the orthodontic microimplant ( mi ) , typical of the skeletal anchorage system , is being increasingly used in contemporary orthodontics . mi has allowed clinicians to overcome problems such as anchorage instability or patient compliance dependency , which occur with traditional anchorage methods.1 - 3 various mi systems , while widening the domain of treatment possibilities , have so far exhibited a fairly high success rate.4 however , these small non - osseointegration - based implants do face problems such as occasional loosening and failure . mi failure tends to occur during the first few weeks following placement.5,6 therefore , improving early - phase stability is an important step toward enhancing the reliability of mi therapy . surface modification of mis is not a new idea , but various methods have already been tested and reported as effective both in animal7,8 and human studies.9,10 most previous studies , however , have focused primarily on improving the quality and speed of osseointegration of dental implants . with the belief that ultimately it is the surface topography that directly affects the retention and biocompatibility of an implant , various methods have been tried , including sandblasting , laser treatment , acid etching , coating with hydroxyapatite , and anodization.11 - 17 among these techniques , the surface anodization method , which is relatively simple , appears to be potentially useful for mi systems . effects of anodization on the long - term performance of dental implants have shown promise . anodized titanium implants induced a strong reinforcement of bone response , resulting in a higher clinical success rate.18,19 however , whether these implants can be used as mis remains to be seen given the differences in size , loading condition , and duration of function as compared to dental implants . mi systems are frequently inserted using the self - drilling method , but in earlier experiments , drilling and self - tapping were used as the modes of implant insertion,13 - 15 and the anodized surface should be proven strong enough to resist the insertion torque . most importantly , anodization should be proven to improve the stability of mi during the first few weeks , i.e. , the critical period for mi success . accordingly , this study was designed to investigate the effect of anodization on the early - phase retention capacity of mi . the removal torque and the interfacial shear strength of anodized and machined mis ( which were identical except for surface characterization ) were compared at 6 weeks after implantation in the rabbit tibia . three mature white rabbits , each 10 months old and weighing 3 - 4 kg , were used as experimental animals . the animals were cared for in compliance with an approved protocol from the institutional animal care committee of kyungpook national university hospital ( daegu , korea ) . daegu , korea ) , made of titanium alloy ( eli , astm grade 23 ) , 6-mm long with a diameter of 1.3 mm at the neck and 1.2 mm at the apex , were used . half had a machined surface ( n = 18 ; machined group ) , and the remaining had an anodized surface ( n = 18 ; anodized group ) . anodization was carried out using the process described by suh et al.20 to form a surface layer that included titanium dioxide enriched with ca and p ions . figure 1 presents the surface textures observed with a scanning electron microscope ( s-4200 ; hitachi , tokyo , japan ) . instead of machining lines , suh et al.20 reported that the surface roughness ( ra ) of an anodized surface was 0.73  0.02 m as compared to 0.24  0.12 m for an as - machined one . each animal received 12 mis : 6 were placed in the diaphysis of each tibia . in each tibia , 3 anodized and 3 machined implants were placed . to ensure an even distribution of bone sites for each group of implants , the implant placement sequence was such that every anodized implant was followed by a machined implant in every row and column as shown in figure 2 . both groups of implants were driven in using the self - drilling method without indentation . when the implants were screwed in , we ensured that they were screwed short of full tightness . the final tightening turn for each implant was made using a custom - made driver connected to a digital torque gauge ( e - mobile tech d 17000 series ; seec , seoul , korea ) that measured the peak insertion torque needed to achieve full tightness . the peak insertion torque at which the implants were fully tightened was measured in ncm units with an accuracy of 0.001 . vaseline gauze dressing was used to cover the sutured area so that the surgical site could be opened later with minimum distraction of the implanted site . the surgeries were performed by an experienced orthodontist . after a healing period of 6 weeks , during which the woven bone was replaced by lamellar bone,21 the animals were killed , and the soft tissue on top of the implants was carefully removed . the maximum torque needed