- Email: [email protected]
Biomechanical three-dimensional finite-element analysis of maxillary prostheses with implants. Design of number and position of implants for maxillary prostheses after hemimaxillectomy
Int. J. Oral Maxillofac. Surg. 2010; 39: 1120–1126 doi:10.1016/j.ijom.2010.06.011, available online at http://www.sciencedirect.com
Research Paper Dental Implants
Biomechanical threedimensional finite-element analysis of maxillary prostheses with implants. Design of number and position of implants for maxillary prostheses after hemimaxillectomy
S. Miyamoto, K. Ujigawa, Y. Kizu, M. Tonogi, G.-Y. Yamane Department of Oral Medicine, Oral and Maxillofacial Surgery, Tokyo Dental College, Chiba, Japan
S. Miyamoto, K. Ujigawa, Y. Kizu, M. Tonogi, G.-Y. Yamane: Biomechanical threedimensional finite-element analysis of maxillary prostheses with implants. Design of number and position of implants for maxillary prostheses after hemimaxillectomy. Int. J. Oral Maxillofac. Surg. 2010; 39: 1120–1126. # 2010 International Association of Oral and Maxillofacial Surgeons. Published by Elsevier Ltd. All rights reserved. Abstract. The present study analyzed stress distributions in craniofacial structures around implant-supported maxillary prostheses. Using post-hemimaxillectomy computed tomography (CT) of a patient, the authors constructed a threedimensional (3D) solid model using Digital Imaging and Communications in Medicine data (DICOM data) for maxillofacial and cranial bones. The effects of different prosthesis designs on stress distributions in craniofacial bones and osseous tissues around the implants were biomechanically investigated using 3D finiteelement analysis. Maxillary prostheses were designed with 2 implants in the zygoma on the affected side and 2–3 implants in the maxillary alveolar bone on the unaffected side, without using a cantilever. Zygomatic implants provided suitable stress dispersal to the zygomatic and craniofacial bones on the affected side. This information is useful for designing maxillary prostheses.
After maxillectomy, the maxilla and the buttresses are lost, and the oral cavity communicates with the nasal cavity and a maxillary sinus21. This operation greatly impairs articulation and mastication13. 0901-5027/1101120 + 07 $36.00/0
Maxillary obturator prostheses have a long history of effectively resolving the functional, cosmetic and psychological problems associated with the defects caused by maxillectomy, but the mobility of max-
Key words: zygomatic implants; maxillectomy; digital biomechanical model of the craniofacial skeleton; three-dimensional finite-element analysis; stress distribution. Accepted for publication 2 June 2010 Available online 11 July 2010
illary obturator prostheses impairs function. Mobility of maxillary prostheses is affected by the size and character of the defect, the height and contours of the residual alveolar ridge and palatal shelf,
# 2010 International Association of Oral and Maxillofacial Surgeons. Published by Elsevier Ltd. All rights reserved.
Analysis of prostheses with implants the availability of undercuts, and most importantly, by the health and position of any remaining teeth12. In cases that have required extensive resections, significant problems regarding the retention, support and stability of maxillary obturator prostheses are encountered after ablation of the retentive maxillary anatomy. In such cases, the main issues are the restoration of functional recovery, for example by adjusting the level of occlusion, and improving the quality of life,17 prognosis and success rates for implanting maxillary prostheses. Zygomatic implants were originally designed for reconstruction of the atrophic, edentulous maxilla. They have also been used to support and aid the retention of maxillary prostheses after maxillectomy8,18, which is otherwise difficult. Clinical application of zygomatic implants has been developed primarily for the treatment of the maxilla showing severe resorption. The extent of stresses such as increasing occlusal force is high, but dynamic techniques for examining stress due to supporting implants are not available. Few biomechanical studies have examined these prostheses with implants. In the present study, the authors analyzed stress distributions in craniofacial structures
[(Fig._1)TD$IG]
around implant-supported maxillary prostheses. Stress was analyzed using three-dimensional (3D) finite-element analysis distributions in craniofacial structures around implant-supported maxillary prostheses. Using post-hemimaxillectomy computed tomography (CT) of a patient, the authors constructed a 3D solid model using Digital Imaging and Communications in Medicine (DICOM) data for maxillofacial and cranial bones. They simulated the prostheses design with implants for this patient with a computer. The information acquired from this study will contribute to designing maxillary prostheses with implants, leading to higher success rate.
Materials and methods Construction of craniofacial bone models
Using 3D finite-element analysis, biodynamics were examined for a maxillary prosthesis with 1 or 2 zygomatic implants after right hemimaxillectomy for malignant maxillary tumor and a left alveolar bone prosthesis with 1–3 implants. Typical occlusal loads were simulated, and stress distributions in the maxillary prosthesis model with zygomatic and normal
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implants and surrounding osseous structures were examined. A 3D, finite-element, solid model of the human skull was constructed based on randomly selected clinical CT data4 from a 63-year-old Japanese man with edentulous maxilla. Data were obtained after right hemimaxillectomy for malignant maxillary tumor. The craniofacial part of the skull was scanned in the transverse plane using a SOMATRON Plus 4 Volume Zoom scanner (Siemens AG, Erlangen, Germany), with 1.0 mm slice thickness and 1.0 mm scan increment, resulting in 180 slice images. Mimics 10.11 image-processing software (Materialise, Leuven, Belgium) was used to generate 3D external shapes of the craniofacial and alveolar bones. Output data were transferred to Solidworks 3D computer-aided design software (SolidWorks Corp., Concord, MA, USA) for conversion to a finite-element solid model (Fig. 1). Maxillary prosthesis models
Bra˚nemark System1 standard implants (Norbel Biocare AB, Gotebo¨rg, Sweden) with dimensions of 3.75 mm 13.0 mm and 3.0 mm high multi-unit abutments were used. The superstructure was mod-
Fig. 1. Front and isometric views of the finite-element craniofacial model after hemimaxillectomy and mesh models of maxillary prostheses. Maxillary prosthesis models with 1 and 2 zygomatic implants and superstructure mesh models.
