Sex and Population Variations in Nasopalatine Canal Dimensions: A CBCT-Based Systematic Review

Introduction

The nasopalatine canal, or incisive canal, is an interosseous conduit passing through the anterior maxilla to connect the oral and nasal cavities. Morphometric variations can affect the canal’s contents, including the nasopalatine nerve and a vascular anastomosis between the greater palatine artery and sphenopalatine artery [1,2]. Knowledge of accurate anatomical characterizations, dimensions, and the potential variation in NPC is essential when performing local anesthetic and several surgical procedures, such as dental implants, impacted tooth extraction, apicoectomy, NPC duct cyst enucleation, and orthognathic surgery [3–8]. In orthodontics, the canal’s dimensions must be considered when planning the movement of anterior upper teeth [9]. The NPC, also known as the incisive canal, is located at the midline, posterior to the upper central incisor teeth. The NPC connects the nasal cavity and the oral cavity through a superior opening called the foramina of Stenson (FS) or nasopalatine foramen and an inferior opening called the incisive foramen (IF) [10–12]. Within the NPC, the descending nasopalatine artery’s terminal branch and the nasopalatine nerve pass through, and the minor salivary glands can be found within the canal [13].

To avoid potential complications, carefully evaluating the NPC is recommended before many dental procedures due to the significant anatomical variation in the canal [9,14]. Therefore, 2D conventional radiography techniques, such as panoramic and intraoral radiography, are not very useful for displaying some intra-bone anatomy. Furthermore, using these tools to plan treatments can result in inaccurate imaging due to magnification, overlap, and distortion [15]. While CT can perform 3D imaging, it entails high radiation dose and cost [16]. Since its introduction, CBCT has become a recommended method for dental diagnosis (such as maxillofacial surgery and orthodontics), including the evaluation of NPC, because of its low cost, high image quality, and minimal radiation exposure [17–20].

The variety of measurement methods used is one of the reasons for the difference in the dimensions of NPC. In some studies, the anterior–posterior diameter of IF is measured horizontally. This measurement method makes the NPC length and IF diameter seem smaller than they actually are [21–23]. A different method is to use the opening slope of IF to determine the actual diameter [24–28]. The IF diameter was 7.99 mm in one study and 1.54 mm in another study. The 2 studies used different methods of measurement [23,29]. Studies in Malaysia and China showed that the FS diameter was 6.06 mm, whereas a study in India showed the FS diameter was 1.99 mm [23,30].

Considering sex, different studies performed in numerous countries assessed the NPC and its associated parameters in the oblique position of the IF to assess differences between males and females, and verified different measurements between sexes [14,26,31]. Moreover, other investigators assessed the NPC in different dental states [14,26,32].

Knowledge about the dimensional differences in NPC and understanding the anatomical variations in NPC parameter measurements between males and females during different dental procedures are necessary. Nevertheless, we were unable to identify any review that analyzed these variations among various populations and sexes. Therefore, this systematic review aimed to evaluate the published literature on variations in anatomy and dimensions of the nasopalatine canal using cone beam computed tomography (CBCT).

Material and Methods

REVIEW QUESTION:

The current systematic review followed the guiding principle of the Preferred Reporting Items for Systematic Review (PRISMA; www.prisma-statement.org) statement. The Population, Intervention, Comparison, Outcome (PICO) setup was utilized to frame the intensive question according to the following: “Is there a difference between the NPC and IF in oblique line (Intervention) in relation to measurements of CBCT from different countries and for different sexes with and/or without the presence of maxillary anterior teeth (Population) that exhibit the same measurements in all involved participants (Outcome) compared between participants from different countries (Comparison)?”

INCLUSION AND EXCLUSION CRITERIA:

The inclusion and eligibility criteria were: (i) clear, readable CBCT for in-vivo clinical investigations assessing the NPC parameters, (ii) for both sexes and age ≤18 years, (iii) measurements of IF assessed obliquely and in micrometers (mm) and counted as mean alone or mean±standard deviation, (iv) using the same machine with their software for measurements, and papers published in English and in 2014 and later. Studies involving NPC and IF measurements other than the oblique plane (the opening slope of IF) or using finite element analysis were excluded. Reviews, case reports, pilot studies, case series, editorials, and research published in languages other than English were also excluded.

