Biomechanical Effects of Different Fixed Partial Denture Designs Planned on Bicortically Anchored Short, Graft-Supported Long, or 45-Degree–Inclined Long Implants in the Posterior Maxilla: A Three-Dimensional Finite Element Analysis Emre S¸eker, DDS, PhD1/Mutahhar Ulusoy, DDS, PhD2/Og˘uz Ozan, DDS, PhD3/ Derya Özdemir Dog˘an, DDS, PhD4/Bas¸ak Kus¸akci S¸eker, DDS, PhD5 Purpose: The purpose of this study was to analyze the functional stresses around implants and supporting tissues placed in different combinations in the grafted and nongrafted atrophic posterior maxilla and to consider the acceptability of various fixed partial denture treatment options. Materials and Methods: A computer model of the atrophic posterior maxilla was created from the computed tomography images of an actual patient. Three different treatment scenarios were modeled with partial denture restorations, grafted and nongrafted maxillary sinuses, and various implant inclinations. Oblique forces were applied to simulate chewing movements. Stress analyses were performed with a three-dimensional finite element analysis computer program, and the von Mises and minimum principal stresses on the implants and supporting tissues were compared. Results: In all models, minimum principal (compressive) stress peak points were the highest within the crestal cortical bone (49.761 MPa), lower within sinus cortical (14.144 MPa) and trabecular bone (4.347 MPa), and lowest within grafted bone (0.049 MPa). The second molar implant in the third model (5 × 11-mm implant, inclined 45 degrees) showed the highest von Mises stresses (499.50 MPa), and the second molar implant in the first model (6 × 5-mm implant) showed the lowest (219.63 MPa) von Mises stresses. Conclusions: The stress absorption capacity of graft material is not sufficient and is much lower than that of other supporting tissues. For a fixed partial prosthesis, the use of short, wide implants with sinus floor bicortical fixation was found to be the most feasible approach for the atrophied posterior maxilla. Int J Oral Maxillofac Implants 2014;29:e1–e9. doi: 10.11607/jomi.3264 Key words: dental implants, finite element analysis, fixed partial prosthesis, maxillary sinus augmentation, short implants
I
n contemporary dentistry, implants are used routinely because they contribute to esthetics in addition to fulfilling the function of missing natural teeth. An appropriate quantity and quality of jawbone at the surgical
1 Assistant
Professor, Department of Prosthodontics, Faculty of Dentistry, Eskis¸ehir Osmangazi University, Eskis¸ehir, Turkey. 2Professor, Department of Prosthodontics, Faculty of Dentistry, Near East University, Mersin, Turkey. 3Associate Professor, Department of Prosthodontics, Faculty of Dentistry, Near East University, Turkey. 4 Assistant Professor, Department of Prosthodontics, Faculty of Dentistry, Cumhuriyet University, Sivas, Turkey. 5Research Assistant, Department of Periodontology, Faculty of Dentistry, Eskis¸ehir Osmangazi University, Eskis¸ehir, Turkey. Correspondence to: Emre S¸eker, Department of Prosthodontics, Faculty of Dentistry, Eskis¸ehir Osmangazi University, Eskis¸ehir, Türkiye. Email: [email protected] ©2014 by Quintessence Publishing Co Inc.
site are important criteria for successful dental implant therapy. Dental implants require sufficient bone to be adequately stabilized, making it challenging to place implants in regions with severe atrophy. Over time, alveolar bone undergoes resorption-related atrophy when teeth have been lost. In the posterior maxilla, the proximity of the maxillary sinus floor to the alveolar bone crest limits implant placement in this area. Conventional implant insertion is not feasible in many patients who require augmentation procedures. Maxillary sinus augmentation has become one of the most common surgical procedures for increasing bone volume for implant placement in the posterior atrophic edentulous maxilla and enabling the placement of longer implants with a higher success rate.1–3 Nevertheless, surgical time, the risk of morbidity, and the cost of this treatment method relative to other alternatives should be taken into account when sinus bone grafting is considered. The International Journal of Oral & Maxillofacial Implants e1
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One alternative treatment modality is the use of inclined implants,4,5 which makes it possible to achieve good implant anchorage by benefiting from more remote available dense bone structures. Another solution is longer implants.3 In the literature, the rate of failure is higher for implants shorter than 7 mm.6,7 An increased implant length could improve bone-to-implant contact, but bone type and engagement of cortical bone may exert more profound effects on implant stability than implant length, especially for bicortically anchored implants.6,8 Bicortical anchorage of implants in crestal bone provides ideal stabilization and fixation, minimizes failure rates during healing, and leads to better osseointegration.9 In addition to these approaches, the use of short implants seems to be an obvious alternative in areas of insufficient bone height. Although in general, short implants were thought to be associated with low success rates, some studies suggest similar clinical success rates for short implants compared with longer ones.9–14 The use of short implants offers a simpler and safer treatment owing to reduced risks of interference with anatomic structures, such as the maxillary sinus.15–17 In the edentulous posterior maxilla, the native bone, which provides primary stability for establishing and maintaining osseointegration, consists of crestal cortical, cancellous, and sinus floor cortical bone tissues.18 The contribution of grafted bone in establishing and maintaining implant stability is not yet well-known. Jensen et al19 achieved a 90% success rate for implants placed in grafted sinuses after 3 years of function. However, there was a statistically significant difference in the implant failure rate when the available bone was 4 mm or less as opposed to 5 mm or more. In addition to bone quantity, the quality of available bone is a decisive factor in treatment planning, implant design, surgical approach, healing time, and prosthetic loading protocol.20,21 Various graft materials and procedural modifications have been proposed to improve the efficacy of the therapy. Autogenous bone, allogeneic materials, alloplastic materials, xenogeneic materials, and combinations of these materials have been utilized.1–3,7,9,19 A complex structure consisting of bone with variable stiffness can be found around implants placed in the posterior maxilla with grafted sinus.1 The load-bearing characteristics of grafted bone depend on the graft material and its maturation process.22 Huang et al3 described two classes of stiffness of grafting material with reference to the maturation process in a finite element analysis (FEA) study. The degree of stiffness was classified as low for lowelastic-modulus tissue (ie, 345 MPa) and high for highelastic-modulus graft tissue (ie, 3,450 MPa). Late implant failures are primarily related to biomechanical complications; thus, the major factor leading
to such failures may be a lack of understanding of biomechanical factors.23,24 The transfer of a load from implants to the surrounding bone depends on the type of loading, the nature of the bone-implant interface, the length and diameter of the implants, angulation of implants in bone, the shape and characteristics of the implant surface, the prosthesis type, and the quantity and quality of the surrounding bone.4,25 Some of these biomechanical factors are inherent with the patients, and others can be controlled by clinicians. Clinical studies provide important information about general trends in biomechanics; however, specific biomechanical variables might be more efficiently examined through in vitro models. Accurately designed and appropriately analyzed in vitro models can be useful in studying biomechanical factors and their relationships. FEA, a numeric modeling system, is routinely used in modern engineering practice to study complex structures and has been long used in implant dentistry research.24–27 Recently, correlation of stress distribution in bone with implant-supported prosthesis design has been investigated primarily by means of twodimensional (2D) and 3D FEA.28 Studies comparing the accuracy of these analyses found that, if detailed stress information is required, then 3D modeling is necessary. The 3D FEA is considered an appropriate method for investigation of the stresses throughout a 3D structure, and therefore, this method was selected for the evaluation of stresses in bone and implants in the present study. There are many studies1,3,9,14,29–32 on single implants in the grafted sinus in FEA, but the efficacy of grafted bone in maintaining an implant-supported fixed partial denture (FPD) is not yet well-known. The purpose of this study was to investigate the biomechanical effects of bicortical fixation and graft tissue through varying the lengths, diameters, and angulations of implants in the atrophic posterior maxilla and to consider the acceptability of various treatment options for an implant-supported FPD.
