DOI 10.1515/bmt-2013-0109 Biomed Tech 2014; 59(3): 203–211
Istabrak Hasan*, Friedhelm Heinemann and Christoph Bourauel
Biomechanical finite element analysis of self-tapping implants with different dimensions inserted in two bone qualities Abstract: Self-tapping dental implants offer the advantage of shortening the surgical insertion time of the implants and improve primary stability in poor bone quality. Using finite element analysis, a series of self-tapping implants with different diameters and lengths have been analysed with respect to their load transfer to the alveolar bone under axial and 45° loading conditions with a total force of 300 N. The implants were inserted in idealised bone beds with cortical thicknesses of 2 and 3 mm. The implants were considered to have osseointegrated condition. A linear decrease of the maximum stresses and strains in the bone around the implants was observed by increasing the diameter of the implants from 3.7 to 5.5 mm regardless of the length of these implants. Lateral loading of the implants caused a critical increase of the stresses and strains in the bone, in particular with the thin cortical layer of 2 mm. The determined biomechanical characteristics of the self-tapping implants showed their applicability in different bone qualities even with extreme reduced length of 7 mm. Keywords: dental implants; finite element method; selftapping; strain; stress. *Corresponding author: Dr.rer.nat. Istabrak Hasan, Endowed Chair of Oral Technology, Rheinische Friedrich-Wilhelms University, Welschnonnenstrasse 17, 53111 Bonn, Germany, Phone: +49 228 2872 2332 2388, E-mail: [email protected] Friedhelm Heinemann: Department of Prosthodontics, Gerodontology and Biomaterials, University of Greifswald, Domstrassee 11, 17489 Greifswald, Germany Christoph Bourauel: Endowed Chair of Oral Technology, Rheinische Friedrich-Wilhelms University, Welschnonnenstrasse 17, 53111 Bonn, Germany
Introduction The biomechanics of dental implants is a critical issue and plays a major role on the long-term success of implant therapy. The mechanism of load transfer from implants to surrounding bone depends on the nature of
loading including the direction and magnitude of the applied force, length and diameter of the implants, the shape and characteristics of the implant surface, the prosthesis type, and the quality of the surrounding bone [2, 23]. The technique of implant placement (self-drilling vs. self-tapping), the diameter of the pilot drill and the intraosseous insertion angle of the implant are frequently cited as the factors that influence the primary stability of implants [3–8, 12, 20]. The geometry of the implants has a significant influence on primary stability. One of these design modifications is the adoption of a self-tapping thread. Self-tapping implants are usually designed with vertical cutting blades in the apical third of the implant. This design would help in eliminating the need for a tapping procedure during the placement surgery and may improve the implant’s primary stability and survival rate [15, 17, 22]. According to Kim and Lim [17], “[t]herefore, many differently designed self-tapping implants are becoming available on the market at present.” The finite element method has the advantage of noninvasively analysing the influence of numerous factors on the success of particular therapy and anticipating its prognosis, in particular, the effect of varying the direction of the applied force on the distribution of the load in the surrounding bone [11]. This study aimed to analyse the relation of stresses and strains in the bone to the diameter and length of self-tapping implants, in particular, by studying an extremely reduced implant length of 7 mm.
Materials and methods Three-dimensional finite element models of nine self-tapping tioLogic©-ST implants (Dentaurum Implants GmbH, Ispringen, Germany) were created (Table 1, Figure 1). The geometries of the implants were constructed from the CAD/CAM data that were generated and provided by the dental implant company and subsequently fed into the FE program MSC Marc/Mentat 2010 (MSC Software, Santa Ana, CA, USA).
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204 I. Hasan et al.: Self-tapping dental implants Table 1 Diameters and lengths of the investigated implants. Implant type S S M M M M M L L
300 N
Diameter (mm)
Length (mm)
3.3 3.3 3.7 3.7 3.7 4.2 4.2 4.8 5.5
9 13 7 9 7 13 13 13 13
The implants were inserted to their total length in idealised bone segments with cortical thicknesses of 2 and 3 mm to study the behaviour of the implants in different bone qualities. The bone segment had width and length of 15 mm and height of 25 mm. The bone was considered as an isometric homogenous linear elastic material. A full osseointegrated condition was considered for the numerical analyses, and for this reason, contact and friction analysis was not considered in this study. Two loading conditions were applied for the implants: axial loading of 300 N and lateral loading of 300 N 45° from the implant’s long axis. The free ends of the bone segment were constrained in three degrees of freedom (Figure 2). The Young’s moduli of the different components were 110 GPa for titanium grade 5 implants, 20 GPa [18, 24] for cortical bone and 300 MPa for cancellous bone [23]. The Poisson’s ratio was 0.3 for all the components of the numerical model. The element type used was a four-noded tetrahedral element. The final models had a total number of about 250,000 elements and 40,000 nodes.
3.3×9 mm
3.3×13 mm
3.7×7 mm
3.7×9 mm
3.7×13 mm
300 N
45°
Figure 2 Boundary conditions of the numerical models. The implant was first loaded axially with 300 N (black arrow) and then in 45° from its long axis (red arrow).
