2010 IEEE EMBS Conference on Biomedical Engineering & Sciences (IECBES 2010), Kuala Lumpur, Malaysia, 30th November - 2nd December 2010.

Effects of Different Zygomatic Implant Body Surface

Roughness and Implant Length on Stress Distribution

Muhammad Ikman Ishak'), Mohammed Rafiq Abdul Kadir2)

1,2Medical Implant Technology Group, Faculty of Biomedical Engineering and Health Sciences, Universiti

Teknologi Malaysia, 81310 UTM Skudai, Johor, MALAYSIA.

Imikman_ishak@yahoo.com, 2rafiq@biomedical.utm.my

Abstract- Among factors to contribute for the primary implant

stability are implant design parameters as well as implant body

surface condition. Through this study, the stress distribution

within bone around implant will be investigated under variation

of implant lengths and surface conditions by using three

dimensional finite element analysis. Six finite element models

involving three different zygomatic implant lengths - 46.5 mm,

41.5 mm and 36.5 mm with a smooth surface and rough surface

of implant body have been analyzed. Three dimensional models

of craniofacial including soft tissue and prosthesis were

constructed from computed tomography (CT) images datasets.

The implant models were developed using computer-aided design

(CAD) software and all models were analyzed via finite element

analysis software. A 230N of vertical force was applied on top

surface of prosthesis in second premolar region and a masseter

load of 300N applied at zygomatic arch. The result showed that

the increase of zygomatic implant length shows an ability to

reduce both cortical and cancellous bone stress. Besides, the use

of rough implant surface has resulted in reduction of stress value

at bone-implant interface as well as at surrounding bone. The

alveolar bone seems to play a lesser significant role in the zygomatic implant anchorage compared to the zygomatic bone.

Keywords-finite element analysis; zygomatic implant; different

lengths; surface roughness; atrophic maxilla; edentulous

L INTRODUCTION

The introduction of zygomatic implant as an alternative procedure to treat atrophic maxilla patients has shown a good outcome in many clinical-based studies [1]. The original purpose of zygomatic implants was to rehabilitate patients who had undergone maxillectomy due to tumor resection, trauma or congenital defects [1-5]. However, the function of this implant had been expanded for rehabilitation of edentulous resorbed maxillae patients [2].

The role of surface roughness towards osseointegration between bone and implant is crucial for the primary and long term implant stability. As the placement of zygomatic implant performs a surface contact with maxilla and zygomatic bone, the strength of engagement at those areas cannot be compromised. In addition, the achievement of primary stability for implant in the bone is mainly depends on several factors such as bone quality, bone quantity, implant design

978-1-4244-7600-8/101$26.00 ©201 0 IEEE 210

Eshamsul Sulaiman3), Norhayaty Abu Kassim4)

3,4Department of Conservative Dentistry, Faculty of Dentistry, Universiti Malaya, Kuala Lumpur, MALAYSIA.

, 4 °eshamar@um.edu.my, nhayaty@um.edu.my

especially in geometrical aspect, implant body surface roughness and surgical procedures [6].

Currently, dental implants specifically have been available in various types of surface roughness characteristics such as machined surface or known as smooth surface [7]. Besides, implant body also comes with coated surface which often known as oxidized or rough surface. Basically, the employment of rough surface implant will increase the implant surface area which is proportional to the increase of bone-to-implant contact surface [6]. As discussed in several clinical-based studies, the increase of mating surface between implant and bone will allow more osseointegration to occur. A study from Ibanez et al has shown a high survival rate of 99.42% by using acid-etched surface implants to treat completely and partially edentulous patients in immediate function. A rough surface implant will develop a firmer mechanical link to the surrounding bone tissue compared to implants with machined surface [6]. According to Bacchelli et ai, the sandblasted surface offered more bone ingrowth through implant threads compared to other implant surface types. As suggested by Carlos Aparicio et aI., there is a necessity to study the effect of oxidized surface on zygomatic implant clinically for a long term performance [8]. Probably, there is a need for a shorter implant with rough surface condition to be used in zygomatic implant application to treat atroph ic edentulous patients [1]. Other than that, it is important to determine the role of alveolar ridge bone in supporting the zygomatic implant.

In zygomatic implant placement, the most strength part for implant anchorage located at zygomatic bone according to some studies. Although the implant placement also covered alveolar ridge region, the engagement of implant at the region still be questionable [8]. Zygomatic bone has a wider and thicker cancellous bone which could allow for implant initial stability [9]. Compared to the other bones especially posterior maxilla, zygomatic bone is free from bone resorption and regeneration that makes it available for the implant anchorage [9]. However, Nkenke et al have revealed that the parameters of zygomatic bone are poor for the implant placement. The success rate of the zygomatic implant can be guaranteed by having four cortical portion engagement of implant insertion

path. Unlike to Stievenart et ai, who claimed that the high success rate of zygomatic implant is because of tricortical anchorage. Therefore, several judgments about the strength of implant anchorage have led this present study to be conducted [9].

