Morphological and Dimensional Analysis of the Nasopalatine Canal: Insights from Cone-Beam Computed Tomography Imaging in a Large Cohort

Introduction

The nasopalatine canal (NPC) is located posterior to the central incisors in the midline of the anterior maxilla [1]. It is a tube that joins the oral and nasal cavities. The NPC has 2 openings: an oral opening called the incisive foramen (IF), which sits just below the incisive papilla, and a nasal opening called the Stenson foramen (SF) [1–3]. Anatomically, the NPC is an essential part of the anterior maxilla [4]. It consists of the terminal branch of the descending nasopalatine artery, fat, fibrous connective tissue, nasopalatine nerve, and seromucous glands [5].

NPC anatomical structures are important in dental practice. A current orthodontic study highlights the importance of planning anterior maxillary tooth movement while considering the morphometric features of NPCs, which reduces the possibility of root resorption and invasion by preventing contact between the maxillary central incisors (MCIs) and the NPC [6]. For accurate treatment planning and implant insertion [7–9], as well as for safe and accurate procedures during local anesthesia [7], prosthetic dentistry [10,11], impacted teeth extractions, periodontal surgery, nasopalatine duct cyst enucleation, surgically assisted rapid palatal expansion, apicoectomy, and orthognathic surgery, a thorough understanding of NPC dimensions and the buccal bone plate (BBP) is crucial [12,13].

Two-dimensional (2D) methods, such as intra-oral radiography and panoramic radiography, are used for the diagnostic imaging of the anterior maxilla [14–17]. However, their limitations, including distortion and magnification, compromise accurate anatomical assessment [18,19]. Cone-beam computed tomography (CBCT) has transformed imaging by offering a 3-dimensional (3D) view in dentistry [20,21]. Accurate diagnosis is aided by the precise imaging of NPC topography and size, made possible by CBCT. When evaluating canal entrance size at the palatal level, where the buccal wall is frequently obscured in sagittal plane 2D images, CBCT is especially helpful [22,23].

The NPC is a highly variable anatomical unit in the anterior maxilla, with notable structural differences [6,22]. Global studies reveal statistically significant variations in measurements after evaluating the NPC in 2 Peruvian cities [24]. Özeren et al and Hakbilen et al recorded different NPC dimensions among the Turkish population [25,26] and concluded that they are less than those in other worldwide studies. NPC measurements of the Iranian population have been reported by Panjnoush et al and Khojastepour et al, who documented lengths of 14.1±3.0 mm and 10.34±2.74 mm, respectively. The IF diameter was measured as 4.7±1.11 mm and 3.17±1.01 mm, respectively [27,28]. Jayasinghe et al in Sri Lanka, López Jornet et al in Spain, Soumya et al in India, and Alasmari in Saudi Arabia reported a variety of NPC lengths, namely, 12.14±2.3 mm, 11.02±2.4 mm, 18.63±2.35 mm, and 13.86±2.68 mm, respectively [29–32]. According to Görürgöz and Öztaş in Turkey [33], López Jornet et al in Spain [30], and Alasmari in Saudi Arabia [32], the recorded IF diameter of patients in the different countries varied, measuring 5.29±1.37 mm, 3.29±1.0 mm, and 4.07±2.06 mm, respectively. Furthermore, investigations conducted in Brazil by Januário et al [34], Peru by Córdova-Limaylla et al [24], Switzerland by Braut et al [35], and Egypt by El Nahass and Naiem [31] reported differences in BBP diameter.

Despite these international research efforts, no previous studies have explicitly evaluated CBCT radiographic morphological and dimensional variances of NPC in Yemeni. Therefore, this retrospective study aimed to assess the anatomical morphology of the NPC according to age, sex, and dental status using CBCT in 335 Yemeni patients.

Material and Methods

STUDY DESIGN AND SAMPLE SIZE CALCULATION:

Ethics approval was obtained from the University of Ibn al-Nafis Ethics Committee (19/February 12/2024), and the study adhered to the Declaration of Helsinki. The present retrospective cross-sectional study was conducted to assess the effects of sex, age group, and dental status on NPC anatomical features by using CBCT scans from patients in Yemeni. A total of 335 CBCTs were counted, based on a confidence level of 95% and an estimated error of 5%. This decision was made after considering the number of men and women in Ibb City, Yemen.

INCLUSION AND EXCLUSION CRITERIA:

The inclusion criteria were patients from both sexes, aged 18 years or older, who were dentate and edentulous patients. All patients should have high-quality CBCT images. All CBCT for concerns of impacted teeth and orthodontic planning were included in this study. Exclusion criteria included participants with poor image quality; diseases affecting NPC, including cysts or tumors; cleft lips; dental implant installation; and orthodontic expanders.

