BIOMECHANICAL ANALYSIS OF THE APPLICATION OF ZYGOMA IMPLANTS FOR PROSTHESIS IN UNILATERAL MAXILLARY DEFECT

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1 Journal of Mechanics in Medicine and Biology Vol. 16, No. 8 (2016) (16 pages) c World Scientific Publishing Company DOI: /S BIOMECHANICAL ANALYSIS OF THE APPLICATION OF ZYGOMA IMPLANTS FOR PROSTHESIS IN UNILATERAL MAXILLARY DEFECT D. WANG* and A. QU School of Mechanical Engineering Shanghai Jiao Tong University Shanghai , P. R. China *dmwang@sjtu.edu.cn; dmw_mail@163.net qq_qal@163.com H. ZHOU Shanghai Testing & Inspection Institute for Medical Devices Shanghai , P. R. China cmtc@cmtc.com.cn M. WANG Department of Oral and Maxillofacial-Head and Neck Oncology Ninth People s Hospital Affiliated to School of Medicine Shanghai Jiao Tong University, Shanghai , P. R. China wmyall@163.com Received 2 March 2016 Revised 17 April 2016 Accepted 26 May 2016 Published 23 September 2016 The objective of this research is to evaluate the biomechanical effect of zygomatic implantsupported obturator prostheses in unilateral maxillary defect. Based on CT data, four 3D numerical models were built. One model was a normal craniofacial complex (model 1) and other three models were structures with unilateral maxilla defect reconstructed using claspretained obturator prosthesis (model 2), one zygomatic implant-supported and clasp-retained prosthesis (model 3), two zygomatic implant-supported and clasp-retained prosthesis (model 4). Bilateral vertical loads of 300 N were imposed and the stress and displacement distribution were calculated, analyzed and compared. The bilateral occlusal forces dispersed along the three-mechanical-pillar of the maxillofacial region and the displacement distributed symmetrically in model 1. Because of mechanical pillars break on the affected side, all occlusal forces were transferred by clasps and abutment teeth in model 2, which induced the increase in stress and displacement level. The zygomatic implant restored mechanical pillars and greatly reduced the stress and displacements levels in models 3 and 4. The stress and displacement distributions on clasps, bones, teeth and periodontal ligaments were more reasonable with the support of *Corresponding author

2 D. Wang et al. zygomatic implants. Therefore, the zygomatic implant-supported and clasp-retained prostheses were found to be more effective for unilateral maxillary defect reestablishment. Keywords: Biomechanical behaviors; finite element method (FEM); unilateral maxilla defect; zygomatic implant; obturator prosthesis. 1. Introduction Although unilateral maxillary defects induced by tumors or traumas are very common in clinic, but the management of maxillectomy patients is the most complicated and controversial not only on terms of disease control or cure but also in the best methods of oral and facial reconstruction and rehabilitation. 1 The unilateral maxillary defect, not involving the orbit, belongs to Class II according to the reported latest classifications of vertical and horizontal maxillectomy and midface defect. 1,2 In this class, very good results can be achieved with prosthetic obturation, 3 local pedicle flap, 4 bone graft with the titanium plate, mesh or zygomatic implant. 5 7 However, regional pedicled flaps can provide little option for a functioning dental prosthesis on the operated side, although close the oroantral and nasal fistulae. And bone grafts increase the patient s suffering due to second operation areas and microvascular anastomosis technique. Therefore, obturator prostheses are considered to be a better and preferred choice for the restoration of acquired maxillary defect. Because of the lack of supporting tissues, occlusal force on obturator prostheses must be transmitted by the abutment teeth and palate of the normal side, which will increase the stress level of the abutment teeth and induce teeth loose or insufficient retention. In order to preserve the remaining supporting structure and enhance retention, the framework design of an obturator was optimized including the application of attachment. 3,8 However, frame design optimization of maxillary obturator prostheses cannot reduce the bite force on the normal maxillary side and reconstruct the mechanical pillar on the defect side. As a result, a protocol for surgical and prosthetic reconstruction with zygomatic implants was presented, which greatly improved the therapeutic effect and was a potential innovation of the maxillary reestablishment following limited tumor resection or trauma Compared with the conventional removable prosthesis (CRP), the advantages of zygomatic implant-supported obturation prostheses (ZSP) have been validated in clinic. 10,11 And the stress distributions on zygomatic implants, supporting skeleton and abutments following maxillary reestablishment with ZSP bearing occlusal force on artificial teeth, were preliminarily analyzed and reported. 12 But the biomechanical effects of ZSP, including displacement and stress distribution on ZSP, supporting bones, abutments, periodontal ligament (PL) and yalveolar ridge under the vertical bite force on both the remaining natural teeth and artificial teeth have not been clear. The aim of the present study was to analyze the biomechanical behaviors of ZSP bearing bilateral vertical bite force. A normal craniofacial complex model including