Results Axial loading of the implants with 300 N The maximum displacements of the implants were higher with a cortical thickness of 2 mm (0.012 mm) than with 3 mm (0.007 mm). The highest displacement was observed with the 3.3 × 9-mm and 3.7 × 7-mm implants. The maximum displacements of the 3.3 × 9-mm to 4.2 × 7-mm implant series were 0.011–0.012 mm.
4.2×7 mm
4.2×13 mm
Figure 1 Overview of implant dimensions used in the study.
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4.8×13 mm
5.5×13 mm
I. Hasan et al.: Self-tapping dental implants 205
maximum stress were decreased from 51 MPa (2-mm cortical layer) and 32 MPa (3-mm cortical layer) with implant diameter of 3.3 × 9 mm to 23 MPa and 14 MPa with implant diameter of 5.5 × 13 mm (Figure 4). The decrease in the maximum strain values in the cancellous bone was seen when both the diameter and the length of the implant were increased. The highest strain was, however, observed with the 3.3 × 9-mm implant (6120 µε), followed by the 3.3 × 13-mm implant (5900 µε) and the 4.2 × 13-mm implant (5800 µε) with 2-mm cortical layer (Figure 5).
The implant displacement was decreased linearly by increasing the diameter of the implant from 4.2 to 5.5 mm for the models with cortical thickness of 2 mm (Figure 3). However, by changing the cortical thickness to 3 mm, a linear decrease of the implant’s maximum displacement was observed even when the diameter of the implant was increased from 3.7 mm (0.006 mm) to 5.5 mm (0.004 mm). The maximum stress in the cortical bone was decreased linearly by increasing the diameter of the implants regardless of their lengths. The values of the
A
0.020
Cortical bone 2 mm Cortical bone 3 mm
Maximum displacement (mm)
0.015
0.010
0.005
0.000
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S3.3
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7
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Axial loading
Maximum displacement (mm)
Loading in 45°
0.015
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-L13
S3.3
3
7-L1
M3.
3
2-L1
M4.
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L5.5
Figure 3 Maximum implant displacements under axial loading. (A) Maximum displacement of the implants with 300 N and cortical thicknesses of 2 and 3 mm under axial loading. (B) The relation of maximum displacement and increasing implant diameter with 300 N and cortical thickness of 2 mm.
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206 I. Hasan et al.: Self-tapping dental implants
A
200
Cortical bone 2 mm Cortical bone 3 mm
180
Maximum stress (MPa)
160 140 120 100 80 60 40 20 0
S3.3
B
3.3×9 mm
-L9 3-L13 .3-L7 .7-L09 .7-L13 .2-L07 .2-L13 4.8-L13 .5-L13 L S3 L5 M3 M4 M3 M4 S3.
0.0
3.3×13 mm
32.5
3.7×7 mm
3.7×9 mm
3.7×13 mm
Equivalent stress
4.2×7 mm
(MPa)
4.2×13 mm
65.0
4.8×13 mm
5.5×13 mm
Figure 4 Stresses of the cortical bone under axial loading. (A) Maximum stresses of the cortical bone under an axial load of 300 N. (B) Longitudinal section through the middle of the model illustrating the distribution of the stresses around the implants with cortical thicknesses of (upper row) 2 and (lower row) 3 mm.
Lateral loading of the implants with 300 N in 45° The maximum displacement of the implants under lateral loading was increased to 0.019 mm. The highest displacement was observed with the 3.7 × 7-mm implant. The linear decrease in the maximum displacement was registered with the 3.7-mm diameter (length 13 mm) to the 5.5-mm diameter. The maximum displacements linearly decreased from 0.016 and 0.012 mm to 0.009 and 0.006 mm with cortical thickness of 2 and 3 mm, respectively (Figure 6). The maximum stresses of the cortical bone were twofold more than those with axial loading. The highest stress was observed with the 3.3 × 9-mm implant (118 MPa). The linear reduction in the stress values by increasing the
implant diameter was steeper in comparison to the axial loading (Figure 7). The maximum strains of the cancellous bone around the 3.3 × 9-mm implant and the 3.7 × 7-mm implant were dramatically increased by loading the implants laterally, in particular with a cortical thickness of 2 mm. The maximum strains of the two mentioned implants were 9400 and 9000 µε, respectively. The highest strains were obtained with the reduced implant lengths, namely, 7 and 9 mm (Figure 8).
