| Objective:Malocclusion affects craniofacial development, oral health and founction, facial esthetics, psysical and psychological health seriously. In China, the prevalence of malocclusion is reported by 72.92%. The number of patients requiring orthodontics thearoy has undergone a marked increase in recent decades with the standard of living and awareness of oral health. Anchorage control is a fundamental aspect of orthodontic biomechanics. Poor anchorage control is one important limiting factor for Orthodontics development. There have been many attempts to divise suitable anchorage methods, including introral and extraoral appliances. All intraoral appliances, however show some loss of anchorage. Extraoral appliances do not privide reliable anchorage without patient's comppliance. Especially, mini-implants have increasingly been used in cesent years for orthodontic anchorage because of their small size, flexible insertion site, ease placement, can immediately loading, and accepeted attention by scholar. However, mini-implants still fail frequently. Miyawaki and Cheng et al repoted that the success rates for mini-implants were 83-89% How to improve implant stability is the research focus.The fell off implants happen mostly before orthodontics loading and during the process of loading. Failure of oral implants may often be attributed to inadequate operation and biomechanical coupling between implant and bone. The stability of implant was influenced by many factors. Implants must satisfy requirements of primary satility, and later they must withstand the srtesses and strains to which they subjected. Good micro-environment is conducive to the formation of implant-bone healing and to withstand load. The choice of implant, implant surgery, initial stability, load direction, and healing time are the important factors on stability of mini-implants. Therefore, the studies that make clear the influencing factors on the stability and standardize the operating system are important to popularize and use mini-implant.This research established some models of mandible with mini-implant including different insertion angles, mini-implant lengths and loading directions. Analyzing the stress distribution and displacement in bone influenced by orthodontics loading. The purpose was to offer a theoretical basis to rational choice and use mini-implant in clinical, reduce stress concentration in bone interface. To Study the effection of different insertion torque on bone healing by means of establishing animal models with different insertion torque and healing periods under loading. This study is to observe the biomechanical and histologica properties of peri-implant bone under different heailing period. It can also provide a reference for chosing rational insertion torque and loading period according to initial stability. To explore the relevant factors of stability through clinical loading tests under different healing period. The results would provide experimental data and theory for the future clinical studies and pro-clinic applies.1 Experiment one:Establishing the FEA models of the jaw with mini-implant1.1 Obtaining three dimensional radiographic data of the jaw by CT scan, the three-dimensional finite element analysis mandible model was established.1.2 The four kinds of mini-implant models were established. And the diameter was 1.6mm, the length were 6mm,8mm,10mm and 12mm respective.1.3 The insertion region of implant was in the between first molar and second premolar. Four lengths of 6,8,10,12mm of implant modles were inserted vertically into designed site of mandibular and 8mm length of implant was embbed vertically,30°tilted mesiolly,60°tilted mesiolly,30° tilted occlusally,60°tilted occlusally, respectively.Four lengths of 6,8,10,12mm of implant-mandible modles were chosen. A force of 1.96N was applied.mesioly and 45°tilted mesiol-vertically in modles. The stress distribution under every condition was recorded and analyzed.The chosen models were that the implants were inserted vertically,30°tilted mesiolly,60°tilted mesiolly,30°tilted occlusally,60°tilted occlusally, respectively. A load of 1.96N was applied mesiolly to the head of implant, caculated the maximum Von-Mises stress.The model with implant inserted 30°tilted mesiolly was loaded by a force of 1.96N mesiolly, mesiol-vertically, vertically, distal-vertically, distally, respectively. The model with implant inserted 30°tilted occlusally was loaded by a "force of 1.96N mesiolly, mesiol-vertically, vertically, respectively. The stress distribution under every condition was recorded and analyzed.5.1 Experimental design:The implants were equally divided into 14±1Ncm and 11±1Ncm insertion torque among 4 adult male Beagle dogs.5.2 Empirical procedure and method:Thirty-six mini-implants (1.5mm in diameter,7mm in length) were placed buccaly at the interradicular with