Incisor Mandibular plane Angle (IMPA), Frankfort Mandibular Plane Angle (FMA) and Lower Incisor to A-pog distance, and their relation in Different Skeletal Classes

Introduction: Tweed’s cephalometric analysis has its beginning in clinical orthodontics where he found that the cases of malocclusion with pleasing outcome, harmonious profiles and stable occlusion following orthodontic treatment had a common consistent feature of occlusion: their mandibular incisors were upright on their skeletal bases. So the correct position of incisors is essential for achieving stable end results and establish harmony and balance of facial aesthetics. Aims and objectives: The purpose of this study was to evaluate the linear and angular position of mandibular incisors over the basal bone and their relationship with frankfort mandibular plane angle in different skeletal classes. Materials and Methods: A total of 60 lateral cephalograms in the age group of 18-30 years of Class I, Class II, Class III subjects were selected and traced as per inclusion criteria. All the linear and angular measurements were done in each group and their relationship and comparison was done in each group by using ANOVA test and Student’s independent t-test. Results: Descriptive statistical analysis was first carried out to find the means of IMPA, FMA angle and lower incisor to A-pog distance. On doing intergroup comparison with ANOVA test and Student’s independent t-test, statistically significant difference was found only in IMPA and lower incisor to A-pog line distance between Class I and III groups and Class II and Class III groups. Conclusion: There appears to exist a demonstrable relationship between the axial inclination of the mandibular incisors and the incisor mandibular plane angle, the greater the procumbency of the mandibular incisors, the greater the incisor mandibular plane angle in excess of 90 degree and vice versa. There also appears to exist a relationship between the incisor mandibular plane angle and the contour of the lower third of the face. The lower incisors tend to be more upright in subgroups with prognathic mandible than subgroups with normal or retrognathic mandible.