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eled as a symmetrically curved gold alloy bar with a section of 10.0 mm 8.0 mm and constructed using Solidworks. Models of maxillary prostheses with different numbers of implants were prepared for the right maxillary defect. On the affected side, 1 or 2 implants were placed in zygomatic bone on the affected side: Z1 group, 1 implant (Z-1); and Z-2 group, 2 implants (medial implant, Z-M; distal implant, Z-D). The following three implants were placed in the left unaffected alveolar bone: L-1, in the maxillary lateral incisor; L-2, in the maxillary first premolar; and L-3, in the maxillary first molar. The maxillary prostheses with 2 zygomatic implants in the zygomatic bone on the affected side were compared with alveolar bone on the unaffected side using different numbers of implants. Type 1
used 3 implants (L-1, L-2 and L-3) in the left alveolar bone. Types 2–4 used two implants (Type 2: L-2 and L-3; Type 3: L-1 and L-3; Type 4: L-1 and L-2). Types 5–7 used 1 implant (Type 5: L-1; Type 6: L-2; Type 7: L-3) (Fig. 2). Under the condition of tilted implant placement, zygomatic implants were tilted 908 relative to the plane of resection, and left implants were tilted 908 relative to the vertical plane. Maxillary prostheses with these designs were compared and analyzed. Craniofacial and maxillary prosthesis models were craniofacial bone model (craniofacial model) and maxillary prosthesis models. These models were observed to be coincidentally related and were read into a finite-elemental program (COSMOS/ Works; Structural Research & Analysis
[(Fig._3)TD$IG]
Corp., Los Angeles, CA, USA) for mesh generation. The number of triangles describing the external shapes needed to be drastically reduced (Fig. 1). Mesh generation yielded a total of approximately 170,000 elements and 260,000 nodes. Average element size was 4.0 mm for the craniofacial model and 1.0 mm for the implant model. The mechanical properties of components of these models were based on the results of previous studies5. Loading
Loading involved the application of a simulated bite force as a distributed vertical load of 150 N to the occlusal surface at the zygomatic implant axis, and a lateral load of 50 N to the palatal surface of the superstructure20. A distributed bite force
[(Fig._2)TD$IG]
Fig. 2. In the maxillary prostheses models with 2 zygomatic implants in the zygomatic bone on the affected side. The alveolar bone on unaffected side with different numbers of implants: L-1 (in the maxillary lateral incisor), L-2 (in the maxillary first premolar), and L-3 (in the maxillary first molar). Type 1 was placed three implants (L-1, L-2 and L-3) in the left alveolar bone. Types 2–4 were two implants (type 2: L-2 and L-3, type 3: L-1 and L-3 and type 4: L-1 and L-2) placed, types 5–7 were one implant (type 5: L-1, type 6: L-2 and type 7: L-3) placed.
Fig. 3. Front and lateral views of craniofacial model after hemimaxillectomy and maxillary prosthesis models. Boundary conditions and external loads are shown.
Analysis of prostheses with implants
[(Fig._4)TD$IG]
of 300 N was applied at the insertion area of the masseter muscle on the zygomatic arch and the zygomatic process of the maxilla to simulate movement of the masseter muscle in occlusion, passing downwards and backwards19. With regard to boundary conditions, a fixed bond (i.e. osseointegrated implants) was assumed between the bone and implants along the entire interface, meaning that under the load applied on the implants, relative motion between the bone and implant did not occur. For the top and back cutting plane, restraints were selected to simulate the presence of the rest of the skull. Both loading and boundary conditions of finiteelement models are shown in Fig. 3. Comparative stress analysis was performed using the 3D finite-element method regarding von Mises stresses, defined by the following equation. The amounts and distributions of main stresses on maxillary prostheses with zygomatic
implants in the craniofacial region were compared for different restorative situations:
The von Mises stress 1 ¼ ½ðs 1 s 2 Þ2 þ ðs 2 s 3 Þ2 2 þ ðs 3 s 1 Þ2 Where s1, s2, s3, is the principal stress and s1 > s2 > s3. Results
Figure 4 shows the visualization of stress using pseudo-colors on the geometrical model. In the maxillary prosthesis model, stress tended to be generated over the zygomatic bone on the affected side and the front of the prosthetic superstructures. Stress was concentrated at the implantabutment joints. In the craniofacial model,
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stress due to occlusal forces was distributed by dispersion to the zygomatic bone on the affected side, transferred predominantly through the infrazygomatic crest, then divided in two directions into the frontal and temporal processes of the zygomatic bone. The craniofacial model allowed better distribution of stress on maxillary alveolar bone on the unaffected side and zygomatic bones on the affected side in maxillary prostheses with zygomatic implants (Fig. 5). Compared with maxillary prostheses with 1 zygomatic implant, maxillary prostheses with 2 zygomatic implants exhibited lower stress distributions at implant-abutments joints and the surrounding zygomatic bone under all loads. The authors compared maxillary prostheses with 2 zygomatic implants in the zygomatic bone on the affected side with alveolar bone on the unaffected side with different numbers of implants (Fig. 6).
