Introduction
The aim of this investigation was to study the relationship between third molar agenesis—including the number of ageneses—and craniofacial structure growth.
Methods
We reviewed 305 clinical histories of patients treated at the Orthodontics Unit of the Faculty of Medicine and Dentistry at the University of Valencia in Spain. This included radiographic records of optimal quality. Of these, 40 patients who had agenesis of at least 1 third molar were included in the study group. A control group was formed with another 40 patients with all 4 third molars present. For both groups, a further criterion for inclusion was cone-beam computed tomography records. The cephalometric analysis was performed with NemoCeph 3D software (version 11.3.1.38; Nemotec, Madrid, Spain).
Results
The only significant differences between the 2 groups were in the total gonial angle and the upper gonial angle ( P ≤0.05), both of which were smaller in the study group.
Conclusions
Third molar agenesis is associated with a reduction in Jarabak’s gonial angle and upper gonial angle, characteristic of patients with a more horizontal or brachyfacial skeletal pattern. No significant differences were found in other measurements.
Highlights
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We studied third molar agenesis and development of craniofacial structures.
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Total and upper gonial angles differed between the agenesis and control groups.
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Third molar agenesis is associated with horizontal or brachyfacial skeletal patterns.
Dental agenesis can be defined as the situation in which at least 1 tooth is absent because it never formed.
The frequency of third molar agenesis ranges between 14% and 51.1%. The wide prevalence range of this anomaly can be attributed to differences in sample selection and examination methods and to the distribution of the subjects by sex, age, and racial origin. The rate in the Spanish population is 17.5%.
Agenesis, also known as hypodontia, can involve any tooth but is more frequent in some than in others. For the white population in general, the order in decreasing frequency is third molar, mandibular second premolar, and maxillary lateral incisor, suggesting that it is the most distal tooth in each group that disappears.
Ageneses do not appear in isolation. They are usually associated with development anomalies such as delayed tooth formation, late exfoliation of deciduous teeth, retention of deciduous teeth, agenesis of other teeth, and poor development of the alveolar bone. However, there is no consensus concerning a possible association between dental agenesis and craniofacial structure.
The alveolar process, the part of the jaw that surrounds and holds the teeth, has been described as depending on the constitution of the crown and the formation of the root of the tooth. Although reduction of the jaw with agenesis could be expected, there is considerable debate concerning the association between dental agenesis and craniofacial structure. Their possible association has been described by several authors, whereas other studies have reported that dental agenesis does not appear to influence the craniofacial structures.
Various researchers have suggested that there is no evidence that the third molar is necessary for maxillary or mandibular basal bone growth. Measurements of mandibular length have found that persons with erupted, functional third molars did not show a different mandibular growth pattern than subjects with third molar agenesis or impaction.
Other studies have stated that third molar agenesis has a slight influence on dentofacial structures or concluded that third molar agenesis is unrelated to mandibular length but is associated with the anteroposterior dimensions of the maxilla.
Several studies have observed an association between agenesis of various teeth and a reduction in maxillary jaw size. According to Vastardis, ageneses are polygenetic, and dental agenesis could act as an early indicator of developmental defects in both jaws. However, a few studies have related third molar agenesis to a specific craniofacial growth pattern, although with different results.
The objective of our investigation was to study the relationship between third molar agenesis and craniofacial structure development.
Material and methods
We reviewed 305 clinical histories of patients treated at the Orthodontics Unit of the Faculty of Medicine and Dentistry at the University of Valencia in Spain; they included radiographic records of optimal quality. Of these, 40 patients with agenesis of at least 1 third molar were included in the study group (13.1%). A further 40 who had all 4 third molars present served as the control group.
The mean age of the sample was 17.2 years. In the study group of patients with agenesis (n = 40), 21 were female and 19 were male, with an average age of 18.1 years. In the control group with no third molar agenesis (n = 40), 21 were female and 19 were male, with an average age of 16.4 years. There were no significant differences in age and sex between the 2 groups ( P ≤0.05).
