Intraoral Radiographs: Identifying Normal Anatomy

Full-mouth intraoral radiographs are an essential part of the evaluation of oral health. Combined with extraoral and intraoral examinations of an anesthetized patient, intraoral radiographs provide information needed to diagnose and treat oral disease. As with any skill in veterinary medicine, interpreting intraoral radiographs improves with time and experience. To appreciate abnormal, you first need to understand normal. This article explains why full-mouth radiographs should be taken for every dentistry patient and later reviews: 

• normal radiographic dental anatomy 
• criteria for diagnostic radiographs
• steps for reading an intraoral radiograph 
• radiographic tooth development in pediatric patients
• normal anatomic structures that may be misinterpreted as pathology

By the end of this article, general practitioners should be able to appreciate the benefits of full-mouth radiographs and know how to recognize normal anatomic structures on intraoral radiographs of dogs and cats. 

Why Take Full-Mouth Radiographs?

To determine the full extent of oral disease and develop a treatment plan for your patients, an oral examination and full-mouth radiography performed with the patient under general anesthesia are required. Not including full-mouth radiographs as the standard of care for every dentistry patient could result in sending patients out the door with undiagnosed, untreated, painful oral disease. Research confirms that the diagnostic yield of full-mouth radiographs is high and that routine use of full-mouth radiographs is justifiable (BOX 1).^1,2 

Use of intraoral radiographs is supported by the American Veterinary Medical Association (AVMA) and the American Animal Hospital Association (AAHA). After a 2015 review of cases of jaw fractures and retained tooth roots, the AVMA professional liability newsletter stated that it would be difficult to defend complications resulting from dentistry procedures if the veterinarian did not have access to dental radiographs.³ The AAHA Guidelines Task Force strongly recommends obtaining full-mouth intraoral dental radiographs for all dentistry patients.⁴ 

Tooth Anatomy

For proper evaluation of intraoral radiographs, knowledge of normal anatomy of the tooth, the mandible, and the maxilla is essential. Recognizing pathology requires familiarity with normal dental anatomy (FIGURE 1). The radiographic components of the tooth and its supporting structures are provided in TABLE 1. 

Figure 1. Radiograph of the right mandibular first molar (409) in a dog. Cementoenamel junction (orange arrows); dentin (black asterisk); pulp cavity (blue arrow); periodontal ligament space (yellow arrow); lamina dura (white arrow); mandibular canal (yellow asterisk); ventral cortex of the mandible (orange asterisk); interradicular bone (blue asterisk); interproximal bone (white asterisk).

Criteria For Diagnostic Radiographs

Criteria 

A diagnostic radiograph must include the following anatomic landmarks (FIGURES 2–4):

• The level of alveolar bone (alveolar margin)
  

  Figure 2. Diagnostic dental radiograph in a dog and cat. Alveolar bone margin (orange arrows), periodontal ligament space (yellow arrows), 2–3 mm around the apex (white arrows). (A) Right maxillary canine tooth (104) in a young dog. Note the wide pulp cavity and open apex consistent with a developing tooth in a young dog.

  

  Figure 2. (B) Right mandibular third and fourth premolars and first molar (407, 408, 409) in a cat. Note tooth resorption at the furcation of 407 (white asterisk).

• The periodontal ligament space
  

  Figure 3. Diagnostic radiograph of the left mandibular first molar (309) in a dog. The alveolar bone margin, periodontal ligament space, and 2–3 mm around the apex of each tooth root are visible. The entire crown is not visible.

• The entire tooth root, including 2 to 3 mm around the apex of each root
• As much of the crown as possible

Figure 4. Diagnostic radiograph of the right and left maxillary first and second incisors (101, 102, 201, 202) in a dog. Nondiagnostic radiograph for the right and left maxillary third incisors (103 and 203) (white asterisks).

Both roots of the same tooth do not need to appear on the same radiograph, nor do the entire crown and root(s). Obtaining a diagnostic image of 1 tooth may require 2 radiographs (e.g., the mandibular first molar in a larger dog [FIGURE 5]). Similarly, 2 radiographic images of a canine tooth in a dog may be required: 1 image of the coronal portion of the root showing the level of alveolar bone to evaluate presence/absence of periodontal disease and 1 image of the most apical portion of the tooth root and the periapical area to evaluate presence/absence of endodontic disease (FIGURE 6).

