Kaohsiung J Med Sci March 2008 • Vol 24 • No 3 138
Dental implantation serves as a foundation for the support of a fixed or removable prosthesis. In recent years, many systems have been introduced to den-tists by various manufacturers. With the progress of implant surface management, high survival rates with easy installation have persuaded most dentists to par-ticipate in this work. Although surgical stent and radiography assist the implant insertion procedure, angulated implant bodies have still appeared in recent studies [1] and clinical situations.
Finite element analysis has been utilized to evaluate the stress induced around the implant and the sur-rounding bone tissue, including maxilla posterior
edentulism [1], mandibular posterior edentulism [2–5], restoration type [6] and implant body arrangement [7]. This study focused on the compressive stress analysis of different implant angulations when vertical and dif-ferent directions of horizontal forces were applied.
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ETHODSAccording to morphologic data on the Asian mandible [8], including bone size, cortical bone thickness and tooth size, a bone block from the canine to the poste-rior border of the mandible was built up with a coor-dinate system. We measured the key point, line, area and volume.
Cortical bone with different thicknesses [8] (Table 1) was defined around the cancellous bone. It was assumed to be isotropic, homogeneous and linearly elastic. The Young’s modulus of cortical and cancel-lous bone was assumed to be 13,700 MPa and Received: Apr 23, 2007 Accepted: Sep 26, 2007
Address correspondence and reprint requests to: Dr Chau-Hsiang Wang, Department of Prosthodontics, Kaohsiung Medical University Hospital, 100 Tzyou 1stRoad, Kaohsiung 807,
Taiwan.
E-mail: chhswa@kmu.edu.tw
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Ting-Hsun Lan,1Heng-Li Huang,2Ju-Hui Wu,1Huey-Er Lee,1,3and Chau-Hsiang Wang1,3
1Department of Prosthodontics, Kaohsiung Medical University Hospital, 3Faculty of Dentistry,
College of Dental Medicine, Kaohsiung Medical University, Kaohsiung, and 2School of Dentistry,
China Medical University, Taichung, Taiwan.
Clinically, many implant cases with different angulation over the lower posterior area have been found. The purpose of this study was to analyze the bony stress with different implant tilting during normal masticatory load using the finite element method (FEM), with the hope of discovering a desir-able installation of implant. A three-dimensional finite element method was employed to analyze the bony stress generated by different angulation designs (15°) of implant bodies. Eight solid models of the mandibular first and second molars were built up and then transferred to a mesh model in FEM (ANSYS) to perform a stress analysis. A simulated load (400 N) was applied to the splinted crowns with vertical and horizontal forces. The loading sites were on the central fossa of the splinted crowns. For stress distribution, some designs will be better than a parallel installation. The results suggested that not all implant bodies tilting with the splinted crowns lead to stress concentration.
Key Words:angulated, finite element method, implant body, stress analysis (Kaohsiung J Med Sci 2008;24:138–43)
1,850 MPa, respectively. Poisson’s ratio was 0.3 for both [7].
Implants of 11 mm in length and 5 mm in diameter were installed into the center of the residual ridge according to the coordinate system and an ideal occlusal plane. The distance from the center of each implant to the center of the other was 10 mm.
Titanium-aluminum-vanadium (Ti-6Al-4V) was used as the implant material, and Young’s modulus and Poisson ratio were assumed to be 117,000 MPa and 0.35, respectively [4]. Splinted full ceramic crowns were selected for the prosthesis, with Young’s modulus and Poisson ratio assumed to be 69,000 MPa and 0.28, respectively [9] (Table 2).
After material properties were applied, a mesh 3-D finite element model was constructed with a tetrahe-dral 10-node 92 element, resulting in 123,172 elements and 183,948 nodes in the ANSYS system (5.6; ANSYS, Canonsburg, PA, USA).
Models were constrained in all directions at the nodes on the mesial and distal border of the bone sur-face. Applied loads can be resolved into minimum principle stress to evaluate the bony response. A 200 N load was applied to the central fossa of each prosthetic construction in vertical and horizontal directions. Implants that were perpendicular to the occlusal plane and parallel to each other were called Model 1. We used model 1 as the standard model to analyze the stress-concentrated areas. Minimum principle stress (σ min) was used to express the peak compressive stress values of alveolar bone.
After a preliminary study, the implant was tilted in the same occlusal plane by 0 or 15 degrees mesially or distally (Figure 1). In addition, three forces were
applied to these eight solid models, including forces in vertical and horizontal directions from buccal to lingual, and in the horizontal direction from lingual to buccal (Table 3).