3 Biomechanical Analysis of the Zygoma Implant for Prosthesis all bone segments, teeth, PL, alveolar ridge and palatal plate was created using the CT scan image data. And unilateral maxilla defect was numerically simulated and reestablished using CRP and ZSP. Bilateral vertical bite forces of 300 N were imposed on the teeth of normal and defect sides. The stress and displacement distributions on the zygomatic implant, supporting skeleton, abutment, PL and alveolar ridge, were studied by finite element method (FEM), thereby the biomechanical effects of reconstruction with CRP and ZSP were further contrasted and analyzed. 2. Materials and Methods 2.1. Construction of a normal 3D FE model The CT image data of craniofacial complex was collected from a subject reported in literature. 12 The volunteer was informed and consented to the use of his data. This research was approved by the ethical committee of Shanghai 9th people s hospital. The images were imported into Mimics (Materialise, Belgium) software. Geometrical models of each piece of bones and teeth, with Stereo Lithographic (STL) format, were constructed by image segmentation, region growing and 3D creation. And then the STL models inducted to Geomagic 10.0 (Geomagic, USA) software to build surface models (Fig. 1(a)). The 3D geometrical model of the craniofacial complex was meshed with HyperMesh 10.0 (Altair, USA). All bone and teeth were meshed with 3D solid element and PLs were meshed with shell element with 0.25 mm thickness (Fig. 1(b)). 13 Mechanical properties of all materials in four models were defined as linear elasticity, homogeneity and isotropy. The parameter values of the material properties were shown in Table 1 referring to earlier reported data (a) (b) (c) Fig. 1. (a) The 3D geometric model of a normal craniofacial complex; (b) The 3D FE model of a normal craniofacial complex; (c) The 3D FE model of the teeth and PL in the normal craniofacial complex

4 D. Wang et al. Table 1. Material properties of analysis objects. Materials Young s modulus (MPa) Poisson s ratio Zygomatic bone 11, Parietal bone 12, Sphenoid bone Frontal bone 11, Occipital bone 18, Temporal bone 10, Maxilla Ethmoid bone Nasal concha Nasal bone Vomer Teeth 26, PL Clasp, denture base (Co Cr alloy) 218, Obturator prostheses (Acrylic resin) Zygomatic implants (Titanium) 117, Construction of maxillary reconstruction model Based on the normal model, three reconstruction models with unilateral maxillary defect and different reconstruction protocols were constructed. So, the four models to study were listed below. Model 1: A normal craniofacial complex (Fig. 2(a)). Model 2: A structure with unilateral maxilla defect reconstructed using CRP consisting of a base tray, a prosthesis and three clasps including I bar (C-1) on central incisor (T1), mesial occlusal rest (C-2) on the first premolar (T4) and combined clasp (C-3) on the first and second molars (T6,T7) of the normal side (Figs. 2(c) and 2(d)). Model 3: A structure with unilateral maxilla defect reconstructed using one-zygomatic-implant-supported and clasp-retained prosthesis. The clasp retention was the same as that in model 2. One Bränemark system zygomatic implant (Z-1) was employed to support the prosthesis. The diameter of the zygoma implant was 4.5 mm, and the distance between zygoma and prosthesis determined the zygoma implant length (Figs. 2(e) and 2(f)). Model 4: A structure with unilateral maxilla defect reconstructed using two-zygomatic-implant-supported and clasp-retained prosthesis. The clasp retention was the same as that in model 2. And two Bränemark system zygomatic implants (Z-21, Z-22) were employed to support the prosthesis (Figs. 2(g) and 2(h)) Loading and constraining A distributed vertical load of 300 N was imposed on the occlusal surface of the bilateral premolar and molar, loading location and directions were shown in

