Finite element analysis of narrow dental implants

Narrow-diameter implants (NDIs) traditionally have been associated to higher rates of failure in comparison with regular-diameter implants (RDIs) and wide-diameter implants (WDIs), since they generate a more unfavorable stress distribution in peri-implant bone. However, it is well known that the load sharing effect associated with prostheses supported by multiple implants (also called splinted prostheses) affords mechanical benefits. The present study involves finite element analysis (FEA) to determine whether the risks linked to NDIs could be mitigated by the mechanical advantages afforded by the splinting concept. For this purpose, a three-dimensional (3D) model of a real maxilla was reconstructed from computed tomography (CT) images, and different implants (NDIs, RDIs and WDIs) and prostheses were created using computer-aided design (CAD) tools. Biting forces were simulated on the prostheses corresponding to three different rehabilitation solutions: single-implant restoration, three-unit bridge and all-on-four treatment. Stress distribution around the implants was calculated, and overloading in bone was quantified within peri-implant volumes enclosed by cylinders with a diameter 0.1 mm greater than that of each implant. The mechanical benefits of the splinting concept were confirmed: the peri-implant overloaded volume around NDIs splinted by means of the three-unit bridge was significantly reduced in comparison with the nonsplinted condition and, most importantly, proved even smaller than that around nonsplinted implants with a larger diameter (RDIs). However, splinted NDIs supporting the all-on-four prosthesis led to the highest risk of overloading found in the study, due to the increase in compressive stress generated around the tilted implant when loading the cantilevered molar.     

人工楔状缺损对牙颈部硬组织应力分布的影响

目的 分析楔状缺损患牙颈部硬组织的应力分布.方法 在下颌前磨牙沿釉牙本质界建立三角形缺损,设计楔状缺损模型,模拟侧方运动中牙齿的受力情况,对下颌前磨牙颈部硬组织行非线性接触分析.结果 斜向载荷下,釉牙本质界缺损的下颌前磨牙缺损区尖端存在明显的拉应力集中.随缺损高度的增加,应力值增大,应力集中程度也增加:应力集中区沿釉牙...     

Finite element analysis of the possible mechanism of cervical lesion formation by occlusal force

Recently, various authors have proposed the interesting idea that occlusal force may be a principal factor in cervical lesions of the tooth. It is speculated that the lateral force in non-ideal mastication causes the tooth to bend and that the resulting tensile stress damages the enamel surface. In this study, we carried out stress analysis on the upper central incisor and the lower first molar using the plastic-elastic deformation theory with two-dimensional finite element method (FEM). The essential feature that the tensile yield strength is much smaller than the compressive one was taken into account. Our results suggested that oblique loading on the tooth stretches the enamel surface near the cemento-enamel junction and causes plastic deformation which eventually leads to the cervical lesion.     

氧化锆陶瓷颈圈牙种植体有限元分析

目的 :探讨氧化锆陶瓷 (ZrO2)颈圈牙种植体在生理加载条件下的生物力学安全性能 ,为该类种植体的结构设计和研究提供数据。方法 :依据螺旋CT针对中切牙位上颌骨扫描数据 ,建立上颌骨及种植体装配的三维有限元模型 ,分别就种植体穿心连接螺钉在不同预紧力 (100N/120N/150N/200N)、不同载荷 (100N/200N/300N)、不同加载角度(50、100、150、200、250、300)条件下进行有限元分析计算。结果 :本文研究条件下 ,连接螺钉的预紧力在120 -150N较合适 ;300N轴线加载条件下氧化锆陶瓷颈圈处于安全范围 ;斜向加载对于陶瓷颈圈中的应力影响较大。加载角度150可满足300N颌力加载 ;加载角度300 时颌力在250N以下才能保证安全性。结论 :在颌力加载不高于250N的情况下,氧化锆陶瓷颈圈牙种植体结构的力学性能可满足要求     

