Abstract:

objective To discuss the morphological changes and related factors of soft and bone tissues in chin in the skeletal class Ⅲ malocclusion patients at three dimensions after orthognathic operation. Methods A total of 19 adult patients underwent orthognathic operation were collected， including 9 males and 10 females. All the patients received craniofacial computed tomography （CT） scans one week before operation（T0） and 12 months after （T1） operation to obtain the facial data of the soft and bone tissues， with a three-dimensional reconstruction and registration by using ProPlan CMF 3.0 software. The measurements and calculations were made on the thickness of chin soft tissue， the horizontal， sagittal， and vertical changes in anatomical landmarks， and the correlation and ratios between them. Results The most significant retrusion of soft tissue after operation was in the lower lip vermilion and the mentolabial fold area. The posterior displacement of soft tissue was gradually decreased， spreading from this area to the periphery. The posterior change in the submental area was relatively minor， with an anterior trend at the submental point. Compared with T0 time point， the soft tissue thickness at the lower lip （LL）at T1 time point was increased （P<0.05）.The linear regression analysis results showed that at the horizontal（X）， as the bone tissue changed to the left，the anatomical landmarks of soft tissue in chin LL （B=0.795， R2=0.832）， mentolabial sulcus （Si） （B=0.876，R2=0.987）， soft tissue pogonion （Pos） （B=0.890， R2=0.971）， and menton of soft tissue（Mes） （B=0.942，R2=0.978） showed negative changes， all moved to the left； the movement directions of soft and bone tissues were consistent. At the sagittal （Y） direction， as the bone tissue moved backward， the LL （B=0.882， R2=0.934）， Si （B=0.946， R2=0.847）， Pos （B=0.839， R2=0.909） and Mes （B=0.666，R2=0.455） all moved backward，and the movement directions of soft and bone tissues were consistent.At the vertical （Z） direction， as the bone tissue moved upwards， the LL （B=0.932， R2=0.686）， Si （B=0.834， R2=0.469）， and Mes （B=0.925， R2=0.709） moved downwards， while the Pos （B=0.487， R2=0.444） moved upwards. In horizontal， sagittal， and vertical directions， the correlation between soft-tissue movement（ΔSM） and bone-tissue movement（ΔBM） of chin anatomical landmarks was generally strong （0.6<r<1.0，P<0.01）. Conclusion The changes of soft and hard tissues in the chin of skeletal class Ⅲ malocclusion patients at horizontal， sagittal， and vertical directions after orthognathic operation are influenced by bone tissue movements， showing the positive correlation. The correlations at the horizontal and sagittal directions are stronger， and the correlation at the vertical direction is weaker.

Key words: Orthognathic surgery, Three-dimension study, Chin, Facial soft tissue, Digital technology