Fig. 4. Isometric and frontal views of craniofacial and maxillary prosthesis models with distribution of von Mises stresses. Dark-blue areas represent unstressed regions, while red areas represent the most stressed regions. All plots were compared on the same chromatic scale, which is shown on the right-hand side. Maxillary prostheses models are proposed to analyze the extent of by support provided 1 and 2 zygomatic implants in the affected side. Simulated bite force was applied to distribute the vertical loads to the occlusal surface. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
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Fig. 5. Comparison of von Mises stresses by position in the craniofacial and maxillary prosthesis model. The Z-1 group has 1 zygomatic implant (Z-1), and the Z-2 group has 2 zygomatic implants (medial, Z-M; distal, Z-D). In the left alveolar bone, the following implants were placed; L-1 (in the maxillary lateral incisor), L-2 (in the maxillary first premolar), and L-3 (in the maxillary first molar). Md is the midpoint of jugale (Ju) and zygomaxilare (Zm) (right side, R-Md; left side, L-Md).
[(Fig._6)TD$IG]
Fig. 6. Comparison of von Mises stresses by position in the craniofacial and maxillary prosthesis model. Using two zygomatic implants, type 1: 3 implants (L-1, L-2 and L-3) placed in the left alveolar bone. Types 2–4: 2 implants (type 2, L-2 and L-3; type 3, L-1 and L-3; type 4, L-1 and L-2), types 5–7: 1 implant (type 5, L-1; type 6, L-2; type 7, L-3).
Analysis of prostheses with implants Stress in the distal abutment was dispersed the most for maxillary prostheses with 3 implants in the alveolar bone on the unaffected side. In cases with 1–2 implants placed in the alveolar bone on the unaffected side using a cantilever, stress was strongly concentrated on this side. Discussion
3D finite-element analysis is a numerical stress-analysis technique6,7 that is widely used to study engineering and biomechanical problems. The advantages lie in the non-invasive nature of the process and the ability to assess theoretically the amount of stress experienced at any given point. Based on CT-DICOM data from the maxillectomy patient, the system allowed visual confirmation and analysis of stress distribution, as well as convenient and simple construction of a digital biomechanical post-maxillectomy mode that provided details regarding anatomical structures in the regions of interest, such as the maxillary sinuses and craniofacial bones. On comparing the distribution of stress in maxillary prostheses with 1–2 zygomatic implants, stress was observed to disperse more for prostheses with 2 implants. In these cases, the maximum stresses did not exceed 25 MPa, which induces bone damage according to BORCH1 ERS and REICHART . The shape of the zygomatic defects complicated the placement of zygomatic implants perpendicular to the occlusal plane, and implants were thus often placed at an angle in relation to the occlusal plane. NAKAMURA and SHIMADA16 reported that when an implant is placed at an angle, vertical load functions as lateral load. This resulted in a bending moment when connections between implant-abutment joints and the interface between the alveolar bone and implants acted as fulcra, and metal fatigue caused abutment loosening11 and fracture. One study discovered that inclinations of 45–758 relative to the occlusal plane did not affect implant prognosis3, and several other studies have documented that the zygomatic bone is anatomically composed of thick trabecular structures that enable the tolerance of occlusal force10,22. When considering long-term prognosis, 2 implants are preferable to 1 implant in order to disperse the stress more efficiently. On comparing maxillary prostheses with 1–3 implants in maxillary alveolar bone on the unaffected side, stress in alveolar bone around the implants exceeded 25 MPa if only 1 implant was
placed in the alveolar bone on the unaffected side or if 2 implants were placed in the maxillary lateral incisor and maxillary first premolar regions with a cantilevertype superstructure. Excessive load can cause microscopic bone damage, which has been shown to increase the risk of bone fracture and make bone more fragile2,14 by damaging osseointegration between the implant and alveolar bone9. The prevention of excessive stress concentrations in osseous tissue around the implants thus requires the placement of 2 zygomatic implants in the zygomatic bone on the affected side and 2–3 implants in alveolar bone on the unaffected side. The superstructure should not be of the cantilever type. The prognosis for maxillary prostheses can also be improved by adjusting the occlusion to disperse stress in the anterior tooth region and affected side with vertical and lateral load application, respectively. Based on this analysis of the effects of zygomatic implants in a digital biomechanical model of the craniofacial skeleton, stress due to occlusal forces was supported by the zygomatic bone in the craniofacial model. This stress distribution in implants resembles the pattern of withstanding occlusal stresses in the dentulous jaw. When zygomatic implants were loaded with occlusal forces, stress was transferred predominantly through the infrazygomatic crest and divided in two directions into the frontal and temporal processes of the zygomatic bone. In maxillary prostheses with zygomatic implants, stress was dispersed around the zygomatic bone on the affected side, and stress distribution to the orbital cavity was confirmed. This suggests that occlusal force affects the orbital cavity. The maximum stress was 2 MPa, suggesting that the effects of occlusal force on the orbital cavity are minimal and that placement of implants in zygomatic bone on the affected side is feasible for dispersion of occlusal force. In the present study, 100% osseointegration between implants and surrounding osseous tissue was assumed. Implants were placed in the zygomatic and maxillary alveolar bones on the affected and unaffected sides, respectively, and different designs of maxillary prostheses were compared. In clinical settings, radio- and chemotherapy can often weaken the bone substance and lower the success rate of implants15, suggesting a reduced degree of osseointegration. In the future, analysis of bone defects and substance in each patient using CT data appears essential.