FIGURE 5. Diagnostic radiographs of mandibular molars in a dog; 2 images of the mandibular first molar are often needed for larger dogs. (A) Right mandibular first molar distal root (409), mandibular second molar (410), and third molar (411). Note: 411 has been removed (white asterisk).

FIGURE 5. (B) Right mandibular fourth premolar distal root (408) and right mandibular first molar mesial root (409).

Figure 6. Two views of the canine tooth in a young dog. (A) Left maxillary canine tooth (204), showing the crown and most coronal portion of the root. Note the persistent deciduous left maxillary canine tooth (604) (white asterisk).

Figure 6. (B) Additional view showing the apical portion of the root and the periapical area. Note the wide pulp cavity and open apex consistent with an immature tooth in a young dog.

Two radiographic views are also needed for complete evaluation of the maxillary canine teeth in dogs and cats (FIGURE 7). The occlusal view is useful for comparing the pulp cavity sizes of the maxillary canine teeth and examining the buccal aspects of the teeth for signs of periodontitis. Lateral views of the right and left maxillary canine teeth are required for accurately evaluating the periapical region without superimposition on the maxillary premolar teeth and for evaluating periodontal status at the mesial and distal aspects of the canine teeth.⁵ The palatal side of maxillary canine teeth cannot be fully evaluated with radiographs; evaluating the maxillary canine tooth for periodontal disease on the palatal side requires use of a periodontal probe. 

FIGURE 7. Radiographs of maxillary canine teeth in a cat. (A) The occlusal view of the maxillary canine teeth is useful for evaluating the pulp cavity size (yellow asterisks) and buccal bone (orange asterisks). The apical portion of the maxillary canine teeth is superimposed on the maxillary premolars and cannot be evaluated.

FIGURE 7. (B) Lateral view of the left maxillary canine tooth (204) is required for evaluating the mesial and distal alveolar bone margins (white asterisks) and periapical area (orange arrow).

Viewing 

Some veterinary software programs will label the image with the tooth number(s) as they are exposed and will orient them correctly on the computer screen. If images are not labeled and correctly oriented by the software, certain clues will help determine whether the image is of the maxillary or mandibular arcade (TABLE 2). 

When viewing intraoral radiographs, orient the image the same way each time (just as you would do with thoracic or abdominal radiographs). If the tooth on the image is a maxillary tooth, then the radiograph should be rotated so the crowns of the teeth point downward (in their natural position) on the computer screen. If the tooth on the image is a mandibular tooth, then the radiograph should be rotated so the crowns of the teeth point upward. After determining if the tooth is in the maxilla or the mandible and orienting the radiograph accordingly, ask yourself whether the radiograph is of the right or left side of the mouth. If the rostral mandibular or maxillary premolars or molars are toward the right side of the radiograph, then you are looking at the right maxillary or mandibular premolars and molars. If the rostral teeth are located toward the left side of the radiograph, then you are looking at the left mandibular or maxillary premolars and molars (FIGURE 8). When viewing incisors, the right maxillary or mandibular incisors are on the left side of the image and the left maxillary or mandibular incisors are on the right side of the image (similar to a ventrodorsal abdominal or thoracic radiograph) (FIGURE 9).

Figure 8. Radiograph of mandibular premolars and molar in a cat. The rostral teeth are toward the left. This radiograph shows the left mandibular third and fourth premolars and first molar (307, 308, 309).

Figure 9. Radiographs of incisors in a cat and dog. (A) Maxillary incisors in a cat. The right maxillary incisors (101, 102, 103) are on the left side of the radiograph (orange asterisks), and the left maxillary incisors (201, 202, 203) are on the right (white asterisks). The palatine fissures are the symmetrical radiolucent structures (orange arrows) distal to the maxillary incisors.

