The Experimental And Clinical Studies Of Midfacial Skeleton Protraction At Its Medium Position

Midface hypoplasia is a common deformity of Craniomaxillofacial regions. The current techniques for this deformity include bone grafting, orthopedic protraction, orthognathic surgery and distraction osteogenesis. Bone grafting is only suitable for mild deformity and exists problems of bone resorption, relapse, and the potential donor site morbidity. Orthognathic surgery advancements couldn't correct severe midface deficiency. When excessive, it may result in large defects between regions of maxillary bone contact, Increasing relapse, and postoperative instability. Distraction osteogenesis can achieves rapid maxillary advancement associated with favorable soft-tissue changes. It is especially suitable for severe maxillary deficiency. However, it is usually more traumatic and treatment time is long. Tooth-borne Orthopedic protraction is less traumatic and can cause the forward displacement of the maxilla. However, the point of force application is at the inferior end of the midfacial skeleton, far away from the center of resistance of the maxilla. The technique usually leads to unfavorable tooth movement, tooth root resorption, and counterclockwise rotation. It often yields smaller skeletal movement.In order to obviate the problems of the traditional techniques, a new concept of midfacial skeleton distraction was developed. The midfacial skeleton was distracted by bone-borne forces other than by teeth-borne forces, which applied at a medium position, near the resistance center of midfacial skeleton. The biomechanical changes of midface protraction at the medium position were evaluated in experiment. Part I Experimental studies of Midface skeleton distraction at its Medium position Objective: 1. To explore a new method for constructing three-dimensionalfinite element model of craniofacial complex. 2. To explore the application of craniofacial suture in the three-dimensional finite element model of craniofacial complex. 3. To investigate the biomechanical changes of midface skeleton distraction at its medium position in the craniofacial complex, using FEM. 4. To investigate the effects of directions of midface skeleton distraction at its medium position.Methods: Combining spiral CT scanning technology with the three-dimensional finite element method, together with DICOM data transferring, a three-dimensional FE model of craniofacial complex of midface hypoplasia was developed. In FE model, craniofacial sutures were constructed. Different distraction forces were applied to the three-dimensional FE model : (1) The first molar, (2) full maxillary arch, and (3) the floor of apertura piriforms. Biomechanical changes from different position distraction were investigated by means of finite element analyses. An anteriorly directed 500g force was applied to the floor of apertura piriforms in directions varying from 0 to -40 degrees to the occlusal plane. The displacement patterns and stress distributions of the craniofacial complex were evaluated.Results: 1. A three-dimensional FE model of craniofacial complex of midface hypoplasia was developed. The model comprised teeth and craniofacial sutures, and consisted of 57481 nodes and 52901 solid elements. 2. The displacement of the skeletal structures with sutures was significantly larger than that without sutures. 3. Distraction forces at the level of the floor of apertura piriforms produced a more forward movement of the upper maxilla in Sagittal direction. Vertical and lateral displacements were less than those in loading with teeth or denture. The region of the Von Mises (VM) concentraction was reduced in midface skeleton protraction at its medium position. Compressive stress on the radix nasi decreased obviously. 4. As the force direction was more downward, displacement of the craniofacial complex became smaller in the Sagittal directions. Vertical displacement changed from upward to downward. Displacements were mosttranslatory in loading with forces applied in the directions ranging from -20 to -30 degrees to the occlusal plane. The downward forces about -20 to -30 degrees to the occlusal plane could generate the uniform stress distribution in the craniofacial sutures and avoid counterclockwise rotation of the maxilla.Conclusion: 1. It could improve the speed of reconstructing 3D finite element model through DICOM data. The model showed excellent geometry similarity. The method was feasible and effective. 2. The construction of craniofacial sutures in the three-dimensional finite element model of craniofacial complex could improve the accuracy of finite element model. 3. Compared with traditional orthopedic protraction, midface skeleton distraction at its medium position could advance maxilla en bloc, Minimize the counterclockwise rotation of the maxilla, and reduce the constriction of the anterior part of the palate. 4. The distraction forces of -20 to -30 degrees, which were applied to the floor of apertura piriforms, may produce most optimal reposition of the craniofacial complex. Part II Clinical studies of Midface Skeleton distraction at its Medium PositionObjective: To evaluate the effects of midface trans-suture Distraction osteogenesis and Distraction osteogenesis at its medium position for treatment of midface hypoplasia.Methods: 1. Ten cases with class III maxillary deficiencies were treated with midface skeleton distraction at its medium position. Mean treatment time was 4-5 months. Lateral cephalometric films were taken and analyzed before and after treatment. 2. Eight cases with class III severe maxillary deficiencies were treated with midface skeleton distraction at its medium position. Following Le Fort III osteotomy, traction hook was hitched to the floor or side of apertura piriforms. Through the nostrils, it connected with face bow or rigid external distraction device. Mean treatment time was 3 months. Lateral cephalometric films and photographs were taken and...