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In conclusion, different designs of implant numbers and placement positions of maxillary prostheses used in occlusal reconstruction following hemimaxillectomy for malignant maxillary tumor were analyzed using 3D finite-element analysis. Placement of 2 implants rather than 1 implant in zygomatic bone on the affected side is preferable, as 2 implants allow stress dispersion to the implant-abutment joint and craniofacial bone. In the case of 2–3 implants placed in alveolar bone on the unaffected side without using a cantilever, stress concentrated around the implants did not damage osseointegration. After implants were placed in zygomatic bone on the affected side, stress distribution was confirmed in ipsilateral craniofacial bones. Effects on peripheral tissues, including the orbital cavity, appeared minimal. Placement of zygomatic implants allows dispersion of stress in the alveolar bone on the unaffected side to the zygomatic bone and craniofacial bones on the affected side. This finding could lead to better prognosis for maxillary prostheses with zygomatic implants. Conflict of interest
The authors have no conflicts of interest to declare. Funding
None. Ethical approval
Not required. References 1. Borchers L, Reichart P. Three-dimensional stress distribution around a dental implant at different stages of interface development. J Dent Res 1988: 62: 155–159. 2. Burr DB, Forwood MR, Fyhrie DP, Martin RB, Schaffler MB, Turner CH. Bone microdamage and skeletal fragility in osteoporotic and stress fractures. J Bone Miner Res 1997: 12: 6–15. 3. Calandriello R, Tomatis M. Simplified treatment of the atrophic posterior maxilla via immediate/early function and tilted implants: a prospective 1-year clinical study. Clin Implant Dent Relat Res 2005: 7: 1–12. 4. Cattaneo PM, Dalstra M, Melsen B. The transfer of occlusal forces through the maxillary molars: a finite element study. Am J Orthod Dentofacial Orthop 2003: 123: 367–373. 5. Geng JP, Tan KB, Liu GR. Application of finite element analysis in implant den-
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6.
7.
8.
9.
10.
11.
12.
Miyamoto et al.
tistry: a review of the literature. J Prosthet Dent 2001: 85: 585–598. Himmlova´ L, Dosta´lova´ T, Ka´covsky´ A, Konvickova´ S. Influence of implant length and diameter on stress distribution: a finite element analysis. J Prosthet Dent 2004: 91: 20–25. Hsu ML, Chen FC, Kao HC, Cheng CK. Influence of off-axis loading of an anterior maxillary implant: a 3-dimensional finite element analysis. Int J Oral Maxillofac Implants 2007: 22: 301–309. Hu YJ, Hardianto A, Li SY, Zhang ZY, Zhang CP. Reconstruction of a palatomaxillary defect with vascularized iliac bone combined with a superficial inferior epigastric artery flap and zygomatic implants as anchorage. Int J Oral Maxillofac Surg 2007: 36: 854–857. Isidor F. Loss of osseointegration caused by occlusal load of oral implants. A clinical and radiographic study in monkeys. Clin Oral Implants Res 1996: 7: 143–152. Kato Y, Kizu Y, Tonogi M, Ide Y, Yamane G. Observation of the internal structures of the zygomatic bone by micro-computed tomography. J Oral Biosci 2004: 46: 523–529. Khraisat A. Stability of implant-abutment interface with a hexagon-mediated butt joint: failure mode and bending resistance. Clin Implant Dent Relat Res 2005: 7: 221–228. Kornblith AB, Zlotolow IM, Gooen J, Huryn JM, Lerner T, Strong EW,
13.
14.
15.
16.
17.
18.
Shah JP, Spiro RH, Holland JC. Quality of life of maxillectomy patients using an obturator prosthesis. Head Neck 1996: 18: 323–334. Kreissl ME, Heydecke G, Metzger MC, Schoen R. Zygoma implant-supported prosthetic rehabilitation after partial maxillectomy using surgical navigation: a clinical report. J Prosthet Dent 2007: 97: 121–128. Martin B. Mathematical model for repair of fatigue damage and stress fracture in osteonal bone. J Orthop Res 1995: 13: 309–316. Moy PK, Medina D, Shetty V, Aghaloo TL. Dental implant failure rates and associated risk factors. Int J Oral Maxillofac Implants 2005: 20: 569–577. Nakamura A, Shimada J. Mechanical effects of mesiodistal tilting of a fixture on the strain distribution in an implant structure and surrounding bone—experimental study using the beagle dog mandible. J Jpn Soc Oral implant 2005: 18: 3–13. Rogers SN, Lowe D, McNally D, Brown JS, Vaughan ED. Health-related quality of life after maxillectomy: a comparison between prosthetic obturation and free flap. J Oral Maxillofac Surg 2003: 61: 174–181. Schmidt BL, Pogrel MA, Young CW, Sharma A. Reconstruction of extensive maxillary defects using zygomatic implants. J Oral Maxillofac Surg 2004: 62: 82–89.