Thus, the aim of this study is to investigate the biomechanical effects of stress distribution on bone surrounding zygomatic implant with different types of implant surface roughness - machined surface and rough surface. A variation of implant lengths - 46.5 mm, 41.5 mm and 36.5 mm also will be considered for each type of surface condition.

II. MATERlAL AND METHODS

A three dimensional (3D) model of edentulous human craniofacial with some degree of resorption was reconstructed from a series of computed tomography (CT) images data set using a materialise software of Mimics. The two dimensional CT images data sheets were applied with threshold feature to create a mask. From the mask, the images would be edited slice by slice and then would be changed into a 3D model. This bone model was assumed to be symmetrical for both side, therefore, only one side of the model would be analysed [10]. The selected region of interest was at the left part, which covers maxilla, infrazygomatic crest and zygomatic arch as well. Both maxilla and zygomatic bone were reconstructed by consisting of two layers, cortical layer and cancellous layer. The cortical model has a thickness ranging from 2.22 mm to 1.36 mm [II]. Besides, a measurement has been conducted on bone model in order to obtain some critical dimensions for zygomatic implant model development [12, 13]. A distance of 48.94 mm from jugale point of zygomatic bone to alveolar crest has been determined to represent the length of implant [13]. The angulation of implant head was about 45.76° has been measured and represented by the angle between the jugale point-alveolar crest distance and the plane through the infraorbital foramen [13].

w (� Fig. I (a) Two dimensional CT image of human craniofacial in coronal view.

(b) 3D solid model of craniofacial with region of interest (red).

The three dimensional model of soft tissue and prosthesis also been considered and reconstructed using the similar CT images. The soft tissue model has a thickness of 0.39 mm to 7.58 mm. A partial prosthetic superstructure with flange was modeled based on original patient's removable denture. The

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prosthesis model used in the present study was a fixed restoration type which tightly connected to the implant abutment by screws [14]. It has modeled with 1.52 mm to 3.46 mm in thickness, 12.45 mm to 19.06 mm in width and 15.41 mm to 18.37 mm in height.

In this study, a concept of one zygomatic implant placed bilaterally in conjunction with two conventional implants placed in anterior region will be used to treat the atrophic maxilla since the height and width of posterior region did not much support for the implant placement. A 5.76 mm and 9.69 mm have been determined for the height and width dimension of alveolar bone in molar region respectively [15, 16]. Therefore, three units of zygomatic implant with different length were employed together with multi-unit implant abutment of similar size. Since both sides of craniofacial were assumed to be symmetrical, each condition will include only a zygomatic implant placement in particular length stabilized by a conventional implant in anterior region. The zygomatic implant and abutment were designed into 3D solid models using a computer-aided design (CAD) software of Solid Works. The conventional implant of 10 mm in length and 4.0 mm in diameter with 30° angulated abutment has been used. All of implant models are products from similar manufacturer which is Branemark System (Nobel Biocare) [14].

The 3D solid models of implant will be converted into surface triangular elements or meshed elements via other CAD software of Abaqus. The elements size has been assigned to 0.5 mm for all models which is corresponding with the size used in Huang et al. study[17]. Surgery simulation or implantation will be performed just after the implant models have been positioned correctly in bone. The surgical approach of extramaxillary was chosen to place the zygomatic implant in bone [12]. It is interesting to note that the surgical approach used, conventional implant and prosthesis design, and material properties of all models have been maintained through all analyses. The triangularized elements for each model were then imported into MARC Mentat software for tetrahedral elements creation. As a result, the total numbers of 394300 elements were generated in models with implant length of 46.5 mm, followed by 389926 elements for 41.5 mm and 379292 elements for the model with 36.5 mm of implant length.

I..: 36.50101 �I

IOm

I 41.50101

� 46.50101

�I 40101

(a) (b)

Fig. 2 (a) Zygomatic implants model in different length with multi-unit abutment. (b) Conventional implant model attached with 30° angulated abutment.

For this finite element study, all models were assumed to be homogenous, static, isotropic and linearly elastic. The material properties of each model are shown in the Table 1.

