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Effect of increased crown height on stress distribution in short dental implant components and their surrounding bone: A finite element analysis
RESEARCH AND EDUCATION
Effect of increased crown height on stress distribution in short dental implant components and their surrounding bone: A finite element analysis Haddad Arabi Bulaqi, MSc,a Mahmoud Mousavi Mashhadi, PhD,b Hamed Safari, DDS, MS,c Mohammad Mahdi Samandari, MSc,d and Farideh Geramipanah, DDS, MSe The use of short implants is ABSTRACT increasing as a result of the Statement of problem. Implants in posterior regions of the jaw require short dental implants with ease of use when anatomic limlong crown heights, leading to increased crown-to-implant ratios and mechanical stress. This can itations prevent conventional lead to fracture and screw loosening. implant placement. However, Purpose. The purpose of this study was to investigate the dynamic nature and behavior of prosshort implants might cause thetic components and preimplant bone and evaluate the effect of increased crown height space biomechanical complications,1 (CHS) and crown-to-implant ratio on stress concentrations under external oblique forces. especially if used with long Material and methods. The severely resorbed bone of a posterior mandible site was modeled with crowns. The crown-to-implant Mimics and Catia software. A second mandibular premolar tooth was modeled with CHS values of ratio (CIR) can affect dental im8.8, 11.2, 13.6, and 16 mm. A Straumann implant (4.1×8 mm), a directly attached crown, and an plants and bone tissue.1-4 Unabutment screw were modeled with geometric data and designed by using SolidWorks software. der oblique forces, increases in Abaqus software was used for the dynamic simulation of screw tightening and the application of an external load to the buccal cusp at a 75.8-degree angle with the occlusal plane. The distribution of CIR contribute to stress accu1-3,5 screw load and member load at each step was compared, and the stress values were calculated mulation. Another relevant within the dental implant complex and surrounding bone. concept, crown height space (CHS), is measured from the Results. During tightening, the magnitude and distribution of the preload and clamp load were uniform and equal at the cross section of all CHSs. Under an external load, the screw load decreased crest of the bone to the occlusal/ 4,6 and member load increased. An increase in the CHS caused the corresponding distribution to incisal planes. The biomebecome more nonuniform and increased the maximum compressive and tensile stresses in the chanics of CHS are related preimplant bone. Additionally, the von Mises stress decreased at the abutment screw and increased to lever arm mechanics.4,7 An at the abutment and fixture. increased CHS causes an exConclusions. Under nonaxial forces, increased CHS does not influence the decrease in screw load or cessive vertical and horizontal increase in member load. However, it contributes to screw loosening and fatigue fracture by skewing cantilever, which could conthe stress distribution to the transverse section of the implant. (J Prosthet Dent 2015;-:---) centrate stress at the implant In a retrospective study, Kim et al12 observed that neck by the lever mechanism4,7 and cause biomechanical screw loosening was significantly correlated with the complications, such as implant fracture, implant failure, or mesiodistal cantilever in single molar implants. Tenscrew loosening.3,8,9 sioning the screw creates a preload that is proportional to Screw loosening is one of the most common problems the tightening torque. To achieve a stable and secure associated with fixed implant-supported prostheses.10,11 a
Graduate student, Department of Mechanical Engineering, School of Mechanics, University of Tehran, Tehran, Iran. Professor, Department of Mechanical Engineering, School of Mechanics, University of Tehran, Tehran, Iran. Assistant Professor, Department of Periodontics, School of Dentistry, Qom University of Medical Sciences, Qom, Iran. d Associate Professor, Implant Research Center, Department of Mechanical Engineering, School of Mechanics, University of Tehran, Tehran, Iran. e Graduate student, Department of Mechanical Engineering, School of Mechanics, University of Tehran, Tehran, Iran. b c
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Clinical Implications Under oblique loads, the use of long crown heights not only increases the stress at the peripheral bone and implant compartments but also causes biomechanical disadvantages, thereby elevating the risk of screw loosening. The use of a suitable crown height could reduce the incidence of biologic and biomechanical complications. joint, the preload should be greater than the external loads.13,14 Before the application of the external compressive load (P), preload (Fi ) and clamp load (-Fi ) are equal but in opposite directions. Upon applying the external load (P), a fraction of the external load (C) is carried by the abutment screw, and the rest (1−C) is carried by the fixture and the abutment, which are collectively referred to as members. Therefore, the resultant load on the abutment screw is Fb (screw load, Eq. (1)), and the resultant load on the members is Fm (member load, Eq. (2)). Note that C is called the stiffness constant of the joint and depends on the geometrical parameters, design, and material properties of the abutment screw and members.15 Fb =−Pb +Fi =−ðC×PÞ+Fi
(1)
Fm =−Pm −Fi =−ðð1−CÞ×PÞ−Fi ;
(2)
where P=External compressive load =Pb +Pm ; Pb =Portion of P taken by bolt=C×P, and Pm =Portion of P taken by members=ð1−CÞ×P Few biomechanical studies have evaluated crown height.3-5 Moreover, few studies have investigated the CIR, and the ones that have are considered controversial.5 Additionally, a potentially more critical factor is CHS,5 because an increase in crown height might be more problematic than an equal reduction in implant length.6 Therefore, CHS alone is more important than the CIR and should thus be studied independently. Yet only a few studies have surveyed the impact of crown height as a factor independent of fixture length.6,8 Furthermore, the effect of force on preloaded implants,10 screw loosening, and prosthetic complications3 has also been assessed by few studies. Finally, the literature lacks studies on the effects of different CIR or CHS values on stress/strain levels and distributions in short implant applications. Investigating the factors that influence stress distribution within the implant complex can provide necessary information to facilitate the design of components with better joint stability and stress distribution. Finite element analysis is an engineering method that is used to calculate stresses and strains within a solid body, thereby making it useful for the resultant and THE JOURNAL OF PROSTHETIC DENTISTRY
Figure 1. Cross-sectional view of three-dimensional computer-aided design models of implant (violet), direct abutment (orange), abutment screw (yellow), metal frame (blue), porcelain (green), and composite resin (red).
contact forces on dental implant components. However, finite element analysis is considered a qualitative and approximation method that is can be used to better recognize a problem. The aim of this study was to investigate the effect of different CHS values on the stress distribution in short dental implant components under oblique loads. Moreover, the effect of different CHS values on the distribution and magnitude of screw load and member load and the compressive and tensile stresses in the surrounding bone was also investigated. MATERIAL AND METHODS In order to model the severely resorbed posterior mandible site, a cone beam computed tomography image and Digital Imaging and Communications in Medicine (DICOM) file were generated. Mimics 10.01 software (Materialise Group) was used to create the cloud points and demarcate the trabecular bone (150 to 900 Hounsfield units [HU]), cortical bone (900 to 1800 HU), and the primary form of the crown (2500 to 3000 HU).9 The cloud points were transformed by the Digitized Shape Editor of Catia v5 R19 (Dassault Systèmes), and computer-aided design files were modeled. By using the primary form of the teeth and proper scaling in all directions, 4 tooth models with CHS values of 8.8, 11.2, 13.6, and 16 mm were modeled (Fig. 1). The resulting CIR values were 1.1, 1.4, 1.7, and 2.0 for an 8 mm short implant. Because the metal frame was simultaneously cast with the abutment, the abutment and the frame models were integrated. The composite material was used to cover the abutment screw access hole. A micrometer projector was used to obtain the geometry and dimensions of a 4.1×8 mm Straumann implant (SLA 043.031S; Institut Straumann), the directly attached crown (048.642, RN SynOcta gold abutment), and the abutment screw (048.356, SynOcta basal screw) (Fig. 1). The models were constructed by using software Bulaqi et al
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Figure 2. Three-dimensional finite element method models of implant complex in embedding cortical and trabecular bones at different crown height spaces (CHS). Loading of implants occurred in axial, lingual, and mesiodistal direction.