Materials and Methods A model of the atrophic posterior maxilla was created via cone beam computed tomography (CBCT) images that were obtained for implant planning of an actual patient and were chosen from the tomography database archive previously stored in the Department of Dentomaxillofacial Radiology at Near East University. Images were obtained with a Newtom 3G CBCT device (Quantitative Radiology) at 120 kVP and 3 to 5 mA using a 9-inch field of view, an axial slice thickness of 0.3 mm, and isotropic voxels. A series of CT images at 1-mm intervals of the posterior edentulous maxilla were converted to medical image formats and
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Fig 1 Three-dimensional merging of supporting tissues, implants, and prosthetic structures. Top row, left to right: Maxillary bone tissue with implants; nickel-chromium alloy FPD substructure; completed FPD restoration. Center row, left to right: Schemes of the relationship between implants and the maxillary sinus for model 1, model 2, and model 3, respectively. Bottom row, left to right: Mesh structures of model 1, model 2, and model 3, respectively. Fig 2 Identification of the residual crest layers: sinus cortical bone (red arrows, 0.5 mm thick); trabecular bone (blue arrows, 3.5 mm thick); crestal cortical bone (yellow arrows, 1 mm thick). Total residual crestal bone thickness was 5 mm.
0.5 mm 3.5 mm 1.0 mm
transferred to a personal computer with commercially available image processing software (3D-Doctor, Able Software). The implants (long: 5 × 11 mm; short: 6 × 5 mm) and abutments were supplied by the manufacturer (Bicon) and were scanned with a 3D scanner (Next Engine Inc) on the macro scale. The obtained image data were imported into Rhinoceros 4.0 modeling software (McNeel & Associates). With the use of same
program, three different treatment scenarios were modeled that incorporated FPDs, long and short implants, angulated implants, and grafted and nongrafted maxillary sinuses (Fig 1). Simulated crestal cortical bone, cancellous bone, and sinus floor cortical bone were fabricated in 1-mm, 3.5-mm, and 0.5-mm heights, respectively, according to the study of Fanuscu et al.1 The total height of the simulated native bone was therefore 5.0 mm (Fig 2). The International Journal of Oral & Maxillofacial Implants e3
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Fig 3 Application of 300 N of occlusal force on each crown in the palatobuccal direction at a 30-degree inclination.
Table 1 Mechanical Properties of the Materials Modeled Young’s Poisson modulus (GPa) ratio
Material Feldspathic porcelain
68.9
0.28
Nickel-chromium alloy
200
0.33
Implant and abutment (Ti-6Al-4V)
110
0.35
Crestal/sinus cortical bone
13.7
0.30
Trabecular bone (D4)
1.10
0.30
Graft material (completely matured)
3.45
0.31
Table 2 Numbers of Elements and Nodes in Each Model Model
No. of elements
No. of nodes
Model 1
109,320
23,400
Model 2
316,213
56,890
Model 3
308,984
56,173
A D4 bone structure was defined for the modeled trabecular bone according to Misch’s classification.33 All of the remaining surfaces of the implants inside the maxillary sinus were coated with grafting material with the use of the “complete peri-implant packing” technique.34 It was assumed that the maxillary sinus grafted bone was fully matured and mineralized.3 The geometry of the simulated crowns was described by Wheeler and Ash.35 The simulated FPD denture consisted of nickelchromium (Remanium CS, Dentaurum) as the framework material and feldspathic porcelain (Ceramco II, Dentsply) for the restorative material. The thicknesses of the metal and porcelain used in this study were 0.8 mm and 2 mm, respectively.29 For all three models, distances of 3 mm and 1.5 mm were left between the two implants and the implant neck–vestibular crestal cortical bone, respectively.36 In model 1, a short implant was placed to provide bicortical fixation from crestal cortical to sinus cortical bone tissue; in model 2, long implants were placed parallel to the axial axis in trabecular and grafted bone tissue; and in model 3, an inclined implant was placed at an angle of 45 degrees relative to the axial axis in trabecular bone.
All materials were presumed to be linearly elastic, homogenous, and isotropic.1,3,29 The interface between the implant and bone was bonded to simulate ideal osseointegration. The interface conditions between the FPD and the abutment, the native bone (crestal cortical, trabecular, and sinus cortical), and the grafted bone were also set as bonded interfaces.3 The corresponding elastic properties such as Young’s modulus and Poisson ratios were determined from values obtained from the literature29 and are summarized in Table 1. For all models, a static oblique force of 300 N37 was applied to each crown at 30 degrees relative to the long axis of the implants in a palatobuccal direction.38 The occlusal load on the crown was divided into equal parts according to the cusp contact/marginal ridge relationship to ensure point loading (second premolar, 300 N/2 = 150 N; first molar, 300 N/5 = 60 N; second molar, 300 N/4 = 75 N) (Fig 3). To obtain results that would be as realistic as possible, four-noded tetrahedral solid elements were used and distributed to all 3D structures homogenously. In all, the numbers of elements ranged from 109,320 to 382,103 and the numbers of nodes ranged from 23,400 to 60,445 for all models (Table 2). Stress analyses were performed on treatment models with Algor Fempro (ALGOR), a 3D FEA program. The stresses that occurred at the implants and supporting tissues were measured and compared by means of von Mises and minimum principal stresses (in megapascals). The results of the FEAs do not have a variance; therefore, there was no need to perform statistical analyses.
Results The properties of the materials were decisive in influencing the obtained results (Table 3). Under occlusal loading, the stresses in the native bone simulants were notably higher than those in the graft simulants. Compared to the minimum principal (compressive) stresses for supporting tissues, the highest stress value was seen within the crestal cortical bone (49.761 MPa) and the lowest stress value was seen within the grafted bone (0.049 MPa) in all models (Fig 4).