TABLE I

MATERJAL PROPERTIES USED IN FINITE ELEMENT ANALYSIS

Material Young Modulus, Poison

References YM (MPa) Ratio, v

Cortical 13,400 0.3 [14, 18] Cancellous 1000 0.3 [19] Soft tissue 2.8 0.4 [20] Prosthesis 100,000 0.3 [14] Implants 110,000 0.33 [14, 18]

Six models involved three zygomatic implants with different lengths (46.5 mm, 41.5 mm and 36.5 mm) will be evaluated based on biomechanical criterion of stress distribution around bone under two types of zygomatic implant body surface roughness condition (smooth or machined surface and rough or oxidized surface). Thus, the properties of implant to bone contact for each surface condition will be different. A friction coefficient of 0.3 was applied to the implant body for smooth surface type and 0.6 for rough surface type. The contact properties between implant body, abutment and prosthesis were assumed with 0.3 of friction coefficient. Note that the immediate function loading has been chosen as the type of implantation procedure for the analysis [6].

The top cutting plane, midsagittal plane and posterior plane were constrainted in all axes (x, y and z) to prevent the model from any movements [21]. A vertical load with value of 230 N was applied on the top surface of prosthesis in second premolar [20]. The loading applied has represented the simulated occlusal forces generated during chewing action. In

representing the existence of masseter muscle attachment on zygomatic arch, a simulated masticatory load with components of 62.12 N in x-axis, -265.20 N in z-axis and 125.69 N in y-axis were applied[IO, 14]. All boundary conditions and loadings configuration showed in Fig. 3.

L

230 N 230N

Fig. 3 Boundary conditions and loadings configuration in FEA models.

III. RESULT

The distribution of von Mises stress within bone tissues (cortical and cancellous bone) has been investigated in the current study. Based on result, a lower cortical bone stress was found at bone-implant interface and at bone region around implant site for the rough implant surface type compared to the smooth surface type. This condition has occurred in all variation of implant lengths. The cortical bone stress seemed to be decreased proportionally with the increase of implant length in both surface types. The use of 41.5mm of implant length exhibited a reduction of 4.71% bone stress in rough surface and 42.5% in smooth surface. For the 46.5mm model, it reduced the bone stress by 12.37% and 41.5% for the rough and smooth surface respectively. As can be seen in Fig. 6, the widest stress concentration area was generated in 36.5mm model indicated by more yellow to grey colour contour presence and the high stress value recorded as well. The implant length of 46.5mm showed the smallest stress distribution within bone, however, the highest stress still located at the bone-implant interface and at bone surrounding.

Fig. 7 illustrated the stress distribution within cancellous bone for the three different implant lengths. There were two portions of cancellous bone involved for the implant anchorage, at the maxilla and zygomatic bone. A similar pattern of stress distribution was observed among the three bone models without major significant differences. However, the stress value varies depending on the implant length and surface roughness. The most affected region was at cancellous periimplant site in maxilla with a more stress localization. The increase of implant length contributed to the decrease of stress values by 5.39% in 41.6mm model, however, 46.5mm model showed an increase by 3.86% for the rough surface type. The cancellous bone in zygoma for the 36.5mm model seemed to have a largest stress distribution than the other bone-implant interface lengths with a maximum stress value of 10.1 MPa. For the smooth surface type, the stress magnitude in the cancellous bone for all implant lengths showed an increase compared to the rough surface type. The percentage of increasing was about 66.3% in 36.5mm model, 56.3% in 41.5mm model and 0.2% in 46.5mm model. The highest stress value was found at cancellous bone-36.5mm implant interface with 30MPa. Fig. 4 and Fig. 5 (graph) showed the comparison of stress distribution among particular positions.

18

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o l-.... .c��--.... c:ll -, Corticrtl ROlle (;(IllceII01I� Rone

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Fig. 4 Comparison of the von Mises stress for rough surface type. Average values (bar) and maximum values (line).

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50

45

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Fig. 5 Comparison of the von Mises stress for smooth surface type. Average values (bar) and maximum values (line).

MPa

(a)

(b)

36.5mm 41.5mm 46.5mm

Fig. 6 von Mises stress distribution within cortical bone for different implant surface types, (a) rough surface and (b) smooth surface among implant lengths.

(a)

(b)

36.5mm 41.5mm 46.5mm

Fig. 7 von Mises stress distribution within cancellous bone in maxilla for different implant surface types, (a) rough surface and (b) smooth surface among

implant length.