(SolidWorks; Dassault Systèmes). The implant components were modeled precisely to simulate the process of screw tightening, preload creation, and osseointegration. The outer surface of the abutment screw and implant were geometrically modeled with a continuous spiral threaded helix, and the internal surface of the implant and bone bore were geometrically modeled with a continuous spiral threaded bore. The implant thread pitch was 1.25 mm, and the abutment screw thread pitch was 0.4 mm. Contact interface and tangential behavior with a specific coefficient of friction were considered for the reciprocal interfacing surfaces. More specifically, the coefficient of kinetic friction (mk ) of 0.12 was applied to simulate the screw tightening, and for the functional condition, the value for the coefficient of static friction was ms =0:16; which was slightly larger than the value for the coefficient of kinetic friction.16,17 The initial condition of the abutment screw within the implant complex was snug. The implant was positioned at the second premolar site. The mesial-axial inclination and the lingual-axial inclination of the implant were both 9 degrees.18 The three-dimensional finite element method model of the implant complex at the 4 different CHS values were shown inserted in the bone (Fig. 2). The bone was sectioned and bounded at the sites depicted in Figure 2 in order to reduce the number of elements considered in the analysis. Because complete osseointegration was supposed to be achieved, the bone-implant contact surface was defined as a “tie.” Software (Abaqus 6.11; Dassault Systèmes Simulia Corp) was used for explicit dynamic simulation. Tetrahedral elements, the free meshing technique, and linear geometric order were used for meshing. The total number of elements used for each model is presented in Table 1. The mechanical behavior of the implant component material was assumed to be in both the elastic and plastic regions. Also, the mechanical properties of the surrounding bone were assumed to be the linear Bulaqi et al
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elastic region. The mechanical properties of materials are presented in Table 2.19-21 All materials were assumed to be both isotropic and homogenous. Three external loads of 114.6 N (axial), 17.1 N (buccal-lingual), and 23.4 N (mesiodistal) were applied to the coronal part of the buccal cusp (Fig. 3). The equivalent of the combination of these loads is a 118.2 N load at 75.8 degrees in relation to the occlusal plane.22 The radius of the load area was 0.4 mm. The analysis included 2 steps. First, the wrench turned the screw until a recommended torque of 35 Ncm was applied and the implant complex was clamped. In the next step, the external load was applied for 0.5 seconds (Fig. 3). The time frequency of the load is an important factor in dynamic analysis. This time was chosen based on the study by Po et al,23 which indicates that each mastication cycle lasts approximately 0.5 seconds (2 Hz). The relationships of the created total torque (Tt ), conical torque (Tc ), thread torque (Tth ), preload (Fi ), screw load (Fb ), and member load (Fm ) with respect to time at 4 CHS values are presented. The maximum amount of external loading influencing the screw load and member load are also shown. The criterion of the von Mises stress for ductile materials, such as titanium and gold, and maximum-normal stress theory for brittle materials, such as cortical and trabecular bone and porcelain, were used.15 The amounts of maximum compressive and tensile stress with respect to time for all CHS values were presented for the surrounding bone in each step of the analysis. Likewise, the amount of von Mises stress at the zone where the maximum influence of external load occurs was presented with respect to time for the abutment screw, abutment, and fixture in all steps. RESULTS The created total, conical, and thread torques with respect to time in 2 steps for all CHS values are presented in Figure 4. The maximum amount of total torque was 35 Ncm, which is the manufacturer’s recommended torque value (step 1). Additionally, at this step, the conical torque was 26.13 Ncm, and the thread torque was 8.87 Ncm. By removing the wrench and applying the external load (step 2), total torque was completely eliminated. The minimum conical torque was -7.03 Ncm, and the minimum thread torque was 7.03 Ncm. Figure 5 presents the amount of load changes in the screw (preload, step 1), the screw load (step 2), the members (clamp load, step 1), and the member load (step 2) with respect to time for all the CHS values. The preload (Fi , tensile) and clamp load (−Fi , compressive) at the end of the screw tightening process were each 500 N. In the second step, an external load was applied in the same direction as the clamp load and the opposite THE JOURNAL OF PROSTHETIC DENTISTRY
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Table 1. Number of tetrahedral elements for each part in models of 4 different CHS CHS (mm)
CIR
Cortical Bone
Trabecular Bone
Abutment Screw
Fixture
Composite
Frame+Abutment
8.8
1.1
39700
55947
32303
78774
1640
19350
Porcelain 24123
11.2
1.4
-
-
-
-
1685
25961
29229
13.6
1.7
-
-
-
-
1713
34098
38889
16
2.0
-
-
-
-
1745
45349
40954
CHS, crown height space; CIR, crown-to-implant ratio.
direction of the preload; consequently, the minimum amount of screw load (Fb ) was 478 N, and maximum amount of member load (Fm ) was -590 N. Figure 6 shows the distribution of the preload and screw load at the transverse section of the abutment screw, and the distribution of the clamp load and member load at the transverse section of the fixture-abutment contact surface with respect to the neutral axis. The minimum, midrange, and maximum values of screw load and member load at the transverse section are presented in Table 3. Figure 7 presents the changes in the compressive and tensile stress values, otherwise known as the Min and Max principle stress values, with respect to time in the surrounding bone during steps 1 and 2 for 4 different crown heights. Table 4 lists the maximum amounts of Min and Max principle stresses for 4 CHS values in the 2 simulated steps. The graphs for the changes in the von Mises stress in the abutment screw, abutment, and fixture with respect to time during the screw tightening process and the application of an external load at 4 different crown heights are presented in Figure 8. Table 4 presents the values of the von Mises stress for the abutment screw, abutment, and fixture at the 4 different CHS values in both simulated steps. DISCUSSION Implant success not only depends on proper osseointegration but also on biomechanical aspects.8 Excessive crown heights can increase the amount of force and stress applied to the implant and the supporting bone through the lever mechanism.3,7 Such increased stresses can lead to crestal bone loss, screw loosening, and implant fracture.9 Additionally, larger crown dimensions with shorter implant lengths might cause greater stress.2 A shorter implant has a greater CIR when compared to a longer implant with a constant crown height. Therefore, complications should be presumably anticipated in short implants. However, previous research has shown that short implants are clinically successful regardless of the CIR.24 A finite element study concluded that increasing the crown height could be destructive to the stress distribution on the screw, especially under oblique loading.3 However, in an investigation of 326 implants with a mean CIR of 1.6, Urdaneta et al25 observed that excessive CIR had no negative effect on the periimplant bone loss but caused more significant THE JOURNAL OF PROSTHETIC DENTISTRY
Table 2. Mechanical properties of materials used for finite element analysis
Material Component
Young Modulus Poisson Density Strength Elongation (GPa) Ratio (g/cm) (MPa) (%)
Gold abutment+frame*
136
0.37
17.5
765
10 Min
Titanium grade 4*
110
0.34
4.5
550
15 Min
13.7
0.30
3
190
2 Max
1.37
0.30
3
10
2 Max
68.9
0.28
2.44
145
2 Max
7
0.20
2.3
480
2 Max
Cortical bone
19
Trabecular bone
19
Porcelain20 Composite resin
21
*According to the manufacturer specifications.
prosthetic complications, such as screw loosening and porcelain fracture. Screw loosening, implant component fracture, and bone resorption are influenced by the magnitude or midrange and distribution or variation in amplitude of loads in the course of function. In this study, the external compressive load was constant for all CHS values, but according to the lever mechanism (M=F×r), an increased arm length (r) or crown height causes bending and alters the stress and strain distribution in the transverse sections of the implant complex. In the first step of this study, which was the tightening of the abutment screw, thread torque was created as a result of rotational resistance at the abutment screw/ abutment interface, and conical torque was created as a result of rotational resistance at the abutment screw/ fixture interface. The sum of these torque values was equal to the recommended or target torque according to the manufacturer’s recommendation (Fig. 4). In this step, the amounts of preload or tensile stress at the transverse section of abutment screw and the clamp load or compressive stress at the transverse section of the contacting surface were equal but in opposite directions and had a uniform distribution (Figs. 5, 6). Therefore, the implant complex components, including the abutment screw, abutment, and fixture, were exposed to stress. Jorn et al26 indicated that the screw preload should be included in an investigation of dental implants for a realistic study of the implant complex. The purpose of applying the recommended torque was to create an appropriate preload and clamp load to achieve a stable joint, thereby forming the implant complex. This sustainable joint will properly bear external loads.13,14 Previous research indicates that the members often bear Bulaqi et al
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120 Axial Load 100
114.6
Mesiodistal Load Buccal-Lingual Load
Load (N)
80 60 40 23.4 20 17.1 0 0.1
0
0.2
0.3
0.4
0.5
Time (s) Figure 3. Dynamic loading in 1 cycle of mastication at 2 Hz.