SETTINGS FOR ACQUIRING CBCT IMAGES:

Two radiologists, A.H. and Al.M., with 5 years of experience evaluated the CBCT images. The CBCT images were evaluated by slice thickness of measurements provided by the system used. Pax-Flex3DP2, manufactured by Vatech in Korea, was utilized, with a 9×12-field of view, 90-kVp tube voltage, 4.0-mA tube current, 24-s scan period, and isotropic voxel size range from 0.160 nm to 0.200 mm. Ez3D-i software (version Ewoosoft, Gyeonggi-do, Korea) was used on a 64-bit Windows 10 Laptop to process CBCT pictures. A Dell 15.6-inch laptop with an AMD Radeon R7 M360, UHD Graphics, and 1920×1080-pixel screen resolution was used for the visualization process, in a dimly lit room.

RELIABILITY AND NORMALITY ASSESSMENTS:

The examiners randomly selected 34 samples (10% of sample size) for the intra-class correlation coefficient with the inter-examiner agreement between A.H. and Al.M. for all NPC parameter measurements and morphological shapes. The same examiners assessed each scan after a 2-week break to determine intra-observer reliability averaging, which was 0.88. This finding indicated virtually perfect agreement and confirmation of meeting the study’s prerequisite for commencement. The Shapiro-Wilk test showed that data for age, sex, and dental status were normally distributed (P=0.521, P=0.642, and P=0.788 respectively).

IMAGE EVALUATIONS AND MEASUREMENTS:

The CBCT images for patients were categorized according to sex (male and female), age group (18–35, 36–50, and ≥51 years), NPC shape types, and dental status, with 2 MCIs (MCI 2), with 1 central incisor (MCI 1), and without 2 central incisors (MCI 0). Every CBCT was assessed and evaluated to record the NPC in 3 planes. Table 1 shows the details of parameters and definition of each NPC parameter and the aspect and/or plane that was used to record it in sagittal, coronal, and axial planes. Figure 1 shows the parameters measured in the sagittal and axial planes, whereas Figures 2 and 3 represent the anatomical structures in the sagittal and coronal planes. Figure 4 characterizes the anatomical structures and shape variation in the axial aspect.

DATA ANALYSIS:

Data were analyzed using the Statistical Package for the Social Sciences version 21. (IBM Corp, Armonk, NY, USA). Intra-observer reliability was checked by using intra-class correlation tests. For qualitative data, descriptive statistics were shown as numbers and percentages. For quantitative data, the expression was the mean±standard deviation (SD). The independent t test was used to evaluate measurement differences based on sex and edentulous patients. The association of NPC classification with sex, age groups, and dental status was examined using the chi-square test. Based on dental state, age groups, and NPC morphology, NPC parameters were compared by one-way ANOVA. To investigate the association between age groups and other NPC parameters, the Pearson correlation test was used. The significance level was set at P<0.05.

Results

PATIENT CHARACTERISTICS AND PARAMETERS OF NPC AND ADJACENT BBP:

Among the 335 CBCT scans assessed, 142 (42.4%) belonged to male and 193 (57.6%) to female patients. The patients were aged between 18 to 68 years, with a mean age of 36.4±14.5 for men and 32.2±11.3 years for women The largest patient age group was 18–35 years of age and numbered 208 (62%) patients, whereas the smallest age group was ≥51 years and numbered 41 (12%) patients. Regarding dental status, 300 cases (89.6%) were with 2 central incisors, 16 (4.8%) were with 1, and 19 (5.7%) were with 2 missing MCIs (Figure 5). At the oral level, the mediolateral (M-L) and labio-palatal means of IF diameters were 3.21±0.96 mm and 5.13±1.45 mm, respectively. At the nasal level, the average anteroposterior diameter of NPC was 2.57±1.25 mm at SF. The NPC total length was 11.79±2.50 mm (Table 2).

ASSOCIATION OF SEX AND AGE GROUPS WITH NPC AND ADJACENT BBP PARAMETERS:

The mean values of the NPC were longer in men, at 12.58±2.45, than in women, at 11.21±2.39, with a statistically significant difference (P=0.000), by using the t test. Additionally, the M-L IF (P=0.000) and SF (P=0.033) values were also shown to be higher in men. The findings were statistically significant. Men further had higher recorded values in BBPs 1, 2, and 3, with a significant difference and P=0.011, P=0.000, and P=0.001, respectively. However, a larger labio-palatal diameter of IF was found among men, and the difference was not statistically significant (P=0.194; Table 3).

Concerning the different age groups, participants aged over 51 years recorded higher values of IF diameter, NPC length, and M-L IF diameter of 5.50±1.37, 13.31±2.13, and 3.57±0.83, with one-way ANOVA revealing a significant difference in P values of 0.023, 0.001, and 0.00, respectively. Participants aged over 51 years had a lower width of BBP1 (5.56±1.54 mm) with significant differences, with P=0.001 (Table 3).