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Table 3 Maximum Values for Minimum Principal Stresses and Maximum Von Mises Stresses According to Implant Locations and Supporting Tissue Type for Each Model Treatment model
Supporting tissue
Model 1
Crestal cortical Trabecular
Model 2
34.43845
Implant locations
Implant diameter × length (mm), position
Maximum von Mises stress (MPa)
Second premolar
5 × 11 mm, straight
292.48
4.34759
Sinus cortical
12.89765
Second molar
6 × 5 mm, straight
219.63
Crestal cortical
46.26138
Second premolar
5 × 11 mm, straight
386.98
Second molar
5 × 11 mm, straight
378.51
Second premolar
5 × 11 mm, straight
376.39
Second molar
5 × 11 mm, inclined
499.50
Trabecular Sinus cortical Graft Model 3
Minimum principal stress max (MPa)
Crestal cortical Trabecular
2.36135 14.14472 0.04991 49.76194 2.55451
N/mm2 0 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10
Model 1
N/mm2 0 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10
Model 2 N/mm2
Model 3
Stresses in the cortical bone tissues (both in the crest and in the sinus) were higher than those in the surrounding tissues, but they were significantly higher than those in only the crestal cortical bone area. This behavior was shown for all simulations. The simulated results show that when force is applied in the palatal direction, the buccal region displays stress in all models. This finding reveals that loading direction and compressive stresses region contrasted with each other, as usual. The maximum compressive stress values were examined on the buccal surface of the second molar implant area in each of the bone types; they were seen in the crestal cortical bone in model 3 (49.761 MPa), in the trabecular bone in model 1 (4.347 MPa), in the sinus cortical bone in model 2 (14.144 MPa), and in the graft material in model 2 (0.049 MPa) (Fig 5). Although graft material
0 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10
Fig 4 Minimum principal stress distribution on supporting tissues (N/mm2) in (top to bottom) model 1, model 2, and model 3. a = crestal cortical bone tissue; b = trabecular bone tissue; c = sinus cortical bone tissue; d = graft material.
and longer implants were used in model 2 and model 3, model 1 showed the lowest overall compressive stresses in the crestal cortical bone tissue. The results of this study indicated that higher stresses occurred in the stiffer bone simulants around the implant. The maximum von Mises stresses for titanium implants were recorded. Von Mises stresses were concentrated in the necks of the implants in all three models. Compared to the von Mises stress for all implants (Fig 6), the 45-degree-inclined implant in model 3 showed the highest stress value (499.505 MPa). Analysis of the maximum von Mises stress values on the necks ofthe second premolar and second molar implants were 292.48 MPa and 219.63 MPa in model 1, 386.98 MPa and 378.51 MPa in model 2, and 376.39 MPa and 499.50 MPa in model 3, respectively (Fig 7). The International Journal of Oral & Maxillofacial Implants e5
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Stresses (MPa)
60 34.43845
40 30 20
14.14472
12.89765 4.34759
4.36135
0.04991
Crestal Trabecular Sinus cortical cortical
Crestal Trabecular Sinus cortical cortical
Graft
Model 1
Model 2
10 0
60 Stresses (MPa)
49.76194
46.26138
50
Crestal Trabecular cortical Model 3
Model 1
46.26138 49.76194
50
2.55451
Model 2 34.43845
40
Model 3
30 12.89765 14.14472
20 10
4.34759
0
Crestal cortical bone
2.36135
Trabecular bone
2.55451
0.04991
Sinus cortical bone
Graft
Fig 5 Graphical representations of the minimum principal stress values for all models (top) according to the different treatment designs and (bottom) according to supporting tissues. Model 1
Model 2
Model 3
N/mm2 200 180 160 140 120 100 80 60 40 20 0
Fig 6 Maximum von Mises stresses on the implants (N/mm2).
Second molar implant Second premolar implant 499.50
Model 3
376.39 378.51 386.98
Model 2
219.63 292.48
Model 1 1
201
401
601
Fig 7 Von Mises stress values for implants in the different models.
Discussion A better understanding of implant biomechanics in the posterior maxilla might result in better treatment planning and outcomes. The effect of grafted bone on implant stability and survival rates in the posterior maxilla is not entirely clear. The most important factors in the outcome according to FEA studies are the mechanical properties of the materials used, such as the modulus of elasticity and Poisson ratio. Only a few studies are available that define the mechanical properties and stress distribution of graft materials.1,3,9,27,30,32 It should be noted that the stiffness of the grafted sinus could be lower or higher than that of the cancellous bone, depending on the graft material and maturation process. Fanuscu et al1 reported that mature graft material showed a homogenous distribution
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of stress at the end of a sufficient recovery period. The present study assumed that the maturation process was complete, and stiffness of the graft was assumed to be greater in light of the available literature. In the present study, the long, inclined implant (model 3) showed the highest stress values in crestal cortical bone in all models. In addition, for the sinus cortical bone, given the incorporation of grafted tissue, the observation of lower stress values in model 2 versus model 1 may lead to further questions regarding the stress absorption capability of graft material. At the same time, although additional support was achieved with the use of graft material and increased implant length in models 2 and 3, model 1 showed the lowest compressive stresses in all supporting bone tissues. This finding could reveal the importance of bicortically anchored short but wide implants in native bone in stress absorption, as shown by many publications.3,6,8,14,26,30,31 In crestal cortical and sinus cortical bone, observed stress values were higher than in trabecular bone tissue. This finding can be explained by the fact that both crestal cortical bone and sinus cortical bone, because they have a higher modulus of elasticity than trabecular bone, are stronger and more resistant to deformation. For this reason, cortical bone will bear a greater load than trabecular bone in clinical situations.9,20,29 This study provides an initial attempt to demonstrate the interactions of bone tissues of varying densities with a loaded implant. It may be assumed that, with respect to its ability to absorb stress, an implant in native bone tissue is more effective than an implant in grafted tissue. In support of the results seen in the present study, Huang et al3 reported that “making the implant longer in grafted bone did not reduce the stress in native bone, but it did decrease the stress in grafted bone.”3 Also, Chang et al30 emphasized that, in the edentulous posterior maxilla, the use of a long implant in the grafted sinus decreased implant stress but increased the bone stress/strain values relative to monocortically or bicortically engaged short and wide implants. Schuller-Götzburg et al9 reported that stresses in a cortical bone block graft were higher than those in the surrounding particulated bone during loading, and they claimed that the inserted bone graft block absorbed stresses. Huang et al32 reported that increasing the stiffness of grafted bone reduced peak bone stresses by 10% to 12% during angulated (30 degrees relative to the vertical axis) loading compared with that of low-stiffness grafted bone. To increase the stress absorption capacity of the graft material in the present study, the stiffness of a completely mature graft was described as high (elastic modulus of 3,450 MPa), as mentioned earlier by Huang et al.32 However, the grafted material was not stiffer than cortical bone tissue, as