The criteria for inclusion in the study group were patients with agenesis of at least 1 third molar and with high-quality radiographic records (cone-beam computed tomography). Agenesis of the third molar was confirmed in the absence of radiographic signs of crown mineralization and with the mandibular second molar at stage 7 or G: in other words, when there is mineralization of the radicular bifurcation, according to the method of Demirjian et al, which classifies the crown and root formation of the mandibular permanent teeth into 8 stages. The criteria for exclusion were patients with congenital deformities, such as a cleft palate, or any syndrome, or agenesis of any tooth other than a third molar.
A cephalometric analysis was performed with NemoCeph 3D software (version 11.3.1.38; Nemotec, Madrid, Spain) to assess craniofacial structure development. As well as the Wits appraisal, lines, angles, and proportions were measured according to the methods of the following authors: Ricketts (mandibular plane, corpus length, lower face height, facial axis, palatal plane), Steiner (SNA, SNB, ANB), and Jarabak (saddle angle, articular angle, gonial angle, upper gonial angle, lower gonial angle, face height ratio, mandibular body length) ( Table I ).
| Measurement | Cephalometic analysis | Definition |
|---|---|---|
| SNA (°) | Steiner | Anteroposterior position of maxilla relative to anterior cranial base |
| SNB (°) | Steiner | Anteroposterior position of mandible relative to anterior cranial base |
| ANB (°) | Steiner | Magnitude of the discrepancy between the maxilla and the mandible |
| Wits (mm) | Wits | Distance from AO-BO on occlusal plane (Ag-Me) |
| Facial axis (°) | Ricketts | Inferior angle formed by the intersection of Ba-N and Pt-Gn |
| Mandibular plane angle (°) | Ricketts | Angle formed by the intersection of mandibular plane (Me-Go) and Frankfort horizontal (FH) |
| Lower face height (°) | Ricketts | Angle formed by the intersection of Xi-ENA and corpus axis (Xi-Pm) |
| Corpus length (°) | Ricketts | Angle formed by the intersection of the condylar axis DC-Xi and the distal extrapolation of the corpus axis |
| Saddle angle (°) | Bjork-Jarabak | Angle formed by union of anterior cranial base (N-S) with posterior cranial base (S-Ar) |
| Articular angle (°) | Bjork-Jarabak | Angle formed by union of posterior cranial base with the ramus height (Ar-Go) |
| Gonial angle (°) | Bjork-Jarabak | Angle formed by the ramus height with the mandibular plane (Go-Me) |
| Sum of S+Ar+Go angles (°) | Bjork-Jarabak | The sum of posterior angles: saddle angle, articulare angle, and gonial angle |
| Upper gonial angle (°) | Bjork-Jarabak | Angle formed by Ar-Go-N, describes how oblique the ramus is |
| Lower gonial angle (°) | Bjork-Jarabak | Angle formed by N-Go-Me, describes the slants of mandibular body |
| Face height ratio (%) | Bjork-Jarabak | Ratio between posterior facial height (S-Go) and anterior facial height (N-Me) |
| Palatal plane length (ANS-PNS) (mm) | Ricketts | Distance between ANS and PNS |
| Mandibular body length (Go-Pog) (mm) | Bjork-Jarabak | Distance between Go and Pog, used in the assessment of proportional averages of bases (cranial base) |
One month after the first measurements, 20 measurements per group were repeated by an author (J.R.-V.). The data obtained intraclass correlation coefficients (ICCs) of between 0.96 and 0.99, showing good reproducibility. A second examiner (E.V.-C.), acting as the gold standard, made the same measurements to check their reliability and obtained ICC ratings between 0.87 and 0.99 for all measurements.
Statistical analysis
The Student t test, analysis of variance (ANOVA), and a test for a linear trend across the categories were used to discover significant differences between the means. The Pearson correlation coefficient was also calculated. Linear regression was performed with the variables that were found to be significant. The significance level was P = 0.05.