to unscrew the implants , i.e. , the peak removal torque , in 3 animals was measured using the digital torque gauge described above . first , the shapiro - wilk normality test was performed to verify normal distribution of the torque data . analysis of covariance ( ancova ) was then employed to compare the group differences in terms of torque measurements . for ancova of the removal torque , the insertion torque was used as a covariate to eliminate possible effects of the initial mechanical pressure imposed on the interfacial bone at the stage of insertion , which might have been influenced by bone characteristics or other surgery factors in addition to surface anodization . statistical significance was determined at p < 0.01 . for a meaningful comparison of the surface characteristics with the results from other studies based on mis of different designs , torque is a product of the interfacial shear resistance and the distance from the interface to implant axis . thus , the amount of torque created by a pitch length of implant , tpitch , is : where the variables used in equation ( 1 ) are as shown in figure 3 . s represents the interfacial shear resistance during insertion or the interfacial shear strength during removal . the amount of torque created by 1 mm length of the implant is obtained by dividing tpitch by the pitch length (= 0.5 mm for sh1312 - 6 mi ) . then , the torque created by cortical bone , tcortical , is : where tc is the cortical bone thickness , which was measured at approximately 1.5 mm . in this study , the insertion and removal torques were assumed to be created by cortical bone only , since all the mis were placed in the tibial diaphysis consisted mainly of cortical bone . finally , s during insertion and/or removal can be calculated using equations ( 1 ) and ( 2 ) . three mature white rabbits , each 10 months old and weighing 3 - 4 kg , were used as experimental animals . the animals were cared for in compliance with an approved protocol from the institutional animal care committee of kyungpook national university hospital ( daegu , korea ) . daegu , korea ) , made of titanium alloy ( eli , astm grade 23 ) , 6-mm long with a diameter of 1.3 mm at the neck and 1.2 mm at the apex , were used . half had a machined surface ( n = 18 ; machined group ) , and the remaining had an anodized surface ( n = 18 ; anodized group ) . anodization was carried out using the process described by suh et al.20 to form a surface layer that included titanium dioxide enriched with ca and p ions . figure 1 presents the surface textures observed with a scanning electron microscope ( s-4200 ; hitachi , tokyo , japan ) . instead of machining lines , suh et al.20 reported that the surface roughness ( ra ) of an anodized surface was 0.73  0.02 m as compared to 0.24  0.12 m for an as - machined one . each animal received 12 mis : 6 were placed in the diaphysis of each tibia . in each tibia , 3 anodized and 3 machined implants were placed . to ensure an even distribution of bone sites for each group of implants , the implant placement sequence was such that every anodized implant was followed by a machined implant in every row and column as shown in figure 2 . both groups of implants were driven in using the self - drilling method without indentation . when the implants were screwed in , we ensured that they were screwed short of full tightness . the final tightening turn for each implant was made using a custom - made driver connected to a digital torque gauge ( e - mobile tech d 17000 series ; seec , seoul , korea ) that measured the peak insertion torque needed to achieve full tightness . the peak insertion torque at which the implants were fully tightened was measured in ncm units with an accuracy of 0.001 . vaseline gauze dressing was used to cover the sutured area so that the surgical site could be opened later with minimum distraction of the implanted site . the surgeries were performed by an experienced orthodontist . after a healing period of 6 weeks , during which the woven bone was replaced by lamellar bone,21 the animals were killed , and the soft tissue on top of the implants was carefully removed . the maximum torque needed to unscrew the implants , i.e. , the peak removal torque , in 3 animals was measured using the digital torque gauge described above . first , the shapiro - wilk normality test was performed to verify normal distribution of the torque data . analysis of covariance ( ancova ) was then employed to compare the group differences in terms of torque measurements . for ancova of the removal torque , the insertion torque was used as a covariate to eliminate possible effects of the initial mechanical pressure imposed on the interfacial bone at the stage of insertion , which might have been influenced by bone characteristics or other surgery factors in addition to surface anodization . statistical significance was determined at p < 0.01 . for a meaningful comparison of the surface characteristics