Introduction
Individuals with Pierre Robin sequence (PRS) frequently have tooth agenesis, especially in the mandible. The aims of this study were to characterize permanent tooth agenesis patterns and to determine their prevalence in patients with nonsyndromic PRS.
Methods
Radiographs and clinic charts of 146 children with nonsyndromic PRS were examined for permanent tooth agenesis, excluding third molars, and the tooth agenesis patterns were identified with the tooth agenesis code.
Results
The overall prevalence of permanent tooth agenesis was 32.9% (48 of 146 patients), with about two thirds having bilateral tooth agenesis. No sex or racial predilection was found for tooth agenesis. The most common tooth agenesis pattern was the absence of both mandibular second premolars. Among the children with tooth agenesis, almost half had agenesis patterns involving both mandibular second premolars. Other common patterns of permanent tooth agenesis involved the maxillary lateral incisors and the maxillary premolars.
Conclusions
Symmetric agenesis of individual tooth types occurs much more frequently in the mandible than in the maxilla in patients with nonsyndromic PRS. Bilateral absence of the mandibular second premolars is the predominant pattern of dental agenesis.
Robin described what is known today as Pierre Robin sequence (PRS) as the triad of micrognathia, glossoptosis, and resultant airway obstruction. The prevalence of PRS has been reported to be between 1 in 8500 and 1 in 20,000 births. A universal consensus on the exact definition of PRS is lacking; however, in current practice, this congenital anomaly is commonly recognized by the triad of micrognathia, cleft palate, and severe respiratory distress caused by airway obstruction in the neonatal period. Tooth agenesis (or hypodontia) is also found commonly in those with PRS.
The prevalence of agenesis of permanent teeth in the general population, excluding third molars, according to a recent meta-analysis, ranges from 3.2% to 7.6%. This prevalence varies according to the tooth type; the mandibular second premolar is most commonly affected (3.1%), followed by the maxillary lateral incisor (1.7%) and the maxillary second premolar (1.5%). Among people with tooth agenesis, 1 tooth is missing in 48%, 2 teeth are missing in 35%, and more than 2 teeth are missing in 17%. Bilateral agenesis of the maxillary lateral incisors occurs more frequently than unilateral agenesis. On the contrary, bilateral agenesis of the mandibular second premolars occurs less frequently than unilateral agenesis. Racial and regional differences in the prevalence of tooth agenesis have been observed; these range from a mean prevalence of 2.5% for a Saudi Arabian population to 3.9% for both North American white and African American populations, 5.5% for a European white population, 6.3% for an Australian white population, and 6.9% for a Chinese population.
In patients with PRS, a much higher prevalence of permanent tooth agenesis has been reported by several authors, with an approximate range of 30% to 50%. In a group of 56 Finnish children with PRS, the prevalence of hypodontia, excluding third molars, was shown to be 50%. In this sample, the mandible was more frequently affected than the maxilla. In a group of 36 patients with PRS in their deciduous dentition, Amaratunga reported that 27.7% had agenesis of a maxillary deciduous lateral incisor. In a group of 14 patients with PRS, Larson et al found a prevalence of hypodontia of approximately 30%. Ross et al found a prevalence of 32.1% in patients with PRS. In a group of 34 Norwegian patients with PRS over the age of 10 years, Andersson et al reported hypodontia in 44.1%, with 80.8% of the missing teeth in the mandible. Of the 26.5% of the subjects with PRS with hypodontia in that study, 60% were missing both mandibular second premolars. According to Laitinen and Ranta, the most commonly congenitally missing teeth in patients with PRS are the mandibular second premolars.
A recent classification system, identifying and coding tooth agenesis patterns, has been developed and called the tooth agenesis code (TAC). The TAC uses a binary system whereby a specific value is assigned to each missing tooth, and a unique value for each tooth agenesis pattern is obtained. The TAC has generated increasing interest in recent years in identifying common patterns of tooth agenesis. Identifying tooth agenesis patterns has clinical interest because subphenotyping of patients with a particular anomaly based on dental development characteristics might identify characteristic subgroups. This could subsequently help to determine specific genetic contributions. We aimed to characterize permanent tooth agenesis patterns and their prevalence in children with nonsyndromic PRS.
