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| 2010-22 |
| 22-1 |
ISSN (Print) 1013-9052
EISSN 1658-3558
P.O. Box 52500,
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Kingdom of Saudi Arabia
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Reliability and accuracy of the lower incisor mandibular plane angle: Proposed correction factor
Mohammed T. Bukhary, BDS, D OrlhoRCS, M OrthoRCS (Eclin), M.OrthoRCS (Lond), MDORCPS(GlasQ), PhD(Lond)
The labiolingual position of the lower incisors is in a zone of balance. Any significant changes in the position lead to relapse, although, in some cases, there are exceptions. In cephalometric analysis, the relative anteroposterior position of the lower incisor is measured by the lower incisor mandibular plane angle (IMPA). However, the morphology of the mandible could vary affecting the value of IMPA. This study is designed to analyze the factors influencing the IMPA, and to introduce a method for adjusting abnormal inclination.
It is accepted that the lower incisors lie in a zone of balance between the tongue and the lips.1 During orthodontic treatment in a majority of patients, the significant changes in the position of the lower incisors are followed by relapse.2 However in some cases, there are exceptions to the above rule.3 The determination of the incisor labiolingual position is part of the cephalometric analysis.4-8 The lower incisor mandibular plane angle (IMPA) is a valuable parameter for clinicians to assess the position of the lower incisors procumbancy relative to the face. In addition some clinicians would find it useful for the determination of teeth extractions and the orientation of the teeth to their upper counterparts during treatment planning. This parameter has gained popularity after its adoption by Tweed in 1946.9 The mean value for the angle of the lower incisor to the mandibular plane was around 900 ± 50.10 The range of normality was almost 100. However, the accuracy of this parameter will depend on the position of the incisor on the mandibular basal bone and the inclination of the lower border of the mandible. The latter is affected by the height of the ramus. The purpose of this paper was to introduce a correction factor for the abnormal inclination of the lower border of the mandible.
In the study, a model template of the mandible was derived through a computer from the lateral skull radiographs of a random sample of 150 school children (75 boys and 75 girls).10 The computer model template was used as a guide to transfer to and correct individual mandibular points for patients and propose an adjustment factor for the lower incisor mandibular plane angle. The method for calculating the correction factor for the lower incisor mandibular plane angle is as follows. In the model template of the mandible, the anatomical points B (supramentale) and Gn (Gnathion) were located. The distance between points B and Gn was found equal to 20 mm in the template. A perpendicular line drop to B-Gn from X (hypothetical point) was made to form X-Gn. The perpendicular X-Gn made an angle of 150 with the mandibular plane on the mean template (Fig. 1). From the above parameters, the value of B-Gn and the angle of X-Gn to mandibular plane in addition to the gonial angle were used to calculate the correction factor for the lower incisor mandibular plane angle.
The results of applying the correction factor are presented in the following cases. Case 1: Measured: Lower incisor mandibular plane angle (IMPA) = 1050 Gonial angle = 1200 Join B-Gn (20 mm) Drop perpendicular X to B-Gn Go Gn X = 130 Difference between measured GoGnX and the model GoGnX = 130 - 150 = -20 Corrected IMPA = 1050 - 20 = 1030 Case 2: Measured: Lower incisor mandibular plane angle (IMPA) = 1010 Gonial angle = 1210 Join B-Gn (20 mm) Drop perpendicular X to B-Gn Go Gn X = 70 Difference between measured GoGnX and the model GoGnX = 70 - 150 = -80 Corrected IMPA = 1010 - 80 = 930 Case 3: Measured: Incisor mandibular plane angle (IMPA) = 700 Gonial angle = 1380 Join B-Gn (20 mm) Drop perpendicular X to B-Gn Go Gn X = 300 Difference between measured GoGnX and the model GoGnX = 300 - 150 = +150 Corrected IMPA = 700 + 150 = 850 Case 4: Measured: Incisor mandibular plane angle (IMPA) = 810 Gonial angle = 131.50 Join B-Gn (20 mm) Drop perpendicular X to B-Gn Go Gn X = 220 Mean Go Gn X = 150 Difference between measured GoGnX and the model GoGnX = 220 - 150 = +70 Corrected IMPA = 810 + 70 = 880 Case 5: Measured: Incisor mandibular plane angle (IMPA) = 90.50 Gonial angle = 1310 Join B-Gn (20 mm) Drop perpendicular X to B-Gn Go Gn X = 160 Difference between measured GoGnX and the model GoGnX = 160 - 150 = +10 Corrected IMPA = 90.50 + 10 = 91.50 CASE 6: Measured: Incisor mandibular plane angle (IMPA) = 1000 Gonial angle = 1300 Join B-Gn (20 mm) Drop perpendicular X to B-Gn Go Gn X = 80 Difference between measured GoGnX and the model GoGnX = 80 - 150 = -70 Corrected IMPA = 1000 - 70 = 930 Case 7: Measured: Incisor mandibular plane angle (IMPA) = 800 Gonial angle = 1300 Join B-Gn (20 mm) Drop perpendicular X to B-Gn Go Gn X = 220 Difference between measured GoGnX and the model GoGnX = 220 - 150 = +70 Corrected IMPA = 800 + 70 = 870 Case 8: Measured: Incisor mandibular plane angle (IMPA) = 820 Gonial angle = 1350 Join B-Gn (20 mm) Drop perpendicular X to B-Gn Go Gn X = 150 Difference between measured GoGnX and the model GoGnX = 150 - 150 = 00 Corrected IMPA = 820 + 00 = 820 In general, the corrected factor for the abnormal inclination of the lower border the mandible can be summarized into the following formula: Corrected IMPA = IMPA + (G - 150) where IMPA = Incisor mandibular plane angle G = measured GoGnX
