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Three-dimensional finite element analysis of stress distribution in retention screws of different crown–implant ratios a
b
b
b
c
S.L.D. Moraes , E.P. Pellizzer , F.R. Verri , J.F. Santiago Jr & J.V.L. Silva a
Department of Prosthodontics, Pernambuco State University, Recife, Brazil
b
Department of Dental Materials and Prosthodontics, Araçatuba Dental School, UNESP – Univ Estadual Paulista, José Bonifácio St, 1193, AraçatubaSão Paulo16015-050, Brazil c
Three-Dimensional Technologies, Archer Information Technology Center, CTI, D. Pedro I (SP – 65) Km 143, 6 CampinasSão Paulo13069-901, Brazil Published online: 15 Aug 2013.
To cite this article: Computer Methods in Biomechanics and Biomedical Engineering (2013): Three-dimensional finite element analysis of stress distribution in retention screws of different crown–implant ratios, Computer Methods in Biomechanics and Biomedical Engineering, DOI: 10.1080/10255842.2013.820719 To link to this article: http://dx.doi.org/10.1080/10255842.2013.820719
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Computer Methods in Biomechanics and Biomedical Engineering, 2013 http://dx.doi.org/10.1080/10255842.2013.820719
Three-dimensional finite element analysis of stress distribution in retention screws of different crown – implant ratios S.L.D. Moraesa1, E.P. Pellizzerb*, F.R. Verrib2, J.F. Santiago Jrb3 and J.V.L. Silvac4 a
Department of Prosthodontics, Pernambuco State University, Recife, Brazil; bDepartment of Dental Materials and Prosthodontics, Arac atuba Dental School, UNESP – Univ Estadual Paulista, Jose´ Bonifa´cio St, 1193, Arac atuba, Sa˜o Paulo 16015-050, Brazil; c Three-Dimensional Technologies, Archer Information Technology Center, CTI, D. Pedro I (SP – 65) Km 143, 6 Campinas, Sa˜o Paulo 13069-901, Brazil
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(Received 20 September 2012; final version received 26 June 2013) The retaining screw of the implant-supported dental prosthesis is the weakest point of the crown/implant system. Furthermore, crown height is another important factor that may increase the lever arm. Therefore, the aim of this study was to assess the stress distribution in implant prosthetic screws with different heights of the clinical crown of the prosthesis using the method of three-dimensional finite element analysis. Three models were created with implants (3.75 mm £ 10 mm) and crowns (heights of 10, 12.5 and 15 mm). The results were visualised by means of von Mises stress maps that increased the crown heights. The screw structure exhibited higher levels of stresses in the oblique load. The oblique loading resulted in higher stress concentration when compared with the axial loading. It is concluded that the increase of the crown was damaging to the stress distribution on the screw, mainly in oblique loading. Keywords: dental implants; implant-supported dental prosthesis; biomechanics; finite element analysis; mechanical stress
1.
Introduction
The crown – implant ratio was established as a biomechanical variable that may have a negative influence on dental implants and bone tissue (Salvi and Bra¨gger 2009; Nissan et al. 2011a). The literature indicates that under non-axial forces, the increase of the crown – implant ratio increases in proportion to stress concentration (Salvi and Bra¨gger 2009; Urdaneta et al. 2010; Nissan et al. 2011b). Furthermore, a greater moment of force or lever arm is formed, which will result in progressive accumulation of stresses in the bone tissue (Urdaneta et al. 2010). The effect of stresses may increase by 20% for each 1 mm in crown height (Bidez and Misch 1992; Urdaneta et al. 2010). Nevertheless, there is no biomechanical agreement about the risk of the crown to implant ratio. The longitudinal clinical follow-up studies of implants with long crowns are rare and state that an increase of the crown does not compromise predictability of the treatment, considering bone loss as the main variable (Rokni et al. 2005; Tawil et al. 2006; Blanes et al. 2007). However, a literature review shows that clinical studies considered the increase in crown following radiographic criterion and did not report mechanical or technical complications (Salvi and Bra¨gger 2009). Clinically, the increase in stresses can cause loosening of the screw and fracture, bone loss, occlusal material
*Corresponding author. Email: [email protected] q 2013 Taylor & Francis
fracture of the prosthesis, loss of osseointegration or fracture of the implants (Misch 2006; Nissan et al. 2011b). Thus, it is relevant to consider the loosening of the prosthetic screw because it occurs frequently (Rangert et al. 1989; Schwarz 2000; Goodacre et al. 2003; Pjetursson et al. 2004). The literature shows that screw loosening occurs in 3.9– 17% (Aglietta et al. 2009) of cases. The biomechanical studies analysing crown height are still rare (Kwan et al. 2004; Nissan et al. 2011a, 2011b). The literature shows that oblique loading can be damaging in this situation (Kwan et al. 2004; Nissan et al. 2011a). However, it is necessary to study the effect of increasing crown height in the internal structures of the prosthesis, especially in the screw because it is the weakest link of the external hexagonal implant and can fracture under fatigue. The three-dimensional (3D) finite element analysis is a method that originated in engineering. It allows analysis of the stresses and strains of a solid body (Su¨tpideler et al. 2004). It is an appropriate methodology for studying the internal performance of dental structures (Matson et al. 2012) and implants (Demenko et al. 2012). Thus, with the purpose of obtaining newer data to understand and correct planning of implant prosthesis, this research aimed at analysing the stress distribution exclusively in screws of implant-supported dental prostheses with crowns of different heights.