described by Schuller-Götzburg et al.9 The conclusion of Fanuscu et al27 should be kept in mind, that “the quality of the graft is great concern when the quantity of existing maxillary bone is limited.” Unfortunately, in the present study, the grafted material did not show the expected effect; the results of this study revealed that stiffness of the graft material could result in differences in the response of the graft to occlusal loading. In another aspect, it has been demonstrated that in many situations, the 2 to 3 mm at the most coronal part of the implant are responsible for the transmission of maximum load to the supporting tissue29,39; these findings may be interpreted as a rationale for selecting short and wide implants, provided that they are well anchored in the residual bone.15 Ivanoff et al40 reported that, to ensure a significant reduction in stresses around the implant, it is critical to provide an implant with a diameter of 6 mm. The results of the current study are in agreement with the findings of these investigators. However, it has been reported that the use of wide implants may increase the risk of failure.3,41 Therefore, consideration of the width of the supporting bone is essential before a wider implant is placed. Also, in case of possible crestal bone resorption, the biomechanical advantage of a long implant should not be overlooked. Several studies4,5,42,43 indicated that inclination of an implant by 30 to 35 degrees was clinically acceptable in the posterior maxilla. A recently published article44 claimed that tilted implants that were splinted to support fixed prostheses had no more marginal bone loss than axial implants, but in the present study, a long implant placed at a 45-degree angle showed the highest values for minimum principal and von Mises stresses of all implants. This finding is in agreement with the study of Lan et al,45 which showed that distal inclination of implant bodies, especially in the second molar area, provided the poorest results. Perhaps this situation may have been caused by the high degree of angulation. It has been reported that implants should be placed on the same axial plane with the prosthesis.45,46 Several studies3,6,8,26,30 have advocated that bicortical fixation could increase implant success rates. Rather than placing a long implant at a 45-degree angle in native bone, placement of a short but wide implant with bicortical fixation or the use of a long implant with sinus grafting appears to be more effective in decreasing stress in the supporting tissues and implants, which may increase the likelihood of successful treatment. On the other hand, it should be kept in mind that long implants in the grafted sinus or placed at an inclination in native bone show reduced bending owing to the decreased crown/implant ratio,47 and this advantage is maintained even in the case of crestal resorption that may occur over time. The International Journal of Oral & Maxillofacial Implants e7
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The maximum von Mises stresses seen in the first and second implants were 292.48 MPa and 219.63 MPa, 386.98 MPa and 378.51 MPa, and 376.39 MPa and 499.50 MPa for model 1, model 2, and model 3, respectively. Schuller-Götzburg et al9 mentioned that titanium tolerates stresses ranging up to 900 MPa without irreversible deformation. In light of this information, the stresses seen in all models were within the range of tolerance and thus should not contribute to mechanical implant failure. Considering the differences in stress values for all models, model 1 can be considered as the most appropriate design because of its lower von Mises stress values. While computer modeling offers many advantages over other methods in considering the complexities that characterize clinical situations, the most common disadvantage in the application of FEA in orthopedic studies of biomechanics is extreme sensitivity to the assumptions made regarding model parameters, such as loading conditions, boundary conditions, and material properties.29,48 These are essential limitations of the present study. On the other hand, the design of the occlusal morphology and the surface properties of an implant are other factors that influence stress distribution patterns. In the current study, the morphology of simulated crowns was taken from Wheeler and Ash,35 and the implant design was taken from that of a popular manufacturer. However, the stress patterns could differ with even moderate changes to the occlusal surfaces of the crowns.29 The occlusal morphologies and implant designs used for this study would not be expected to be the same for all posterior implantsupported FPDs. Although a static oblique load has been suggested to represent an occlusal load,26 the presence of chewing movements means that dynamic loading simulations need to be performed in future studies.
Conclusion As a consequence of bicortical anchorage, a short and wide implant may reduce the transmitted stress in the surrounding bone in comparison to long implants placed in the grafted sinus or placed at an angle in native bone. In cases of possible crestal bone resorption, however, the biomechanical advantage of a longer implant should not be overlooked. In the case of limited bone height, model 1, which featured one short and one long implant, was found to be the most feasible approach in the posterior maxilla. Otherwise, when longer implants will be used, rather than an inclined implant (model 3), a grafting solution (model 2) will be more appropriate. The stress absorption capacity of grafting material is not sufficient in comparison with
other supporting tissues. Future investigations of the clinical performance of long implants associated with grafting in the maxillary sinus may be necessary.
Acknowledgment The project was financed by the Department of Scientific Research, Turkish Republic (grant no. NEU/2010-2-11). The authors reported no conflicts of interest related to this study.
REFERENCES 1. Fanuscu MI, Iida K, Caputo AA, Nishimura RD. Load transfer by an implant in a sinus-grafted maxillary model. Int J Oral Maxillofac Implants. 2003;18:667–674. 2. Cawood JI, Howell RA. A classification of the edentulous jaws. Int J Oral Maxillofac Surg 1988;17:232–236. 3. Huang HL, Fuh LJ, Ko CC, Hsu JT, Chen CC. Biomechanical effects of a maxillary implant in the augmented sinus: A three-dimensional finite element analysis. Int J Oral Maxillofac Implants 2009;24:455–462. 4. Krekmanov L, Kahn M, Rangert B, Lindström H. Tilting of posterior mandibular and maxillary implants for improved prosthesis support. Int J Oral Maxillofac Implants 2000;15:405–414. 5. Aparicio C, Perales P, Rangert B. Tilted implants as an alternative to maxillary sinus grafting: A clinical, radiologic, and periotest study. Clin Implant Dent Relat Res 2001;3:39–49. 6. Bahat O. Treatment planning and placement of implants in the posterior maxillae: Report of 732 consecutive Nobelpharma implants. Int J Oral Maxillofac Implants 1993;8:151–161. 7. Blomqvist JE, Alberius P, Isaksson S. Two-stage maxillary sinus reconstruction with endosseous implants: A prospective study. Int J Oral Maxillofac Implants 1998;13:758–766. 8. Brånemark PI, Adell R, Albrektsson T, Lekholm U, Lindström J, Rockler B. An experimental and clinical study of osseointegrated implants penetrating the nasal cavity and maxillary sinus. J Oral Maxillofac Surg 1984;42:497–505. 9. Schuller-Götzburg P, Entacher K, Petutschnigg A, Pomwenger W, Watzinger F. Sinus elevation with a cortical bone graft block: A patient-specific three-dimensional finite element study. Int J Oral Maxillofac Implants 2012;27:359–368. 10. Mertens C, Meyer-Bäumer A, Kappel H, Hoffmann J, Steveling HG. Use of 8-mm and 9-mm implants in atrophic alveolar ridges: 10year results. Int J Oral Maxillofac Implants 2012;27:1501–1508. 11. Fugazzotto PA, Beagle JR, Ganeles J, Jaffin R, Vlassis J, Kumar A. Success and failure rates of 9 mm or shorter implants in the replacement of missing maxillary molars when restored with individual crowns: Preliminary results 0 to 84 months in function. A retrospective study. J Periodontol 2004;75:327–332. 12. Friberg B, Gröndahl K, Lekholm U, Brånemark PI. Long-term follow-up of severely atrophic edentulous mandibles reconstructed with short Brånemark implants. Clin Implant Dent Relat Res 2000;2:184–189. 13. Deporter D, Todescan R, Caudry S. Simplifying management of the posterior maxilla using short, porous-surfaced dental implants and simultaneous indirect sinus elevation. Int J Periodontics Restorative Dent 2000;20:476–485. 14. Chang SH, Lin CL, Hsue SS, Lin YS, Huang SR. Biomechanical analysis of the effects of implant diameter and bone quality in short implants placed in the atrophic posterior maxilla. Med Eng Phys 2012;34: 153–160. 15. Renouard F, Nisand D. Short implants in the severely resorbed maxilla: A 2-year retrospective clinical study. Clin Implant Dent Relat Res 2005;7:104–110. 16. Atieh MA, Zadeh H, Stanford CM, Cooper LF. Survival of short dental implants for treatment of posterior partial edentulism: A systematic review. Int J Oral Maxillofac Implants 2012;27:1323–1331.
