Journal of Mechanical Science and Technology 26 (3) (2012) 759~766

www.springerlink.com/content/1738-494x DOI 10.1007/s12206-011-1107-3

Effects of material properties and hole designs of the jig on the fatigue life of

dental implants† Jong-Su Bae and Hyun-Yong Jeong*

School of Mechanical Engineering, Sogang University, Seoul, 121-742, Korea

(Manuscript Received July 20, 2011; Revised October 23, 2011; Accepted November 20, 2011)

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Abstract A purpose of this paper is to analyze the effects of material properties and hole designs of the jig on fatigue test results of dental im-

plants. An implant fatigue test method is described in ISO14801, which requires that the jig should firmly hold the implant and the elastic modulus of the jig should be more than 3 GPa. However, these requirements are insufficient to represent a dental implant in the jawbone as the fixture is osseointegrated in the jawbone that comprises of the cortical bone and the cancellous bone. In this paper, three different materials for the jig and two different hole designs of the jig for holding the implant were examined in FE simulations and fatigue tests. From the simulation and test results, the effects of material properties and the hole designs of the jig were evaluated in the light of fatigue life of dental implants.

Keywords: Dental implant; Fatigue test; Fatigue test jig; Finite element simulation ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 1. Introduction

A dental implant refers to an artificial dental root planted in the jawbone in place of a lost natural tooth. Since the osseoin-tegration concept of directly fusing organic tissues with metals was introduced by Branemark in 1952, single tooth implants and completely edentulous implants have become the most commonly practiced methods [1, 2]. In an implant, a part planted inside of the jawbone for the purpose of providing physical support is referred to as the fixture. A screw is used to connect this artificial dental root with an abutment which is attached to an artificial tooth. This is a popular method at the present time as it enables one to recover to the dental state prior to the loss and to achieve outstanding results in both functional and esthetic qualities. However, as this method is associated with planting into the jawbone, a continual usage of the artificial tooth induces repetitive loads to be exerted due to the masticatory movements.

Consequently, long-term stability needs to be assured against fatigue damage, and this leads to the need for estab-lishing the structural stability of an implant itself along with the stability of adjacent osseous tissues in adhesion of fixture and jawbone [3]. Good jaw bone conditions lead to high sta-bility of the implant, while thinness of the cortical bone along

with low rigidity and low density of the jaw bone tends to introduce to low stability of the implant [4]. In addition, a number of previous studies have revealed that stresses are largely concentrated in the cortical bone and that there is in-significant amount of the stress concentration in the cancellous bone, while the density of jawbone affects the stresses [5-7].

With more studies on implants having been conducted, the success rate of implant practice has been largely enhanced. However, there are still reports of problems occurring in im-plants. The problems can be categorized into the following three causes. The first is the bone resorption between the fix-ture and the associated jaw bone [8, 9]. The second is raveling of the screw which is used to connect the fixture and the abutment [10, 11]. The third is severe deformation and frac-ture of the implant and dental prosthesis [12, 13]. Since an implant is supposed to have direct adhesion of fixture and jawbone, a fracture in the fixture can be a large burden to the patient. Consequently, studies to enhance the success rate in various bone qualities while avoiding problems such as bone resorption, raveled screws, and fracture are continually in demand.

Various studies have been conducted for dental implants, and new designs have been introduced. Thus, for the selection of implants among various types of implants which can result in the higher success rates in practice, a testing method which can better simulate the practice conditions needs to be con-structed. The fatigue test presently being conducted for per-formance testing of implants is specified in ISO14801 [14].

† This paper was recommended for publication in revised form by Associate EditorJin Weon Kim

*Corresponding author. Tel.: +82 2 705 8640, Fax.: +82 2 712 0799 E-mail address: jeonghy@sogang.ac.kr

© KSME & Springer 2012

760 J.-S. Bae and H.-Y. Jeong / Journal of Mechanical Science and Technology 26 (3) (2012) 759~766

ISO14801 specifies that fatigue tests should be performed with conditions of constant bone resorption of 3 mm along with 30° inclination of the load from the axial direction of the implant. However, these requirements are insufficient to evaluate fatigue resistances under the various bone resorption amounts. In addition, implants provide support through the osseointegration of the fixture threads after being planted into the jawbone, but ISO14801 does not specify an explicit jig hole design for fatigue tests. Consequently, there are fatigue test reports produced by using various jigs such as cylindrical hole steel jigs, resins and artificial bones [15-17]. Such fatigue test jigs fail to properly represent the jawbone, and thus they are limited in providing accurate evaluation of the fatigue life of implants. Therefore, a test method which can consistently and accurately represent the osseointegration and the proper-ties of the jawbone is further required.