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Table 1.
S.L.D. Moraes et al. Description of models.
Models
Description
A B C
External hex implant with crown height of 10 mm External hex implant with crown height of 12.5 mm External hex implant with crown height of 15 mm
2. Methods 3D models were created (Table 1) to simulate a mandibular bone section with an implant, crown and screw. The bone block was composed of trabecular bone in the centre, surrounded by 1 mm of cortical bone, obtained from decomposition of computerised tomography (sagittal section) of the second molar region. A series of 2D tomographic slices were transferred to InVesaliusw software (CTI, Campinas, Brazil) for conversion into a 3D model. The resulting model was transferred to 3D CAD software (Rhinoceros 4.0w NURBS Modeling for Windows, Seattle, WA, USA) to simplify the geometry of the design and accuracy. The implant geometry (3.75 mm diameter and 10 mm length) was obtained in similar specifications for a screw-shaped dental implant and its corresponding prosthetic component (Conexa˜ow Sistemas de Pro´tese, Aruja´, Brazil), and was designed using 3D CAD software (SolidWorks Corpw, Concord, MA, USA). Three screw-retained single crowns were modelled with different heights (10, 12.5 and 15 mm), following a study by Misch (2005). The crown design, modelled after an artificial second mandibular molar from a dental model (Odontofixw, Ribeira˜o Preto, Brazil), was digitised using a 3D scanner (MDX-20w, Roland DG, Sa˜o Paulo, Brazil). Feldsphatic porcelain (1.2 mm thick) was used for the occlusal surface. The crown framework was constructed of nickel– chromium alloy. The models were exported to Rhinoceros 4.0w (NURBS Modeling for Windows, Seattle, WA, USA) CAD software for modelling, and
Figure 1.
occlusal surface details were added using SolidWorksw 3D CAD (SolidWorks Corpw, Concord, MA, USA) software (Falco´n-Antenucci et al. 2010). The geometries of 3D models (10, 12.5 and 15 crowns) were transferred to finite element software (NEINastramw 9.2, Noran Engineering, Inc., Westminster, CA, USA). Then the finite element meshes were generated with solid elements for a parabolic crown of 10 mm (Figure 1(A)), a crown of 12.5 mm (Figure 1(B)), a crown of 15 mm (Figure 1(C)), an implant (Figure 1(D)) and a screw (Figure 2). The number of nodes and elements for each model were 432,738 nodes and 287,331 elements (Model A), 446,288 nodes and 295,607 elements (Model B) and 434,122 nodes and 287,534 elements (Model C). The structural properties, such as Young’s modulus and Poisson’s ratio, were obtained for each material from the literature (Table 2). All materials were presumed to be linear, elastic, homogeneous and isotropic. Boundary conditions were established by fixing the bone block in the x, y and z directions by the lateral faces, leaving the base freely suspended. A load of 200 N (Morneburg and Pro¨schel 2002) was applied toward axial direction, and a load of 100 N was applied toward oblique direction, guided to the lingual cusps, simulating eccentric contacts. Those forces were divided among the cusps: axial load (four regions) and oblique load (two regions). Analysis was generated in the finite element programme (FEMAP 10.0w, Siemens PLM Software, Inc., Santa Ana, CA, USA) and exported to the calculation programme NeiNastranw Version 9.2 (Noran Engineering, Inc., Westminster, CA, USA) running on a workstation (Sun Microsystemsw, Inc., Sa˜o Paulo, Brazil). The results were imported again into the FEMAPw 10.0 (Siemens PLM Software, Inc., Santa Ana, CA, USA) programme for viewing and post-processing of the maps. The results were visualised using von Mises stress maps to display stress values and patterns of stress concentration.
(A) Mesh of the crown (10 mm) and cortical/trabecular bone; (B) crown (12.5 mm); (C) crown (15 mm) and (D) implant.
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Computer Methods in Biomechanics and Biomedical Engineering
Figure 2.
