Malocclusion is defined as an abnormal occlusion in which teeth are not in a normal position in relation to adjacent teeth in the same jaw and/or the opposing teeth when the jaws are closed¹. Skeletal class III malocclusion exists in three forms: maxillary retrognathism, mandibular excess, or a combination of both². Any such disparities in the maxilla and/or mandible appear to be compensated for by the dentoalveolar segments to achieve a harmonious interarch relationship between the defective jaw bases. This is referred to as “dentoalveolar compensation”.

Thus, dentoalveolar compensation refers to ‘a system which attempts to maintain normal interarch relationship under varying jaw relationships’³. Goldsman⁴ explained that when one facial dimension shows an obvious discrepancy, one or more of the other structures will be altered to minimize the effects of the dimensions displaying the obvious discrepancy. There are several factors that influence dentoalveolar compensation. These include the influence of the normal eruptive system, dental equilibrium, and the neighboring teeth⁵.

However, there is a limit to the compensation, and that is what defines whether the teeth will be in normal occlusion despite a jaw mal-relationship or whether a malocclusion less severe than the jaw discrepancy will develop⁶. The pattern of dentoalveolar compensations observed in class III skeletal malocclusion caused by retrognathic maxilla is proclination of maxillary incisors in the sagittal plane and buccal tipping of maxillary molars in the transverse plane, whereas in a skeletal class III malocclusion due to a prognathic mandible, compensation involves retroclination of mandibular incisors in the sagittal plane and lingual tipping of mandibular molars in the transverse plane⁷. With regards to a combination class III malocclusion, the above-mentioned compensations may also appear in combination.

However, review of the literature suggests that such compensations do not always happen in the patterns described above, and one jaw might compensate more than the other^8,9. Additionally, most, if not all, authors who have described dentoalveolar compensations in skeletal class III malocclusions have not quantified it. To the best of our knowledge, only a few studies have investigated the association between dentoalveolar compensation and skeletal base severity. The authors of these studies described the association between these two factors^9,10. In more detail, Ishikawa et al.⁹ studied incisal compensation in 44 adult females with either a class I or class III skeletal base based on the mean ANB angle of the Japanese population and concluded that incisal compensation tended to occur by labial tipping of upper incisors and lingual tipping of lower incisors. The magnitude of compensation was also specified in his study. However, it was exclusively studied in females and included both class I and class III skeletal bases. Mathapun and Charoemratrote¹⁰ studied the incisor relation in class III malocclusion based on different positions of maxillary and mandibular jaw bases. They concluded that upper incisors showed no compensation in prognathic maxilla and only limited compensation even with progressive maxillary retrognathism. However, the magnitude of this compensation was not studied. Also, These studies did not address confounding entities like sex and race which might influence dentoalveolar compensation. As the etiology of class III malocclusion is multi factorial with contributions from genetic, ethnic, environmental, and habitual factors, the scale of compensation might vary across races, and applying the results of these studies to the general human population is not appropriate.

This study was approved by the Institutional Ethics Committee of Sri Ramachandra Institute of Higher Education and Research (SRIHER) (CSP/21/DEC/103/613). This cross-sectional study was carried out using the pre-treatment lateral cephalograms of 57 orthodontic patients seen in the Department of Orthodontics and Dentofacial Orthopedics, Sri Ramachandra Institute of Higher Education and Research (SRIHER), Chennai, India. The written informed consent was waived due to the study’s retrospective design.

Age of the included subjects ranged from 8-30 years with a mean age of 17.9±5.2 years. Thirty-three male and 24 female subjects were included in this study. Of 57 samples, 22 had a retrognathic maxilla and 35 had a prognathic mandible that contributed to their skeletal class III malocclusion. A total of 57 maxillary and mandibular incisor inclinations were studied. Means and standard deviations of all parameters used to assess upper and lower incisor inclination were calculated.(Table 2) Upper incisors proclined and were forwardly placed in the skeletal class III population. However, no change in lower incisor inclination in terms of compensation was noted.