In this study, finite element simulations were performed to analyze the effects of different material properties and hole designs of the jig on the stress distribution and deformation occurring in the implant. In addition, fatigue tests using jigs with three different materials and two different hole designs were conducted to evaluate the effects of different material properties and hole designs on the fatigue life of implants.

2. Materials and method

2.1 Implant

The implant evaluated in the fatigue tests is a product of a manufacturer located in Korea. Its fixture includes a platform of 3.6 mm width and 10 mm length. This product is a two stage type with a fixture and an abutment. The connection between a fixture and a connecting part of an abutment is through a hexagon-shaped internal type with a taper. The fix-ture and the abutment are fastened using a ratchet capable of exerting the maximum torque of 30 Ncm. During the experi-ment, a semi-globe shaped cap was employed to reduce the friction between the tester ram and the implant. Table 1 shows the material properties of the implant parts used in this study [5, 6, 18-20].

2.2 Jigs

Based on the tensile strength, modulus of elasticity, and density of the jawbone, PPS (Polypropylene sulfide) and PP (Polypropylene) were selected to represent the cortical bone and the cancellous bone, respectively [6, 21-24]. The material properties of PPS and PP are compared with those of the cor-tical bone and the cancellous bone in Table 2 and 3 [25, 26]. Since the jawbone has been reported to have the cortical bone 1~2 mm thick, the thickness of the cortical bone was assumed to be 2 mm in this study [6, 15, 20]

Five different jigs using three different materials and two different hole designs are shown in Fig. 1. Fig. 1(a), (b), (c), (d) and (e) show a jig made of PPS with a threaded hole, a jig made of PPS+PP with a threaded hole, a jig made of PP with a

threaded hole, a jig made of PPS with a cylindrical hole, and a jig made of steel with a cylindrical hole, respectively.

The PPS plate at the top of the PPS+PP jig shown in Fig. 1 (b), which most accurately represents the jawbone, was ad-hered to the PP block using polymer-based hardening agents and adhesives. The PPS jig shown in Fig. 1(a), which repre-sents the cortical bone, and the steel jig shown in Fig. 1(e), which has been mostly used in previous studies, were also used in this study [12, 13, 27]. As for jig hole designs, the threaded hole was employed to represent the osseointegration between an implant and the jawbone. The cylindrical hole was

Table 1. Material properties of an implant used in this study.

Parts/materials Young's modulus (GPa) Poisson's ratio

Fixture 116 0.34

Screw 116 0.34

Abutment

Titaniumalloy

Grade Ⅳ 116 0.34

Table 2. Mechanical properties of PPS and the cortical bone.

Property PPS (GF/MF 40%) Cortical bone

Yield strength 140 MPa 120~140 MPa

Elastic modulus 14 GPa 13~14.5GPa

Density 1.64 g/cm3 1.54~2g/cm3

Table 3. Mechanical properties of PP and the cancellous bone.

Property PP Cancellous bone

Yield strength 30 MPa 1~30 MPa

Elastic modulus 1.6 GPa 1~2.5 GPa

Density 0.91 g/cm3 0.7~0.95 g/cm3

(a) PPS threaded jig (b) PPS+PP threaded jig (c) PP threaded jig

(d) PPS cylindrical jig (e) Steel cylindrical jig Fig. 1. Flow chart.

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selected because it is mostly used in implant fatigue tests. The threaded or cylindrical hole in the jigs was produced in

the following procedure. Using a milling machine, a puncture of 2.85 mm diameter and 7 mm depth was created at the cen-ter. By using a proper tap, a screw thread was formed. To simulate an implant planted in the jawbone, the same drills and taps used in the actual practice were used. For the cylin-drical type jigs, a puncture was made at the center using a milling machine with a drill of 3.4 mm diameter, which is equal to the diameter of fixture at 3 mm.