Mesh of the screw.
3.
Table 2. Properties of the materials. Elastic modulus Young’s modulus Poisson’s (l) (GPa) ratio (v)
Structures Trabecular bone Cortical bone Titanium NiCr alloy
1.37 13.7 110.0 206.0
0.30 0.30 0.35 0.33
Feldspathic porcelain
82.8
0.35
Figure 3.
3
Results
The results of this study were plotted on maps of von Mises stress. The tension unit was Mega Pascal (MPa). References Sertgo¨z (1997) Sertgo¨z (1997) Sertgo¨z (1997) Anusavice and Hojjatie (1987) Sertgo¨z (1997)
3.1
General map
In the application of axial load (Figure 3), the highest stresses concentrated at the crown/implant interface and in the first threads of the implant (until the seventh thread), with a similar distribution pattern, each delimited by the screw region, which had a high-stress concentration. In
von Mises stress distribution for crown/implant (axial load).
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Figure 4.
S.L.D. Moraes et al.
von Mises stress distribution crown/implant (oblique load).
oblique loading (Figure 4), higher stresses have concentrated on the screw neck of the crown, opposite to the load application and the interface-crown in the platform of the implant. The stress concentration has been increased in the region of the screw with the increasing height of the crown.
3.2 Screw Analysis of the distribution of stresses in both loads revealed that the highest tension was concentrated in the middle screw (Figures 5 and 6). The oblique loading originated the largest concentration for all the models examined (Figure 6). In oblique load, it was observed that by increasing the length of the crown, the stress concentration has been increased, especially in the model with a crown of 15 mm (Model C), the middle portion (neck region) and the first screw thread, as shown in Figure 7. In analysis of the middle portion of the screws (mm), it was observed that the model with the larger crown (C) showed the largest area of high-stress concentrations (250 MPa). The axial forces do not increase the stress concentration to increase the height of the crown (Figure 8).
4.
Discussion
The use of von Mises stress analysis allows identification of a stress value proportional to an applied force, using a linear analysis. The use of this criterion is relevant for measurement of tension in solid and ductile metal structures (Baggi et al. 2008; Demenko et al. 2012), such as implants and screws. Moreover, the finite element analysis has proven effective in solving problems involving biomechanics of this type of problem (Su¨tpideler et al. 2004; Baggi et al. 2008; Demenko et al. 2012; Matson et al. 2012). By analysing the axial loading it was found that increasing the crown from 10 to 15 mm was not significant; this is consistent with the literature. The direction of the force occurs in the long axis of the implant in the bone, but no tension is increased significantly (Fugazzotto et al. 2004; Su¨tpideler et al. 2004; Misch 2005). However, in oblique loads, the area of stress concentration in the screw with the increase of the crown from 10 to 15 mm increased greatly. This result may be supported by the theoretical analysis of Rangert et al. (1989) that suggests that the moment caused by the oblique forces is more severe (Papavasiliou et al. 1996),
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Computer Methods in Biomechanics and Biomedical Engineering
Figure 5.
von Mises distribution for screw (axial load).
Figure 6.
von Mises distribution for screw (oblique load).
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S.L.D. Moraes et al.
Figure 7. Maximum von Mises stress distribution along the length of the screw (upper, middle and inferior portions) to Models A, B and C under axial and oblique loadings.
emphasising the strength potential of increasing the concentration of tension in the implant and bone in oblique loading. These results are consistent with Fugazzotto et al. (2004), Misch (2005), Rokni et al. (2005), Tawil et al. (2006), Blanes et al. (2007), Schulte et al. (2007), Salvi and Bra¨gger (2009), Birdi et al. (2010), Gomez-Polo et al. (2010), Urdaneta et al. (2010), Nissan et al. (2011a, 2011b), who all described the potential of increasing the crown in the transmission of occlusal forces. Regarding the increase of the crown, the clinical condition is more favourable for the crown height of 10 mm length, which increased tension in the screw more than five times. Similarly, Su¨tpideler et al. (2004) reported
Figure 8.
that the oblique loading increased stress distribution proportionally when the crown height increased from 6 mm (17.72 MPa) to 12 mm (30.09 MPa). Separate analysis of the screw showed that the higher stress concentration occurred in the central region, which showed a similar distribution of stresses in the axial loading for all crown heights. However, the stress concentration was significantly increased with an improved crown in oblique loading. The stress concentration in this region of the neck screw occurs because different structures are in contact, which is in agreement with the results of Cehreli et al. (2004) that emphasise the higher stress concentration in the regions of material interfaces. Furthermore, Rangert et al. (1989) reported that the components of the implant external hexagon have the weakest point in the screw connection. Overloading this screw may produce adverse effects, the most common being loosening or fracture of the same. This is confirmed in studies by Schwarz (2000), Pjetursson et al. (2004), Aglietta et al. (2009) and Urdaneta et al. (2010) that explained the loosening or fracture as a frequent complication in implant external hexagon. It should be emphasised that due to bone loss in the posterior mandible, it is not possible to manufacture crowns with reduced heights, thus increasing the lever arm. Another possibility would be the use of intermediate elements, which could be a practical choice (Rossi et al. 2010). The prosthesis was prepared exclusively with UCLA’s component; however, the use of other components that place these screws closer to the occlusal
Maximum von Mises stress distribution along the length of the a/b line of the screw (Models A, B and C).