35
35
Total Torque
30
Thread Torque 26.13
Torque (Ncm)
25 20
Total Torque
Conical Torque
step 1
Conical Torque
step 2
15 8.87
10
Thread Torque
7.03
5 0 0
0.15
0.3
0.45
0.75
0.6
–5
Abutment Screw
–7.03 –10
Time (s) Figure 4. Torque as a function of time; location and direction of torque are marked with blue markers on right scheme.
600
0 500
478 –100 –200
Member Load 300
–300 step 1
200
step 2
100
–400 –500
–590 –500
Preload
0 0
0.15
Clamp Load
0.3
0.45
0.6
External Load
Screw Load
400
Member Load (N)
Screw Load (N)
500
–600 0.75
Time (s) Figure 5. Screw load and member load as a function of time; directions of preload, clamp load and axial component of external-load are marked with red, green, and blue markers on right scheme.
Bulaqi et al
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Path Length
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Neutral Axis
G
G
Clamp load distribution at step 1
Screw load distribution at step 2
Member load distribution at step 2
Dotted blue line is midrange of screw load at step 2
Dotted blue line is midrange of members load at step 2 1
0 1
0 1
H
Path Length
Path Length
CHS = 13.6 mm
0
0
1
1
J
Path Length
CHS = 16 mm
0
Path Length
F
Path Length
CHS = 11.2 mm
0 1
G 1
I
D
Path Length
CHS = 8.8 mm
1
Path Length
E
B
Preload distribution at step 1
Path Length
C
Transverese Section of Abutment-Fixture Contact Surface
Transverese Section G-G
A
0
0 420
440
460
480
500
520
Load/Length
–900
–800
–700
–600
–500
–400
–300
Load/Length
Figure 6. Distribution of A, screw load at abutment-screw and B, member load at abutment (C and D, CHS= 8.8 mm; E and F, CHS= 11.2 mm; G and H, CHS= 13.6 mm; I and J, CHS= 16 mm). Path of evaluations are marked with dotted line on top schemes.
over 80% (C = 0.2) of the external load.15 Additionally, in order to study the dynamic effect of CHS on all implant complex compartments, a stable assembly must be achieved.
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By removing the wrench (step 2), total torque was eliminated, and conical and thread torques neutralize each other (Fig. 4). When applying an oblique external load of 118.2 N with a 75.8 degree angle with respect Bulaqi et al
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Table 3. Values of screw load and member load distributions for each CHS at step 2 Screw (N)
Member Load (N)
CHS (mm)
CIR
Minimum
Midrange
Maximum
Variation of Amplitude
Maximum
Midrange
Minimum
Variation of Amplitude
8.8
1.1
469
478
487
18
-677
-590
-503
174
11.2
1.4
463
478
493
30
-720
-590
-460
260
13.6
1.7
452
478
504
52
-773
-590
-407
366
16
2.0
436
478
520
84
-837
-590
-343
494
CHS, crown height space; CIR, crown-to-implant ratio.
Time (s) 0
Min Principle Stress (MPa)
A
0.15
0.3
0.45
0.6
0.75
0 step 1
step 2 –14.5
–20
–47 –40 CHS = 8.8 mm –60
–61.8
CHS = 11.2 mm
–53.5
CHS = 13.6 mm CHS = 16 mm
Bone
–71.5
–80
Max Principle Stress (MPa)
B 50
51.9
CHS = 8.8 mm CHS = 11.2 mm
40
40.9
33.6
CHS = 13.6 mm CHS = 16 mm
30.4
30 20 8
10
Bone
0 0
0.15 Step 1
0.3
0.45
Time (s)
0.6 Step 2
0.75
Figure 7. Maximum stress (MPa) values as function of time for each bone: A, Minimum principle. B, Maximum principle; locations of evaluations are marked on right schemes.
to the occlusal plane (step 2), 112 N of effective axial load was transferred to the joint, from which 19.6% (C = 0:196) was carried by the abutment screw (Pb =22 N), and 80.4% (1−C = 0:804) was carried by the members (Pm =90 N). Because the external compressive load was in the opposite direction of the preload but in the same direction of the clamp load, the screw load (Fb ) decreased and the member load (Fm ) increased (Fig. 5). Additionally, changes in the CHS did not affect the midrange of the screw load and member load. However, as the CHS increased, the arm length also increased, and Bulaqi et al
with a higher level of bending, the distribution of these load values becomes nonuniform with respect to the neutral axis. In other words, the amplitude variations of the screw load increased 366% and the member load increased 184% (Fig. 6, Table 3). With the more nonuniform and enlarged stress and force magnitude and distribution with respect to the neutral axis, the probability of screw loosening and fracture increases. Budynas and Nisbett15 have ascribed fatigue fracture to the increased variation in the amplitude of loads and have also attributed screw loosening to THE JOURNAL OF PROSTHETIC DENTISTRY
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Table 4. Stress values (MPa) at components for each CHS Min Principle
Max Principle
Bone
Bone
Von Mises (MPa) Abutment Screw
Frame+Abutment
Fixture
CHS (mm)
CIR
Step 1
Step 2
Step 1
Step 2
Step 1
Step 2
Step 1
Step 2
Step 1
8.8
1.1
-14.5
-47.0
8
30.4
540
453
90
125.5
180
266.5
11.2
1.4
-
-53.5
-
33.6
-
451
-
135.5
-
278.0
13.6
1.7
-
-61.8
-
40.9
-
443
-
151.0
-
291.5
16
2.0
-
-71.5
-
51.9
-
429
-
168.5
-
313.5
Step 2
CHS, crown height space; CIR, crown-to-implant ratio.
the reduction in the midrange screw load. In the second step of this study, the screw load and member load distributions became nonuniform at the cross section through the bending mechanism compared to the first step. Simultaneously, the stress increased on one side of the neutral axis and decreased on the other side. At the abutment screw, the increase in the stress induced by the bending force on one side of the neutral axis was nearly neutralized by the stress reduction induced by decreasing the screw load as much as 22 N. This was true even at the CHS of 16 mm. However, the stress reduction induced by the bending force on other side of the neutral axis alongside the stress reduction induced by decreasing the screw load could be considered a disadvantage that may lead to the increased probability of screw loosening and fatigue fracture. At the abutment and fixture, the stress enhancement induced by the bending force on one side of the neutral axis alongside the stress enhancement induced by the increase in the member load by as much as 90 N could also lead to fracture (Fig. 8). With increasing CHS, tension increases in the prosthetic components, that is, the fixture and frame abutment, and the periimplant bone, but tension decreases in the abutment screw. By increasing the CHS from 8.8 to 16 mm, the von Mises stress decreased 5.6% at the abutment screw, increased 34.2% at the frame abutment, and increased 17.6% at the fixture (Table 4). In a study conducted by Nissan et al,5 a marked increase in stress and fracture was reported for the abutment screw with off-axis force application; however, insignificant stress was also reported when load was applied along the vertical long axis because the arm length was zero. In this study, a 112 N effective axial load is applied from a 118.2 N oblique external load at a 75.8-degree angle with respect to the occlusal plane (given that the mesial and lingual axial inclination of the implant is 9 degrees); consequently, the load was near the axial because the majority of the load was converted to the axial component. In other studies,3,8 the external load was applied to the system without considering the preload. The differences in the results reported in the literature might be rooted in the use of various methods; different implant brands with different geometries, shapes, and angles of stiffness; various angles of load exertion; or confounding THE JOURNAL OF PROSTHETIC DENTISTRY
factors present in the clinical studies. Only off-axis loads were able to cause abutment screw fracture, in contrast to axial loads, which only cause minimal tension. Implants lack periodontal ligaments and were nonmobile;4,8 therefore, the interface was rigid, and the forces applied to the implant/prosthesis system were directly transferred to the bone.8 Moreover, nonaxial loads were concentrated at the abutment junction and the bone crest.4 The abutment screw underwent the highest stress concentration.8 This result was similar to that of a few previous studies8 and confirms that overloading the screw might lead to loosening or fatigue fracture.3,8,9,25 In this study, the stress concentrated at the screw decreased slightly with increases in the crown height. By increasing the CHS, other parts bore increasingly more stress, which might justify a slight reduction in the stress exerted on the screw. Nevertheless, this result was in contrast with the result from a previous study that reported increases in screw stress concentrations with increasing crown height.8 In this simulation, screw tightening produced a miniscule amount of compressive and tensile stress remaining in the surrounding bone, which in reality diminish as a result of bone remodeling. This remaining stress was small and was not considered in this study. At the second step when the oblique external load was applied, an increase in the CHS from 8.8 to 16 mm resulted in the maximum compressive stress increasing by 52% and maximum tensile stresses increasing by 70.7% (Fig. 7, Table 4), enhancing the probability of bone resorption around the dental implants. This study was limited by the fact that it is an in vitro study and should be confirmed with clinical studies. Moreover, the finite element method, like all other computer simulation studies, has limitations. For example, the properties of the tested materials were considered isotropic, homogeneous, and linear, which vary from the clinical situation and the real properties of bone tissue.8 However, finite element analysis is an accurate mathematical model that approximates and analyzes the geometry and loading conditions of stresses/ strains in a solid body while eliminating numerous confounding variables that might exist in a clinical situation.3 It also provides a visualization of the load magnitudes and directions and has been used many times in the fields of dental and implant research.3 Thus, this Bulaqi et al
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A 540
Von Mises Stress (MPa)
500
453
451
400
429
443
300
CHS = 8.8 mm CHS = 11.2 mm
200
CHS = 13.6 mm CHS = 16 mm
100 Step 1 0
0
Step 1
Step 2
Step 2 0.15
0.3
0.45
0.6
0.75
Time (s)
B
Von Mises Stress (MPa)
168.5
CHS = 8.8 mm
150
151
CHS = 11.2 mm 125
135.5
CHS = 13.6 mm CHS = 16 mm
100
125.5 90
75
Step 1
50 25
Step 1
Step 2
Step 2
0 0
0.15
0.3
0.45
0.6
0.75
Time (s)
C
Von Mises Stress (MPa)
300
CHS = 8.8 mm
313.5
278
291.5
CHS = 11.2 mm
250
CHS = 13.6 mm CHS = 16 mm
200
266.6
150 180 100 50 0
Step 1 0
Step 1
Step 2 0.15
0.3
0.45
0.6
Step 2
0.75
Time (s) Figure 8. Von Mises stress (MPa) as function of time at the maximum influenced location of external force: A, abutment screw; B, abutment; and C, fixture. Locations of evaluations for each step are marked on right schemes.