FIGURE 9. (B) Mandibular incisors in a dog. The right mandibular incisors (401, 402, 403) are on the left (orange asterisks), and the left mandibular incisors (301, 302, 303) are on the right (white asterisks). The fibrocartilaginous mandibular symphysis joins the 2 mandibles and appears as a radiolucent line between the central incisors (blue arrows).

The number of full-mouth radiographs needed to see all the teeth, including the level of alveolar bone and the periapical area around each tooth root, can vary with patient size (the larger the patient, the more images usually needed). Mount full-mouth radiographs so that the patient’s right maxilla and right mandible are on the viewer’s left side and the patient’s left maxilla and left mandible are on the right side (FIGURE 10).

Figure 10. Full mouth radiographs of a cat.

Steps For Reading An Intraoral Radiograph

Every time you read an intraoral radiograph, follow the same steps in the same order, just as you do for thoracic or abdominal radiographs. 

1. Evaluate the level of alveolar bone around each tooth; alveolar bone should be 1 to 2 mm apical to the cementoenamel junction.
2. Follow the periodontal ligament space around each root, pausing at the apex to evaluate the periapical area. The periodontal ligament space should be of uniform width around the entire tooth root.
3. Evaluate the size of the pulp cavity of each tooth. The pulp cavity size should be consistent with the age of the patient and with the contralateral tooth.
4. Look at all the bone and soft tissue visible on the radiograph; additional pathology may be present in these tissues.

If you are not sure of the significance of a radiographic finding, take another view or look at the radiograph of the contralateral tooth. Remember that normal anatomy can sometimes mimic pathology.

Radiographs Of Pediatric Patients

Deciduous Dentition

Interpreting intraoral radiographs of pediatric patients can be challenging because of the presence of deciduous teeth and permanent tooth buds. The permanent maxillary canine teeth erupt mesial to the deciduous maxillary canine teeth. All other permanent teeth erupt lingual or palatal to their deciduous tooth counterparts. There are no deciduous precursors for the permanent first premolar or molars. In dogs, the maxilla and mandible have 3 deciduous premolars: the second, third, and fourth premolars, named for the teeth that succeed them in the jaw rather than their function or anatomy. The deciduous maxillary third premolar is anatomically similar to the permanent maxillary fourth premolar. The deciduous maxillary fourth premolar is anatomically similar to the permanent maxillary first molar (3 roots and a large occlusal surface). The deciduous mandibular fourth premolar has the function and morphology of a molar but is replaced by the permanent fourth premolar (FIGURE 11).⁶ 

Figure 11. Deciduous and permanent teeth in a young dog. Note: There are no deciduous precursors for the first premolar or molar teeth. (A) Left maxilla. Permanent tooth buds (asterisks): left maxillary canine (204) (red); first premolar (205) (black); second premolar (206) (yellow); third premolar (207) (blue); and fourth premolar (208) (orange). Note: the permanent left maxillary first and second molar teeth (209, 210) are not visible in this radiograph. Deciduous teeth (arrowheads): left maxillary canine (604) (red); second premolar (606) (yellow); third premolar (607) (blue); and fourth premolar (608) (orange).

Figure 11. (B) Left mandible. Permanent teeth (asterisks): left mandibular second premolar (306) (yellow); third premolar (307) (blue); fourth molar (308) (orange); first molar (309) (green); and second molar (310) (white). Note: the permanent mandibular first premolar and third molar teeth are not visible in this radiograph. Deciduous teeth (arrowheads): second premolar (706) (yellow); third premolar (707) (blue); fourth premolar (708) (orange).

Permanent Tooth Development 

At the time of permanent tooth eruption, the apex is incomplete, the pulp cavity is wide, and the primary dentin layer is thin. As the tooth develops, the apex closes and secondary dentin is produced by odontoblasts within the pulp cavity. Apical closure occurs by 7 to 9.5 months of age in dogs⁷ and 7 to 11 months in cats.⁸ As the dog or cat continues to mature, the pulp cavity continues to get smaller as the thickness of the secondary dentin layer increases (FIGURE 12).

Figure 12. Development of left maxillary canine tooth (204) in the cat. (A) Immature tooth with wide pulp cavity, thin dentin layer, and open apex.