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Computer Methods in Biomechanics and Biomedical Engineering surface of the restoration may reduce stress to the screws (Su¨tpideler et al. 2004). A restrictive factor is the material of the screw, which was not changed. This study was modelled on titanium, which is the most common use. Nevertheless, the literature states that other materials may yield better preloaded screws (Binon 2000; Pjetursson et al. 2007), and this will be the focus of future studies as well as the real inclination of the implant with prosthetic crowns. In oblique loading, it was observed that the maximum von Mises stresses were in a concentration of 250 MPa. Considering the maximum coefficient of tensile strength and yield of titanium from 240 to 550 MPa (Geetha et al. 2009), clinically there is a greater tendency to fracture or loosening these screws that are installed with prostheses with high crowns, which are indispensable in these situations of exact occlusal adjustment. Some clinical studies did not show longitudinal bone loss or mechanical complications related to the increase of the crown height. However, most of these studies used the geometry of internal connection that can reduce the stresses in the system (Blanes et al. 2007; Schulte et al. 2007; Birdi et al. 2010; Urdaneta et al. 2010), and clinical research studies of crown height with external hexagon implant are rare. Thus, additional longitudinal studies are required. Finally, we believe that a meticulous occlusal adjustment is important. This is relevant because the overload generated by the oblique loading added to other biomechanical and systemic factors of the patient may lead to failure of the screw, which when not observed can lead to system failures.
5.
Conclusion
Based on the methodology used, the results indicate that the increase of the crown increases the strength of stress in the screw with relevance to the oblique loading.
Acknowledgements The authors would like to express gratitude to the State of Sao Paulo Research Foundation (FAPESP) and for grant support (2009/16164-7) provided, in addition to company Connection Implant Systems Ltd. (Aruja´, Sa˜o Paulo, Brazil), and ThreeDimensional Technologies Division, Center for Information Technology Renato Archer, Campinas, Brazilian Ministry of Science and Technology (MCT), Sa˜o Paulo, Brazil.
Notes 1. 2. 3. 4.
Email: [email protected] Email: [email protected] Email: [email protected] Email: [email protected]
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Misch CE. 2005. Short dental implants: a literature review and rationale for use. Dent Today. 24(64 –66):68. Misch CE. 2006. Consideration of biomechanical stress in treatment with dental implants. Dent Today. 25(80):84– 85. Morneburg TR, Pro¨schel PA. 2002. Measurement of masticatory forces and implant loads: a methodologic clinical study. Int J Prosthodont. 15:20 – 27. Nissan J, Ghelfan O, Gross O, Priel I, Gross M, Chaushu G. 2011b. The effect of crown/implant ratio and crown height space on stress distribution in unsplinted implant supporting restorations. J Oral Maxillofac Surg. 69:1934– 1939. Nissan J, Gross O, Ghelfan O, Priel I, Gross M, Chaushu G. 2011a. The effect of splinting implant-supported restorations on stress distribution of different crown – implant ratios and crown height spaces. J Oral Maxillofac Surg. 69:2990– 2994. Papavasiliou G, Kamposiora P, Bayne SC, Felton DA. 1996. Three-dimensional finite element analysis of stress-distribution around single tooth implants as a function of bony support, prosthesis type, and loading during function. J Prosthet Dent. 76:633 –640. Pjetursson BE, Bra¨gger U, Lang NP, Zwahlen M. 2007. Comparison of survival and complication rates of toothsupported fixed dental prostheses (FDPs) and implantsupported FDPs and single crowns (SCs). Clin Oral Implants Res. 18:97 – 113. Pjetursson BE, Tan K, Lang NP, Bra¨gger U, Egger M, Zwahlen M. 2004. A systematic review of the survival and complication rates of fixed partial dentures (FPDs) after an observation period of at least 5 years. Clin Oral Implants Res. 15:667 – 676. Rangert B, Jemt T, Jo¨rneus L. 1989. Forces and moments on Branemark implants. Int J Oral Maxillofac Implants. 4:241 – 247.