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17. Srinivasan M, Vazquez L, Rieder P, Moraguez O, Bernard JP, Belser UC. Efficacy and predictability of short dental implants (< 8 mm): A critical appraisal of the recent literature. Int J Oral Maxillofac Implants 2012;27:1429-1437. 18. Friberg B, Sennerby L, Roos J, Lekholm U. Identification of bone quality in conjunction with insertion of titanium implants. Clin Oral Implant Res 1995;6:213–219. 19. Jensen OT, Shulman LB, Block MS, Iacono VJ. Report of the Sinus Consensus Conference of 1996. Int J Oral Maxillofac Implants 1998; 13:758–766. 20. Cochran DL. The scientific basis for and clinical experiences with Straumann implants including the ITI Dental Implant System: A consensus report. Clin Oral Implants Res 2000;11:33–58. 21. Quirynen M, Naert I, van Steenberghe D. Fixture design and overload influence marginal bone loss and fixture success in the Brånemark system. Clin Oral Implants Res 1992;3:104–111. 22. Jensen OT, Sennerby L. Histologic analysis of clinically retrieved titanium microimplants placed in conjunction with maxillary sinus floor augmentation. Int J Oral Maxillofac Implants 1998;13:513–521. 23. Taylor TD, Agar JR, Vogiatzi T. Implant prosthodontics: Current perspective and future directions. Int J Oral Maxillofac Implants 2000;15:66–75. 24. Iplikcioglu H, Akca K, Cehreli MC, Sahin S. Comparison of non-linear finite element stress analysis with in vitro strain gauge measurements on a Morse taper implant. Int J Oral Maxillofac Implants 2003;18:258–265. 25. Rangert B, Sennerby L, Nilson H. Load factor analysis for implants in the resorbed posterior maxilla. In: Jensen OT (ed). The Sinus Bone Graft. Chicago: Quintessence,1999:167–176. 26. Geng JP, Tan KBC, Liu GR. Application of finite element analysis in implant dentistry: A review of the literature. J Prosthet Dent 2001;85:585–598. 27. Fanuscu MI, Vu HV, Poncelet B. Implant biomechanics in grafted sinus: A finite element analysis. J Oral Implantol 2004;30:59–68. 28. Ismail YH, Pahountis LN, Fleming JF. Comparison of two-dimensional and three-dimensional finite element analysis of a blade implant. J Oral Implantol 1987;4:25–31. 29. Sevimay M, Turhan F, Kılıçarslan MA, Eskitaşcıog˘lu G. Three-dimensional finite element analysis of the effect of different bone quality on stress distribution in an implant supported crown. J Prosthet Dent 2005;93:227–234. 30. Chang SH, Lin CL, Lin YS, Hsue SS, Huang SR. Biomechanical comparison of a single short and wide implant with monocortical or bicortical engagement in the atrophic posterior maxilla and a long implant in the augmented sinus. Int J Oral Maxillofac Implants 2012;27:102–111. 31. Li T, Yang X, Zhang D, et al. Analysis of the biomechanical feasibility of a wide implant in moderately atrophic maxillary sinus region with finite element method. Oral Surg Oral Med Oral Pathol Oral Radiol 2012;114:1–8. 32. Huang HL, Fuh LJ, Hsu JT, Tu MG, Shen YW, Wu CL. Effects of implant surface roughness and stiffness of grafted bone on an immediately loaded maxillary implant: A 3D numerical analysis. J Oral Rehabil 2008;35(4):283–290.
33. Misch CE. Bone character: Second vital implant criterion. Dent Today 1988:39–40. 34. Tepper G, Haas R, Zechner W, Krach W, Watzek G. Three-dimensional finite element analysis of implant stability in atrophic posterior maxilla. Clin Oral Implants Res 2002;13:657–665. 35. Wheeler RC, Ash MM. An Atlas of Tooth Form, ed 4. Philadelphia: Saunders, 1969:68. 36. Misch CE. Dental Implant Prosthetics, ed 9. St Louis: Elsevier Mosby, 2005:109. 37. Mericske-Stern R, Zarb GA. In vivo measurements of some functional aspects with mandibular fixed prostheses supported by implants. Clin Oral Implants Res 1996;7:153–161. 38. Imanishi A, Nakamura T, Ohyama T, Nakamura T. 3-D finite element analysis of all-ceramic posterior crowns. J Oral Rehabil 2003; 30:818–822. 39. Pierrisnard L, Renouard F, Renault P, Barquins M. Influence of implant length and bicortical anchorage on implant stress distribution. Clin Implant Dent Relat Res 2003;5:254–262. 40. Ivanoff CJ, Sennerby L, Johansson C, Rangert B, Lekholm U. Influence of implant diameters on the integration of screw implants. An experimental study in rabbits. Int J Oral Maxillofac Surg 1997; 26:141–148. 41. Eckert SE, Meraw SJ, Weaver AL, Lohse CM. Early experience with wide-platform Mk II implants. Part I: Implant survival. Part II: Evaluation of risk factors involving implant survival. Int J Oral Maxillofac Implants 2001;16:208–216. 42. Nocini PF, Albanese M, Fior A, De Santis D. Implant placement in the maxillary tuberosity: The Summers technique performed with modified osteotomes. Clin Oral Implants Res 2000;11:273–278. 43. Venturelli A. A modified surgical protocol for placing implants in the maxillary tuberosity: Clinical results at 36 months after loading with fixed partial dentures. Int J Oral Maxillofac Implants 1996;11:743–749. 44. Monje A, Chan HL, Suarez F, Galindo-Moreno P, Wang HL. Marginal bone loss around tilted implants in comparison to straight implants: A meta-analysis. Int J Oral Maxillofac Implants 2012;27:1576–1583. 45. Lan TH, Huang HL, Wu JH, Lee HE, Wang CH. Stress analysis of different angulations of implant installation: The finite element method. Kaohsiung J Med Sci 2008;24:138–143. 46. Lin CL, Chang SH, Chang WJ, Kuo YC. Factorial analysis of variables influencing mechanical characteristics of a single tooth implant placed in the maxilla using finite element analysis and the statisticsbased Taguchi method. Eur J Oral Sci 2007;115:408–416. 47. Davarpanah M, Martinez H, Kebir M, Etienne D, Tecucianu JF. Widediameter implants: New concepts. Int J Periodontics Restorative Dent 2001;21:149–159. 48. Sato Y, Wadamoto M, Tsuga K, Teixeira ER. The effectiveness of element downsizing on a three-dimensional finite element model of bone trabeculae in implant biomechanics. J Oral Rehabil 1999;26: 288–291.
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I
n contemporary dentistry, implants are used routinely because they contribute to esthetics in addition to fulfilling the function of missing natural teeth. An appropriate quantity and quality of jawbone at the surgical
1 Assistant
Professor, Department of Prosthodontics, Faculty of Dentistry, Eskis¸ehir Osmangazi University, Eskis¸ehir, Turkey. 2Professor, Department of Prosthodontics, Faculty of Dentistry, Near East University, Mersin, Turkey. 3Associate Professor, Department of Prosthodontics, Faculty of Dentistry, Near East University, Turkey. 4 Assistant Professor, Department of Prosthodontics, Faculty of Dentistry, Cumhuriyet University, Sivas, Turkey. 5Research Assistant, Department of Periodontology, Faculty of Dentistry, Eskis¸ehir Osmangazi University, Eskis¸ehir, Turkey. Correspondence to: Emre S¸eker, Department of Prosthodontics, Faculty of Dentistry, Eskis¸ehir Osmangazi University, Eskis¸ehir, Türkiye. Email: [email protected] ©2014 by Quintessence Publishing Co Inc.
