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J Periodontol • March 2011The Biomechanical Analysis of RelativePosition Between Implant and AlveolarBone: Finite Element MethodCheng-Chun Huang,*†‡ Ting-Hsun Lan,§ Huey-Er Lee,§ and Chau-Hsiang Wang§ Background: The purpose of this study is to analyze bio- mechanical interactions in the alveolar bone surrounding implants with smaller-diameter abutments by changing posi- tion of the ﬁxture–abutment interface, loading direction, and thickness of cortical bone using the ﬁnite element method. Methods: Twenty different ﬁnite element models including four types of cortical bone thickness (0.5, 1, 1.5, and 2 mm) E xcellent outcomes for implants and ﬁve implant positions relative to bone crest (subcrestal have been documented, yet im- 1, implant shoulder 1 mm below bone crest; subcrestal 0.5, plant failures are still reported.1 implant shoulder 0.5 mm below bone crest; at crestal implant Implant failures after loading primarily shoulder even with bone crest; supracrestal 0.5, implant result from cortical bone loss,2 and one shoulder 0.5 mm above bone crest; and supracrestal 1, im- important factor contributing to cortical plant shoulder 1 mm above bone crest) were analyzed. All bone loss is the position of the ﬁxture– models were simulated under two different loading angles abutment interface relative to the alveo- (0 and 45 degrees) relative to the long axis of the implant, lar crest.3,4 Buser et al.5 indicated that respectively. The three factors of implant position, loading the ﬁxture–abutment interface should type, and thickness of cortical bone were computed for all be placed subcrestally to compensate for models. the loss of vertical bone height in the ﬁrst Results: The results revealed that loading type and implant year after implant placement. Davar- position were the main factors affecting the stress distribution panah et al.6 found that a supracrestal in bone. The stress values of implants in the supracrestal 1 position of the ﬁxture–abutment interface position were higher than all other implant positions. Addition- is favorable for prosthetic fabrication. ally, compared with models under axial load, the stress values Furthermore, two studies7,8 emphasized of models under off-axis load increased signiﬁcantly. that inﬂammatory cells aggregate in the Conclusions: Both loading type and implant position were microgap between the ﬁxture and abut- crucial for stress distribution in bone. The supracrestal 1 im- ment, which leads to bone loss. Placing plant position may not be ideal to avoid overloading the alve- implants subcrestally relative to the olar bone surrounding implants. J Periodontol 2011;82:489- initial cortical bone crest resulted in 496. greater bone loss than placing implants supracrestally.9,10 Broggni et al.8 inves- KEY WORDS tigated bone loss among implants with Abutment; biomechanics; bone loss; dental implant; various apico-coronal locations of the ﬁnite element analysis. ﬁxture–abutment interface and found that subcrestal interfaces accumulated more * Department of Dentistry, Chang Gung Memorial Hospital, Kaohsiung, Taiwan. neutrophils than supracrestal interfaces, † Graduate Institute of Dental Sciences, Kaohsiung Medical University, Kaohsiung, Taiwan. ‡ Department of Stomatology, National Cheng Kung University Hospital, Tainan, Taiwan. resulting in signiﬁcant bone loss. Thus, § Department of Prosthodontics, School of Dentistry, Kaohsiung Medical University a supracrestal position of the ﬁxture– Hospital, Kaohsiung Medical University. abutment interface not only diminished the amount of bone loss but facilitated the fabrication of prosthesis. In esthetic sites, a more apically posi- tioned interface is advised to avoid the unesthetic appearance of metal crown doi: 10.1902/jop.2010.100388 489
Biomechanical Analysis of Different Implant Positions Volume 82 • Number 3 margin, especially for patients with a high smiling line. Besides, placing the implant shoulder subcres- tally is favorable for an ideal emergence proﬁle.11 However, subcrestal location of the ﬁxture–abutment interface leads to a greater amount of bone loss than placing the interface supracrestally. It is generally believed that the vertical position of the ﬁxture–abutment interface greatly inﬂuences bone resorption and soft tissue dimensions.3,4,12 The per- spective of implant designs should reduce the amount of bone resorption that results from the microgap be- tween ﬁxture and abutment to improve the esthetic outcome. Placing a smaller-diameter abutment rela- tive to the platform of the ﬁxture is applied progres- sively to decrease bone loss. This is referred to as ‘‘platform switching.’’13 Lazzara and Porter14 reported that the amount of bone loss was less when using im- plants with non-matching diameters of ﬁxture and abutment rather than using the same diameters. Hurzeler et al.15 indicated that changing the horizontal ¨ relationship between ﬁxture and abutment by reposi- tioning