Rokni S, Todescan R, Watson P, Pharoah M, Adegbembo AO, Deporter D. 2005. An assessment of crown-to-root ratios with short sintered porous-surfaced implants supporting prostheses in partially edentulous patients. Int J Oral Maxillofac Implants. 20:69 – 76. Rossi F, Ricci E, Marchetti C, Lang NP, Botticelli D. 2010. Early loading of single crowns supported by 6-mm-long implants with a moderately rough surface: a prospective 2-year follow-up cohort study. Clin Oral Implants Res. 21:937– 943. Salvi GE, Bra¨gger U. 2009. Mechanical and technical risks in implant therapy. Int J Oral Maxillofac Implants. 24:69– 85. Schulte J, Flores AM, Weed M. 2007. Crown-to-implant ratios of single tooth implant-supported restorations. J Prosthet Dent. 98:1 –5. Schwarz MS. 2000. Mechanical complications of dental implants. Clin Oral Implants Res. 11:156– 158. Sertgo¨z A. 1997. Finite element analysis study of the effect of superstructure material on stress distribution in an implantsupported fixed prosthesis. Int J Prosthodont. 10:19 – 27. Su¨tpideler M, Eckert SE, Zobitz M, An KN. 2004. Finite element analysis of effect of prosthesis height, angle of force application, and implant offset on supporting bone. Int J Oral Maxillofac Implants. 19:819 – 825. Tawil G, Aboujaoude N, Younan R. 2006. Influence of prosthetic parameters on the survival and complication rates of short implants. Int J Oral Maxillofac Implants. 21:275 – 282. Urdaneta RA, Rodriguez S, McNeil DC, Weed M, Chuang SK. 2010. The effect of increased crown-to-implant ratio on single-tooth locking-taper implants. Int J Oral Maxillofac Implants. 25:729 – 743.
Computer Methods in Biomechanics and Biomedical Engineering Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/gcmb20
Three-dimensional finite element analysis of stress distribution in retention screws of different crown–implant ratios a
b
b
b
c
S.L.D. Moraes , E.P. Pellizzer , F.R. Verri , J.F. Santiago Jr & J.V.L. Silva a
Department of Prosthodontics, Pernambuco State University, Recife, Brazil
b
Department of Dental Materials and Prosthodontics, Araçatuba Dental School, UNESP – Univ Estadual Paulista, José Bonifácio St, 1193, AraçatubaSão Paulo16015-050, Brazil c
Three-Dimensional Technologies, Archer Information Technology Center, CTI, D. Pedro I (SP – 65) Km 143, 6 CampinasSão Paulo13069-901, Brazil Published online: 15 Aug 2013.
To cite this article: Computer Methods in Biomechanics and Biomedical Engineering (2013): Three-dimensional finite element analysis of stress distribution in retention screws of different crown–implant ratios, Computer Methods in Biomechanics and Biomedical Engineering, DOI: 10.1080/10255842.2013.820719 To link to this article: http://dx.doi.org/10.1080/10255842.2013.820719
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
Computer Methods in Biomechanics and Biomedical Engineering, 2013 http://dx.doi.org/10.1080/10255842.2013.820719
Three-dimensional finite element analysis of stress distribution in retention screws of different crown – implant ratios S.L.D. Moraesa1, E.P. Pellizzerb*, F.R. Verrib2, J.F. Santiago Jrb3 and J.V.L. Silvac4 a
Department of Prosthodontics, Pernambuco State University, Recife, Brazil; bDepartment of Dental Materials and Prosthodontics, Arac atuba Dental School, UNESP – Univ Estadual Paulista, Jose´ Bonifa´cio St, 1193, Arac atuba, Sa˜o Paulo 16015-050, Brazil; c Three-Dimensional Technologies, Archer Information Technology Center, CTI, D. Pedro I (SP – 65) Km 143, 6 Campinas, Sa˜o Paulo 13069-901, Brazil
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(Received 20 September 2012; final version received 26 June 2013) The retaining screw of the implant-supported dental prosthesis is the weakest point of the crown/implant system. Furthermore, crown height is another important factor that may increase the lever arm. Therefore, the aim of this study was to assess the stress distribution in implant prosthetic screws with different heights of the clinical crown of the prosthesis using the method of three-dimensional finite element analysis. Three models were created with implants (3.75 mm £ 10 mm) and crowns (heights of 10, 12.5 and 15 mm). The results were visualised by means of von Mises stress maps that increased the crown heights. The screw structure exhibited higher levels of stresses in the oblique load. The oblique loading resulted in higher stress concentration when compared with the axial loading. It is concluded that the increase of the crown was damaging to the stress distribution on the screw, mainly in oblique loading. Keywords: dental implants; implant-supported dental prosthesis; biomechanics; finite element analysis; mechanical stress
1.