second, third, forth premolars and first molar bilaterally of the mandibular of dogs.14±1Ncm insertion torque and 11±1Ncm insertion torque were made on two groups at the time of fixture placement. Bone tissue responses were evaluated by histological analysis at 7 days and 28 days after implant placement. Following an observation period of 7 and 28 days, the mini-implants and the surrounding bone were removed. Undecalcified serial sections were made and the degree of ossointeration studied. All measurements were statistically evaluated using independent t-ests to determine any difference in insertion torques and histomorphometric analysis, (bone-implant contact and bone area). A P value less than 0.05 was considered significant.6.1 Experimental design:8 twelve month-old male beagle dogs were divided into two groups. The test mini-implants remain in the low jaw for an additional 10 wks of force application (100 g) after 0,1,3 and 8 weeks of bone healing. Healing control (HC) implants were further divided into 4 groups (1,3,8 and 10 weeks). It is important to note that the HC implants to termination of the experiment were placed in the jaw for 1,3,8, or 10 wks prior so that bone could be assessed at the initiation of loading.6.2 Empirical procedure and method:Sixty-four mini-implants were placed in low jaw among 8 twelve month-old male beagle dogs. The location and surgical procedure of this study were sismilar to which in study four. The insertion torques was 12±1Ncm. Recorded the insertion torques while the implants were embedded. The test mini-implants of force application (100 g) after different bone healing period according to disign. Maximum removal torque (MRT) testing was performed to evaluate the interfacial shere strength of each test groups. Surface analysis of removed implants were performed by SEM. MRT of two groups after different healing period were compared with the use of ANOVE. A P value less than 0.05 was considered significant。Forty-six patients who required skeletal anchorage for orthodontic therapy were included in a prosective study. A total of 109 mini-implants (1.5mm in diameter,7mm in length) were placed in 46 patients (11 males,35 females, and average age of 17.8 years old). A variety of orthodontic loading were applied after varied healing period including early and delayed loading. Possibe correlation between various clinical parameters and mini-implant failure and complications were evaluated by the chi-aquare or Fisher exact tests where appropriate. AP value less than 0.05 was considered significant.1 Experiment one:The three-dimentional FEA models of mandible and mini-implants were established:Four lengths of mini-implant models obtained were 6mm,8mm,10mm and 12mm. Five insertion angle models of mini-implant with 8 mm of length were established, embedding angle were vertical,30°tilted occlusally,60°tilted occlusally,3°tilted mesiolly,60°tilted mesiolly, respectively. The geometric, biomechanics similarity and clinical adaptability of modles established achieved the test requirements.2 Experiment two:Stress distribution on bone under different mini-implant lengths:The results showed that the peak stress occurred at the cervical bone margin adjacent to the implants on different loading conditions. The change of the length didn't show much influence on the stress ditribution. When given load mesiolly, the maximum stress varied from 3.765Mpa to 3.5Mpa, the maximum displacement varied from 1.288μm tol.266μm. When load was applied 45 degree mesiol-vertically, the maximum stress varied from 4.51Mpa to 4.075 Mpa, maximum displacement varied from 1.694μm to 1.668μm.3 Experiment three:Stress distribution on bone adjacent to a mini-implant under different embeded direction:The results showed that the peak stress occurred at the cervical bone margin adjacent to the implants. When inserted with 30°ilted mesiolly, the peak stress is less than other models obviously and the stress distributions of 60°ilted mesiolly was more even secondly. When mini-implants inserted with angle of 90°nd 60°ilted occlusally, the stress concentration was more obvious and the peak stress was the highest.4 Experiment four:Stress distribution on bone under different loading directions of mini-implan:The rerults showed that when mini-implants inserted with angle of 30°tilted occlusally, stress distribution of loading vertically was evener than loading mesiol-vertically, loading mesiolly was the unevenest. When inserted with 30°tilted mesiolly, stress distributions of loading mesiolly and distally were more even, and the difference with them was little. The strees distribution of vertical loading was the unevenest.5.1Clinical observation when mini-implants inserted with 11±1Ncm and 14±1Ncm torque value:A total of 36 implants were placed in mandible of 4 beagle dogs,2 of them failed. The survival rates of implants with 