LITERATURE SEARCH:

The search was accomplished using mixtures of medical subject heading terms and free keywords together with Boolean operators (AND, OR, and NOT) concerning the PICO question. An electronic literature search was conducted between January 2023 and February 2023 by 2 operators utilizing Clarivate Analytics’ Web of Science, Elsevier’s Scopus, and PubMed (MEDLINE) without a restriction to studies after the 2014 publication year. A manual search was then conducted by checking the bibliographies of all the initially selected studies to identify articles that may have been missed during the electronic search. The keywords used were nasopalatine canal length, incisive foramen, foramina of Stenson, buccal bone plate, oblique measurements, and sagittal or axial plane.

STUDY SELECTION:

The choice and verification of papers and references went through 3 stages: (i) choice based on title and its relevance to NPC measurements in the oblique plane, (ii) collection constructed on abstract significance, and (iii) full-text screening. All involved papers saved by manual and electronic explorations were grouped and evaluated for inclusion according to the eligibility criteria.

DATA EXTRACTION AND ANALYSIS:

A uniform worksheet (Microsoft Office Excel software) was used to extract data of interest from the selected papers published between 2014 and 2024. The collected data comprised: (i) CBCT studies that included researcher(s) name, publication years, population, and country where the study was conducted, the age of participants, the number of CBCTs involved, and dental status (2 central incisors present, 1 central incisor missing, 2 central incisors missing, presence of central and lateral incisors, and presence and/absence of anterior teeth). Measurements of NBC parameters in the oblique line technique were in mm for IF anterior–posterior diameter, FS anterior–posterior diameter, middle of NPC diameter, mediolateral diameter of IF, mediolateral diameter of FS, length of NPC, and buccal bone plate (BBP).

QUALITY ASSESSMENT:

The quality of studies was assessed using the parameters of previously published relevant systematic reviews involving CBCT studies [9,13,31,40]. The risk of bias was judged according to the following variables: measurements by 2 operators, standardization of plan of measurements, presence or absence of anterior teeth, standardization of CBCT preparation and mounting, and calculation of sample size. A “yes” classification was assigned to a judged parameter if it was existing in paper; otherwise, a “no” classification was assigned. The risk of bias of each study was characterized based on the overall number of “yes” classifications received as follows: 1–3 (low quality/high risk of bias), 4–6 (medium quality/moderate risk of bias), and 7–8 (high quality/low risk of bias).

Results

LITERATURE SEARCH AND STUDIES SELECTIONS:

The primary search keywords yielded 1404 studies. After removing 1322 irrelevant and duplicate studies and papers, the abstracts of 182 studies were read to exclude ineligible ones. A total of 127 studies were excluded, and 55 other studies were selected for full-text retrieval. Finally, 14 full-length papers were included in the present review [14,25,26,28,32–41]. These studies measured the NBC parameters concerning gender from different countries and populations and in the presence of diverse dental statuses in the axial and sagittal planes. The flowchart of the literature exploration method is shown in Figure 1.

CHARACTERISTICS OF THE STUDIES:

The 14 papers included were studies that used CBCT [14,25,26,28,32–41]. The highest number of publications (10 studies [67%]), was from the years 2014–2019 [14,25,26,36–41] and only 5 studies (33%) were published in 2020 and later [28,32–35]. There were 3 studies from Turkey [25,26,41], 3 from Iran [36–38], and 1 study each from Germany, Yemen, Pakistan, Saudi Arabia, Brazil, Korea, India, and Spain [14,28,32–35,39,40]. The highest number of examined and analyzed CBCT was 619, followed by 490 and 401 [25,26,33], whereas the lowest number was only 48 topographies [36]. Seven studies assessed the existing dental statutes in the form of the presence of 2 central incisors, and 1 or 2 centrals missing/total edentulous [25,26,28,35,38,40,41], whereas 4 studies assessed the dental statutes in the form of the presence or absence of anterior teeth [14,26,34,37]. Two studies stated the presence of central and lateral incisors [32,33], 1 in the total edentulous [36], and 1 study assessed the NBC parameters when missing 1 or 2 central incisors [39]. Table 1 presents the details of the studies included in the present systematic review.