2.3 Test procedure

INSTRON 8871 model was used for the fatigue tests. In conducting a fatigue test, it is essential to precisely control the load. In addition, a real time monitoring needs to be provided in order to examine whether the load is provided at the right frequency. INSTRON 8871 has a maximum load of 1000 kgf, and it is adequate to perform the fatigue test for implants.

According to ISO14801, 3 mm of bone resorption was as-sumed to exist in each of the jig, meaning that the top of the fixture has a distance of 3 mm to the top of the jig. An implant was fixated at 30 degrees declination from the direction of the load, and the jig and the exerting point were 11 mm apart. In addition, the load was exerted repeatedly at a frequency of 3 Hz. ISO14801 requires that the frequency should be from 3 to 15 Hz. In this study the lowest frequency was selected because a low frequency fatigue test of metal specimens is known to result in a low fatigue life. The overall test set-up is depicted in Fig. 2.

A Jig and a semi-globe-shaped cap were glued using RelyX Luting Cement from 3M. To maintain the constant tempera-ture during the experiment, a thermostat was set to 20°C. For Group A, the maximum load of 250 N was used, which is the smallest load required by the KFDA (Korea Food & Drug Administration) test protocol [28]. Group B was different from Group A only in the load conditions. To select the proper load conditions for Group B, a static compressive load test was performed as recommended in ISO14801 three times, and the load-displacement curves are depicted in Fig. 3. From the tests performed with the cylindrical steel jig, the maximum

load (approximately 600 N) was obtained. ISO14801 specifies that the maximum load used in a fatigue test should be 80% of the maximum load obtained in a static test, and the minimum load should be 10% of the maximum load. Thus, 480 N and 48 N were determined as the maximum load and the mini-mum load, respectively, for Group B. According to Duyck et al., the maximum masticatory force applied to the implant is 450 N [29]. Thus, the test for Group B may represent fatigue under repetitive loads close to the maximum masticatory load. All the test conditions are summarized in Table 4.

2.4 Implant FE model

Prior to performing the fatigue tests, in order to evaluate the effects of the materials and the hole design of the jig on the deformation and the stress distribution through a finite ele-ment (FE) analysis, an FE model of an implant in the jig was required. An FE analysis enables to exclude unintended exter-nal factors, and it also requires relatively less efforts in modi-fying the structure of an implant, allowing comparison of the structure before and after modification. In addition, it holds an advantage that the stress distribution in an implant is more readily obtained [30]. For the simplification of the FE model and numerical calculations, the materials were assumed to be homogenous isotropic linear elastic.

An FE model for an implant in a jig subjected a load was created as depicted in Fig. 4. The overall shape of the jig was cubical with a dimension of 14 mm x 14 mm x 11 mm. To simulate the different material properties of the jawbone, the cortical bone thickness was assumed to be 2 mm, and the rest was assumed to be the cancellous bone. The point of the load was selected to be 11 mm from the top of the jig and at the

Fig. 2. Fatigue test device.

Table 4. Fatigue test conditions.

Test condition

Group A Group B

Implant length 11 mm (without cap) 14 mm (with cap)

11 mm (without cap) 14 mm (with cap)

Frequency 3 Hz 3 Hz Compressive

load 25 N ~ 250 N 48 N ~ 480 N

Temperature 20 ℃ 20 ℃

Fig. 3. Load-displacement curves of three static compressive tests.

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center on the top surface. ISO14801 specifies that the line of the action of the load should pass the center even with a cap on top of the implant. Thus, in the FE model the load was applied at the center although the cap was removed to reduce the computation time. The upper part of abutment covered with a prosthesis was modeled as a rigid body, and the right and bottom surfaces of the block were fixed in all six degrees of freedom because the block was pushed on those two sur-faces against the outside steel block as shown in Fig. 2. In addition, a complete osseointegration was assumed between the fixture and the block, using the tie option.

