| 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, when excessive, 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 it is also difficult to control the distraction vector. Long-term stability and further growth of the maxilla are still to be studied. 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. Patients with cleft palate frequently present with horizontal and vertical maxillary deficiency, scarred palatal mucosa, and a compromisedblood supply, which can cause the difficulty of treatment.In order to obviate the problems of traditional technique, a new concept of midfacial skeleton protraction was developed. The midfacial skeleton was protracted by bone-borne forces other than by teeth-borne forces, which applied at a medium position, near the resistance center of midfacial skeleton. In this experiment, the possibility of midface protraction at the medium position was explored and the factor of animal age related to protraction response was evaluated.Part I Protraction of midfacial skeleton at its medium position: The possibility of a new conceptA protraction system was developed, it consists of three parts: a rigid external framwork, a bone hook, and elastic. Four 12-week mongrel dogs were randomly assigned into two groups: the control (n=l) and the experimental groups (n=3). Four pairs of titanium bone markers were fixed on either side of suture of all animals. The control group was normally fed. the experimental group was fitted with a distraction device and a forward elastic force was exerted for 1 month. Clinical observation and X-ray examination were performed at the beginning and end of distraction period. All animals were killed at the end of the force application period. Histological study was made on the new bone specimen. Results: All experimental animals appearances a progressively increasing anterior movement of maxilla and the occlusal relationship were in an Angle Class II until the end of the experiment. The increasing distance between every pair of bone markers showed that premaxilla advancement was more thanthat of the maxilla. Next was the zygoma. There was no obviously change in the basilar-maxillary angle and counterclockwise rotation between the control and the experimental group animals. There was active osteogenesis in sutures of experimental animals, especially in premaxillar-maxillary sutures. The number of osteoblasts and fibroblasts increased dramatically and new bone formation was found at the edge of the suture. The osteoblasts and fibroblasts apposited along the force direction. Conclusion: This technique can provide significant advancement of maxilla. The force was direct, which can avoid the counterclockwise rotation of maxilla.Part II Midface skeleton protraction : Responses of animals at different ages and Long-term effectsThe protraction system was the same as study Part I. Four 12-week mongrel dogs in the same nest were Randomly divided into dog A, dog B, dog C, and dog D. dog A was served as the control. Midfacial skeleton protraction was applied to dog B, C, and D at different age. In dog B, it started at 12 weeks...
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