Rehabilitation of the Atrophic Mandible with Short Implants in Different Positions: a Finite Elements Study

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Rehabilitation of the atrophic mandible with short implants in different positions: A finite elements study

Article · April 2017 DOI: 10.1016/j.msec.2017.03.310

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Rehabilitation of the atrophic mandible with short implants in different positions: A ﬁnite elements study

Hugo E. Peixoto a, Paulo R. Camati b, Fernanda Faot c, Bruno S. Sotto-Maior d,f, Elizabeth F. Martinez e,DaianeC.Peruzzoe,⁎ a São Leopoldo Mandic Dental Institute and Research Center, Campinas, SP, Brazil b Curitiba, PR, Brazil c Federal University of Pelotas, Department of Restorative Dentistry, Pelotas, RS, Brazil d Federal University of Juiz de Fora, Department of Restorative Dentistry, Juiz de Fora, MG, Brazil e São Leopoldo Mandic Dental Institute and Research Center, Campinas, SP, Brazil f Brazil and São Leopoldo Mandic Dental Institute and Research Center, Campinas, SP, Brazil article info abstract

Article history: Objective: The aim of this study was to analyze whether the use of inclined short implants without lower Received 19 September 2016 transcortical involvement (test model - SI), thus preserving the mandibular lower cortical bone, could optimize Received in revised form 7 March 2017 stress distribution. Accepted 12 March 2017 Materials and methods: Six identical atrophic mandible models were created featuring 8 mm of height at the sym- Available online 07 April 2017 physis. Two study factors were evaluated: implant length and angulation. Implant length was represented either by short implants (7 mm) with preservation of the mandibular lower cortical bone or standard implants (9 mm) Keywords: with a bicortical approach and 3 possible implant positioning conﬁgurations: 4 distally-inclined implants at 45° Atrophic mandible Dental implants (experimental model), all-on-four, 4 vertical implants. All tridimensional (3D) models were analyzed using the Finite element analysis Finite Element Method (FEM) and the Ansys Workbench software. Results: The maximum stress on the bone at the cervical region of the implants in the experimental model was 132 MPa and transcortical involvement with implant inclination yielded higher values (171 MPa). Regarding von Mises stress on the retaining screw of the prosthesis, 61 MPa was recorded for the experimental model while upright implants had the highest values (223 MPa). At the acrylic base, 4 MPa was recorded for the experimental model whereas models with upright implants showed the highest stress values (11 MPa). Conclusion: Rehabilitation of severely resorbed mandibles with 4 short implants placed distally at 45°, without lower transcortical involvement, were biomechanically more favorable, generating lower stress peaks, than the models with short implants on an all-on-four, or on an upright conﬁguration, with or without lower transcortical involvement. © 2017 Published by Elsevier B.V.

1. Introduction report pain, difﬁculty chewing and speech impairment, which compro- mise their quality of life [2–6]. The increase in life expectancy in recent decades described by the The use of osseointegrated titanium implants is considered an effec- World Health Organization has increased the number of people wearing tive method for functional and esthetic replacement of lost teeth. Treat- lower dentures for many years [1]. As alveolar ridge volume is dictated ment success has always been related to variables such as volume and by the presence of natural teeth, a lack thereof inevitably triggers bone anatomy of the remaining bone, the longer the implant the more favor- resorption. In the case of complete teeth loss, replacement of natural able the prognosis [2,7,8]. However, in many situations, placement of teeth using soft tissue supported dentures generates unfavorable forces long implants is hindered by severely resorbed alveolar ridges, anatom- that may accelerate this process, exacerbating the instability of full den- ical limitations such as the mental foramen or inferior alveolar nerve tures as well as the degree of patient dissatisfaction. Patients often and mandibular canal and shape of the mandible [1,9–14]. Placement of short implants (≤8 mm long) may be considered an effective option to rehabilitate edentulous patients whenever conven- ⁎ Corresponding author at: Rua José Rocha Junqueira, 13, Campinas, SP, Brazil. tional implants cannot be placed without prior bone augmentation E-mail address: [email protected] (D.C. Peruzzo). procedures [3,4,9–18], such as autologous bone graft, osteogenic