Figure 12. (B) Maturing tooth, smaller pulp cavity, thicker dentin layer, and closed apex.

Figure 12. (C) Mature tooth, small pulp cavity, thick dentin layer, and closed apex.

Why is the tooth development process important? Radiographs of a tooth in a mature dog or cat should show a small pulp cavity and a thick layer of dentin. In a mature dog or cat, if the radiograph of a tooth shows a large pulp cavity and a narrow layer of dentin, development of that tooth has arrested and the tooth is nonvital. When the tooth dies, the pulp cavity of that tooth stays the same size for the rest of the animal’s life. 

Normal Anatomy That May Mimic Pathology

Two “White Lines” in the Maxilla

For dogs, 2 radiopaque lines appear on radiographs of the maxillary canine and premolar teeth: the junction of the vertical body of the maxilla and its palatine process and the maxillary conchal crest. These normal anatomic structures can mimic the lamina dura and interfere with interpretation of apical structures. 

• The junction of the vertical body of the maxilla and its palatine process is visualized as a radiopaque line that crosses the midroot of the maxillary canine tooth and continues caudally and is apical to the roots of the maxillary premolars. 
• The maxillary conchal crest is seen as a radiopaque line that begins just mesial to the root of the canine tooth and continues caudally to reach the level of the third premolar. Because of its location close to the apex of the maxillary canine tooth, it is difficult to change its location by changing angulation of the x-ray beam (FIGURE 13A).⁹

Figure 13. Radiographs of maxillary canine teeth in a dog and cat, showing linear opacities (white lines). (A) Left maxillary canine tooth (204) in a dog. Conchal crest (white arrow), junction of the vertical body of the maxilla and its palatine process (yellow arrow).

For cats, the 2 lines are also formed by the junction of the vertical body of the maxilla and its palatine process and the conchal crest.

• The vertical body of the maxilla and its palatine process is visualized as a radiopaque line that crosses the middle third of the maxillary canine tooth root. 
• The conchal crest appears as a radiopaque line that begins just mesial to the root apex and ends at the third premolar. Because of the anatomic location of these 2 structures close to the apex of the maxillary canine tooth, the linear opacities always appear close to the tooth root (FIGURE 13B).¹⁰

Figure 13. (B) Left maxillary tooth (204) in a cat. Conchal crest (white arrow), junction of the vertical body of the maxilla and its palatine process (yellow arrow). Note: the teeth are slightly elongated, and the left maxillary third premolar (207) has a supernumerary (additional) root. The zygomatic arch is the radiopaque structure superimposed on the roots of 207, 208, and 209.

Chevron

A chevron artifact is a very regular chevron-shaped radiolucency commonly associated with the maxillary incisors and canine teeth in dogs (FIGURE 14); radiographically, it looks similar to lesions of endodontic origin. This effect is created by trabecular bone and vascular channels around the apices, contrasted with the dense compact bone of the alveolar walls and incisive bone.¹¹

Figure 14. Chevron lucencies associated with right maxillary incisors (101, 102, 103). Note the regular shape of the lucency and the intact lamina dura around the apex of the tooth (arrows). Courtesy Chanda Miles, DVM, DAVDC.

In general, periapical lesions associated with endodontic disease will be more circular or irregularly shaped lucencies and the periodontal ligament space will not be visible around the root tip. The root tip of an endodontically diseased tooth may exhibit signs of apical root resorption. 

Summation

Other lucencies and opacities are created by the summation effect of superimposed structures, projecting overlying anatomy in a way that makes the structure appear to be associated with a tooth root or its supporting bone.¹¹ Summation of the mandibular first molar apices superimposed over the mandibular canal can be confused as an endodontic lesion (FIGURE 15). BOX 2 lists clues for differentiating summation of normal structures from pathology. 

FIGURE 15. Radiographs showing periapical lucencies in a dog. (A) Right mandibular first molar (409) showing the summation effect resulting from superimposition of the apex of the tooth roots over the mandibular canal (arrows).

FIGURE 15. (B) Black dashed line indicates the dorsal border of the mandibular canal. Orange dashed lines indicate the periodontal ligament extension lines. The lucency is contained within the extension lines and therefore is probably the result of superimposition of the apices over the mandibular canal.

FIGURE 15. (C) Left mandibular first molar (309). The periapical lucency is irregular in shape, wider than the root tip, and extends beyond the extension lines of the periodontal ligament space. There is evidence of apical root resorption. These findings are consistent with endodontic disease and are not the result of superimposition of the apices of 309 on the mandibular canal.

When in doubt, consult with a board-certified veterinary dentist or take follow-up radiographs in 3 to 6 months to evaluate changes in the appearance of the periapical lucency.

Foramina

The caudal and middle mental foramina may be mistaken for periapical pathology associated with the rostral mandibular premolars. 

In the dog, the middle mental foramen is ventral to the mesial root of the second premolar and the caudal mental foramen is ventral to the mandibular third premolar (FIGURE 16).

Figure 16. Mental foramina of the dog. (A) Rostral mandible of a dog skull, showing the location of the rostral (white asterisk), middle (black asterisk), and caudal (orange asterisk) mental foramina. The mandibular first premolar is missing. Courtesy Chanda Miles, DVM, DAVDC.

Figure 16. (B) Radiograph of right mandibular premolars, showing the middle mental foramen ventral to the mesial root of the mandibular second premolar (406) (black arrow) and caudal mental foramen ventral to the mandibular third premolar (407) (orange arrow). Courtesy Chanda Miles, DVM, DAVDC.

In the cat, the middle mental foramen is in the interdental space between the mandibular canine tooth and third premolar and the caudal mental foramen is ventral to the mandibular third premolar (FIGURE 17). To distinguish between a foramen and a periapical radiolucency, change the angle of the tube head when obtaining the radiograph. The foramen will move relative to the root as the angle of the tube head is changed. If the lucency remains associated with the apex of the tooth, then it is periapical pathology. 

Figure 17. Mental foramina of the cat. (A) Rostral mandible of cat skull, showing the location of the middle (black asterisk) and caudal (orange asterisk) mental foramina. Courtesy Chanda Miles, DVM, DAVDC.

Figure 17. (B) Radiograph of left mandibular premolars showing the middle mental foramen in the interdental space between the left mandibular canine (304) and third premolar teeth (307) (black arrow) and the caudal mental foramen ventral to the mandibular third premolar (307) (orange arrow). Courtesy Chanda Miles, DVM, DAVDC.

Radicular Groove of Mandibular First Molar

The periodontal ligament space on the distal aspect of the mesial root of the mandibular first molar in a dog may appear as a double line or shadow. This double line represents the radicular groove (FIGURE 18).⁶ Incorrectly interpreting this normal anatomic structure may lead you to believe that there are 2 mesial roots in the first molar. 

Figure 18. Radicular groove in a dog. (A) Radiograph showing double line resulting from the radicular groove on the distal side of the mesial root of the right mandibular first molar (409) (orange arrows).

Figure 18. (B) Extracted mesial root of the right mandibular first molar, with the radicular groove visible on the distal side of the mesial root. Black arrows point to the edges of the radicular groove that cause the double line seen in (A).

Conclusions

To obtain essential information for determining appropriate treatment for each tooth in each patient, full-mouth radiographs are required. 

As with most things in veterinary medicine, the first step in learning how to interpret intraoral radiographs is becoming familiar with normal dental anatomy. Radiographs are 2-dimensional, and superimposed structures may cause misinterpretation of normal anatomy as pathology. The second step in learning how to interpret intraoral radiographs is understanding radiographic findings associated with oral diseases. To read about pathology seen on intraoral radiographs, see “Intraoral Radiographs: Identifying Common Pathology” in the July/August 2022 issue of Today’s Veterinary Practice. The more intraoral radiographs you read, the more disease you will find and the more you will believe in the value of full-mouth intraoral radiographs for every dentistry patient.

Author’s note: Special thanks to the veterinary nurses who obtained these radiographs and are an integral part of the oral healthcare team.