Introduction
The crown – implant ratio was established as a biomechanical variable that may have a negative influence on dental implants and bone tissue (Salvi and Bra¨gger 2009; Nissan et al. 2011a). The literature indicates that under non-axial forces, the increase of the crown – implant ratio increases in proportion to stress concentration (Salvi and Bra¨gger 2009; Urdaneta et al. 2010; Nissan et al. 2011b). Furthermore, a greater moment of force or lever arm is formed, which will result in progressive accumulation of stresses in the bone tissue (Urdaneta et al. 2010). The effect of stresses may increase by 20% for each 1 mm in crown height (Bidez and Misch 1992; Urdaneta et al. 2010). Nevertheless, there is no biomechanical agreement about the risk of the crown to implant ratio. The longitudinal clinical follow-up studies of implants with long crowns are rare and state that an increase of the crown does not compromise predictability of the treatment, considering bone loss as the main variable (Rokni et al. 2005; Tawil et al. 2006; Blanes et al. 2007). However, a literature review shows that clinical studies considered the increase in crown following radiographic criterion and did not report mechanical or technical complications (Salvi and Bra¨gger 2009). Clinically, the increase in stresses can cause loosening of the screw and fracture, bone loss, occlusal material
*Corresponding author. Email: [email protected] q 2013 Taylor & Francis
fracture of the prosthesis, loss of osseointegration or fracture of the implants (Misch 2006; Nissan et al. 2011b). Thus, it is relevant to consider the loosening of the prosthetic screw because it occurs frequently (Rangert et al. 1989; Schwarz 2000; Goodacre et al. 2003; Pjetursson et al. 2004). The literature shows that screw loosening occurs in 3.9– 17% (Aglietta et al. 2009) of cases. The biomechanical studies analysing crown height are still rare (Kwan et al. 2004; Nissan et al. 2011a, 2011b). The literature shows that oblique loading can be damaging in this situation (Kwan et al. 2004; Nissan et al. 2011a). However, it is necessary to study the effect of increasing crown height in the internal structures of the prosthesis, especially in the screw because it is the weakest link of the external hexagonal implant and can fracture under fatigue. The three-dimensional (3D) finite element analysis is a method that originated in engineering. It allows analysis of the stresses and strains of a solid body (Su¨tpideler et al. 2004). It is an appropriate methodology for studying the internal performance of dental structures (Matson et al. 2012) and implants (Demenko et al. 2012). Thus, with the purpose of obtaining newer data to understand and correct planning of implant prosthesis, this research aimed at analysing the stress distribution exclusively in screws of implant-supported dental prostheses with crowns of different heights.
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Table 1.
S.L.D. Moraes et al. Description of models.
Models
Description
A B C
External hex implant with crown height of 10 mm External hex implant with crown height of 12.5 mm External hex implant with crown height of 15 mm
2. Methods 3D models were created (Table 1) to simulate a mandibular bone section with an implant, crown and screw. The bone block was composed of trabecular bone in the centre, surrounded by 1 mm of cortical bone, obtained from decomposition of computerised tomography (sagittal section) of the second molar region. A series of 2D tomographic slices were transferred to InVesaliusw software (CTI, Campinas, Brazil) for conversion into a 3D model. The resulting model was transferred to 3D CAD software (Rhinoceros 4.0w NURBS Modeling for Windows, Seattle, WA, USA) to simplify the geometry of the design and accuracy. The implant geometry (3.75 mm diameter and 10 mm length) was obtained in similar specifications for a screw-shaped dental implant and its corresponding prosthetic component (Conexa˜ow Sistemas de Pro´tese, Aruja´, Brazil), and was designed using 3D CAD software (SolidWorks Corpw, Concord, MA, USA). Three screw-retained single crowns were modelled with different heights (10, 12.5 and 15 mm), following a study by Misch (2005). The crown design, modelled after an artificial second mandibular molar from a dental model (Odontofixw, Ribeira˜o Preto, Brazil), was digitised using a 3D scanner (MDX-20w, Roland DG, Sa˜o Paulo, Brazil). Feldsphatic porcelain (1.2 mm thick) was used for the occlusal surface. The crown framework was constructed of nickel– chromium alloy. The models were exported to Rhinoceros 4.0w (NURBS Modeling for Windows, Seattle, WA, USA) CAD software for modelling, and
Figure 1.