technique is supported by the literature on bone biomechanics and reveals data that are complementary to that gathered in clinical follow-up.8 However, the simulation of more implant brands would improve the generalizability of the results. Bulaqi et al
CONCLUSIONS Under a 75.8-degree oblique load, the frame abutment fixture bears 80% of the effective axial external load, and the remaining 20% was placed on the abutment screw.
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This result depends on the stiffness constant of the contact members. Although an increase in the CHS has no influence on the midrange screw load and member load, the corresponding distributions become more uneven. Moreover, the tension concentrates on the fixture and abutment and decreases in abutment screw. Therefore, the probability of fatigue fracture might be increased. By increasing the CHS, tension increased in the periimplant bone. REFERENCES 1. Quaranta A, Piemontese M, Rappelli G, Sammartino G, Procaccini M. Technical and biological complications related to crown to implant ratio: a systematic review. Implant Dent 2014;23:180-7. 2. Himmlova L, Dostalova T, Kacovsky A, Konvickova S. Influence of implant length and diameter on stress distribution: a finite element analysis. J Prosthet Dent 2004;91:20-5. 3. Moraes SL, Pellizzer EP, Verri FR, Santiago JF Jr, Silva JV. Three-dimensional finite element analysis of stress distribution in retention screws of different crown-implant ratios. Comput Methods Biomech Biomed Engin 2015;18: 689-96. 4. Nissan J, Gross O, Ghelfan O, Priel I, Gross M, Chaushu G. The effect of splinting implant-supported restorations on stress distribution of different crown-implant ratios and crown height spaces. J Oral Maxillofac Surg 2011;69:2990-4. 5. Nissan J, Ghelfan O, Gross O, Priel I, Gross M, Chaushu G. The effect of crown/implant ratio and crown height space on stress distribution in unsplinted implant supporting restorations. J Oral Maxillofac Surg 2011;69: 1934-9. 6. Blanes RJ. To what extent does the crown-implant ratio affect the survival and complications of implant-supported reconstructions? A systematic review. Clin Oral Implants Res 2009;20(suppl 4):67-72. 7. English CE. Biomechanical concerns with fixed partial dentures involving implants. Implant Dent 1993;2:221-42. 8. de Moraes SL, Verri FR, Santiago JF Jr, Almeida DA, de Mello CC, Pellizzer EP. A 3-D finite element study of the influence of crown-implant ratio on stress distribution. Braz Dent J 2013;24:635-41. 9. Papakostas GI, McGrath P, Stewart J, Charles D, Chen Y, Mischoulon D, et al. Psychic and somatic anxiety symptoms as predictors of response to fluoxetine in major depressive disorder. Psychiatry Res 2008;161:116-20. 10. Alkan I, Sertgoz A, Ekici B. Influence of occlusal forces on stress distribution in preloaded dental implant screws. J Prosthet Dent 2004;91:319-25. 11. Romanos GE, Gupta B, Eckert SE. Distal cantilevers and implant dentistry. Int J Oral Maxillofac Implants 2012;27:1131-6. 12. Kim YK, Kim SG, Yun PY, Hwang JW, Son MK. Prognosis of single molar implants: a retrospective study. Int J Periodontics Restorative Dent 2010;30: 401-7.
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13. Jorneus L, Jemt T, Carlsson L. Loads and designs of screw joints for single crowns supported by osseointegrated implants. Int J Oral Maxillofac Implants 1992;7:353-9. 14. Patterson EA, Johns RB. Theoretical analysis of the fatigue life of fixture screws in osseointegrated dental implants. Int J Oral Maxillofac Implants 1992;7:26-33. 15. Budynas RG, Nisbett JK. Shigley’s mechanical engineering design. 9th ed. New York: McGraw-Hill; 2011. p. 221-35. 435-7. 16. Bowden FP, Tabor D. The friction and lubrication of solids. Oxford: Oxford University Press; 2001. 17. Haack JE, Sakaguchi RL, Sun T, Coffey JP. Elongation and preload stress in dental implant abutment screws. Int J Oral Maxillofac Implants 1995; 10:529-36. 18. Nelson SJ, Ash MM, Ash MM. Wheeler’s dental anatomy, physiology, and occlusion. 9th ed. St Louis: Saunders/Elsevier; 2010. p. 284-6. 19. Borchers L, Reichart P. Three-dimensional stress distribution around a dental implant at different stages of interface development. J Dent Res 1983;62:155-9. 20. Lewinstein I, Banks-Sills L, Eliasi R. Finite element analysis of a new system (IL) for supporting an implant-retained cantilever prosthesis. Int J Oral Maxillofac Implants 1995;10:355-66. 21. Craig R. Improved materials pique interest. Dentist 1989;67(15):18-9. 22. Mericske-Stern R, Zarb GA. In vivo measurements of some functional aspects with mandibular fixed prostheses supported by implants. Clin Oral Implants Res 1996;7:153-61. 23. Po JM, Kieser JA, Gallo LM, Tesenyi AJ, Herbison P, Farella M. Time-frequency analysis of chewing activity in the natural environment. J Dent Res 2011;90:1206-10. 24. Tawil G, Aboujaoude N, Younan R. Influence of prosthetic parameters on the survival and complication rates of short implants. Int J Oral Maxillofac Implants 2006;21:275-82. 25. Urdaneta RA, Rodriguez S, McNeil DC, Weed M, Chuang SK. The effect of increased crown-to-implant ratio on single-tooth locking-taper implants. Int J Oral Maxillofac Implants 2010;25:729-43. 26. Jorn D, Kohorst P, Besdo S, Rucker M, Stiesch M, Borchers L. Influence of lubricant on screw preload and stresses in a finite element model for a dental implant. J Prosthet Dent 2014;112:340-8.
Corresponding author: Dr Farideh Geramipanah Implant Research Center Dentistry Research Institute Tehran University of Medical Sciences North Amir-Abad Tehran IRAN Email: [email protected] Acknowledgment Computing support of this research was done at the High-Performance Computing Research Center of the Amirkabir University of Technology. Copyright © 2015 by the Editorial Council for The Journal of Prosthetic Dentistry.