with the results from other studies based on mis of different designs , the interfacial shear resistance between the implant and bone was assessed . torque is a product of the interfacial shear resistance and the distance from the interface to implant axis . thus , the amount of torque created by a pitch length of implant , tpitch , is : where the variables used in equation ( 1 ) are as shown in figure 3 . s represents the interfacial shear resistance during insertion or the interfacial shear strength during removal . the amount of torque created by 1 mm length of the implant is obtained by dividing tpitch by the pitch length (= 0.5 mm for sh1312 - 6 mi ) . then , the torque created by cortical bone , tcortical , is : where tc is the cortical bone thickness , which was measured at approximately 1.5 mm . in this study , the insertion and removal torques were assumed to be created by cortical bone only , since all the mis were placed in the tibial diaphysis consisted mainly of cortical bone . finally , s during insertion and/or removal can be calculated using equations ( 1 ) and ( 2 ) . all the implants were placed without fracture during the insertion and remained stable during the 6 weeks that followed . figures 4 and 5 show the magnitude of the peak insertion and removal torque values for individual implants . as compared to the insertion torque , the removal torque , in general , was reduced at 6 weeks after implant placement , except in the case of 3 implants . these 3 implants , which showed an increase in torque at removal , were in the anodized group . the shapiro - wilk normality test revealed that all the torque data were compatible with a normal distribution . the p values for insertion torque data were 0.175 and 0.248 for the machined and anodized implants , respectively . with regard to removal torque data , the p - values were 0.753 and 0.228 for the machined and anodized implants , respectively . no significant difference in the insertion torque of the machined and anodized implants was indicated . in contrast , a significant difference was noted in the magnitude of the removal torque recorded for anodized ( 3.79  1.39 ) and machined ( 2.05  1.07 ) group implants ( p < 0.01 ) ; these values were obtained under adjustment of the peak insertion torque . the mean interfacial shear strength between the implant and cortical bone at insertion was calculated to be 19.72 and 20.22 mpa for the anodized and machined implants , respectively , and at removal , the values changes to 10.6 and 5.74 mpa , respectively . the drop in the removal torque for most of the implants as compared to their respective insertion torque ( figures 4 and 5 ) can be related to peri - implant bone remodeling . when an implant is placed , it is retained by mechanical purchase from the normal bone . the insertion torque , therefore , is a result of the press fit between the implant and well - calcified bone . during the healing period , as the peri - implant bones undergo remodeling , the pressure at the implant / bone interface falls , and implant retention is increasingly gained from the newly formed bone . the removal torque value becomes dependent more on the extent of bone formation and healing and the extent of mineralization of the newly formed bone . the fall in torque can also be attributed to the occasional presence of soft tissue between the bone and implant . a localized inflammatory response might be a contributing factor.22 - 24 the effects of mi surface topology with regard to bone remodeling needs attention . as shown in figure 1 , the surface of the anodized layer had more porosities and unevenness in comparison with the machined surface . these microfine irregularities , similar to those reported in previous studies,20,25,26 might have created an uneven stress field in the peri - implant bone : higher stresses near elevations and lower stresses near depressions on the mi surface . this uneven stress could have triggered differential remodeling of the interfacial bone , which could have led to a three - dimensional bone structure at the implant / bone interface , which can in turn play an important role in enhancing mechanical locking during removal . effect of surface roughness was more evident in the initial phase following the implant placement . an experiment spanning 1 year27 revealed that bone formation around implants placed in rabbit bone did not show much difference in terms of surface roughness , except in the initial phase . lack of an active response in the later phase may be because after a turn - over of the interfacial bone , the stress field on the interfacial bone became rather homogeneous and so did the bone remodeling there until eventually it reduced to a level that maintained homeostasis . on the other hand , it may indicate that anodization - driven surface irregularities are effective for enhancing the early - phase retention ability of mis . the overall level of insertion torque , or the