Introduction

The maxillary nerve is one of the branches of the trigeminal nerve, being exclusively sensitive and ramifying into superior posterior alveolar nerves, nasopalatine nerve, major palatine nerve and infra-orbital nerve. The infra-orbital nerve passes along the infra-orbital foramen and has a lateral branch called canalis sinuosus (CS), through which the anterior superior alveolar nerve passes. The CS is a neurovascular bundle that emerges from the posterior portion of the infra-orbital foramen and descends below the inferior wall of the orbit and medially towards the anterior wall of the maxillary sinus, bypassing the lateral and inferior limits of the nasal fossa, with the canal opening laterally to the nasal septum in front of the nasopalatine canal or in the anterior region of the maxilla. The insertion point is anterior to the incisive canal, and at this point, the CS commonly presents anatomical variations in the anterior palate, called accessory canals [4, 5, 12, 13]. Figure 1 shows the CS leaving the bilateral infra-orbital canals with some accessory canals at the end of their trajectory.

Fig. 1
figure 1

Bilateral canalis sinuosus leaving the bilateral infra-orbital canals, with some accessory canals at the end of their trajectory (by Machado et al. [9])

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The neurovascular branches in the CS form the dental plexus in the canine region. CS enables sensibility of anterior teeth, nasal fossa floor and maxillary sinuses. The lack of knowledge of the positioning of CS can bring risks during dental surgical procedures and may cause pain, local infection and even paresthesia [12].

Some studies show that the presence of CS can mimic a periapical lesion and lead the dentist to perform inappropriate endodontic treatment [3], causes pain due to injuries to nerves during the placement of dental implants [4] or to have even paresthesia following a surgical procedure [1].

Cone beam computed tomography (CBCT) has been the most used examination to investigate and diagnose the presence of anatomical variations and structures, such as CS, because periapical and panoramic radiographs often cannot delineate and show this structure in detail [1, 2, 9, 10, 12]. In this way, CBCT is fundamental for diagnosing the presence of CS and the relationship between this structure and symptoms found [1, 9, 17].

The study of anatomical variations is important not only to collect anthropometric data but also to improve clinical protocols. CS has been little explored and many practitioners have no knowledge of its presence and location. In addition, the objective of the present study was to verify the presence, spatial location, the end of the CS trajectory and size of CS using CBCT to characterise it as either a structure or an anatomical variation.

Materials and methods

After approval by the Research Ethics Committee of the School of Dentistry of the City of São Paulo University (UNICID), 206 CBCT images of the maxilla of patients aged 18–85 years corresponding to the period from April to August 2018 were randomly selected from a clinical radiology (Itajai, Santa Catarina, Brazil) and prospectively evaluated.

All CBCT images were acquired with a scanner (Prexion® Corporation, Tokyo, Japan) operating with FOV of 8.1 × 7.5 cm, 90 KVp, 4.0 mA, focal distance of 0.15 mm and voxel size of 0.16 mm. All individuals submitted to CBCT examination whose field of vision (FOV) covered the maxillary sinuses entirely (i.e. from the upper border above the inferior ridge of the orbit to the lower border beneath the superior alveolar ridge, including dental arches in the posterior region of the maxilla) were included.

The tomographic examinations were interpreted by a trained examiner specialist in dental radiology and imagenology with more than 10 years of experience with CBCT. Images with patient motion, artefacts, bone lesions in the maxillary region and unsatisfactory quality or images suggesting surgery or trauma, presence of plates or screws, syndromes or malformations and bone graft material in the anterior maxilla were excluded.

The images were analysed by the examiner in the scanner’s workstation on a dynamic basis using the Prexion 3D viewer software. Axial, coronal and sagittal sections were analysed to verify presence or absence of CS and whether it was unilateral or bilateral, the end of the CS trajectory (i.e. regions of central and lateral incisors, nasopalatine canal, canine and first pre-molar) and measure the size of CS at the level of its mouth as viewed in the axial sections.

The resulting data were descriptively analysed and correlated using Fischer’s exact and Chi-square tests. GraphPad software (GraphPad Prism version 5.0 for Windows, GraphPad Software, San Diego, California USA) was used for statistical analyses at a significance level of 5% (P < 0.05).

Results

A total of 206 CBCT images were assessed, but six were discarded according to the exclusion criteria. Of these 200 images of the maxilla, 107 (53.5%) were from female and 93 (46.5%) from male individuals aged between 18 and 85 years, with median age of 53 years. CS was found in 133 (66.5%) patients, being more frequently observed in males (P < 0.05), as seen in Table 1. No relationship between the presence of CS and patient’s age was found (P > 0.05%), as seen in Table 2.

