Nasopalatine canal

The nasopalatine canal, also called the incisive canal, is a bony canal through the anterior maxilla that connects the oral and nasal cavities [1]. On a periapical radiograph its oral opening, the incisive foramen, shows as a radiolucency in the midline between the roots of the maxillary central incisors [1][4]. Its shape and width vary from person to person [9][12].

Synonyms: incisive canal

What is the nasopalatine canal?

The nasopalatine canal runs through the anterior maxilla from the floor of the nose to the palate [1]. It carries the nasopalatine nerve and an anastomosis between the greater palatine and sphenopalatine arteries [1]. The canal sits lingual to the maxillary central incisors, 11 and 21, and opens into the mouth at the incisive fossa beneath the incisive papilla [1][11].

At the nasal floor the canal often splits into two openings, a Y-shaped canal, and sometimes into three or four [9].

Where does the nasopalatine canal appear on a periapical radiograph?

Look in the midline, between the roots of 11 and 21. The incisive foramen shows as a radiolucency between their apices [1][4].

It won't be on every periapical radiograph of the central incisors. One study of 100 patients saw it on 74% of radiographs [4].

What shapes and sizes does a normal nasopalatine canal take?

On a periapical the radiolucency may be round, oval, lobular or heart-shaped. Which one you see depends on how the anterior nasal spine overlaps it [1].

The canal behind that outline varies too. A systematic review of 10 CBCT studies with 1,697 participants found the cylindrical shape and the single canal the most common [11]. A CT study of 34 scans often found the Y-shaped form [9]. A review also describes banana, spindle and hourglass shapes [1]. The shapes vary between populations, which makes a standard classification hard to establish [11].

Every normal size figure below comes from CT or CBCT. None was measured on a periapical, which adds magnification and distortion of its own [12].

Canals are typically larger in males, and their dimensions differ between populations [12]. On CT, canals are wider in older patients, while their shapes do not differ significantly with age [8].

How do you tell a normal canal from a nasopalatine duct cyst?

The traditional rule is width. A nasopalatine duct cyst, also called an incisive canal cyst, is suspected when the incisive foramen is wider than 6 mm on an intraoral radiograph, a value taken as the upper limit of normal [2].

Width alone separates them less well than that rule suggests. In one CBCT study of 19 cysts and 164 controls, the smallest cyst was 7.8 mm wide, while the oral opening of normal canals reached 11.5 mm [2]. A CT study of 220 normal subjects and 40 cysts found the 6 mm cut-off failed in patients over 60 and proposed 7.1 mm for that age group [8]. The authors of the CBCT study describe the published normal widths as differing widely between studies [2].

Look at the rest of the image as well. On intraoral radiographs a nasopalatine duct cyst typically shows as a round or heart-shaped, well-defined radiolucency in the maxillary midline between the central incisors [2][15], bordered by a radiopaque margin [5]. The heart shape often comes from the anterior nasal spine overlapping it [18]. A normal canal can take the same heart shape for the same reason [1].

Cysts come in a wide range of sizes. Of 102 operated cysts measured on the surgical specimen, 21.6% were under 10 mm and 23.5% were over 15 mm [7]. On CBCT, cyst widths ran from 7.8 mm to 19.6 mm [2].

On CBCT, a cyst expands the anterior wall of the canal and widens its oral opening and midsection [2]. Combining those three measurements, one study separated cysts from normal canals [2]. On anterior wall expansion, a cyst too small to expand bone may be missed [2]. On CT, a wider canal with an hourglass shape was more often a cyst [8].

What is the nasopalatine canal confused with?

Why does the canal look different on two radiographs of the same patient?

A periapical flattens a three-dimensional canal into two dimensions. The canal's variations can't be appreciated on it [13], and the projection magnifies and distorts [12]. CBCT removes the superimposition [1].

What overlaps the canal changes its outline. How the anterior nasal spine superimposes decides whether it looks round, oval, lobular or heart-shaped [1]. The foramen can also project over the apex of a central incisor [14].

The technique changes whether you see it at all. Two studies found the foramen more often on paralleling-technique periapicals than on bisecting-angle ones [3][4]. Both compared different patients for each technique, so this evidence is early. A comparison on the same patients would show how much the technique alone accounts for.

The canal itself also differs with age and history. It is wider in older patients on CT [8], in edentulous maxillae on CBCT [16], and in patients with a history of dentoalveolar injury on CBCT [17]. Each of these studies compared groups of patients at one point in time. None followed one canal over the years.

What it means clinically

A canal projected over an apex can mimic apical periodontitis, and endodontic treatment or retreatment may then be prescribed that the tooth doesn't need [14]. A case report describes a canal variant misdiagnosed as a periapical cyst [13].

A nasopalatine duct cyst often causes no symptoms. In a systematic review of 2,734 cases, most lesions were asymptomatic, and the periapical cyst was the second most frequent clinical diagnostic hypothesis [6]. On CBCT, a periapical radiolucency on a central incisor over 5 mm in diameter tends to merge with the canal [17]. Histopathology confirms a nasopalatine duct cyst [14].

How to be sure

• A clinical reference work and a review advise vitality testing of 11 and 21 to exclude a periapical lesion of pulpal origin [18][1].
• Two small studies saw the foramen more often with the paralleling technique than with the bisecting-angle technique [3][4].
• In one series of 22 cysts, periapical, occlusal and panoramic radiographs identified most lesions, and CT was used where doubt remained [15].
• In one CBCT study of 19 cysts and 164 controls, expansion of the canal's anterior wall helped separate cysts from normal canals [2].

Common errors

• Reading the canal over the apex of 11 or 21 as apical periodontitis [14].
• Taking a heart-shaped radiolucency as proof of a cyst [18]. The normal canal can take the same shape [1].
• Applying the 6 mm width cut-off unchanged to patients over 60. On CT it failed in that age group [8].
• Quoting a CT or CBCT diameter as if it had been measured on a periapical [9][10][12].
• Expecting the incisive foramen on every periapical of the central incisors [4].