An Experimental Study Of A Modified Distraction Implant

Dental implants have been extensively used as replacements of lost ordamaged natural teeth, which have better functional and aesthetic resultscompared with other kinds of restorations. However, insufficient alveolar heightresulting from atrophy, trauma, congenital malformation, and resection oftumors limits implant placement. As a result, various augmentation techniques,such as bone grafting, guided bone regeneration (GBR) and alveolar distractionosteogenesis (ADO) have been used to correct this deformity. Among thevarious augmentation techniques, ADO is a technique of gradual bonelengthening allowing the body’s natural healing mechanisms to generate newbone. It offers significant advantages including predictability, elimination of adonor site for autogenous grafts, and simultaneous bone and soft-tissueregeneration. Currently, ADO has been demonstrated to be a promisingtechnique and has gained increasing acceptance.Currently available alveolar distraction designs include extraosseous distractors placed on the lateral side of the bone, endosseous distracters insertedinto the bone segments, and distraction implant (DI). Among them, DI canfunction as both a distractor and an implant eventually. Its predominantadvantage is the single-step surgical technique, which not only simplifies theoperation and minimizes the trauma, but also dramatically reduces the treatmenttime, thus satisfying both patients and dentists.However, DI has its own drawbacks like narrow indication, high risk ofcomplications and unfavorable biomechanical properties due to the complexstructure. In addition, the device will prove difficult to remove if it fails with anincrease in morbidity of the procedure for removal. Therefore, furtherimprovement of the DI devices is in a great need.In this study, a modified DI device was developed. Finite element analyseswere performed to optimize some parameters of DI, which could providevaluable reference for DI’s improvement. Then in vitro and in vivo studies wereperformed to evaluate the modified DI’s properties. The aim of this study was todevelop a useful DI device which can have better distraction and implantationperformance.Part one: Finite element analysesExperiment one:3D model designMethod：a DI system was developed by the Zhongbang Company (Xi’an,China). This system had dual functions for the distractor and prosthetic implant.A posterior mandible segment with a DI and a superstructure were modeled on apersonal computer using a3D program (Pro/E Wildfire). Forces of100N and30N were applied axially and45°buccolingually on the buccal cusp, respectively.The maximum equivalent Von Mises stress in the jaw bones and DI-abutmentcomplex, and the Max displacement of the DAC were used to evaluate the reliability of the model.Results: a posterior mandible segment with a DI and a superstructure weremodeled successfully and the model’s reliability was conformed.Experiment two: biomechanical optimization of the diameter (D) of thedistraction screw (DS) by three-dimensional finite element analysisMethod：in Ansys DesignXplorer, the D of DS (ranging from1mm~3mm)was set as input variable. The maximum (Max) equivalent Von Mises (EQV)stress in cortical bone, cancellous bone, and distraction screw, and the Maxdisplacement of distraction screw were set as output variables to evaluate theeffect of different distraction screw diameters on the jaw bones and DI.Results: the results showed that under axial load, the maximum equivalentstresses in the cortical bone, cancellous bone, and distraction screw decreased by21.83%,69.60%, and62.67%respectively with increasing diameter, while underbuccolingual load, these values decreased by13.30%,31.69%, and44.92%respectively. The maximum displacements in the distraction screw decreased by22.22%and12.75%under axial and buccolingual loads respectively. When thediameter exceeded2.0mm, the stress in the jaw bones and the displacement inthe distraction screw reached the minimum.Experiment three: biomechanical optimization of the length ratio of thetwo endosseous portions in distraction implants by three-dimensional finiteelement analysisMethod：in Ansys DesignXplorer, the length ratio (R) of the two endosseousportions of distraction implant (ranging from5:5~9:1) was set as an inputvariable. The Max EQV stress in the jaw bones and DAC, and the Maxdisplacement of the DAC were set as output variables to evaluate the effect ofdifferent ratios between the lengths of TP and SP in the jaw bones and the DI. Results: the results showed that under axial load, the maximum equivalentstress in the cortical bone, cancellous bone, and distraction implant-abutmentcomplex decreased by9.19%,19.25%, and32.97%respectively with increasinglength ratio, while under buccolingual load, these values decreased by10.94%,67.32%, and12.63%respectively. The maximum displacements in thedistraction implant-abutment complex decreased by6.18%and9.36%underaxial and buccolingual loads respectively. When the length ratio was at8:2, thestress in the jaw bones and the displacement in the distraction implant-abutmentcomplex reached the minimum.Conclusions of part one1. A posterior mandible segment with a DI and a superstructure weremodeled successfully.2. D≥2mm and R=8:2were the optimal choices.Part two：In vitro study of DIExperiment four: DI’s design, manufacture, and biomechanical analysisMethod: according to the optimization results above, we designed andmanufactured a modified DI. Axial pull-out test and fatigue test were conductedto compare the biomechanical properties between the DI with6mm distractionheight and a normal implant.Results: axial pull-out test results showed that the max axial pull-outstrengths were1106±75.22N and1094±114.3N for the DI and the implantrespectively (P>0.05). The fatigue test results showed that none of the samples(DIs and implants) failed after2.4million’s loading cycles (about10years’chewing simulation).Experiment five: In vitro distraction testMethod：DI’s insertion and distraction procesures were simulated in a fresh mandible of a dog: horizontal osteotomy-preparation of planting hole-DI’sinsertion-vertical osteotomy-distraction screw’s replacement-distractionResults: the surgical procedures were smoothly, the transport bone wassuccessfully lifted.Conclusions of part two1. The modified DI with6mm distraction height’s biomechnical property isas good as the normal implant.2. The modified DI has reliable distraction osteogenesis performance.Part three: animal experimentExperiment six: animal models’ setting up and DI’s insertion operationMethod: three adult mongrel dogs were used in this experiment. Allmandibular premolors were extracted and an alveoloplasty was performed.Three months later, the animal models were set up. Then, a new surgicalapproach was applied to insert DIs.Results：three months later, the animal models with atrophic alveolar ridgewere successfully set up by both the general observarion and X ray evaluation.The new surgical approach is easy to handle with less bleeding. The DIs weresuccessfully inserted with this approach.Experiment seven: in vivo evaluation of bone regeneration around DI inthe early stage of healingMethod：five days after DIs’ insertion, the initial connection apparatus wasreplaced by the distraction screw. Distraction was then carried out at a rate of1mm per two days for12consecutive days to achieve6mm distraction height.Thirty days later, the dogs were sacrificed and the mandible segments includeddistracted bone and DIs were harvested. X ray, CT, Micro-CT and histologicalanalysis were used to evaluate the bone regeneration. Results: all of the DIs were in position and none of them was lost or looseduring the healing. The new generated tissue over the distraction site showeddark red color. The buccal surface of the new generated tissue was rough, whilethe lingual surface was smooth. The osteotomy line could not be seen with thenaked eye. The results of X ray, CT, Micro-CT and histological analysis showedactive osteogenesis in the distraction area where new generated bone had beenformed. Histological analysis for gingival tissue showed that most of thegingival tissue had healed well while only limited gingival tissue showedincreased epithelial thickness and more layers of epithelial cells.Conclusions of part three1. The new surgical approach can minimize the trauma, which is beneficialto the stability and survival of the transport bone.2. The in vivo experiment results showed that the modified DI had reliabledistraction osteogenesis performance.