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Effect of increased crown height on stress distribution in short dental implant components and their surrounding bone: A finite element analysis Haddad Arabi Bulaqi, MSc,a Mahmoud Mousavi Mashhadi, PhD,b Hamed Safari, DDS, MS,c Mohammad Mahdi Samandari, MSc,d and Farideh Geramipanah, DDS, MSe The use of short implants is ABSTRACT increasing as a result of the Statement of problem. Implants in posterior regions of the jaw require short dental implants with ease of use when anatomic limlong crown heights, leading to increased crown-to-implant ratios and mechanical stress. This can itations prevent conventional lead to fracture and screw loosening. implant placement. However, Purpose. The purpose of this study was to investigate the dynamic nature and behavior of prosshort implants might cause thetic components and preimplant bone and evaluate the effect of increased crown height space biomechanical complications,1 (CHS) and crown-to-implant ratio on stress concentrations under external oblique forces. especially if used with long Material and methods. The severely resorbed bone of a posterior mandible site was modeled with crowns. The crown-to-implant Mimics and Catia software. A second mandibular premolar tooth was modeled with CHS values of ratio (CIR) can affect dental im8.8, 11.2, 13.6, and 16 mm. A Straumann implant (4.1×8 mm), a directly attached crown, and an plants and bone tissue.1-4 Unabutment screw were modeled with geometric data and designed by using SolidWorks software. der oblique forces, increases in Abaqus software was used for the dynamic simulation of screw tightening and the application of an external load to the buccal cusp at a 75.8-degree angle with the occlusal plane. The distribution of CIR contribute to stress accu1-3,5 screw load and member load at each step was compared, and the stress values were calculated mulation. Another relevant within the dental implant complex and surrounding bone. concept, crown height space (CHS), is measured from the Results. During tightening, the magnitude and distribution of the preload and clamp load were uniform and equal at the cross section of all CHSs. Under an external load, the screw load decreased crest of the bone to the occlusal/ 4,6 and member load increased. An increase in the CHS caused the corresponding distribution to incisal planes. The biomebecome more nonuniform and increased the maximum compressive and tensile stresses in the chanics of CHS are related preimplant bone. Additionally, the von Mises stress decreased at the abutment screw and increased to lever arm mechanics.4,7 An at the abutment and fixture. increased CHS causes an exConclusions. Under nonaxial forces, increased CHS does not influence the decrease in screw load or cessive vertical and horizontal increase in member load. However, it contributes to screw loosening and fatigue fracture by skewing cantilever, which could conthe stress distribution to the transverse section of the implant. (J Prosthet Dent 2015;-:---) centrate stress at the implant In a retrospective study, Kim et al12 observed that neck by the lever mechanism4,7 and cause biomechanical screw loosening was significantly correlated with the complications, such as implant fracture, implant failure, or mesiodistal cantilever in single molar implants. Tenscrew loosening.3,8,9 sioning the screw creates a preload that is proportional to Screw loosening is one of the most common problems the tightening torque. To achieve a stable and secure associated with fixed implant-supported prostheses.10,11 a
Graduate student, Department of Mechanical Engineering, School of Mechanics, University of Tehran, Tehran, Iran. Professor, Department of Mechanical Engineering, School of Mechanics, University of Tehran, Tehran, Iran. Assistant Professor, Department of Periodontics, School of Dentistry, Qom University of Medical Sciences, Qom, Iran. d Associate Professor, Implant Research Center, Department of Mechanical Engineering, School of Mechanics, University of Tehran, Tehran, Iran. e Graduate student, Department of Mechanical Engineering, School of Mechanics, University of Tehran, Tehran, Iran. b c
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Clinical Implications Under oblique loads, the use of long crown heights not only increases the stress at the peripheral bone and implant compartments but also causes biomechanical disadvantages, thereby elevating the risk of screw loosening. The use of a suitable crown height could reduce the incidence of biologic and biomechanical complications. joint, the preload should be greater than the external loads.13,14 Before the application of the external compressive load (P), preload (Fi ) and clamp load (-Fi ) are equal but in opposite directions. Upon applying the external load (P), a fraction of the external load (C) is carried by the abutment screw, and the rest (1−C) is carried by the fixture and the abutment, which are collectively referred to as members. Therefore, the resultant load on the abutment screw is Fb (screw load, Eq. (1)), and the resultant load on the members is Fm (member load, Eq. (2)). Note that C is called the stiffness constant of the joint and depends on the geometrical parameters, design, and material properties of the abutment screw and members.15 Fb =−Pb +Fi =−ðC×PÞ+Fi
(1)
Fm =−Pm −Fi =−ðð1−CÞ×PÞ−Fi ;
(2)
where P=External compressive load =Pb +Pm ; Pb =Portion of P taken by bolt=C×P, and Pm =Portion of P taken by members=ð1−CÞ×P Few biomechanical studies have evaluated crown height.3-5 Moreover, few studies have investigated the CIR, and the ones that have are considered controversial.5 Additionally, a potentially more critical factor is CHS,5 because an increase in crown height might be more problematic than an equal reduction in implant length.6 Therefore, CHS alone is more important than the CIR and should thus be studied independently. Yet only a few studies have surveyed the impact of crown height as a factor independent of fixture length.6,8 Furthermore, the effect of force on preloaded implants,10 screw loosening, and prosthetic complications3 has also been assessed by few studies. Finally, the literature lacks studies on the effects of different CIR or CHS values on stress/strain levels and distributions in short implant applications. Investigating the factors that influence stress distribution within the implant complex can provide necessary information to facilitate the design of components with better joint stability and stress distribution. Finite element analysis is an engineering method that is used to calculate stresses and strains within a solid body, thereby making it useful for the resultant and THE JOURNAL OF PROSTHETIC DENTISTRY
Figure 1. Cross-sectional view of three-dimensional computer-aided design models of implant (violet), direct abutment (orange), abutment screw (yellow), metal frame (blue), porcelain (green), and composite resin (red).
contact forces on dental implant components. However, finite element analysis is considered a qualitative and approximation method that is can be used to better recognize a problem. The aim of this study was to investigate the effect of different CHS values on the stress distribution in short dental implant components under oblique loads. Moreover, the effect of different CHS values on the distribution and magnitude of screw load and member load and the compressive and tensile stresses in the surrounding bone was also investigated. MATERIAL AND METHODS In order to model the severely resorbed posterior mandible site, a cone beam computed tomography image and Digital Imaging and Communications in Medicine (DICOM) file were generated. Mimics 10.01 software (Materialise Group) was used to create the cloud points and demarcate the trabecular bone (150 to 900 Hounsfield units [HU]), cortical bone (900 to 1800 HU), and the primary form of the crown (2500 to 3000 HU).9 The cloud points were transformed by the Digitized Shape Editor of Catia v5 R19 (Dassault Systèmes), and computer-aided design files were modeled. By using the primary form of the teeth and proper scaling in all directions, 4 tooth models with CHS values of 8.8, 11.2, 13.6, and 16 mm were modeled (Fig. 1). The resulting CIR values were 1.1, 1.4, 1.7, and 2.0 for an 8 mm short implant. Because the metal frame was simultaneously cast with the abutment, the abutment and the frame models were integrated. The composite material was used to cover the abutment screw access hole. A micrometer projector was used to obtain the geometry and dimensions of a 4.1×8 mm Straumann implant (SLA 043.031S; Institut Straumann), the directly attached crown (048.642, RN SynOcta gold abutment), and the abutment screw (048.356, SynOcta basal screw) (Fig. 1). The models were constructed by using software Bulaqi et al
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Figure 2. Three-dimensional finite element method models of implant complex in embedding cortical and trabecular bones at different crown height spaces (CHS). Loading of implants occurred in axial, lingual, and mesiodistal direction.