19. Tuxen A, Bakke M, Pinholt EM. Comparative data from young men and women on masseter muscle fibres, function and facial morphology. Arch Oral Biol 1999: 44: 509–518. 20. Ujigawa K, Kato Y, Kizu Y, Tonogi M, Yamane G. Three-dimensional finite elemental analysis of zygomatic implants in craniofacial structures. Int J Oral Maxillofac Surg 2007: 36: 620– 625. 21. Yamamoto Y, Kawashima K, Sugihara T, Nohira K, Furuta Y, Fukuda S. Surgical management of maxillectomy defects based on the buttress reconstruction. Head Neck 2004: 26: 247–256. 22. Yoshino M, Kato Y, Kizu Y, Tonogi M, Abe S, Ide Y, Yamane G. Study on internal structure of zygomatic bone using micro-finite element analysis model-differences between dentulous and edentulous dentition in Japanese cadavers. Bull Tokyo Dent Coll 2007: 48: 129–134. Address: Satoshi Miyamoto Department of Oral Medicine Oral and Maxillofacial Surgery Tokyo Dental College 5-11-13 Sugano Ichikawa city Chiba 272-8513 Japan Fax: +81 47 324 8577 E-mail: [email protected]
Research Paper Dental Implants
Biomechanical threedimensional finite-element analysis of maxillary prostheses with implants. Design of number and position of implants for maxillary prostheses after hemimaxillectomy
S. Miyamoto, K. Ujigawa, Y. Kizu, M. Tonogi, G.-Y. Yamane Department of Oral Medicine, Oral and Maxillofacial Surgery, Tokyo Dental College, Chiba, Japan
S. Miyamoto, K. Ujigawa, Y. Kizu, M. Tonogi, G.-Y. Yamane: Biomechanical threedimensional finite-element analysis of maxillary prostheses with implants. Design of number and position of implants for maxillary prostheses after hemimaxillectomy. Int. J. Oral Maxillofac. Surg. 2010; 39: 1120–1126. # 2010 International Association of Oral and Maxillofacial Surgeons. Published by Elsevier Ltd. All rights reserved. Abstract. The present study analyzed stress distributions in craniofacial structures around implant-supported maxillary prostheses. Using post-hemimaxillectomy computed tomography (CT) of a patient, the authors constructed a threedimensional (3D) solid model using Digital Imaging and Communications in Medicine data (DICOM data) for maxillofacial and cranial bones. The effects of different prosthesis designs on stress distributions in craniofacial bones and osseous tissues around the implants were biomechanically investigated using 3D finiteelement analysis. Maxillary prostheses were designed with 2 implants in the zygoma on the affected side and 2–3 implants in the maxillary alveolar bone on the unaffected side, without using a cantilever. Zygomatic implants provided suitable stress dispersal to the zygomatic and craniofacial bones on the affected side. This information is useful for designing maxillary prostheses.
After maxillectomy, the maxilla and the buttresses are lost, and the oral cavity communicates with the nasal cavity and a maxillary sinus21. This operation greatly impairs articulation and mastication13. 0901-5027/1101120 + 07 $36.00/0
Maxillary obturator prostheses have a long history of effectively resolving the functional, cosmetic and psychological problems associated with the defects caused by maxillectomy, but the mobility of max-
Key words: zygomatic implants; maxillectomy; digital biomechanical model of the craniofacial skeleton; three-dimensional finite-element analysis; stress distribution. Accepted for publication 2 June 2010 Available online 11 July 2010
illary obturator prostheses impairs function. Mobility of maxillary prostheses is affected by the size and character of the defect, the height and contours of the residual alveolar ridge and palatal shelf,
# 2010 International Association of Oral and Maxillofacial Surgeons. Published by Elsevier Ltd. All rights reserved.
Analysis of prostheses with implants the availability of undercuts, and most importantly, by the health and position of any remaining teeth12. In cases that have required extensive resections, significant problems regarding the retention, support and stability of maxillary obturator prostheses are encountered after ablation of the retentive maxillary anatomy. In such cases, the main issues are the restoration of functional recovery, for example by adjusting the level of occlusion, and improving the quality of life,17 prognosis and success rates for implanting maxillary prostheses. Zygomatic implants were originally designed for reconstruction of the atrophic, edentulous maxilla. They have also been used to support and aid the retention of maxillary prostheses after maxillectomy8,18, which is otherwise difficult. Clinical application of zygomatic implants has been developed primarily for the treatment of the maxilla showing severe resorption. The extent of stresses such as increasing occlusal force is high, but dynamic techniques for examining stress due to supporting implants are not available. Few biomechanical studies have examined these prostheses with implants. In the present study, the authors analyzed stress distributions in craniofacial structures
[(Fig._1)TD$IG]
around implant-supported maxillary prostheses. Stress was analyzed using three-dimensional (3D) finite-element analysis distributions in craniofacial structures around implant-supported maxillary prostheses. Using post-hemimaxillectomy computed tomography (CT) of a patient, the authors constructed a 3D solid model using Digital Imaging and Communications in Medicine (DICOM) data for maxillofacial and cranial bones. They simulated the prostheses design with implants for this patient with a computer. The information acquired from this study will contribute to designing maxillary prostheses with implants, leading to higher success rate.
Materials and methods Construction of craniofacial bone models
Using 3D finite-element analysis, biodynamics were examined for a maxillary prosthesis with 1 or 2 zygomatic implants after right hemimaxillectomy for malignant maxillary tumor and a left alveolar bone prosthesis with 1–3 implants. Typical occlusal loads were simulated, and stress distributions in the maxillary prosthesis model with zygomatic and normal
1121
implants and surrounding osseous structures were examined. A 3D, finite-element, solid model of the human skull was constructed based on randomly selected clinical CT data4 from a 63-year-old Japanese man with edentulous maxilla. Data were obtained after right hemimaxillectomy for malignant maxillary tumor. The craniofacial part of the skull was scanned in the transverse plane using a SOMATRON Plus 4 Volume Zoom scanner (Siemens AG, Erlangen, Germany), with 1.0 mm slice thickness and 1.0 mm scan increment, resulting in 180 slice images. Mimics 10.11 image-processing software (Materialise, Leuven, Belgium) was used to generate 3D external shapes of the craniofacial and alveolar bones. Output data were transferred to Solidworks 3D computer-aided design software (SolidWorks Corp., Concord, MA, USA) for conversion to a finite-element solid model (Fig. 1). Maxillary prosthesis models
Bra˚nemark System1 standard implants (Norbel Biocare AB, Gotebo¨rg, Sweden) with dimensions of 3.75 mm 13.0 mm and 3.0 mm high multi-unit abutments were used. The superstructure was mod-
Fig. 1. Front and isometric views of the finite-element craniofacial model after hemimaxillectomy and mesh models of maxillary prostheses. Maxillary prosthesis models with 1 and 2 zygomatic implants and superstructure mesh models.
