Dental Tribune Indian Edition

15Dental Tribune Indian Edition - September 2013 Authors_Prof. Amr Abdel Azim, Dr Amani M. Zaki & Dr Mohamed I. El­Anwar, Egypt _The single­tooth restoration has become one of the most widely used procedures in implant dentistry.1 In the posterior region of the oral cavity, bone volume and density are often compromised. Occlusal forces are greater in this region and, with or without parafunctional habits, can easily compromise the stability of the restorations (Fig. 1).2,3 The single-molar implant- supported restoration has historically presented a challenge in terms of form and function. The mesiodistal dimensions of a molar exceed that of most standard implants (3.75 to 4.0 mm), creating the possibility of functional overload resulting in the failure of the retaining components or the failure of the implant (Figs. 2 & 3).4 Wider-diameter implants have a genuine use in smaller molar spaces (8.0 to 11.0 mm) with a crestal width greater than or equal to 8 mm (Fig. 4 a).5 Clinical parameters governing the proposed restoration should be carefully assessed in light of the availability of implants and components that provide a myriad of options in diameter, platform configurations and prosthetic connections. Many of the newer systems for these restorations are showing promising results in recent clinical trials.6-8 It has further been suggested by Davarpanah and others,9 Balshi and others,2 English and others10 and Bahat and Handelsman11 that the use of multiple implants may be the ideal solution for single-molar implant restorations (Figs. 4 b & c). Most standard implants and their associated prosthetic components, when used to support a double implant molar restoration, will not fit in the space occupied by a molar unless the space has been enlarged (12 mm or larger).4 Moscovitch suggests that the concept of using 2 implants requires the availability of a strong and stable implant having a minimum diameter of 3.5 mm. Additionally, the associated prosthetic components should ideally not exceed this dimension.2 Finite element analysis (FEA) is an engineering method that allows investigators to assess stresses and strains within a solid body.10-13 FEA provides calculation ofstresses and deformations of each element alone and the net of all elements. A finite element model is constructed by breaking a solid object into a number of discrete elements that are connected at common nodal points. Each element is assigned appropriate material properties that correspond to the properties of the structure to be modeled. Boundary conditions are applied to the model to stimulate interactions with the environment.14 This model allows simulated force application to specific points in the system, and it provides the resultant forces in the surrounding structures. FEA is particularly useful in the evaluation of dental prostheses supported by implants.13-16 Two models were subjected to FEA study to compare between a wide implant restoration versus the two implant restoration of lower first molar. _Material and Methods Three different parts were modeled to simulate the studied cases; the jaw bones, implant/abutment assembly, and crown. Two of these parts (jaw bone and implant/abutment) were drawn in three dimensions by commercial general purpose CAD/ CAM software “AutoDesk Inventor” version 8.0. These parts are regular, symmetric, and its dimensions can be simply measured with their full details. On the other hand, crown is too complicated in its geometry therefore it was not possible to draw it in three dimensions with sufficient accuracy. Crown was modeled by using three- dimensional scanner, Roland MDX- 15, to produce cloud of points or triangulations to be trimmed before using in any other application. The second phase of difficulty might appear for solving the engineering problem, is importing and manipulating three parts one scanned and two modeled or drawn parts on a commercial FE package. Most of CAD/CAM and graphics packages deal with parts as shells (outer surface only). On the other hand the stress analysis required in this study is based on volume of different materials.3 Therefore set of operations like cutting volumes by the imported set of surfaces in addition to adding and subtracting volumes can ensure obtaining three volumes representing the jaw bone, implant/ abutment assembly, and crown.2 Bone was simulated as cylinder that consists of two parts. The inner part represents the spongy bone (diameter 14 mm and height 22 mm) that filling the internal space of the other part (shell of 1 mm thickness) that represents cortical bone (diameter 16 mm and height 24 mm). Two implants were modeled one of 3.7 mm diameter and the other of 6.0 mm. The implants/abutment design and geometry were taken from Zimmer dental