INCISIVE FORAMEN (IF): Fourteen studies assessed the IF parameter. Two studies recorded marginally equal values of (6.08±1.57 and 6.09±2.03) and (7.52±2.08 and 7.49±1.85) for males and females [32,38] in studies in Pakistan and Iran. One study recorded a higher value in females than in males 6.48±2.26 and 7.22±2.46 mm among patients in Iran [36]. In other studies, the mean values of IF were slightly lower than those of the above, and was 6.21 in Turkish males [26] and the mean value for females 6.01.

FORAMINA OF STENSON (FS): For FS, 3 studies recorded marginally equal values of (2.31±1.07 and 2.39±1.09), (3.54 and 3.47), and (3.12 and 3.03) for males and females [26,34,41] in studies performed in patients in Brazil and Turkey. One study recorded a higher value in females than in males (3.96±1.81 and 3.38±1.12 mm) in patients in Iran [36]. The highest value of FS was recorded as 3.63 in males in India [39]; for females, the maximum value was recorded as 3.96±1.81 in patients in Iran [36].

MID-MEDIOLATERAL DIAMETER NASOPALATINE CANAL (MID-NPC): Most of the studies did not measure this parameter, although it was higher in males than in females in all included studies. The highest measurement values recorded in males was 2.82±0.89 in populations in Pakistan, and the lowest was 1.87±0.93 in Yemen [28,32] compared with 2.34±1.01 and 1.75±0.86 for females in the same populations. In-between values were recorded in studies performed in Germany and Brazil [14,34].

NASOPALATINE CANAL LENGTH: Among males, 2 studies recorded NPC lengths of 14.74 and 14.00±2.63 in Turkey and Saudi Arabia [26,33], respectively. Most of the studies recorded this parameter as 13± mm, and only 2 studies verified this parameter with 10.20 and 10.08±2.44 among populations in Turkey and Brazil, respectively [34,41]. Among the same studies, the recorded values were 12.83 and 13.71±2.73 for females in Turkey and Saudi Arabia, respectively [26,33], while the lowest values were 9.04, 8.84±2.31, and 9.52±1.98 mm in females in Turkey, Brazil, and Iran, respectively [34,37,41]. Almost all studies documented higher values in males than in females.

MEDIOLATERAL DIAMETER OF THE INCISIVE FORAMEN (M-L IF): Only 3 studies measured this parameter and documented values of 3.45±0.91, 4.93±1.36, and 4.41±1.00 mm in males in Yemen, Korea, and Iran, respectively, and 3.04±0.96, 4.58±1.08, and 3.92±0.09 mm in females in Yemen, Korea, and Iran, respectively [28,35,38]. In all studies, measurements were higher in males than in females.

MID-MEDIOLATERAL DIAMETER FORAMINA OF STENSON (M-L FS): Three studies assessed this parameter and recorded 2.43±1.19, 4.75±1.59, and 4.76 in males in Brazil, Iran, and Germany, respectively, and 2.42±1.26, 4.36±1.37, and 4.42 mm in females in Brazil, Iran, and Germany, respectively [14,34,38]. In all studies, measurements were higher in males than in females.

AT IF LEVEL: Five studies assessed the BBP at the IF level, and 1 of them found that the measurement was 7.78±2.15 mm in the examined patients in Korean [35]. Among male patients, the highest recorded values were 7.10±1.58 in Dutch patients [14], and the lowest value was 6.16±1.45 among Yemeni patients [28]. For females, the highest and lowest values were 6.36±1.54 and 5.77 among Dutch and Turkish populations, respectively [14,26].

MIDWAY, AT INCISIVE FORAMEN (IF) PALATAL BORDER: Only 4 studies measured the BBP at the IF palatal border. In relation to sex, in male patients, the highest recorded values were 7.05±1.46 in Dutch patients [14], whereas the lowest value was 6.16±1.45 in Yemeni patients [28]. Nevertheless, the highest values were recorded as 6.37±1.54 and the lowest values were 5.83±1.14 amongst female Dutch and Yemeni patients [14,28].