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eled as a symmetrically curved gold alloy bar with a section of 10.0 mm 8.0 mm and constructed using Solidworks. Models of maxillary prostheses with different numbers of implants were prepared for the right maxillary defect. On the affected side, 1 or 2 implants were placed in zygomatic bone on the affected side: Z1 group, 1 implant (Z-1); and Z-2 group, 2 implants (medial implant, Z-M; distal implant, Z-D). The following three implants were placed in the left unaffected alveolar bone: L-1, in the maxillary lateral incisor; L-2, in the maxillary first premolar; and L-3, in the maxillary first molar. The maxillary prostheses with 2 zygomatic implants in the zygomatic bone on the affected side were compared with alveolar bone on the unaffected side using different numbers of implants. Type 1
used 3 implants (L-1, L-2 and L-3) in the left alveolar bone. Types 2–4 used two implants (Type 2: L-2 and L-3; Type 3: L-1 and L-3; Type 4: L-1 and L-2). Types 5–7 used 1 implant (Type 5: L-1; Type 6: L-2; Type 7: L-3) (Fig. 2). Under the condition of tilted implant placement, zygomatic implants were tilted 908 relative to the plane of resection, and left implants were tilted 908 relative to the vertical plane. Maxillary prostheses with these designs were compared and analyzed. Craniofacial and maxillary prosthesis models were craniofacial bone model (craniofacial model) and maxillary prosthesis models. These models were observed to be coincidentally related and were read into a finite-elemental program (COSMOS/ Works; Structural Research & Analysis
[(Fig._3)TD$IG]
Corp., Los Angeles, CA, USA) for mesh generation. The number of triangles describing the external shapes needed to be drastically reduced (Fig. 1). Mesh generation yielded a total of approximately 170,000 elements and 260,000 nodes. Average element size was 4.0 mm for the craniofacial model and 1.0 mm for the implant model. The mechanical properties of components of these models were based on the results of previous studies5. Loading
Loading involved the application of a simulated bite force as a distributed vertical load of 150 N to the occlusal surface at the zygomatic implant axis, and a lateral load of 50 N to the palatal surface of the superstructure20. A distributed bite force
[(Fig._2)TD$IG]
Fig. 2. In the maxillary prostheses models with 2 zygomatic implants in the zygomatic bone on the affected side. The alveolar bone on unaffected side with different numbers of implants: L-1 (in the maxillary lateral incisor), L-2 (in the maxillary first premolar), and L-3 (in the maxillary first molar). Type 1 was placed three implants (L-1, L-2 and L-3) in the left alveolar bone. Types 2–4 were two implants (type 2: L-2 and L-3, type 3: L-1 and L-3 and type 4: L-1 and L-2) placed, types 5–7 were one implant (type 5: L-1, type 6: L-2 and type 7: L-3) placed.
Fig. 3. Front and lateral views of craniofacial model after hemimaxillectomy and maxillary prosthesis models. Boundary conditions and external loads are shown.
Analysis of prostheses with implants
[(Fig._4)TD$IG]
of 300 N was applied at the insertion area of the masseter muscle on the zygomatic arch and the zygomatic process of the maxilla to simulate movement of the masseter muscle in occlusion, passing downwards and backwards19. With regard to boundary conditions, a fixed bond (i.e. osseointegrated implants) was assumed between the bone and implants along the entire interface, meaning that under the load applied on the implants, relative motion between the bone and implant did not occur. For the top and back cutting plane, restraints were selected to simulate the presence of the rest of the skull. Both loading and boundary conditions of finiteelement models are shown in Fig. 3. Comparative stress analysis was performed using the 3D finite-element method regarding von Mises stresses, defined by the following equation. The amounts and distributions of main stresses on maxillary prostheses with zygomatic
implants in the craniofacial region were compared for different restorative situations:
The von Mises stress 1 ¼ ½ðs 1 s 2 Þ2 þ ðs 2 s 3 Þ2 2 þ ðs 3 s 1 Þ2 Where s1, s2, s3, is the principal stress and s1 > s2 > s3. Results
Figure 4 shows the visualization of stress using pseudo-colors on the geometrical model. In the maxillary prosthesis model, stress tended to be generated over the zygomatic bone on the affected side and the front of the prosthetic superstructures. Stress was concentrated at the implantabutment joints. In the craniofacial model,
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stress due to occlusal forces was distributed by dispersion to the zygomatic bone on the affected side, transferred predominantly through the infrazygomatic crest, then divided in two directions into the frontal and temporal processes of the zygomatic bone. The craniofacial model allowed better distribution of stress on maxillary alveolar bone on the unaffected side and zygomatic bones on the affected side in maxillary prostheses with zygomatic implants (Fig. 5). Compared with maxillary prostheses with 1 zygomatic implant, maxillary prostheses with 2 zygomatic implants exhibited lower stress distributions at implant-abutments joints and the surrounding zygomatic bone under all loads. The authors compared maxillary prostheses with 2 zygomatic implants in the zygomatic bone on the affected side with alveolar bone on the unaffected side with different numbers of implants (Fig. 6).