Material and methods
After approval by the institutional research ethics board, a retrospective chart review was undertaken to identify treatment charts and records of nonsyndromic children with PRS born between 1970 and 2006 whose diagnostic records would allow the identification of permanent tooth agenesis. Diagnostic records included dental history and examination notes, and panoramic or periapical series radiographs taken at the age of 6 years and older, which were available in the archives of the craniofacial center of the Hospital for Sick Children, Toronto. Exclusion criteria were the following: children with other craniofacial anomalies or syndromes in addition to PRS (such as Stickler’s or velocardiofacial syndrome), children who had undergone neonatal mandibular distraction osteogenesis (because tooth bud injury can be associated with this procedure), children whose radiographs were not of diagnostic quality, children who had unclear notes in their medical records regarding whether permanent teeth had been extracted because of dental decay or orthodontic treatment or after trauma, and children whose diagnostic records showed persistent doubt regarding tooth agenesis. Data on sex and race were collected for each child. These data were categorized according to racial groups based on the guidelines published by the US Department of Health and Human Services Food and Drug Administration into the following racial groups: American Indian or Alaska Native, Asian, black, Native Hawaiian or other Pacific Islander, and white.
This study was conducted in one of the largest craniofacial centers in the world, where an interdisciplinary team has functioned for many decades. Due to the nosologic confusion surrounding the classification of PRS, for the purposes of this study, PRS was defined as a cleft palate along with a micrognathic mandible at birth and respiratory distress caused by airway obstruction in the neonatal period. With the participation of experts from many disciplines involved in craniofacial care and treatment of patients at the center, it is reasonable that the sample consisted of children who had been accurately diagnosed with nonsyndromic PRS. Although this was a hospital-based sample of patients who had received craniofacial treatment, it was a fair representation of children with PRS because it was a large sample and, under a government-sponsored program, all children born with clefts and craniofacial anomalies in the large province of Ontario in Canada are supported for receiving craniofacial treatment, which also includes orthodontic and dental treatment. Thus, most children born with PRS in this province would most likely have been examined or have received treatment at the craniofacial center of the hospital.
A thorough radiographic examination (using all available panoramic, periapical, and occlusal radiographs) of eligible subjects was conducted to confirm tooth agenesis in the permanent dentition. Permanent tooth agenesis (hypodontia) in this study was recognized as the congenital absence of at least 1 permanent tooth, other than the third molar. Agenesis was diagnosed based on the lack of any differentially calcified tissue (pointing to enamel or dentin) in the area of the corresponding tooth. Sequential radiographs were used when available to confirm tooth agenesis. Although odontogenesis of the second premolars begins in most children at the age of 3 to 3½ years, this has been found to vary widely; therefore, agenesis of these teeth can be more accurately confirmed by the age of 8 or 9 years. Ranta observed a delay in the formation of the second premolars in patients with clefts; this was even greater in those with clefts and hypodontia. Because premolar tooth buds can sometimes develop late, in our study, if agenesis of these teeth was determined from radiographs taken before the age of 10 years, it was confirmed only if the contralateral tooth was already present, or if the tooth buds of the permanent second molars, which normally form after premolars, could be visualized, according to the methods described by de Lima Pedro et al. Findings from the radiographic examinations were confirmed by evaluation of the patients’ charts, intraoral photographs, and plaster models (dental casts). All records were examined by the same investigator (G.S.A.).