the ﬁxture–abutment interface inwardly could effectively abate bone resorption. Jung et al.16 and Figure 1. Cochran et al.17 evaluated the bone loss around the The three-dimensional ﬁnite element models of an implant-supported implants with non-matching diameters of ﬁxtures system used in the study. A) Abutment. B) Fixture. C) Metal framework. D) Porcelain. E) Cortical and cancellous bone. F) All models were and abutments by radiographic and histologic ana- combined by Boolean operations. lyses. The ﬁxture–abutment interfaces were placed at three different locations: 1 mm above bone crest, even with bone crest, and 1 mm below bone crest. and decreasing the thickness of cortical bone would Both radiographic and histologic results indicated that lead to more stress concentration. bone loss surrounding the ﬁxture with a smaller-diam- eter abutment was much less than the implant with MATERIALS AND METHODS a butt–joint connection between ﬁxture and abutment According to the mandibular buccal and lingual mean regardless of the implant position. In addition, radio- cortical thickness over the cervical area,18 a three- graphic analyses revealed that no signiﬁcant dif- dimensional FE model of a mandibular segment from ferences in bone loss in various positions of the second premolar to second molar was constructed ﬁxture–abutment interface were observed,16 but the using a computer-aided design program.¶ A solid histologic results did identify signiﬁcant differences screw-type implant model# with a narrow-diameter in the amount of bone loss among implants with dif- abutment that combined a horizontal offset and a ferent positions.17 Morse taper connection was placed in the mandibular According to the studies of Jung et al.16 and ﬁrst molar area. The thickness of cortical bone was Cochran et al.,17 application of the implant with non- changed to 0.5, 1, 1.5, and 2 mm to investigate the matching diameters of ﬁxture and abutment could effect on cortical bone thickness. The geometry of diminish bone loss. The position of ﬁxture–abutment the implant-supported crown of the mandibular ﬁrst surface would signiﬁcantly affect the bone loss around molar was created as previously described.19 The implants. However, there is insufﬁcient biomechanical simulated crown consisted of framework material evidence concerning implants with non-matching di- and porcelain, and the porcelain thickness used in ameters of ﬁxture and abutment. The purpose of this this study was 1.5 mm (Fig. 1). study is to analyze the stress distribution in the bone The effect of various positions of the ﬁxture–abut- surrounding implants with smaller-diameter abutments ment interface relative to the alveolar bone crest to investigate the effects of loading direction, position of and the thickness of cortical bone were investigated the ﬁxture–abutment interface, and thickness of cortical in 20 FE models. The models were divided into ﬁve bone by the ﬁnite element (FE) method. The interac- groups based on the position of the ﬁxture–abutment tions between these three factors were also evaluated. The hypothesis of the study was that placing the ﬁx- ¶ Pro/ENGINEER, Parametric Technology, Boston, MA. ture–abutment interface in a supracrestal position # 3.5 mm in diameter and 11 mm in length, Ankylos, Mannheim, Germany.490
J Periodontol • March 2011 Huang, Lan, Lee, WangTable 1. compressive stresses are more substantial than ten- sile stresses and provide reliable information forDescription of the Five Different Groups analyzing bone resorption leading to the loss of os-Used in the Study seointegration between alveolar bone and implants.22 Therefore, this study investigates the stress distribu- Group Description tion of cortical bone by peak compressive stress. To A Subcrestal 1: the position of ﬁxture–abutment simplify the results, the main effect of each level of interface was 1 mm below alveolar bone crest the three investigated factors (position of ﬁxture– abutment interface, loading type, and thickness of B Subcrestal 0.5: the position of ﬁxture–abutment cortical bone) was analyzed statistically.20,26 The interface was 0.5 mm below alveolar bone crest data from simulated results were compared using C At crestal: the position of ﬁxture–abutment a three-way analysis of variance (ANOVA) by the interface was even with alveolar bone crest statistical program.†† D Supracrestal 0.5: the position of ﬁxture–abutment RESULTS interface was 0.5 mm above alveolar bone crest The peak compressive stress values of cortical bone E Supracrestal 1: the position of ﬁxture–abutment under axial and off-axis loads are illustrated in Figure interface was 1 mm above alveolar bone crest 2. The compressive stress distribution of cortical bone with a thickness of 2 mm and different implant posi- tions under axial and off-axis load are illustrated ininterface. These were designated with a ﬁrst symbol of Figure 3. The maximum stress in the alveolar bone‘‘A,’’ ‘‘B,’’ ‘‘C,’’ ‘‘D,’’ and ‘‘E,’’ respectively, as de- was concentrated at the buccal and lingual cervicalscribed in Table 1. In addition, a second group of sym- areas in the cortical bone when axial and off-axisbols (1 through 4) represented the thickness of loads were applied, respectively. The stress valuescortical bone (0.5, 1, 1.5, and 2 mm, respectively). of models with the at crestal implant position were After all models were assembled by Boolean oper- lower than models with other implant positions (Fig.ations, a convergence test was conducted by applying 3). To evaluate the relative importance of the investi-element reﬁnement methodology. The criterion be- gated factors and their interaction effects, ANOVAtween mesh reﬁnements was a change of <5% for was performed, and the results are summarized inmodels with variant mesh size.20 According to the re- Table 4. The relative importance of each factor thatsults of the convergence test, all models were meshed affects the stress values was expressed as a per-by the FE program** with a mesh size of 0.8 mm. centage of the total sum of squares. 20,27 Loading The interface between implant and alveolar bone type was the main factor affecting stress distributionwas bonded to simulate ideal osseointegration. An of cortical bone. The results revealed that loading typeocclusal force of 100 N was applied to the mesio-buc- signiﬁcantly (P <0.01) dominated the magnitude ofcal and disto-buccal cusps axially and at 45 degrees the peak compressive stress values and the percent-to the long axis of the implant from the buccal to lin- age contribution was 62.39% (Table 4). The highgual side, respectively. Table 2 provides a detailed value of percentage of the total sum of squares meansclassiﬁcation of the thickness of cortical bone, the po- that loading type was a crucial factor determining thesition of the ﬁxture–abutment interface, and loading stress distribution relative to other factors. Generally,types for all FE models. All materials were presumed off-axis load evidently increased the peak compres-to be linear elastic, homogeneous, and isotropic; the sive stress values regardless of the position of ﬁxture–material properties are described in Table 3.21-23 In abutment interface and the thickness of cortical boneaddition, nodes over the mesial and distal border compared to the axial load.surfaces of the bone model were constrained in all The position of the ﬁxture–abutment interface sig-directions as the boundary conditions. niﬁcantly affected the peak compressive stress values Presently, the ideal stresses used in the calcula- of cortical bone (P = 0.02) and the percentage con-tions are not clearly deﬁned. Based on previous re- tribution was 30.49%. The peak compressive stresssearch,22,24,25 von Mises stress values are deﬁned as values of the group C models were smaller than thethe ductile material, such as metallic implants, and other groups (Figs. 2A and 2B), which indicated thatprincipal stress offers the possibility of making a dis- the stress values were lowest when the position oftinction between tensile and compressive stress. Posi- ﬁxture–abutment interface was at the crest regard-tive values of principle stress represent tensile stress; less of the loading types and thickness of corticalnegative values represent compressive stresses. Thatis, the most negative stress (minimum principal stress) ** ANSYS, v11.0, Swanson Analysis System, Houston, PA.stands for the peak compressive stress. In general, †† SPSS, v11.0, IBM, Chicago, IL. 491
Biomechanical Analysis of Different Implant Positions Volume 82 • Number 3 Table 2. Detailed Thickness of Cortical Bone, Position of Fixture–Abutment Surface, Loading Types, and Sequence of Simulated Finite-Element Models in This Study Position of Fixture–Abutment Interface (A, subcrestal 1 mm; B, subcrestal 0.5 mm; C, at crestal; D, supracrestal Thickness of Cortical Bone 0.5 mm; E, supracrestal 1 mm) Sequences: Vertical Load Sequences: Off-Axis Load 1) Thickness of cortical bone: 0.5 mm A 1 21 B 2 22 C 3 23 D 4 24 E 5 25 2) Thickness of cortical bone: 1 mm A 6 26 B 7 27 C 8 28 D 9 29 E 10 30 3) Thickness of cortical bone: 1.5 mm A 11 31 B 12 32 C 13 33 D 14 34 E 15 35 4) Thickness of cortical bone: 2 mm A 16 36 B 17 37 C 18 38 D 19 39 E 20 40 Table 3. bone thickness, the ANOVA results failed to identify Material Properties Used in the any apparent effect on the stress values of cortical Finite-Element Models bone and the percentage contribution was only 1.78%. The interaction effects among the three factors (load- Young Modulus ing type, position of the ﬁxture–abutment interface, and Materials (MPa) Poisson Ratio References thickness of cortical bone) were also investigated and Porcelain 69,000 0.28 20 the results are summarized in Table 4. The cofactor (loading type · position) was a signiﬁcant factor for Titanium 117,000 0.35 21 the stress value in cortical bone (P <0.01), but only Trabecular bone 1,850 0.30 21 a small percentage contribution (3.47%) was noted. Cortical bone 13,700 0.30 21 DISCUSSION Low-gold alloy 120,000 0.33 22 Excessive stress at the implant–bone interface has (Au-Pd-Pt) been considered a potential cause for peri-implant bone loss and failure of osseointegration. Based on previous studies, the magnitude of the stresses in bone. Post hoc analyses suggested that the stress bone was highly correlated with the thickness of cor- values of models with the supracrestal 1 position were tical bone. As the thickness of cortical bone increased, signiﬁcantly greater than the models with the position the maximum stress values concentrated in the corti- of supracrestal 0.5 and at crestal (Fig. 4); however, cal bone decreased.28,29 In this study, as the thickness some group differences were marginally signiﬁcant of cortical bone increases from 0.5 to 2 mm, peak (i.e., group A versus group E and group B versus compressive stress reduces despite the loading type; group E, both P = 0.07). As to the effect of cortical however, no signiﬁcant difference is observed among492
J Periodontol • March 2011 Huang, Lan, Lee, Wang Figure 2. A) The stress values of cortical bone in all models under axial load. B) The stress values of cortical bone in all models under off-axis load. Figure 3. A) The compressive stress distribution and values of cortical bone with a thickness of 2 mm and different implant positions under axial load. The peak compressive stress positions were located at the buccal cervical area in the cortical bone of the implant side. B) The peak compressive stress distribution and values of cortical bone with a thickness of 2 mm and different implant positions under off-axis load. The peak compressive stress positions were located at the lingual cervical area in the cortical bone of the implant side.the models with different cortical bone thickness. ing moment that increased stress compared to thatThis might be because the force applied in this study generated by axial load. This result is in agreementis too small to present the loading effect. Moreover, with previous reports,20,25,27,30 which found loadingthe von Mises stress was adopted in previous studies type was one important factor affecting the stress dis-to evaluate the condition of stress distribution in tribution for alveolar bone.bone, but the peak compressive stress is instead Clinically, placement of the ﬁxture–abutment inter-used in this study, which might inﬂuence the results. face needs to take into account anatomic limitationsThe study also found that loading type was a critical and esthetic requirements. Placing the ﬁxture–abut-factor for stress distribution. The peak compressive ment interface below the alveolar crest (a subcrestalstress values were signiﬁcantly higher in models un- placement) could achieve satisfying esthetic outcomeder off-axis load than models under axial load, which and a favorable emergence proﬁle, which is desirableimplied that an off-axis load generated a larger bend- for esthetic and hygienic reasons.11 However, the 493
Biomechanical Analysis of Different Implant Positions Volume 82 • Number 3 Table 4. Summary of the Analysis of Variance Showing the Statistical Results of Peak Compressive Stress With Respect to Cortical Bone Source df SS MS % TSS P Value Loading type 1 129,857.65 129,857.65 62.39 <0.01 Position 4 63,469.56 15,867.39 30.49 0.02 Thickness of cortical bone 3 3,694.80 1,231.60 1.78 0.90 Loading type · position 3 7,224.05 1,806.01 3.47 <0.01 Loading type · thickness of cortical bone 4 1,453.93 484.64 0.70 0.80 Position · thickness of cortical bone 6 2,433.70 202.81 1.17 1.00 Total 208,133.69 100 df = degrees of freedom; SS = sum of square; MS = mean square; % TSS = total sum of squares. The major ﬁnding of this study is that the position of the ﬁxture–abutment interface signiﬁcantly affects the magnitude of the peak compressive stress for cortical bone. Post hoc analyses revealed the stress values of models with a supracrestal 1 position were signiﬁ- cantly greater than models with the supracrestal 0.5 and at crestal positions. Furthermore, marginal signif- icant differences were noted between groups A and E and between groups B and E. These results implied that under the same magnitude of loading, the peak compressive stress values were higher in models with a supracrestal 1 position than in models with other positions despite the thickness of cortical bone and loading type. Hansson24 also found that placing the ﬁxture–abutment interface supracrestally caused a higher peak compressive value in bone than that even Figure 4. with alveolar bone crest. Eccentric loading applied Means and standard errors of stress values in models with different to the occlusal plane of the implant-supported pros- implant positions (* P <0.05). thesis causes a bending of the implant, and a bending moment for cortical bone was generated. The bending subcrestal microgap was thought to promote a moment was greater in models with supracrestal 1 remarkably greater amount of inﬂammatory reaction implant position because of the longer resistance correlated with bone destruction than supracrestal arm. In addition, there is a direct connection between microgaps.7,8 Placing the microgap at a ﬁxture– alveolar bone and the implants used in our study. It abutment connection subcrestally had been per- was assumed that the force applied to the implant- ceived as a contraindication for maintaining vertical supported prosthesis would be transferred directly bone height until the concept of platform switching to the alveolar bone. Therefore, the amount of contact was introduced. Platform switching means that the area between the implant and alveolar bone could abutment with narrower diameter is connected to inﬂuence the stress distribution in alveolar bone. The the ﬁxture, which has been reported to decrease the overall area of the implant–bone interface was smaller vertical bone loss.14,15 Maeda et al.31 indicated that in models with the supracrestal 1 position than in any the platform switching conﬁguration has the biome- of the other models. This might explain why models chanical advantage of shifting the stress concentra- with the supracrestal 1 position had the highest peak tion away from the bone–implant interface. In our compressive stress. study, a ﬁxture with narrower diameter of abutment In the present study, the position of the ﬁxture– connection is used to simulate the platform switching abutment interface has a signiﬁcant impact on stress structure. values of alveolar bone. It is generally believed that494
J Periodontol • March 2011 Huang, Lan, Lee, Wangexcessive stress concentration causes bone destruc- accuracy of our study. The bending of the mandibletion.28 Jung et al.16 assessed the amount of bone loss during mastication is not considered in our study,by radiography to determine the bone response to and it is impossible to simulate entire chewing pat-various positions of the ﬁxture–abutment interfaces. terns by the FE method. The assumptions of loadingThis research group reported that position of the ﬁx- types in this study are simpliﬁed and represent onlyture–abutment interface did not signiﬁcantly affect two possible occlusal contacts in clinical situations.bone loss, which was inconsistent with our ﬁndings. From a biomechanical viewpoint, FE analyses pro-Image distortion, insufﬁcient resolution, and poor re- vide a general idea regarding bone response to occlu-sponse to minor bony changes lead to errors in den- sal force. Further studies involving different implanttal radiography,32,33 which might potentially explain positions and long-term clinical results are required.why the results of Jung et al.16 differ from the presentstudy. In contrast, Cochran et al.17 investigated bone CONCLUSIONSloss by histologic analyses for implants with platform Considering the limitations of the study, we concludeswitching conﬁguration and found that position of the the following: 1) the position of the ﬁxture–abutmentimplant shoulder was an important factor affecting bone interface had an important role on the stress distri-destruction. Moreover, placing the implant shoulder 1 bution in alveolar bone; 2) the stress values of themm above the bone crest resulted in mild bone growth models under off-axis load were higher than those un-surrounding the implant instead of bone loss. Our re- der axial load; 3) the cofactor (loading type · position)sults suggested that peak compressive stress values was a prominent factor affecting stress distribution;were highest in models with the supracrestal 1 posi- and 4) realizing how clinical variables affect stress dis-tion. This implied that the bone loss surrounding the tribution facilitates optimal prosthesis fabrication andimplants in this position would be more severe than may lead to a decrease in mechanical complicationsin other positions. The differences between the pres- and improve implant longevity. According to theent study and the results of Cochran et al.17 are likely simulation results, locating the ﬁxture–abutment in-attributable to various factors. Cochran et al.17 used terface 1 mm above the bone crest may not be andogs in their study, and differences in chewing pat- appropriate option to prevent the bone surroundingterns, the different cortical thickness of the implant implants from overloading.placing areas, and loading types were some of themain factors that were difﬁcult to control and could ACKNOWLEDGMENTSexplain, at least in part, the different results. Drs. Ting-Hsun Lan and Chau-Hsiang Wang equally Another key ﬁnding of this study is that the cofactor contributed to this article. The authors thank National(loading type · position) is a crucial factor affecting Kaohsiung University of Applied Science, Kaohsiung,the stress values. Both loading type and the position Taiwan, for technical support. The authors report noof ﬁxture–abutment interface greatly inﬂuenced the conﬂicts of interest related to this study.stress values. 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