(SolidWorks; Dassault Systèmes). The implant components were modeled precisely to simulate the process of screw tightening, preload creation, and osseointegration. The outer surface of the abutment screw and implant were geometrically modeled with a continuous spiral threaded helix, and the internal surface of the implant and bone bore were geometrically modeled with a continuous spiral threaded bore. The implant thread pitch was 1.25 mm, and the abutment screw thread pitch was 0.4 mm. Contact interface and tangential behavior with a specific coefficient of friction were considered for the reciprocal interfacing surfaces. More specifically, the coefficient of kinetic friction (mk ) of 0.12 was applied to simulate the screw tightening, and for the functional condition, the value for the coefficient of static friction was ms =0:16; which was slightly larger than the value for the coefficient of kinetic friction.16,17 The initial condition of the abutment screw within the implant complex was snug. The implant was positioned at the second premolar site. The mesial-axial inclination and the lingual-axial inclination of the implant were both 9 degrees.18 The three-dimensional finite element method model of the implant complex at the 4 different CHS values were shown inserted in the bone (Fig. 2). The bone was sectioned and bounded at the sites depicted in Figure 2 in order to reduce the number of elements considered in the analysis. Because complete osseointegration was supposed to be achieved, the bone-implant contact surface was defined as a “tie.” Software (Abaqus 6.11; Dassault Systèmes Simulia Corp) was used for explicit dynamic simulation. Tetrahedral elements, the free meshing technique, and linear geometric order were used for meshing. The total number of elements used for each model is presented in Table 1. The mechanical behavior of the implant component material was assumed to be in both the elastic and plastic regions. Also, the mechanical properties of the surrounding bone were assumed to be the linear Bulaqi et al
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elastic region. The mechanical properties of materials are presented in Table 2.19-21 All materials were assumed to be both isotropic and homogenous. Three external loads of 114.6 N (axial), 17.1 N (buccal-lingual), and 23.4 N (mesiodistal) were applied to the coronal part of the buccal cusp (Fig. 3). The equivalent of the combination of these loads is a 118.2 N load at 75.8 degrees in relation to the occlusal plane.22 The radius of the load area was 0.4 mm. The analysis included 2 steps. First, the wrench turned the screw until a recommended torque of 35 Ncm was applied and the implant complex was clamped. In the next step, the external load was applied for 0.5 seconds (Fig. 3). The time frequency of the load is an important factor in dynamic analysis. This time was chosen based on the study by Po et al,23 which indicates that each mastication cycle lasts approximately 0.5 seconds (2 Hz). The relationships of the created total torque (Tt ), conical torque (Tc ), thread torque (Tth ), preload (Fi ), screw load (Fb ), and member load (Fm ) with respect to time at 4 CHS values are presented. The maximum amount of external loading influencing the screw load and member load are also shown. The criterion of the von Mises stress for ductile materials, such as titanium and gold, and maximum-normal stress theory for brittle materials, such as cortical and trabecular bone and porcelain, were used.15 The amounts of maximum compressive and tensile stress with respect to time for all CHS values were presented for the surrounding bone in each step of the analysis. Likewise, the amount of von Mises stress at the zone where the maximum influence of external load occurs was presented with respect to time for the abutment screw, abutment, and fixture in all steps. RESULTS The created total, conical, and thread torques with respect to time in 2 steps for all CHS values are presented in Figure 4. The maximum amount of total torque was 35 Ncm, which is the manufacturer’s recommended torque value (step 1). Additionally, at this step, the conical torque was 26.13 Ncm, and the thread torque was 8.87 Ncm. By removing the wrench and applying the external load (step 2), total torque was completely eliminated. The minimum conical torque was -7.03 Ncm, and the minimum thread torque was 7.03 Ncm. Figure 5 presents the amount of load changes in the screw (preload, step 1), the screw load (step 2), the members (clamp load, step 1), and the member load (step 2) with respect to time for all the CHS values. The preload (Fi , tensile) and clamp load (−Fi , compressive) at the end of the screw tightening process were each 500 N. In the second step, an external load was applied in the same direction as the clamp load and the opposite THE JOURNAL OF PROSTHETIC DENTISTRY
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Table 1. Number of tetrahedral elements for each part in models of 4 different CHS CHS (mm)
CIR
Cortical Bone
Trabecular Bone
Abutment Screw
Fixture
Composite
Frame+Abutment
8.8
1.1
39700
55947
32303
78774
1640
19350
Porcelain 24123
11.2
1.4
-
-
-
-
1685
25961
29229
13.6
1.7
-
-
-
-
1713
34098
38889
16
2.0
-
-
-
-
1745
45349
40954
CHS, crown height space; CIR, crown-to-implant ratio.
direction of the preload; consequently, the minimum amount of screw load (Fb ) was 478 N, and maximum amount of member load (Fm ) was -590 N. Figure 6 shows the distribution of the preload and screw load at the transverse section of the abutment screw, and the distribution of the clamp load and member load at the transverse section of the fixture-abutment contact surface with respect to the neutral axis. The minimum, midrange, and maximum values of screw load and member load at the transverse section are presented in Table 3. Figure 7 presents the changes in the compressive and tensile stress values, otherwise known as the Min and Max principle stress values, with respect to time in the surrounding bone during steps 1 and 2 for 4 different crown heights. Table 4 lists the maximum amounts of Min and Max principle stresses for 4 CHS values in the 2 simulated steps. The graphs for the changes in the von Mises stress in the abutment screw, abutment, and fixture with respect to time during the screw tightening process and the application of an external load at 4 different crown heights are presented in Figure 8. Table 4 presents the values of the von Mises stress for the abutment screw, abutment, and fixture at the 4 different CHS values in both simulated steps. DISCUSSION Implant success not only depends on proper osseointegration but also on biomechanical aspects.8 Excessive crown heights can increase the amount of force and stress applied to the implant and the supporting bone through the lever mechanism.3,7 Such increased stresses can lead to crestal bone loss, screw loosening, and implant fracture.9 Additionally, larger crown dimensions with shorter implant lengths might cause greater stress.2 A shorter implant has a greater CIR when compared to a longer implant with a constant crown height. Therefore, complications should be presumably anticipated in short implants. However, previous research has shown that short implants are clinically successful regardless of the CIR.24 A finite element study concluded that increasing the crown height could be destructive to the stress distribution on the screw, especially under oblique loading.3 However, in an investigation of 326 implants with a mean CIR of 1.6, Urdaneta et al25 observed that excessive CIR had no negative effect on the periimplant bone loss but caused more significant THE JOURNAL OF PROSTHETIC DENTISTRY
Table 2. Mechanical properties of materials used for finite element analysis
Material Component
Young Modulus Poisson Density Strength Elongation (GPa) Ratio (g/cm) (MPa) (%)
Gold abutment+frame*
136
0.37
17.5
765
10 Min
Titanium grade 4*
110
0.34
4.5
550
15 Min
13.7
0.30
3
190
2 Max
1.37
0.30
3
10
2 Max
68.9
0.28
2.44
145
2 Max
7
0.20
2.3
480
2 Max
Cortical bone
19
Trabecular bone
19
Porcelain20 Composite resin
21
*According to the manufacturer specifications.
prosthetic complications, such as screw loosening and porcelain fracture. Screw loosening, implant component fracture, and bone resorption are influenced by the magnitude or midrange and distribution or variation in amplitude of loads in the course of function. In this study, the external compressive load was constant for all CHS values, but according to the lever mechanism (M=F×r), an increased arm length (r) or crown height causes bending and alters the stress and strain distribution in the transverse sections of the implant complex. In the first step of this study, which was the tightening of the abutment screw, thread torque was created as a result of rotational resistance at the abutment screw/ abutment interface, and conical torque was created as a result of rotational resistance at the abutment screw/ fixture interface. The sum of these torque values was equal to the recommended or target torque according to the manufacturer’s recommendation (Fig. 4). In this step, the amounts of preload or tensile stress at the transverse section of abutment screw and the clamp load or compressive stress at the transverse section of the contacting surface were equal but in opposite directions and had a uniform distribution (Figs. 5, 6). Therefore, the implant complex components, including the abutment screw, abutment, and fixture, were exposed to stress. Jorn et al26 indicated that the screw preload should be included in an investigation of dental implants for a realistic study of the implant complex. The purpose of applying the recommended torque was to create an appropriate preload and clamp load to achieve a stable joint, thereby forming the implant complex. This sustainable joint will properly bear external loads.13,14 Previous research indicates that the members often bear Bulaqi et al
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120 Axial Load 100
114.6
Mesiodistal Load Buccal-Lingual Load
Load (N)
80 60 40 23.4 20 17.1 0 0.1
0
0.2
0.3
0.4
0.5
Time (s) Figure 3. Dynamic loading in 1 cycle of mastication at 2 Hz.