The choice of a sample size of 40 subjects in each group was based on a 2-tailed test with a 95% confidence interval (CI) for statistical power of 80%, to achieve a minimum difference of 5° between the 2 groups for a 30° cephalometric measurement of the mandibular plane in the Spanish population with a variance of 64, obtained from the study by Sánchez et al.
Results
Table II shows the distribution of the cephalometric measurements by group (with and without third molar agenesis). The only significant differences between the groups were in the gonial angle and the upper gonial angle ( P ≤0.05).
| Agenesis group, n = 40 | Control group, n = 40 | Student t test P value | |
|---|---|---|---|
| SNA (°) | 81.30 (79.84-82.76) | 82.08 (80.88-83.27) | 0.41 |
| SNB (°) | 77.53 (76.22-78.83) | 78.43 (77.17-79.68) | 0.32 |
| ANB (°) | 3.60 (2.72-4.48) | 3.68 (2.86-4.49) | 0.90 |
| Wits (mm) | −0. 41 (−1.70-0.88) | −0.72 (−1.83-0.38) | 0.71 |
| Facial axis (°) | 88.45 (86.91-89.99) | 88.55 (87.05-90.05) | 0.93 |
| Mandibular plane angle (°) | 24.23 (22.46-25.99) | 24.50 (22.49-26.51) | 0.84 |
| Lower face height (°) | 43.73 (42.28-45.17) | 44.05 (42.44-45.66) | 0.76 |
| Corpus length (°) | 33.95 (32.20-35.70) | 31.93 (29.90-33.95) | 0.13 |
| Saddle angle (°) | 123.85 (122.31-125.39) | 122.75 (121.10-124.40) | 0.33 |
| Articular angle (°) | 145.68 (143.50-147.85) | 143.68 (141.58-145.77) | 0.19 |
| Gonial angle (°) | 122.38 (120.13-124.62) | 126.05 (123.77-128.33) | 0.02 ∗ |
| Sum of S+Ar+Go angles (°) | 391.91 (389.95-393.84) | 392.48 (390.18-394.77) | 0.70 |
| Upper gonial angle (°) | 49.20 (47.82-50.58) | 51.55 (50.50-52.60) | 0.01 ∗ |
| Lower gonial angle (°) | 73.10 (71.41-74.79) | 74.18 (72.35-76.00) | 0.39 |
| Face height ratio (%) | 66.91 (65.58-68.23) | 67.05 (65.22-68.88) | 0.90 |
| Palatal plane length (ANS-PNS) (mm) | 54.2 (53.12-55.33) | 55.07 (53.87-56.23) | 0.42 |
| Mandibular body length (Go-Pog) (mm) | 76.1 (75.21-77.23) | 76.2 (75.11-77.18) | 0.92 |
In the distribution of the measurements by sex, only the lower gonial angle showed a significant difference ( P ≤0.05): it was smaller in the males. By location, 56.6% of the ageneses were maxillary and 43.4% were mandibular.
The distribution of the cephalometric measurements by number of ageneses can be seen in Table III . ANOVA showed significant differences in the gonial angle and the upper gonial angle ( P ≤0.05). We also found an inverse association between the number of ageneses and the degrees of both angles: the greater the number of absent third molars, the smaller both the gonial angle and the upper gonial angle (linear trend test, P ≤0.05).
