The Experimental And Clinical Studies Of Reconstruction Of Mandibular Defects Using Rapid Prototyped Titanium Tray And The Optimized Design

Mandible reconstruction poses a vast challenge to maxillofacial surgeons. There is no ideal method that can achieve satisfactory esthetic and functional results. Free bone grafting, including non-vascularized and vascularized bone grafting, is the most commonly used method for reconstruction of mandibular defects. However, for the non-vascularized bone grafting, resorption of the block bone graft remains unsolved, and it’s not suitable for the large defects. Moreover, it is difficult to achieve ideal aesthetic reconstruction, because of that the bone grafts were manually shaped and fixed intraoperatively. Success rates of over 90% have been achieved with vascularized bone grafting. However, harvesting a vascularized graft requires complicated operative techniques and takes lots of time during the operation. And to reconstruct symmetrical facial appearance is extremely difficult.The reverse engineering (RE), CAD/CAM, and rapid prototyping (RP) technologies, originated in the field of industrial fabrication, have been employed to construct craniofacial prostheses. Their feasibility and efficiency have been well demonstrated. However, mandibular segmental defects reconstruction demands not only appearance reconstruction, but also bony continuity to facilitate prosthetic dental rehabilitation. In this study, a customized bone grafting tray was manufactured using the RE-CAD-RP technology, aiming to reconstruct the mandibular shape and the bony continuity at one time. Preshaped titanium mesh tray has been demonstrated to be feasible to carry the autologous cancellous bone grafts to restore bone defects. Thus the bone grafts in the tray was expected to be remodeled and serve as a bony support for the further implant dental implantation.This study includes 5 experiments:Experiment 1. Reconstruction of Mandibular Defects with the Combination of Rapid Prototyped Tray and Autologous Iliac Bone Grafting: Animal Experiment in Hybrid Dogs.The purpose of this experiment is to repair a mandibular discontinous defect using Computer-Aided Design (CAD) and Rapid Prototyping (RP) technique,for satisfactory aesthetic and functional restoration. 10 hybrid dogs were used for this experiment. Helical CT scanning was performed for each subject, and 3D digital model of the skulls of the dogs was constructed with the CT slices in Mimics. After that, segmental mandible dissection was simulated on the model in Geomagic, and individualized bone grafting tray was designed based on the digital. A prototyped tangible resin model was then obtained using the Stereolithography technology. A custom titanium tray was then cast for each animal, and then an operation was done on the each subject. A Discontinous defect in the mandibular body was created according to the CAD proposal. The defect was restored immediately using the tray combining autologous iliac cancellous bone grafting. Sequential radionuclide bone imaging was performed in 2nd,4th,8th,12th,24th week to evaluate the bone graft. Semi-quantitative analysis was done by comparing the ratio of activity between the grafted and the host left mandible. The radionuclide tracer uptake greatly increased in the grafted mandible over a follow-up period of 24 week. the mean activity ratio between the grafted and the contralateral host mandible was greatest at 2 week. The results suggested that the bone graft survived and active modification occurred. The study demonstrates that customized CAD-RP titanium tray combining autologous cancellous bone grafting is feasible for the reconstruction of mandibular discontinuous defect. And radionuclide bone imaging is a sensitive and efficient means for evaluating the bone graft in the Titanium tray. [1]Experiment 2. Accurate Reconstruction of Discontinuous Mandible Using a Rapid Prototyped Tray in Combination with Autologous Bone Grafting: A Preliminary Clinical StudySix patients who had undergone various block resection of the mandible were restored using a custom titanium tray combining autogenous iliac grafts. The custom titanium tray was fabricated using the rapid prototyping technique. A CAD model of the mandible was constructed in MIMICS with the CT scanning data. The opposite side (unaffected side) of the mandible was mirrored to cover the defect area. A bone grafting tray was designed from the mirrored image. Rapid prototyped model was then obtained and used for casting to get a titanium tray. The mandibular defects were restored using the custom titanium trays in combination of autologous iliac grafting. For 1 of the 6 patients, an implant denture was made to rehabilitate the occlusion, at 24 weeks postoperatively. Results revealed that the prototyped trays fit well in all 6 patients. Satisfactory mandibular symmetries were restored for all of the 6 patients. The reconstructive procedures were straight, simple and time saving. No severe complications ware noted in the other 5 patients without occlusion rehabilitation during a mean 50-month follow-up period. The reconstruction in the patient with occlusion rehabilitation lasted only 1 year and failed eventually due to bone resorption and infection. This case was restored after another 6 months, using vascularized bone grafting. This study suggested that the Rapid Prototyping technique facilitated the clinical mandibular reconstruction and secured satisfactory esthetic reconstruction. However, the rigidity of the cast titanium tray could cause severe stress shielding to the grafts filled in the tray, which could lead to disuse atrophy. Therefore, some modification is needed for better functional reconstruction.[2]Experiment 3. customized grafting plate for mandibular defects.To reduce the area of the grafting tray, we cut off the lingual and bottom wall of the U shaped tray. A customized tray that only has a buccal wall, which we called“customized grafting plate”, was computer-aided designed and rapid prototyped. A titanium plate was used to reconstruct discontunous mandible, by combining with autologous iliac grafting in dogs. The results revealed that, the grafted bone which is opposite to the plate survived and got corticalized, while the bone graft close to the plate was observed absorbed or osteonecrosis formed. These results suggested that, the customized grafting plate could increase the blood supply of the lingual side but not the buccal side. Furthermore, the plate can’t carry the crushed iliac bone graft, thus iliac cancellous bone block was attached onto the plate, which is not good for nutrient penetration.Experiment 4. Optimum Designed Rapid Prototyping Tray for Reconstruction of Mandibular Defects.To diminish the stress shield that the tray affect to the inner bone graft,the rapid prototyping tray was optimum designed to decrease the rigidity of the cast titanium tray. ANSYS 6.0 was used to construct the CAD model of the mandible and the Titanium prosthesis,and mechanical simulation investigation was done. Optimum design in the body part of the prosthesis was made on the base of the routine tray.Experiment 5. Reconstruction of Mandibular Defects with the Optimum Rapid Prototyped Tray. Animal experiments were done to compare the advantages of the optimum designed tray to the routine U shaped tray in reconstruction of mandibular defects. Nine hybrid dogs were used to investigate the difference between the optimumized tray and the routine tray. 3D digital model was made through helical CT scanning for each subject. Segmental mandible dissection was simulated on the model. And individualized Titanium bone grafting tray was made utilizing CAD-RP technique. Optimumize tray for the 9 subjects in the experimental group,routine tray for the control group. And then an operation was done on the each subject. Discontinuity defect in the mandibular body was created in accordance with the CAD proposal. The defect was restored immediately using the tray combining autologous iliac cancellous bone graft. Sequential radionuclide bone imaging was performed in 2nd,4th,8th,12th,24th week to evaluate the bone graft. And quantitative analysis was done by comparing the ratio of activity between the grafted and the contralateral host mandible. Gross observation and Three-point bending test was done for each sample after sacrifice and harvesting. The radionuclide bone imaging revealed that the bone graft in the optimumized tray has a higher activity than that in the routine tray. The optimumized tray fractured in one of the 10 subjects at about 1 month postoperatively,while the gross observation of the bone sample showed that the bone grafted survived and the rebuilt in a good condition. Infection and osteonecrosis occurred in 2 subjects after the breakage of the alveolar mucosa. Partly osteonecrosis occurred in the inner part of bone graft in the tray,and the bone block covered on the top the tray survived in 2 subjects. Fibrous connective tissue was observed between the bone and the tray in 3 samples. The rest 3 samples were found good. The optimum designed trays could effectively diminish the stress shield and other adverse effect to the in-filled bone graft. They are more profit for the survival and remolding of the bone graft comparing to the routine trays.