35
35
Total Torque
30
Thread Torque 26.13
Torque (Ncm)
25 20
Total Torque
Conical Torque
step 1
Conical Torque
step 2
15 8.87
10
Thread Torque
7.03
5 0 0
0.15
0.3
0.45
0.75
0.6
–5
Abutment Screw
–7.03 –10
Time (s) Figure 4. Torque as a function of time; location and direction of torque are marked with blue markers on right scheme.
600
0 500
478 –100 –200
Member Load 300
–300 step 1
200
step 2
100
–400 –500
–590 –500
Preload
0 0
0.15
Clamp Load
0.3
0.45
0.6
External Load
Screw Load
400
Member Load (N)
Screw Load (N)
500
–600 0.75
Time (s) Figure 5. Screw load and member load as a function of time; directions of preload, clamp load and axial component of external-load are marked with red, green, and blue markers on right scheme.
Bulaqi et al
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Path Length
Neutral Axis
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Neutral Axis
G
G
Clamp load distribution at step 1
Screw load distribution at step 2
Member load distribution at step 2
Dotted blue line is midrange of screw load at step 2
Dotted blue line is midrange of members load at step 2 1
0 1
0 1
H
Path Length
Path Length
CHS = 13.6 mm
0
0
1
1
J
Path Length
CHS = 16 mm
0
Path Length
F
Path Length
CHS = 11.2 mm
0 1
G 1
I
D
Path Length
CHS = 8.8 mm
1
Path Length
E
B
Preload distribution at step 1
Path Length
C
Transverese Section of Abutment-Fixture Contact Surface
Transverese Section G-G
A
0
0 420
440
460
480
500
520
Load/Length
–900
–800
–700
–600
–500
–400
–300
Load/Length
Figure 6. Distribution of A, screw load at abutment-screw and B, member load at abutment (C and D, CHS= 8.8 mm; E and F, CHS= 11.2 mm; G and H, CHS= 13.6 mm; I and J, CHS= 16 mm). Path of evaluations are marked with dotted line on top schemes.
over 80% (C = 0.2) of the external load.15 Additionally, in order to study the dynamic effect of CHS on all implant complex compartments, a stable assembly must be achieved.
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By removing the wrench (step 2), total torque was eliminated, and conical and thread torques neutralize each other (Fig. 4). When applying an oblique external load of 118.2 N with a 75.8 degree angle with respect Bulaqi et al
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Table 3. Values of screw load and member load distributions for each CHS at step 2 Screw (N)
Member Load (N)
CHS (mm)
CIR
Minimum
Midrange
Maximum
Variation of Amplitude
Maximum
Midrange
Minimum
Variation of Amplitude
8.8
1.1
469
478
487
18
-677
-590
-503
174
11.2
1.4
463
478
493
30
-720
-590
-460
260
13.6
1.7
452
478
504
52
-773
-590
-407
366
16
2.0
436
478
520
84
-837
-590
-343
494
CHS, crown height space; CIR, crown-to-implant ratio.
Time (s) 0
Min Principle Stress (MPa)
A
0.15
0.3
0.45
0.6
0.75
0 step 1
step 2 –14.5
–20
–47 –40 CHS = 8.8 mm –60
–61.8
CHS = 11.2 mm
–53.5
CHS = 13.6 mm CHS = 16 mm
Bone
–71.5
–80
Max Principle Stress (MPa)
B 50
51.9
CHS = 8.8 mm CHS = 11.2 mm
40
40.9
33.6
CHS = 13.6 mm CHS = 16 mm
30.4
30 20 8
10
Bone
0 0
0.15 Step 1
0.3
0.45
Time (s)
0.6 Step 2
0.75
Figure 7. Maximum stress (MPa) values as function of time for each bone: A, Minimum principle. B, Maximum principle; locations of evaluations are marked on right schemes.
to the occlusal plane (step 2), 112 N of effective axial load was transferred to the joint, from which 19.6% (C = 0:196) was carried by the abutment screw (Pb =22 N), and 80.4% (1−C = 0:804) was carried by the members (Pm =90 N). Because the external compressive load was in the opposite direction of the preload but in the same direction of the clamp load, the screw load (Fb ) decreased and the member load (Fm ) increased (Fig. 5). Additionally, changes in the CHS did not affect the midrange of the screw load and member load. However, as the CHS increased, the arm length also increased, and Bulaqi et al
with a higher level of bending, the distribution of these load values becomes nonuniform with respect to the neutral axis. In other words, the amplitude variations of the screw load increased 366% and the member load increased 184% (Fig. 6, Table 3). With the more nonuniform and enlarged stress and force magnitude and distribution with respect to the neutral axis, the probability of screw loosening and fracture increases. Budynas and Nisbett15 have ascribed fatigue fracture to the increased variation in the amplitude of loads and have also attributed screw loosening to THE JOURNAL OF PROSTHETIC DENTISTRY
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Table 4. Stress values (MPa) at components for each CHS Min Principle
Max Principle
Bone
Bone
Von Mises (MPa) Abutment Screw
Frame+Abutment
Fixture
CHS (mm)
CIR
Step 1
Step 2
Step 1
Step 2
Step 1
Step 2
Step 1
Step 2
Step 1
8.8
1.1
-14.5
-47.0
8
30.4
540
453
90
125.5
180
266.5
11.2
1.4
-
-53.5
-
33.6
-
451
-
135.5
-
278.0
13.6
1.7
-
-61.8
-
40.9
-
443
-
151.0
-
291.5
16
2.0
-
-71.5
-
51.9
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429
-
168.5
-
313.5
Step 2
CHS, crown height space; CIR, crown-to-implant ratio.
the reduction in the midrange screw load. In the second step of this study, the screw load and member load distributions became nonuniform at the cross section through the bending mechanism compared to the first step. Simultaneously, the stress increased on one side of the neutral axis and decreased on the other side. At the abutment screw, the increase in the stress induced by the bending force on one side of the neutral axis was nearly neutralized by the stress reduction induced by decreasing the screw load as much as 22 N. This was true even at the CHS of 16 mm. However, the stress reduction induced by the bending force on other side of the neutral axis alongside the stress reduction induced by decreasing the screw load could be considered a disadvantage that may lead to the increased probability of screw loosening and fatigue fracture. At the abutment and fixture, the stress enhancement induced by the bending force on one side of the neutral axis alongside the stress enhancement induced by the increase in the member load by as much as 90 N could also lead to fracture (Fig. 8). With increasing CHS, tension increases in the prosthetic components, that is, the fixture and frame abutment, and the periimplant bone, but tension decreases in the abutment screw. By increasing the CHS from 8.8 to 16 mm, the von Mises stress decreased 5.6% at the abutment screw, increased 34.2% at the frame abutment, and increased 17.6% at the fixture (Table 4). In a study conducted by Nissan et al,5 a marked increase in stress and fracture was reported for the abutment screw with off-axis force application; however, insignificant stress was also reported when load was applied along the vertical long axis because the arm length was zero. In this study, a 112 N effective axial load is applied from a 118.2 N oblique external load at a 75.8-degree angle with respect to the occlusal plane (given that the mesial and lingual axial inclination of the implant is 9 degrees); consequently, the load was near the axial because the majority of the load was converted to the axial component. In other studies,3,8 the external load was applied to the system without considering the preload. The differences in the results reported in the literature might be rooted in the use of various methods; different implant brands with different geometries, shapes, and angles of stiffness; various angles of load exertion; or confounding THE JOURNAL OF PROSTHETIC DENTISTRY
factors present in the clinical studies. Only off-axis loads were able to cause abutment screw fracture, in contrast to axial loads, which only cause minimal tension. Implants lack periodontal ligaments and were nonmobile;4,8 therefore, the interface was rigid, and the forces applied to the implant/prosthesis system were directly transferred to the bone.8 Moreover, nonaxial loads were concentrated at the abutment junction and the bone crest.4 The abutment screw underwent the highest stress concentration.8 This result was similar to that of a few previous studies8 and confirms that overloading the screw might lead to loosening or fatigue fracture.3,8,9,25 In this study, the stress concentrated at the screw decreased slightly with increases in the crown height. By increasing the CHS, other parts bore increasingly more stress, which might justify a slight reduction in the stress exerted on the screw. Nevertheless, this result was in contrast with the result from a previous study that reported increases in screw stress concentrations with increasing crown height.8 In this simulation, screw tightening produced a miniscule amount of compressive and tensile stress remaining in the surrounding bone, which in reality diminish as a result of bone remodeling. This remaining stress was small and was not considered in this study. At the second step when the oblique external load was applied, an increase in the CHS from 8.8 to 16 mm resulted in the maximum compressive stress increasing by 52% and maximum tensile stresses increasing by 70.7% (Fig. 7, Table 4), enhancing the probability of bone resorption around the dental implants. This study was limited by the fact that it is an in vitro study and should be confirmed with clinical studies. Moreover, the finite element method, like all other computer simulation studies, has limitations. For example, the properties of the tested materials were considered isotropic, homogeneous, and linear, which vary from the clinical situation and the real properties of bone tissue.8 However, finite element analysis is an accurate mathematical model that approximates and analyzes the geometry and loading conditions of stresses/ strains in a solid body while eliminating numerous confounding variables that might exist in a clinical situation.3 It also provides a visualization of the load magnitudes and directions and has been used many times in the fields of dental and implant research.3 Thus, this Bulaqi et al
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A 540
Von Mises Stress (MPa)
500
453
451
400
429
443
300
CHS = 8.8 mm CHS = 11.2 mm
200
CHS = 13.6 mm CHS = 16 mm
100 Step 1 0
0
Step 1
Step 2
Step 2 0.15
0.3
0.45
0.6
0.75
Time (s)
B
Von Mises Stress (MPa)
168.5
CHS = 8.8 mm
150
151
CHS = 11.2 mm 125
135.5
CHS = 13.6 mm CHS = 16 mm
100
125.5 90
75
Step 1
50 25
Step 1
Step 2
Step 2
0 0
0.15
0.3
0.45
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Time (s)
C
Von Mises Stress (MPa)
300
CHS = 8.8 mm
313.5
278
291.5
CHS = 11.2 mm
250
CHS = 13.6 mm CHS = 16 mm
200
266.6
150 180 100 50 0
Step 1 0
Step 1
Step 2 0.15
0.3
0.45
0.6
Step 2
0.75
Time (s) Figure 8. Von Mises stress (MPa) as function of time at the maximum influenced location of external force: A, abutment screw; B, abutment; and C, fixture. Locations of evaluations for each step are marked on right schemes.