| Agenesis 0, n = 40 | Agenesis 1, n = 22 | Agenesis 2, n = 6 | Agenesis 3, n = 6 | Agenesis 4, n = 6 | Linear trend test | ANOVA | |
|---|---|---|---|---|---|---|---|
| SNA (°) | 82.08 (80.88-83.27) | 81.45 (79.55-83.36) | 82.67 (76.63-88.70) | 79.50 (74.91-84.09) | 81.17 (75.88-86.45) | 0.36 | 0.65 |
| SNB (°) | 78.43 (77.17-79.68) | 78.00 (76.06-79.94) | 78.50 (73.96-83.04) | 75.67 (73.12-78.21) | 76.67 (72.13-81.20) | 0.15 | 0.53 |
| ANB (°) | 3.68 (2.86-4.49) | 3.36 (2.21-4.52) | 4.33 (0.66-8.01) | 2.83 (1.03-4.64) | 4.50 (0.70-8.30) | 0.67 | 0.77 |
| Wits (mm) | −0.72 (−1.83-0.38) | −1.27 (−2.76-0.21) | 1.51 (−5.08-8.12) | 0.16 (−3.41-3.75) | 0.26 (−4.31-4.84) | 0.36 | 0.52 |
| Facial axis (°) | 88.55 (87.55-90.05) | 88.41 (86.12-90.70) | 87.17 (82.90-91.44) | 88.83 (84.03-93.64) | 89.50 (83.88-95.12) | 0.63 | 0.94 |
| Mandibular plane angle (°) | 24.50 (22.49-26.51) | 24.82 (22.13-27.51) | 24.33 (17.88-30.79) | 23.00 (19.43-26.57) | 23.17 (17.56-28.78) | 0.45 | 0.95 |
| Lower face height (°) | 44.05 (42.44-45.66) | 44.55 (42.59-46.50) | 43.00 (36.53-49.47) | 42.67 (39.37-45.96) | 42.50 (37.50-47.50) | 0.30 | 0.82 |
| Corpus length (°) | 31.93 (29.90-33.95) | 33.23 (30.63-35.82) | 34.17 (29.23-39.11) | 34.67 (30.04-39.30) | 35.67 (28.81-42.52) | 0.14 | 0.53 |
| Saddle angle (°) | 122.75 (121.10-124.40) | 124.55 (122.13-126.97) | 121.83 (118.62-125.05) | 122.67 (116.67-128.66) | 124.50 (122.04-126.96) | 0.74 | 0.60 |
| Articular angle (°) | 143.68 (141.58-145.77) | 144.73 (141.48-147.97) | 146.67 (140.82-152.52) | 149.68 (143.67-155.66) | 144.17 (137.10-151.24) | 0.37 | 0.32 |
| Gonial angle (°) | 126.05 (123.77-128.33) | 123.95 (120.34-127.57) | 123.33 (115.35-131.32) | 119.33 (117.07-121.60) | 118.67 (117.23-120.10) | 0.00 ∗ | 0.06 |
| Sum of S+Ar+Go angles (°) | 392.47 (390.18-394.76) | 393.22 (390.36-396.09) | 391.83 (387.41-396.25) | 391.66 (387.28-396.05) | 387.33 (380.31-394.34) | 0.07 | 0.42 |
| Upper gonial angle (°) | 51.55 (50.50-52.60) | 50.09 (48.07-52.12) | 49.33 (44.80-53.87) | 47.67 (44.18-51.16) | 47.33 (42.85-51.82) | 0.00 ∗ | 0.03 ∗ |
| Lower gonial angle (°) | 74.18 (72.35-76.05) | 73.64 (70.98-76.29) | 74.17 (67.68-80.65) | 71.67 (68.58-74.76) | 71.50 (67.70-75.30) | 0.19 | 0.72 |
| Face height ratio (%) | 67.04 (65.22-68.87) | 66.16 (63.98-68.33) | 68.85 (65.80-71.89) | 66.91 (63.71-70.11) | 67.68 (65.01-70.36) | 0.68 | 0.82 |
| Palatal plane length (ANS-PNS) (mm) | 45.25 (43.07-47.43) | 45.64 (44.06-47.22) | 46.12 (40.56-52.43) | 45.64 (43.06-48.21) | 43.35 (38.31-48.75) | 0.53 | 0.71 |
| Mandibular body length (Go-Pog) (mm) | 70.09 (66.10-74.07) | 64.34 (60.27-68.41) | 63.10 (59.21-67.12) | 66.67 (60.92-72.42) | 62.98 (49.85-76.11) | 0.30 | 0.29 |
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