ASSOCIATION BETWEEN MCI STATUS ON NPC AND ADJACENT BBP PARAMETERS:

Data on dental status and whether MCIs were present are shown in Table 4. A gradual reduction in BB1 and BBP2 widths when MCIs were extracted (MCI 0) with mean values and SD of 4.06±2.01 mm and 4.90±1.88 mm was found, and the differences were statistically significant, with P=0.000 for BBP1 and BBP2, respectively. Furthermore, edentulous patients exhibited significantly decreased BBP dimensions. Participants without 2 central incisors (MCI 0) exhibited a slightly wider M-L IF diameter of 3.74±0.71 mm than did those with 2 central incisors of 3.17±0.95 mm, with statistical significance (P=0.012). However, NPC length was higher among participants in the presence of 2 central incisors (MCI 2; 11.73±2.54 mm), with a statistically significant difference from other dental status (P=0.028; Table 4).

ASSOCIATION OF MORPHOLOGICAL VARIANTS OF NPC IN AXIAL, SAGITTAL, AND CORONAL PLANES BASED ON SEX, DENTAL STATUS, AND AGE GROUP:

Figure 6 shows that, in the axial plane and from the CBCT image, the most common morphological shape of the NPC was round (243; 42.7%), whereas the least common was oval (44; 13.1%). Correlation tests showed no significant differences in sex, age group, and dental status, with P=0.518, P=0.106, and P=0.231, respectively (Table 5). Along the sagittal plane, the morphology of NPC was a funnel shape in 165 (49.3%), followed by a cylindrical shape in 94 (28.1%). The chi-square test of the morphological shape of the NPC in the sagittal plane was found to be statistically insignificant in sex, age group, and dental status (P=0.069, P=0.702, and P=0,583, respectively), as presented in Table 5. In the coronal plane, there were 166 cases (49.6%) of Y-type canals, 153 cases (45.7%) of a single canals, and 16 cases (4.8%) of 2 parallel canals (Figure 6). The chi-square test between sex, dental status, and age group and NPC morphological variation in the coronal plane was found to have no significant relationship (Table 5).

ASSOCIATION OF NPC SHAPE ON THE NPC AND ADJACENT BBP IN MM:

In the correlation between NPC shape and other parameters, a statistically significant relation was observed in the axial plane between canal type and SF (P=0.034), NPC length (P=0.003), mid-level of NPC (P=0.000), M-L IF (P=0.000), and BBP1 (P=0.009). The heart-shaped NPC had the highest M-L IF diameter, followed by oval, triangle, and round NPC. All NPC and BBP diameters except the IF diameter, BBP1, and BBP2 were significantly affected by the form of the NPC on the sagittal plane, with statistical significance (P=0.000 and P=0.000, respectively). The highest mid-level and M-L IF diameters had a banana shape. Although the highest IF diameters were banana-shaped, the difference was not statistically significant (P=0.089). The canal of this type had a longer NPC length than the other types, with statistical significance (P=0.008). In the coronal plane, NPC shape indicated that the single canal group had a longer NPC length than the other types. A significant difference in NPC length also existed between the groups of single, 2 parallel canals, and Y-type canals (P=0.000). Additionally, the NPC form in the coronal plane significantly affected the IF, SF, and M-L IF diameters, as well as BBP2 and BBP3 (Table 6).

Discussion

The premaxilla with its structural anatomy, such as NPC and its associated structures, plays a significant role in daily dental procedures. Thus, understanding the anatomy of these structures is crucial because of clinical effects during implant insertion procedures, orthodontic root movement of the upper central incisor, safe and accurate procedures during local anesthesia, and other procedures [7,12,13,22]. The assessments of variations in morphological NPC features among different populations are essential and can be performed by CBCT for an accurate diagnosis and proper selection among different types of dental treatments.

To the best of our knowledge, this study is the first to focus on the Yemeni population, which has dimensions and morphological characteristics different from those in other regions of the world. The present work served as a morphological and dimensional variation investigation of the NPC using CBCT. The adjacent BBP diameter was also evaluated and analyzed by CBCT imaging. The overall NPC and its associated structures (diameter, length, and shape) among sex, age group, and dental status were consistent with the results of most studies conducted in Arab and other countries [8,9,18–20,22–28,32,36,37]. The disagreement could be related to the different races of the studies conducted worldwide. Also, the methods of measurements of the dimensional variances of the NPC parameters and IF dimensions can play a role.

The CBCT system is particularly useful for assessing the canal entrance size at the palatal level, where the buccal wall is often concealed in sagittal-plane 2D images [15–17,22,23]. Furthermore, difficulty is encountered in using 2D radiography to deliver precise information about anatomical structures under evaluation, because of its intrinsic limitations [14,17–19].

