Preliminary Experimental Study And Biomechanics Optimum Analysis Of A New Distraction Implant

Dental rehabilitation with implants has been accepted by more and more doctors and patients in the last few decades, with reliable long-term functional and aesthetic results. However, alveolar ridge defects resulting from atrophy of edentulous segments after tooth extraction, trauma, periodontitis, tumor resection in the mandible and maxilla limit implant placement. Many surgical procedures are proposed to correct the poor local condition, including onlay or sandwich bone graft[1, 2], guided bone regeneration[3, 4], distraction osteogenesis[5-7](DO) and so on. Among them alveolar distraction osteogenesis (ADO) has received considerable interest since 1996, when Block[8] and Chin reported the first experimental and clinical results by this technique, respectively. Compared with other methods of alveolar ridge augmentation, ADO has many advantages including both bone and soft tissue augment simultaneously, without bone graft so that no bone harvesting is necessary and morbidity of the donor site can be avoided, low degree of bone resorption than free bone graft[5, 9-11] and so on. ADO distractor has been paid close attention.Currently, the conventional method of alveolar distraction includes a 2 or 3-step procedure: placement and removal of the distractor, and insertion of the implant 8-16 weeks after distraction. Another 4-6 months of integration is needed before application of prosthethese[5, 12, 13]. That is to say, the overall treatment time is at least 6-10 months. To simplify the operation and shorten the overall treatment time, several researchers[14]tried to distract alveolar ridge by implant directly or by distraction implant(DI), However, studies on alveolar ridge distraction in one-step are scarce. None type of DI in the domestic market, and there is only a few of types in the foreign, including DISSIS, 3I and so on. Among them only one research team guided by Gaggl described to augment alveolar ridge vertically by DISSIS in experimental and clinical studies, and all the others are short-term clinical case report. Hence, in the first part of this research, we aimed to develop a new type of DI and investigate whether the bone healing process of distraction gap and the osseointegration process could be combined together, in order to examine the suitability and the double functions of it in the biological aspect.Part one Development of the new DI and preliminary experimental study Experiment one Design and development of the new DI systemObjective: To development a new type of DI system with dual functions of distractor and prosthetic implant. Methods: Based on the detailed comparison and analysis of the current ADO distractors, to design a new type of DI, and made it with TA2 titanium. Results: The new DI system included main components and tools. The former comprised three main components: the main body, distraction screw and consolidation screw. The main body included the 5 mm-long coronal transport portion, the 5.5 mm-long apical support portion. The coronal portion engages the bony transport segment with external threads. The apical portion has an internal thread hole in which the distraction screw could sit and engage for the distraction process. In the undistracted state ready for insertion, the DI is cylinder-shaped, threaded (0.7 mm in pitch), 10.5 mm in length, 4.0 mm in outer diameter and 2.0 mm in inner diameter. The length of the distraction screw ranges from 12 mm to 22 mm,corresponding to a 5-15mm distraction distance, and a right turn of 360 degrees corresponds to a 0.4 mm elevation. The distraction screw could be replaced by the consolidation screw after distraction so that the disconnected coronal and apical parts could be combined together and the transformation from a distractor to a dental implant is available. Besides, there are several types of abutments in different length. Conclusion: The new DI has dual functions of distractor and implant, but its efficiency need further tested.Experiment two Establishment of an experimental model for ADO by the new DIObjective: To test the basic distract function of the new DI, and investigate the process and standards of the operation. Methods: Simulating the process of ADO by the new DI in vitro using a piece of mandible bone of dog. The radiographs were taken after the operation. Results: Observation in general and radiograph showed the new DI can distract the bone vertically without displacement of itself. Conclusion: ADO model has been established successfully. The new DI had the quality of a distractor, process and standards had been made, which could be helpful for the next experimental study. Experiment three Preliminary experimental study of ADO by the new DIObjective: To test the dual functions of distractor and implant of the new DI, in order to provide support in theory. Methods: ADO was practiced by DI in six dogs. Two DIs were installed into one lateral of edentulous mandibular ridge after osteotomy. The DIs were left to integrate into the bone after 8 days of distraction at a rate of 1 mm per day. The specimens were obtained 5, 8, 12 weeks after distraction and examined radiopacically, histologically, and histomophologically. Results: Radiopacity of the regenerated bone increased steadily up during consolidation period. After 12 weeks, a homogeneous