Discussion The main factors that ensure a secure bone-implant interface include implant design, diameter and length of the
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I. Hasan et al.: Self-tapping dental implants 207
A10,000
Cortical bone 2 mm Cortical bone 3 mm
9000
Maximum strain (µstrain)
8000 7000 6000 5000 4000 3000 2000 1000 0
-L9
S3.3
B
3.3×9 mm
3 -L13 S3.3-L7 3.7-L09 3.7-L13 4.2-L07 4.2-L1 4.8-L13 5.5-L13 M L L M M M
S3.3
0
3.3×13 mm
3250 Equivalent strain (µStratin)
3.7×7 mm
3.7×9 mm
3.7×13 mm
4.2×7 mm
4.2×13 mm
6500
4.8×13 mm
5.5×13 mm
Figure 5 Strains of the cancellous bone under axial loading. (A) Maximum strains of the cancellous bone under an axial load of 300 N. (B) Longitudinal section through the middle of the model illustrating the distribution of the strains around the implants with cortical thicknesses of (upper row) 2 and (lower row) 3 mm.
implant as well as bone density [19]. The indications of certain implant diameters are based on both surgical and prosthetic requirements. Finite element studies suggest that implants with a wider diameter are more favourable in reducing the stress distribution in the bone surrounding the implants when sufficient alveolar bone is available [13, 21]. The known advantages of using wide-diameter implants include more bone-to-implant contact, bicortical engagement, immediate placement in failure sites and reduction in abutment stresses and strains. Therefore, more contact area provides increased initial stability and reduces stresses. Improved implant strength and
resistance to fracture can be attained by increasing the diameter of implants [14]. The common lengths of conventional dental implants range from 8 to 13 mm, which correspond closely to normal root length. It has been suggested that as the length of the surface area increases, the stress levels in the surrounding bone for a given applied load on the implant decrease. This consequently improves the mechanical resistance to occlusal forces. The selection of implant length depends entirely upon the amount of available bone [14]. The results of the present study showed an obvious effect of increasing the diameter of the implant on
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208 I. Hasan et al.: Self-tapping dental implants 0.020
Cortical bone 2 mm Cortical bone 3 mm
Maximum displacement (mm)
0.015
0.010
0.005
0.000
-L9
S3.3
-L13
S3.3
7-L7
M3.
9
7-L0
M3.
3
7-L1
M3.
7
2-L0
M4.
3
2-L1
M4.
3
-L13 L5.5-L1
L4.8
Figure 6 Maximum displacement of the implants with 300 N and cortical thicknesses of 2 and 3 mm under lateral loading of 45°.
decreasing the stress within the surrounding bone. By loading the implant axially, a slight decrease in the maximum stress was obtained by increasing the diameter of the implants regardless of their length. The maximum stresses of the cortical bone were twofold higher than those with axial loading. The highest stress was observed with the 3.3 × 9-mm implant (118 MPa). The linear reduction in the maximum stress values by increasing the implant diameter was steeper in comparison to the axial loading. Under an axial load, the highest strain was observed with the 3.3 × 9-mm implant (6120 µε), followed by the 3.3 × 13-mm implant (5900 µε) and the 4.2 × 13-mm implant (5800 µε) with 2-mm cortical layer. By loading the implant laterally, the maximum strains of the cancellous bone around the 3.3 × 9-mm and the 3.7 × 7-mm implant were dramatically increased, in particular with the cortical thickness of 2 mm. The maximum strains of the two mentioned implants were 9400 and 9000 µε, respectively. The highest strains were obtained with the reduced implant lengths, namely, 7 and 9 mm. According to the obtained stress and strain results for the different diameters and lengths of the investigated implants, desirable outcomes could be obtained (the stresses were below 200 N, i.e., within the physiological rage of the bone) as well using self-tapping implants with shortened length, in our case 7 mm, when more implants are inserted to compensate for the reduction of the length and improve their stability.
Demenko et al. [9] suggested that an increase in implant length and diameter leads to a reduction in stress magnitudes within cortical bone. The results of our study are in agreement with the numerical results of Demenko’s study and the conclusion of Baggi et al. [2] that implant diameter can be considered a more effective design parameter than implant length [9]. In clinical situations where soft bone quality is present, primary stability may be enhanced following drill preparations without bone tapping. In denser bone, however, the standard procedure, including bone pretapping, is recommended. Therefore, without the tapping procedure, Kim and Lim [17], in their study, assumed that the implant with self-cutting blades could be expected to achieve better values for primary stability. Moreover, they suggested that implants without self-cutting blades could also yield higher primary stability even in medium-density bone such as quality 2. This is related to the increased surface arising from the design of the implant that lacked a self-tapping blade. According to Kim and Lim [17], [t]he cutting blades reduce the thread surface area and may thus minimise implant-bone contact in the apical half.” In our study, the reduction in the maximum stress of the cortical bone of about 20% was obtained by increasing the thickness of the cortical layer from 2 to 3 mm. Al-Nawas et al. [1] found that the Brånemark system with self-cutting blades reveals higher insertion torque and implant stability quotient values than the Straumann
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I. Hasan et al.: Self-tapping dental implants 209
A
200
Cortical bone 2 mm Cortical bone 3 mm
180 160
Maximum stress (MPa)
140 120 100 80 60 40 20 0
-L9
S3.3
B
3.3×9 mm
-L13
S3.3
-L7
S3.3
0.0
3.3×13 mm
9
7-L0
M3.