5 Biomechanical Analysis of the Zygoma Implant for Prosthesis Fig. 3(a). The surface nodes of the foramen magnum and the surface nodes of the attachment area of the masseter muscle on the zygomatic arch were all constrained for six degrees of freedom (Fig. 3(b)). Stress calculations and analyses were performed in the FE package, ABAQUS V6.10 (Simulia, Dassault Systems, USA). A comparison was made in terms of Von Mises stress and sum displacement pattern to evaluated the biomechanical behaviors of bones and implant reactions under the applied simulated mastication loading conditions. The Von Mises stress and sum displacement levels on the bones, (a) (b) (c) (d) Fig. 2. (a) The FE model of the normal craniofacial complex; (b) The model of the craniofacial complex with left maxilla defect; (c) The FE model of the craniofacial complex with left maxilla defect reconstructed with CRP; (d) The FE model of the CRP; (e) The FE model of the craniofacial complex with left maxilla defect reconstructed with CRP and one zygoma implant; (f) The FE model of the CRP and one zygoma implant for prosthesis retention; (g) The FE model of the craniofacial complex with left maxilla defect reconstructed with CRP and two zygoma implants; (h) The FE model of the CRP and two zygoma implant for prosthesis retention

6 D. Wang et al. (e) (g) Fig. 2. (Continued) (f) (h) (a) (b) Fig. 3. (a) 300 N forces were loaded on the occlusal surface of the bilateral premolar and molar; (b) Boundary constraint and loading position

7 Biomechanical Analysis of the Zygoma Implant for Prosthesis Fig. 4. The concerned position in models: (A) Frontal bone; (B) Anterior nasal spine (ANS); (C) Anterior point of alveolar process; (D) Middle point of alveolar process; (E) Posterior point of alverolar process; (F) Orbital rim (OR); (G) Maxillary anterior border; (H) Zygomatic bone; (I) Zygomatic arch; (J) Temoral bone. teeth, PL, clasps, prosthesis and implants were to be analyzed and contrasted (Fig. 4). 3. Results In Fig. 5, the pseudo-color visualization of Von Mises stress value was shown on all models. The dark-blue color presented zero stress while the gray and red color represented the highest stress value. Maximum, medium and minimum principal stress vectors were displayed as red, green and blue arrows, respectively. The Von Mises stress and principal stress vector distributions on model 1 showed that occlusal forces were dissipated upward mostly through the alveolar ridge and then divided into three paths. One conduction path was along macilla frontal process to front bone, one path along zygoma frontosphenoidal process to frontal bone and the other path along the zygoma to skull base (Figs. 5(a) and 5(b)). And the Von Mises stress and principal stress vector distributions of model 1 were asymmetric. In model 2 (Figs. 5(c) and 5(d)), stress level was greatly increased and the highest Von Mises stress exceeding 112 MPa was occurred at metal palate plate. Stress was concentrated at the prosthesis-tray joints. Stress concentration was also generated at the tooth cervix and alveolar bone of the abutment on the normal side. The prostheses were supported by implants and stress levels of the models were greatly reduced in models 3 and 4 (Figs. 5(e) and 5(h)). The occlusal forces on the affected side were shared simultaneously by the implants and clasps. And two zygomatic mechanical pillars on the affected side were reconstructed (Figs. 5(f) and 5(h)). As shown in Fig. 6, the sum displacement distributions on the four models were visualized employing pseudo-colors. The dark-blue color presented zero

8 D. Wang et al. (a) (c) (b) (d) (e) (f) Fig. 5. (a) Model 1 with Von Mises stress distribution; (b) Model 1 with Principal stress vector distribution; (c) Model 2 with Von Mises stress distribution; (d) Model 2 with Principal stress vector distribution; (e) Model 3 with Von Mises stress distribution; (f) Model 3 with Principal stress vector distribution; (g) Model 4 with Von Mises stress distribution; (h) Model 4 with Principal stress vector distribution