11±1Ncm and 14±1Ncm torque value were 100%,88.9%, respectively. There were no significant differences between two groups. The total success rate was 94.4%.5.2Observation by light microscope when mini-implants inserted with 11±1Ncm and 14±1Ncm torque value:The histomorphologic analysis showed that diffuse staining and cross-hatch staining were discovered in cortical bone of implant neck or apex with 14±1Ncm torque after 7 day. The microdamage still exist after 28 day. In contrast, cross-hatch staining was discovered in cortical bone of implant neck with 11±1Ncm torque after 7 day, but disappeared after 28 day.5.3The histomorphometry analysis:The results showed that there were no significant differences for the BIC between 11±1Ncm group and 14±1Ncm group at 7 day and 28 day. But the BA within 50μm and 150μm for the 11±1Ncm group (53.15%,51.53%, respectivly) were great than 14±1Ncm group (32.56%,34.55%, respectivly) at 28 day.6.1 Clinical observation:A total of 64 implants were placed in mandible of 8 beagle dogs,3 of them failed. The success rates were 96.9% for loading mini-implants,90.6% for unloadings, and 93.8% for overall.6.2SEM observations:The results indicate that the mineral formation in the nanosized layer adjacent to the titanium surface of mini-implant increases over time for loading and unloading groups. The striking finding was the loading promotes the bone healing around implants, except healing of 3 wks.6.3Removeal torque value changes:There were no signifacant differences for removeal torque value of unloading implants, mean of 2.42±0.29Ncm. The mean removal torque values for the loading implants were 4.10±0.39Ncm at 0 week,4.25±0.70Ncm at 1 week,2.42±0.44Ncm at 3 weeks and 4.42±0.38 Ncm at 8 weeks. During the process of healing, the removal torque values of tested implants were significant highter than control implants except at 3 weeks.Implant mobility or complete exfoliation was found for ten implants among 109 mini-implants. Six of them failed before the appilication of orthodontic load, one implant was lost after loading of less than 1 month, and three were lost during delayed loading period. The overal mini-implants success rate was 90.83%. Mini-implants on the buccal and labial side of the mandible and palatal side of maxilla had a higher failure rate. There was no significant difference between early loading and delayed loading. Root contact during placement of mini-implants and inflammation increased significantly the possibility of implant failure.1 Insertion direction and angle of the mini-implant can affect the stress distribution of infacial bone. Embbeding direction of the implant shoud be accordance,with loadingdirection as much as possible, and decrease the angle between bone sueface to long axies of implant.2 The change of the length within 6mm-12mm didn't show much influence on the stress distribution. There is no use to select longer mini-implants strictly. 3 Loading direction and angle of the implant can affect the stress distribution of infacial bone. Loading direction of implant should be accordance with or contrary to the insertion direction of implant for orthodontic anchorage. When loading derection was perpendicular with long axis of the implant, the interfacial bone will result in stress concentration.4 Primary stability is basis of osseointergation formation of mini-implants. Suitable insertion torque of mini-implant is in favor of good micro-environment of peri-implant bone healing. The strees between the implant and infacial bone should Control within certain limits to guarantee blood supply for bone. Appropriate insertion torque produce bone interface with little microdamage, and healing speedy. Heavy insertion torque can cause intense bone microdamage around interface of implant-bone by strong stress concentration, damnify the healing of the implant-bone interface. The mini-implants should be placed using an appropriate insertion torque, over-tighting should be avoided.5 Suitable mechanical stimulation contributed to healing of bone-implants after mini-implant was inserted.6 There are a stable dangerous period, after mini-implant was inserted and before the bone remodeling to complete. As a function of healing time, orthodontics loading immediately or 1 week,8 week after implant was inserted appear a positive effect on peri-implant bone remodeling and implant stability except at 3 weeks of healing period.7 Initial stability is an important condition for early loading of mini-implant. When implant inserted with good primary stability, early orthodontics loading is considered no impact to implant-bone healing.8 To reduce the failure rate, the inflammation around the implant body should be controled, particularly mini-implants were inserted on the buccal and labial side of the mandible and palatal side of maxilla.
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