This retrospective study included CBCT images of 19 patients (12 (63.2%) males, 7 (36.8%) females, mean age 44.7 ± 13.3) with NPD cysts diagnosed and treated between 2015 and 2022. The control group of 164 adult patients (72 (43.9%) males, 92 (56.1%) females, mean age 47.25 ± 17.74) was selected from the CBCT database stored in the Center for Diagnostic Radiology of the same institution. All patients were referred to CBCT imaging due to various indications: 27 patients for anterior restorations, 29 patients due to impacted teeth, 34 patients due to prosthetic restorations, 48 patients for orthodontic treatment, and missing relevant teeth in 26 patients. Patient selection for the control group was carried out according to the following inclusion criteria: (1) no signs of NPD cyst on clinical examination (data obtained from the medical records), (2) the frontal region of the maxilla in the field of view on CBCT images, (3) the absence of dental implants in the anterior teeth region, (4) absence of extensive bone lesions affecting the nasopalatine canal integrity, (5) no history of trauma, (6) no history of developmental jaw anomalies, and (7) good quality of CBCT images (no severe artifacts, motion blur).

CBCT imaging was carried out on Scanora 3Dx device (Soredex Co., Tuusula, Finland). Different fields of view were used for scanning: 50 × 50 mm (S field), 50 × 100 mm (S + field), and 80 × 100 mm (M field). The following scanning parameters were applied: voltage of 90 kVp, tube current of 8–10 mA, and the voxel size ranged from 0.15 mm to 0.4 mm (resolution mode marked as “standard”). Section thickness was in the range of 0.15–0.4 mm, depending on the voxel size. Evaluation of CBCT images and measurements in sagittal, coronal, and axial planes were performed in the OnDemand3D Viewer (Cybermed Inc., Seoul, Korea).

The morphology of the nasopalatine canal was assessed by measuring linear and angular parameters in both the patient and control groups (Figs. 1 and 2). There were no NPD cysts with extensive bone destruction in our series that limited CBCT analysis. Many of the assessed CBCT parameters were measured according to the protocol described in the literature [13, 16]. The definition of each parameter and a brief description of the measurement technique are summarized in Table 1.

The anteroposterior diameter of the nasal opening (APNO, in mm), anteroposterior diameter of the oral opening (APOO, in mm), nasopalatine canal length (L, in mm), the mid-level anteroposterior diameter (midAP, in mm), and the nasopalatine canal angle (A, in degree) were measured in a sagittal plane [13, 16]. A reference sagittal image was set running through the longitudinal axis of the nasopalatine canal using a 3D navigation tool that simultaneously shows the canal position in the axial and coronal plane (Fig. 1A-C). The anterior and posterior nasal spines were used as reference points in the axial plane. For two nasal openings (Y-shaped canal in coronal view), sagittal images were scrolled laterally to show both nasal openings and to measure AP diameters. The mean value was calculated and used in the further analysis. In the case of the double nasopalatine canal, both canals were measured, and mean values were used.

The expansion of the anterior wall of the nasopalatine canal (AWE, in mm) was introduced by the authors of the current study as a new parameter. The AWE was used to quantify a focal bulging of the anterior wall of the nasopalatine canal typically found in patients with an NPD cyst (Fig. 1C). The AWE measurement was performed on the same sagittal reference image (Fig. 1C). During the first step, the line connecting the anterior edges of the nasal and oral opening was drawn (white line in Fig. 1C). The second, the distance between this line and the most prominent point on the anterior wall of the canal was defined as AWE. The measurement was taken perpendicular to the white line, as shown in Fig. 1C. This parameter was also measured in normal canals since certain anatomical variations of the canal shape may have a positive AWE („banana“-shaped canal, spindle-shaped canal).

The mediolateral diameter of the nasal opening (MLNO, in mm) and the minimum mediolateral diameter (minML, in mm) were measured on the oblique coronal plane. A reference coronal plane was set by tilting the line showing the coronal section on a sagittal reformated image to fit the longitudinal axis of the canal. The reference coronal plane passed through the middle of the APNO and APOO (Fig. 1D). The mediolateral diameter of the oral opening (MLOO) was measured on the axial plane (Fig. 1E). When setting a reference axial plane, the same sagittal reformated image was used to tilt the line corresponding to the axial section. The line was running through the anterior and posterior edges of the oral opening.

Morphological parameters of the nasopalatine canal measured on reference CBCT section images. A, B. Measurements in a reference sagittal plane: APNO (in mm) - anteroposterior diameter of the nasal opening, APOO (in mm) - anteroposterior diameter of the oral opening, midAP (in mm) - mid-level anteroposterior diameter of the canal, L (in mm) – length of the canal, A (in degrees) – the angle between the canal axis and the nasal floor. C. AWE (in mm) – anterior wall expansion measured on the same sagittal image. D. Measurements in a reference coronal plane: MLNO (in mm) – mediolateral diameter of the nasal opening, minML (in mm) – minimum mediolateral diameter. E. Measurements in a reference axial plane: MLOO (in mm) – mediolateral diameter of the oral opening

CBCT parameters measured in a patient with NPD cyst. (A) Sagittal plane measurements. Note the increase in size of the APOO and midAP caused by a cyst. (B) Coronal plane measurements. MinML was also increased due to the cyst. (C) Axial plane measurement

Two radiologists, experts in dento-maxillofacial radiology with more than ten years of experience, made the measurement. They performed CBCT analysis independently and were blind to each other’s measurement outcomes. Inter-observer reliability was estimated using interclass correlation coefficients.

Obtained linear and angular CBCT parameters were analyzed in SPSS software (version 17.0; SPSS Inc., Chicago, IL, USA). Mean and standard deviation were calculated for each CBCT parameter. The Kolmogorov-Smirnov test evaluated the distribution of the data normality. The Student’s t-test assessed the differences in CBCT parameters between patients with NP cysts and the control group. Discriminant functional analysis was applied to find CBCT parameters that best differentiate the NP cyst from the normal NP canal. The level of significance was set at 0.05.