Fig. 4. Isometric and frontal views of craniofacial and maxillary prosthesis models with distribution of von Mises stresses. Dark-blue areas represent unstressed regions, while red areas represent the most stressed regions. All plots were compared on the same chromatic scale, which is shown on the right-hand side. Maxillary prostheses models are proposed to analyze the extent of by support provided 1 and 2 zygomatic implants in the affected side. Simulated bite force was applied to distribute the vertical loads to the occlusal surface. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
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Fig. 5. Comparison of von Mises stresses by position in the craniofacial and maxillary prosthesis model. The Z-1 group has 1 zygomatic implant (Z-1), and the Z-2 group has 2 zygomatic implants (medial, Z-M; distal, Z-D). In the left alveolar bone, the following implants were placed; L-1 (in the maxillary lateral incisor), L-2 (in the maxillary first premolar), and L-3 (in the maxillary first molar). Md is the midpoint of jugale (Ju) and zygomaxilare (Zm) (right side, R-Md; left side, L-Md).
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Fig. 6. Comparison of von Mises stresses by position in the craniofacial and maxillary prosthesis model. Using two zygomatic implants, type 1: 3 implants (L-1, L-2 and L-3) placed in the left alveolar bone. Types 2–4: 2 implants (type 2, L-2 and L-3; type 3, L-1 and L-3; type 4, L-1 and L-2), types 5–7: 1 implant (type 5, L-1; type 6, L-2; type 7, L-3).
Analysis of prostheses with implants Stress in the distal abutment was dispersed the most for maxillary prostheses with 3 implants in the alveolar bone on the unaffected side. In cases with 1–2 implants placed in the alveolar bone on the unaffected side using a cantilever, stress was strongly concentrated on this side. Discussion
3D finite-element analysis is a numerical stress-analysis technique6,7 that is widely used to study engineering and biomechanical problems. The advantages lie in the non-invasive nature of the process and the ability to assess theoretically the amount of stress experienced at any given point. Based on CT-DICOM data from the maxillectomy patient, the system allowed visual confirmation and analysis of stress distribution, as well as convenient and simple construction of a digital biomechanical post-maxillectomy mode that provided details regarding anatomical structures in the regions of interest, such as the maxillary sinuses and craniofacial bones. On comparing the distribution of stress in maxillary prostheses with 1–2 zygomatic implants, stress was observed to disperse more for prostheses with 2 implants. In these cases, the maximum stresses did not exceed 25 MPa, which induces bone damage according to BORCH1 ERS and REICHART . The shape of the zygomatic defects complicated the placement of zygomatic implants perpendicular to the occlusal plane, and implants were thus often placed at an angle in relation to the occlusal plane. NAKAMURA and SHIMADA16 reported that when an implant is placed at an angle, vertical load functions as lateral load. This resulted in a bending moment when connections between implant-abutment joints and the interface between the alveolar bone and implants acted as fulcra, and metal fatigue caused abutment loosening11 and fracture. One study discovered that inclinations of 45–758 relative to the occlusal plane did not affect implant prognosis3, and several other studies have documented that the zygomatic bone is anatomically composed of thick trabecular structures that enable the tolerance of occlusal force10,22. When considering long-term prognosis, 2 implants are preferable to 1 implant in order to disperse the stress more efficiently. On comparing maxillary prostheses with 1–3 implants in maxillary alveolar bone on the unaffected side, stress in alveolar bone around the implants exceeded 25 MPa if only 1 implant was
placed in the alveolar bone on the unaffected side or if 2 implants were placed in the maxillary lateral incisor and maxillary first premolar regions with a cantilevertype superstructure. Excessive load can cause microscopic bone damage, which has been shown to increase the risk of bone fracture and make bone more fragile2,14 by damaging osseointegration between the implant and alveolar bone9. The prevention of excessive stress concentrations in osseous tissue around the implants thus requires the placement of 2 zygomatic implants in the zygomatic bone on the affected side and 2–3 implants in alveolar bone on the unaffected side. The superstructure should not be of the cantilever type. The prognosis for maxillary prostheses can also be improved by adjusting the occlusion to disperse stress in the anterior tooth region and affected side with vertical and lateral load application, respectively. Based on this analysis of the effects of zygomatic implants in a digital biomechanical model of the craniofacial skeleton, stress due to occlusal forces was supported by the zygomatic bone in the craniofacial model. This stress distribution in implants resembles the pattern of withstanding occlusal stresses in the dentulous jaw. When zygomatic implants were loaded with occlusal forces, stress was transferred predominantly through the infrazygomatic crest and divided in two directions into the frontal and temporal processes of the zygomatic bone. In maxillary prostheses with zygomatic implants, stress was dispersed around the zygomatic bone on the affected side, and stress distribution to the orbital cavity was confirmed. This suggests that occlusal force affects the orbital cavity. The maximum stress was 2 MPa, suggesting that the effects of occlusal force on the orbital cavity are minimal and that placement of implants in zygomatic bone on the affected side is feasible for dispersion of occlusal force. In the present study, 100% osseointegration between implants and surrounding osseous tissue was assumed. Implants were placed in the zygomatic and maxillary alveolar bones on the affected and unaffected sides, respectively, and different designs of maxillary prostheses were compared. In clinical settings, radio- and chemotherapy can often weaken the bone substance and lower the success rate of implants15, suggesting a reduced degree of osseointegration. In the future, analysis of bone defects and substance in each patient using CT data appears essential.