insertion stresses at the interfacial bone , is an important ingredient in mi stability as an increase in stresses ( or strains ) beyond a certain threshold can result in microdamage and accelerate bone resorption,28 deteriorating implant stability . in order to keep the interfacial stresses within the physiological range , the insertion torque can be used as a safe guide . motoyoshi et al.29 recommended using an insertion torque in the range of 5 to 10 ncm for a 1.6-mm - wide and 8-mm - long mi . based on a first - order approximation assuming a proportionality between the torque and length , and between the torque and square of implant diameter,30 the appropriate torque for a 1.3-mm - wide and 6-mm - long implant would be in the range of 2.5 to 5 ncm . in this study , the mean peak insertion torque of most implants were twofold higher than this value ( table 1 ) , which might have led to initial bone damage during insertion . on the other hand , the insertion torque of approximately 2 ncm exhibited by 2 implants ( # 7 and # 13 ) in the anodized group appeared to be acceptable . thus , the rise in the magnitude of the removal torque observed on 3 occasions , including these 2 , may reflect the absence of a retarding factor such as excessive stresses that might have aided in bone - implant association . in an attempt to rule out the possibility of error induced in the conclusion due to possible inaccuracies in reading results for these 3 anodized implants some researchers31,32 believe that ca and p enrichment of the anodized surface layer helps achieve a chemical bond between the implant and bone , resulting in stronger interfacial shear strength between the anodized implant and bone . from our results , it is difficult to decide whether a chemical bond was formed . determination of the exact contact surface area between the implant and bone using histomorphometry and determination of the extent of calcification of the bone could be used as confirmatory tests , but these were not used in this study . as seen in equation ( 1 ) , torque is composed of 2 factors : the implant design factors or the terms enclosed in parenthesis { } and the interfacial shear strength s. for a quantitative assessment of the effects of the surface characteristics of implants , one needs to isolate s from the torque equation . although not many studies have dealt with this subject , a recent study by morais et al.33 measured s between the rabbit cortical bone and a machined mi of ti-6al-4v material , 2 mm in diameter . after a healing period of 12 weeks , interfacial strength as high as 12.34 mpa was reported , and this was much higher than the values recorded for machined implants and even higher than that recorded for anodized implants in this study . a thorough examination , however , revealed that the formula used by morais et al.33 was imperfect . recalculation using equations ( 1 ) and ( 2 ) yielded a value of 5.46 mpa instead of 12.34 mpa , which is close to the results we obtained for the machined implant and suggested that the present findings were reliable . this animal experiment revealed that anodized implants have a retentive advantage at 6 weeks after placement . however , the anodization procedure increases the manufacturing costs of implants and consequently the total cost incurred by patients . on the basis of our 6-week observation , we concluded that anodizing titanium alloy orthodontic mis significantly increased their interfacial shear strength and/or removal torque as compared with machined ones . this increase might enable anodized mis to withstand orthodontic loading more successfully by providing better mechanical resistance during the important early phase after mi placement .
objectiveto determine the effect of surface anodization on the interfacial strength between an orthodontic microimplant ( mi ) and the rabbit tibial bone , particularly in the initial phase after placement.methodsa total of 36 mis were driven into the tibias of 3 mature rabbits by using the self - drilling method and then removed after 6 weeks . 
 half the mis were as - machined ( n = 18 ; machined group ) , while the remaining had anodized surfaces ( n = 18 ; anodized group ) . the peak insertion torque ( pit ) and the peak removal torque ( prt ) values were measured for the 2 groups of mis . 
 these values were then used to calculate the interfacial shear strength between the mi and cortical bone.resultsthere were no statistical differences in terms of pit between the 2 groups . 
 however , mean prt was significantly greater for the anodized implants ( 3.79  1.39 ncm ) than for the machined ones ( 2.05  1.07 ncm ) ( p < 0.01 ) . 
 the interfacial strengths , converted from prt , were calculated at 10.6 mpa and 5.74 mpa for the anodized and machined group implants , respectively.conclusionsanodization of orthodontic mis may enhance their early - phase retention capability , thereby ensuring a more reliable source of absolute anchorage .