2.5 Analysis results

To determine the mesh size that offers an accurate result in a reasonable amount of computation time, the number of ele-ments was increased to double each time from models 1, 2 and 3 as shown in Table 5. The mesh sensitivity analysis depicted in Fig. 5 shows that there occurred an insignificant difference in the maximum von Mises stress between model 2 and model 3, although there was a noticeable difference between model 1 and model 2. Since model 2 had fewer elements than model 3, it needed less computation time. Thus, model 2 was deter-mined to be reliable and selected for the further simulations.

The FE analysis was conducted under the identical condi-tions for the fatigue tests. Fig. 6(a) and (b) represent the stress distributions when the implant was subjected to the load of

250 N and 480 N, respectively. The sequence of the results depicted in the figure is identical to the sequence illustrated in Fig. 1; PPS threaded type, PPS+PP threaded type, PP threaded type (top three figures), PPS cylindrical type and Steel cylin-drical type (bottom two figures). The area which exceeds the yield stress of an implant (650Mpa) is depicted in gray color, and the maximum stress exerted in each implant is compared in Fig. 7.

The analysis results of Group A and Group B shown in Fig. 6 illustrate that there are little differences in stress distributions due to the different material properties, but there are signifi-cant differences in stress distributions due to the different jig hole designs. In threaded jigs the stress was more widely dis-tributed toward the bottom of the fixture, but in cylindrical jigs the stress was more concentrated in the upper part of the fix-ture and around the top surface of the jig. This difference came from the fact that the contact area was larger in the threaded jigs than in the cylindrical jigs. As shown in Fig. 7, the maximum stresses in the PPS and steel cylindrical type jigs were significantly larger than those of the other types, proving that the jig hole design plays a bigger role in the maximum stress.

Even though there were significant differences in the maximum stresses occurring in the implants due to the jig hole designs, the area where stress concentration occurred re-

Fig. 4. Schematic diagram of FE analysis.

Table 5. Number of nodes and elements, and element types used in the FE models.

Number of nodes

Number of elements Element type

Fixture 1. 3347 2. 4718 3. 8732

1. 10470 2. 15802 3. 31939

C3D4 (tetrahedron)

Abutment 1. 1213 2. 1536 3. 3183

1. 3715 2. 1935 3. 10951

C3D4 (tetrahedron)

Screw 1. 904 2. 1334 3. 2341

1. 2869 2. 4624 3. 8542

C3D4 (tetrahedron)

Threaded type jig 9153 40365 C3D4 (tetrahedron)

Cylindrical type jig 13639 11748 C3D8 (hexahedron)

Fig. 5. Mesh sensitivity results in terms of the maximum von Mises stress.

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mained around the implant adjacent to the top surface of the jig.

The magnitude of the maximum stress and the location of the stress concentration directly affect fatigue life in the fa-tigue test. Thus, the FE analysis predicted that the jig hole design is more dominant than the jig material properties in the aspect of fatigue life as well. In addition, fracture is predicted to occur around the area of the implant adjacent to the top surface of the jig.

3. Fatigue test results

According to the fatigue test conditions and procedures mentioned above, each test piece was tested three times, a total of 30 fatigue tests. According to ISO14801, the required number of loadings is to be 5 million cycles. However, due to excessive testing time, 1 million cycles was adopted in this study. It is noteworthy that the purpose of this test is to evalu-ate the effects of the material properties and the hole design of the jig on the fatigue life. In addition, two groups of tests were conducted; Group A and B tests were conducted under the

maximum load of 250 N and 480 N, respectively. Test sequences were randomized to disperse the effects of

unknown factors. Data for each test piece were extracted at every 1,000 cycles to measure the maximum and minimum loads, displacements, and the displacement amplitude. As an example, Fig. 8 depicts the load being exerted on to the im-plant planted in the cylindrical PPS jig of Group B with a rapid drop at the end of the curves. This rapid drop was due to fracture of the implant, and the number of loading cycles was interpreted as its fatigue life. The fatigue life data for a total of 30 test pieces are depicted in Fig. 9. All the tests for Group A were completed successfully for one million cycles, meaning that under 250 N the material properties of the jig and the jig hole design do not play a significant role in the fatigue life of the implant. However, only tests with PP threaded jig and PPS+PP threaded jig among Group B successfully concluded one million cycles. Tests with PPS threaded jig also survived more than 650,000 cycles. In contrast, the tests with PPS cy-

(a) Group A simulation results for the maximum load of 250N

(b) Group B simulation results for the maximum load of 480N

Fig. 6. von Mises stress distributions in the implant and the jig.