3.2.0
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7
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3.7×13 mm
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2-L1
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4.2×7 mm
-L13
L4.8
4.2×13 mm
-L13
L5.5
65.0
4.8×13 mm
5.5×13 mm
Figure 7 Stresses of the cortical bone under loading in 45°. (A) Maximum stresses of the cortical bone under a lateral load of 300 N in 45°. (B) Longitudinal section through the middle of the model illustrating the distribution of the stresses around the implants with cortical thicknesses of (upper row) 2 and (lower row) 3 mm.
system without self-cutting blades. However, higher insertion torque values of implants without self-cutting blades have been shown in another study by Rabel et al. [25]. In these previous studies, experiments were performed using different implant systems [17]. In the study of Khayat et al. [16], the influence of increasing the insertion torque on the bone around tapered screw implants was prospectively investigated. They concluded that the use of high insertion torques (up to 176 N cm) did not prevent osseointegration, and marginal bone
levels in the control and experimental groups were similar both at the time of loading and after 1 year. Based on the obtained numerical results, self-tapping implants under axial or later loading provide an acceptable loading of the bone that was within the physiological range [10]. Even by reducing the length to 7 mm, the stress of the surrounding bone was comparable to the values obtained with conventional lengths. However, one of the limitations of this study is that these are numerical investigations that need to be validated with long-term clinical
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210 I. Hasan et al.: Self-tapping dental implants
A
10,000
Cortical bone 2 mm Cortical bone 3 mm
9000 8000
Maximum strain (µstrain)
7000 6000 5000 4000 3000 2000
1000 0
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3250 Equivalent strain (µStratin)
3.3×13 mm
3.7×7 mm
3.7×9 mm
3.7×13 mm
4.2×7 mm
4.2×13 mm
6500
4.8×13 mm
5.5×13 mm
Figure 8 Strains of the cancellous bone under loading in 45°. (A) Maximum strains of the cancellous bone under a lateral load of 300 N in 45°. (B) Longitudinal section through the middle of the model illustrating the distribution of the strains around the implants with cortical thicknesses of (upper row) 2 and (lower row) 3 mm.
studies. A further limitation in this study was the use of an idealised homogenous bone segment. In addition, the quality of the bone where the selftapping implants are to be inserted is important. More implants are recommended to be inserted in case of short implants (7 mm) to compensate for their reduced length and to improve their stability.
Acknowledgments: The authors wish to thank Dentaurum Implants Company for their kind cooperation in providing implant geometries for the FE models.
Received October 15, 2013; accepted February 3, 2014; online first April 1, 2014
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I. Hasan et al.: Self-tapping dental implants 211
References [1] Al-Nawas B, Wagner W, Grötz KA. Insertion torque and resonance frequency analysis of dental implant systems in an animal model with loaded implants. Int J Oral Maxillofac Implants 2006; 21: 726–732. [2] Baggi L, Cappelloni I, Di Girolamo M, Maceri F, Vairo G. The influence of implant diameter and length on stress distribution of osseointegrated implants related to crestal bone geometry: a three-dimensional finite element analysis. J Prosthet Dent 2008; 100: 422–431. [3] Baumgaertel S. Predrilling of the implant site: is it necessary for orthodontic mini-implants? Am J Orthod Dentofacial Orthop 2010; 137: 825–829. [4] Çehreli S, Arman-Özçırpıcı A. Primary stability and histomorphometric bone-implant contact of self-drilling and self-tapping orthodontic microimplants. Am J Orthod Dentofacial Orthop 2012; 141: 187–195. [5] Chen Y, Kyung HM, Gao L, Yu WJ, Bae EJ, Kim SM. Mechanical properties of self-drilling orthodontic micro-implants with different diameters. Angle Orthod 2010; 80: 821–827. [6] Chen Y, Lee JW, Cho WH, Kyung HM. Potential of self-drilling orthodontic microimplants under immediate loading. Am J Orthod Dentofacial Orthop 2010; 137: 496–502. [7] Chen Y, Shin HI, Kyung HM. Biomechanical and histological comparison of self-drilling and self-tapping orthodontic microimplants in dogs. Am J Orthod Dentofacial Orthop 2008; 133: 44–50. [8] Deguchi T, Nasu M, Murakami K, Yabuuchi T, Kamioka H, TakanoYamamoto T. Quantitative evaluation of cortical bone thickness with computed tomographic scanning for orthodontic implants. Am J Orthod Dentofacial Orthop 2006; 129: 721.e7–12. [9] Demenko V, Linetskiy I, Nesvit K, Hubalkova H, Nesvit V, Shevchenko A. Importance of diameter-to-length ratio in selecting dental implants: a methodological finite element study. Comput Methods Biomech Biomed Eng 2012; 5: 1–7. [10] Frost HM. Bone’s mechanostat: a 2003 update. Anat Rec A Discov Mol Cell Evol Biol 2003, 275: 1081–1101. [11] Gedrange T, Bourauel C, Kobel C, Harzer W. Three-dimensional analysis of endosseous palatal implants and bones after vertical, horizontal, and diagonal force application. Eur J Orthod 2003, 25: 109–115. [12] Heidemann W, Terheyden H, Gerlach KL. Analysis of the osseous/metal interface of drill free screws and self-tapping screws. J Craniomaxillofac Surg 2001; 29: 69–74. [13] Himmlova L, Dostalova T, Kacovsky A, Konvickova S. Influence of implant length and diameter on stress distribution: a finite element analysis. J Prosthet Dent 2004; 91: 20–25.
[14] Hoon LJ, Frias V, Woo LK, Wright RF. Effect of implant size and shape on implant success rates: a literature review. J Prosthet Dent 2005; 94: 377–381. [15] Ivanoff CJ, Grondahl K, Bergstrom C, Lekholm U, Brånemark PI. Influence of bicortical or monocortical anchorage on maxillary implant stability: a 15-year retrospective study of Brånemark system implants. Int J Oral Maxillofac Implants 2000; 15: 103–110. [16] Khayat PG, Arnal HM, Tourbah BI, Sennerby L. Clinical outcome of dental implants placed with high insertion torques (up to 176 Ncm). Clin Implant Dent Relat Res 2013; 15: 227–233. [17] Kim YS, Lim YJ. Primary stability and self-tapping blades: biomechanical assessment of dental implants in mediumdensity bone. Clin Oral Implants Res 2011; 22: 1179–1184. [18] Makoto S, Kenji S, Koichi K. Measurements of mechanical properties of cortical bone using nanoindentation tests. Proc Annu Meet Jpn Soc Orthopaed Biomech 2003; 24: 1–5. [19] Martinez H, Davarpanah M, Missika P, Celletti R, Lazzara R. Optimal implant stabilization in low density bone. Clin Oral Implants Res 2001; 12: 423–432. [20] Mischkowski RA, Kneuertz P, Florvaag B, Lazar F, Koebke J, Zoller JE. Biomechanical comparison of four different miniscrew types for skeletal anchorage in the mandibulomaxillary area. Int J Oral Maxillofac Surg 2008; 37: 948–954. [21] Mohammed Ibrahim M, Thulasingam C, Nasser KS, Balaji V, Rajakumar M, Rupkumar P. Evaluation of design parameters of dental implant shape, diameter and length on stress distribution: a finite element analysis. J Indian Prosthodont Soc 2011; 11: 165–171. [22] Olsson M, Friberg B, Nilson H, Kultje C. MkII – a modified self-tapping Brånemark implant: 3-year results of a controlled prospective pilot study. Inter J Oral Maxillofac Implants 1995; 10: 15–21. [23] Perez MA, Prados-Frutos JC, Bea JA, Doblare M. Stress transfer properties of different commercial dental implants: a finite element study. Comput Methods Biomech Biomed Eng 2010; 17: 1–11. [24] Poumarat G, Squire P. Comparison of mechanical properties of human, bovine bone and a new processed bone xenograft. Biomaterials 1993; 14: 337–340. [25] Rabel A, Köhler SG, Schmidt-Westhausen AM. Clinical study on the primary stability of two dental implant systems with resonance frequency analysis. Clin Oral Invest 2007; 11: 257–265.
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Istabrak Hasan*, Friedhelm Heinemann and Christoph Bourauel
Biomechanical finite element analysis of self-tapping implants with different dimensions inserted in two bone qualities Abstract: Self-tapping dental implants offer the advantage of shortening the surgical insertion time of the implants and improve primary stability in poor bone quality. Using finite element analysis, a series of self-tapping implants with different diameters and lengths have been analysed with respect to their load transfer to the alveolar bone under axial and 45° loading conditions with a total force of 300 N. The implants were inserted in idealised bone beds with cortical thicknesses of 2 and 3 mm. The implants were considered to have osseointegrated condition. A linear decrease of the maximum stresses and strains in the bone around the implants was observed by increasing the diameter of the implants from 3.7 to 5.5 mm regardless of the length of these implants. Lateral loading of the implants caused a critical increase of the stresses and strains in the bone, in particular with the thin cortical layer of 2 mm. The determined biomechanical characteristics of the self-tapping implants showed their applicability in different bone qualities even with extreme reduced length of 7 mm. Keywords: dental implants; finite element method; selftapping; strain; stress. *Corresponding author: Dr.rer.nat. Istabrak Hasan, Endowed Chair of Oral Technology, Rheinische Friedrich-Wilhelms University, Welschnonnenstrasse 17, 53111 Bonn, Germany, Phone: +49 228 2872 2332 2388, E-mail: [email protected] Friedhelm Heinemann: Department of Prosthodontics, Gerodontology and Biomaterials, University of Greifswald, Domstrassee 11, 17489 Greifswald, Germany Christoph Bourauel: Endowed Chair of Oral Technology, Rheinische Friedrich-Wilhelms University, Welschnonnenstrasse 17, 53111 Bonn, Germany
Introduction The biomechanics of dental implants is a critical issue and plays a major role on the long-term success of implant therapy. The mechanism of load transfer from implants to surrounding bone depends on the nature of
loading including the direction and magnitude of the applied force, length and diameter of the implants, the shape and characteristics of the implant surface, the prosthesis type, and the quality of the surrounding bone [2, 23]. The technique of implant placement (self-drilling vs. self-tapping), the diameter of the pilot drill and the intraosseous insertion angle of the implant are frequently cited as the factors that influence the primary stability of implants [3–8, 12, 20]. The geometry of the implants has a significant influence on primary stability. One of these design modifications is the adoption of a self-tapping thread. Self-tapping implants are usually designed with vertical cutting blades in the apical third of the implant. This design would help in eliminating the need for a tapping procedure during the placement surgery and may improve the implant’s primary stability and survival rate [15, 17, 22]. According to Kim and Lim [17], “[t]herefore, many differently designed self-tapping implants are becoming available on the market at present.” The finite element method has the advantage of noninvasively analysing the influence of numerous factors on the success of particular therapy and anticipating its prognosis, in particular, the effect of varying the direction of the applied force on the distribution of the load in the surrounding bone [11]. This study aimed to analyse the relation of stresses and strains in the bone to the diameter and length of self-tapping implants, in particular, by studying an extremely reduced implant length of 7 mm.
Materials and methods Three-dimensional finite element models of nine self-tapping tioLogic©-ST implants (Dentaurum Implants GmbH, Ispringen, Germany) were created (Table 1, Figure 1). The geometries of the implants were constructed from the CAD/CAM data that were generated and provided by the dental implant company and subsequently fed into the FE program MSC Marc/Mentat 2010 (MSC Software, Santa Ana, CA, USA).
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204 I. Hasan et al.: Self-tapping dental implants Table 1 Diameters and lengths of the investigated implants. Implant type S S M M M M M L L
300 N
Diameter (mm)
Length (mm)
3.3 3.3 3.7 3.7 3.7 4.2 4.2 4.8 5.5
9 13 7 9 7 13 13 13 13
The implants were inserted to their total length in idealised bone segments with cortical thicknesses of 2 and 3 mm to study the behaviour of the implants in different bone qualities. The bone segment had width and length of 15 mm and height of 25 mm. The bone was considered as an isometric homogenous linear elastic material. A full osseointegrated condition was considered for the numerical analyses, and for this reason, contact and friction analysis was not considered in this study. Two loading conditions were applied for the implants: axial loading of 300 N and lateral loading of 300 N 45° from the implant’s long axis. The free ends of the bone segment were constrained in three degrees of freedom (Figure 2). The Young’s moduli of the different components were 110 GPa for titanium grade 5 implants, 20 GPa [18, 24] for cortical bone and 300 MPa for cancellous bone [23]. The Poisson’s ratio was 0.3 for all the components of the numerical model. The element type used was a four-noded tetrahedral element. The final models had a total number of about 250,000 elements and 40,000 nodes.
3.3×9 mm
3.3×13 mm
3.7×7 mm
3.7×9 mm
3.7×13 mm
300 N
45°
Figure 2 Boundary conditions of the numerical models. The implant was first loaded axially with 300 N (black arrow) and then in 45° from its long axis (red arrow).
Results Axial loading of the implants with 300 N The maximum displacements of the implants were higher with a cortical thickness of 2 mm (0.012 mm) than with 3 mm (0.007 mm). The highest displacement was observed with the 3.3 × 9-mm and 3.7 × 7-mm implants. The maximum displacements of the 3.3 × 9-mm to 4.2 × 7-mm implant series were 0.011–0.012 mm.
4.2×7 mm
4.2×13 mm
Figure 1 Overview of implant dimensions used in the study.
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4.8×13 mm
5.5×13 mm
I. Hasan et al.: Self-tapping dental implants 205
maximum stress were decreased from 51 MPa (2-mm cortical layer) and 32 MPa (3-mm cortical layer) with implant diameter of 3.3 × 9 mm to 23 MPa and 14 MPa with implant diameter of 5.5 × 13 mm (Figure 4). The decrease in the maximum strain values in the cancellous bone was seen when both the diameter and the length of the implant were increased. The highest strain was, however, observed with the 3.3 × 9-mm implant (6120 µε), followed by the 3.3 × 13-mm implant (5900 µε) and the 4.2 × 13-mm implant (5800 µε) with 2-mm cortical layer (Figure 5).
The implant displacement was decreased linearly by increasing the diameter of the implant from 4.2 to 5.5 mm for the models with cortical thickness of 2 mm (Figure 3). However, by changing the cortical thickness to 3 mm, a linear decrease of the implant’s maximum displacement was observed even when the diameter of the implant was increased from 3.7 mm (0.006 mm) to 5.5 mm (0.004 mm). The maximum stress in the cortical bone was decreased linearly by increasing the diameter of the implants regardless of their lengths. The values of the
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Figure 3 Maximum implant displacements under axial loading. (A) Maximum displacement of the implants with 300 N and cortical thicknesses of 2 and 3 mm under axial loading. (B) The relation of maximum displacement and increasing implant diameter with 300 N and cortical thickness of 2 mm.
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206 I. Hasan et al.: Self-tapping dental implants
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Figure 4 Stresses of the cortical bone under axial loading. (A) Maximum stresses of the cortical bone under an axial load of 300 N. (B) Longitudinal section through the middle of the model illustrating the distribution of the stresses around the implants with cortical thicknesses of (upper row) 2 and (lower row) 3 mm.
Lateral loading of the implants with 300 N in 45° The maximum displacement of the implants under lateral loading was increased to 0.019 mm. The highest displacement was observed with the 3.7 × 7-mm implant. The linear decrease in the maximum displacement was registered with the 3.7-mm diameter (length 13 mm) to the 5.5-mm diameter. The maximum displacements linearly decreased from 0.016 and 0.012 mm to 0.009 and 0.006 mm with cortical thickness of 2 and 3 mm, respectively (Figure 6). The maximum stresses of the cortical bone were twofold more than those with axial loading. The highest stress was observed with the 3.3 × 9-mm implant (118 MPa). The linear reduction in the stress values by increasing the
implant diameter was steeper in comparison to the axial loading (Figure 7). The maximum strains of the cancellous bone around the 3.3 × 9-mm implant and the 3.7 × 7-mm implant were dramatically increased by loading the implants laterally, in particular with a cortical thickness of 2 mm. The maximum strains of the two mentioned implants were 9400 and 9000 µε, respectively. The highest strains were obtained with the reduced implant lengths, namely, 7 and 9 mm (Figure 8).
Discussion The main factors that ensure a secure bone-implant interface include implant design, diameter and length of the
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I. Hasan et al.: Self-tapping dental implants 207
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Figure 5 Strains of the cancellous bone under axial loading. (A) Maximum strains of the cancellous bone under an axial load of 300 N. (B) Longitudinal section through the middle of the model illustrating the distribution of the strains around the implants with cortical thicknesses of (upper row) 2 and (lower row) 3 mm.
implant as well as bone density [19]. The indications of certain implant diameters are based on both surgical and prosthetic requirements. Finite element studies suggest that implants with a wider diameter are more favourable in reducing the stress distribution in the bone surrounding the implants when sufficient alveolar bone is available [13, 21]. The known advantages of using wide-diameter implants include more bone-to-implant contact, bicortical engagement, immediate placement in failure sites and reduction in abutment stresses and strains. Therefore, more contact area provides increased initial stability and reduces stresses. Improved implant strength and
resistance to fracture can be attained by increasing the diameter of implants [14]. The common lengths of conventional dental implants range from 8 to 13 mm, which correspond closely to normal root length. It has been suggested that as the length of the surface area increases, the stress levels in the surrounding bone for a given applied load on the implant decrease. This consequently improves the mechanical resistance to occlusal forces. The selection of implant length depends entirely upon the amount of available bone [14]. The results of the present study showed an obvious effect of increasing the diameter of the implant on
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208 I. Hasan et al.: Self-tapping dental implants 0.020
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Figure 6 Maximum displacement of the implants with 300 N and cortical thicknesses of 2 and 3 mm under lateral loading of 45°.
decreasing the stress within the surrounding bone. By loading the implant axially, a slight decrease in the maximum stress was obtained by increasing the diameter of the implants regardless of their length. The maximum stresses of the cortical bone were twofold higher than those with axial loading. The highest stress was observed with the 3.3 × 9-mm implant (118 MPa). The linear reduction in the maximum stress values by increasing the implant diameter was steeper in comparison to the axial loading. Under an axial load, the highest strain was observed with the 3.3 × 9-mm implant (6120 µε), followed by the 3.3 × 13-mm implant (5900 µε) and the 4.2 × 13-mm implant (5800 µε) with 2-mm cortical layer. By loading the implant laterally, the maximum strains of the cancellous bone around the 3.3 × 9-mm and the 3.7 × 7-mm implant were dramatically increased, in particular with the cortical thickness of 2 mm. The maximum strains of the two mentioned implants were 9400 and 9000 µε, respectively. The highest strains were obtained with the reduced implant lengths, namely, 7 and 9 mm. According to the obtained stress and strain results for the different diameters and lengths of the investigated implants, desirable outcomes could be obtained (the stresses were below 200 N, i.e., within the physiological rage of the bone) as well using self-tapping implants with shortened length, in our case 7 mm, when more implants are inserted to compensate for the reduction of the length and improve their stability.
Demenko et al. [9] suggested that an increase in implant length and diameter leads to a reduction in stress magnitudes within cortical bone. The results of our study are in agreement with the numerical results of Demenko’s study and the conclusion of Baggi et al. [2] that implant diameter can be considered a more effective design parameter than implant length [9]. In clinical situations where soft bone quality is present, primary stability may be enhanced following drill preparations without bone tapping. In denser bone, however, the standard procedure, including bone pretapping, is recommended. Therefore, without the tapping procedure, Kim and Lim [17], in their study, assumed that the implant with self-cutting blades could be expected to achieve better values for primary stability. Moreover, they suggested that implants without self-cutting blades could also yield higher primary stability even in medium-density bone such as quality 2. This is related to the increased surface arising from the design of the implant that lacked a self-tapping blade. According to Kim and Lim [17], [t]he cutting blades reduce the thread surface area and may thus minimise implant-bone contact in the apical half.” In our study, the reduction in the maximum stress of the cortical bone of about 20% was obtained by increasing the thickness of the cortical layer from 2 to 3 mm. Al-Nawas et al. [1] found that the Brånemark system with self-cutting blades reveals higher insertion torque and implant stability quotient values than the Straumann
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I. Hasan et al.: Self-tapping dental implants 209
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Figure 7 Stresses of the cortical bone under loading in 45°. (A) Maximum stresses of the cortical bone under a lateral load of 300 N in 45°. (B) Longitudinal section through the middle of the model illustrating the distribution of the stresses around the implants with cortical thicknesses of (upper row) 2 and (lower row) 3 mm.
system without self-cutting blades. However, higher insertion torque values of implants without self-cutting blades have been shown in another study by Rabel et al. [25]. In these previous studies, experiments were performed using different implant systems [17]. In the study of Khayat et al. [16], the influence of increasing the insertion torque on the bone around tapered screw implants was prospectively investigated. They concluded that the use of high insertion torques (up to 176 N cm) did not prevent osseointegration, and marginal bone
levels in the control and experimental groups were similar both at the time of loading and after 1 year. Based on the obtained numerical results, self-tapping implants under axial or later loading provide an acceptable loading of the bone that was within the physiological range [10]. Even by reducing the length to 7 mm, the stress of the surrounding bone was comparable to the values obtained with conventional lengths. However, one of the limitations of this study is that these are numerical investigations that need to be validated with long-term clinical
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210 I. Hasan et al.: Self-tapping dental implants
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Figure 8 Strains of the cancellous bone under loading in 45°. (A) Maximum strains of the cancellous bone under a lateral load of 300 N in 45°. (B) Longitudinal section through the middle of the model illustrating the distribution of the strains around the implants with cortical thicknesses of (upper row) 2 and (lower row) 3 mm.
studies. A further limitation in this study was the use of an idealised homogenous bone segment. In addition, the quality of the bone where the selftapping implants are to be inserted is important. More implants are recommended to be inserted in case of short implants (7 mm) to compensate for their reduced length and to improve their stability.
Acknowledgments: The authors wish to thank Dentaurum Implants Company for their kind cooperation in providing implant geometries for the FE models.
Received October 15, 2013; accepted February 3, 2014; online first April 1, 2014
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[14] Hoon LJ, Frias V, Woo LK, Wright RF. Effect of implant size and shape on implant success rates: a literature review. J Prosthet Dent 2005; 94: 377–381. [15] Ivanoff CJ, Grondahl K, Bergstrom C, Lekholm U, Brånemark PI. Influence of bicortical or monocortical anchorage on maxillary implant stability: a 15-year retrospective study of Brånemark system implants. Int J Oral Maxillofac Implants 2000; 15: 103–110. [16] Khayat PG, Arnal HM, Tourbah BI, Sennerby L. Clinical outcome of dental implants placed with high insertion torques (up to 176 Ncm). Clin Implant Dent Relat Res 2013; 15: 227–233. [17] Kim YS, Lim YJ. Primary stability and self-tapping blades: biomechanical assessment of dental implants in mediumdensity bone. Clin Oral Implants Res 2011; 22: 1179–1184. [18] Makoto S, Kenji S, Koichi K. Measurements of mechanical properties of cortical bone using nanoindentation tests. Proc Annu Meet Jpn Soc Orthopaed Biomech 2003; 24: 1–5. [19] Martinez H, Davarpanah M, Missika P, Celletti R, Lazzara R. Optimal implant stabilization in low density bone. Clin Oral Implants Res 2001; 12: 423–432. [20] Mischkowski RA, Kneuertz P, Florvaag B, Lazar F, Koebke J, Zoller JE. Biomechanical comparison of four different miniscrew types for skeletal anchorage in the mandibulomaxillary area. Int J Oral Maxillofac Surg 2008; 37: 948–954. [21] Mohammed Ibrahim M, Thulasingam C, Nasser KS, Balaji V, Rajakumar M, Rupkumar P. Evaluation of design parameters of dental implant shape, diameter and length on stress distribution: a finite element analysis. J Indian Prosthodont Soc 2011; 11: 165–171. [22] Olsson M, Friberg B, Nilson H, Kultje C. MkII – a modified self-tapping Brånemark implant: 3-year results of a controlled prospective pilot study. Inter J Oral Maxillofac Implants 1995; 10: 15–21. [23] Perez MA, Prados-Frutos JC, Bea JA, Doblare M. Stress transfer properties of different commercial dental implants: a finite element study. Comput Methods Biomech Biomed Eng 2010; 17: 1–11. [24] Poumarat G, Squire P. Comparison of mechanical properties of human, bovine bone and a new processed bone xenograft. Biomaterials 1993; 14: 337–340. [25] Rabel A, Köhler SG, Schmidt-Westhausen AM. Clinical study on the primary stability of two dental implant systems with resonance frequency analysis. Clin Oral Invest 2007; 11: 257–265.
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