,27,29,30,31,32,33,34,35,36,37,38,40,41,42,43,45,46,47,48,49,51,52,55,56,57,58,59,61,62,65,67,71,73,74,75,76,77,79,81,82,83,84,85,86,87,88]. The second most frequently observed classification was 1-1-1/1 (type I according to Weine and type I according to Vertucci) with a frequency of 1.1–72% [28,39,54,63,72,78,80]. The RCC 2-2-1/1 (type II according to Weine and type II according to Vertucci) was reported as the most common configuration in just one article, with a frequency of 23.8% [44]. 1-1-2/2 (Weine type IV and Vertucci type V) was reported as the most common RCC in four articles, with a frequency of up to 58.5% [22,50,60,64]. 3-3-3/3 (type VIII according to Vertucci) was observed with a frequency of only up to 11.7%.

3.3. Comparative Studies

This review also includes comparative studies. The differences between women and men [13,17,23,26,31,34,37,43,45,47,48,52,59,61,62,64,65,71,72,74,75,76,79,81,82,83,84,85,86], the left and right first premolars in the maxilla [26,31,45,47,52,56,59,62,65,67,69,70,71,75] and between different populations [15,38,60,83,85] were investigated. Two studies compared RCCs within different age groups [36,74,79,81,86].

The data were analyzed by meta-analysis and grouped by geographical location (Asia, Europe, North America, South America, Africa) (Figure 2a–h).

4. Discussion

The aim of this paper was to summarize the possible variations in RCC of the maxillary first premolar in the form of a systematic review and to provide the practitioner with an understanding and knowledge of what to expect during a root canal treatment.

4.1. Number of Roots and Root Canal Configuration (RCC)

This systematic review shows that the first maxillary premolar usually has two roots. The lowest reported frequency was 7.1% [72], while the highest frequency was 96.2% [60]. However, many studies reported only one root with frequencies between 3.8% and 93.8%. The lowest and highest frequencies were again reported by Alnaqbi et al. [60] and Shah [72], respectively. Three roots were found in many studies, but with a lower frequency of between 0.4–6.5% [17,69]. A RCC of 2-2-2/2 (type IV according to Vertucci and type II according to Weine) was the most frequently reported RCC with a frequency of up to 80.5% [69]. In teeth with two roots, the frequency of the configuration can be as high as 99.6% [47]. However, in some of the included studies, for example, by Bulut et al., the frequency was very low at 1.9% [28]. The range of results is thus very wide, and the values are heterogeneous. Several studies [28,39,54,63,72] report that a RCC of 1-1-1/1 is the most common RCC in their samples. The frequency ranges between 45.4 and 72.0%, whereby it was striking that three of the five studies were conducted on a Pakistani population. The RCCs 2-2-1/2 (type II according to Vertucci) and 1-1-2/2 (type V according to Vertucci) were reported as the most common RCC in only five studies. First maxillary premolars with three roots and the configuration 1-1-3/3 were even described as being present in 100% of cases in various populations [25,27,35,40,47,48]. Only one study [33] found other, not further classified, RCCs in three-rooted teeth with a frequency of 14.3%. All other RCCs, such as 1-2-1/1, 2-1-2/2 and 1-2-1/2, were also reported in most papers but occur less frequently.

4.2. Differences Between Women and Men

This systematic review examined sex-specific differences in the anatomy of the first maxillary premolar, with 20 studies included in the meta-analysis. Except for the studies by Sert et al. [13], Ng’Ang’A et al. [17] and Peiris et al. [23], all were conducted using CBCT. The included studies show that maxillary first premolars with only one root are more common in women [17,23,31,34,37,43,61,74,85,86]. The frequency is up to 100% [23]. In contrast, maxillary first premolars with two or three roots are more common in men [17,23,31,34,37,43,61,65,74,80,83,84] with a frequency of 62.1% [37] and 83.1% [17], respectively. Men also more often had additional RCCs that Vertucci could not define [13,23,37,43,79,86], with Martins et al. [37] reporting 20%. A significantly higher number of the RCCs 1-1-1/1 (Vertucci Type I), 2-2-1/1 (Vertucci Type II), 1-2-1/1 (Vertucci Type III), 1-1-2/2 (Vertucci Type V) and 2-1-2-/2 (Vertucci Type VI) and (Vertucci Type VII) were observed in females. However, the RCCs 2-2-2/2 (Vertucci Type IV) and 1-1-3/3 (Vertucci Type VIII) were observed with a higher number in males. For each RCC, sex-specific differences were observed. The age of the patients was not reported in all the studies included; therefore, the age was not included in the meta-analysis either.