bone density was observed. Bone mineral density (BMD) of the distraction gap in the experimental lateral at 8 week was not only lower than that in the contrast lateral, but also lower than the transport and basic bone significantly. While at 12 week, no significant difference of BMD in all areas was found. Histologic and scanning electron microscope (SEM) examination showed osseointegration of the whole DI in the region of macro-thread in the native bone and micro-thread in the regenerated area 12 weeks later,BIC of the two areas are 60.48±6.12% and 51.65±3.83% respectively, and no significant difference was found. Conclusion: Our results suggest that simultaneously bone regeneration in distracted gap and osseointegration of DIs in augmented alveolar ridge can be achieved. The overall treatment time could be shortened into a half by this method. This new DI was promising.Part two Biomechanics optimum design and analysis about the distraction screw of the new DI after osseointegration Favorable biomechanics compatibility is one of the most important factions for the long-term success rate of implants. As a new apparatus, the DI should be analyzed optimumly before applying in clinic so that the potential problems could be found out and solved. Finite Element Method (FEM) is an effective method for biomechanical research of implant. The second part of this research aimed to perform biomechanics optimum design and analysis about the distraction screw of the new DI after osseointegration, in order to recognize the special characteristic of the new DI, and provide detailed parameters for the clinical practice.Experiment one Establishment of Self-adapting assembled 3-D FEM models of DI-bone complexesObjective: To establish self-adapting assembled 3-D FEM models of DI-bone complexes. Methods: In a computer, 3-D models of cortical bone, cancellous bone, thread DI and superstructure were constructed by Pro/E modeling software, the DI complexes were assembled based on parameters of the distraction screw of DI by self-adapting assembled programme of Pro/E. Then the models were imported to Ansys Workbench software by bidirectional parameters transmitting of the two softwares, and the accuracy of the models was also evaluated. Results: 3-D FEM models of cortical bone, cancellous bone, thread DI and superstructure were constructed. Conclusion: The self-adapting assembled models provide the technical platform for further implant optimum design and analysis.Experiment two Biomechanics optimum design and analysis about the length of the distraction screwObjective: To evaluate the effect of the length of the distraction screw (L) to OBJ (Objective Function). Methods: L was set as DV (design variable), which ranged form 10.0-20.0mm. The Max EQV stresses, Max total deformation in jaw bone and DI-abutment complex were set as OBJ. Results: Under AX (Axical) load and BL (Buccolingual) Load, the Max EQV stresses and Max total deformation in the inferior part of DI all decreased 50-60% obviously. Under AX load, decreasing degree of Max total deformation of all the other parts are about 5% equivalently except the inferior part. The tangent slope rate of OBJ showed: Under AX load, the Max EQV stresses decreased with the increasing of L in jaw and apical part of DI, when L≥11.8mm, the values are low and gently. Under BL load, Max total deformation in all the other parts decreased with the increasing of L except the superior part of DI, when L≥14.0mm, the values are low and gently. Conclusion: Abilities of anti-AX and anti-BL of DI all increased with the increasing of L until it amounts to 14mm long. The results imply that the length of distraction screw shouldn't shorter than 14mm, which corresponding to 6-7mm distraction distance.Experiment three Biomechanics optimum design and analysis about the diameter of the distraction screwObjective: To evaluate the effect of the diameter of the distraction screw (D) to OBJ. Methods: D was set as DV, which ranged form 1.4-2.8mm. Others were as same as experiment two. Results: Under AX load and BL Load, the Max EQV stresses in the middle part of DI decreased 50-60% obviously. But under both loads, there were contrary changes of deformation between the inferior and superior part of DI, and under BL load, the Max EQV stresses also showed contrary changes between the superior and middle part of DI. The tangent slope rates showed: Under AX load, both of the stress and deformation decreased with the increasing of D in jaw, when D≥2.1mm, the values are low and gently. Under both loads, the Max EQV stresses in the middle part of DI also decreased with the increasing of D, the values are low and gently when D≥2.02mm. Under both loads, contrary changes of deformation were showed between the superior and inferior parts, when D≤2.4mm, the values of the superior part are low and gently. Conclusion: The middle part of DI effected by D obviously. Possibility of loosing or breaking decreased with the increasing of D. D should be design in the range of 2.0-2.4mm in order to get an optimum biomechanical result.In a conclusion, the function and suitability of the new DI has been evaluated from the two aspects of biologics and biomechanics. On biomechanical point, the new DI was more suitable to distraction a longer distance than 6-7mm, and the diameter of DI was reasonable. ADO with the new DI was promising for clinical application.