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ESULTSVertical load
The peak compressive stress values were predomi-nantly found in cortical bone around the cervical region of the implants. The value that appeared in the standard model was 17.43 MPa located in the second implant alveolar bone; models 2, 4 and 8 all showed a similar situation. The highest value, which appeared in model 7, was 27.14 MPa, and the lowest value, which appeared in model 6, was 13.66 MPa, located at the first implant alveolar bone. Models 3 and 5 showed a similar situation. We set the eight models in the same bony stress value from 0 to −13 and com-pared the distributions (Figure 2).
Horizontal load (buccal to lingual)
The peak compressive stress values were also pre-dominantly found in cortical bone around the cervi-cal region of the implants. The value appearing in the standard model was 86.38 MPa at the second implant alveolar bone. The highest value appeared in model 8, and was 93.99 MPa. The lowest value appeared in model 6, and was 77.11 MPa. We set the eight models in the same bony stress value from 15 to −75 and compared the distributions (Figure 3).
Horizontal load (lingual to buccal)
The results were similar to those obtained when a horizontal load was applied from buccal to lingual. The value appearing in the standard model was 73.72 MPa located at the second implant alveolar bone. The highest value appeared in model 8, and was 77.18 MPa. The lowest value appeared in model 5, and was 69.77 MPa. We set the eight models in the same bony stress value from 15 to −60 and compared the distributions (Figure 4).
Table 1.Mandibular buccal and lingual mean cortical thickness over cervical area (mm)
Canine 1stpremolar 2ndpremolar 1stmolar 2ndmolar 3rdmolar
Buccal 0.60 0.61 1.05 1.36 2.18 2.78
Lingual 1.54 1.82 2.23 2.39 2.71 2.09
Table 2.Material properties
Material Young’s Poisson’s
modulus E (MPa) ratio
Cortical bone 13,700 0.30
Cancellous bone 1,850 0.30
Ti-6Al-4V 117,000 0.35
Figure 5 shows the compressive stresses on corti-cal bone around the implants, for all three direction loadings.
D
ISCUSSIONThe finite element method [10,11] has advantages for stress analysis, but the thickness of cortical bone is hard to differentiate when using computed tomogra-phy [3,12] or surface scanners [4]. Thus, we used a coordinate system to build up a solid model. From the key point, line, and area, volume to model, we obtained different cortical bone dimensions [8] to perform this study.
Identifying the term of overload creates some dif-ficulty, but we could predict the worse tendency for implant failure. For posterior short-span prostheses, by raising the potential negative effects of bending
with offset loads as well as lateral force, bending that elevates stresses to the implant and bone should be minimized [13]. In our study, we found the mean peak stress from a horizontal direction was four to six times that in the vertical direction. As theoretical calculation of load distribution on partial prostheses has demonstrated load increases from cantilevers, offset implants and cusp inclination [7], from the angulated installation of implant bodies, we found the lowest peak stress in model 6, but not model 1. In addition, the distal inclination of implant bodies, especially in the second molar area, revealed the worst results.
Aspects to consider for sufficient bone support are primary cortical anchorage and healing time before loading the implants. Reliance on the existing cortical bone seems to be the most predictable condi-tion. This means great care must be taken to preserve any available outer cortex in the posterior regions for anchorage of implant threads. The different thick-nesses of cortical bone between the buccal and lin-gual sides may affect the prognosis of implant. In the same force but in the opposing direction, the thicker the cortical bone, the more the peak value of stress. On the basis of this finding, we suggest that clini-cians carefully note the balance side interference from occlusion. Because occlusion may change over time, it is important during follow-up to check these parameters in situations where other load factors are present [7].
Implant treatment is multifaceted: a number of load factors of smaller magnitude may be acceptable. The ability to actively control the occlusal condition Table 3. Peak compressive stresses on cortical bone
around implants
Model Vertical Horizontal
B L L B 1 17.43 86.38 73.72 2 18.01 86.05 73.81 3 26.31 87.06 73.95 4 25.68 93.83 76.92 5 18.23 77.28 69.77 6 13.66 77.11 70.09 7 27.14 77.59 70.08 8 24.98 93.99 77.18 B = buccal; L = lingual. 1 2 3 4 5 6 7 8
Figure 1. Eight different mesiodistally-angulated models according to the same prosthetic design. Left implant replaces first molar, and right one replaces second molar. From left to right: top row are models 1, 2, 3; middle row are models 4, 5, 6; and bottom row are models 7, 8.
