| Objective: At present, the thread of implant is designed mostly simply. The implant with smaller pitch thread on the neck cortical area and wider pitch thread on inferior part, which is designed for different elastic modulus of cortical bone and cancellous bone, was seldom reported. In this experiment, we contrast the rule of the implant-bone-interface stress distribution by changing the distribution style of thread, the shape of implant and the loading style, and analyze which design is more reasonable and biocompatible by three-dimensional finite element analysis. Through contrasting and analyzing the rule of the implant-bone-interface stress distribution on the implant with compound thread or simple thread, the optimal thread style of implant would be chosen, and theoretical evidence could be provided for implant production and design and its clinical application.Materials and methods: In this experiment, we used Unigraghics NX software (to abbreviate UGNX) to set up the model. NX Nastran software was used to calculate and analyze. One section of mandible Model was made. The model is 8.1mm length, 25mm tall and 8.1-12.1mm buccolingual wide, and the length of implant is 12mm. The collar of the implant is 2mm and the superstructure was simplified into a cylinder with 7mm height. The models of implants were drawn, and the implants were installed in the middle of the bone model.(1) Shade : cylindrical implant /root form implant(2) Diameter: 4.1mm(3) Cortical thickness: 2mm(4) Thread distribution:①uniform thread②the superior extremity is 2mm of small pitch thread and the inferior part is wider pitch thread.③the superior extremity is 4mm of small pitch thread and the inferior part is wider pitch thread . The uniform thread′s pitch is 1.0mm . The peak of thread is 0.3mm height, and the angle is 86°. In the superior extremity, the compound thread′s pitch is 0.3mm, the peak of thread is 0.12mm height. and the angle is 70°。In the inferior part, the thread parameters are same as uniform thread.The dental corona was simplified a cylinder with non-anatomical occlusion surface. The data of the six models were kept with one accord. The data were taken into UGNX software. The software can export the numbers of the elements and the nodes automatically.Two kinds of loading: (1) axial loading. (2) Oblique loading (30°). The loading, 100N, was concentrated on the center of occlusion surface.By using NX Nastran software to analyze and calculate, Von-Mises stress values of every model were gotten, and maximum Von-Mises stress values in the neck, the middle and the bottom of the bone tissue were found out .The stress values were list and compared.Results:1. With the vertical loading, the Von-Mises stress peak value at the neck and the bottom of the implant-bone interface on the cylinder implant: the superior extremity is 4mm of small pitch thread and the inferior part is wider pitch thread > uniform thread > the superior extremity is 2mm of small pitch thread and the inferior part is wider pitch thread .The bone stress values at the neck and bottom of the implant-bone interface are obviously higher than those at the middle. The maximum Von-Mises stress values at the middle of the implant-bone interface have no obvious difference. 2 . With the oblique (30°) loading, the Von-Mises stress peak value at the neck and the bottom of the implant-bone interface on the cylinder implant: the superior extremity is 4mm of small pitch thread and the inferior part is wider pitch thread > uniform thread > the superior extremity is 2mm of small pitch thread and the inferior part is wider pitch thread .The bone stress values at the neck of the implant-bone interface are obviously higher than those at the middle and the bottom. The maximum Von-Mises stress values at the middle and the bottom of the implant-bone interface have no obvious difference.3. With the vertical loading, the Von-Mises stress peak value at the neck of the implant-bone interface on the root-form implant: the superior extremity is 4mm of small pitch thread and the inferior part is wider pitch thread > the superior extremity is 2mm of small pitch thread and the inferior part is wider pitch thread > uniform threadWith the vertical loading, the Von-Mises stress peak value at the bottom of the implant-bone interface on the root-form implant: the superior extremity is 2mm of small pitch thread and the inferior part is wider pitch thread > the superior extremity is 4mm of small pitch thread and the inferior part is wider pitch thread > uniform threadThe maximum Von-Mises stress values at the middle of the implant-bone interface have no obvious difference.4. With the oblique ( 30°) loading, the Von-Mises stress peak value at the neck of the implant-bone interface on the root-form implant: the superior extremity is 4mm of small pitch thread and the inferior part is wider pitch thread > the superior extremity is 2mm of small pitch thread and the inferior part is wider pitch thread > uniform threadThe maximum Von-Mises stress values at the middle of the implant-bone interface have no obvious difference.With the oblique ( 30°) loading, the Von-Mises stress peak value at the bottom of the implant-bone interface on the root-form implant: uniform thread > the superior extremity is 4mm of small pitch thread and the inferior part is wider pitch thread > the superior extremity is 2mm of small pitch thread and the inferior part is wider pitch thread5. With vertical loading and identical thread distribution, the Von-Mises stress peak value at the neck of the implant-bone interface on the cylinder implant is less than that on the root-form implant. Moreover, with compound thread distribution, the Von-Mises stress peak value at the neck of the implant-bone interface on the cylinder implant is obviously less than that on the root-form implant.6. With oblique (30°) loading, the stress value at the neck of the implant-bone interface on the uniform-thread cylinder implant is slightly larger than that on the root-form implant, but the stress value at the bottom is obviously less than that on the root-form implant. With 2mm of small pitch thread on the superior extremity and wide pitch thread on the inferior part, the stress value at the neck of the implant-bone interface on the cylinder implant is obviously less than the root-form implant. The maximum Von-Mises stress values at the middle of the implant-bone interface have no obvious difference.7. The stress values at the neck of the implant-bone interface on the cylinder implant with oblique (30°) loading are 5~15 times than those with vertical loading, but when the superior extremity is 2mm of small pitch thread and the inferior part is wide pitch thread, they are only about 5 times. The maximum Von-Mises stress values at the middle and the bottom of the implant-bone interface on the cylinder implant have no obvious difference. Conclusions:1. The design of compound thread with small pitch thread in corresponding to cortical thickness and the inferior part is wider pitch thread is surpass the design of uniform thread on the cylinder implant. It can decrease the bone stress around the neck of the implant efficiently.2. The maximum Von-Mises stress concentrates on the cortical area of the neck of the implant, especially with oblique loading. It is suggested that we may reduce the cusp inclination reasonably so as to decrease the stress on the cortical area of the neck of the implant clinically.3. The design of compound thread with small pitch thread in corresponding to cortical thickness and the inferior part is wider pitch thread is not suitable for the root-form implant.4. The biomechanical property of cylinder implant is prior to the root-form implant.
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