| Because of its unique comfort and stability, implant denture has been widely accepted by the clinicians and patients since its introduction 30 years ago. An ideal implant denture should have a choice implant together with a precise superstructure to ensure its good long-term performance. Implant superstructure can be retained by either cement or screw. Screw-retaining is the prevalent retention form since its introduction. Its advantage is that the superstructure can be removed and cleaned periodically. If the superstructure is damaged it can be repaired easily. It can provide enough retention when the occlusal space is insufficient. Additionally, it can protect the integrity of the implant-bone interface. But the loosening of the retaining screw and its negative effect on the success rate is still a problem. So, it has been a focus of many researches. The retaining screw is a weak part of the whole superstructure .The causes of the screw loosening and fracture are complex and difficult to control. Among the factors, the preload is a key factor. Almost all the causes of the screw loosening and fracture can be attributed to the preload problems. Preload is the spring force when the screw is fastened to fix the components of the superstructure. When the preload lost, the close contact between the components of the superstructure come down, consequently, the screw loosen. The loosened screw will sustain the whole load applied on the superstructure, and the longevity of the screw will be cut greatly. So, to retain the preload is the key to avoid the screw loosening. When the occlusal force is applied on the implant denture , the denture will develop elastic deformation . If the force can create little relative slide between the screw and the abutment, the preload or the tensile force loses partly. This is the beginning of the screw loosening. Tensile is pro rata to the resistance . When the tensile is well below the critical value, force and vibration can cause the screw reverse, loosen and out of function. Previous researches indicated that many factors could affect the preload, such as the fit of the interface , the anti-rotational designs, material, the diameter , length and shape of the screw, the design of neck, the rigidity of the screw, the surfaces roughness of the screw, the quality and quantity of lubricant, and the speed of applying torque. But there are few systematic reports about the effects of different material combinations of the superstructure , the different screw head angles and different preloads on the retention of the screw.This study utilized 3-D finite element analysis to investigate the effects of retaining screw head angle, materials combination of the retaining screw and the inner crown , and the preload on the retention of the screw. Aim to optimize the design of the superstructure.According to the original designs of several commercial available implant systems, including ITI,BLB,Branemark,established the geometric configuration. The screw head angles were 5,8,10,12 degree respectively, the material of the inner crown was gold alloy, Ni-Cr alloy and castable ceramic , the materials of the screw were Ni-Cr alloy and Ti alloy. Preloads were 5N,10N,20N,30N and 40 N respectively. There were 120 mechanical models in this investigation. When the occlusal force of 150N was applied on the restoration, the preload decreased. The residual preloads of the each combination were calculated and the stress distribution photographs were drawn. The variations of the residual preload and the interfacial stress were investigated.The results were demonstrated by the 3-D digital photographs and charts. The 3-D digital photographs can reflect the characteristics and tendency of stress contribution under different conditions. Under each screw head angles , the residual preload- preload curves of the 6 material combinations were drawn respectively . Afterwards, selected one material combination randomly, and depicted the residual preload-preload curves u...
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