technique is supported by the literature on bone biomechanics and reveals data that are complementary to that gathered in clinical follow-up.8 However, the simulation of more implant brands would improve the generalizability of the results. Bulaqi et al
CONCLUSIONS Under a 75.8-degree oblique load, the frame abutment fixture bears 80% of the effective axial external load, and the remaining 20% was placed on the abutment screw.
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This result depends on the stiffness constant of the contact members. Although an increase in the CHS has no influence on the midrange screw load and member load, the corresponding distributions become more uneven. Moreover, the tension concentrates on the fixture and abutment and decreases in abutment screw. Therefore, the probability of fatigue fracture might be increased. By increasing the CHS, tension increased in the periimplant bone. REFERENCES 1. Quaranta A, Piemontese M, Rappelli G, Sammartino G, Procaccini M. Technical and biological complications related to crown to implant ratio: a systematic review. Implant Dent 2014;23:180-7. 2. Himmlova L, Dostalova T, Kacovsky A, Konvickova S. Influence of implant length and diameter on stress distribution: a finite element analysis. J Prosthet Dent 2004;91:20-5. 3. Moraes SL, Pellizzer EP, Verri FR, Santiago JF Jr, Silva JV. Three-dimensional finite element analysis of stress distribution in retention screws of different crown-implant ratios. Comput Methods Biomech Biomed Engin 2015;18: 689-96. 4. Nissan J, Gross O, Ghelfan O, Priel I, Gross M, Chaushu G. The effect of splinting implant-supported restorations on stress distribution of different crown-implant ratios and crown height spaces. J Oral Maxillofac Surg 2011;69:2990-4. 5. Nissan J, Ghelfan O, Gross O, Priel I, Gross M, Chaushu G. The effect of crown/implant ratio and crown height space on stress distribution in unsplinted implant supporting restorations. J Oral Maxillofac Surg 2011;69: 1934-9. 6. Blanes RJ. To what extent does the crown-implant ratio affect the survival and complications of implant-supported reconstructions? A systematic review. Clin Oral Implants Res 2009;20(suppl 4):67-72. 7. English CE. Biomechanical concerns with fixed partial dentures involving implants. Implant Dent 1993;2:221-42. 8. de Moraes SL, Verri FR, Santiago JF Jr, Almeida DA, de Mello CC, Pellizzer EP. A 3-D finite element study of the influence of crown-implant ratio on stress distribution. Braz Dent J 2013;24:635-41. 9. Papakostas GI, McGrath P, Stewart J, Charles D, Chen Y, Mischoulon D, et al. Psychic and somatic anxiety symptoms as predictors of response to fluoxetine in major depressive disorder. Psychiatry Res 2008;161:116-20. 10. Alkan I, Sertgoz A, Ekici B. Influence of occlusal forces on stress distribution in preloaded dental implant screws. J Prosthet Dent 2004;91:319-25. 11. Romanos GE, Gupta B, Eckert SE. Distal cantilevers and implant dentistry. Int J Oral Maxillofac Implants 2012;27:1131-6. 12. Kim YK, Kim SG, Yun PY, Hwang JW, Son MK. Prognosis of single molar implants: a retrospective study. Int J Periodontics Restorative Dent 2010;30: 401-7.
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13. Jorneus L, Jemt T, Carlsson L. Loads and designs of screw joints for single crowns supported by osseointegrated implants. Int J Oral Maxillofac Implants 1992;7:353-9. 14. Patterson EA, Johns RB. Theoretical analysis of the fatigue life of fixture screws in osseointegrated dental implants. Int J Oral Maxillofac Implants 1992;7:26-33. 15. Budynas RG, Nisbett JK. Shigley’s mechanical engineering design. 9th ed. New York: McGraw-Hill; 2011. p. 221-35. 435-7. 16. Bowden FP, Tabor D. The friction and lubrication of solids. Oxford: Oxford University Press; 2001. 17. Haack JE, Sakaguchi RL, Sun T, Coffey JP. Elongation and preload stress in dental implant abutment screws. Int J Oral Maxillofac Implants 1995; 10:529-36. 18. Nelson SJ, Ash MM, Ash MM. Wheeler’s dental anatomy, physiology, and occlusion. 9th ed. St Louis: Saunders/Elsevier; 2010. p. 284-6. 19. Borchers L, Reichart P. Three-dimensional stress distribution around a dental implant at different stages of interface development. J Dent Res 1983;62:155-9. 20. Lewinstein I, Banks-Sills L, Eliasi R. Finite element analysis of a new system (IL) for supporting an implant-retained cantilever prosthesis. Int J Oral Maxillofac Implants 1995;10:355-66. 21. Craig R. Improved materials pique interest. Dentist 1989;67(15):18-9. 22. Mericske-Stern R, Zarb GA. In vivo measurements of some functional aspects with mandibular fixed prostheses supported by implants. Clin Oral Implants Res 1996;7:153-61. 23. Po JM, Kieser JA, Gallo LM, Tesenyi AJ, Herbison P, Farella M. Time-frequency analysis of chewing activity in the natural environment. J Dent Res 2011;90:1206-10. 24. Tawil G, Aboujaoude N, Younan R. Influence of prosthetic parameters on the survival and complication rates of short implants. Int J Oral Maxillofac Implants 2006;21:275-82. 25. Urdaneta RA, Rodriguez S, McNeil DC, Weed M, Chuang SK. The effect of increased crown-to-implant ratio on single-tooth locking-taper implants. Int J Oral Maxillofac Implants 2010;25:729-43. 26. Jorn D, Kohorst P, Besdo S, Rucker M, Stiesch M, Borchers L. Influence of lubricant on screw preload and stresses in a finite element model for a dental implant. J Prosthet Dent 2014;112:340-8.
Corresponding author: Dr Farideh Geramipanah Implant Research Center Dentistry Research Institute Tehran University of Medical Sciences North Amir-Abad Tehran IRAN Email: [email protected] Acknowledgment Computing support of this research was done at the High-Performance Computing Research Center of the Amirkabir University of Technology. Copyright © 2015 by the Editorial Council for The Journal of Prosthetic Dentistry.
Bulaqi et al