The diameter of IF up to 6 mm is believed to be within the normal range, but if the diameters exceed 10 mm, it can be a sign of pathological development, more specifically, a nasopalatine duct cyst [14]. According to the results of the present study, the average labio-palatal IF and SF diameters were 5.13±1.45 mm and 2.57±1.25 mm, respectively. Compared with the SF diameter, the mean labio-palatal IF diameter was greater. These outcomes were consistent with earlier outcomes documented by Bornstein et al [20]. Furthermore, this finding was comparable to the average labio-palatal IF and SF diameters recorded in Serbia, which were 5.04±0.12 mm and 2.93±0.01 mm, respectively [8]. The results of our IF and SF diameters were less than the Iranian labio-palatal IF diameter of 7.5±1.95 mm, Turkish labio-palatal IF diameter of 6.47±1.41, Iranian SF diameter of 3.32±1.16 mm, and Turkish SF diameter of 4.13±1.08 mm [23,37]. They were also greater than the results documented by Özeren et al of a labio-palatal IF diameter of 3.54 mm and SF diameter of 2.53 mm [25].

Additionally, we discovered a statistically significant relationship between sex and the diameters of SF and M-L IF. The average NPC length we measured was 11.79 mm, consistent with the measurements reported by previous studies conducted in different countries [20,38–40]. Higher values were discovered in Saudi Arabia (13.86 mm) and in 2 Mongoloid groups (16.33 mm) in other research [32,40]. Additional studies demonstrate lower values, for example, 9.49 mm and 9.62 mm, in Turkish populations [19,25]. In our work, we found a significant difference in NPC length between male and female patients. This is in parallel with previous studies investigating the relationship to patient sex [20,25,32].

Age was found to be significantly correlated with the numerical variables in our study, including IF diameter, NPC length, and M-L IF diameter, which increased with age. Previous results have shown that age significantly affected IF diameter and that other parameter averages often increased with age [39]. Additional findings indicated that the mean values generally decreased with increased age [20]. No age-related differences have been observed by other researchers, such as Panjnoush et al and Al-Amery et al [27,41]. In the present work, the diameter of the BBP1 significantly decreased in patients older than 50 years. These results were consisted with those of previous studies [26,27].

According to our findings, patients in Yemeni exhibited a reabsorption crest of the BBP following the loss of one or both MCIs, as clearly shown by patients who were edentulous. These results were consistent with those of previous studies [20,25,39].

At the sagittal plane, the funnel canals had the highest percentage, consistent with those found in other studies conducted on individuals in Saudi Arabia and Serbia [8,42]. Conversely, Turkey, India, Iran, and Spain reported the highest prevalence of cylinder-shaped NPCs [23,25,38,40]. The most prevalent type of NPC described by Etoz et al [43] is the hourglass-shaped NPC. In terms of variations in the axial plane shape of the NPC, a round canal was the most commonly observed, consistent with previous findings [37]. Y-type canals were the most prevalent anatomic variant type in the coronal plane in the present investigation, accounting for 166 cases (49.6%). The second most prevalent type was the single canal, with 153 cases (45.7%), whereas 2 separate parallel canals were present in 16 cases (4.8%). The findings of previous investigations are comparable with ours [23,25,38,40]. However, some studies have found higher numbers of single canals [20,39]. The NPC shape was compared in 3 planes with sex, dental status, and age group, with no significant relationship found, consistent with previous studies [23,25,28,43,44].

Regarding the NPC morphology and its relationship with the NPC and adjacent BBP parameters, we found a relationship between canal shape and the NPC and BBP parameters. Previous investigations documented by Milanovic et al, Hakbilen et al, and Alasmari also revealed a statistically significant difference [8,26,32]. Meanwhile, other investigators [38,40] have shown statistically significant differences in the coronal cross-section but not in the sagittal plane.

To the best of our knowledge, the present study is the first to provide information about the relationship between shapes in the axial plane at the level of incisive foramen with M-L IF diameters. The patients were also divided into age groups, which may offer some information about the changes related to the NPC and its associated structures with increased age. Through the discrepancy shown by this study compared with others, the results supported that the 3D assessment of the NPC was essential in preventing challenges before many dental procedures in the anterior maxilla, such as apicoectomy, impacted teeth extractions, immediate implantation, pathology removal in the NPC area, and orthognathic surgery, as well as local anesthesia and its related procedures.

Our study’s primary limitation was its single-city focus, which limited the generalizability of our findings. Including more cities would offer more diverse insights and a more comprehensive view of such important dental anatomy and associated structures. Our work was also unable to estimate the time elapsed after missing or extracted central incisors.

Conclusions

From the present CBCT study, the following conclusions can be drawn: