• View in gallery

    Lateral radiographic view of the mandibular teeth of a dog with external surface resorption. There is a shallow resorption lacuna with discernible periodontal ligament space located on the lateral surface of the mesial root of the right mandibular first molar tooth (black arrows). Notice that the periodontal ligament space is not visible around the roots of the right mandibular fourth premolar tooth (white arrowheads) and that the surrounding alveolar bone is replacing the root structure, which is consistent with external replacement resorption.

  • View in gallery

    Lateral radiographic view of the mandibular teeth of a dog with external replacement resorption. There is loss of periodontal ligament space around the roots of the right mandibular first molar tooth (black arrows) as the surrounding alveolar bone replaces the dental structures. The roots of the right mandibular fourth premolar tooth (white arrowheads) have a more advanced stage of the same type of tooth resorption.

  • View in gallery

    Lateral radiographic view of the mandibular teeth of a dog with external inflammatory resorption. Notice the areas of bone loss around the resorbing roots of the right mandibular first molar tooth (black arrows). There is a clearly circumscribed periapical lucent area in the distal root, and the lesion in the mesial root is consistent with a combined periodontal-endodontal lesion.

  • View in gallery

    Lateral radiographic view of the mandibular teeth of a dog with external cervical root surface resorption. A deep lesion (black arrowheads) penetrates into the pulp cavity of the right mandibular second molar tooth. Notice the cervical location of the lesion with invasion of the crown and intact alveolar bone.

  • View in gallery

    Lateral radiographic view of the maxillary teeth of a dog with internal surface resorption. An oval-shaped resorption lacuna (black arrow) is evident in the apical third of the root canal of the distal root of the left maxillary fourth premolar tooth. There is no evidence of endodontal or periodontal disease.

  • View in gallery

    Lateral radiographic view of the maxillary teeth of a dog with internal inflammatory resorption. An oval-shaped resorption area (black arrows) is evident in the middle third of the root canal of the distal root of the left maxillary fourth premolar tooth. Notice the crown fracture indicated by a missing middle cusp (white arrowhead).

  • View in gallery

    Lateral radiographic view of the maxillary teeth of a dog with unclassifiable tooth resorption. Several resorption lacunae are visible at the left maxillary canine tooth. Two separate areas of tooth resorption (black arrowheads) are evident at the cervical and apical thirds of the tooth. It is unclear whether the tooth resorption process is of an internal or external nature. A widened root canal (black arrows) is suggestive of internal resorption. Notice the reduced opacity of the surrounding alveolar bone and the resorbing roots of the adjacent third incisor tooth and first and second premolar teeth.

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Radiographic evaluation of the types of tooth resorption in dogs

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  • 1 Dentistry and Oral Surgery Service, William B. Pritchard Veterinary Medical Teaching Hospital, School of Veterinary Medicine, University of California-Davis, Davis, CA 95616.
  • | 2 Departments of Surgical and Radiological Sciences, School of Veterinary Medicine, University of California-Davis, Davis, CA 95616.
  • | 3 Population Health and Reproduction, School of Veterinary Medicine, University of California-Davis, Davis, CA 95616.

Abstract

Objective—To determine the prevalence of tooth resorption in dogs and to evaluate whether a classification system for tooth resorption in humans is applicable in this species.

Animals—224 dogs > 1 year old admitted for periodontal treatment or other dental procedures in 2007

Procedures—Full-mouth radiographs of all dogs were reviewed for evidence of tooth resorption. Tooth resorption was classified in accordance with radiographic criteria described for use in humans. Patient signalment and concurrent dental conditions were recorded and tabulated.

Results—Tooth resorption was detected in 120 of 224 (53.6%) dogs and 943 of 8,478 (11.1%) teeth. The classification system for use in humans was applicable in 908 of 943 (96.3%) affected teeth. Tooth resorption was more frequent among older and large-breed dogs; no significant differences were found among sex categories. The 2 most common types of tooth resorption were external replacement resorption (77/224 [34.4%] dogs and 736/8,478 [8.7%] teeth) and external inflammatory resorption (58/224 [25.9%] dogs and 121/8,478 [1.4%] teeth). External cervical root surface resorption was detected in 13 of 224 (5.8%) dogs; external surface resorption was detected in 10 of 224 (4.5%) dogs, and internal inflammatory resorption and internal surface resorption were detected in 9 of 224 (4.0%) and 1 of 224 (0.4%) dogs, respectively. Internal replacement resorption was not detected.

Conclusions and Clinical Relevance—The classification of tooth resorption in humans was applicable to tooth resorption in dogs. Resorption lesions, in general, and external replacement and external inflammatory resorption, in particular, were frequently detected in dogs.

Abstract

Objective—To determine the prevalence of tooth resorption in dogs and to evaluate whether a classification system for tooth resorption in humans is applicable in this species.

Animals—224 dogs > 1 year old admitted for periodontal treatment or other dental procedures in 2007

Procedures—Full-mouth radiographs of all dogs were reviewed for evidence of tooth resorption. Tooth resorption was classified in accordance with radiographic criteria described for use in humans. Patient signalment and concurrent dental conditions were recorded and tabulated.

Results—Tooth resorption was detected in 120 of 224 (53.6%) dogs and 943 of 8,478 (11.1%) teeth. The classification system for use in humans was applicable in 908 of 943 (96.3%) affected teeth. Tooth resorption was more frequent among older and large-breed dogs; no significant differences were found among sex categories. The 2 most common types of tooth resorption were external replacement resorption (77/224 [34.4%] dogs and 736/8,478 [8.7%] teeth) and external inflammatory resorption (58/224 [25.9%] dogs and 121/8,478 [1.4%] teeth). External cervical root surface resorption was detected in 13 of 224 (5.8%) dogs; external surface resorption was detected in 10 of 224 (4.5%) dogs, and internal inflammatory resorption and internal surface resorption were detected in 9 of 224 (4.0%) and 1 of 224 (0.4%) dogs, respectively. Internal replacement resorption was not detected.

Conclusions and Clinical Relevance—The classification of tooth resorption in humans was applicable to tooth resorption in dogs. Resorption lesions, in general, and external replacement and external inflammatory resorption, in particular, were frequently detected in dogs.

The loss of tooth structure as a result of osteoclastic activity may be physiologic, as during the exfoliation process of deciduous dentition, or pathological, as in patients with internal and external resorption of permanent teeth. Pathological tooth resorption has been described in several species,1–3 including dogs,4–8 cats,9–13 and humans.14–17 It represents one of the most common dental conditions in cats (ie, resorption lesions).9,12 Some types of tooth resorption are progressive in nature and may result in associated pain, eventual loss of teeth as a result of exfoliation, or the imminent need for tooth extraction.16,18 methods.14–17,19 The most widely accepted classification method is that proposed in 1 study.19 The authors of that report describe 7 types of resorption, as determined on the basis of the radiographic appearance: external surface resorption, external replacement resorption, external inflammatory resorption, external cervical root surface resorption, internal surface resorption, internal replacement resorption, and internal inflammatory resorption. Ultimately, each type of tooth resorption can only be distinguished histologically; however, from a clinical standpoint, the diagnosis is considered dependent on radiographic examination.19

In contrast, classifications of tooth resorption in domestic animals are based on clinical and radiographic findings and traditionally have been intended for staging of resorption lesions in cats. The emphasis has been on the extent of the resorptive process, rather than on the exact location and type of lesions; investigators have described 4 stages12 or 5 stages20,21 of tooth resorption. The 4-stage method12 is as follows: stage 1, shallow enamel or cementum lesions that do not involve the dentin; stage 2, lesions that have progressed to affect the dentin but that do not involve the pulp; stage 3, lesions that have progressed to involve the pulp; and stage 4, lesions in which there is massive loss of tooth substance with or without resultant root ankylosis.

In 2007, the Nomenclature Committee of the AVDC adopted a 5-stage classification method that was based on the extent of resorption lesions22; however, there is no mention regarding possible causes and radiographic patterns. To our knowledge, there have been no published studies in which investigators have evaluated the radiographic patterns and types of tooth resorption in dogs. Given the clinical importance of some types of tooth resorption, it appears relevant to determine these aspects in dogs. Although some types of tooth resorption are believed to be incidental radiographic findings (ie, external replacement resorption),4,23 others are associated with common dental conditions (ie, inflammatory resorption) that may cause pain.15 The identification of various types of tooth resorption in dogs will allow clinicians to make specific diagnoses and provide appropriate treatment recommendations.

In veterinary medicine, most of the research has focused on tooth resorption in cats. Several studies9,11–13,24–26 have been conducted to investigate epidemiological and risk factors in this species; however, the causes remain elusive. In contrast to tooth resorption in cats and humans, tooth resorption in dogs has been poorly evaluated and the epidemiological aspects are unclear. The frequency of external replacement resorption was examined in a group of 33 dogs > 10 years old, and 6 of the dogs were affected in that study.4 In another study,8 tooth resorption was detected radiographically in 29 of 162 (17.9%) dogs randomly selected for postmortem evaluation; however, the radiographic patterns and classification criteria were vague. Root resorption-ankylosis was detected as an incidental finding during examination of routine full-mouth dental radiographs in 49 of 226 (21.7%) dogs, but no distinction was made regarding radiographic patterns or classification of tooth resorption.23 Other information on tooth resorption in dogs is limited to case reports.5–7

Given the fact that dogs share some of the risk factors associated with tooth resorption in humans (ie, common periodontal and endodontal inflammatory conditions), it appears likely that tooth resorption lesions would be frequent in dogs and that they would possibly share some of the radiographic characteristics of tooth resorption in humans. The purpose of the study reported here was to evaluate whether the classification method for tooth resorption in humans that is based on types of tooth resorption described in 1 study19 would be applicable to dogs and to determine the prevalence of tooth resorption in dogs.

Materials and Methods

Full-mouth radiographs of 224 dogs admitted to the University of California-Davis William B. Pritchard Veterinary Medical Teaching Hospital during 2007 for periodontal treatment or other dental procedures were prospectively reviewed for evidence of tooth resorption. Included in the study were dogs > l year old for which diagnostic-quality full-mouth radiographs (including lateral views of the canine teeth) were obtained. The diagnosis of tooth resorption was performed on a patient-by-patient and on a tooth-by-tooth basis. Types of tooth resorption (Figures 1–7) were classified in accordance with the radiographic criteria described by Andreasen and Andreasen19 (Appendix). The radiographic findings of dogs in which tooth resorption was diagnosed were evaluated in conjunction with other dental findings and other information in the medical record. Signalment and concurrent dental conditions were recorded and tabulated. Previous intraoral radiographs were not used for comparison purposes because they were only available for a limited number of dogs. When considered pertinent, additional radiographic projections were obtained. Teeth with readily apparent radiographic signs of tooth resorption but that did not fulfill the radiographic criteria for any of the types of tooth resorption were considered unclassifiable.

Figure 1—
Figure 1—

Lateral radiographic view of the mandibular teeth of a dog with external surface resorption. There is a shallow resorption lacuna with discernible periodontal ligament space located on the lateral surface of the mesial root of the right mandibular first molar tooth (black arrows). Notice that the periodontal ligament space is not visible around the roots of the right mandibular fourth premolar tooth (white arrowheads) and that the surrounding alveolar bone is replacing the root structure, which is consistent with external replacement resorption.

Citation: American Journal of Veterinary Research 71, 7; 10.2460/ajvr.71.7.784

Figure 2—
Figure 2—

Lateral radiographic view of the mandibular teeth of a dog with external replacement resorption. There is loss of periodontal ligament space around the roots of the right mandibular first molar tooth (black arrows) as the surrounding alveolar bone replaces the dental structures. The roots of the right mandibular fourth premolar tooth (white arrowheads) have a more advanced stage of the same type of tooth resorption.

Citation: American Journal of Veterinary Research 71, 7; 10.2460/ajvr.71.7.784

Figure 3—
Figure 3—

Lateral radiographic view of the mandibular teeth of a dog with external inflammatory resorption. Notice the areas of bone loss around the resorbing roots of the right mandibular first molar tooth (black arrows). There is a clearly circumscribed periapical lucent area in the distal root, and the lesion in the mesial root is consistent with a combined periodontal-endodontal lesion.

Citation: American Journal of Veterinary Research 71, 7; 10.2460/ajvr.71.7.784

Figure 4—
Figure 4—

Lateral radiographic view of the mandibular teeth of a dog with external cervical root surface resorption. A deep lesion (black arrowheads) penetrates into the pulp cavity of the right mandibular second molar tooth. Notice the cervical location of the lesion with invasion of the crown and intact alveolar bone.

Citation: American Journal of Veterinary Research 71, 7; 10.2460/ajvr.71.7.784

Figure 5—
Figure 5—

Lateral radiographic view of the maxillary teeth of a dog with internal surface resorption. An oval-shaped resorption lacuna (black arrow) is evident in the apical third of the root canal of the distal root of the left maxillary fourth premolar tooth. There is no evidence of endodontal or periodontal disease.

Citation: American Journal of Veterinary Research 71, 7; 10.2460/ajvr.71.7.784

Figure 6—
Figure 6—

Lateral radiographic view of the maxillary teeth of a dog with internal inflammatory resorption. An oval-shaped resorption area (black arrows) is evident in the middle third of the root canal of the distal root of the left maxillary fourth premolar tooth. Notice the crown fracture indicated by a missing middle cusp (white arrowhead).

Citation: American Journal of Veterinary Research 71, 7; 10.2460/ajvr.71.7.784

Figure 7—
Figure 7—

Lateral radiographic view of the maxillary teeth of a dog with unclassifiable tooth resorption. Several resorption lacunae are visible at the left maxillary canine tooth. Two separate areas of tooth resorption (black arrowheads) are evident at the cervical and apical thirds of the tooth. It is unclear whether the tooth resorption process is of an internal or external nature. A widened root canal (black arrows) is suggestive of internal resorption. Notice the reduced opacity of the surrounding alveolar bone and the resorbing roots of the adjacent third incisor tooth and first and second premolar teeth.

Citation: American Journal of Veterinary Research 71, 7; 10.2460/ajvr.71.7.784

Statistical analysis of the collected data included χ2 tests of homogeneity to compare numbers of affected and nonaffected dogs with respect to nominal variables (eg, sex), Kruskal-Wallis 1-way ANOVA to compare numbers of affected and nonaffected dogs with respect to ordinal categorical variables (eg, body weight and age), and Cochran-Armitage tests to compare proportions of affected dogs across ordinal categorical variables (eg, body weight and age). Mann-Whitney tests were used to compare the distribution of continuous values between patient subpopulations. Mean, SD, and range values were reported for continuous data. Values of P < 0.05 were considered significant.

Results

Study group—The study group consisted of 224 dogs. This included 117 (52.2%) males (89 [39.7%] castrated and 28 [12.5%] sexually intact) and 107 (47.8%) females (92 [41.1%] spayed and 15 [6.7%] sexually intact). Dogs ranged from 1 to 19 years of age (mean ± SD, 7.7 ± 3.7 years). The dogs were assigned to 1 of 6 age groups as follows: age group 0 (1 to 3 years old) consisted of 32 (14.3%) dogs, age group 1 (4 to 6 years old) consisted of 51 (22.8%) dogs, age group 2 (7 to 9 years old) consisted of 71 (31.7%) dogs, age group 3 (10 to 12 years old) consisted of 48 (21.4%) dogs, age group 4 (13 to 15 years old) consisted of 18 (8.0%) dogs, and age group 5 (> 16 years old) consisted of 4 (1.8%) dogs. Similarly, dogs were assigned to 1 of 4 body weight groups as follows: group 0 (< 12 kg) consisted of 101 (45.1%) dogs, group 1 (13 to 25 kg) consisted of 53 (23.7%) dogs, group 2 (26 to 45 kg) consisted of 62 (27.7%) dogs, and group 3 (> 46 kg) consisted of 8 (3.6%) dogs (Table 1). The 224 dogs in the study group had 8,478 teeth, with 9 to 47 teeth/dog (mean ± SD, 37.8 ± 6.0 teeth/dog).

Table 1—

Frequency of tooth resorption in 224 dogs categorized on the basis of age group, body weight group, and sex category.

VariableDogs with tooth resorptionExternal surface resorptionExternal replacement resorptionExternal inflammatory resorptionExternal cervical root surface resorptionInternal surface resorptionInternal replacement resorptionInternal inflammatory resorptionUnclassifiable
Age group*
   0(n = 32)6 (18.8)1 (3.1)1 (3.1)6 (18.8)0 (0)0 (0)0 (0)00
   1 (n = 51)17 (33.3)1 (2.0)10 (19.6)7 (13.7)2 (3.9)0 (0)0 (0)3 (5.9)3 (5.9)
   2(n = 71)43 (60.6)5 (7)26 (36.6)23 (32.4)5 (7.0)1 (1.4)0 (0)00
   3(n = 48)35 (72.9)2 (4.2)25 (52.1)15 (31.3)3 (6.3)0 (0)0 (0)5 (10.4)3 (6.3)
   4 (n = 18)15 (83.3)1 (5.6)13 (72.2)5 (27.8)1 (5.6)0 (0)0 (0)1 (5.6)3 (16.7)
   5 (n = 4)4 (100)0 (0)2 (50.0)2 (50.0)2 (50.0)0 (0)0 (0)0 (0)1 (25)
Body weight groupt
   0(n = 101)47 (46.5)1 (1.0)21 (20.8)24 (23.8)6 (5.9)0 (0)0 (0)5 (5)7 (6.9)
   1 (n = 53)26 (49.1)3 (5.7)20 (37.7)11 (20.8)4 (7.5)0 (0)0 (0)1 (1.9)1 (1.9)
   2 (n = 62)40 (64.51)4 (6.5)30 (48.4)19 (30.6)3 (4.8)1 (1.6)0 (0)3 (4.8)1 (1.6)
   3 (n = 8)7 (87.5)2 (25.0)6 (75.0)4 (50.0)0 (0)0 (0)0 (0)0 (0)1 (12.5)
Sex category
   All males (n = 117)68 (58.1)5 (4.3)45 (38.5)34 (29.1)9 (7.7)1 (0.9)0 (0)6 (5.1)9 (7.7)
   Castrated (n = 89)58 (65.2)4 (4.5)38 (42.7)29 (32.6)8 (9.0)1 (1.2)0 (0)5 (5.6)7 (7.9)
   Sexually intact (n = 28)10 (35.7)1 (3.6)7 (25.0)5 (17.9)1 (3.6)0 (0)0 (0)1 (3.6)2 (7.1)
    All females (n = 107)52 (48.6)5 (4.7)32 (29.9)24 (22.4)4 (3.7)0 (0)0 (0)3 (2.8)1 (0.9)
      Spayed (n = 92)46 (40.0)5 (5.4)30 (32.6)20 (21.7)3 (3.3)0 (0)0 (0)3 (3.3)1 (1.1)
      Sexually intact (n = 15)6 (50.0)0 (0)2 (13.3)4 (26.7)1 (6.7)0 (0)0 (0)0 (0)0 (0)

Values reported are number (%) of dogs.

Age groups were as follows: 0 = 1 to 3 years, 1 = 4 to 6 years, 2 = 7 to 9 years, 3 = 10 to 12 years, 4 = 13 to 15 years, and 5 = > 16 years. tBody weight groups were as follows: 0 = < 12 kg, 1 = 13 to 25 kg, 2 = 26 to 45 kg, and 3 = > 46 kg. n = Number of dogs.

Frequency of tooth resorption—Of the 224 dogs in the study group, 120 (53.6%) had radiographic evidence of tooth resorption and 104 (46.4%) did not have radiographic evidence of tooth resorption. Of the 8,478 teeth in the study group, 943 (11.1%) had evidence of tooth resorption, whereas 7,535 (88.9%) did not have evidence of tooth resorption. Affected dogs had 9 to 47 teeth/dog (mean, 38.3 ± 5.7 teeth/dog) and 1 to 38 affected teeth/dog (mean ± SD, 7.9 ± 7.9 affected teeth/dog). Nonaffected dogs had 11 to 44 teeth/dog (mean ± SD, 37.3 ± 6.4 teeth/dog). There was no significant (P = 0.36) difference in the number of teeth between affected and nonaffected dogs. Mean ± SD age of dogs with tooth resorption was 9.2 ± 3.5 years, whereas mean age of unaffected dogs was 6.0 ± 3.1 years; age of dogs differed significantly (P < 0.001) between affected and nonaffected dogs.

The percentage of affected dogs increased significantly (P < 0.001) with increasing age. There were 6 of 32 (18.8%) dogs with tooth resorption in age group 0, whereas there were 4 of 4 (100%) dogs with tooth resorption in age group 5. Similarly, the percentage of affected dogs increased significantly (P = 0.006) with increasing body weight. There were 47 of 101 (46.5%) dogs with tooth resorption in body weight group 0, whereas there were 7 of 8 (87.5%) dogs with tooth resorption in body weight group 3. Tooth resorption was detected in 68 of 117 (58.1%) males and 52 of 107 (48.6%) females; these percentages did not differ significantly (P = 0.18). However, there was a significant (P = 0.008) difference in the percentage of affected dogs between sexually intact males (10/28 [35.7%] dogs) and castrated males (58/89 [65.2%] dogs). Among affected females, no significant (P = 0.58) difference was found between the percentage of sexually intact females (6/15 [40.0%] dogs) and spayed females (49/92 [53.3%] dogs).

A significant (P = 0.002) difference was found for the distribution of lesions between the maxillary and mandibular teeth, with 495 of 4,039 (12.3%) maxillary teeth and 448 of 4,439 (10.1%) mandibular teeth affected. The tooth most frequently affected in the maxilla was the first premolar tooth (73/368 [19.8%] teeth), whereas the teeth least frequently affected in the maxilla were the first (15/413 [3.6%]) and second (10/386 [2.6%]) molar teeth. Tooth resorption was evident in the remainder of the maxillary teeth at a frequency ranging from 39 of 402 (9.7%) to 70 of 413 (16.9%). The tooth most frequently affected in the mandible was the second premolar tooth (88/391 [22.5%] teeth), whereas the teeth least frequently affected in the mandible were the incisor teeth and second and third molar teeth (frequency ranging from 5/331 [1.5%] to 13/405 [3.2%]). Frequency of tooth resorption among the first, third, and fourth premolar teeth and the first molar tooth ranged from 50 of 437 (11.4%) to 81 of 407 (19.9%). There was a significant (P < 0.001) difference in the frequency of tooth resorption between mandibular (27/444 [6.1%]) and maxillary (70/439 [15.9%]) canine teeth. Similarly, the frequency of tooth resorption in mandibular incisor teeth (11/408 [2.7%] to 13/405 [3.2%]) was significantly (P < 0.001) lower than that for the maxillary incisor teeth (52/427 [12.2%] to 62/413 [15.0%]; Table 2).

Table 2—

Frequency of tooth resorption in 224 dogs categorized on the basis of tooth type.

ToothExternal surface resorptionExternal replacement resorptionExternal nflammatory resorptionExternal cervical root surface resorptionInternal surface resorptionInternal replacement resorptionInternal inflammatory resorptionUnclassifiableTotal
All teeth (n = 8,478)17 (0.2)736 (8.7)121 (1.4)21 (0.2)1 (< 0.1)0 (0)12 (0.1)35 (0.4)943 (11.1)
Maxilla (n = 4,039)7 (0.2)368 (9.1)79 (2.0)7 (0.2)1 (< 0.1)0 (0)7 (0.2)26 (0.6)495 (12.3)
First incisor (n = 398)0 (0)47 (11.8)8 (2.0)0 (0)0 (0)0 (0)0 (0)1 (0.3)56 (14.1)
Second incisor (n = 413)1 (0.2)49 (11.9)10 (2.4)0 (0)0 (0)0 (0)0 (0)2 (0.5)62 (15)
Third incisor (n = 427)1 (0.2)48 (11.2)1 (0.2)0 (0)0 (0)0 (0)0 (0)2 (0.5)52 (12.2)
Canine (n = 439)0 (0)59 (13.4)7 (1.6)1 (0.2)0 (0)0 (0)0 (0)3 (0.7)70 (15.9)
First premolar (n = 368)1 (0.3)66 (17.9)3 (0.8)0 (0)0 (0)0 (0)1 (0.3)2 (0.5)73 (19.8)
Second premolar (n = 380)1 (0.3)25 (6.6)6 (1.6)4 (1.1)0 (0)0 (0)3 (0.8)9 (2.4)48 (12.6)
Third premolar (n = 402)0 (0)27 (6.7)8 (2.0)0 (0)0 (0)0 (0)2 (0.5)2 (0.5)39 (9.7)
Fourth premolar (n = 413)3 (0.7)31 (7.5)29 (7.0)2 (0.5)1 (0.2)0 (0)1 (0.2)3 (0.7)70 (16.9)
First molar (n = 413)0 (0)8 (1.9)6 (1.5)0 (0)0 (0)0 (0)0 (0)1 (0.2)15 (3.6)
Second molar (n = 386)0 (0)8 (2.1)1 (0.3)0 (0)0 (0)0 (0)0 (0)1 (0.3)10 (2.6)
Mandible (n = 4,439)10 (0.2)368 (8.3)42 (0.9)14 (0.3)0 (0)0 (0)5 (0.1)9 (0.2)448 (10.1)
First incisor (n = 384)0 (0)9 (2.3)2 (0.5)0 (0)0 (0)0 (0)0 (0)0 (0)11 (2.9)
Second incisor (n = 408)0 (0)10 (2.5)1 (0.2)0 (0)0 (0)0 (0)0 (0)0 (0)11 (2.7)
Third incisor (n = 405)0 (0)11 (2.7)2 (0.5)0 (0)0 (0)0 (0)0 (0)0 (0)13 (3.2)
Canine (n = 444)0 (0)19 (4.3)4 (0.9)1 (0.2)0 (0)0 (0)1 (0.2)2 (0.5)27 (6.1)
First premolar (n = 379)4 (1.1)62 (16.4)2 (0.5)0 (0)0 (0)0 (0)0 (0)4 (1.1)72 (19)
Second premolar (n = 391)1 (0.3)69 (17.6)9 (2.3)5 (1.3)0 (0)0 (0)2 (0.5)2 (0.5)88 (22.5)
Third premolar (n = 434)3 (0.7)68 (15.7)4 (0.9)2 (0.5)0 (0)0 (0)1 (0.2)0 (0)78 (18)
Fourth premolar (n = 407)1 (0.2)74 (18.2)4 (1)1 (0.2)0 (0)0 (0)0 (0)1 (0.2)81 (19.9)
First molar (n = 437)1 (0.2)40 (9.2)4 (0.9)4 (0.9)0 (0)0 (0)1 (0.2)0 (0)50 (11.4)
Second molar (n = 419)0 (0)2 (0.5)9 (2.1)1 (0.2)0 (0)0 (0)0 (0)0 (0)12 (2.9)
Third molar (n = 331)0 (0)4 (1.2)1 (0.3)0 (0)0 (0)0 (0)0 (0)0 (0)5 (1.5)

Values reported are number (%) of teeth. n = Number of teeth.

Types of tooth resorption—Every type of tooth resorption described in the classification method for humans,19 except for internal replacement resorption, was found in the dogs reported here. In decreasing order, the prevalence of each type of tooth resorption was external replacement resorption (77 [34.4%] dogs), external inflammatory resorption (58 [25.9%] dogs), external cervical root surface resorption (13 [5.8%] dogs), external surface resorption (10 [4.5%] dogs), internal inflammatory resorption (9 [4.0%] dogs), and internal surface resorption (1 [0.4%] dog; Table 3). Of the 120 dogs with tooth resorption, 77 (64.2%) had only 1 type of resorption, 30 (25.0%) had 2 types of resorption, 11 (9.2%) had 3 types of resorption, and 2 (1.7%) had 4 types of resorption. Tooth resorption was evident but considered unclassifiable in 10 of the 120 (8.3%) affected dogs and in 35 of the 943 (3.7%) affected teeth.

Table 3—

Mean ± SD age and number of teeth affected per dog for various types of tooth resorption.

Type of tooth resorptionNo. (%) of dogs affectedAge (y)No. of affected teeth/dog
External surface resorption10 (4.5)7.7 ± 3.01.7 ± 1.6
External replacement resorption77 (34.4)9.7 ± 3.29.7 ± 8.3
External inflammatory resorption58 (25.9)8.5 ± 3.52.1 ± 2.0
External cervical root surface resorption13 (5.8)10.2 ± 4.31.6 ± 1.4
Internal surface resorption1 (0.4)1*7*
Internal replacement resorption0 (0)NANA
Internal inflammatory resorption9 (4.0)9.1 ± 3.71.3 ± 0.5

Represents the result for the only dog in the category. NA = Not applicable.

External surface resorption was detected in 17 of 8,478 (0.2%) teeth, which represented 7 of 4,039 (0.2%) maxillary teeth and 10 of 4,439 (0.2%) mandibular teeth. Affected maxillary teeth included the second and third incisor teeth and the second, third, and fourth premolar teeth, with frequency ranging from 1 of 368 (0.3%) for the first premolar tooth to 3 of 413 (0.7%) for the fourth premolar tooth. In the mandible, affected teeth included all 4 premolar teeth and the first molar tooth, with the frequency ranging from 1 of 407 (0.2%) for the first molar tooth to 4 of 379 (1.1%) for the first premolar tooth. External surface resorption was found in 1 to 6 teeth/affected dog (mean ± SD, 1.7 ± 1.6 teeth/affected dog). Only 2 of 17 (11.8%) lesions were in teeth with concurrent alveolar bone loss, although in both cases, this was considered mild (bone loss < 25%). Two (11.8%) other lesions were in teeth that had concurrent endodontal disease (ie, complicated crown fractures). One (5.9%) tooth had radiographic evidence of abrasion or attrition.

Frequency of external surface resorption increased significantly (P = 0.006) in proportion to body weight of the dogs (group 0, 1/101 [1.0%] dogs; group 1, 3/53 [5.7%] dogs; group 2, 4/62 [6.5%] dogs; and group 3, 2/8 [25.0%] dogs). Age did not appear to affect the frequency of external surface resorption because the frequency ranged from 1 of 51 (2.0%) dogs in age group 1 to 5 of 71 (7.0%) dogs in age group 2; this type of tooth resorption was not detected in age group 5. Dogs with external surface resorption ranged from 3 to 14 years of age (mean ± SD, 7.7 ± 3.0 years). The frequency of this type of tooth resorption did not differ significantly (P = 0.85) among dogs on the basis of sex category (sexually intact males, 1/28 [3.6%]; castrated males, 4/89 [4.5%]; sexually intact females, 0/15 [0%]; and spayed females, 5/92 [5.4%]).

External replacement resorption was detected in 736 of 8,478 (8.7%) teeth, which represented 368 of 4,039 (9.1%) maxillary teeth and 368 of 4,439 (8.3%) mandibular teeth. The maxillary tooth most frequently affected was the first premolar tooth (66/368 [17.9%]), whereas the maxillary teeth least frequently affected were the first (8/413 [1.9%]) and second (8/386 [2.1%]) molar teeth; the frequency for all other maxillary teeth ranged from 25 of 380 (6.6%) to 59 of 439 (13.4%). In the mandible, the tooth most frequently affected was the fourth premolar tooth (74/407 [18.2%]), whereas the teeth least frequently affected were the second (2/419 [0.5%]) and third (4/331 [1.2%]) molar teeth. The frequency for the mandibular incisor teeth ranged from 9 of 384 (2.3%) to 11 of 405 (2.7%), was 19 of 444 (4.3%) for the mandibular canine tooth, and ranged from 40 of 437 (9.2%) to 69 of 391 (17.6%) for the remainder of the mandibular teeth. External replacement resorption was detected in 1 to 38 teeth/affected dog (mean ± SD, 9.7 ± 8.3 teeth/affected dog).

Alveolar bone loss was inversely related to the frequency of external replacement resorption. Of the 736 affected teeth, 576 (78.3%) had no radiographic evidence of alveolar bone loss. Of 160 (21.7%) teeth with alveolar bone loss, 133 (18 .1%) had alveolar bone loss of < 25%, 25 (3.4%) had alveolar bone loss of 25% to 50%, and 2 (0.3%) had alveolar bone loss of > 50%.

Regarding nonperiodontal dental findings, 649 (88.2%) teeth appeared grossly normal; evidence of tooth wear indicative of abrasion or attrition was identified in 42 (5.7%) teeth, and 45 (6.1%) teeth had miscellaneous dental conditions (eg, crown fractures or root fractures).

Among age groups 0 to 4, there was a significant (P < 0.001) positive association with external replacement resorption. In age group 0, only 1 of 32 (3.1%) dogs had this type of tooth resorption, whereas 25 of 48 (52.1%) and 13 of 18 (72.2%) dogs had this type of lesion in age groups 3 and 4, respectively. Only 2 of 4 (50.0%) dogs in age group 5 had external replacement resorption. Age of dogs with this type of tooth resorption ranged from 1 to 17 years (mean ± SD, 9.7 ± 3.2 years). Body weight groups were also significantly (P = 0.003) positively associated with external replacement resorption (group 0, 21/101 [20.8%] dogs; group 1, 20/53 [37.7%] dogs; group 2, 30/62 [48.4%] dogs, and group 3, 6/8 [75.0%] dogs). This type of tooth resorption was detected in 45 of 117 (38.5%) males and 32 of 107 (29.9%) females; these proportions did not differ significantly (P = 0.21). Sexually intact males had this type of tooth resorption less frequently than did castrated males (7/28 [25.0%] vs 38/89 [42.7%] dogs, respectively); however, these proportions did not differ significantly (P = 0.12). Sexually intact females had this type of tooth resorption less frequently than did spayed females (2/15 [13.3%] vs 30/92 [32.6%] dogs, respectively); however, these proportions did not differ significantly (P = 0.22).

External inflammatory resorption was detected in 121 of 8,478 (1.4%) teeth, which represented 79 of 4,039 (2.0%) maxillary teeth and 42 of 4,439 (0.9%) mandibular teeth. The maxillary tooth most frequently affected was the fourth premolar tooth (29/413 [7.0%]), and the maxillary teeth least frequently affected were the third incisor tooth (1/427 [0.2%]) and second molar tooth (1/386 [0.3%]); the frequency in all other maxillary teeth ranged from 3 of 368 (0.8%) to 10 of 413 (2.4%). The mandibular teeth most frequently affected were the second premolar tooth (9/391 [2.3%]) and second molar tooth (9/419 [2.1%]); the frequency among the remainder of the mandibular teeth ranged from 1 of 408 (0.2%) to 4 of 407 (1.0%). This type of tooth resorption was found in 1 to 13 teeth/affected dog (mean ± SD, 2.1 ± 2.0 teeth/affected dog). External inflammatory resorption was associated with alveolar bone loss (61/121 [50.4%] affected teeth), endodontal disease (48/121 [39.7%] affected teeth), and a combination of endodontal and periodontal disease (12/121 [9.9%] affected teeth).

Among the age groups, external inflammatory resorption was significantly (P = 0.039) more common in older dogs, with the frequency ranging from 5 of 18 (27.8%) for group 4 to 2 of 4 (50.0%) for group 5. In the younger dogs of groups 0 and 1, the frequency was 6 of 32 (18.8%) and 7 of 51 (13.7%), respectively. Age of dogs with this type of tooth resorption ranged from 1 to 17 years (mean ± SD, 8.5 ± 3.5 years). There was not a significant (P = 0.16) association between body weight and external inflammatory resorption. Frequency of external inflammatory resorption varied from 11 of 53 (20.8%) to 4 of 8 (50.0%) for body weight groups 1 and 3, respectively. Castrated males and sexually intact females had this type of lesion most frequently (29/89 [32.6%] and 4/15 [26.7%] dogs, respectively). The frequency among sexually intact males and spayed females was 5 of 28 (17.9%) and 20 of 92 (21.7%), respectively. However, proportions did not differ significantly (P = 0.28) among these 4 sex categories.

External cervical root surface resorption was detected in 24 of 8,478 (0.3%) teeth, which represented 7 of 4,039 (0.2%) maxillary teeth and 14 of 4,439 (0.3%) mandibular teeth. In the maxilla, this type of tooth resorption was detected in the canine tooth (1/439 [0.2%] teeth) and the second (4/380 [1.1%] teeth) and fourth (2/413 [0.5%] teeth) premolar teeth. In the mandible, it was detected in the canine tooth (1/444 [0.2%] teeth); the second (5/391 [1.3%] teeth), third (2/434 [0.5%] teeth), and fourth (1/407 [0.2%] teeth) premolar teeth; and the first (4/437 [0.9%] teeth) and second (1/419 [0.2%] teeth) molar teeth. This type of tooth resorption affected 1 to 6 teeth/affected dog (mean ± SD, 1.6 ± 1.4 teeth/affected dog). No concurrent dental conditions were found on teeth affected with this type of tooth resorption.

External cervical root surface resorption was most frequently detected in dogs of body weight group 1 (5/53 [9.4%] dogs). No lesions were found among the dogs in body weight group 3. Body weight groups 0 and 2 had a frequency of 6 of 101 (5.9%) dogs and 3 of 62 (4.8%) dogs, respectively. Among age groups, frequency varied between 2 of 51 (3.9%) and 5 of 71 (7.0%) for groups 1 and 2; no lesions were found in age group 0, and age group 5 had the highest frequency with 2 of 4 (50%) dogs affected. Dogs with external cervical root surface resorption ranged from 5 to 19 years of age (mean ± SD, 10.2 ± 4.3 years). Among sex categories, those most frequently affected were castrated males (8/89 [9.0%] dogs), followed by sexually intact females (1/15 [6.7%] dogs), sexually intact males (1/28 [3.6%] dogs), and spayed females (3/92 [3.3%] dogs); however, these proportions did not differ significantly (P = 0.39).

Internal surface resorption was detected only in 1 maxillary tooth (a fourth premolar tooth) of a 7-year-old castrated male dog in body weight group 2. The tooth had no signs of concurrent periodontal or endodontal problems. Internal inflammatory resorption was detected in 12 of 8,478 (0.1%) teeth, which represented 7 of 4,039 (0.2%) maxillary teeth and 5 of 4,439 (0.1%) mandibular teeth. Affected maxillary teeth included the first (1/368 [0.3%] teeth), second (3/380 [0.8%] teeth), third (2/402 [0.5%] teeth), and fourth (1/413 [0.2%] teeth) premolar teeth; no other maxillary teeth were affected. Affected mandibular teeth included the canine tooth (1/444 [0.2%] teeth), the second (2/391 [0.5%] teeth) and third (1/434 [0.2%] teeth) premolar teeth, and the first molar tooth (1/437 [0.2%] teeth); no other mandibular teeth were affected. Number of teeth with this type of tooth resorption ranged from 1 to 2/affected dog (mean ± SD, 1.3 ± 0.5 teeth/affected dog). One (8.3%) of the affected teeth had concurrent endodontal disease (complicated crown fracture), whereas 7 (58.3%) affected teeth had concurrent periodontal disease (ie, alveolar bone loss).

Internal inflammatory resorption and body weight were not significantly associated; the proportions of dogs with this type of lesion were as follows: group 0, 5/101 (5.0%) dogs; group 1, 1/53 (1.9%) dogs; and group 2, 3/62 (4.8%) dogs. No lesions were detected in dogs of group 3. Among the age groups, this type of tooth resorption was detected in 3 of 51 (5.9%) dogs in group 1, 5/48 (10.4%) dogs in group 3, and 1/18 (5.6%) dogs in group 4. Age of dogs with this type of tooth resorption ranged from 4 to 13 years (mean ± SD, 9.1 ± 3.7 years). Six of 117 (5.1%) males had this type of tooth resorption; of these, 1 (3.6%) was a sexually intact male and 5 (5.6%) were castrated males. Of the 107 females, 3 (2.8%) had internal inflammatory resorption; all 3 (3.3%) dogs were spayed females.

Of the 35 teeth that were considered to be unclassifiable, 15 were in a dog with a history of radiotherapy for a nasal lymphoma 2 years prior to admission to the study reported here. Another 6 unclassifiable teeth were adjacent to bilateral mandibular dentigerous cysts associated with unerupted first premolar teeth in 1 dog. The remaining 14 teeth were detected in 8 dogs (11 [78.6%] were maxillary teeth and 3 [21.4%] were mandibular teeth). Of these 14 teeth, 6 (42.9%) had evidence of concurrent periodontal disease, 1 (7.1%) had endodontal disease, and 1 (7.1%) was associated with an odontogenic tumor.

Discussion

The radiographic and epidemiological aspects of tooth resorption have been described in cats9–13,20,21,24–26 and humans,14–17,19 but only limited information is available for dogs.4–8,23 Although tooth resorption can sometimes be detected clinically, the diagnosis typically is made on the basis of radiographic findings.10,16,19 In the study reported here, we evaluated radiographic patterns of tooth resorption in dogs and determined that the classification criteria described for tooth resorption in humans19 are applicable. In addition, we provided epidemiological data that indicate these lesions are common in this species.

Given the fact that the study group comprised dogs admitted to a veterinary dental referral service, the reported prevalence may be overestimated, compared with the prevalence for the general canine population. Similarly, epidemiological evaluation of populations of healthy cats has revealed a lower prevalence of tooth resorption when compared with that in animals examined at dental referral services.21 However, as reported here, not all types of tooth resorption are associated with concurrent common dental conditions (eg, endodontal and periodontal disease) or tooth wear (ie, external replacement resorption). Moreover, the prevalences of periodontal and endodontal conditions found in the general canine population, and that could be associated with certain types of tooth resorption (ie, external inflammatory resorption), are generally considered high8,23 and probably do not differ substantially from those of the group of dogs used in our study.

Regardless of the type of tooth resorption, the frequency of tooth resorption in dogs in the study reported here was high (120/224 [53.6%] dogs and 943/8,478 [11.1%] teeth). These results are consistent with those reported in another study8 in which investigators found tooth resorption in up to 43.5% of randomly selected canine cadaver specimens. Although the frequency of tooth resorption did not differ significantly between males and females or between sexually intact and spayed females, castrated males had a significantly higher frequency of tooth resorption than did the sexually intact males. It is unknown whether hormonal differences between sexually intact and castrated animals play a role in the pathogenesis of tooth resorption in male dogs. Consistent with results of the study reported here, investigators in that other study8 detected a positive association between tooth resorption and age; however, the different types of tooth resorption were not determined, and their possible association with body weight and sex of the animals was not investigated. Unexpectedly, we found no significant difference in the number of teeth in affected and nonaffected dogs. It can be inferred that the possible progression of the condition is slow relative to the life span of dogs and thus seldom results in the loss of affected teeth.

Frequency of tooth resorption differed significantly between maxillary and mandibular teeth. This can be explained by significant differences among individual teeth. For example, there was a significant difference between tooth resorption in mandibular and maxillary canine teeth. Also, a significantly lower frequency of tooth resorption in mandibular incisor teeth was detected, compared with the frequency in their maxillary counterparts. It is unknown whether these differences reflected a lower propensity of these teeth to develop tooth resorption or whether the relative low frequencies detected were the result of radiographic limitations associated with the close proximity and overlapping of the roots of mandibular incisor teeth. Such radiographic limitation does not apply to mandibular canine teeth; thus, functional and anatomic factors are more likely to play a role. The latter could also explain the fact that the 2 teeth most frequently affected were the first premolar tooth in the maxilla (73/368 [19.8%]) and the second premolar tooth in the mandible (88/391 [22.5%]), in contrast to the frequency for the maxillary and mandibular molar teeth excluding the mandibular first molar (the highest of which was the maxillary first molar tooth [15/413 {3.6%}]).

We found that the classification criteria described for use in humans19 were applicable in 908 of 943 (96.3%) of the affected teeth. Other classification methods have been proposed in human dentistry14–16; however, the classification method used in that study19 is the most widely accepted.14

By far, the 2 most common types of tooth resorption in the 8,478 teeth of the 224 dogs were external replacement and external inflammatory resorption, which represented 77 (34.4%) and 58 (25.9%) dogs, respectively, and 736 (8.7%) and 121 (1.4%) teeth, respectively. In comparison, the number of dogs with other types of tooth resorption was considerably lower: external cervical root surface resorption, 13 (5.8%); external surface resorption, 10 (4.5%); internal inflammatory resorption, 9 (4%); and internal surface resorption, 1 (0.4%).

Except for external replacement resorption, these findings were not surprising, compared with the findings in humans. For example, all teeth with endodontal disease in humans are assumed to have a certain degree of apical inflammatory root resorption (evident in approx 70% of teeth with endodontal disease).16 Periodontal disease can also lead to this type of tooth resorption.15,27 Because these 2 conditions are common in dogs, it is expected that external inflammatory resorption is a frequent event. Conversely, external surface resorption is considered a reversible and infrequent finding in humans. Because of the relatively small size of the lesions, they are not always detected during examination of radiographs.16,19 Our findings indicate that they are also infrequently encountered in dogs. Similarly, although internal resorption is typically associated with endodontal disease, it is considered a rare condition in humans with permanent dentition.16,19 The results of the study reported here indicate that in dogs, internal tooth resorption is also an infrequent condition.

Interestingly and in contrast with our findings, external replacement resorption is considered an infrequent condition in humans and usually has a poor long-term prognosis because of its progressive nature and the limited treatment options.14–17,19 It is believed to be the result of necrosis of the periodontal ligament and is usually associated with intrusive injuries and delayed replantation of avulsed teeth.16 Contrary to findings in humans, a study21 in a colony of 109 healthy cats found that external replacement resorption accounted for 60% of all the teeth with tooth resorption. This would suggest that cats and dogs share risk factors and the etiopathogenesis associated with this type of tooth resorption.

A significant positive association was detected between external replacement resorption, age, and body weight of the affected dogs. Similar to results of another study,4 we did not detect an association with concurrent dental disease. Because this type of tooth resorption was found to affect multiple teeth in the same dog but rarely affected molar teeth, it can be inferred that both systemic (ie, age-related) and local (ie, occlusal surfaces or increased bite force in larger dogs) factors may be associated with this type of tooth resorption.

Despite its low frequency and similar to external replacement resorption, external surface resorption was positively associated with body weight, had no association with sex category, and was found more frequently in maxillary and mandibular premolar teeth. Moreover, external surface resorption was often found in dogs with external replacement resorption that affected other teeth. One difference was that the mean age of the affected dogs (7.7 ± 3.0 years) was lower than that of dogs with external replacement resorption (9.7 ± 3.2 years). These findings suggested that contrary to the situation in humans in which this type of tooth resorption is usually considered reversible,16,17,19 it represents the initial stages of what will probably progress to external replacement resorption in dogs.

External inflammatory resorption was always associated with periodontal disease, endodontal disease, or a combination of both, and the frequency of this type of tooth resorption increased with age. The maxillary fourth premolar tooth is a tooth that is often endodontally compromised in dogs, and not surprisingly, it was by far the most frequently affected tooth in this study. These findings are consistent with reported causes of external inflammatory root resorption in humans.15,27 The frequent occurrence of this type of tooth resorption is consistent with the prevalent nature of endodontal and periodontal disease in adult dogs.

Although external cervical root surface resorption has been described in dogs,5,7 the prevalence of this type of tooth resorption in dogs is unknown. In humans, it is considered a rare condition by some28,29; however, others believe that this type of tooth resorption is underdiagnosed because it is easily clinically mistaken for internal resorption as a result of the common pink appearance of the crowns of affected teeth.14,16,17 The causes of this type of tooth resorption remain unknown, but the condition typically is progressive and aggressive, which oftentimes leads to massive loss of tooth substance and eventually involves the pulp cavity.14,16,17,30–32 To our knowledge, this is the first report on the prevalence of this type of tooth resorption in dogs. There was an inconsistent distribution among the age groups, and no sex category or body weight predilection was detected. The mean ± SD age of affected dogs was 10.2 ± 4.3 years. Affected teeth included maxillary premolar and canine teeth and mandibular premolar, molar, and canine teeth. Because of the low frequency of these lesions (13/224 [5.8%] dogs), it cannot be ruled out that other teeth not reported here are equally susceptible to this type of tooth resorption. The mean number of affected teeth per dog was 1.6 ± 1.4, but the range (1 to 6 affected teeth/dog) reveals that multiple teeth may be affected in the same dog, as has been described elsewhere.5,7

Internal resorption traditionally has been associated with chronic endodontal disease.16,17 Consistent with the low frequency found in dogs in the study reported here, this type of tooth resorption is reported to be rare in humans.16 Of the 3 subcategories, internal surface resorption has been described as the only transient condition (lasting a few months until resolution) and is typically the result of luxation injuries.19 Only 1 tooth was affected with this type of internal tooth resorption in the dogs of our study, and not surprisingly, that tooth had no signs of endodontal or periodontal disease, which makes a historical luxation injury the most likely cause. The second category of internal tooth resorption, internal replacement resorption, was not detected in any of the dogs in this study; therefore, it is assumed to be a rare condition. Finally, inflammatory root resorption, which in humans is typically associated with necrotic pulp,14,16,19 was the subcategory that was most commonly detected. However, we found this type of tooth resorption to be more frequently associated with periodontal disease than with radiographically evident endodontal conditions. It is likely that the periodontally compromised teeth had concurrent pulp involvement that was not evident at the time of diagnosis. Similar to external inflammatory resorption, no association with age, sex, or body weight was detected. In addition, the mean ± SD age of affected dogs was 9.1 ± 3.7 years, and the mean number of teeth affected per dog was 1.3 ± 0.5.

Although there were 35 teeth with radiographic signs of tooth resorption that were considered unclassifiable on the basis of the criteria used for humans,19 some of these corresponded to types of tooth resorption that have been described by other authors. For example, tooth resorption in the 6 teeth associated with dentigerous cysts could be considered pressure resorption by some authors.14,15 The atypical tooth resorption patterns seen in the 15 teeth of the dog with a history of nasal lymphoma are most likely a consequence of local osteoclastic activity as an adverse effect of radiotherapy. A similar case has been described in the literature.33 The remaining unclassifiable 14 teeth were assumed to be the result of the inherent limitations of conventional radiography when localizing resorption lesions in a tooth.34,35 Although in typical clinical circumstances the diagnosis of tooth resorption is based on radiographic findings,19 such a diagnostic tool has limitations.34,35 For example, early lesions may not be detectable by use of conventional radiography.36 Also, certain anatomic variations or developmental abnormalities of teeth may radiographically resemble tooth resorption. When tooth resorption is detected, it may be necessary to make comparisons with previously obtained radiographs or with additional radiographic views of the tooth in question. When diagnosing tooth resorption, the use of parallel radiographic techniques is preferred because radiographs obtained via the bisecting-angle technique may not be reproducible.34,35

Previous radiographs were generally not available for the dogs included in this study, and the parallel intraoral radiographic technique is only applicable to caudal mandibular premolar and molar teeth in dogs.37 Therefore, to minimize erroneous diagnoses or misclas-sification, all tooth resorptions were evaluated in conjunction with the medical records of each dog as well as with other dental abnormalities found during oral examination and examination of intraoral radiographs. When questionable results were obtained, additional radiographic views were obtained.

We conclude that the radiographic criteria described for the classification of tooth resorption in humans19 are applicable to dogs. Resorption lesions in general, and external replacement and external inflammatory resorption in particular, are frequent conditions in this species.

ABBREVIATIONS

AVDC

American Veterinary Dental College

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Appendix

Descriptions of the types of tooth resorption in dogs based on a classification method used for humans.19

Type of tooth resorption 
External surface resorptionNot always visible on radiographs. Radiographically characterized by shallow resorption lacunae that affect the cementum and dentin. Lesions are usually located along the lateral margins of the root, and the periodontal ligament space and lamina dura are not affected. In humans, this type of tooth resorption may be the result of mild trauma to the tooth, and it is considered a self-limiting process. No clinical signs are associated with this type of tooth resorption.
External replacement resorptionEvident radiographically as a gradual disappearance of the periodontal ligament space with progressive replacement of root tissues by the surrounding alveolar bone. It is associated with injuries that lead to necrosis of the periodontal ligament fibers. In humans, it primarily causes no clinical signs; it is considered an untreatable condition, and the long-term prognosis is poor.
External inflammatory resorptionRadiographic characteristic is loss of dental tissues adjacent to areas of loss of alveolar bone secondary to inflammatory conditions (ie, endodontal or periodontal disease). All endodontally compromised teeth are considered to have some degree of external inflammatory resorption. Treatment is targeted to resolve the underlying disease process.
External cervical root surface resorptionRadiographically characterized by an invasive resorption process that starts at the cervical area of the tooth and invades in both the coronal and apical directions, sometimes creating tunnels along affected tissues. There may be massive loss of tooth substance, and the crown may appear pink, which often leads to a clinical misdiagnosis of internal resorption. Treatment requires thorough debridement of the affected areas and restoration with or without endodontal therapy. The cause of this type of tooth resorption is unclear.
Internal surface resorptionRadiographically characterized by an oval-shaped enlargement located in the apical third of the root canal. It is interpreted as a sign of active revascularization and is considered a self-limiting process that results from mild traumatic injuries.
Internal replacement resorptionRadiographically characterized by an irregular enlargement with a tunnel-like appearance adjacent to the root canal. The lesion typically is located on the coronal fragment of root fractures, although it may also result from luxation injuries. It is not considered a progressive condition, and no specific treatment is indicated.
Internal inflammatory resorptionRadiographically characterized by an oval-shaped enlargement typically found in the cervical third of the root canal. It is the result of inflammatory conditions of the pulp (ie, endodontal disease), and treatment is targeted to resolve the underlying disease process.

Contributor Notes

Dr. Peralta's present address is Servicio de Odontología y Cirugía Oral Veterinaria, Carrera 15 No. 36–16, Bogotá, Colombia.

Address correspondence to Dr. Verstraete (fjverstraete@ucdavis.edu).