下顎拡大術の許容限界

Permissible limit for mandibular expansion.

抄録

近年、矯正治療と並行して下顎拡大術が行われることが多くなっている。しかし、下顎拡大に伴う臼歯の側方傾斜は、過度の拡大により頬側歯牙の過度の傾斜を招き、咬合関係を乱す可能性があるため、注意が必要である。本研究では、Schwarz装置を用いて下顎拡大時の臼歯の動揺を定量的に明らかにし、傾斜動揺の臨床的指標として下顎拡大の許容限界を決定することを目的とした。第一大臼歯の咀嚼面の傾斜と臼歯間幅を拡大前（T1）、拡大後（T2）、エッジワイズ治療前（T3）に測定した。エキスパンションプレートで治療した被験者29名の下顎石膏模型を使用し、正常咬合の対照被験者11名の模型と比較した。臼歯間幅の治療による変化（T1-T2）は平均5.42mm（標準偏差1.98）、頬側歯牙傾斜角の平均は10.16度（標準偏差3.83）であった。治療前の年齢と治療期間を臼歯間幅の増加および傾斜角度と比較したところ、有意な相関は認められなかった。保定期間と拡大量には有意な正の相関が認められた。拡大時の頬側歯牙傾斜角と臼歯間幅の増分の回帰係数は約0.2であった。これは、1mmの拡大は5度の臼歯側方傾斜を伴うことを意味する。この係数は、下顎の拡大許容限度を推定する上で臨床的に有用である。

In recent years, mandibular expansion has been increasingly performed in conjunction with orthodontic treatment. Lateral tipping of the molars associated with mandibular expansion should, however, be considered, because excessive expansion may result in excessive buccal tooth inclination, which may disturb the occlusal relationship. This study was conducted to quantitatively clarify molar movement during mandibular expansion using the Schwarz appliance to determine the permissible limit of mandibular expansion as a clinical index for inclination movement. Inclinations in the masticatory surface of the first molar and intermolar width were measured before expansion (T1), after expansion (T2), and before edgewise treatment (T3). Lower plaster models from 29 subjects treated with expansion plates were used and compared with models from 11 control subjects with normal occlusion. The average treatment change (T1-T2) in intermolar width was 5.42 mm (standard deviation 1.98), and the average angle of buccal tooth inclination was 10.16 degrees (standard deviation 3.83). No significant correlation was found between age prior to treatment and the treatment period when they were compared with the intermolar width increments and inclination angles. There was a significant positive correlation between retention duration and the amount of expansion. The regression coefficient of the angle of buccal tooth inclination during expansion to the increment of the intermolar width was approximately 0.2. This means that 1 mm of expansion is accompanied by 5 degrees of molar lateral tipping. This coefficient is clinically useful for estimating the permissible limit for mandibular expansion.