Clinical fit of four‐unit zirconia posterior fixed dental prostheses

The objective of this clinical study was to assess the internal and marginal accuracy of computer‐aided design/computer‐aided manufacturing (CAD/CAM)‐generated four‐unit all‐ceramic posterior fixed dental prostheses (FDPs). The data were compared with the results of three‐unit metal‐ceramic and all‐ceramic FDPs that were obtained in a previous study. Twenty‐four patients were provided with all‐ceramic posterior four‐unit FDPs made from semisintered blank zirconia material. Prior to definitive insertion the accuracy was evaluated using a replica technique with a light body silicone that was stabilized with a heavy body material. The replica samples were examined using microscopy. The median marginal gap of the 24 four‐unit FDPs was 77 μm. The median gap widths were 87 μm at the midaxial wall, 167 μm at the axio‐occlusal transition of the abutments, and 170 μm at centro‐occlusal location. Although the marginal accuracy of the four‐unit FDPs differed significantly from that of the three‐unit metal‐ceramic FDPs (median 54 μm), the values obtained were clinically satisfactory and showed that semisintered zirconia blanks could be used for the fabrication of four‐unit FDPs.     

Three-dimensional finite element analysis of buccally cantilevered implant-supported prostheses in a severely resorbed mandible

PURPOSEThe aim of the study was to compare the lingualized implant placement creating a buccal cantilever with prosthetic-driven implant placement exhibiting excessive crown-to-implant ratio.MATERIALS AND METHODSBased on patient''s CT scan data, two finite element models were created. Both models were composed of the severely resorbed posterior mandible with first premolar and second molar and missing second premolar and first molar, a two-unit prosthesis supported by two implants. The differences were in implants position and crown-to-implant ratio; lingualized implants creating lingually overcontoured prosthesis (Model CP2) and prosthetic-driven implants creatingan excessive crown-to-implant ratio (Model PD2). A screw preload of 466.4 N and a buccal occlusal load of 262 N were applied. The contacts between the implant components were set to a frictional contact with a friction coefficient of 0.3. The maximum von Mises stress and strain and maximum equivalent plastic strain were analyzed and compared, as well as volumes of the materials under specified stress and strain ranges.RESULTSThe results revealed that the highest maximum von Mises stress in each model was 1091 MPa for CP2 and 1085 MPa for PD2. In the cortical bone, CP2 showed a lower peak stress and a similar peak strain. Besides, volume calculation confirmed that CP2 presented lower volumes undergoing stress and strain. The stresses in implant components were slightly lower in value in PD2. However, CP2 exhibited a noticeably higher plastic strain.CONCLUSIONProsthetic-driven implant placement might biomechanically be more advantageous than bone quantity-based implant placement that creates a buccal cantilever.     

上切牙区双端悬臂梁种植固定桥的应力分析

目的：通过分析双端悬臂梁种植义齿修复上颌切牙区牙列缺损的种植体在不同加载条件下的周围骨应力分布及位移量,进而探讨临床修复的可行性。方法：应用CT断层扫描技术对上颌骨及上颌牙列的大致轮廓进行三维重建,后于上中切牙区植入种植体,完成牙冠修复并建立模型。通过三维有限元技术模拟载荷,比较不同加载条件下种植体的周围骨组织应力分布及位移量。结果：经统计学分析,种植体-骨界面的皮质骨唇侧、舌侧、近中、远中、根尖部五个部位应力值的差异有统计学意义（P〈0.001）,五个部位的位移量的差异也有统计学意义（P〈0.001）。模型在各种加载条件下的应力分布及位移量的特征大致相同,且变化趋势相似,种植体颈部及周围骨皮质为应力集中区。在各种不同的加载条件中,30°加载角度时种植体周围骨应力分布更为科学。结论：在适当的负重条件下,双端悬臂梁种植义齿是临床修复切牙区缺失可供选择的一种设计方案。     

The effect of retainer thickness on posterior resin-banded prostheses: a finite element study