The incisor mandibular plane angle is a useful parameter in indicating the relative anteroposterior position of the lower incisor. However, in previous studies4,6 the range of normality of this parameter was around 100. This wide range did not permit the clinician to utilize this parameter accurately; therefore, if this range was narrowed a somewhat more precise diagnosis could be achieved. The IMPA simply is the intersection between the long axis of the lower central incisor and the inclination of the lower border of the mandible. In the majority of cases, the mandible has normal dimensions which will not alter related parameters, i.e. FMA, SNMP, IMPA...etc. In some cases, the mandible could acquire short or long ramii, short or long anterior border (B-Gn) or a combination of both. However, the incisor mandibular plane angle (IMPA) will be affected by the relative labiolingual position of the lower central incisor and the inclination of the lower border of the mandible. This is explained in Fig. 2 by the tracing of two cases with the same lower incisor position relative to the face. The shorter the ramus relative to the anterior border of the mandible (B-Gn), the larger the gonial angle, the longer the ramus relative to the anterior border of the mandible, the smaller the gonial angle. However, the above factors, which alter the mandibular parameters, did not change the position of the lower central incisor relative to the facial plane or Frankfort horizontal and if the anterior border of the mandible used as a reference the mandibular structures could be adjusted in order to obtain an accurate IMPA. The labiolingual inclination of the lower incisor is the target angle for measurement and therefore should be accepted. The inclination of the lower border of the mandible alters the angle. In addition, the inclination of the lower border of the mandible varies according to the gonial angle. For example a steep mandibular lower border will be associated with a large gonial angle, if the lower central incisor is reasonably upright relative to the face, the incisor mandibular plane angle will be less than its true value. While a decreased gonial angle will give an increased value for incisor mandibular plane angle if the lower incisor is an upright position relative to the face. Therefore, the inclination of the mandibular plane will vary according to the height of the ramus relative to the height of the anterior border of the mandible (Fig. 2). From the above, the incisor mandibular plane angle's value will vary according to the inclination of the mandibular plane, which in turn affected by the ramal height independent of the true inclination of the lower central incisor. Therefore, since the position of the lower central incisor anteroposteriorly is significant in orthodontic diagnosis and treatment planning independent of other related parameters as the mandibular plane, the inclination of the lower border of the mandible should be corrected so that the true position of the lower central incisor anteroposteriorly can be assessed. In the present study, a model mandible derived from a mean template was used as a guide to transfer to and correct individual mandibular points for patients and propose an adjustment factor for the lower incisor mandibular plane angle. The mean template used was a combination of a random sample for boys and girls aged 9-12 years. The choice of this model enabled this study to assess mandibular inclination for both sex's fairly accurate. Further standardization of the height of the anterior border of the mandible B-Gn at 20 mm in this study confirms that an accurate inclination of the mandibular plane will be recorded independent of the border of the mandible. Furthermore, using point Gnathion and not Menton in this study was to avoid morphological abnormality at the mandibular symphysis. However, a clinical study may be needed to assess the reliability of this correction.
In conclusion, the true inclination of the lower central incisor in relation to the face could be measured using the above method. This may help in the treatment planning to change the angulation of the lower incisor in relation to the mandibular plane. The following correction factor for the abnormal IMPA was introduced: Corrected IMPA = IMPA + (G - 150) where IMPA = Incisor mandibular plane angle G = measured GoGnX
I would like to express my deep appreciation to Dr. Omar Sarhan for the encouragement he has given me to write this paper and for allowing me to use the template of the mandible. References
1.Tulley WJ. The Tongue: That unruly member? Dent Practic 1964; 15: 1-5.
2.Sadowsky C and Sakls EI. Long-term assessment oforthodontic relapse. Am J Orthod 1982; 82: 456-463. 3.Mills JRE. The long term results of the proclination of lower incisors. Brit Dent J 1966; 120: 355-364. 4.Downs WB. Variation in facial relationship: Their significance in treatment and progress. Am J Orthod 1948; 34: 812-840. 5.Downs WB. Analysis of the dentofacial profile. AngleOrthod 1956; 26: 191-212 6.Tweed CH. The Frankfort mandibular plane angle inorthodontic diagnosis, classification and treatmentplanning and prognosis. Am J Orthod 1946; 32(4):175-230. 7.Steiner CC. Cephalometric for you and me. Am. J Orthod 1953; 39: 279-285. 8.Ricketts RM. Perspectives in the clinical application of cephalometrics. Angle Orthodon 198; 51:115-130. 9.Tweed CH. Frankfort mandibular plane angle in diagnosis, treatment planning and prognosis. Angle Orthod 1946; 24: 121-169. 10. Sarhan OA. Rotational effects of S-N on thedentoskeletal pattern within the range of normal.Angle Orthod 1988; 59 (1): 43.
Address reprint requests to: Dr. Mohammed T. Bukhary
P.O. Box 60169, Riyadh 11545
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