One way to improve the distribution of functional forces on the prosthesis, implants and peri-implant tissues in patients with little bone is the use of angled implants. Biomechanical justiﬁcation for distal inclination of implants is based on decreased distal extension of the prosthesis and favorable anteroposterior distribution of implants. Furthermore, the use of tilted implants can increase its prima- ry cortical anchoring and stability, allowing the use of longer Fig. 1. Illustration of the 6 models generated: SI - short inclined implants; IT – inclined implants [1,23–26]. transcortical implants; SA - short implants positioned as all-on-four; TA – transcortical The all-on-four concept consists of a relatively less invasive treat- short implants positioned as all-on-four; SU - short upright implants; UT - upright ment option with a high success rate [27–29]. Distally inclined im- transcortical implants. plants at 45° decrease the concentration of stresses on the peri- implant bone [23] and compressive forces on the distal bone/implant distraction or mental foramen transposition, which increase surgical interface compared to the conﬁguration with four upright implants morbidity and treatment time [1,3,4,9–12,14–16,19]. [1,13,24,25,30–33]. A survey on the occurrence of mandibular fractures associated with Based on the aforementioned arguments, the aim of this study dental implants concluded that mandibles with b10 mm of bone height was to evaluate stress distribution on severely resorbed mandibles, at the region of the symphysis are at risk of fracture and associated com- with 8 mm of remaining bone height at the symphysis, restored plications [20]. One factor that may favor the occurrence of a mandibu- with implants at varying angulations and length (bicorticated and lar fracture is the penetration of the lower mandibular cortical ridge, by non-bicorticated) using the tridimensional Finite Element Method the implant, i.e. bicortication. Thus, the installation of shorter implants, (FEM) [33–35]. The null-hypothesis tested was that different im- keeping the integrity of the lower mandibular cortex, might reduce plant angulations and length would not affect biomechanical the risk of mandibular fragilization [21,22]. behavior.

Fig. 2. Implant dimensions. An implant with an external hexagonal interface Titamax TI Cortical (Neodent®, Curitiba, Brasil) was used. 124 H.E. Peixoto et al. / Materials Science and Engineering C 80 (2017) 122–128

2. Materials and methods

2.1. Experimental design

This study involved the establishment of models of the mandible in silico with severe alveolar bone resorption and average bone height of 8 mm in the region of the symphysis. The mandibles were dentally reha- bilitated using implant-supported ﬁxed dentures over 4 implants. Both the mandible and denture models were the same for all simulations. The study factors were the angulation of implants and implant length. Three possibilities were considered for the angulation of implants: (1) vertically positioned implants; (2) all-on-four (2 vertical anterior implants and 2 posterior implants distally inclined at 45° inclined); (3) 4 implants distally inclined implants at 45°. In addition, for each implant angulation model two implant lengths were used, a 7-mm short implant placement preserving the lower cortical bone of the mandible and a 9- mm implant model, which penetrates the lower cortical bone of the mandible. Six models were therefore generated: SI - short inclined implants; IT – inclined transcortical implants; SA - short implants positioned as all-on-four; TA – transcortical short implants positioned as all-on-four; SU - short upright implants; UT - upright transcortical implants. The models can be viewed in Fig. 1. Tridimensional Finite Ele- ment Method was used to determine the maximum principal stress in Fig. 3. a) Straight intermediate used on vertical implants - Mini-Pilar Cônico SF (Neodent®, bone tissue and von Mises values for prosthesis, prosthetics screw and Curitiba, Brazil); b) The inclined implants were bent in 45° and received angled implants. intermediates at a 30° angle - Mini-Pilar CônicoAngulado (Neodent®, Curitiba, Brasil).

2.2. Modeling

The structures were processed in the software Rhinoceros 4.0 SR 5 2.4. Analysis (Robert McNeel& Associates, Seattle WA, USA). The model of the atrophic mandibular bone structure, type II, according to the classiﬁcation Quantitative analysis was performed using the numerical values by Lekholm&Zarb, was obtained following measurements obtained for the von Mises stress peaks forecast for the implant, prosthetic using cone beam computed tomography scans of edentulous mandi- screw and the prosthesis. Measurements of stress within the mandibles. The models were later edited to transform the height to 8 mm at bleweremadefromtheforecastpeaks of maximum and minimum the anterior region. For the design of implants (Titamax TI Cortical, stress, as they allow the distinction between tensile and compressive Neodent®, Curitiba, Brasil) (Fig. 2) and prosthetic abutments, measure- strength peaks and are more effective to identify possible overload ments from commercially available abutments were used (Mini-Pilar regions on the peri-implant bone [30,33]. Combination of variables Cônico SF, Neodent®, Curitiba, Brasil). In the models of inclined im- (implant length and angulation of implants) were analyzed statisti- plants, abutments angled at 30° were used (Mini-Pilar Cônico Angulado, cally using main effect and interactions with Two-Way ANOVA and Neodent®, Curitiba, Brazil), as seen in Fig. 3. The design of the metal bar linear regression in SAS/STAT® Statistical Analysis Software. The sig- and the prosthetic structure were made so as to suit the dimensions of niﬁcance level was set at 5%. the mandible and the dimensions of the 6 CAD models.

2.3. Numeric analysis 3. Results

After construction, the CAD models were exported to Ansys Work- The highest von Mises equivalent stress values predicted for the bench10.0 FEA software (Swanson Analysis Inc.) to finite element anal- prosthesis, the prosthetic screw and the implant after load applica- ysis. To improve accuracy and ensure comparable results, the analysis tion as well as the maximum principal stress values found in the was accomplished by mesh refinement at a 5% level. Quadratic tetrahe- bone structure for each model are described in Chart 1. Table 3 dral element was used for meshing. The number of nodes and elements shows the average stress values predicted for each region of the all in each model are shown in Table 1. Properties were assigned to the ma- models and their respective standard deviations. Table 4 illustrates terials, as shown in Table 2 [35–39]. The models were considered homo- the relationship between the highest stress values found for each geneous, isotropic and linearly elastic. Movement restriction was applied to the angle of the mandible, similar to the insertion of the masseter and medial pterygoid muscles. The Table 1 surfaces of the cortical and cancellous bone were regarded as perfectly Number of nodes and elements generated in each model. SI - short inclined implants; united. Similarly, the interface between the implant and the mandible, IT – inclined transcortical implants; SA - short implants positioned as all-on-four; TA – as well as the interface between metal bar of the prosthesis and the transcortical short implants positioned as all-on-four; SU - short upright implants; acrylic structure were considered perfectly bonded. The interfaces be- UT - upright transcortical implants. tween the prosthetic screws, prosthetic implants and intermediates Models Nodes Elements were considered frictionless/frictional (0.3). SU 3,925,685 2,644,022 A load of 100 N was applied at a right angle with the prosthetic struc- UT 4,205,230 2,818,916 ture at the lower right first molar region [26]. Unilateral load is intended AS 3,393,666 2,281,347 to simulate the interposition of food during mastication. The values of TA 4,257,065 2,850,704 maximum principal stress for bone tissue and von Mises for the implant, SI 3,956,037 2,660,433 IT 3,778,176 2,512,778 prosthesis and prosthetic screw were obtained for all models. H.E. Peixoto et al. / Materials Science and Engineering C 80 (2017) 122–128 125

Table 2 Material attributed to each region of the model and properties of the materials used [35–39].

Component Material Modulus of elasticity (MPa) Poisson coefﬁcient

Prosthesis Acrylic resin [35] 2700 0.35 Mandible Cortical bone [36] 13,700 0.3 Mandible Cancellous bone [36] 1370 0.3 Bar Cr Co alloy [37] 218,000 0.33

Mini screws [1,2,3,4] Ti6AlV4 alloy [38,39] 110,000 0.28

Shoulder [1,2,3,4] Ti6AlV4 alloy [38,39] 110,000 0.28

Screws [1,2,3,4] Ti6AlV4 alloy [38,39] 110,000 0.28 Implants [1,2,3,4] Ti cp IV [38,39] 110,000 0.33 model and the average stress value encountered across the 6 models 3.2. Implants by region. Equivalent von Mises stress values, below the average for all models, were found in the implants from the models SI, IT and TA, whereas in SA 3.1. Bone tissue models, SU and UT, stress performed above average. Thus, in the region of the implants, models with 4 inclined implants, both short (SI) and The highest von Mises stress values were found at the cervical transcortical (IT) had lower than average values, while models featuring bone region of the distal implant on the same side as the load appli- upright implants (SU and UT) showed higher than average stress values. cation (Fig. 4), where the UT model showed the highest value On models featuring the all-on-four layout, those containing short im- (351.6 MPa). On this same model, UT, the highest von Mises stress plants (SA) showed higher than average stress values, while the values for the retaining screw region (202.56 MPa) were also found model with transcortical implants (UT) showed lower than average as well as for the region of the acrylic resin (11.5 MPa). Regarding values. the bone structure, the highest maximum stresses were observed in the SA model (171.69 MPa). For the principal maximum stress values in the mandible, models 3.3. Prosthesis and prosthetics screw with inclined transcortical implants (IT = 170,69 MPa and TA = 171.69 MPa) showed higher than average values (149.85 MPa). The In the region of the prosthesis retaining screw, models with 4 in- only model with transcortical implants showing values below average clined implants (SI and IT) showed below average stress values, was the UT model (135.82 MPa) in which the implants were placed while models with upright implants (SU and UT) presented higher upright. than average values for both models with short implants and transcortical implants, which is similar to the actual stress values observed for implants. For models with the all-on-four configuration, a change in pattern was detected, where the transcortical implant model (TA) showed higher than average stress values, while the model with short implants (SA) showed lower than average values, differently from the behavior of stresses seen with implants. In the region of the acrylic material of the prosthesis, models with 4 inclined implants (SI and IT) and those with all-on-four configuration (SA and TA) showed lower than average values, while models with upright implants (SU and UT) had higher than average stress values. The interaction of the variables “implant inclination” and “implant length” did not reach significance among the groups with short and long implants (p N 0.05). To compare the influence of implant positioning (upright, all-on-four, inclined) in the peak stress values predicted for each region of the models, ANOVA was used. A significant difference (p b 0.05) was observed between different positions of the implants and the maximum von Mises stress values predicted for the prosthetic screw. Models with upright implants had higher stress values (UT = 202.56 MPa; SU = 200.05 MPa) than models with the all-on-four configuration (TA = 198.76 MPa; SA = 80.37 MPa) with the latter showing higher stress values than the models with inclined implants (IT = 51.17 MPa, SI = 61.65 MPa). For implant and prosthesis, no statistical significance was observed (p N 0.05).

Table 3 Mean peak von Mises stress values (MPa) found for prosthesis, screw and implant in the 6 models and mean peak maximum stress (MPa) found for the bone structure.

Chart 1. Highest von Mises stress values (MPa) observed at prosthesis, prosthetic screw Mean stress values Standard deviation and implants for each model, and highest maximum values of stress (MPa) found in the Prosthesis 6.618333 1.431972 bone structure for each model after load application. SI - short inclined implants; IT – Screw 132.4283 30.66592 inclined transcortical implants; SA - short implants positioned as all-on-four; TA – Implant 235.3033 30.85476 transcortical short implants positioned as all-on-four; SU - short upright implants; UT - Mandible 149.95 7.113002 upright transcortical implants. 126 H.E. Peixoto et al. / Materials Science and Engineering C 80 (2017) 122–128

Table 4 Characteristics of the highest stress values observed for each model compared to the mean stress values across all models. Maximum stress values for bone structure and von Mises stress for implant, screw and prosthesis observed for each model compared to the mean stress values across all models. A minus sign (−) indicates that the model presented lower stress values than the mean. A plus (+) indicates that the models presented higher stress values than the mean of all models. SI - short inclined implants; IT – inclined transcortical implants; SA - short implants positioned as all-on-four; TA – transcortical short implants positioned as all-on-four; SU - short upright implants; UT - upright transcortical implants.

SI IT SA TA SU UT Mean stress values (MPa)

Bone structure −132.45 MPa +170.69 MPa −139.51 MPa +171.69 MPa −149.54 MPa −135.82 MPa 149.65 MPa Implant −176.88 MPa −171.35 MPa +297.27 MPa −174.72 MPa +240 MPa +351.6 MPa 235.30 MPa Screw −61.65 MPa −51.17 MPa −80.37 MPa +198.76 MPa +200.05 MPa +202.56 MPa 132.42 MPa Prosthesis −4.34 MPa −4.41 MPa −4.29 MPa −4.40 MPa +10.66 MPa +11.59 MPa 6.61 MPa

Linear regression was performed to verify the relationship be- bone and reducing the risk of fracture [9–12,14,15,17,21,22].Such tween the variables: positioning of the implants (upright, all-on- model also promotes greater implant-bone contact, because implant in- four, inclined) and implant length (short, long/transcortical) with clination permits the use of longer implants while preserving the lower peak stress values for each region of the models. A significant differ- cortical bone of the mandible [1,13,23–27,31–33]. ence for the von Mises stress concentration was found for the pros- This study was based on previous clinical [1,11,23,26,27,29], thetic screw (p b 0.05 R2 = 81%), however, implant length did not photoelastic [41,44] and numeric studies [1,13,24,25,30–32,45–47], influence the biomechanical behavior (p = 0.35) in any of the which demonstrated that rehabilitation with inclined implants opti- models. For the remaining models, no significant difference was mizes stress distribution when compared to vertically positioned im- found between the variables: positioning of implants and length plants. It was also based on studies showing favorable results using of the implants in relation to stress peaks (p N 0.05). short implants [9–12,14,15,17] with reported success rates similar to conventional long implants. The results of the present study cor- 4. Discussion roborate those by Dejak and Mlotkowski, [48] who reported that maximum principal stress is adequate to establish the stress behav- Dental rehabilitation using fixed prosthesis over implants in the se- ior in brittle materials such as bone tissue, while von Mises stress verely atrophic mandible poses difficulties both in surgical and pros- may be used to evaluate the behavior of ductile materials such as im- thetic procedures. Scarcity of bone requires the use of shorter plants and prosthetic screws. implants than the gold standard, which reduces the contact between The model SI was the only to present peak von Mises stress values bone surface and implant, increasing the risk of mechanical overload as well as maximum stress values below average in the four regions on the implants [2,7,8]. Moreover, implant installation may further studied: mandible, implant, prosthetic screw and acrylic base, which weaken the already fragile bone structure, increasing the risk of fracture reiterates the promising findings for the four inclined short implants [21,22]. Fixed prostheses with a distal cantilever induces considerable configuration. stress to the distal implant on the side of load application [40,41], In agreement with other studies [24,25,29,32,45–47], the region which may result in technical problems, such as fracture of the prosthet- with the highest risk of overload at the bone/implant interface was ic screw or the acrylic resin teeth [42].Incasesofsevereatrophy,there- the cervical region of the distal implant on the load application side, par- lationship between implant length and distance from the implant to the ticularly during compression. Assuming the ultimate bone strength as a occlusal plane is compromised because the implants are shorter and the physiological limit, overloading occurs at the cortical bone when the prosthesis longer, resulting in unfavorable biomechanics [17,20,43]. maximum stress exceeds 170–190 MPa [29,32]. In the SI model, the ex- The idea proposed in the SI model using 4 distally inclined short im- pected peak stress values to the bone structure was 132.45 MPa, illus- plants aimed to preserve the lower cortical bone of the mandible, in trating a low risk of overloading the mandibular bone, preventing cases of severely resorbed mandibles, preventing weakening of the peri-implant bone loss and implant failure. On the other hand, the IT

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