9 Biomechanical Analysis of the Zygoma Implant for Prosthesis (g) Fig. 5. (Continued) displacement and red color presented the large displacement. In model 1, the displacement distribution was basically symmetrical. The sum displacement level increased in model 2 because of cantilever beam effect of the prosthesis. The sum displacement levels on the bone of models 3 and 4 decreased greatly and were similar with that of model 1. The maximum Von Mises stresses and sum displacements of the 10 concerned positions, teeth and PL on the unaffected side of the four models were illustrated and compared as Fig. 7. The results indicated that maxillary reconstruction with CRP led to a large increase in the stress and displacement on the structure of normal (h) (a) (b) Fig. 6. (a) Model 1 with sum displacement distribution; (b) Model 2 with sum displacement distribution; (c) Model 3 with sum displacement distribution; (d) Model 4 with sum displacement distribution

10 D. Wang et al. (c) (a) Fig. 6. (Continued) (d) (b) (c) (d) Fig. 7. (a), (b), (c) Under bilateral Vertical loads, the maximum Von Mises stress values of the same position on the unaffected side of the four models were contrasted; (d), (e), (f) Under bilateral Vertical loads, the maximum sum displacement values of the same position on the unaffected side of the four models were contrasted

11 Biomechanical Analysis of the Zygoma Implant for Prosthesis (e) (f) Fig. 7. (Continued) side while the zygomatic implant-supported prostheses could make the stress and displacement level on the bones, teeth and PLs of the normal side more rational. The stressed and displacements of the concerned positions on the affected side, clasps, trays and implants in the four models were contrasted (Fig. 8). In model 2, the maximum stress and displacement values were the highest on all positions except zygoma. In models 3 and 4, the implants made the occlusal forces on the affected side retransferred to the zygoma and effectively shared the forces on the clasps and trays. Reaction forces calculated from the constrained nodes on the masseter attachment areas were listed in Table 2. The masseter forces on both sides were slightly (a) (b) Fig. 8. (a) Bilateral Vertical loading, the maximum Von Mises stress values of the same position on the affected side of the four models were contrasted; (b) Under bilateral Vertical loads, the maximum Von Mises stress values of the clasps, trays and implants in the left maxillary reconstruction models were contrasted; (c) Under bilateral Vertical loads, the maximum sum displacement values of the same position on the affected side of the four models were contrasted; (d) Under bilateral Vertical loads, the maximum sum displacement values of the clasps, trays and implants in the left maxillary reconstruction models were contrasted

12 D. Wang et al. (c) (d) Table 2. The constraint reaction force magnitude and three components on masseter attachment areas in the four models. increased in reconstruction models when providing the same bite forces. The masseter forces on the affected side increased greater than those on the unaffected side. 4. Discussion Masseter-L (N) Fig. 8. (Continued) Masseter-R (N) Model Fx Fy Fz Amplitude Fx Fy Fz Amplitude M M M M Several studies have been conducted to investigate the biomechanical behaviors and occlusal force transmission capability of the zygomatic implant. 5 7,13,16 The biomechanical effects of reconstructions in unilateral maxillary defect, including obturator prosthesis, implant-supported maxillary prostheses and autogenous bone graft, have been analyzed using 3D FEM. 3,6,17 The biomechanical effects of zygomatic implant-supported obturator protheses designs in a unilaterally maxillary defect was firstly evaluated by Korkmaz et al. 16 in 2012 using simplified models and 3D FEM. Wang et al. 12 established a 3D FE model of the normal human craniofacial complex, simulated unilateral maxillary defect, designed obturator prosthesis supported by zygomatic implant and clasps for the maxillary reconstruction and analyzed the biomechanical effects of the reconstruction under the vertical and lateral loads on the affected side. In this study, Wang s models were further improved by adding the PL tissues. The 300 N loads were imposed on the bilateral sides of the normal and reconstructed models. The biomechanical effects of the