catalogue (Fig. 5). Linear static analysis was performed. The solid modeling and finite element analysis were performed on a personal computer Intel Pentium IV, processor 2.8 GHz, 1.0 GB RAM. The meshing software was ANSYS version 9.0 and the used element in meshing all three dimensional model is eight nodes Brick element (SOLID45 ), which has three degrees of freedom (translations in the global directions). Listing of the used materials in this analysis is found in Table 1. The two models were subjected to 120 N vertical load equally distributed (20 N on six points simulate the occlusion; one on each cusp and one in the central fossa). On the other hand, the base of the cortical bone cylinder was fixed in all directions as a boundary condition.17-21 _Results and Discussion Results of FEA showed a lot of details about stresses and deformations in all partsofthetwomodelsunderthescope of this study. Figures 6a & b showed a graphical comparison between the crowns of the two models which are safe under this range of stresses (porcelain coating, gold crown, and implants showed the same ranges of safety). No critical difference can be noticed on these parts of the system. All differences might be found are due to differences in supporting points and each part volume to absorb load energy (equation 2).** Generally a crown placed on two implants is weaker than the same crown placed on one implant. This fact is directly reflected on porcelain coating and the two implants that have more deflections. Comparing wide implant model with the two implants from the geometrical point of view it is simply noted that cross sectional area was reduced by 43.3 % while the side area increased by 6.5 %. Using one implant results as a reference in a detailed comparison between the two models by using equation (1) resulted in Table 2 for porcelain coating, gold crown, implant(s), spongy and cortical bones respectively. Difference % = {One implant Result— Two implants Result}*100/One implant Result…(1) Spongy bone deformation and stresses (Table 2) seems to be the same in the two cases. Simple and fast conclusion can be taken that using one wide implant is equivalent to using two conventional implants. On the other hand a very important conclusion can be exerted that, under axial loading, about 10% increase in implant side area can overcome reduction of implant cross section area by 50%. In other words, effectiveness of increasing implant side area might be five times higher than the increasing of implant cross section area on spongy bone stress level under axial loading. Starting from Figure 7 a & b, slight differences can be noticed on spongy bone between the two models results. The stresses on the spongy bone are less by about 5% in the two implants model than the one wide diameter implant. The exceptions are the relatively increase in maximum compressive stresses and deformations of order 12% and 0.3% respectively. The bone is known to respond the best to compressive and the least to shear stresses22 , so considering the difference in compressive stresses less significant, the two implants were found to have a better effect on spongy bone. Contrarily, Figures 8a & b, showed better performance with cortical bone in case of using one wide implant over using two implants, that, deformations Table 2: Results Differences % Porcelain coating (1 mm) Gold crown Implants Spongy bone Cortical bone Usum -17.86 -16.70 -8.18 -0.28 -19.57 Uz -11.10 -11.10 -2.72 -0.03 -19.62 S1 31.59 -179.99 -6.72 5.96 -37.17 S3 0.71 -33.44 -310.74 -11.24 -70.43 Sint -1.26 -18.08 -166.39 4.75 -31.82 Seqv 0.25 -10.22 -196.86 4.00 -39.17 Single Molar Restoration Single molar restoration—Wide implant versus two conventional Figure 1: Load distribution during masti- cation shows marked increase in the mo- lar and premolar area.23 Figure 2: Occlusal view showing a mis- sing first molar. The mesio-distal width is very wide and restoration couldn’t com- pensate it leaving a space distally. Figure 4B: Buccal view of 2 standard 20-degree abutments on 3.5 mm Astra Tech implants for restoration of mandibu- lar right first molar.1,24 Figure 4C: Radiographic view of the re- storation.1,24 Figure 3: Proximal cantilever shown ra- diographic view of maxillary right first molar on standard Brånemark implant with standard abutment (Nobel Biocare).1 Figure 4A: Radiographic view of wide im- plants used to restore missing lower first molars.1,24 Figure 5: Crown, implants and bone assembled in a model (FEA software). Figure 6A & B: Von Mises stress on crown (A) wide implant; (B) two implants. Table 1: Material Properties Material Poisson’s ratio Young’s modulus MPa Coating (Porcelain) 0.3 67,200 Restoration (Gold) 0.3 96,000 Implants (Titanium) 0.35 110,000 Spongy bone 0.3 150 Cortical bone 0.26 1,500 → DT page 22