Wang Rui-Feng, Kang Byungsik, Lang Lisa A, Razzoog Michael E
Department of Biological and Material Sciences, School of Dentistry, University of Michigan, 1011 N University Ave, Ann Arbor, MI 48103, USA.
J Prosthet Dent. 2009 Jun;101(6):359-71. doi: 10.1016/S0022-3913(09)60079-2.
A fundamental problem in fully understanding the dynamic nature of implant loading is the confusion that exists regarding the torque load delivered to the implant complex, the initial force transformation/stress/strain developed within the system during the implant complex assembly, and how the clamping forces at the interfaces and the preload stress impact the implant prior to any external loading.
The purpose of this study was to create an accurately dimensioned finite element model with spiral threads and threaded bores included in the implant complex, positioned in a bone model, and to determine the magnitude and distribution of the force transformation/stress/strain patterns developed in the modeled implant system and bone and, thus, provide the foundational data for the study of the dynamic loading of dental implants prior to any external loading.
An implant (Brånemark Mark III), abutment (CeraOne), abutment screw (Unigrip), and the bone surrounding the implant were modeled using HyperMesh software. The threaded interfaces between screw/implant and implant/bone were designed as a spiral thread helix assigned with specific coefficient of friction values. Assembly simulation using ABAQUS and LS-DYNA was accomplished by applying a 32-Ncm horizontal torque load on the abutment screw (Step 1), then decreasing the torque load to 0 Ncm to simulate the wrench removal (Step 2). The postscript data were collected and reviewed by HyperMesh. A regression analysis was used to depict the relationships between the torque load and the mechanical parameters.
During the 32-Ncm tightening sequence, the abutment screw elongated 13.3 mum. The tightening torque generated a 554-N clamping force at the abutment/implant interface and a 522-N preload. The von Mises stress values were 248 MPa in the abutment at the abutment-implant interface, 765 MPa at the top of the screw shaft, 694 MPa at the bottom of the screw shaft, 1365 MPa in the top screw thread, and 21 MPa in the bone at the top of the implant-bone interface. This study also identified various characteristic isosurface stress patterns. The maximum stress magnitude to complete the von Mises stress joint pattern in the present model was 107 MPa during screw tightening, and was reduced to 104 MPa with removal of the wrench. Various specific stress patterns were identified within all elements of the implant complex during the assembly simulation.
During the torque moment application, the abutment screw was elongated, and every 1.0-mum elongation of the screw was equivalent to a 47.9-N increase of the preload in the implant complex. The ideal index to determine the preload amount was the contact force at the interface between the screw threads and the threaded screw bore. The isosurface mode identified various characteristic stress patterns developed within the implant complex at the various interfaces during the assembly simulation. These patterns are the (1) spiral and ying-yang pattern of the XY stress, (2) spring, cap, clamping, and preload pattern of the ZZ stress, and (3) bone holding and joint pattern of the von Mises stress.
全面理解种植体加载的动态特性存在一个基本问题，即对于传递到种植体复合体的扭矩负载、在种植体复合体组装过程中系统内产生的初始力转化/应力/应变，以及在任何外部加载之前界面处的夹紧力和预紧应力如何影响种植体，存在混淆。
本研究的目的是创建一个尺寸精确的有限元模型，该模型包含种植体复合体中的螺旋螺纹和螺纹孔，置于骨模型中，以确定在模拟的种植体系统和骨中产生的力转化/应力/应变模式的大小和分布，从而为在任何外部加载之前研究牙种植体的动态加载提供基础数据。
使用HyperMesh软件对种植体（Brånemark Mark III）、基台（CeraOne）、基台螺钉（Unigrip）以及种植体周围的骨进行建模。螺钉/种植体和种植体/骨之间的螺纹界面设计为具有特定摩擦系数值的螺旋线螺旋。使用ABAQUS和LS-DYNA进行装配模拟，方法是在基台螺钉上施加32 N·cm的水平扭矩负载（步骤1），然后将扭矩负载降至0 N·cm以模拟扳手拆卸（步骤2）。由HyperMesh收集并审查后处理数据。使用回归分析来描述扭矩负载与力学参数之间的关系。
在32 N·cm的拧紧过程中，基台螺钉伸长了13.3μm。拧紧扭矩在基台/种植体界面产生了554 N的夹紧力和522 N的预紧力。在基台-种植体界面处的基台中，von Mises应力值为248 MPa，在螺钉轴顶部为765 MPa，在螺钉轴底部为694 MPa，在顶部螺纹处为1365 MPa，在种植体-骨界面顶部的骨中为21 MPa。本研究还识别出了各种特征性等表面应力模式。在本模型中，拧紧螺钉时完成von Mises应力联合模式的最大应力大小为107 MPa，移除扳手后降至104 MPa。在装配模拟过程中，在种植体复合体的所有单元内识别出了各种特定的应力模式。
在施加扭矩时，基台螺钉伸长，螺钉每伸长1.0μm相当于种植体复合体中的预紧力增加47.9 N。确定预紧量的理想指标是螺纹与螺纹螺孔之间界面处的接触力。等表面模式识别出了在装配模拟过程中种植体复合体在各个界面处产生的各种特征性应力模式。这些模式包括：（1）XY应力的螺旋和阴阳模式；（2）ZZ应力的弹簧、帽、夹紧和预紧模式；（3）von Mises应力的骨固定和联合模式。