site are important criteria for successful dental implant therapy. Dental implants require sufficient bone to be adequately stabilized, making it challenging to place implants in regions with severe atrophy. Over time, alveolar bone undergoes resorption-related atrophy when teeth have been lost. In the posterior maxilla, the proximity of the maxillary sinus floor to the alveolar bone crest limits implant placement in this area. Conventional implant insertion is not feasible in many patients who require augmentation procedures. Maxillary sinus augmentation has become one of the most common surgical procedures for increasing bone volume for implant placement in the posterior atrophic edentulous maxilla and enabling the placement of longer implants with a higher success rate.1–3 Nevertheless, surgical time, the risk of morbidity, and the cost of this treatment method relative to other alternatives should be taken into account when sinus bone grafting is considered. The International Journal of Oral & Maxillofacial Implants e1
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One alternative treatment modality is the use of inclined implants,4,5 which makes it possible to achieve good implant anchorage by benefiting from more remote available dense bone structures. Another solution is longer implants.3 In the literature, the rate of failure is higher for implants shorter than 7 mm.6,7 An increased implant length could improve bone-to-implant contact, but bone type and engagement of cortical bone may exert more profound effects on implant stability than implant length, especially for bicortically anchored implants.6,8 Bicortical anchorage of implants in crestal bone provides ideal stabilization and fixation, minimizes failure rates during healing, and leads to better osseointegration.9 In addition to these approaches, the use of short implants seems to be an obvious alternative in areas of insufficient bone height. Although in general, short implants were thought to be associated with low success rates, some studies suggest similar clinical success rates for short implants compared with longer ones.9–14 The use of short implants offers a simpler and safer treatment owing to reduced risks of interference with anatomic structures, such as the maxillary sinus.15–17 In the edentulous posterior maxilla, the native bone, which provides primary stability for establishing and maintaining osseointegration, consists of crestal cortical, cancellous, and sinus floor cortical bone tissues.18 The contribution of grafted bone in establishing and maintaining implant stability is not yet well-known. Jensen et al19 achieved a 90% success rate for implants placed in grafted sinuses after 3 years of function. However, there was a statistically significant difference in the implant failure rate when the available bone was 4 mm or less as opposed to 5 mm or more. In addition to bone quantity, the quality of available bone is a decisive factor in treatment planning, implant design, surgical approach, healing time, and prosthetic loading protocol.20,21 Various graft materials and procedural modifications have been proposed to improve the efficacy of the therapy. Autogenous bone, allogeneic materials, alloplastic materials, xenogeneic materials, and combinations of these materials have been utilized.1–3,7,9,19 A complex structure consisting of bone with variable stiffness can be found around implants placed in the posterior maxilla with grafted sinus.1 The load-bearing characteristics of grafted bone depend on the graft material and its maturation process.22 Huang et al3 described two classes of stiffness of grafting material with reference to the maturation process in a finite element analysis (FEA) study. The degree of stiffness was classified as low for lowelastic-modulus tissue (ie, 345 MPa) and high for highelastic-modulus graft tissue (ie, 3,450 MPa). Late implant failures are primarily related to biomechanical complications; thus, the major factor leading
to such failures may be a lack of understanding of biomechanical factors.23,24 The transfer of a load from implants to the surrounding bone depends on the type of loading, the nature of the bone-implant interface, the length and diameter of the implants, angulation of implants in bone, the shape and characteristics of the implant surface, the prosthesis type, and the quantity and quality of the surrounding bone.4,25 Some of these biomechanical factors are inherent with the patients, and others can be controlled by clinicians. Clinical studies provide important information about general trends in biomechanics; however, specific biomechanical variables might be more efficiently examined through in vitro models. Accurately designed and appropriately analyzed in vitro models can be useful in studying biomechanical factors and their relationships. FEA, a numeric modeling system, is routinely used in modern engineering practice to study complex structures and has been long used in implant dentistry research.24–27 Recently, correlation of stress distribution in bone with implant-supported prosthesis design has been investigated primarily by means of twodimensional (2D) and 3D FEA.28 Studies comparing the accuracy of these analyses found that, if detailed stress information is required, then 3D modeling is necessary. The 3D FEA is considered an appropriate method for investigation of the stresses throughout a 3D structure, and therefore, this method was selected for the evaluation of stresses in bone and implants in the present study. There are many studies1,3,9,14,29–32 on single implants in the grafted sinus in FEA, but the efficacy of grafted bone in maintaining an implant-supported fixed partial denture (FPD) is not yet well-known. The purpose of this study was to investigate the biomechanical effects of bicortical fixation and graft tissue through varying the lengths, diameters, and angulations of implants in the atrophic posterior maxilla and to consider the acceptability of various treatment options for an implant-supported FPD.
Materials and Methods A model of the atrophic posterior maxilla was created via cone beam computed tomography (CBCT) images that were obtained for implant planning of an actual patient and were chosen from the tomography database archive previously stored in the Department of Dentomaxillofacial Radiology at Near East University. Images were obtained with a Newtom 3G CBCT device (Quantitative Radiology) at 120 kVP and 3 to 5 mA using a 9-inch field of view, an axial slice thickness of 0.3 mm, and isotropic voxels. A series of CT images at 1-mm intervals of the posterior edentulous maxilla were converted to medical image formats and
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Fig 1 Three-dimensional merging of supporting tissues, implants, and prosthetic structures. Top row, left to right: Maxillary bone tissue with implants; nickel-chromium alloy FPD substructure; completed FPD restoration. Center row, left to right: Schemes of the relationship between implants and the maxillary sinus for model 1, model 2, and model 3, respectively. Bottom row, left to right: Mesh structures of model 1, model 2, and model 3, respectively. Fig 2 Identification of the residual crest layers: sinus cortical bone (red arrows, 0.5 mm thick); trabecular bone (blue arrows, 3.5 mm thick); crestal cortical bone (yellow arrows, 1 mm thick). Total residual crestal bone thickness was 5 mm.
0.5 mm 3.5 mm 1.0 mm
transferred to a personal computer with commercially available image processing software (3D-Doctor, Able Software). The implants (long: 5 × 11 mm; short: 6 × 5 mm) and abutments were supplied by the manufacturer (Bicon) and were scanned with a 3D scanner (Next Engine Inc) on the macro scale. The obtained image data were imported into Rhinoceros 4.0 modeling software (McNeel & Associates). With the use of same
program, three different treatment scenarios were modeled that incorporated FPDs, long and short implants, angulated implants, and grafted and nongrafted maxillary sinuses (Fig 1). Simulated crestal cortical bone, cancellous bone, and sinus floor cortical bone were fabricated in 1-mm, 3.5-mm, and 0.5-mm heights, respectively, according to the study of Fanuscu et al.1 The total height of the simulated native bone was therefore 5.0 mm (Fig 2). The International Journal of Oral & Maxillofacial Implants e3
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Fig 3 Application of 300 N of occlusal force on each crown in the palatobuccal direction at a 30-degree inclination.
