Abstract
Harmony between the size of the supporting bone in which the teeth are positioned and the size of the dental arch is essential for proper occlusion. The authors reported in a previous paper that the extent of growth of the supporting bone capable of containing the dentition (S.C.M.) varied during treatment with the types (Class I, Class II, and Class III) of malocclusion and with extraction. For this paper, studies by the types of malocclusion were made on the difference in the sum of the mesiodistal crown diameters (T.M.S.), ratios of the maxilla and mandible T.M.S., and the relations after treatment between S.C.M. and T.M.S. (excluding the mesiodistal crown diameters of the extracted teeth of extraction cases). The subjects examined were the same as for the previous paper : a total of 68 female patients (with an average age of nine and with an average treatment period of five years) comprising 15 Class I Cases, 21 Class II cases, and 32 Class III cases. The results were as follows : 1. As for T.M.S. (6-6), Class I cases showed the highest values (maxilla : 101.0mm ; mandible : 93.1mm) for both maxilla and mandible. No significant difference existed between Class II (maxilla : 97.7mm ; mandible : 89.4mm) and Class III (maxilla : 96.7mm ; mandible : 89.6mm). Each case showed higher values than those of normal occlusion (maxilla : 94.26mm ; mandible : 84.00mm). Comparison of extraction and non-extraction cases revealed that the extraction cases showed higher values than the non-extraction cases for both Class II and Class III. 2. As for tooth size ratios, no significant difference was found between individual cases and their values were approximately the same as those of normal occlusion. 3. No characteristic trend was indicated for each case in the difference between S.C.M. and T.M.S. (excluding the mesiodistal crown diameters of the extracted teeth of extraction cases) after treatment. The extraction cases (maxilla : 11.0mm ; mandible : 9.5mm) showed higher values than non-extraction cases (maxilla : 4.2mm ; mandible : 2.7mm). The investigation, as stated, of the extent of increase in S.C.M. before and after treatment revealed that no particular trend existed between T.M.S. and S.C.M., and that the extent of increase varied with the type, maxilla, and mandible, with extraction affecting the difference substantially. Consequently, in deciding on the matter of extraction or non-extraction with due consideration on values of S.C.M. after treatment and on their balance with the values of T.M.S., it is deemed more appropriate to make estimates of the extent of increase of S.C.M. after treatment after classifying the cases by type of malocclusion and by extraction or non-extraction than to make reference to the S.C.M. values of nornal occlusion. The average annual increases by types of malocclusion were as follows : Maxilla Mandible Extraction Cases Class I 1.1mm 1.6mm Class II 1.0mm 0.8mm Class III 1.5mm 1.0mm Non-extraction Cases Class II 1.6mm 2.5mm Class III 2.2mm 2.1mm
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