Patterns of tooth agenesis were identified and recorded using the TAC. The TAC uses a binary system: 0 denotes presence and 1 denotes absence of a tooth, and a specific value is assigned to each missing tooth. This specific value corresponding to a missing tooth is determined by calculating 2 (n–1) , in which n is the tooth number. The teeth are numbered 1 to 8: 1 represents a central incisor and 8 a third molar. The sum of these values is calculated for each quadrant, representing a unique value for each tooth agenesis pattern, the TAC. According to this system, a quadrant without tooth agenesis would have a TAC value of 0, and a quadrant with complete tooth agenesis would have a TAC value of 255 ( Table I ). Patterns of tooth agenesis of the whole dentition of a subject are described using the TAC overall , which is composed of the TAC value for each quadrant, providing a unique number for the particular tooth agenesis pattern. The TAC overall score is presented in the format TAC q1 , TAC q2 , TAC q3 , and TAC q4 , representing the first to fourth quadrants (q1-q4) of the dentition.
| Maxilla right (q1) | Maxilla left (q2) | |||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| A | 18 ∗ | 17 | 16 | 15 | 14 | 13 | 12 | 11 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28 ∗ |
| B | 128 | 64 | 32 | 16 | 8 | 4 | 2 | 1 | 1 | 2 | 4 | 8 | 16 | 32 | 64 | 128 |
| A | 48 ∗ | 47 | 46 | 45 | 44 | 43 | 42 | 41 | 31 | 32 | 33 | 34 | 35 | 36 | 37 | 38 ∗ |
| Mandible right (q4) | Mandible left (q3) | |||||||||||||||
Statistical analysis
All data were analyzed using the Statistical Package for the Social Sciences (version 21.0 for Windows; SPSS, Chicago, Ill). Frequencies (prevalence rates) and distributions were calculated for the patterns of tooth agenesis. Chi-square and Fisher exact tests were used to investigate statistically significant differences between the prevalence of tooth agenesis and sex, race, side (left vs right), and jaw (maxilla vs mandible).
Intraexaminer reliability was assessed using kappa statistics, having the same investigator reevaluate 20 randomly selected patients after a 1-week interval. Agreement was calculated for the observations of missing teeth. No disagreements between the first and second examinations were found (kappa value, 1.00).
Results
A total of 318 children born between 1970 and 2006, who had been diagnosed with PRS, were identified. Of these, 146 children (76 boys, 70 girls) met the inclusion criteria. Of these subjects, 3 (2.1%) were born in the 1970s, 32 (21.9%) in the 1980s, 84 (57.5%) in the 1990s, and 27 (18.5%) in the 2000s. The mean age at radiographic examination was 11.1 years. Among these, 118 (80.8%) children were white, 20 (13.7%) were Asian, and 8 (5.5%) were black. Because most of the children were white, analyses were carried out for the total sample as well as for the white subgroup.
The overall prevalence of tooth agenesis in the total sample was 32.9% (48 of 146 patients). No significant differences were found when comparing the sexes, both of which showed a prevalence of tooth agenesis of 32.9% (23 of 70 boys and 25 of 76 girls). When comparing children of different racial origins, no significant differences in the prevalence of tooth agenesis were found. The prevalence of tooth agenesis in each racial group was the following: white (36 of 118, or 30.5%), Asian (9 of 20, or 45.0%), and black (3 of 8, or 37.5%). Similarly, no significant differences were found when comparing patients born in different decades of the time period of data collection.
The number of missing teeth ranged from 1 to 6. Of the 48 patients who had hypodontia in the total sample of 146 patients, 17 (35.4%) had agenesis of 1 tooth, 18 (37.5%) had agenesis of 2 teeth, 6 (12.5%) had agenesis of 3 teeth, 5 (10.4%) had agenesis of 4 teeth, and 2 (4.2%) had agenesis of 5 or more teeth.
Twenty-seven (18.5%) patients in the total sample had maxillary tooth agenesis, 35 (24.0%) patients had mandibular tooth agenesis, and 14 (9.6% of the total sample; 30% of those with tooth agenesis) patients had both maxillary and mandibular tooth agenesis. There was no statistically significant difference between tooth agenesis in the maxilla or the mandible ( P = 0.3165). Forty-one (28.1%) patients had left-sided tooth agenesis, and 38 (26.0%) had right-sided tooth agenesis, with no statistically significant difference between the left and right sides ( P = 0.7923).
