| Application of oral implantology successfully solved the difficulties of poor retention casued by traditional complete denture and improved the masticatory efficiency. However, lack of sufficient bone for implant placement may pose problems when the implants are to be placed in proximity to anatomical structures such as the mandibular nerve or the maxillary sinus. Therefore, Malo et al proposed All-on-4 concept, which means to rehabilitate the ededtulous with just four implants, 2 vertical implants in the anterior region and 2 tilted implants in the posterior region. Many clinical studies of All-on-4 concept have showed that the survival rate of tilted implants and vertical implants has no significant difference. The use of tilt implants can avoid the maxillary sinus and inferior alveolar nerve and decrease the distal cantilever length. However,for the All-on-4 restoration, in order to decrease the stress distribution around the crest of implant, the suitable cantilever length is also needed to be further researched. Photoelastic is commonly used in biomechanical analysis. However, the fabrication of the photoelastic model is complex and low accuracy. Therefore, the study intends to fabricate the All-on-4 implant photoelastic models using Computer Aided Design(CAD) and Rapid Prototyping(RP) technology and analysis the stress of peri-implant bone of the All-on-4 photoelastic models with different cantilever length and inclination of distal implants so as to provide reference for clinical therapy.Objective: To investigate the feasibility of the method to fabricate the edentulous mandibular photoelastic model with CAD/RP and to analysis the stress distribution around the implants in the All-on-4 edentulous mandibular photoelastis models.Methods: The edentulous mandible specimen was scanned by CT. And then the data was input in the Mimics software to obtain the 3D model. After that, the 3D model was edited with Geomagic Studio software and the data between bilateral mandibular ramus and at the plane of mental foramen was intercepted. Then heighten the mandible model to 35 mm. In the study, the implants were simulated with 13 mm cylinders and for each model, the four implants was placed between the interminal area and two of which were vertical paralled to each other. For one of the model, the distal implants were placed vertically. For the other models, the posterior implants were inclined by 15°, 30°and 45°, respectively from 6mm before the mental foramen and the corresponding cantilever length were 20 mm, 18.3mm, 15 mm and 9.8mm. The 4 models were manufactured by RP and then the All-on-4 edentulous mandibular photoelastis models were made by traditional ways.The metal frameworks were also designed 3mm height and 4mm distance from the surface of the mandible using Geomagic software according to the shape of mandibular alveolar ridge morphology and 0 degrees, 30 degrees multiunit abutments. In order to indicate the loading site, four disc-shaped structures were desinged on the surface of the 4 frameworks at 5mm,10 mm, 15 mm cantilever length and the distal fossa of the first molar respectively. Then 4 resin frameworks were made using the SLA rapid prototyping machine, and the metal frameworks were made by traditional method. After the 4 distal implants were respectively cemented in the photoelastic models, a vertical load of 5kg, 10 kg, 15 kg and 20 kg was applied on the implant and the stress distribution around the implant was photographed. The other implants were cemented in the models using the same method and the multiunit abutments were fixed to the frameworks by 20 Ncm torque screws with a manual torque wrench. After that, a vertical load of 5kg-20 kg was applied on framework at the site of the distal implant. Then, a vertical load of 15 kg was applied on the framework at 5mm,10 mm, 15 mm cantilever length and the distal fossa of the first molar. The stress distribution around the implant was also photographed.Results:1.The ethoxyline resin models featured faintly yellowish, bright surface, homogeneous structure, high optical sensibility and with no natural sress. The position and inclination of implants is accutate and the frameworks can be passive emplacement.2. In the study about the stress distribution around single tilted implant and All-on-4 distal tilted implant, with the inclination of the implant increase,both of the two design showed that the sress around the crest of the implant increased and the stress at the apex of the implant decreased. However, the stresses around the All-on-4 distal tilted implant are lower conpared to single tilted implant with the same implant inclination.3. In this study, the stresses around the distal tilted implant increased as the length of cantilever increased when the inclination of the implant remained unchanged. With the same cantilever length, the peri-implant bone srtess increased with the inclination of implant increased. For the model in which the distal implants were placed vertical, stresses were mainly concentrated in the apex of the distal implant. However, for the other three All-on-4 models, both the neck and apex of the distal tilted implant can be observed stress concentration.4. In this study, the peri-implant bone stress concentrated both in the crest and apex of the distal implant. With the inclination of the distal implant increased the cantilever length declined, and the stress around the distal implant reduced.Conclusion: The results showed that, the increase of the inclination of single tilted implant and All-on-4 distal tilted implant leaded to the peri-implant bone stress increase. However, compared to the single implant, the tilted implant in All-on-4 produced lower stress with the same implant inclination. When the distal tilted implant remained the inclination unchanged, the peri-implant bone stress increased with the cantilever length increased. In All-on-4 concept, the use of distal tilted implant induced the cantilever length, which produced the lower peri-implant bone stress. And this illustrated that the influence of cantilever on stress distribution was greater than the influence of implant inlination. |
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