To assess if the compensation that occurred was influenced by the skeletal base, we performed correlation analysis between the severity of the class III skeletal base and the deviation of the upper incisor values from the norms. Similarly, to assess if the compensation was related to the maxillary skeletal base only, correlation analysis was performed between the SNA and U1-NA values of the subjects.(Table 3)

With regards to Pearson’s correlation coefficient, a positive correlation between the change in U1 inclination and the severity of ANB would mean that the former was influenced by the latter, indicating compensation. To extrapolate, if the ANB angle was reduced and if the upper incisors were proclined, then compensation was considered present. Similarly, a negative correlation between U1 inclination and SNA would mean the former was affected by the latter. We found that in terms of upper incisor inclination, U1-APog (liner and angular) (r=–0.321; P=0.015 and r=–0.509; P<0.001, respectively) and incisal edge to N-Pog line (r=–0.549; P<0.001) showed only a weak and moderately negative correlation with the severity of class III malocclusions, respectively.(Table 3) There was no significant correlation between U1-NA and SNA (r=0.003; P=0.980).(Table 3)

These results indicate that though the upper incisors of the study subjects were proclined, which can be seen from the mean values, there was no correlation between the quantum of upper incisor proclination and the quantum of class III skeletal base. In fact, the negative association seen for some of the parameters indicates that as the class III skeletal base increased, upper incisor compensation decreased.

Pearson correlation analysis of the L1 parameter and ANB severity showed weak positive correlations with regards to incisor mandibular plane angle (IMPA) (r=0.305; P=0.021), L1-GoGn (r=0.311, P=0.018), and L1-GoMe (r=0.305, P=0.021) and a weak negative correlation with L1-APog (r=–0.283, P=0.033).(Table 4)

These findings indicate that although the lower incisors were upright and did not compensate as much as the upper incisors (Table 2), the lower incisors began to compensate as the severity of the class III malocclusion increased, as evident from the significantly weak positive association. L1-NB and SNB were not significantly correlated (r=–0.061, P=0.655).(Table 4)

After we determined the presence of an association between lower incisor proclination and skeletal class III malocclusion severity, we performed linear regression analysis for IMPA, L1-GoMe, and L1-GoGn. The results of the linear regression analysis are shown in Table 5, Fig. 2, and Fig. 3.

Student’s t-test showed no differences in dentoalveolar compensation of the maxillary incisors between males and females (P>0.05).(Table 6) However, comparison of lower incisors compensation between males and females revealed males had higher compensation of the lower incisors compared to females with regards to L1-NB (mm) (P=0.008).

Dentoalveolar compensation plays an important role in the exhibition of malocclusion as it masks the underlying severity of the skeletal malocclusion. There are several factors that play roles in dentoalveolar adaptation, such as a normal eruptive system without any systemic factors affecting the eruption pattern, forces exerted by the soft tissue envelope on the dental arches, swallowing patterns, non-nutritive habits, the influence of the neighboring teeth during eruption, and the inclined-plane effect of opposing teeth during occlusion and mastication⁵. Several studies have investigated dentoalveolar compensations, especially in class III malocclusions. Ishikawa et al.⁹ performed a study in the Japanese female population to determine the cephalometric parameters that quantitatively describe dental compensations. However, in his study, the population did not solely represent class III malocclusion individuals as subjects with class I and class III malocclusions were included, as the cut-off ANB angle used in that study was 3.39°. We conducted this study to evaluate the quantum of dentoalveolar compensation in class III malocclusions in the Indian population, as ethnic origin and race contribute to the etiology of class III malocclusions. Different ethnicities may have different incisal compensations with different magnitudes of compensation. There are several cephalometric variables available to assess the axial inclinations of the upper and lower incisors, and not all parameters are equally correlated with prediction of the compensatory axial inclination of the incisors⁹. Hence, in our study, we evaluated eight dental parameters for upper and lower incisor position and inclination to see which parameter showed the strongest correlation with the jaw base for dentoalveolar compensation. We used the ANB angle to measure the skeletal base. ANB was chosen because it is the most common method of measuring the maxillomandibular differential¹¹ and also was considered to be the most valid cephalometric parameter in angle’s class III subjects with counter-clockwise rotation and a flattened occlusal plane¹². The current study was a self-controlled study, in which the deviation in inclination of the incisors was calculated by subtraction from norm values. This way, we did not have to include a separate control group in which the inherent intra-arch abnormalities present could lead to a change in the axial inclination of the incisors regardless of the compensatory mechanism and thereby influence the results.

Interestingly, we found that the upper incisors compensated by proclination whereas the lower incisors were almost upright and showed no compensation. This indicates that the upper incisors responded more to the jaw discrepancy than the lower incisors. This is in accordance with the study by Kim et al.⁷ who evaluated dentoalveolar compensation in skeletal class III malocclusions with both positive and negative overjets. They showed that the upper incisors were more flared in the positive overjet group than the negative overjet group, whereas no difference was found in lower incisor inclination. The authors suggested that this was because the position of the mandibular incisors is firmly regulated by the labiolingual muscular environment, so that incisal occlusion does not affect the position of incisors. In our study, the lower incisors were in fact more upright than retroclined. We came to this conclusion by comparison to normal values for all parameters obtained through literature review. However, the norms for axial inclination differ between various ethnic and racial groups¹³. This could have been one of the reasons why the lower incisors did not show significant compensation in our study.

In contrast to our findings, Al-Kadhim et al.¹⁴ showed that dentoalveolar compensation in class III skeletal pattern was by retroclination of the lower incisors. These differences in findings could be due to the different ethnicities of our respective study populations.

Furthermore, correlation analysis to identify the influence of incisor compensation showed in that as class III severity increased, the upper incisors became less compensatory (as seen by U1-APog and incisal edge to N-Pog values). Additionally, the maxillary base did not affect upper incisor compensation. This is consistent with the findings of Alhammadi¹⁵, namely that compensation is not affected by the holding jaw base. However, they found a moderate correlation between ANB and upper and lower incisor inclination. We attribute this discrepancy in results to the fact that their study evaluated the association between ANB and incisor inclination whereas we investigated the association between the magnitude of incisor compensation and severity of class III malocclusions. To the best of our knowledge, none of the studies performed to date have evaluated the correlation between the severity of class III skeletal malocclusions and the magnitude of incisor compensation.

Although the lower incisors were upright and did not retrocline in mild class III cases, as the class III malocclusion became more severe, the lower incisors began to compensate by retroclination as seen from the positive association between L1-GoMe, L1-GoGn, and IMPA. The magnitude of compensation revealed that as the ANB decreased by 1 degree from normal, IMPA and L1-GoMe decreased by 0.79° and L1-GoGn decreased by 0.81°.

Together, the results from our study and those of previous studies indicate that it is not always only anteroposterior relationships that influence dentoalveolar compensation, but also several other factors such as vertical growth pattern¹⁵, transverse relationships, the inherent relationship of the jaw with the skull base¹⁶, and the effects of oral musculature pressure, upper lip thickness, and lip height¹⁷.

The limitations were (1) lack of cephalometric norms for specific ethnic populations to evaluate incisor inclination, (2) lack of equal sample sizes for retrognathic maxillae and prognathic mandibles, and (3) small sample size.

In class III skeletal malocclusions, the upper incisors contribute more to compensation by proclination than the lower incisors until a certain limit. However, as the severity of class III malocclusion increases, the upper incisors stop compensating and the lower incisors began to exhibit more compensation. For every degree of reduction in ANB angle, IMPA and L1-GoMe decreased by 0.79°, respectively, and L1-GoGn decreased by 0.81°. There was a difference in lower incisor compensation but not upper incisor compensation between males and females.