AT THE MIDDLE OF THE CANAL: Out of the total 15 studies, 4 studies measured the BBP in the middle of the canal. However, male patients and among Turkish and Yemeni patients had the highest (7.90) and lowest (7.19±1.54) values, respectively [26,28]. However, for females, the highest and lowest values in mm were recorded in studies performed on patients from Turkey and had values of 7.29 and 6.53, respectively [26,41].

BIAS ASSESSMENT:

To assess the risk of bias in the included studies, the Newcastle-Ottawa Scale tool [31,42] was employed. This tool is commonly used to assess the quality and risk of bias in cohort studies. Out of the studies, 8 had a low risk of bias [25,26,28,32,35,36,40,41], while the remaining scored an equal number with high [34,37,39], and unclear [14,33,38] risk of bias. Table 2 shows the quality bias assessment of the CBCT studies involved in this systematic review.

Discussion

An exact understanding of anatomical structures and variations of NPC is very important in diverse surgical procedures to avoid complications [10,15]. Furthermore, a recent trend in the literature is to use CBCT for NPC studies because it is low radiation and facilitates accurate assessment of bone structures. Moreover, CBCT is a highly effective technique for 3D imaging for NPC. CBCT provides accurate, reliable linear measurements of maxillofacial and dental structures. As a result, the increased use of CBCT in dentistry makes it a preferred method for evaluating NPC and its associated structures [43,44]. The application of various measurement methods is one of the potential factors that influence NPC dimensions. The diameter of the IF as well as the length of the NPC appear shorter if the measurement is employed horizontally. To avoid this error, studies measuring the IF diameter obliquely were included in this review.

The studies revealed that males had higher IF measurements than females. In 6 studies [12,23,31,32,35,39], males had significantly higher IF values; in 4 studies [26,28,35,39], males had higher IF values but were not statistically significant. Furthermore, 2 studies showed marginally equal values between sexes [32,38]. Regarding the FS parameter, 8 studies conducted in Yemen, Saudi Arabia, Iran, Turkey, Pakistan, India, and Germany recorded higher values in males than in females, and 5 of them were statistically significant [14,25,28,32,33,37–39]. In addition, 3 studies recorded marginally equal values for males and females [26,34,41]. One study showed that females had higher IF and FS measurements than males, but statistical significance was not met. This outcome may be explained by the fact that the study’s sample included 29 males and 19 females [36].

Regarding the NPC length, almost all studies documented higher values in males than in females. Moreover, a clear association was observed between sex and NPC length, as indicated by the significant influence found in the various studies included in the present review [14,25,26,28,32,37,39–41]. In addition, assessment of the relationship between sex and (mid-NPC, M-L IF, and M-L FS) showed higher value in males.

According to the studies involved in the current review, individuals from diverse populations also had varied dimensional analyses of NPC, and environmental and gene-related factors may influence these variations. However, within the Iranian population, 2 studies reported an average of 7.52 and 3.27 mm in males and 7.22 and 2.78 mm in females for IF diameter [37,38]. The reason for this difference in IF diameter between males and females in the same population may be related to the failure of the study conducted by Nikkerdar et al [37] using the oblique method, where an inconsistency was evident regarding the technique that was selected versus what was employed.

Studies measured BBP diameters at 3 levels: BBP width at the slope of the IF, BBP width in the midway, at the IF palatal border, and BBP width in the middle of the canal. The results showed a lower value in females than in males with statistical significance for studies conducted in Yemen, Turkey, and Germany [14,26,28]. All those dimensions of NPC were measured in a proper way by using CPCT.

The overall dimension and shape of the NPC are differed among countries, depending mainly on gene-related factors, as mentioned recently in a study that used CBCT to examine the NPC dimension in horizontal plan in North Cyprus. They found a slight difference in NPC dimensions from the established norm and existing standards found in the published literature [45].

One of the limitations of this review is that it included only articles published in English. The methodologies used had high heterogeneity among the involved papers and research. In addition, more than half of the included studies had “high” quality bias. Further reviews are needed to study the NPC measurements, and studies of NPC shapes in relation to sex, publication, and age parameters are also strongly recommended.

Conclusions

The following conclusions can be drawn: