Journal of Biomechanics 48 (2015) 138–145
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Biomechanical influence of crown-to-implant ratio on stress distribution over internal hexagon short implant: 3-D finite element analysis with statistical test Fellippo Ramos Verri a,n, Joel Ferreira Santiago Junior a, Daniel Augusto de Faria Almeida a, Guilherme Bérgamo Brandão de Oliveira a, Victor Eduardo de Souza Batista a, Heitor Marques Honório b, Pedro Yoshito Noritomi c, Eduardo Piza Pellizzer a a
Department of Dental Materials and Prosthodontics, Aracatuba Dental School, UNESP—Univ Estadual Paulista, Aracatuba, SP, Brazil Department of Pediatrics Dentistry, Orthodontics and Public Health, Discipline of Research Methodology and Statistics, University of São Paulo, Bauru, SP, Brazil c Renato Archer Research Center (CTI), Campinas, SP, Brazil b
art ic l e i nf o
a b s t r a c t
Article history: Accepted 16 October 2014
The study of short implants is relevant to the biomechanics of dental implants, and research on crown increase has implications for the daily clinic. The aim of this study was to analyze the biomechanical interactions of a singular implant-supported prosthesis of different crown heights under vertical and oblique force, using the 3-D finite element method. Six 3-D models were designed with Invesalius 3.0, Rhinoceros 3D 4.0, and Solidworks 2010 software. Each model was constructed with a mandibular segment of bone block, including an implant supporting a screwed metal–ceramic crown. The crown height was set at 10, 12.5, and 15 mm. The applied force was 200 N (axial) and 100 N (oblique). We performed an ANOVA statistical test and Tukey tests; po0.05 was considered statistically significant. The increase of crown height did not influence the stress distribution on screw prosthetic (p4 0.05) under axial load. However, crown heights of 12.5 and 15 mm caused statistically significant damage to the stress distribution of screws and to the cortical bone (p o0.001) under oblique load. High crown to implant (C/I) ratio harmed microstrain distribution on bone tissue under axial and oblique loads (po 0.001). Crown increase was a possible deleterious factor to the screws and to the different regions of bone tissue. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Finite element analysis Biomechanics Dental implants Analysis of variance Implant-Supported Prosthesis Dental
1. Introduction The predictability of dental implants is affected by biologic, technical, and biomechanical factors (Wennerberg and Albrektsson, 2011). The control of these factors is important to bone preservation (Lin et al., 2010a, 2010b) and prosthetic complication reduction (Goodacre et al., 2003), which could extend the implant-supported rehabilitation success rate (Manfredini et al., 2012). In this way, the use of implants to rehabilitate posterior regions of maxillae or mandibles often requires the use of short implants to short implants (Srinivasan et al., 2012) and to the fabrication of a higher prosthetic crown (Schulte et al., 2007; Tawil. et al., 2006). From a biomechanical point of view, the literature indicates that n Corresponding author at: Department of Dental Materials and Prosthodontics, UNESP—Univ Estadual Paulista, José Bonifácio St, 1193, Aracatuba 16015-050, SP, Brazil. Tel.: þ 55 1836363292; fax: þ55 1836363245. E-mail address: [email protected] (F. Ramos Verri).
http://dx.doi.org/10.1016/j.jbiomech.2014.10.021 0021-9290/& 2014 Elsevier Ltd. All rights reserved.
increase in prosthetic crown height can be a dangerous factor to the stress distribution as determined by mechanical technique analysis (Nissan et al., 2011) and by 3-D finite element analysis (Sütpideler et al., 2004). On the other hand, clinical longitudinal studies and systematic revision did not show differences in clinical performance of implants in rehabilitations with high crown height prostheses (Birdi et al., 2010; Blanes et al., 2007, 2009; Gomez-Polo et al., 2010; Schneider et al., 2012). However, Urdaneta et al. (2010), in a longitudinal study, found a higher likelihood of prosthetic complications in prostheses with high crown height. In fact, there is evidence of prosthetic complications like screw loosening and occlusal veneer fracture (Perelli et al., 2012) over short implants placed in maxillae and mandibular posterior regions, even with high treatment predictability (Esposito et al., 2012; Lops et al., 2012; Perelli et al., 2012). The use of 3-D finite element methodology to analyze biomechanical clinical situations has been documented previously (de Faria
F. Ramos Verri et al. / Journal of Biomechanics 48 (2015) 138–145
Almeida et al., 2014; Faegh and Müftü, 2010; Pellizzer et al., 2012). Furthermore, the use of statistical analysis has been considered an effective tool in the finite element method (Chou and Müftü, 2013; de Faria Almeida et al., 2014). Therefore, this study aimed to evaluate the stress distribution in single implant-supported prostheses placed over short implants and with different crown heights as the stress was transmitted to the bone tissue. Our hypothesis is that increasing prosthetic crown damages the stress distribution to the fixation screw and to the peri-implant bone tissue.
2. Materials and methods 2.1. Experimental design This research was designed to consider two factors: prosthetic screw and bone tissue. In one specific analysis of the prosthetic screw, we considered three levels of crown height (10 mm, 12.5 mm and 15 mm) and two levels of loading (axial and oblique). Therefore, a two-way analysis of variance was chosen. For bone tissue, this study considered two factors: crown height at three levels (10 mm, 12.5 mm, and 15 mm) and region at four levels (mesial, distal, buccal, and lingual). Consequently, in this case, a two-way analysis of variance was chosen for axial and oblique loading for analysis of maximum principal stress (MPa), and other two-way analyses of variance were conducted for analysis of microstrain (με) to the bone tissue in both loadings. 2.2. Models Our methodology follows that of previous studies (de Faria Almeida et al., 2014; de Moraes et al., 2013; Moraes et al., 2013Santiago Junior et al., 2013; Pellizzer et al., 2012). For this study, six 3-D models were constructed (Table 1). Each model presented a mandibular bone block from the second molar region and one implant of 4.0 8.5 mm supporting a single fixed prosthesis (Fig. 1). The bone tissue design was obtained by CT scan recomposition using InVesalius software (CTI, Campinas, São Paulo, Brazil) and line simplification using Rhinoceros 4.0 (Seattle, WA, USA)
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software. This bone tissue was modeled showing cortical and trabecular bone, presenting properties of type III bone (Lekholm and Zarb, 1985). The implant design was obtained by simplification of one connection original design (Conexão Sistemas de Prótese Ltda., Aruja, Sao Paulo, Brazil) of an internal hexagon measuring 3.75 8.5 mm, along with a prosthetic UCLA abutment. All implants and abutment designs were simplified using SolidWorks 2010 (SolidWorks Corp, Waltham, MA, USA) and Rhinoceros 4.0 software, but they maintained their similarity to the real models of the company. The crown height was modeled at 10 mm, 12.5 mm, and 15 mm (de Moraes et al., 2013; Moraes et al., 2013); the dimensions and shape of the structures associated with the crown (infrastructure, screws) were similar to the real models. All prosthetic crowns were simulated with screw fixation. All geometries were exported to the finite element software FEMAP 10.2 (Siemens PLM Software Inc., Santa Ana, CA, USA). All mechanical properties of each simulated material were attributed to the meshes using literature values (Anusavice and Hojjatie, 1987; Eraslan et al., 2005; Sertgöz, 1997; Sevimay et al., 2005). The modulus of elasticity and Poisson’s ratio for trabecular bone (Sevimay et al., 2005) and cortical bone (Sertgöz, 1997) were 1.37 GPa and 0.3 and 13.7 GPa and 0.3, respectively. Titanium was assumed with an elastic modulus of 110 GPa and Poisson’s ratio of 0.35 (Sertgöz, 1997). The materials were assumed to crown with elastic modulus and Poisson’s ratio: 82.8 GPa and 0.35 for feldspathic porcelain (Eraslan et al., 2005), 206 GPa and 0.33 for NiCr alloy (Anusavice and Hojjatie, 1987), respectively. All materials were considered isotropic, homogeneous, and linearly elastic. All contacts were simulated by symmetric weld with the exception of the abutment/implant contact, which was simulated by symmetric contact. The boundary conditions were established as fixed in all axes (x, y, and z) at both bone block sections (anterior and posterior). All other model parts were under free restrictions. The applied force was 200 N in an axial direction, with 50 N at each cusp tip, and 100 N oblique, with 50 N at each lingual cusp tip. The solid mesh convergence error shows a higher value on the cortical bone of model with crown height of 15 mm under oblique loading (0.011–1.11%) with an average value of 0.007 (0.7%) for all models. In the prosthetic screw, the higher value was obtained on the model of crown height of 10 mm under axial load (0.003–0.3%) with an average value of 0.006 (0.6%). The stress maps were plotted for qualitative analysis. Von Mises analysis was used to evaluate the stress distribution in the implant, retaining-screw, and crown; von Mises interpretation is adequate for analysis of ductile material (Barão et al., 2013). A high stress concentration should reveal the probability of failure (Spazzin et al., 2013). Maximum principal stress and μ-strain were used to evaluate the cortical bone tissue
Table 1 Pecifications of the models. Model
Load
Description
1 2 3 4 5 6
AXIAL
Dental Dental Dental Dental Dental Dental
Oblique
implant implant implant implant implant implant
(4 8.5 mm), (4 8.5 mm), (4 8.5 mm), (4 8.5 mm), (4 8.5 mm), (4 8.5 mm),
internal internal internal internal internal internal
hexagon, hexagon, hexagon, hexagon, hexagon, hexagon,
and and and and and and
crown crown crown crown crown crown
height height height height height height
of of of of of of
Fig. 1. (A) Views of the solid model illustrating the different height of the crown; (B) finite element model; and (C) zoom of the analyzed area.
10 mm 12.5 mm 15 mm 10 mm 12.5 mm 15 mm
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surrounding the dental implant. These assessments were chosen because they show local risk indicators of physiological bone failure and of the activation of bone resorption/deposition (Baggi et al., 2008; Spazzin et al., 2013). 2.3. Statistical analysis The two-way variance analysis (ANOVA two-way) was performed to verify the influence of crown increase (10, 12.5, and 15 mm) and loading direction (axial and oblique load) on von Mises stress distribution (screw). ANOVA two-way was performed to verify the influence of crown increase (10, 12.5, and 15 mm), different regions (mesial, distal, buccal, lingual) for maximum principal stress (MPa) and microstrain (με) distribution (bone tissue), under axial and oblique loads. A Tukey post-hoc test was used to analyze significant results. All statistical analyses were performed with Sigma Plot 12.3 software (Sao Jose, USA).
3. Results 3.1. Crown, implant, and screw von Mises analysis Fig. 2a illustrates the von Mises stress distribution with regard to axial loading. All models presented a similar distribution pattern with some stress concentration. Oblique loading tends to increase the stress concentration as crown height increases from 10 to 15 mm, mainly in the screw, implant, and abutment/implant interfaces (Fig. 2b and c: M4, M5, and M6; 2D: M4, M5, and M6). Fig. 2c shows detailed von Mises stress maps for retaining-screw. The high C/I ratio increased the area of stress concentration on the screws (m5 and m6), mainly in the
Fig. 2. (a) Von Mises stress distribution on implant/crown set—axial loading. (b) Von Mises stress distribution on implant/crown set—oblique loading. (c) Von Mises stress distribution on retaining-screw—axial and loading. (d) Von Mises stress distribution on dental implant—axial and loading.
F. Ramos Verri et al. / Journal of Biomechanics 48 (2015) 138–145
oblique load. The model with crown 15 mm (M6) showed a higher stress concentration in the coronal third of the implant (Fig. 2d). After observing great stress concentration on contact interfaces, we analyzed the screw distribution while increasing the crown height from 10 to 15 mm under axial and oblique loading. We conducted a statistical analysis of average stresses located at the prosthetic screws in all simulated models. ANOVA two-way analysis of these interactions is summarized in Fig. 3 and Table 2. The increase of crown height did not worsen the stress distribution on the screws under axial loading. However, there was a statistically significant difference between oblique loading and axial loading (po0.001), mainly in model C (po0.001). The highest stress concentration was observed in the crown of 15 mm under oblique loading (19.17 MPa), as shown in Fig. 3 and Table 2. The 15-mm crowns exhibited a significant difference in
Fig. 3. Average and standard deviation of von Mises stress on prosthetic screw. Two-way statistical analysis (ANOVA) and post-hoc Tukey tests revealed significant results for different letters (b, c, d) (p o 0.001) and non-significant results for similar letters (a,a) (p4 0.05).
Table 2 Values of the mean, p-value, and standard deviation of the von Mises stress to prosthetic screw of crown at 10 mm, 12.5 mm, and 15 mm and load (axial and oblique). Prosthetic screw
Load
Crown
Screw
Axial
10 mm 12.5 mm 15 mm 10 mm 12.5 mm 15 mm p:o 0.001 SS: 47.561
Oblique
*
p :o 0.001 SS: 2669.079
Mean (MPa) 1.84 1.89 1.94 14.40 16.85 19.17
SD
0.39 0.39 0.44 0.55 0.66 0.74
n Two way analysis of variance (significant difference for po 0.05). SS ¼Sum of square.
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stress concentration when compared with the 10-mm crown and 12.5-mm crown (po0.001) under oblique loading. 3.2. Bone tissue analysis (maximum principal stress and microstrain) Fig. 4a and b illustrates the compressive/tractive stresses and microstrain values observed in the peri-implant bone tissue for axial and oblique loading. The axial loading map showed a similar distribution pattern of compression and traction stress around the implant (Fig. 4a). However, under oblique loading, we observed a higher stress traction concentration area at the opposite side from the load application (Fig. 4b). Similarly, microstrain maps indicated an increase in μ-strain in peri-implant bone tissue when compared to different crowns (10 mm to 15 mm), mainly in the oblique load (Fig. 4b, m6). The values of maximum principal stress of cortical bone were measured at mesial, distal, buccal, and lingual regions of the implant in each model. Axial loading produced consistently lower values than oblique loading (Fig. 4a and Table 3). The increase of crown height increased the stress concentration area of the distal region (7.125 MPa), mainly under oblique loading (Fig. 4b). The microstrain values monitored in the cortical bone tissue are shown in Table 4. The high crown/implant ratio increased bone microstrain, and 15-crown presented the highest values of microstrain (lingual region: 1528 με) when compared with other models in oblique loading. The main results of the ANOVA test are summarized in Figs. 5 and 6 and Tables 3 and 4. Under axial loading, the increase of crown height did not show significant differences in stress distribution at the cortical bone (p ¼0.146) (Fig. 5 and Table 3). However, under oblique loading, 12.5 (b) and 15 mm (b) crowns presented significantly higher stress concentrations at bone tissue when compared to 10 mm (B) high crowns, according to a posthoc Tukey test with a value of p o0.001 (Fig. 5). The microstrain values indicated a significant difference for all models analyzed (Fig. 6 and Table 4) for both loadings. The oblique loading increased microstrain in the bone tissue (Table 4); the 10-mm crown presented the lowest values of microstrain (116.427 με) under axial load; and 15-mm crown showed the highest values of microstrain (712.52c με) under oblique load (Fig. 6).
4. Discussion The literature has shown that crown height increase is a dangerous factor in stress distribution over implant-supported prostheses (Nissan et al., 2011; de Moraes et al., 2013; Moraes et al., 2013). Moreover, Sütpideler et al. (2004) stated that a crown increase of 6 to 12 mm
Fig. 4. (a) Superior view of the cortical bone—Maximum (Max.) principal stress and microstrain—axial loading (M1 to M3). (b) Superior view of the cortical bone—Maximum (Max.) principal stress and microstrain—oblique loading (M1 to M3).
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Table 3 Values of the mean and standard deviation of the maximum principal stress to implants with 10 mm, 12.5 mm, and 15 mm crown, regions (mesial, distal, buccal, and lingual). Axial and oblique loading. Short implant
Crowns
Regions
Mean (MPa)
SD
Axial load (Crown: p ¼ 0.146) (Region: p o 0.001)
10 mm
Mesial Distal Buccal Lingual Mesial Distal Buccal Lingual Mesial Distal Buccal Lingual Mesial Distal Buccal Lingual Mesial Distal Buccal Lingual Mesial Distal Buccal Lingual
0.256 0.300 0.073 0.117 0.280 0.371 0.158 0.122 0.343 0.413 0.202 0.168 2.177 4.057 3.187 0.275 2.755 7.125 4.134 1.022 3.435 6.691 4.883 1.421
0.28 0.19 0.16 0.20 0.33 0.21 0.21 0.20 0.42 0.22 0.24 0.23 1.15 0.52 0.32 0.93 1.67 1.24 0.41 1.30 2.14 0.65 0.50 1.90
12.5 mm
15 mm
Oblique load (Crown: p o 0.001) (Region: p o 0.001)
10 mm
12.5 mm
15 mm
Table 4 Values of the mean and standard deviation of the microstrain to implants with 10 mm, 12.5 mm, and 15 mm crown, regions (mesial, distal, buccal, and lingual). Axial and oblique loading. Short implant
Crowns
Regions
Mean (MPa)
SD
Axial load (Crown: p o 0.001) (Region: p o 0.001)
10 mm
Mesial Distal Buccal Lingual Mesial Distal Buccal Lingual Mesial Distal Buccal Lingual Mesial Distal Buccal Lingual Mesial Distal Buccal Lingual Mesial Distal Buccal Lingual
125.978 94.852 101.236 143.640 128.729 106.467 131.365 158.733 148.289 112.227 147.361 181.760 404.335 397.356 178.877 602.579 451.078 563.129 232.586 1020 501.084 545.781 274.504 1528
39.73 14.815 13.42 22.32 43.00 17.18 14.59 19.05 49.18 18.55 23.78 26.37 62.71 18.72 13.57 155.07 68.75 78.76 16.13 164.19 75.20 28.54 18.32 236.55
12.5 mm
15 mm
Oblique load (Crown: p o 0.001) (Region: p o 0.001)
10 mm
12.5 mm
15 mm
increased the stress concentration in splinted implant-supported prostheses over implants of 3.75 10 mm. The results of our study are in agreement with these studies since the oblique loading induced higher stress and microstrain concentrations. The hypothesis established was accepted, increasing crown/implant ratio damage and stress distribution in the retaining-screw, implant, and peri-implant bone tissue. Therefore, a high C/I ratio should be considered a risk of biomechanical complications in dental implants. Crown to implant ratio was evaluated in systematic reviews (Quaranta et al., 2014; Garaicoa-Pazmiño et al., 2014). A systematic review indicated that the prosthesist should provide careful planning with the implant-abutment interface when rehabilitating dental
implants with unfavorable ratios C/I (Quaranta et al., 2014). Garaicoa-Pazmiño et al. (2014) stated that biomechanics and occlusal considerations are important factors; few clinical studies specified implant system used, opposing arch, and type of restoration. Our results indicated an increase in stress and strain on cortical bone under high crown implant (C/I) ratio; the implant screw was harmed with long crown, especially in oblique loading. Therefore, careful planning is needed when conducting rehabilitation with patients using short implants and higher C/I ratios. Shorter length implants can be correlated to an increase in the implant failure rate (Chrcanovic et al., 2014), so a reverse planning should be developed in the use of these implants. It is well known that
F. Ramos Verri et al. / Journal of Biomechanics 48 (2015) 138–145
Fig. 5. Average and standard error of the maximal principal stress for axial and oblique loading. A two-way ANOVA test showed that significant differences did not exist for crown increase by axial loading. Identical letters indicate that a significant result did not exist (a); post hoc Tukey tests showed significant differences between upper case/lower case letters (Bb). Equal lower-case letters indicate non-significant results (aa; bb) (p4 0.05).
Fig. 6. Average and standard error of the maximum microstrain for axial and oblique loading. A two-way ANOVA test showed that significant differences exist for crown increase by axial and oblique loading. Upper case letters indicate a significant result (AA; AB; BB).
the largest stress concentration tends to locate around the periimplant bone region (Rodríguez-Ciurana et al., 2009). Therefore, occlusal overload in bone tissue can result in implant failure (Garaicoa-Pazmiño et al., 2014), mainly in short dental implants. Our results indicated physiologically acceptable values for stress distribution and strain in bone tissue (de Faria Almeida et al., 2014, Santiago Junior et al., 2013). However, considering the posterior mandible region showing stronger action of masticatory forces, parafunctional habits such as bruxism (Hsu et al., 2012) and associated unfavorable ratios C/I may cause failure of implants. In implant-supported prosthesis biomechanics, the prosthetic screw is considered the most fragile structure even when compared to external hexagon implants (Binon, 2000), and various studies have examined the prosthetic screw (Bozkaya and Müftü, 2003; Boskaya and Müftü, 2005). Some studies focusing on internal hexagon implants reported some advantages of these implants because there is a decrease of 4.5% in the screw-loosening index of implant-supported prostheses (Krennmair et al., 2010). Although there were reports that the sharp corners of internal external hexagon implants set up stress concentration points during lateral and oblique loading that could result in wall fracture during fatigue loading (Binon, 1996), Khayat and Milliez (2007) stated that the strong internal hexagon connection allows torques of up to approximately 125 N/cm without fixture-mount fracture. In this study, the resultant high-insertion torques did not appear to result in greater bone resorption or lack of osseointegration. Thus, in our study, even without risk of fracture in implant walls, it was shown that the internal hexagon geometry tends to increase the stress concentration on the prosthetic screw and associated structures around it that could lead to biomechanical complications, particularly in short implants and higher crowns.
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Finite element studies analyzed the effect of increasing the crown from 10 to 15 mm in external hexagon implants. de Moraes et al. (2013) suggested that the increase in crown height enhanced stress concentration at the implant/bone tissue and increased displacement in the bone tissue, mainly under oblique loading. In another study with the objective of measuring the stress distribution in implant prosthetic screws with different heights of the clinical crown, it was concluded that the increase of the crown was damaging to the stress distribution on the screw, mainly in oblique loading (Moraes et al., 2013). In fact, external hexagon implants have a higher concentration of stresses in the cortical bone prosthesis screw, abutment/implant interface (de Faria Almeida et al., 2014). The internal connection geometry used in this study allowed the reduction of the magnitude of stresses and microstrain in the cortical bone, compared with the results already published regarding external hexagon implants (de Moraes et al., 2013; Moraes et al., 2013). The lower magnitude of stresses located in the cortical bone of the internal hexagon implants is due to the higher stability of the system, anti-rotational mechanism, distribution of lateral load along the axis of the implant, and long internal wall engagements, which create stiff and resistant association to chewing forces (Binon 2000). The results showed that oblique loading was dangerous for stress distribution; this data agrees with earlier studies that reported higher stress concentrations under oblique loading (Tawil. et al., 2006; Pellizzer et al., 2012; Lin et al., 2010a). The peri-implant bone tissue presented an increase in average stress as the crown height increased, becoming statistically significant for maximum principal stress and microstrain. However, the obtained stress and strain values are acceptable as physiologic values in the literature (Baggi et al., 2008; Papavasiliou et al., 1996; Frost, 2003). The results indicated that maximum principal stress values in the peri-implant bone ranged from a minimum of 0.413 MPa to a maximum of 0.073 MPa in the axial load; and a minimum of 0.275 MPa and maximum of 7.125 MPa in the oblique load (Table 3). Therefore, for all the models analyzed, areas of compressive stress and tensile were located around the peri-implant region, and the stress values were within physiological limits described in the literature because the ultimate strength of human cortical bone ranges from 72 to 76 MPa in tension and from 140 to 170 MPa in compression (Papavasiliou et al., 1996). The level of strain on the cortical bone was also relevant to the analysis. Masticatory forces induce strains in the bone, causing stimule modeling and remodeling due to the magnitude of stress, strain, and frequency (dos Santos et al., 2014; Frost, 2003). According to Frost’s Mechanostatic Theory, the acceptable limits of microstrains should not exceed 3000 με (Frost, 2003), and our results indicated microstrain values in the periimplant bone ranging from 94.85 to 181.760 με in the axial load and 178.87 to 1528 με (Table 4). This data is within the permitted biological limits (dos Santos et al., 2014). However, an extrapolation of these findings must be carefully evaluated as the biological response that causes such resorption and remodeling is not completely determined (Spazzin et al., 2013). Different methods (dos Santos et al., 2014) and clinical studies should be realized to validate this data. The results are in accordance with the applied load level. The implant survival rate for higher crowns was recently analyzed. Studies testing survival rates for 1.3:1 (Schulte et al., 2007), 0.8:1 to 3.0:1 (Rokni et al., 2005), and 2:1 and 3:1 (Blanes et al., 2007) implant-to-crown ratios found similar results, with bone resorption at normal levels. However, while Urdaneta et al. (2010) reported an increase in crown height, there was a significant effect on screw loosening in the prosthetic abutments (po0.0001) and a significant correlation with 2 mm abutment fractures in the posterior region of the mouth (p¼ 0.03). The finite element method allows a biomechanical analysis providing information about stresses and microstrains on bone tissue.
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This data allows a critical analysis of the clinical situation. Computer simulation is appropriate for the study of internal structures, which could not be presented in other biomechanical tests (Moraes et al., 2013). This methodology provides evidence of the more favorable clinical option for the longevity of treatment with dental implants. The 3-D finite element method is considered an effective tool for biomechanical analysis (Chou and Müftü, 2013; de Faria Almeida et al., 2014; Faegh and Müftü, 2010; Pellizzer et al., 2012; Kong et al. (2008)). However, there are some limitations due to material properties attributed (de Faria Almeida et al., 2014; Barão et al., 2013). These limitations are related to the environment of a computer simulation; the material properties were assumed to be linear, isotropic, and homogeneous and are subjected to a static occlusal loading. But bone modeling with anisotropic properties is more upscale and problematic and consumes more time for development (Barão et al., 2013). However, the implants showed a small margin of simplification, approaching the real clinical situation, as in previous studies (de Faria Almeida et al., 2014; Santiago Junior et al., 2013; de Moraes et al., 2013; Moraes et al., 2013). Therefore, this data allows for trend analysis, which, when combined with well-designed clinical studies, will serve as a guideline for implant dentistry. The analysis would recommend a careful occlusal adjustment for control of masticatory forces in clinical cases involving short implants and higher crowns through a palliative control method. Finally, controlled clinical studies should be conducted in order to verify these biomechanical effects.
5. Conclusions The increase of crown height significantly increased the stress concentration on the prosthetic screw and bone tissue. The increase of crown height significantly increased the level of microstrain of bone tissue under axial and oblique loading. The oblique loading was more dangerous to structures analyzed.
Conflict of interest statement All authors declare no financial and personal conflict of interest in this study.