occlusal surface details were added using SolidWorksw 3D CAD (SolidWorks Corpw, Concord, MA, USA) software (Falco´n-Antenucci et al. 2010). The geometries of 3D models (10, 12.5 and 15 crowns) were transferred to finite element software (NEINastramw 9.2, Noran Engineering, Inc., Westminster, CA, USA). Then the finite element meshes were generated with solid elements for a parabolic crown of 10 mm (Figure 1(A)), a crown of 12.5 mm (Figure 1(B)), a crown of 15 mm (Figure 1(C)), an implant (Figure 1(D)) and a screw (Figure 2). The number of nodes and elements for each model were 432,738 nodes and 287,331 elements (Model A), 446,288 nodes and 295,607 elements (Model B) and 434,122 nodes and 287,534 elements (Model C). The structural properties, such as Young’s modulus and Poisson’s ratio, were obtained for each material from the literature (Table 2). All materials were presumed to be linear, elastic, homogeneous and isotropic. Boundary conditions were established by fixing the bone block in the x, y and z directions by the lateral faces, leaving the base freely suspended. A load of 200 N (Morneburg and Pro¨schel 2002) was applied toward axial direction, and a load of 100 N was applied toward oblique direction, guided to the lingual cusps, simulating eccentric contacts. Those forces were divided among the cusps: axial load (four regions) and oblique load (two regions). Analysis was generated in the finite element programme (FEMAP 10.0w, Siemens PLM Software, Inc., Santa Ana, CA, USA) and exported to the calculation programme NeiNastranw Version 9.2 (Noran Engineering, Inc., Westminster, CA, USA) running on a workstation (Sun Microsystemsw, Inc., Sa˜o Paulo, Brazil). The results were imported again into the FEMAPw 10.0 (Siemens PLM Software, Inc., Santa Ana, CA, USA) programme for viewing and post-processing of the maps. The results were visualised using von Mises stress maps to display stress values and patterns of stress concentration.
(A) Mesh of the crown (10 mm) and cortical/trabecular bone; (B) crown (12.5 mm); (C) crown (15 mm) and (D) implant.
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Computer Methods in Biomechanics and Biomedical Engineering
Figure 2.
Mesh of the screw.
3.
Table 2. Properties of the materials. Elastic modulus Young’s modulus Poisson’s (l) (GPa) ratio (v)
Structures Trabecular bone Cortical bone Titanium NiCr alloy
1.37 13.7 110.0 206.0
0.30 0.30 0.35 0.33
Feldspathic porcelain
82.8
0.35
Figure 3.
3
Results
The results of this study were plotted on maps of von Mises stress. The tension unit was Mega Pascal (MPa). References Sertgo¨z (1997) Sertgo¨z (1997) Sertgo¨z (1997) Anusavice and Hojjatie (1987) Sertgo¨z (1997)
3.1
General map
In the application of axial load (Figure 3), the highest stresses concentrated at the crown/implant interface and in the first threads of the implant (until the seventh thread), with a similar distribution pattern, each delimited by the screw region, which had a high-stress concentration. In
von Mises stress distribution for crown/implant (axial load).
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Figure 4.
S.L.D. Moraes et al.
von Mises stress distribution crown/implant (oblique load).
oblique loading (Figure 4), higher stresses have concentrated on the screw neck of the crown, opposite to the load application and the interface-crown in the platform of the implant. The stress concentration has been increased in the region of the screw with the increasing height of the crown.
3.2 Screw Analysis of the distribution of stresses in both loads revealed that the highest tension was concentrated in the middle screw (Figures 5 and 6). The oblique loading originated the largest concentration for all the models examined (Figure 6). In oblique load, it was observed that by increasing the length of the crown, the stress concentration has been increased, especially in the model with a crown of 15 mm (Model C), the middle portion (neck region) and the first screw thread, as shown in Figure 7. In analysis of the middle portion of the screws (mm), it was observed that the model with the larger crown (C) showed the largest area of high-stress concentrations (250 MPa). The axial forces do not increase the stress concentration to increase the height of the crown (Figure 8).
4.
Discussion
The use of von Mises stress analysis allows identification of a stress value proportional to an applied force, using a linear analysis. The use of this criterion is relevant for measurement of tension in solid and ductile metal structures (Baggi et al. 2008; Demenko et al. 2012), such as implants and screws. Moreover, the finite element analysis has proven effective in solving problems involving biomechanics of this type of problem (Su¨tpideler et al. 2004; Baggi et al. 2008; Demenko et al. 2012; Matson et al. 2012). By analysing the axial loading it was found that increasing the crown from 10 to 15 mm was not significant; this is consistent with the literature. The direction of the force occurs in the long axis of the implant in the bone, but no tension is increased significantly (Fugazzotto et al. 2004; Su¨tpideler et al. 2004; Misch 2005). However, in oblique loads, the area of stress concentration in the screw with the increase of the crown from 10 to 15 mm increased greatly. This result may be supported by the theoretical analysis of Rangert et al. (1989) that suggests that the moment caused by the oblique forces is more severe (Papavasiliou et al. 1996),
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Computer Methods in Biomechanics and Biomedical Engineering
Figure 5.
von Mises distribution for screw (axial load).
