primary stability is the most important factor for the survival of mini - implants and is obtained by mechanical interlock between bone and mini - implant [ 13 ] . most of orthodontic mini - implants are either self - tapping ( st ) or self - drilling ( sd ) type . a st mini - implant requires predrilling and a sd mini - implant is placed without predrilling [ 47 ] . the st system has been used for a long time but complications can occur during predrilling such as thermal damage , root damage , and a drill fracture . placement of the sd mini - implant is simple and takes less time and thermal damage can be avoided . this system also enhances primary stability by compressing bones during implantation and contact surface of bone to implant is wider . therefore , primary stability of sd mini - implants is affected by intimate bony contact during placement . micromobility of a mini - implant is minimized proper stability , which enhances new bone formation . the sd system is advantageous with better stability , especially in bone with low density such as maxilla and adolescent patients [ 6 , 7 , 9 , 10 ] . in high density bone or thick cortical bone , however , the sd system is disadvantageous in obtaining good primary stability by inducing excessive pressure that can cause microfracture , adjacent cell damage , and other complications . but there has been dramatic increase in the number of adults seeking orthodontic treatment in the last 20 years . and department of health and human services , 15,600 young people ( younger than 20 years of age ) were newly diagnosed with type 1 diabetes annually in the united states during 20022005 . therefore , clinicians are required to understand effects of diabetes in orthodontic treatment , especially when placing mini - implants . although it is still controversial , diabetes is known to disrupt blood supply by inducing microvascular complications and delay wound healing and increase susceptibility to infection and periodontal disease [ 11 , 12 ] . it was reported that the stability of mini - implant was not obtained in diabetic patients because of poor bone quality and chronic hyperglycemia , which suppressed osteoblast differentiation . many previous literatures reported that mineral metabolism and formation of osteoid and bone were reduced in diabetic hyperglycemic state . the sd system , which can enhance bone - implant contact by compressing bone , is advantageous to obtain good primary stability . however , it has not been studied how the sd mini - implant affects stability in diabetic patients . the purpose of this study was to evaluate effects of type 1 diabetes mellitus ( dm ) and mini - implant type ( self - drilling and self - tapping ) on primary stability of mini - implants . comparative analysis between intentionally induced diabetic rabbits and control group was performed 4 weeks after placement in mechanical stability and microstructural / histological differences . six healthy controls and six diabetic new zealand white rabbits were used in this study . those mini - implants were removed in 4 control rabbits and 4 diabetic rabbit following 4 weeks of healing . rabbits were lightly anesthetized with ketamine hydrochloride 30  mg / kg and xylazine 3  mg / kg ( i m ) . alloxan monohydrate ( sigma aldrich chemical , saint louis , mo , usa ) was dissolved in sterile normal saline to achieve concentration of 5% ( w / v ) , and 100  mg / kg was immediately administered intravenously via marginal ear vein for 2 minutes . 4 , 8 , and 12 hours after alloxan injection , 10  ml of glucose ( 5% w / v ) was administered subcutaneously . four days after the injection , blood samples were collected from aural vein and blood glucose level was monitored using accu - check performa ( roche diagnostics , mannheim , germany ) . if glucose level was over 200  mg / dl , diabetes was diagnosed . blood glucose level and weight of rabbits ( ti - grade - v - alloy , jinbiomed , bucheon , korea ) used in this study were modified cylindrical type ( external diameter : 1.6  mm at neck , 1.5  mm at apex , length : 6  mm ) with cutting flutes at the apex ( figure 1 ) . two implants in one tibia were placed and a total of 4 implants were placed on each rabbit . forty - eight implants ( 24 for diabetic group and 24 for control group ) were placed . the self - tapping ( st ) mini - implants were placed after predrilling with 1.0  mm diameter pilot drill ( figure 2 ) . depth of placement was up to smooth surface below the threaded end of mini - implant head . a surgical engine ( elcomed sa-200c , w&h , burmoos , austria ) was used to record torque value in every 0.125 second during insertion and removal of mini - implants . after the surgery , analgesics ( ketoprophen 1  mg / kg , q.d . ) and antibiotics ( gentamicin4  mg / kg , q.d . ) were subcutaneously administered for 3 days . the rabbits were sacrificed following four weeks of healing using an overdose of anesthetics to induce heart failure . micro - ct images ( pixel size7  m ) were obtained using skyscan-1172 high - resolution micro - ct ( skyscan , n.v . , kontich , belgium ) after fixation of specimens by 10% neutral formalin for 2 days . , kontich , belgium ) . tissue volume ( tv ) , bone volume ( bv ) , bone - volume fraction ( bv / tv ) , trabecular thickness , trabecular number , and trabecular separation in 1  mm area around implants were analyzed with ctan ( skyscan n.v . , kontich , belgium ) software . after micro - ct scanning , tissue specimens were fixed again in 10% neutral formalin for 2 weeks and dehydrated with ethanol series and cured specimens were cut into 220  20  mu thick and were ground into 40  5  mu thick using exakt system ( kulzer exakt400cs , germany ) and stained with he solution . the specimens were examined under a light microscope ( bx51 , olympus , japan ) with ccd camera ( diagnostic instrument , usa ) . image analysis was performed with designated software , spot software v4.0 ( diagnostic instrument , usa ) and image pro plus ( mediacybernetics , usa ) ( figure 4 ) . two - way anova was performed to compare effects of diabetes and placement methods [ 17 , 18 ] . mann - whitney test was used to confirm significant differences of values , bone - volume fraction , trabecular thickness , trabecular number , trabecular separation , bone to implant contact , and bone density , according to the presence of diabetes and placement methods . controls showed continuous increase in weights except at four weeks when implants were placed ( figure 5(a ) ) . high blood glucose level over 300  mg / dl was achieved after injection of alloxan and maintained throughout the experiment . controls showed less variable blood glucose level ranging from 105 to 143  mg / dl ( figure 5(b ) ) . as for bone - volume fraction and trabecular number , similar result was found in cortical bone between the controls and diabetic rabbits . in bone marrow , however , diabetic rabbits showed smaller bone - volume fraction and trabecular number than the controls . trabecular thickness was slightly higher in both cortical bone and bone marrow between the control and diabetic groups . diabetic rabbits had less trabecular separation in cortical bone , but more trabecular separation in bone marrow ( table 1 ) . when compared to the controls , maximum insertion torque was smaller in diabetic rabbits without statistical significance . in diabetic group , maximum insertion torque was higher in sd system without statistical significance . in control group , total insertion energy was significantly higher in sd system in both diabetic and control groups ( p value < 0.01 ) . maximum removal torque and total removal energy were similar regardless of placement method and diabetics ( table 2 ) . mini - implants in this study maintained stability until the completion of the experiment and showed 100% success rate . in cortical bone , all values were similar regardless of placement method and diabetes ( table 3 ) . trabecular separation was significantly higher in diabetic rabbits than in controls ( p < 0.05 ) . bone volume / tissue volume and trabecular thickness were smaller in a diabetic group . in both diabetic and control groups , in diabetic group , trabecular was more abundant in sd system and in control group . no significant difference was found between sd and st mini - implants ( table 4 ) . in the histometrical analysis , bic was smaller in diabetic group than in control group . in controls , bic was higher in sd system . in diabetic rabbits , bics in sd system and bone density ( bv / tv% ) was also smaller in diabetic group ( table 5 ) . bone formation and osteoid volume are reduced early in the course of this disorder [ 19 , 20 ] . the effect of diabetes on osseointegration , however , has not thoroughly verified , especially for orthodontic mini - implants . as use of orthodontic mini - implants is gaining popularity , studies were required to increase success rate and stability of mini - implants in increasing diabetic patients . stability is determined by the shape of mini - implant as well as bone quality , primary stability , and surgical protocol [ 1921 ] . when placing mini - implants in adolescent patients , thin cortical bone , or low bone density such as maxilla , self - drilling mini - implant can enhance primary stability by condensing bone . on the contrary , when placing mini - implants in patients with thick cortical bone or high density bone such as mandible , self - drilling procedure reduced stability by inducing excessive stress to outer surface of the cortical bone [ 5 , 22 , 23 ] . sowden and schmitz reported greater bone damage when placing self - drilling mini - implants when compared with self - tapping mini - implant using scanning electron microscopy . the endosteal surface of the self - drilling system showed large voids adjacent to the screw threads at the interface and microfractures of the bone at bone - implant interface . investigated the relationship between the success rate and torque when tightening into the buccal alveolar bone of posterior regions . to improve success rate of 1.6  mm - diameter mini - implants , they recommended the placement torque to be from 5 to 10  ncm . self - drilling system and tapered mini - implants may increase primary stability , but harmful stress can be generated if the torque is excessive [ 24 , 25 ] . in micro - ct image analysis , cortical bone did not show significant difference between sd and st groups and diabetic and control groups . in bone marrow , this coincides with the previous study that reported advanced trabecular bone resorption in a diabetic state . no significant difference was observed between the controls and the diabetic group as well as self - drilling and self - tapping systems . more significant difference , however , was found in bone marrow . the controls showed higher bone - volume fraction , trabecular thickness , and trabecular number than the diabetic group . in both groups , the self - drilling system was higher in bone - volume fraction , trabecular thickness , and trabecular number than the self - tapping system but there were no statistical differences . the diabetic group showed higher trabecular separation than the controls and the difference was significant . from these findings , it can be assumed that placing mini - implants in area where there is more cortical bone will be more advantageous in diabetic patients . bone to implant contact ( bic% ) and bone density ( bv / tv% ) were smaller in diabetic state than the controls . this resulted from reduced direct bone contact due to decreased total amount of bone in diabetic rabbits . when using the self - drilling system , however , it showed more bone contact between mini - implant and surrounding bone . it can be assumed that the mechanical property of cortical bone was weakened due to diabetes , however , no statistical difference was found . in this study , if this period increases , the effect of diabetes on bone will increase as well . the self - drilling and self - tapping systems showed a similar result in maximum insertion torque in both controls and diabetic group . this result did not match with the expectation that the self - drilling system , which has no predrilling procedure , will have higher maximum insertion torque than the self - tapping system . total insertion energy is the energy absorbed by bone from the beginning to the maximum torque value of mini - implant insertion . it can be calculated by measuring torque value continuously during insertion . in both controls and diabetic group , total insertion energy was higher in sd system than stsystem ( p < 0.001 ) . this can be considered that more energy was absorbed into surrounding bone when using sd system . there was no significant difference between diabetic and control groups in maximum removal torque and total removal energy , regardless of placement system . small sample size due to difficulties of maintaining diabetic state and variability of measurements make it difficult to confirm the statistical significance . therefore , further studies with more animals and longer duration of diabetic condition are required . moreover , further study needs to be conducted in regard to potential effect of loading on these mini - implants , since they will be loaded ultimately . in this animal study , higher bone to implant contact ratio and bone density were observed through the self - drilling method than self - tapping method in controls and diabetic rabbits . this result had no statistical significance , but it can be assumed that bone was damaged or deformed due to absorbed energy . in this study , diabetes was induced through medication with animals ; thus it might differ from the actual responses in human body . however , it can be concluded that the presence of diabetes and placement system do not affect the initial stability of orthodontic mini - implants .
objective . this study aimed to evaluate effects of type 1 diabetes mellitus and mini - implant placement method on the primary stability of mini - implants by comparing mechanical stability and microstructural / histological differences . methods . 
 after 4 weeks of diabetic induction , 48 mini - implants ( 24 self - tapping and 24 self - drilling implants ) were placed on the tibia of 6 diabetic and 6 normal rabbits . 
 after 4 weeks , the rabbits were sacrificed . 
 insertion torque , removal torque , insertion energy , and removal energy were measured with a surgical engine on 8 rabbits . 
 remaining 4 rabbits were analyzed by microcomputed tomography ( micro - ct ) and bone histomorphometry . 
 results . 
 total insertion energy was higher in self - drilling groups than self - tapping groups in both control and diabetic groups . 
 diabetic groups had more trabecular separation in bone marrow than the control groups in both sd and st groups . 
 micro - ct analysis showed deterioration of bone quality in tibia especially in bone marrow of diabetic rabbits . 
 however , there was no statistically significant correlation between self - drilling and self - tapping group for the remaining measurements in both control and diabetic groups . conclusions . 
 type 1 diabetes mellitus and placement method of mini - implant did not affect primary stability of mini - implants .