13 Biomechanical Analysis of the Zygoma Implant for Prosthesis reconstructions for unilateral maxillary defect were evaluated by calculating and analyzing the Von Mises stress and displacement of the bone, teeth, PL, clasps, metal palatal plate, implants and prostheses. A vertical load of 150 N was imposed on the defective side according to the reported maximal bite force value of the patients with osseointegrated oral implants 18 and loading method in literatures. 5 7,12,16 But differently, a 150 N vertical load was simultaneously imposed on the non-defective side in order to evaluate the biomechanical behaviors of the reconstructed models during symmetric bite. On the non-defective side in model 1, the maximal Von Mises stress was MPa occurring on the zygoma, position H. In model 2, the maximal Von Mises stress was MPa occurring on the mesial crown of the central incisor, while occurring on the position H in models 3 and 4 with the values of MPa and MPa respectively. In model 2, the maximal Von Mises stress values of every positions (A G, I J) except H were all increased. And the stress levels were also greatly increased in all remaining teeth and PLs, which were consistent with previous finding. 3 The stress increments were larger on the anterior teeth than those on the premolars and molars, which would lead to the abutment teeth loosening. Compared with model 2, the stress levels on bone segments and abutment teeth decreased a lot in models 3 and 4. On the defective side in model 2, the stress level on the zygomatic bone and zygomatic arch significantly reduced because of mechanical pillar interruption caused by maxillary defect. The occlusal forces on defective side were dispersed by clasps and metal palatal plate. There was obvious stress concentration on the metal palatal plate. The maximal Von Mises stress on the metal palatal plate was MPa. In models 3 and 4, the stress level on clasps and metal palatal plate greatly decreased. The maximal Von Mises stress values on the metal palatal plate in models 3 and 4 were MPa and MPa, respectively. The maximal Von Mises stress value on implant in model 3 was MPa while those on Z-21 and Z- 22 in model 4 were MPa and MPa, respectively. The maximal Von Mises stress values of all zygomatic implants occurred in the middle of the zygomatic implants, which was consistent well with the reported results. 5 It is clear that multiimplants provide better stability, retention and more reasonable stress level. However, considering the costs, patient comfort, and the associated risks of the surgery, the number of implants used and their necessity require a rational decision. Furthermore, one implant can decrease the stress level of the clasps, metal palatal plate, bone, teeth and PLs to be an acceptable value. In terms of stress levels, two implants did not show obvious advantages. In model 2, the displacement level was much higher than that in model 1. In models 3 and 4, the displacement levels were close to that in model 1, which indicated that the zygomatic implant effectively increased the stability of obturator prostheses retention. In addition, the masseter was considered as a constraint condition in this study. The reaction force was regarded as the masseter muscle force to generate the

14 D. Wang et al. corresponding occlusal forces. The calculated average value of bilateral muscle force was N, which coincided with the relationship between masseter muscle force and occlusal force The FEM, as a numerical computing and analysis technique and a powerful research tool, is reliable and non-invasive. The method can be used to develop simulations of different defects and simulate corresponding reconstruction designs based on the same geometric model. Furthermore, the model is vivid and the analyzed results are visualized. The stress and deformation on the any interest regions can be most easily evaluated and examined. The model validation, including the sensitivity analysis on material properties and grid, should be performed in order to ensure the reliable and accurate calculated results. Compared with the experimental method, the anatomic structure, constraint boundary condition, loading and material properties of the FE model are simplified to a certain extent. In this study, the normal craniofacial complex model was built using CT image data. The anatomical profiles of bone segments and teeth were extracted integrally. In order to simplify calculation, mechanical properties of all materials in four models were defined as linear elasticity, homogeneity and isotropy. The stress level and distribution on the models were compared with the reported results in literatures 3,5,19 21 and the consistency analysis results further showed the reliabilities of the models. Because this study focused on the comparative analysis of different reconstruction schemes and the mesh density in each model was sufficient, the sensitivity analysis on grid was not specifically performed. In this study, 100% osseointegration between implants and surrounding osseous tissue was assumed and static load was imposed on the models. The ends of implant were placed in the zygomatic and prosthesis, respectively. There was not relative motion between clasps and teeth. And only masseter contribution to bite force was considered. Although all these limitation should affect the accuracy of calculated results, contrastive analysis results may help doctors optimize the design of prosthesis and retention in clinic to improve the living quality of the patients as far as possible. The biomechanical behaviors of the zygomatic implant-supported obturator protheses under dynamic chewing load deserve further investigation considering masticatory muscle forces and the micro-motion between teeth and clasps. The long-term effects of the zygomatic implant-supported obturator protheses should be paid more attention through numerical simulation analysis, animal experiments and clinical data analysis. 5. Conclusions The clasp-retained prothesis for maxillary reconstruction can lead to stress and displacement level increase and stress concentration on the clasps and abutments. Prosthesis retained with the clasp and zygomatic implant can disperse occlusal forces on affected side, rebuild mechanical pillars and greatly decrease the stress level of the clasps, abutment teeth and bones. Therefore, zygomatic implant-supported

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