Figure 6.
von Mises distribution for screw (oblique load).
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S.L.D. Moraes et al.
Figure 7. Maximum von Mises stress distribution along the length of the screw (upper, middle and inferior portions) to Models A, B and C under axial and oblique loadings.
emphasising the strength potential of increasing the concentration of tension in the implant and bone in oblique loading. These results are consistent with Fugazzotto et al. (2004), Misch (2005), Rokni et al. (2005), Tawil et al. (2006), Blanes et al. (2007), Schulte et al. (2007), Salvi and Bra¨gger (2009), Birdi et al. (2010), Gomez-Polo et al. (2010), Urdaneta et al. (2010), Nissan et al. (2011a, 2011b), who all described the potential of increasing the crown in the transmission of occlusal forces. Regarding the increase of the crown, the clinical condition is more favourable for the crown height of 10 mm length, which increased tension in the screw more than five times. Similarly, Su¨tpideler et al. (2004) reported
Figure 8.
that the oblique loading increased stress distribution proportionally when the crown height increased from 6 mm (17.72 MPa) to 12 mm (30.09 MPa). Separate analysis of the screw showed that the higher stress concentration occurred in the central region, which showed a similar distribution of stresses in the axial loading for all crown heights. However, the stress concentration was significantly increased with an improved crown in oblique loading. The stress concentration in this region of the neck screw occurs because different structures are in contact, which is in agreement with the results of Cehreli et al. (2004) that emphasise the higher stress concentration in the regions of material interfaces. Furthermore, Rangert et al. (1989) reported that the components of the implant external hexagon have the weakest point in the screw connection. Overloading this screw may produce adverse effects, the most common being loosening or fracture of the same. This is confirmed in studies by Schwarz (2000), Pjetursson et al. (2004), Aglietta et al. (2009) and Urdaneta et al. (2010) that explained the loosening or fracture as a frequent complication in implant external hexagon. It should be emphasised that due to bone loss in the posterior mandible, it is not possible to manufacture crowns with reduced heights, thus increasing the lever arm. Another possibility would be the use of intermediate elements, which could be a practical choice (Rossi et al. 2010). The prosthesis was prepared exclusively with UCLA’s component; however, the use of other components that place these screws closer to the occlusal
Maximum von Mises stress distribution along the length of the a/b line of the screw (Models A, B and C).
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Computer Methods in Biomechanics and Biomedical Engineering surface of the restoration may reduce stress to the screws (Su¨tpideler et al. 2004). A restrictive factor is the material of the screw, which was not changed. This study was modelled on titanium, which is the most common use. Nevertheless, the literature states that other materials may yield better preloaded screws (Binon 2000; Pjetursson et al. 2007), and this will be the focus of future studies as well as the real inclination of the implant with prosthetic crowns. In oblique loading, it was observed that the maximum von Mises stresses were in a concentration of 250 MPa. Considering the maximum coefficient of tensile strength and yield of titanium from 240 to 550 MPa (Geetha et al. 2009), clinically there is a greater tendency to fracture or loosening these screws that are installed with prostheses with high crowns, which are indispensable in these situations of exact occlusal adjustment. Some clinical studies did not show longitudinal bone loss or mechanical complications related to the increase of the crown height. However, most of these studies used the geometry of internal connection that can reduce the stresses in the system (Blanes et al. 2007; Schulte et al. 2007; Birdi et al. 2010; Urdaneta et al. 2010), and clinical research studies of crown height with external hexagon implant are rare. Thus, additional longitudinal studies are required. Finally, we believe that a meticulous occlusal adjustment is important. This is relevant because the overload generated by the oblique loading added to other biomechanical and systemic factors of the patient may lead to failure of the screw, which when not observed can lead to system failures.
5.
Conclusion
Based on the methodology used, the results indicate that the increase of the crown increases the strength of stress in the screw with relevance to the oblique loading.
Acknowledgements The authors would like to express gratitude to the State of Sao Paulo Research Foundation (FAPESP) and for grant support (2009/16164-7) provided, in addition to company Connection Implant Systems Ltd. (Aruja´, Sa˜o Paulo, Brazil), and ThreeDimensional Technologies Division, Center for Information Technology Renato Archer, Campinas, Brazilian Ministry of Science and Technology (MCT), Sa˜o Paulo, Brazil.
Notes 1. 2. 3. 4.
Email: [email protected] Email: [email protected] Email: [email protected] Email: [email protected]
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