The Stability Of Mini-Implant In Maxillary And Mandible-An Experimental Study In The Pig

Anchorage, defined as a resistance to unwanted tooth movement, is a prerequisite for the orthodontic treatment of dental and skeletal malocclusions. Tooth, dental arch, muscles of oral or cranial bones et al is generally used as the anchorage. The traditional methods to strengthen anchorage are palatal bar, Nance arch, lingual arch and facebow et al. However, these methods need the corporation of patients so as to the effects can not be guaranteed. And In that case, the displacement of anchor-tooth is likely take place. Therefore, a simple, comfortable and efficient pattern of anchorage is needed urgently in the present.With the development of oral implantology, Gainsforth who placed vitallium screws in the ascending ramus of 6 dogs to retract their canines in 1945, got the idea of using screws fixed to bone to obtain absolute anchorage. And then, implant used as anchor had become a new field of study. The implant can provide anchorage efficiently and avoid the unexpected displacement of teeth; hence the effect of treatment had been improved. This pattern of anchorage can overcome many defaults of traditional one and don't need the corporation of patients, so it is welcomed badly by orthodontics and patients and obtained a repaid development.The success of implant anchor lies in the stability under load. At present, the success rate of implant anchor is at a low level. Therefore it has became a hot spot of study over the world. A lot of elements can influence the stability of implant anchor. Many investigations have been processed on the shape, diameter, length, taper, coating of surface of implant and also the load, implant site and combination style with the surrounding tissues. There are rare reports on the healing style and healing process of implant anchor in the present.In general, maxillary and mandible is the common implant site. Their bone quality and volume is suit for the implantation. But maxillary is mainly composed of spongy bone which is loose and its cortical bone is thin. However the mandible is mainly composed of compact bone which is compact and its cortical bone is thick, i.e. obviously difference exits between the maxillary and mandible. In the present studies, the implantation process and healing time in the maxillary and mandible is no difference. That maybe decreased the success rate of implant on some reports. It has not been clear that whether or not the difference between maxillary and mandible can influence the healing process of implants.Objective: To investigate the effects of different bone structure between maxillary and mandible on the healing process of mini-implant for the purpose of theory for clinical and improving stability of implant.Method: All premolars and first molars were extracted from 4 pigs. After 3 months'healing time, 76 mini-implants were surgically placed in the edentulous area of the pigs. All the implants were divided into 6 groups. 3 of them were named as 0-week group, 3-week group and 12-week group, respectively. Each group consisted of two subgroups and each subgroup contained 7 mini-implants. The mini-implants in these three groups experienced several hours, 3 weeks or 12 weeks respectively before the animal killed. The other 3 groups were 6-week group, 9-week group and 12-week group. Each group consisted of two subgroups and each subgroup contained 6 mini-implants. After 4 weeks unloading period, the mini-implants in these 3 groups were loaded for another 2 weeks, 5 weeks or 8 weeks by 100g force respectively before the animals killed. After all the animals killed, their maxillary and mandibles were dissected. The specimens were made into section, and stained by toluidine blue. We studied the implant-to-bone surface on histology of the sections.Result: 1 After the implantation, part the surface of implant combined closely with bone tissues but there was an obvious space between other part and surrounding tissues. In the space there were blood cells, tissue fragment and so on. After 3 weeks of healing time, the most of surface of mini-implant was surrounded by granulation tissues which had transformed to the bone tissue in the vicinity of old bone edge. In the 6th week, the surface of mini-implant had been surrounded by new bone tissue absolutely. In the 9th week, the transformation of new bone to lamellar bone and Haversian lamella had been completed. In the 12th week groups with the load, the boundary of new and old bone had not been clear, and also, the direction of lamella was consistent with the loading force. However it was confused compared with the 12th week unloading group.2 The BIC% of mandible was greater than maxillary in all groups except for the 6 week one.3 Percentages of peri-implant within 50um calcified area (CA% within 50um) and BIC% reached the minimum at the 3rd week and the peak at the 6th week. There was no difference within 500um calcified area in all groups.4 The result of polarized light indicated that the direction of lamella was confused in the unloading groups and consisted with the loading force with loading groups.Conclusion: 1 Min-implant can remain stability under the 100g force after 4 week healing time.2 BIC% and CA% of mandible was greater than maxillary at the same time.3 The direction of lamella, which was effected by load, was consisted with the force direction in order to provide the maximum resistance to the loading force.