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Garaicoa-Pazmiño, C., Suarez, F., Monje, A., Catena, A., Ortega-Oller, I., Galindo-Moreno, P., Wang, H.L., 2014. Influence of crown-implant ratio upon marginal bone loss. A systematic review (Jan 20. [Epub ahead of print]). J. Periodontol. Gomez-Polo, M., Bartens, F., Sala, L., Tamini, F., Celemin, A., Del Rio, J., 2010. The correlation between crown-implant ratios and marginal bone resorption: a preliminary clinical study. Int. J. Prosthod. 23, 33–37. Goodacre, C.J., Bernal, G., Rungcharassaeng, K., Kan, J.Y.K., 2003. Clinical complications with implants and implant prostheses. J. Prosthet. Dent. 90, 121–132. Hsu, Y.T., Fu, J.H., Al-Hezaimi, K., Wang, H.L., 2012. Biomechanical implant treatment complications: a systematic review of clinical studies of implants with at least 1 year of functional loading. Int. J. Oral Maxillofac. Implants 27, 894–904. Khayat, P.G., Milliez, S.N., 2007. Prospective clinical evaluation of 835 multithreaded tapered screw-vent implants: results after two years of functional loading. J. Oral Implantol. 33, 225–231. Kong, L., Sun, Y., Hu, K., Liu, Y., Li, D., Qiu, Z., Liu, B, 2008. Selections of the cylinder implant neck taper and implant end fillet for optimal biomechanical properties: a three-dimensional finite element analysis. J. Biomech. 41, 1124–1130. Krennmair, G., Seemann, R., Schmidinger, S., Ewers, R., Piehslinger, E., 2010. Clinical outcome of root-shaped dental implants of various diameters: 5-year results. Int. J. Oral Maxillofac. Implants 25, 357–366. Lekholm, U., Zarb, G.A., 1985. Patient selection and preparation. In: Brånemark, P.I., Zarb, G.A., Albrektsson, T. (Eds.), Tissue-integrated Prostheses. Osseointegration in Clinical Dentistry, Quintessence, Chicago, pp. 199–209. Lin, C.L., Lin, Y.H., Chang, S.H., 2010a. Multi-factorial analysis of variables influencing the bone loss of an implant placed in the maxilla: prediction using FEA and SED bone remodeling algorithm. J. Biomech. 43, 644–651. Lin, D., Li, Q., Li, W., Duckmanton, N., Swain, M., 2010b. Mandibular bone remodeling induced by dental implant. J. Biomech. 43, 287–293. Lops, D., Bressan, E., Pisoni, G., Cea, N., Corazza, B., Romeo, E., 2012. Short implants in partially edentuolous maxillae and mandibles: a 10 to 20 years retrospective evaluation. Int. J. Dent. , http://dx.doi.org/10.1155/2012/351793. Manfredini, D., Poggio, C.E., Lobbezoo, F., 2012. Is bruxism a risk factor for dental implants? A systematic review of the literature. Clin. Implant Dent. Relat. Res. , http://dx.doi.org/10.1111/cid.12015. Moraes, S.L., Pellizzer, E.P., Verri, F.R., Santiago Jr., J.F., Silva, J.V., 2013. Threedimensional finite element analysis of stress distribution in retention screws of different crown-implant ratios. Comput. Methods Biomech. Biomed. Eng. 2013 (Aug), 15 ([Epub ahead of print]). Nissan, J., Ghelfan, O., Gross, O., Priel, I., Gross, M., Chaushu, G., 2011. 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. Papavasiliou, G., Kamposiora, P., Bayne, S.C., Felton, D.A., 1996. Three-dimensional finite element analysis of stress-distribution around single tooth implants as a
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Contents lists available at ScienceDirect
Journal of Biomechanics journal homepage: www.elsevier.com/locate/jbiomech www.JBiomech.com
Biomechanical influence of crown-to-implant ratio on stress distribution over internal hexagon short implant: 3-D finite element analysis with statistical test Fellippo Ramos Verri a,n, Joel Ferreira Santiago Junior a, Daniel Augusto de Faria Almeida a, Guilherme Bérgamo Brandão de Oliveira a, Victor Eduardo de Souza Batista a, Heitor Marques Honório b, Pedro Yoshito Noritomi c, Eduardo Piza Pellizzer a a
Department of Dental Materials and Prosthodontics, Aracatuba Dental School, UNESP—Univ Estadual Paulista, Aracatuba, SP, Brazil Department of Pediatrics Dentistry, Orthodontics and Public Health, Discipline of Research Methodology and Statistics, University of São Paulo, Bauru, SP, Brazil c Renato Archer Research Center (CTI), Campinas, SP, Brazil b
art ic l e i nf o
a b s t r a c t
Article history: Accepted 16 October 2014
The study of short implants is relevant to the biomechanics of dental implants, and research on crown increase has implications for the daily clinic. The aim of this study was to analyze the biomechanical interactions of a singular implant-supported prosthesis of different crown heights under vertical and oblique force, using the 3-D finite element method. Six 3-D models were designed with Invesalius 3.0, Rhinoceros 3D 4.0, and Solidworks 2010 software. Each model was constructed with a mandibular segment of bone block, including an implant supporting a screwed metal–ceramic crown. The crown height was set at 10, 12.5, and 15 mm. The applied force was 200 N (axial) and 100 N (oblique). We performed an ANOVA statistical test and Tukey tests; po0.05 was considered statistically significant. The increase of crown height did not influence the stress distribution on screw prosthetic (p4 0.05) under axial load. However, crown heights of 12.5 and 15 mm caused statistically significant damage to the stress distribution of screws and to the cortical bone (p o0.001) under oblique load. High crown to implant (C/I) ratio harmed microstrain distribution on bone tissue under axial and oblique loads (po 0.001). Crown increase was a possible deleterious factor to the screws and to the different regions of bone tissue. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Finite element analysis Biomechanics Dental implants Analysis of variance Implant-Supported Prosthesis Dental
1. Introduction The predictability of dental implants is affected by biologic, technical, and biomechanical factors (Wennerberg and Albrektsson, 2011). The control of these factors is important to bone preservation (Lin et al., 2010a, 2010b) and prosthetic complication reduction (Goodacre et al., 2003), which could extend the implant-supported rehabilitation success rate (Manfredini et al., 2012). In this way, the use of implants to rehabilitate posterior regions of maxillae or mandibles often requires the use of short implants to short implants (Srinivasan et al., 2012) and to the fabrication of a higher prosthetic crown (Schulte et al., 2007; Tawil. et al., 2006). From a biomechanical point of view, the literature indicates that n Corresponding author at: Department of Dental Materials and Prosthodontics, UNESP—Univ Estadual Paulista, José Bonifácio St, 1193, Aracatuba 16015-050, SP, Brazil. Tel.: þ 55 1836363292; fax: þ55 1836363245. E-mail address: [email protected] (F. Ramos Verri).
http://dx.doi.org/10.1016/j.jbiomech.2014.10.021 0021-9290/& 2014 Elsevier Ltd. All rights reserved.
increase in prosthetic crown height can be a dangerous factor to the stress distribution as determined by mechanical technique analysis (Nissan et al., 2011) and by 3-D finite element analysis (Sütpideler et al., 2004). On the other hand, clinical longitudinal studies and systematic revision did not show differences in clinical performance of implants in rehabilitations with high crown height prostheses (Birdi et al., 2010; Blanes et al., 2007, 2009; Gomez-Polo et al., 2010; Schneider et al., 2012). However, Urdaneta et al. (2010), in a longitudinal study, found a higher likelihood of prosthetic complications in prostheses with high crown height. In fact, there is evidence of prosthetic complications like screw loosening and occlusal veneer fracture (Perelli et al., 2012) over short implants placed in maxillae and mandibular posterior regions, even with high treatment predictability (Esposito et al., 2012; Lops et al., 2012; Perelli et al., 2012). The use of 3-D finite element methodology to analyze biomechanical clinical situations has been documented previously (de Faria
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Almeida et al., 2014; Faegh and Müftü, 2010; Pellizzer et al., 2012). Furthermore, the use of statistical analysis has been considered an effective tool in the finite element method (Chou and Müftü, 2013; de Faria Almeida et al., 2014). Therefore, this study aimed to evaluate the stress distribution in single implant-supported prostheses placed over short implants and with different crown heights as the stress was transmitted to the bone tissue. Our hypothesis is that increasing prosthetic crown damages the stress distribution to the fixation screw and to the peri-implant bone tissue.