Fig. 7. Comparison of the maximum stresses.

Fig. 8. Fatigue test results.

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lindrical jig and steel cylindrical jig survived less than only 150,000 cycles, which is approximately 7 times earlier than those with the threaded jig types. Note that PP and PPS have similar material properties to those of the cancellous bone and the cortical bone, respectively, and the threaded jig mimics the complete osseointegration between the jaw bone and the im-plant fixture. Thus, it can be derived that the design of the jig hole largely affects the fatigue life of an implant as in the case of the maximum stresses shown in Fig. 7.

The average displacement amplitude is depicted in Fig. 10. Note that the displacement amplitude indicates the stiffness of the jig, showing that the stiffness of the PP threaded jig was the lowest, that of the steel cylindrical jig was the highest, and those of the rest three jigs were similar to one another as ex-pected.

As previously mentioned, the stress exerted on implants which are planted in the jaw bone is largely concentrated in the cortical bone, whereas there is no stress concentration in the cancellous bone [5-7]. The fact that the displacement am-plitude for the PPS+PP threaded jig was similar to that of the PPS threaded jig implies that the PPS slice glued on top of the PP block in the PPS+PP threaded jig supported almost all of the load.

Furthermore, another noticeable result from the fatigue tests was the fracture location. According to the FE analysis results, the maximum stress point was independent of the material

properties and the hole design of the jig, and the stress was the biggest at the implant adjacent to the top surface of the jig. The fractured implants, which are illustrated in Fig. 11, also show that fractures were centered on the fixture adjacent to the top surface of the jig. According to ISO14801, 3 mm of bone resorption was assumed in this study. Since the maximum stress and consequent fracture occurred at the implant adjacent to the top surface of the jig, the amount of bone resorption is important in light of the location of the maximum stress and consequent fracture. Due to the improvement of material and surface treatment of the fixture, the amount of bone resorption is usually less than 3 mm [31]. Thus, the effect of the bone resorption on the fatigue life of implants will be examined in the future.

4. Conclusions

The following conclusions can be derived from this study. (1) Three different materials (PP, PPS and steel) and two

different jig hole designs (threaded hole and cylindrical hole) were proposed to evaluate the effects of the material proper-ties and the jig hole design on the fatigue life of implants.

(2) The threaded jigs and the cylindrical jigs showed sig-nificant differences in the fatigue life, about 7 times different

(a) Fatigue life of Group A

(b) Fatigue life of Group B

Fig. 9. Test results of fatigue life.

Fig. 10. Average displacement amplitude.

Fig. 11. Specimens fractured in the fatigue test.

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fatigue lives. Based on these test results, it can be derived that the performance of an implant in an actual practice environ-ment is largely affected by the adhesion conditions between a jawbone and an implant.

(3) The fatigue life using the PPS+PP threaded jig was al-most the same as that using the PPS threaded jig because the stress exerted onto an implant is largely concentrated on the upper cortical bone, whereas there is little stress concentration at the cancellous bones.

(4) Fracture occurred at the implant adjacent to the top sur-face area of the jig regardless of the hole designs or material properties. This infers that the stress concentration largely affects the fatigue life. It can be also determined that if the weak point of an implant is around 3 mm of bone resorption, the performance of the implant is underestimated than the actual performance. Similarly, when the weak point is higher or lower than the mentioned point, the performance is overes-timated. Therefore, a fatigue test simulating different degrees of bone resorption is further required to provide more accurate evaluation of the performance of an implant.

Acknowledgment

This work supported by Research Program supported by Dentium, S. Korea. The financial and material support from Dentium is highly appreciated.

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Jong-Su Bae received his B.S. degree in Mechanical Engineering at Dong-a University in 2009. He is now enrolled in the Master program in Mechanical Engineering at Sogang University. His research interests include biomechanics and structural analysis.