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IV. DISCUSSION

The present study was conducted to investigate the influence of implant lengths and implant surface roughness towards the stability of implant in rehabilitating the edentulous atrophic maxilla patients. The primary stability of implant is likely to be contributed by several factors such as surgical technique, implant design, implant surface roughness, bone quality and bone quantity [22]. The implant length plays an important role in order to determine the success rate of dental implant in delay loading or immediate loading especially at where bone quality is compromise. Degidi et al. reported a 99.5% survival rate for implants (standard length and longer implants) in 3 years time-period follow-up study [23]. The study showed that a higher failure rate was recorded by using standard length implant compared to longer implant in immediate loading due to the poor bone quality[23]. Basically, the short implant is not preferable to be used especially in poor bone condition as the force applied will be concentrated mostly at the crest bone region rather than distributed throughout the implant body [18, 24].

In this present study, a shorter implant with surface treated is expected to have more engagement to the bone tissue and resulted in decreasing bone stress [7]. However, our result showed that the use of longer zygomatic implant has decreased the stress magnitude generated within cortical bone in both surface roughness types, rough surface and smooth surface. The reduction of bone stress occurred probably due to the increase of bone-implant interface generated from more cortical layer penetration [25]. The shorter implant used, 36.5mm has provided bone-implant contact at maxilla and slightly at maxillary sinus wall. In comparison with longer implant used, 41.5mm and 46.5mm, the zygomatic implant had a larger mating surface between bone and implant. The implant seemed to have a higher percentage of cortical mating surfaces with implant body since the insertion path of implant also covers the cortical layer in zygomatic bone [9]. This is in agreement with the literature reviews done by Lee et al. who reported that the cortical bone engagement would be more important factors rather than implant length [25]. In zygomatic implant application, the apical part of implant body should be anchored in zygomatic bone and the coronal part will engage in alveolar crest bone. However, a study from Carlos et al. reported the role of alveolar bone in osseointegration still being questionable [8]. Through this study, it was clearly shown that the cortical bone in maxilla specifically at crestal region sustains more stresses by using shorter implant compared to longer implant. The high generated stress at alveolar bone around implant neck should be avoided as it will lead to the bone loss in a long term treatment [26]. It might shows that adequate strength for anchorage in zygomatic bone is achievable for using longer implant by leaving behind a less widespread of stress distribution in the maxilla [9]. This result could show that the alveolar bone offers less implant anchorage strength than in zygomatic bone.

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The use of rough implant surface has provided lower bone stress generated either in cortical or cancellous bone compared to the smooth implant surface. The rough implant surface offers more bone-implant contacts since the implant surface area increases due to the applied coating layer [6, 7]. The coating layer allows a strong physical interlock with the cancellous bone and therefore encourages increased bone­implant contact [7].

The stress value and distribution within cancellous bone were minimal for both surface types. This is likely due to the modulus of elasticity of cancellous bone is lower than cortical bone, therefore the cancellous bone is weaker than cortical bone in resisting the deformation [26].

The depth of implant penetration through cancellous bone is similar among the three implant lengths and has less significant differences in average stress values. In this study, the shortest implant used is 36.5mm and the apical part of implant has penetrated and anchored in a small volume of the cortical layer in maxillary sinus wall. There is no engagement of implant apical portion to the cancellous bone in the zygoma compared to the 41.5mm and 46.5mm model. It can be said that the influence of cortical bone layer engagement is more significant and plays an important role in determining the strength of implant anchorage.

The role of rough surface gives a better chance for the bone to growth and integrates within implant body and resulted in a higher survival rate.

For all implant lengths and surface roughness tested, the highest stresses value were recorded within implant body. This could be because of a high titanium modulus of elasticity of 11 OMPa and peaked at 76MPa in the analysis. The maximum stress values generated within all zygomatic implant bodies (36.5mm, 41.5mm and 46.5mm) have no tendency to the implant failure since titanium alloys are known can tolerate stresses of up to 900MPa [26]. The stress magnitude could increased if the zygomatic implant used alone without stabilized by conventional implant in the anterior region [14].

V. CONCLUSION

In conclusion, the increase of zygomatic implant length shows an ability to reduce both cortical and cancellous bone stress. The strength of zygomatic implant anchorage in rehabilitating the atrophic maxilla is mostly depending on the volume of cortical bone penetration rather than affected by implant length variations. It might shows that the morphology and configuration of zygomatic bone plays a significant role in the primary implant stability. The implant anchorage within alveolar bone is lower than it does in the zygomatic bone. Moreover, the use of rough implant surface has resulted in reduction of stress value at bone­implant interface and surrounding bone which can allow more osseointegration to occur due to high bone-implant contact generated.

ACKNOWLEDGE MENT

An appreciation is given to Medical Implant Technology Group (MEDITEG) and Universiti Teknologi Malaysia.

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