Table 1 Mechanical Properties of the Materials Modeled Young’s Poisson modulus (GPa) ratio
Material Feldspathic porcelain
68.9
0.28
Nickel-chromium alloy
200
0.33
Implant and abutment (Ti-6Al-4V)
110
0.35
Crestal/sinus cortical bone
13.7
0.30
Trabecular bone (D4)
1.10
0.30
Graft material (completely matured)
3.45
0.31
Table 2 Numbers of Elements and Nodes in Each Model Model
No. of elements
No. of nodes
Model 1
109,320
23,400
Model 2
316,213
56,890
Model 3
308,984
56,173
A D4 bone structure was defined for the modeled trabecular bone according to Misch’s classification.33 All of the remaining surfaces of the implants inside the maxillary sinus were coated with grafting material with the use of the “complete peri-implant packing” technique.34 It was assumed that the maxillary sinus grafted bone was fully matured and mineralized.3 The geometry of the simulated crowns was described by Wheeler and Ash.35 The simulated FPD denture consisted of nickelchromium (Remanium CS, Dentaurum) as the framework material and feldspathic porcelain (Ceramco II, Dentsply) for the restorative material. The thicknesses of the metal and porcelain used in this study were 0.8 mm and 2 mm, respectively.29 For all three models, distances of 3 mm and 1.5 mm were left between the two implants and the implant neck–vestibular crestal cortical bone, respectively.36 In model 1, a short implant was placed to provide bicortical fixation from crestal cortical to sinus cortical bone tissue; in model 2, long implants were placed parallel to the axial axis in trabecular and grafted bone tissue; and in model 3, an inclined implant was placed at an angle of 45 degrees relative to the axial axis in trabecular bone.
All materials were presumed to be linearly elastic, homogenous, and isotropic.1,3,29 The interface between the implant and bone was bonded to simulate ideal osseointegration. The interface conditions between the FPD and the abutment, the native bone (crestal cortical, trabecular, and sinus cortical), and the grafted bone were also set as bonded interfaces.3 The corresponding elastic properties such as Young’s modulus and Poisson ratios were determined from values obtained from the literature29 and are summarized in Table 1. For all models, a static oblique force of 300 N37 was applied to each crown at 30 degrees relative to the long axis of the implants in a palatobuccal direction.38 The occlusal load on the crown was divided into equal parts according to the cusp contact/marginal ridge relationship to ensure point loading (second premolar, 300 N/2 = 150 N; first molar, 300 N/5 = 60 N; second molar, 300 N/4 = 75 N) (Fig 3). To obtain results that would be as realistic as possible, four-noded tetrahedral solid elements were used and distributed to all 3D structures homogenously. In all, the numbers of elements ranged from 109,320 to 382,103 and the numbers of nodes ranged from 23,400 to 60,445 for all models (Table 2). Stress analyses were performed on treatment models with Algor Fempro (ALGOR), a 3D FEA program. The stresses that occurred at the implants and supporting tissues were measured and compared by means of von Mises and minimum principal stresses (in megapascals). The results of the FEAs do not have a variance; therefore, there was no need to perform statistical analyses.
Results The properties of the materials were decisive in influencing the obtained results (Table 3). Under occlusal loading, the stresses in the native bone simulants were notably higher than those in the graft simulants. Compared to the minimum principal (compressive) stresses for supporting tissues, the highest stress value was seen within the crestal cortical bone (49.761 MPa) and the lowest stress value was seen within the grafted bone (0.049 MPa) in all models (Fig 4).
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Table 3 Maximum Values for Minimum Principal Stresses and Maximum Von Mises Stresses According to Implant Locations and Supporting Tissue Type for Each Model Treatment model
Supporting tissue
Model 1
Crestal cortical Trabecular
Model 2
34.43845
Implant locations
Implant diameter × length (mm), position
Maximum von Mises stress (MPa)
Second premolar
5 × 11 mm, straight
292.48
4.34759
Sinus cortical
12.89765
Second molar
6 × 5 mm, straight
219.63
Crestal cortical
46.26138
Second premolar
5 × 11 mm, straight
386.98
Second molar
5 × 11 mm, straight
378.51
Second premolar
5 × 11 mm, straight
376.39
Second molar
5 × 11 mm, inclined
499.50
Trabecular Sinus cortical Graft Model 3
Minimum principal stress max (MPa)
Crestal cortical Trabecular
2.36135 14.14472 0.04991 49.76194 2.55451
N/mm2 0 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10
Model 1
N/mm2 0 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10
Model 2 N/mm2
Model 3
Stresses in the cortical bone tissues (both in the crest and in the sinus) were higher than those in the surrounding tissues, but they were significantly higher than those in only the crestal cortical bone area. This behavior was shown for all simulations. The simulated results show that when force is applied in the palatal direction, the buccal region displays stress in all models. This finding reveals that loading direction and compressive stresses region contrasted with each other, as usual. The maximum compressive stress values were examined on the buccal surface of the second molar implant area in each of the bone types; they were seen in the crestal cortical bone in model 3 (49.761 MPa), in the trabecular bone in model 1 (4.347 MPa), in the sinus cortical bone in model 2 (14.144 MPa), and in the graft material in model 2 (0.049 MPa) (Fig 5). Although graft material
0 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10
Fig 4 Minimum principal stress distribution on supporting tissues (N/mm2) in (top to bottom) model 1, model 2, and model 3. a = crestal cortical bone tissue; b = trabecular bone tissue; c = sinus cortical bone tissue; d = graft material.
and longer implants were used in model 2 and model 3, model 1 showed the lowest overall compressive stresses in the crestal cortical bone tissue. The results of this study indicated that higher stresses occurred in the stiffer bone simulants around the implant. The maximum von Mises stresses for titanium implants were recorded. Von Mises stresses were concentrated in the necks of the implants in all three models. Compared to the von Mises stress for all implants (Fig 6), the 45-degree-inclined implant in model 3 showed the highest stress value (499.505 MPa). Analysis of the maximum von Mises stress values on the necks ofthe second premolar and second molar implants were 292.48 MPa and 219.63 MPa in model 1, 386.98 MPa and 378.51 MPa in model 2, and 376.39 MPa and 499.50 MPa in model 3, respectively (Fig 7). The International Journal of Oral & Maxillofacial Implants e5
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Stresses (MPa)
60 34.43845
40 30 20
14.14472
12.89765 4.34759
4.36135
0.04991
Crestal Trabecular Sinus cortical cortical
Crestal Trabecular Sinus cortical cortical
Graft
Model 1
Model 2
10 0
60 Stresses (MPa)
49.76194
46.26138
50
Crestal Trabecular cortical Model 3
Model 1
46.26138 49.76194
50
2.55451
Model 2 34.43845
40
Model 3
30 12.89765 14.14472
20 10
4.34759
0
Crestal cortical bone
2.36135
Trabecular bone
2.55451
0.04991
Sinus cortical bone
Graft
Fig 5 Graphical representations of the minimum principal stress values for all models (top) according to the different treatment designs and (bottom) according to supporting tissues. Model 1
Model 2
Model 3
N/mm2 200 180 160 140 120 100 80 60 40 20 0
Fig 6 Maximum von Mises stresses on the implants (N/mm2).
Second molar implant Second premolar implant 499.50
Model 3
376.39 378.51 386.98
Model 2
219.63 292.48
Model 1 1
201
401
601
Fig 7 Von Mises stress values for implants in the different models.
Discussion A better understanding of implant biomechanics in the posterior maxilla might result in better treatment planning and outcomes. The effect of grafted bone on implant stability and survival rates in the posterior maxilla is not entirely clear. The most important factors in the outcome according to FEA studies are the mechanical properties of the materials used, such as the modulus of elasticity and Poisson ratio. Only a few studies are available that define the mechanical properties and stress distribution of graft materials.1,3,9,27,30,32 It should be noted that the stiffness of the grafted sinus could be lower or higher than that of the cancellous bone, depending on the graft material and maturation process. Fanuscu et al1 reported that mature graft material showed a homogenous distribution
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of stress at the end of a sufficient recovery period. The present study assumed that the maturation process was complete, and stiffness of the graft was assumed to be greater in light of the available literature. In the present study, the long, inclined implant (model 3) showed the highest stress values in crestal cortical bone in all models. In addition, for the sinus cortical bone, given the incorporation of grafted tissue, the observation of lower stress values in model 2 versus model 1 may lead to further questions regarding the stress absorption capability of graft material. At the same time, although additional support was achieved with the use of graft material and increased implant length in models 2 and 3, model 1 showed the lowest compressive stresses in all supporting bone tissues. This finding could reveal the importance of bicortically anchored short but wide implants in native bone in stress absorption, as shown by many publications.3,6,8,14,26,30,31 In crestal cortical and sinus cortical bone, observed stress values were higher than in trabecular bone tissue. This finding can be explained by the fact that both crestal cortical bone and sinus cortical bone, because they have a higher modulus of elasticity than trabecular bone, are stronger and more resistant to deformation. For this reason, cortical bone will bear a greater load than trabecular bone in clinical situations.9,20,29 This study provides an initial attempt to demonstrate the interactions of bone tissues of varying densities with a loaded implant. It may be assumed that, with respect to its ability to absorb stress, an implant in native bone tissue is more effective than an implant in grafted tissue. In support of the results seen in the present study, Huang et al3 reported that “making the implant longer in grafted bone did not reduce the stress in native bone, but it did decrease the stress in grafted bone.”3 Also, Chang et al30 emphasized that, in the edentulous posterior maxilla, the use of a long implant in the grafted sinus decreased implant stress but increased the bone stress/strain values relative to monocortically or bicortically engaged short and wide implants. Schuller-Götzburg et al9 reported that stresses in a cortical bone block graft were higher than those in the surrounding particulated bone during loading, and they claimed that the inserted bone graft block absorbed stresses. Huang et al32 reported that increasing the stiffness of grafted bone reduced peak bone stresses by 10% to 12% during angulated (30 degrees relative to the vertical axis) loading compared with that of low-stiffness grafted bone. To increase the stress absorption capacity of the graft material in the present study, the stiffness of a completely mature graft was described as high (elastic modulus of 3,450 MPa), as mentioned earlier by Huang et al.32 However, the grafted material was not stiffer than cortical bone tissue, as
described by Schuller-Götzburg et al.9 The conclusion of Fanuscu et al27 should be kept in mind, that “the quality of the graft is great concern when the quantity of existing maxillary bone is limited.” Unfortunately, in the present study, the grafted material did not show the expected effect; the results of this study revealed that stiffness of the graft material could result in differences in the response of the graft to occlusal loading. In another aspect, it has been demonstrated that in many situations, the 2 to 3 mm at the most coronal part of the implant are responsible for the transmission of maximum load to the supporting tissue29,39; these findings may be interpreted as a rationale for selecting short and wide implants, provided that they are well anchored in the residual bone.15 Ivanoff et al40 reported that, to ensure a significant reduction in stresses around the implant, it is critical to provide an implant with a diameter of 6 mm. The results of the current study are in agreement with the findings of these investigators. However, it has been reported that the use of wide implants may increase the risk of failure.3,41 Therefore, consideration of the width of the supporting bone is essential before a wider implant is placed. Also, in case of possible crestal bone resorption, the biomechanical advantage of a long implant should not be overlooked. Several studies4,5,42,43 indicated that inclination of an implant by 30 to 35 degrees was clinically acceptable in the posterior maxilla. A recently published article44 claimed that tilted implants that were splinted to support fixed prostheses had no more marginal bone loss than axial implants, but in the present study, a long implant placed at a 45-degree angle showed the highest values for minimum principal and von Mises stresses of all implants. This finding is in agreement with the study of Lan et al,45 which showed that distal inclination of implant bodies, especially in the second molar area, provided the poorest results. Perhaps this situation may have been caused by the high degree of angulation. It has been reported that implants should be placed on the same axial plane with the prosthesis.45,46 Several studies3,6,8,26,30 have advocated that bicortical fixation could increase implant success rates. Rather than placing a long implant at a 45-degree angle in native bone, placement of a short but wide implant with bicortical fixation or the use of a long implant with sinus grafting appears to be more effective in decreasing stress in the supporting tissues and implants, which may increase the likelihood of successful treatment. On the other hand, it should be kept in mind that long implants in the grafted sinus or placed at an inclination in native bone show reduced bending owing to the decreased crown/implant ratio,47 and this advantage is maintained even in the case of crestal resorption that may occur over time. The International Journal of Oral & Maxillofacial Implants e7
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S¸eker et al
The maximum von Mises stresses seen in the first and second implants were 292.48 MPa and 219.63 MPa, 386.98 MPa and 378.51 MPa, and 376.39 MPa and 499.50 MPa for model 1, model 2, and model 3, respectively. Schuller-Götzburg et al9 mentioned that titanium tolerates stresses ranging up to 900 MPa without irreversible deformation. In light of this information, the stresses seen in all models were within the range of tolerance and thus should not contribute to mechanical implant failure. Considering the differences in stress values for all models, model 1 can be considered as the most appropriate design because of its lower von Mises stress values. While computer modeling offers many advantages over other methods in considering the complexities that characterize clinical situations, the most common disadvantage in the application of FEA in orthopedic studies of biomechanics is extreme sensitivity to the assumptions made regarding model parameters, such as loading conditions, boundary conditions, and material properties.29,48 These are essential limitations of the present study. On the other hand, the design of the occlusal morphology and the surface properties of an implant are other factors that influence stress distribution patterns. In the current study, the morphology of simulated crowns was taken from Wheeler and Ash,35 and the implant design was taken from that of a popular manufacturer. However, the stress patterns could differ with even moderate changes to the occlusal surfaces of the crowns.29 The occlusal morphologies and implant designs used for this study would not be expected to be the same for all posterior implantsupported FPDs. Although a static oblique load has been suggested to represent an occlusal load,26 the presence of chewing movements means that dynamic loading simulations need to be performed in future studies.
Conclusion As a consequence of bicortical anchorage, a short and wide implant may reduce the transmitted stress in the surrounding bone in comparison to long implants placed in the grafted sinus or placed at an angle in native bone. In cases of possible crestal bone resorption, however, the biomechanical advantage of a longer implant should not be overlooked. In the case of limited bone height, model 1, which featured one short and one long implant, was found to be the most feasible approach in the posterior maxilla. Otherwise, when longer implants will be used, rather than an inclined implant (model 3), a grafting solution (model 2) will be more appropriate. The stress absorption capacity of grafting material is not sufficient in comparison with
other supporting tissues. Future investigations of the clinical performance of long implants associated with grafting in the maxillary sinus may be necessary.
Acknowledgment The project was financed by the Department of Scientific Research, Turkish Republic (grant no. NEU/2010-2-11). The authors reported no conflicts of interest related to this study.
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