According to its design concept, a resin-bonded prosthesis, compared with the conventional fixed partial denture, is a weak and unstable structure. Therefore, a resin-bonded prosthesis induces a higher failure rate, especially in the posterior region. Recently, adhesion agents have been profoundly improved. However, the design guidelines of posterior resin-bonded prostheses (RBP) have seldom been evaluated from a biomechanical perspective. The objective of this study was to investigate the biomechanical effects of the retainer thickness on posterior RBP using the finite element method. A solid model of a posterior mandibular resin-bonded prosthesis, which employed the second molar and second premolar as the abutment teeth, was constructed and meshed with various retainer thickness (ranging from 0.2 to 1.0 mm). Horizontal and vertical loadings of 200 N were applied respectively at the central fossa of the pontic to examine the stress level at the interface between the retainer and abutment teeth. All exterior nodes in the root, below the cementoenamel junction were fixed as the boundary condition. The results showed that horizontal loading would induce higher interfacial stresses than the vertical loading which indicated that the horizontal component of the occlusal force plays a more important role in evaluating the debonding phenomenon. Further, the peak interfacial stresses increased as the retainer thickness decreased and, based on the fitted relation between retainer thickness and interfacial stresses, a 0.4 mm retainer thickness was suggested as the minimum required to prevent severe interfacial stresses increasing.     

3-D Finite element analysis of all-ceramic posterior crowns

The purpose of this study was to evaluate the stress distribution under various loading conditions within posterior all-ceramic crowns. A three-dimensional finite element model representing a lower first molar was constructed. Variations of the model had two types of single layer all-ceramic crowns (Dicor and Empress) and two types of double layer all-ceramic crowns (In-Ceram and Empress2) cemented. A load of 600 N, simulating the maximum bite force, was applied vertically to the crowns. Loads of 225 N, simulating masticatory force, were applied from three directions (vertically, at a 45 degrees angle, and horizontally). In the test simulating maximum bite force, the maximum tensile stresses on all crowns (17.4-19.4 MPa) concentrated around the loading points. In the masticatory force simulation test, the specimens experienced maximum tensile stresses of 19.7-27.0 MPa under a horizontal load and 10.8-10.9 MPa under a vertical load. When the load was applied horizontally, the maximum tensile stress was observed around the loading points on the surface in the case of the single layer crowns, and of the cervical area of the inner core of the double layer crowns. Within the limitation of this study, it was found that the strength of occlusal contact points is important to the integrity of posterior all-ceramic crowns and that bite forces applied from the horizontal direction are a critical factor.     

Influence of connector design and material composition and veneering on the stress distribution of all-ceramic fixed dental prostheses: A finite element study

Objectives

Finite element analysis is a standard method to simulate the stress distribution in all-ceramic dental restorations in order to estimate the loading capacity of the brittle components. The hypothesis of this study was that stresses in the connector area of a veneered FDP are strongly influenced by the framework dimensions and the veneering material.

Methods

Finite element analyzes of bilayered fixed dental prostheses with three different framework-designs and three different veneering materials were conducted, applying the loads onto the veneering as well as directly onto the framework. The outer shape of the veneering ceramic remained constant for all cases.

Results

The maximum first principal stresses in the framework of the fixed dental prostheses (FDP) decreased with smaller framework dimensions when the load was applied on the veneering. By applying the load directly onto the framework of the FDP without veneering a converse tendency was found. The variation of the veneering material lead to the conclusion that stresses in the framework became higher with decreasing Young's modulus of the veneer, while the stresses in the veneer increased at the same time.

Significance

The veneering material plays a significant role for the failure of a FDP and cannot be neglected neither in testing nor in simulation. Thus the loading capacity of dental restorations can only be reasonably evaluated when the whole restoration is taken into account, including framework and veneering.     

Accuracy of implant‐supported prostheses in the edentulous jaw. Analysis of precision of fit between cast gold‐alloy frameworks and master casts by means of a three‐dimensional photogrammetric technique.