4.3. Sample Size and Research Methods

The number of teeth examined in the studies included in this work varies strongly. The sample sizes varied between 24 [6] and 1387 [35] examined teeth. Most studies investigated more than 100 teeth. Different techniques were used to examine the anatomy of the samples, for example, X-ray [2,4,5,6,7,8], grinding [9], staining and clearing [2,10,11,12,13,14,15,16,17,18,19,20,21,22,23], microscopy [9,11,12,13,14,18,20,23], cross-sectioning [2,5,8,22] and micro-CT [87,88]. CBCT imaging was used in most of the studies examined in this systematic review [25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86]. Although micro-CT is considered the gold standard in terms of accuracy and resolution, it is limited to ex vivo application and thus only used in research. It should also be mentioned that scanning and reconstructing a single tooth from the sample is very time-consuming, depending on the resolution, and that artifacts, e.g., due to drying of the teeth, can lead to misinterpretations or errors in the analysis [95,96,97,98]. The other methods mentioned, such as X-rays, grinding, staining and clearing, the use of a microscope and the examination of cross-sections, now appear to have been largely replaced by CBCT imaging as a modern three-dimensional technique in both research and practice. As these previous techniques are used less frequently in more recent studies, especially for issues that cannot be resolved with two-dimensional techniques, three-dimensional techniques are intended to provide additional information. The most popular way of describing RCCs was the method according to Vertucci [1]. Only three papers used the methods of Weine et al. [2,5,22], and one paper used the method of Briseño-Marroquín et al. [3].

4.4. Strengths and Limitations

A limitation of the present work arises from the fact that, due to the diversity of the studies included and the different research methods, the results must be considered with caution. The studies examine different populations, ethnic groups and various countries and often work with age groups that are not clearly defined, and information on sex, age and position in the maxilla (right, left) is often missing. The sample sizes examined also differ significantly, with most studies not calculating the sample size. Although several classification systems for root canal configuration have been proposed over the years, we focused our analysis on the three most used and widely accepted systems, which may be considered a limitation of the search strategy. The strength of this work lies in the large amount of literature from both retrospective studies and in vivo studies that could be included and summarized in this work. Teeth from 32 different populations were examined, so the results can be better applied to the general population of the respective geographic regions and are useful for clinical practice.

5. Conclusions

The internal morphology in teeth can be examined using various methods, with CBCT being the most widely utilized. Maxillary first premolars predominantly exhibit two roots, with reported frequencies ranging from 7.1–96.2%. The most common root canal configuration (RCC) is 2-2-2/2 (Vertucci type IV, Weine type III), followed by 1-1-1/1 (Vertucci type I, Weine type I). Other observed RCCs include 1-2-1/1 (Vertucci type III), 2-1-2/2 (Vertucci type VI), and 1-2-1/2 (Vertucci type VII).

Sex differences were evident: male patients more frequently presented with two or three-rooted maxillary first premolars, with RCCs such as 2-2-2/2 (Vertucci type IV) and 1-1-3/3 (Vertucci type VIII) observed more often. In contrast, females exhibited higher frequencies of RCCs like 1-1-1/1 (Vertucci type I), 2-2-1/1 (Vertucci type II), 1-2-1/1 (Vertucci type III), 1-1-2/2 (Vertucci type V), and 2-1-2/2 (Vertucci type VI).

This systematic review and meta-analysis provide valuable insights into the internal morphology of maxillary first premolars, aiming to help clinicians in optimizing treatment strategies. However, findings should be interpreted cautiously due to inherent limitations. Variations in RCCs, along with patient-specific factors such as ethnicity, sex, and age, must be considered to minimize errors and avoid iatrogenic complications during root canal treatments.