The Effect Of Varying Bone Quantity Endured Loading On Orthodontic Mini-implant Stability: A Biomechanical Analysis

Objective:This study adopts the method of in vitro experiments. Themini-implants were inserted into pig iliac bone blocks depending on thedifferent distances to the edge of bone. The aim of this study was to evaluatethe effect of different bone quantity endured loading on the anchorage abilityof titanium mini-implants used for orthodontic anchorage.Methods:1Making bone block modelThe ilium of country pigs was chosen as the bone block model, once allthe soft tissue and cartilage had been removed with a size22scalpel, abandsaw was used to saw the ilium into blocks measuring11cm long and9cmwide. All bone/mini-implant blocks were subsequently transfered into10%buffered formalin at4°C for fixation.2Mini-implant insertion2.1Marking the implant siteAfter the soft tissue had been removed, eigth positions were marked onthe surface of all the test blocks for mini-implant placement. Marking twopoints respectively at3mm，4mm，5mm and6mm from the edge of the bone.The lateral distance between adjacent points is10mm. Two points of the samedistance from the edge, the left site is planned to inserted vertically and theright45°tilted occlusally.2.2Making the pre-drillingThe handpiece was operated with a dental implantation motor. Setting therotate speed of handpiece as1100rpm. The pilot drilling were performed inthe direction of the planned mini-implant insertion. As recommended by themanufacturers, the pilot holes were drilled under cooling with physiologicalsaline The drilling depths were adjusted at7mm with the predrilling diameters1.2mm.2.3Screwing the mini-implantsAll mini-implants were manually inserted into the pilot hole using ahandheld screwdriver in clockwise direction. Finally, setting the insertiontorque as10Ncm with tortion tester.3Biomechanical testAfter all mini-implants were inserted into bone blocks, pull strengthtesting was performed by using a microcomputer control electronic universaltesting machine. The testing machine tightly clamp the bone/implantsspecimen and pull the mini-implant head by a stainless steel wire. The pullforce should be parallel to the bone surface and perpendicular to the edge ofbone. The mini-implants were pulled out at a crosshead speed of0.05mm/s.The specific information was displayed on the computer screen in the form ofload-displacement curve. When the curve showed a trend of falling sharply,switched off the testing machine. The applied load was monitored, and thepeak load at extraction (maximum pull strength, Fmax) was obtained from thedata file.4CBT EvaluationAfter the biomechanical test, the bone/implants specimen were carefullysectioned into small blocks, each containing1mini-implant, which wassurrounded by<3mm of bone. The CBT all around the small blocks wasevaluated using the vernier caliper. Record the average CBT of the sides ofevery mini-implant.Results:1Morphological changes of the mini-implantOne implant was broken and two slightly bent, two were pulled out of thebone during pull strength test. Other mini-implants still remain in the boneafter the test occuring only slight movement or tilting. 2Comparison of implant biomechanics performance varying bonequantity endured loading：2.1When mini-implants inserted vertically to the bone, maximum pullstrength increasing with the increase of distance from the edge of the bone（P<0.05）.2.2When mini-implants inserted45°tilted occlusally to the bone,maximum pull strength increasing with the increase of distance from the edgeof the bone（P<0.05）.3Comparison of implant biomechanics performance during differentinsertion angles：3.1When the distance between mini-implants and the edge of bone is3-5mm, maximum pull strength of the vertically groups were less than the45°tilted occlusally groups（P<0.05）.3.2When the distance between mini-implants and the edge of bone is6mm, There was no significant difference between the vertically groups andthe45°tilted occlusally groups in term of maximum pull strength（P=0.052）.4Comparison of cortical bone thickness during different group: Therewas no significant difference during the vertically groups and the45°tiltedocclusally groups (P>0.05).Conclusion:1The bone quantity endured loading significantly affect thebiomechanics performance of mini-implant/bone: Within a certain range,biomechanics performance gradually increased with the increase of bonequantity endured loading. Increasing the bone quantity is conducive to theprimary stability of mini-implant.2The implanting angle affect the biomechanics performance ofmini-implant/bone: When the implant position to alveolar crest is nearer,inserting45°tilted occlusally is more beneficial to the stability than verticallyunder loading occlusally.