1 2 3 4 5 6 7 8 Figure 3. Horizontal load (buccal to lin-gual). The models are arranged as in Figure 1. Model 8 has the highest value (thick arrow) and model 6 has the lowest value (thin arrow). 2 3 4 7 6 1 5 8
Figure 2. Peak of maximum compressive stress in eight solid models. The models are arranged as in Figure 1. Model 7 has the highest value (thick arrow) and model 6 has the lowest value (thin arrow).
1 2 3 4 5 6 7 8
Figure 4.Horizontal load (lingual to buccal). The models are arranged as in Figure 1. Model 8 has the highest value (thick arrow) and model 5 has the lowest value (thin arrow).
in situations with multiple geometric load factors seems to be especially significant. It is important to react to mechanical problems, as well as excessive bone resorption, with an appropriate response when they occur, for example, by eliminating cantilevers [14], narrowing the buccolingual width or mesiodistal length of the teeth, flattening cuspal inclination, and centering the occlusal contact. In our study, we found better bony responses to angulation of implant bod-ies (model 6), which might prevent future overload and bending overload.
Implant overload in posterior partial restorations should be prevented by screening patients for load factors. It is possible to identify potential overload situations in advance of treatment.
This result suggests that not all implant bodies tilting with the splinted crowns lead to stress concen-tration. In changeable clinical situations, preventing worsening angulation (models 4, 8) and achieving better angulation (models 6, 5, 1) is our goal.
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CKNOWLEDGMENTSThis work was supported by a grant from the National Science Council of Taiwan (NSC 92-2320-B-037-032).
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EFERENCES1. Caglar A, Aydin C, Ozen J, et al. Effects of mesiodistal
inclination of implants on stress distribution in implant-supported fixed prostheses. Int J Oral Maxillofac Implants 2006;21:36–44.
2. Tuncelli B, Poyrazoglu E, Koyluoglu AM, et al.
Com-parison of load transfer by implant abutments of various diameters. Eur J Prosthodont Restor Dent 1997;5:79–83.
3. Akca K, Iplikcioglu H. Evaluation of the effect of the
residual bone angulation on implant-supported fixed prostheses in mandibular posterior edentulism. Part I: spiral computed tomography study. Implant Dent 2001; 10:216–22.
4. Akca K, Iplikcioglu H. Evaluation of the effect of the residual bone angulation on implant-supported fixed prosthesis in mandibular posterior edentulism. Part II: 3-D finite element stress analysis. Implant Dent 2001; 10:238–45.
5. van Zyl PP, Grundling NL, Jooste CH, et al.
Three-dimensional finite element model of a human mandible incorporating six osseointegrated implants for stress analysis of mandibular cantilever prostheses. Int J Oral Maxillofac Implants 1995;10:51–7.
6. Stegaroiu R, Sato T, Kusakari H, et al. Influence of
restoration type on stress distribution in bone around implants: a three-dimensional finite element analysis. Int J Oral Maxillofac Implants 1998;13:82–90.
7. Rangert BR, Sullivan RM, Jemt TM. Load factor control
for implants in the posterior partially edentulous seg-ment. Int J Oral Maxillofac Implants 1997;12:360–70.
8. Yasuhiko K. Clinical and Applied Craniology. Tokyo:
Anatomy Publishing, 1966.
9. Kamposiora P, Papavasilious G, Bayne SC, et al. Finite
element analysis estimates of cement microfracture under complete veneer crowns. J Prosthet Dent 1994; 71:435–41.
10. DeTolla DH, Andreana S, Patra A, et al. Role of the finite element model in dental implants. J Oral Implantol 2000;26:77–81.
11. Baiamonte T, Abbate MF, Pizzarello F, et al. The experi-mental verification of the efficacy of finite element modeling to dental implant systems. J Oral Implantol 1996;22:104–10.
12. Van Oossterwyck H, Duyck J, Vander Sloten J, et al. Use of microfocus computerized tomography as a new technique for characterizing bone tissue around oral implants. J Oral Implantol 2000;26:5–12.
13. Rangert B, Jemt T, Jorneus L. Forces and moments on Branemark implants. Int J Oral Maxillofac Implants 1989;4:241–7.
14. Krekmanov L, Kahn M, Rangert B, et al. Tilting of pos-terior mandibular and maxillary implants for improved prosthesis support. Int J Oral Maxillofac Implants 2000; 15:405–14. Horizontal B→L Vertical Horizontal L→B 0 20 40 60 80 100 1 2 3 4 5 6 7 8 Model
Compressive stress (MPa)
Figure 5.Compressive stresses on cortical bone around implants from three direction loadings. B= buccal; L = lingual.
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