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In conclusion, different designs of implant numbers and placement positions of maxillary prostheses used in occlusal reconstruction following hemimaxillectomy for malignant maxillary tumor were analyzed using 3D finite-element analysis. Placement of 2 implants rather than 1 implant in zygomatic bone on the affected side is preferable, as 2 implants allow stress dispersion to the implant-abutment joint and craniofacial bone. In the case of 2–3 implants placed in alveolar bone on the unaffected side without using a cantilever, stress concentrated around the implants did not damage osseointegration. After implants were placed in zygomatic bone on the affected side, stress distribution was confirmed in ipsilateral craniofacial bones. Effects on peripheral tissues, including the orbital cavity, appeared minimal. Placement of zygomatic implants allows dispersion of stress in the alveolar bone on the unaffected side to the zygomatic bone and craniofacial bones on the affected side. This finding could lead to better prognosis for maxillary prostheses with zygomatic implants. Conflict of interest
The authors have no conflicts of interest to declare. Funding
None. Ethical approval
Not required. References 1. Borchers L, Reichart P. Three-dimensional stress distribution around a dental implant at different stages of interface development. J Dent Res 1988: 62: 155–159. 2. Burr DB, Forwood MR, Fyhrie DP, Martin RB, Schaffler MB, Turner CH. Bone microdamage and skeletal fragility in osteoporotic and stress fractures. J Bone Miner Res 1997: 12: 6–15. 3. Calandriello R, Tomatis M. Simplified treatment of the atrophic posterior maxilla via immediate/early function and tilted implants: a prospective 1-year clinical study. Clin Implant Dent Relat Res 2005: 7: 1–12. 4. Cattaneo PM, Dalstra M, Melsen B. The transfer of occlusal forces through the maxillary molars: a finite element study. Am J Orthod Dentofacial Orthop 2003: 123: 367–373. 5. Geng JP, Tan KB, Liu GR. Application of finite element analysis in implant den-
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6.
7.
8.
9.
10.
11.
12.
Miyamoto et al.
tistry: a review of the literature. J Prosthet Dent 2001: 85: 585–598. Himmlova´ L, Dosta´lova´ T, Ka´covsky´ A, Konvickova´ S. Influence of implant length and diameter on stress distribution: a finite element analysis. J Prosthet Dent 2004: 91: 20–25. Hsu ML, Chen FC, Kao HC, Cheng CK. Influence of off-axis loading of an anterior maxillary implant: a 3-dimensional finite element analysis. Int J Oral Maxillofac Implants 2007: 22: 301–309. Hu YJ, Hardianto A, Li SY, Zhang ZY, Zhang CP. Reconstruction of a palatomaxillary defect with vascularized iliac bone combined with a superficial inferior epigastric artery flap and zygomatic implants as anchorage. Int J Oral Maxillofac Surg 2007: 36: 854–857. Isidor F. Loss of osseointegration caused by occlusal load of oral implants. A clinical and radiographic study in monkeys. Clin Oral Implants Res 1996: 7: 143–152. Kato Y, Kizu Y, Tonogi M, Ide Y, Yamane G. Observation of the internal structures of the zygomatic bone by micro-computed tomography. J Oral Biosci 2004: 46: 523–529. Khraisat A. Stability of implant-abutment interface with a hexagon-mediated butt joint: failure mode and bending resistance. Clin Implant Dent Relat Res 2005: 7: 221–228. Kornblith AB, Zlotolow IM, Gooen J, Huryn JM, Lerner T, Strong EW,
13.
14.
15.
16.
17.
18.
Shah JP, Spiro RH, Holland JC. Quality of life of maxillectomy patients using an obturator prosthesis. Head Neck 1996: 18: 323–334. Kreissl ME, Heydecke G, Metzger MC, Schoen R. Zygoma implant-supported prosthetic rehabilitation after partial maxillectomy using surgical navigation: a clinical report. J Prosthet Dent 2007: 97: 121–128. Martin B. Mathematical model for repair of fatigue damage and stress fracture in osteonal bone. J Orthop Res 1995: 13: 309–316. Moy PK, Medina D, Shetty V, Aghaloo TL. Dental implant failure rates and associated risk factors. Int J Oral Maxillofac Implants 2005: 20: 569–577. Nakamura A, Shimada J. Mechanical effects of mesiodistal tilting of a fixture on the strain distribution in an implant structure and surrounding bone—experimental study using the beagle dog mandible. J Jpn Soc Oral implant 2005: 18: 3–13. Rogers SN, Lowe D, McNally D, Brown JS, Vaughan ED. Health-related quality of life after maxillectomy: a comparison between prosthetic obturation and free flap. J Oral Maxillofac Surg 2003: 61: 174–181. Schmidt BL, Pogrel MA, Young CW, Sharma A. Reconstruction of extensive maxillary defects using zygomatic implants. J Oral Maxillofac Surg 2004: 62: 82–89.
19. Tuxen A, Bakke M, Pinholt EM. Comparative data from young men and women on masseter muscle fibres, function and facial morphology. Arch Oral Biol 1999: 44: 509–518. 20. Ujigawa K, Kato Y, Kizu Y, Tonogi M, Yamane G. Three-dimensional finite elemental analysis of zygomatic implants in craniofacial structures. Int J Oral Maxillofac Surg 2007: 36: 620– 625. 21. Yamamoto Y, Kawashima K, Sugihara T, Nohira K, Furuta Y, Fukuda S. Surgical management of maxillectomy defects based on the buttress reconstruction. Head Neck 2004: 26: 247–256. 22. Yoshino M, Kato Y, Kizu Y, Tonogi M, Abe S, Ide Y, Yamane G. Study on internal structure of zygomatic bone using micro-finite element analysis model-differences between dentulous and edentulous dentition in Japanese cadavers. Bull Tokyo Dent Coll 2007: 48: 129–134. Address: Satoshi Miyamoto Department of Oral Medicine Oral and Maxillofacial Surgery Tokyo Dental College 5-11-13 Sugano Ichikawa city Chiba 272-8513 Japan Fax: +81 47 324 8577 E-mail: [email protected]