Table II shows the prevalence of tooth agenesis according to tooth type in the total sample. The most commonly absent teeth were the mandibular second premolars followed by the maxillary second premolars and the maxillary lateral incisors. In 45.8% of patients with hypodontia, both mandibular second premolars were absent ( Fig 1 ). No statistically significant differences in the prevalence of missing teeth between the left and right quadrants of the same jaw were observed. When the prevalences of missing teeth between the maxilla and the mandible were compared, statistically significant differences were found for agenesis of the second premolars ( P = 0.0003). Differences between the prevalence of other teeth were not statistically significant. As seen in Table II , the prevalence of agenesis of the second premolars was higher in the mandible on both the left ( P = 0.0295) and right ( P = 0.0197) sides. Table III shows the prevalence of tooth agenesis according to tooth type in the white racial subgroup (n = 118).
| Maxilla right (q1) | Maxilla left (q2) | ||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0.0% | 0.0% | 7.5% | 0.0% | 0.7% | 3.4% | 0.0% | 0.7% | 6.8% | 0.0% | 0.0% | 7.5% | 0.0% | 0.0% | ||
| 8 ∗ | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 ∗ |
| 0.0% | 0.0% | 17.1% | 0.0% | 0.0% | 4.8% | 0.7% | 0.7% | 2.7% | 0.7% | 0.0% | 16.4% | 0.0% | 0.0% | ||
| Mandible right (q4) | Mandible left (q3) | ||||||||||||||
| Maxilla right (q1) | Maxilla left (q2) | ||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0.0% | 0.0% | 8.5% | 0.0% | 0.0% | 2.5% | 0.0% | 0.8% | 4.2% | 0.0% | 0.0% | 7.6% | 0.0% | 0.0% | ||
| 8 ∗ | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 ∗ |
| 0.0% | 0.0% | 16.9% | 0.0% | 0.0% | 5.1% | 0.8% | 0.8% | 3.4% | 0.8% | 0.0% | 16.1% | 0.0% | 0.0% | ||
| Mandible right (q4) | Mandible left (q3) | ||||||||||||||
Table IV shows the different tooth agenesis patterns using the TAC in the total sample. The frequencies and percentages of tooth agenesis patterns using the TAC per quadrant are given in Table V . The mandibular second premolars were involved in 11 of 21 patterns of agenesis (6 of which were bilateral), and maxillary second premolars were involved in 8 of 21 patterns of agenesis (only 1 of which was bilateral), although this difference between unilateral and bilateral patterns of dental agenesis between the maxilla and the mandible was not statistically significant ( P = 0.0931). Maxillary lateral incisors were involved in 8 of 21 patterns of agenesis (3 of which were bilateral). The frequencies and percentages for tooth agenesis patterns with the TAC, as well as patterns per quadrant for the white racial subgroup of the sample (n = 118), are presented in Tables VI and VII .
