Effects Of Mechanical Force On Remodeling Of Premaxillary Suture And Zygomaticomaxillary Suture In Rabbits

[Objective] In this study, we established the animal model of premaxillary retraction with micro-implants placed in maxillary as the anchorage in rabbits. Effects of mechanical force on suture remodeling in vivo can be examined through histological, cytological, protein and molecular ways, which will provide deeply theoretical evidence and scientific basis for investigating the mechanism of maxillary orthopedics.[Methods] Part1:Animal model's establishment. The male Japanese rabbits in2.5month were used in the experiments and were randomly divided into3groups experimental groups, placed in micro-implant without force group (no force group) and blank control group. The average weight range was between2and2.5kg. In experimental groups, micro-implants were placed in premaxillary and maxillary respectively as the anchorage. Mechanical force produced by nickel titanium spring was exerted to the premaxillary suture on both side immediately, which will bring compressive force to premaxillary suture and tensile force to zygomaticomaxillary suture. The same procedure was operated to no force group. And nothing would be done in blank control group. Part2:Different forces (50,100,150,200,250,300g/side) was exerted to the premaxillary suture on both side in different little experimental groups in7days. All of the rabbits were killed after7days. Overbite and overjet of their incisors were measured. Bone tissues of premaxillary and zygomaticomaxillary suture were manufactured into paraffin section. The histological change of suture was observed by hematoxylin-eosin (HE) staining and light microscope (x100). Osteoclast was observed by tartrate-resistant acid phosphatase (TRAP) staining. Matrix metalloproteinase-9(MMP-9) and bone morphogenetic protein-2(BMP-2) were detected by immunohistochemistry to show the change of bone resorption and formation, which can indirectly prove which force is the best one for suture remodeling in rabbits. Part3:According to previous research, the best force of250g/side was exerted to the premaxillary suture on both side. All of the rabbits in experimental groups were killed after1,3,5,7,14and21days. Also, overbite and overjet of their incisors were measured. The histological change of suture and osteoclast were observed by HE and TRAP staining. MMP-9and BMP-2were also detected by immunohistochemistry.[Results] Part1:The weights of rabbits used in part2experiment didn't change significantly among the experimental groups, no force group and blank control group before and after the experiment (P>0.05). The weights of rabbits used in part3experiment increased significantly with time (14,21days group) among these three groups before and after the experiment (P<0.05). Part2:After7days, overbite didn't change significantly (P>0.05), but overjet decreased significantly under different force (P<0.05). Immunohistochemical staining showed that, MMP-9mainly appeared in the cytoplasm of osteoclast and the matrix of bone cell. BMP-2primarily expressed in the cytoplasm of osteoblast and the matrix of bone cell. In premaxillary suture, MMP-9expressed increasingly with the increase of force. The expression of MMP-9reached its peak, while the force was250g/side on both side. Then its expression declined with force increasing continuously. In the beginning of loading mechanical force, BMP-2didn't express obviously. With the force enhanced to150g/side, it began to appear and increased with the force raising. Its expression kept almost in the same level until250g/side force was exerted. In zygomaticomaxillary suture, MMP-9enhanced with force increasing without descent. BMP-2expressed increasingly with the increase of force. TRAP staining showed that, osteoclast looked like fried eggs with dark wine-red cytoplasm and blue nucleus, which had two or more nuclei. The appearance and distribution of osteoclast had the same alteration with MMP-9's expression. Comparing with no force group and blank control group, all these indexes in experimental groups had statistical significance (P<0.05). Part3:Overjet of rabbits'incisors decreased gradually with time under250g/side force on both side (P<0.05). Overbite also changed. Edge-to-edge bite of incisors appeared in14days group. Crossbite showed in21days group (P<0.05). In premaxillary suture, MMP-9expressed increasingly with time, which reached its peak in7days and decreased in14days. It fell into the lowest in21days. BMP-2showed slightly in the first day after loading force, but reduced to the minimum gradually till7days. Then its expression rebounded in14days and reached the maximum in21days. In zygomaticomaxillary suture, MMP-9reduced in the first day, and fell into the lowest in3days, then enhanced gradually with time. BMP-2showed slightly in the beginning and fell into the lowest in3days. It gradually strengthened after7days. TRAP staining showed that, the appearance and distribution of osteoclast had the same alteration with MMP-9's expression. Comparing with no force group and blank control group, all these indexes in experimental groups had statistical significance (P<0.05).[Conclusions] The animal model of premaxillary retraction with micro-implants placed in maxillary as the anchorage in rabbits has the excellent feasibility and repeatability. The best mechanical force loading on suture of rabbit is250g/side. The activity period of maxillary orthopedics after suture loaded on force is7days, and decreases in14days. The whole period of suture bone tissues reconstruction is21days.