Table 1 Presence of CS according to gender
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Table 2 Presence of CS according to age group
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Of the 133 patients with CS, 72 (54.14%) had it bilaterally and 61 (45.86%) unilaterally. In Figs. 2 and 3, one can observe the presence of bilateral and unilateral CS, respectively. The end of the CS trajectory was more frequently found in the regions of central incisor (n = 91; 44.39%) (Fig. 2), followed by lateral incisor (n = 45; 21.95%) and canine (n = 29; 14.15%) (Fig. 3). Figure 4 shows the end of the CS trajectory of the 205 canals. No statistically significant relationship was found between the end of the CS trajectory and its location (i.e. bilateral or unilateral).

Fig. 2
figure 2

Axial, coronal and sagittal sections showing bilateral positional with the end of the CS trajectory towards the lateral and central incisors

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Fig. 3
figure 3

Axial, coronal and sagittal sections showing unilateral positional with the end of the CS trajectory towards the canine

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Fig. 4
figure 4

The end of the CS trajectory (n = 205)

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The diameter of CS was found to be homogeneous along its entire trajectory to the region of the mouth and whose size was measured, showing that the majority of CS had a diameter of up to 1 mm (n = 198/205; 96.6%) and only seven (3.4%) were greater than 1 mm. No statistically significant relationship was found between the diameter of CS and its location (i.e. bilateral or unilateral). Also, both gender and age had no influence on diameter, spatial location and mouth of the canal (P > 0.05%).

Discussion

The knowledge of structures and anatomical variations is crucial for invasive dental procedures to prevent complications and improve prognosis. Despite being little studied, there are some works showing complications resulting from the lack of anatomical knowledge of CS, mainly in dental implant surgeries involving the anterior maxillary region, such as pain and paresthesia [1, 11].

The function of CS is to assist in the sensitivity of anterior teeth, nasal fossa floor and maxillary sinus, according to their anatomical spatial location. Damage to any part of the CS trajectory will inevitably lead to problems in the affected region. Thus, it is very important to know its existence and location to prevent possible injuries to this structure.

CBCT is the best examination to determine the location of CS, thus being very useful for the diagnosis and evaluation of patients [3, 15]. Because periapical and panoramic radiographs cannot often delineate and show this structure in detail [10, 12, 17], we have chosen to evaluate CS using CBCT. The CBCT allows individualization and visualisation of the CS throughout its trajectory in three dimensions: axial, coronal and sagittal.

In our search for studies on CS, we have found 19 works on the PubMed database, where 11 were case reports, 6 were prospective studies and 2 were retrospective studies (Table 3). The prospective studies assessed the presence of CS using CBCT and the number of individuals ranged from 100 to 1000. The presence of CS was also very variable, ranging from 15.7% to 100%. Our study assessed 200 CBCT images, where 133 (66.5%) showed presence of CS. Some studies reporting a few findings considered only those cases of CS with diameter greater than 1 mm [13, 19].

Table 3 Studies on CS found in the literature
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We have found a higher frequency of CS in males in the present study, a finding also corroborated by Von Arx et al. [19] and Machado et al. [9]. On the other hand, no relationship between the presence of CS and patient’s age was found, as in the studies by Oliveira-Santos et al. [13] and Wanzeler et al. [20].

With regard to the location of CS, 72 (54.14%) patients had it bilaterally and 61 (45.86%) unilaterally, totalling 205 cases in our sample. However, several findings are reported in the literature: Oliveira-Santos et al. [13] found 6/28 bilateral (21.4%) and 22/28 unilateral cases, whereas Ghandourah et al. [3] and Guler et al. [4] reported 219 (100%) and 111 (100%) cases of bilateral CS, respectively.

The end of the CS trajectory was found in several sites, being more frequent in the regions of central incisor (n = 91; 44.39%), followed by lateral incisor (n = 45; 21.95%) and canine (n = 29; 14.15%). These findings are similar to those reported by Oliveira-Santos et al. [13], Von Arx et al. [19], Machado et al. [9] and Ghandourah et al. [3], who found higher frequencies of CS in the anterior region of the maxilla.

Only seven (3.4%) patients had CS with diameter greater than 1 mm. The majority of the studies in the literature describe cases of CS with diameter greater than or equal to 1 mm [4, 9], whereas only Ghandourah et al. [3] found results similar to ours, that is, 82.1% of the cases had a diameter less than or equal to 1 mm.

We have found no relationship between gender and diameter of CS, which was also corroborated by Von Arx et al. [19], although Machado et al [9] and Gurler et al. [4] showed greater diameters in male individuals.

Conclusion

The results from the present study have shown that CS is an anatomical structure as most (66.5%) of the study population had CS. In addition, it was observed that there was a higher frequency of CS in male individuals, but no relationship with age. Gender and age had no influence on diameter, location and the end of the CS trajectory either. Therefore, the use of CBCT to identify CS before invasive dental procedures in the region of anterior maxilla can prevent many complications and provide a better prognosis for the patient.