| TAC value | Number | Tooth/teeth missing ∗ | Frequency | Percentage (%) |
|---|---|---|---|---|
| 0.0.0.0 | 0 | None | 98 | 67.1 |
| 0.0.16.16 | 2 | 35, 45 | 12 | 8.2 |
| 0.2.0.0 | 1 | 22 | 5 | 3.4 |
| 16.16.16.16 | 4 | 15, 25, 35, 45 | 5 | 3.4 |
| 0.16.0.0 | 1 | 25 | 3 | 2.1 |
| 0.0.2.2 | 2 | 32, 42 | 3 | 2.1 |
| 0.0.0.2 | 1 | 42 | 2 | 1.4 |
| 16.0.0.0 | 1 | 15 | 2 | 1.4 |
| 0.0.16.0 | 1 | 35 | 2 | 1.4 |
| 16.0.16.16 | 3 | 15, 35, 45 | 2 | 1.4 |
| 0.1.0.0 | 1 | 21 | 1 | 0.7 |
| 2.0.0.0 | 1 | 12 | 1 | 0.7 |
| 0.0.0.16 | 1 | 45 | 1 | 0.7 |
| 2.2.0.0 | 2 | 12, 22 | 1 | 0.7 |
| 2.0.16.0 | 2 | 12, 35 | 1 | 0.7 |
| 0.2.0.16 | 2 | 22, 45 | 1 | 0.7 |
| 2.2.0.2 | 3 | 12, 22, 42 | 1 | 0.7 |
| 16.16.0.16 | 3 | 15, 25, 45 | 1 | 0.7 |
| 0.16.16.16 | 3 | 25, 35, 45 | 1 | 0.7 |
| 0.18.0.16 | 3 | 22, 25, 45 | 1 | 0.7 |
| 0.0.7.3 | 5 | 33, 32, 31, 41, 42 | 1 | 0.7 |
| 22.2.16.16 | 6 | 15, 13, 12, 22, 35, 45 | 1 | 0.7 |
| Total | – | – | 146 | 100.0 |
| TAC | Tooth type | q1 | q2 | q3 | q4 | ||||
|---|---|---|---|---|---|---|---|---|---|
| n | % | n | % | n | % | n | % | ||
| 0 | none | 131 | 89.7 | 125 | 85.6 | 118 | 80.8 | 114 | 78.1 |
| 1 | I 1 | 0 | 0.0 | 1 | 0.7 | 0 | 0.0 | 0 | 0.0 |
| 2 | I 2 | 4 | 2.7 | 9 | 6.2 | 3 | 2.1 | 6 | 4.1 |
| 3 | I 1 + I 2 | 0 | 0.0 | 0 | 0.0 | 0 | 0.0 | 1 | 0.7 |
| 7 | I 1 + I 2 + C | 0 | 0.0 | 0 | 0.0 | 1 | 0.7 | 0 | 0.0 |
| 16 | P 2 | 10 | 6.8 | 10 | 6.8 | 24 | 16.4 | 25 | 17.1 |
| 18 | I 2 + P 2 | 0 | 0.0 | 1 | 0.7 | 0 | 0.0 | 0 | 0.0 |
| 22 | I 2 + C + P 2 | 1 | 0.7 | 0 | 0.0 | 0 | 0.0 | 0 | 0.0 |
| Total | 146 | 100.0 | 146 | 100.0 | 146 | 100.0 | 146 | 100.0 | |
| TAC value | Number | Tooth/teeth missing ∗ | Frequency | Percentage |
|---|---|---|---|---|
| 0.0.0.0 | 0 | None | 82 | 69.5 |
| 0.0.16.16 | 2 | 35, 45 | 9 | 7.6 |
| 16.16.16.16 | 4 | 15, 25, 35, 45 | 5 | 4.2 |
| 0.0.2.2 | 2 | 32, 42 | 3 | 2.5 |
| 0.2.0.0 | 1 | 22 | 2 | 1.7 |
| 16.0.0.0 | 1 | 15 | 2 | 1.7 |
| 0.0.16.0 | 1 | 35 | 2 | 1.7 |
| 16.0.16.16 | 3 | 15, 35, 45 | 2 | 1.7 |
| 0.16.0.0 | 1 | 25 | 1 | 0.8 |
| 0.0.0.2 | 1 | 42 | 1 | 0.8 |
| 0.1.0.0 | 1 | 21 | 1 | 0.8 |
| 2.0.0.0 | 1 | 12 | 1 | 0.8 |
| 0.0.0.16 | 1 | 45 | 1 | 0.8 |
| 2.2.0.0 | 2 | 12, 22 | 1 | 0.8 |
| 2.2.0.2 | 3 | 12, 22, 42 | 1 | 0.8 |
| 16.16.0.16 | 3 | 15, 25, 45 | 1 | 0.8 |
| 0.16.16.16 | 3 | 25, 35, 45 | 1 | 0.8 |
| 0.18.0.16 | 3 | 22, 25, 45 | 1 | 0.8 |
| 0.0.7.3 | 5 | 33, 32, 31, 41, 42 | 1 | 0.8 |
| Total | – | – | 118 | 100.0 |
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