2. Materials and methods 2.1. Experimental design This research was designed to consider two factors: prosthetic screw and bone tissue. In one specific analysis of the prosthetic screw, we considered three levels of crown height (10 mm, 12.5 mm and 15 mm) and two levels of loading (axial and oblique). Therefore, a two-way analysis of variance was chosen. For bone tissue, this study considered two factors: crown height at three levels (10 mm, 12.5 mm, and 15 mm) and region at four levels (mesial, distal, buccal, and lingual). Consequently, in this case, a two-way analysis of variance was chosen for axial and oblique loading for analysis of maximum principal stress (MPa), and other two-way analyses of variance were conducted for analysis of microstrain (με) to the bone tissue in both loadings. 2.2. Models Our methodology follows that of previous studies (de Faria Almeida et al., 2014; de Moraes et al., 2013; Moraes et al., 2013Santiago Junior et al., 2013; Pellizzer et al., 2012). For this study, six 3-D models were constructed (Table 1). Each model presented a mandibular bone block from the second molar region and one implant of 4.0 8.5 mm supporting a single fixed prosthesis (Fig. 1). The bone tissue design was obtained by CT scan recomposition using InVesalius software (CTI, Campinas, São Paulo, Brazil) and line simplification using Rhinoceros 4.0 (Seattle, WA, USA)
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software. This bone tissue was modeled showing cortical and trabecular bone, presenting properties of type III bone (Lekholm and Zarb, 1985). The implant design was obtained by simplification of one connection original design (Conexão Sistemas de Prótese Ltda., Aruja, Sao Paulo, Brazil) of an internal hexagon measuring 3.75 8.5 mm, along with a prosthetic UCLA abutment. All implants and abutment designs were simplified using SolidWorks 2010 (SolidWorks Corp, Waltham, MA, USA) and Rhinoceros 4.0 software, but they maintained their similarity to the real models of the company. The crown height was modeled at 10 mm, 12.5 mm, and 15 mm (de Moraes et al., 2013; Moraes et al., 2013); the dimensions and shape of the structures associated with the crown (infrastructure, screws) were similar to the real models. All prosthetic crowns were simulated with screw fixation. All geometries were exported to the finite element software FEMAP 10.2 (Siemens PLM Software Inc., Santa Ana, CA, USA). All mechanical properties of each simulated material were attributed to the meshes using literature values (Anusavice and Hojjatie, 1987; Eraslan et al., 2005; Sertgöz, 1997; Sevimay et al., 2005). The modulus of elasticity and Poisson’s ratio for trabecular bone (Sevimay et al., 2005) and cortical bone (Sertgöz, 1997) were 1.37 GPa and 0.3 and 13.7 GPa and 0.3, respectively. Titanium was assumed with an elastic modulus of 110 GPa and Poisson’s ratio of 0.35 (Sertgöz, 1997). The materials were assumed to crown with elastic modulus and Poisson’s ratio: 82.8 GPa and 0.35 for feldspathic porcelain (Eraslan et al., 2005), 206 GPa and 0.33 for NiCr alloy (Anusavice and Hojjatie, 1987), respectively. All materials were considered isotropic, homogeneous, and linearly elastic. All contacts were simulated by symmetric weld with the exception of the abutment/implant contact, which was simulated by symmetric contact. The boundary conditions were established as fixed in all axes (x, y, and z) at both bone block sections (anterior and posterior). All other model parts were under free restrictions. The applied force was 200 N in an axial direction, with 50 N at each cusp tip, and 100 N oblique, with 50 N at each lingual cusp tip. The solid mesh convergence error shows a higher value on the cortical bone of model with crown height of 15 mm under oblique loading (0.011–1.11%) with an average value of 0.007 (0.7%) for all models. In the prosthetic screw, the higher value was obtained on the model of crown height of 10 mm under axial load (0.003–0.3%) with an average value of 0.006 (0.6%). The stress maps were plotted for qualitative analysis. Von Mises analysis was used to evaluate the stress distribution in the implant, retaining-screw, and crown; von Mises interpretation is adequate for analysis of ductile material (Barão et al., 2013). A high stress concentration should reveal the probability of failure (Spazzin et al., 2013). Maximum principal stress and μ-strain were used to evaluate the cortical bone tissue
Table 1 Pecifications of the models. Model
Load
Description
1 2 3 4 5 6
AXIAL
Dental Dental Dental Dental Dental Dental
Oblique
implant implant implant implant implant implant
(4 8.5 mm), (4 8.5 mm), (4 8.5 mm), (4 8.5 mm), (4 8.5 mm), (4 8.5 mm),
internal internal internal internal internal internal
hexagon, hexagon, hexagon, hexagon, hexagon, hexagon,
and and and and and and
crown crown crown crown crown crown
height height height height height height
of of of of of of
Fig. 1. (A) Views of the solid model illustrating the different height of the crown; (B) finite element model; and (C) zoom of the analyzed area.
10 mm 12.5 mm 15 mm 10 mm 12.5 mm 15 mm
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surrounding the dental implant. These assessments were chosen because they show local risk indicators of physiological bone failure and of the activation of bone resorption/deposition (Baggi et al., 2008; Spazzin et al., 2013). 2.3. Statistical analysis The two-way variance analysis (ANOVA two-way) was performed to verify the influence of crown increase (10, 12.5, and 15 mm) and loading direction (axial and oblique load) on von Mises stress distribution (screw). ANOVA two-way was performed to verify the influence of crown increase (10, 12.5, and 15 mm), different regions (mesial, distal, buccal, lingual) for maximum principal stress (MPa) and microstrain (με) distribution (bone tissue), under axial and oblique loads. A Tukey post-hoc test was used to analyze significant results. All statistical analyses were performed with Sigma Plot 12.3 software (Sao Jose, USA).
3. Results 3.1. Crown, implant, and screw von Mises analysis Fig. 2a illustrates the von Mises stress distribution with regard to axial loading. All models presented a similar distribution pattern with some stress concentration. Oblique loading tends to increase the stress concentration as crown height increases from 10 to 15 mm, mainly in the screw, implant, and abutment/implant interfaces (Fig. 2b and c: M4, M5, and M6; 2D: M4, M5, and M6). Fig. 2c shows detailed von Mises stress maps for retaining-screw. The high C/I ratio increased the area of stress concentration on the screws (m5 and m6), mainly in the
Fig. 2. (a) Von Mises stress distribution on implant/crown set—axial loading. (b) Von Mises stress distribution on implant/crown set—oblique loading. (c) Von Mises stress distribution on retaining-screw—axial and loading. (d) Von Mises stress distribution on dental implant—axial and loading.
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oblique load. The model with crown 15 mm (M6) showed a higher stress concentration in the coronal third of the implant (Fig. 2d). After observing great stress concentration on contact interfaces, we analyzed the screw distribution while increasing the crown height from 10 to 15 mm under axial and oblique loading. We conducted a statistical analysis of average stresses located at the prosthetic screws in all simulated models. ANOVA two-way analysis of these interactions is summarized in Fig. 3 and Table 2. The increase of crown height did not worsen the stress distribution on the screws under axial loading. However, there was a statistically significant difference between oblique loading and axial loading (po0.001), mainly in model C (po0.001). The highest stress concentration was observed in the crown of 15 mm under oblique loading (19.17 MPa), as shown in Fig. 3 and Table 2. The 15-mm crowns exhibited a significant difference in
Fig. 3. Average and standard deviation of von Mises stress on prosthetic screw. Two-way statistical analysis (ANOVA) and post-hoc Tukey tests revealed significant results for different letters (b, c, d) (p o 0.001) and non-significant results for similar letters (a,a) (p4 0.05).
Table 2 Values of the mean, p-value, and standard deviation of the von Mises stress to prosthetic screw of crown at 10 mm, 12.5 mm, and 15 mm and load (axial and oblique). Prosthetic screw
Load
Crown
Screw
Axial
10 mm 12.5 mm 15 mm 10 mm 12.5 mm 15 mm p:o 0.001 SS: 47.561
Oblique
*
p :o 0.001 SS: 2669.079
Mean (MPa) 1.84 1.89 1.94 14.40 16.85 19.17
SD
0.39 0.39 0.44 0.55 0.66 0.74
n Two way analysis of variance (significant difference for po 0.05). SS ¼Sum of square.
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stress concentration when compared with the 10-mm crown and 12.5-mm crown (po0.001) under oblique loading. 3.2. Bone tissue analysis (maximum principal stress and microstrain) Fig. 4a and b illustrates the compressive/tractive stresses and microstrain values observed in the peri-implant bone tissue for axial and oblique loading. The axial loading map showed a similar distribution pattern of compression and traction stress around the implant (Fig. 4a). However, under oblique loading, we observed a higher stress traction concentration area at the opposite side from the load application (Fig. 4b). Similarly, microstrain maps indicated an increase in μ-strain in peri-implant bone tissue when compared to different crowns (10 mm to 15 mm), mainly in the oblique load (Fig. 4b, m6). The values of maximum principal stress of cortical bone were measured at mesial, distal, buccal, and lingual regions of the implant in each model. Axial loading produced consistently lower values than oblique loading (Fig. 4a and Table 3). The increase of crown height increased the stress concentration area of the distal region (7.125 MPa), mainly under oblique loading (Fig. 4b). The microstrain values monitored in the cortical bone tissue are shown in Table 4. The high crown/implant ratio increased bone microstrain, and 15-crown presented the highest values of microstrain (lingual region: 1528 με) when compared with other models in oblique loading. The main results of the ANOVA test are summarized in Figs. 5 and 6 and Tables 3 and 4. Under axial loading, the increase of crown height did not show significant differences in stress distribution at the cortical bone (p ¼0.146) (Fig. 5 and Table 3). However, under oblique loading, 12.5 (b) and 15 mm (b) crowns presented significantly higher stress concentrations at bone tissue when compared to 10 mm (B) high crowns, according to a posthoc Tukey test with a value of p o0.001 (Fig. 5). The microstrain values indicated a significant difference for all models analyzed (Fig. 6 and Table 4) for both loadings. The oblique loading increased microstrain in the bone tissue (Table 4); the 10-mm crown presented the lowest values of microstrain (116.427 με) under axial load; and 15-mm crown showed the highest values of microstrain (712.52c με) under oblique load (Fig. 6).
4. Discussion The literature has shown that crown height increase is a dangerous factor in stress distribution over implant-supported prostheses (Nissan et al., 2011; de Moraes et al., 2013; Moraes et al., 2013). Moreover, Sütpideler et al. (2004) stated that a crown increase of 6 to 12 mm
Fig. 4. (a) Superior view of the cortical bone—Maximum (Max.) principal stress and microstrain—axial loading (M1 to M3). (b) Superior view of the cortical bone—Maximum (Max.) principal stress and microstrain—oblique loading (M1 to M3).
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Table 3 Values of the mean and standard deviation of the maximum principal stress to implants with 10 mm, 12.5 mm, and 15 mm crown, regions (mesial, distal, buccal, and lingual). Axial and oblique loading. Short implant
Crowns
Regions
Mean (MPa)
SD
Axial load (Crown: p ¼ 0.146) (Region: p o 0.001)
10 mm
Mesial Distal Buccal Lingual Mesial Distal Buccal Lingual Mesial Distal Buccal Lingual Mesial Distal Buccal Lingual Mesial Distal Buccal Lingual Mesial Distal Buccal Lingual
0.256 0.300 0.073 0.117 0.280 0.371 0.158 0.122 0.343 0.413 0.202 0.168 2.177 4.057 3.187 0.275 2.755 7.125 4.134 1.022 3.435 6.691 4.883 1.421
0.28 0.19 0.16 0.20 0.33 0.21 0.21 0.20 0.42 0.22 0.24 0.23 1.15 0.52 0.32 0.93 1.67 1.24 0.41 1.30 2.14 0.65 0.50 1.90
12.5 mm
15 mm
Oblique load (Crown: p o 0.001) (Region: p o 0.001)
10 mm
12.5 mm
15 mm
Table 4 Values of the mean and standard deviation of the microstrain to implants with 10 mm, 12.5 mm, and 15 mm crown, regions (mesial, distal, buccal, and lingual). Axial and oblique loading. Short implant
Crowns
Regions
Mean (MPa)
SD
Axial load (Crown: p o 0.001) (Region: p o 0.001)
10 mm
Mesial Distal Buccal Lingual Mesial Distal Buccal Lingual Mesial Distal Buccal Lingual Mesial Distal Buccal Lingual Mesial Distal Buccal Lingual Mesial Distal Buccal Lingual
125.978 94.852 101.236 143.640 128.729 106.467 131.365 158.733 148.289 112.227 147.361 181.760 404.335 397.356 178.877 602.579 451.078 563.129 232.586 1020 501.084 545.781 274.504 1528
39.73 14.815 13.42 22.32 43.00 17.18 14.59 19.05 49.18 18.55 23.78 26.37 62.71 18.72 13.57 155.07 68.75 78.76 16.13 164.19 75.20 28.54 18.32 236.55
12.5 mm
15 mm
Oblique load (Crown: p o 0.001) (Region: p o 0.001)
10 mm
12.5 mm
15 mm
increased the stress concentration in splinted implant-supported prostheses over implants of 3.75 10 mm. The results of our study are in agreement with these studies since the oblique loading induced higher stress and microstrain concentrations. The hypothesis established was accepted, increasing crown/implant ratio damage and stress distribution in the retaining-screw, implant, and peri-implant bone tissue. Therefore, a high C/I ratio should be considered a risk of biomechanical complications in dental implants. Crown to implant ratio was evaluated in systematic reviews (Quaranta et al., 2014; Garaicoa-Pazmiño et al., 2014). A systematic review indicated that the prosthesist should provide careful planning with the implant-abutment interface when rehabilitating dental
implants with unfavorable ratios C/I (Quaranta et al., 2014). Garaicoa-Pazmiño et al. (2014) stated that biomechanics and occlusal considerations are important factors; few clinical studies specified implant system used, opposing arch, and type of restoration. Our results indicated an increase in stress and strain on cortical bone under high crown implant (C/I) ratio; the implant screw was harmed with long crown, especially in oblique loading. Therefore, careful planning is needed when conducting rehabilitation with patients using short implants and higher C/I ratios. Shorter length implants can be correlated to an increase in the implant failure rate (Chrcanovic et al., 2014), so a reverse planning should be developed in the use of these implants. It is well known that
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Fig. 5. Average and standard error of the maximal principal stress for axial and oblique loading. A two-way ANOVA test showed that significant differences did not exist for crown increase by axial loading. Identical letters indicate that a significant result did not exist (a); post hoc Tukey tests showed significant differences between upper case/lower case letters (Bb). Equal lower-case letters indicate non-significant results (aa; bb) (p4 0.05).
Fig. 6. Average and standard error of the maximum microstrain for axial and oblique loading. A two-way ANOVA test showed that significant differences exist for crown increase by axial and oblique loading. Upper case letters indicate a significant result (AA; AB; BB).
the largest stress concentration tends to locate around the periimplant bone region (Rodríguez-Ciurana et al., 2009). Therefore, occlusal overload in bone tissue can result in implant failure (Garaicoa-Pazmiño et al., 2014), mainly in short dental implants. Our results indicated physiologically acceptable values for stress distribution and strain in bone tissue (de Faria Almeida et al., 2014, Santiago Junior et al., 2013). However, considering the posterior mandible region showing stronger action of masticatory forces, parafunctional habits such as bruxism (Hsu et al., 2012) and associated unfavorable ratios C/I may cause failure of implants. In implant-supported prosthesis biomechanics, the prosthetic screw is considered the most fragile structure even when compared to external hexagon implants (Binon, 2000), and various studies have examined the prosthetic screw (Bozkaya and Müftü, 2003; Boskaya and Müftü, 2005). Some studies focusing on internal hexagon implants reported some advantages of these implants because there is a decrease of 4.5% in the screw-loosening index of implant-supported prostheses (Krennmair et al., 2010). Although there were reports that the sharp corners of internal external hexagon implants set up stress concentration points during lateral and oblique loading that could result in wall fracture during fatigue loading (Binon, 1996), Khayat and Milliez (2007) stated that the strong internal hexagon connection allows torques of up to approximately 125 N/cm without fixture-mount fracture. In this study, the resultant high-insertion torques did not appear to result in greater bone resorption or lack of osseointegration. Thus, in our study, even without risk of fracture in implant walls, it was shown that the internal hexagon geometry tends to increase the stress concentration on the prosthetic screw and associated structures around it that could lead to biomechanical complications, particularly in short implants and higher crowns.
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Finite element studies analyzed the effect of increasing the crown from 10 to 15 mm in external hexagon implants. de Moraes et al. (2013) suggested that the increase in crown height enhanced stress concentration at the implant/bone tissue and increased displacement in the bone tissue, mainly under oblique loading. In another study with the objective of measuring the stress distribution in implant prosthetic screws with different heights of the clinical crown, it was concluded that the increase of the crown was damaging to the stress distribution on the screw, mainly in oblique loading (Moraes et al., 2013). In fact, external hexagon implants have a higher concentration of stresses in the cortical bone prosthesis screw, abutment/implant interface (de Faria Almeida et al., 2014). The internal connection geometry used in this study allowed the reduction of the magnitude of stresses and microstrain in the cortical bone, compared with the results already published regarding external hexagon implants (de Moraes et al., 2013; Moraes et al., 2013). The lower magnitude of stresses located in the cortical bone of the internal hexagon implants is due to the higher stability of the system, anti-rotational mechanism, distribution of lateral load along the axis of the implant, and long internal wall engagements, which create stiff and resistant association to chewing forces (Binon 2000). The results showed that oblique loading was dangerous for stress distribution; this data agrees with earlier studies that reported higher stress concentrations under oblique loading (Tawil. et al., 2006; Pellizzer et al., 2012; Lin et al., 2010a). The peri-implant bone tissue presented an increase in average stress as the crown height increased, becoming statistically significant for maximum principal stress and microstrain. However, the obtained stress and strain values are acceptable as physiologic values in the literature (Baggi et al., 2008; Papavasiliou et al., 1996; Frost, 2003). The results indicated that maximum principal stress values in the peri-implant bone ranged from a minimum of 0.413 MPa to a maximum of 0.073 MPa in the axial load; and a minimum of 0.275 MPa and maximum of 7.125 MPa in the oblique load (Table 3). Therefore, for all the models analyzed, areas of compressive stress and tensile were located around the peri-implant region, and the stress values were within physiological limits described in the literature because the ultimate strength of human cortical bone ranges from 72 to 76 MPa in tension and from 140 to 170 MPa in compression (Papavasiliou et al., 1996). The level of strain on the cortical bone was also relevant to the analysis. Masticatory forces induce strains in the bone, causing stimule modeling and remodeling due to the magnitude of stress, strain, and frequency (dos Santos et al., 2014; Frost, 2003). According to Frost’s Mechanostatic Theory, the acceptable limits of microstrains should not exceed 3000 με (Frost, 2003), and our results indicated microstrain values in the periimplant bone ranging from 94.85 to 181.760 με in the axial load and 178.87 to 1528 με (Table 4). This data is within the permitted biological limits (dos Santos et al., 2014). However, an extrapolation of these findings must be carefully evaluated as the biological response that causes such resorption and remodeling is not completely determined (Spazzin et al., 2013). Different methods (dos Santos et al., 2014) and clinical studies should be realized to validate this data. The results are in accordance with the applied load level. The implant survival rate for higher crowns was recently analyzed. Studies testing survival rates for 1.3:1 (Schulte et al., 2007), 0.8:1 to 3.0:1 (Rokni et al., 2005), and 2:1 and 3:1 (Blanes et al., 2007) implant-to-crown ratios found similar results, with bone resorption at normal levels. However, while Urdaneta et al. (2010) reported an increase in crown height, there was a significant effect on screw loosening in the prosthetic abutments (po0.0001) and a significant correlation with 2 mm abutment fractures in the posterior region of the mouth (p¼ 0.03). The finite element method allows a biomechanical analysis providing information about stresses and microstrains on bone tissue.
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This data allows a critical analysis of the clinical situation. Computer simulation is appropriate for the study of internal structures, which could not be presented in other biomechanical tests (Moraes et al., 2013). This methodology provides evidence of the more favorable clinical option for the longevity of treatment with dental implants. The 3-D finite element method is considered an effective tool for biomechanical analysis (Chou and Müftü, 2013; de Faria Almeida et al., 2014; Faegh and Müftü, 2010; Pellizzer et al., 2012; Kong et al. (2008)). However, there are some limitations due to material properties attributed (de Faria Almeida et al., 2014; Barão et al., 2013). These limitations are related to the environment of a computer simulation; the material properties were assumed to be linear, isotropic, and homogeneous and are subjected to a static occlusal loading. But bone modeling with anisotropic properties is more upscale and problematic and consumes more time for development (Barão et al., 2013). However, the implants showed a small margin of simplification, approaching the real clinical situation, as in previous studies (de Faria Almeida et al., 2014; Santiago Junior et al., 2013; de Moraes et al., 2013; Moraes et al., 2013). Therefore, this data allows for trend analysis, which, when combined with well-designed clinical studies, will serve as a guideline for implant dentistry. The analysis would recommend a careful occlusal adjustment for control of masticatory forces in clinical cases involving short implants and higher crowns through a palliative control method. Finally, controlled clinical studies should be conducted in order to verify these biomechanical effects.
5. Conclusions The increase of crown height significantly increased the stress concentration on the prosthetic screw and bone tissue. The increase of crown height significantly increased the level of microstrain of bone tissue under axial and oblique loading. The oblique loading was more dangerous to structures analyzed.
Conflict of interest statement All authors declare no financial and personal conflict of interest in this study.
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