Distortions of 15 routine implant‐supported prostheses were measured in relation to the master casts after completion by means of a 3‐dimensional (3‐D) photogrammetric technique. All prostheses were designed as one‐piece gold‐alloy castings with resin teeth. Five of the prostheses were placed in the edentulous maxilla, and the remaining were placed in the lower jaw. Distortion of the cylinders was mostly observed in the horizontal plane (x‐and y‐axis) while the vertical aspect seemed to be more stable. The mean 3‐D center point distortion was 42 (SD 15) and 74 (SD 38)μm for the upper and lower jaws, respectively. The measurements revealed a range of3‐D center point distortion from 16 to 80 and 15 to 165 μm for the different jaws, respectively. The corresponding 3‐D mean angular distortion of the cylinders was 51 (SD 35)μm in lower and 70 (SD 75)μm in the upper jaws. A correlation was found between 3‐D center point distortion and the width as well as the curvature of the implant arch, indicating more displacement the wider and the more curved the arch was. The 3‐D center point distortion was also significantly higher in the upper jaws which could possibly be explained by the curvature of the implant arch and higher numbers of implants in the upper jaws. Further problems with the fit of upper jaw castings could be related to more alloy in the 1 castings and poor alignment of implants.     

Evaluation of design parameters of osseointegrated dental implants using finite element analysis

Finite element analyses were performed for various shapes of dental implant to study effects on stress distribution generated in the surrounding jaw bone and to determine an optimal thread shape for even stress distribution. It was found that the square thread shape filleted with a small radius was more effective on stress distribution than other dental implants used in the analyses. Additional analyses were performed on the implant with the thread shape obtained from previous analyses for varying other design parameters, such as the width of thread end and height of thread for various load directions, to determine the optimal dimensions of the implant. Stress distribution was more effective in the case when the width of thread end and the height of thread were 0.5p and 0.46p, respectively, where p is the screw pitch. Then, using the optimal implant thread dimensions determined previously, stress analyses were performed with various screw pitches and implant lengths, to investigate effects on stress distribution and to find the way to reduce the maximum effective stress generated in the jaw bone. Results show that the maximum effective stress decreased not only as screw pitch decreased gradually but also as implant length increased.     

单侧加载对髁突表面应力影响的三维有限元研究

目的：用三维有限元法探讨下颌单侧加载时对髁突表面应力分布的影响，以期为临床提供有益的参考。方法：利用所建的下颌骨三维有限元模型，约束模型颞骨区上表面全部节点的自由度，限制下颌运动；约束后牙咬合面节点和垂直方向的自由度。分别观察单侧（右）第二磨牙区和双侧第二磨牙区加载时，两侧髁状突表面22个区域应力分布变化情况。结果：双侧加载时在髁突表面产生的负荷，左右侧分布对称；单侧加载时，两侧髁状突表面应力分布不对称：工作侧髁突承受的最大拉应力大于非工作侧，而非工作侧髁突表面的最大压应力大于工作侧，非工作侧所有区域的Von Mises应力和最小主应力均大于工作侧。结论：单侧加载可造成双侧髁突表面应力不对称改变，非工作侧髁突表面产生的负荷大于工作侧，提示临床下颌单侧受力可能是颞下颌关节紊乱综合病的病因之一。     

种植体螺纹位置对应力分布影响的有限元研究

目的研究集中载荷下,螺纹不同位置设计对种植体及其周围骨组织应力分布的影响,探讨种植体表面螺纹分布的优化设计。方法应用Solidworks 2005 plus自动化软件和Cosmos/works 7.0分析软件比较在垂直和斜向45°载荷下,螺纹分别位于种植体上1/3（模型A）、中1/3（模型B）、下1/3（模型C）以及遍及整个种植体（模型D）4种情况下种植体-骨界面应力分布状况。结果模型C颈部皮质骨Von-Mises应力、拉应力、压应力峰值最低,但斜向载荷下模型C种植体和松质骨应力显著高于模型A。模型B应力分布明显集中,垂直载荷下各应力均显著高于其他3种模型。模型A和D应力分布较均匀。应力集中主要出现在种植体颈部、皮质骨上缘与种植体接触处和种植体底部最下一个螺纹。斜向载荷下界面的应力显著高于垂直载荷下应力。结论螺纹位置影响种植体-骨界面的应力分布,种植体设计时应谨慎考虑,斜向载荷在种植修复中应尽可能避免。