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Biomechanical evaluation of a novel hybrid reconstruction plate for mandible segmental defects: A finite element analysis and fatigue testing
Accepted Manuscript Biomechanical evaluation of a novel hybrid reconstruction plate for mandible segmental defects: A finite element analysis and fatigue testing Cheng-Hsien Wu, DDS, Ph.D, Yang-Sung Lin, Ph.D, Yu-Shen Liu, MS, Chun-Li Lin, Ph.D, Professor PII:
S1010-5182(17)30248-2
DOI:
10.1016/j.jcms.2017.07.010
Reference:
YJCMS 2733
To appear in:
Journal of Cranio-Maxillo-Facial Surgery
Received Date: 17 November 2016 Revised Date:
18 June 2017
Accepted Date: 18 July 2017
Please cite this article as: Wu C-H, Lin Y-S, Liu Y-S, Lin C-L, Biomechanical evaluation of a novel hybrid reconstruction plate for mandible segmental defects: A finite element analysis and fatigue testing, Journal of Cranio-Maxillofacial Surgery (2017), doi: 10.1016/j.jcms.2017.07.010. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Biomechanical evaluation of a novel hybrid reconstruction plate for mandible segmental defects: A finite element analysis and fatigue testing Cheng-Hsien Wu, DDS, Ph.D.1, Yang-Sung Lin, Ph.D.2, Yu-Shen Liu, MS3 and Chun-Li Lin, Ph.D.4,*
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1 Name: Cheng-Hsien Wu Highest academic degree(s): DDS, Ph.D. Institution: Attending Surgeon, Oral & Maxillofacial Surgery, Taipei Veterans General Hospital Assistant Professor, School of Dentistry, National Yang-Ming University, Taipei, Taiwan Tel: 886-2-28213792 Email: [email protected] Address：No.155, Sec.2, Linong Street, Taipei, 112
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2 Name: Yang-Sung Lin Highest academic degree(s): Ph.D. Institution: Post-doctor Researcher, Department of Biomedical Engineering, National Yang-Ming University, Taipei, Taiwan. Tel: 886-2-28213792 Email: [email protected] Address: No.155, Sec.2, Linong Street, Taipei, 112
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3 Name: Yu-Shen Liu Highest academic degree(s): MS Institution: Department of Biomedical Engineering, National Yang-Ming University, Taipei, Taiwan. Tel: 886-2-28213792 Email: [email protected] Address: No.155, Sec.2, Linong Street, Taipei, 112 4 Name: Chun-Li Lin Highest academic degree(s): Ph.D. Institution: Professor, Department of Biomedical Engineering, National Yang-Ming University, Taipei, Taiwan. Tel: 886-2-28267000 ext 7039 Fax: 886-2-28210847 Email: [email protected] Address: No.155, Sec.2, Linong Street, Taipei, 112
*Corresponding author: Chun-Li Lin, Ph.D., Professor, Department of Biomedical Engineering,
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National Yang-Ming University, Taipei, Taiwan. Tel: 886-2-28267000 ext 7039 Fax: 886-2-28210847 Email: [email protected] Address: No.155, Sec.2, Linong Street, Taipei, 112
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Sources of support in the form of grants: This study is one part of the Master Thesis of YS Yu (2015) at the Department of Biomedical Engineering at National Yang-Ming University, Taiwan and supported in part by MOST project
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103-2221-E-010-012-MY3 of the Ministry of Science and Technology, Taiwan.
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Summary
Purpose: This study develops a novel hybrid (NH) reconstruction plate that can provide load-bearing strength, secure the bone transplant at the prosthesis favored position, and also
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maintain the facial contour in a mandibular segmental defect. A new patient-match bending technique which uses a three-dimensional printing (3DP) stamping process is developed to increase the interfacial fit between the reconstruction plate and mandibular bone.
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Materials and Methods: The NH reconstruction plate was designed to produce a continuous profile with non-uniform thickness and triangular cross-screw patterns with a locking-screw feature
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at the plate base. Two mandible segmental defect finite element models including the NH reconstruction plate to secure a bone flap for occlusal requirement and the commercial straight (CS) reconstruction plate to secure a bone flap along the lower mandible border were generated for biomechanical fatigue testing.
Results: The simulated results showed that the maximum von Mises stresses of the reconstruction
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plate for CS secured model are about 4.5 times more than the NH secured model. The bone strains around the fixation screws showed that the CS secured model was meaningfully higher than that of
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the NH secured model and exceeded the bone limit value. No fracture of any component was found in any sample in the fatigue testing.
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Conclusion: In conclusion, the newly developed NH reconstruction plate can secure the transplant position in accordance to the individual occlusal requirements without sacrificing the maintenance of facial contour. Finite element−based biomechanical evaluation demonstrates superior mechanical strength compared to commercial standard plates.
Keywords: reconstruction plate, finite element, segmental defect, 3D printing, fatigue
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Large mandibular defects resulting from trauma, tumor or osteoradionecrosis are common findings in the maxillofacial area. Failure to correct such defects will jeopardize functions such as masticating, swallowing, pronounciation, and esthetics, to different degrees (Chenping et al., 2012;
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Zhao et al., 2014). Using osteosynthesis devices to bridge the defect with or without bone grafting is fundamental to mandible reconstruction. Among the techniques used, the free vascularized fibular flap has become the preferred method for mandibular reconstruction, owing to the advantages of
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adequate bone length, low donor site morbidity and an abundant periosteal blood supply (Cordeiro et al., 1999; Kim and Blackwell, 2007; He et al., 2011; Chenping et al., 2012). One of the
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disadvantages of fibular bone grafts is the insufficient bone height for implant-supported prosthesis design (Yim and Wei, 1994; He et al., 2011). Meanwhile, the conventional plate design for segmental mandibular defects places the device and secures the bone graft in the lower border of mandible, which is not suitable for prosthesis fabrication (Chiapasco et al., 2008; Chenping et al., 2012). Several options have be developed to increase the fibula vertical height, such as the
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double-barrel technique, second-stage bone graft and distraction osteogenesis (Chang et al., 2008; Goh et al., 2008; Cho-Lee et al., 2011; Chenping et al., 2012). However, these methods may
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increase the flap failure rate, the need for secondary operation, and extended time before oral rehabilitation. (Goh et al., 2008; He et al., 2011).
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The purpose of reconstruction plate is not only to bridge the mandibular stumps, it should also bear the occlusal load to maintain the fixation stability during the bone healing phase (Vajgel et al., 2013; Bujtár et al., 2014; Li et al., 2014). Two major concerns are frequent encountered when using reconstruction plate technique. Firstly, the current commercial reconstruction plates needs manual contouring to fit the individual mandibular geometry profiles. This procedure is quite experience related and technique sensitive, which means that, for inexperienced surgeons or in the case of an extensive mandibular defect, the trial-and-error plate contouring (bending) approach may generate residual stress to predispose to bone plate fracture and fatigue failure (Goh et al., 2008). Secondly,
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the poor adaptation of the reconstruction plate will impair the esthetic outcome and exert unfavorable stress concentrated around the fixation screws when occlusal load is applied. This will ultimately result in screw loosening and prosthesis failure (Kimura et al., 2006). This study develops a novel hybrid (NH) reconstruction plate that can provide load-bearing
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strength, secure the bone transplant at the prosthesis favored position and also maintain the facial contour in the mandibular segmental defect. A patient-matched bending technique was developed to increase the interfacial fit adaptation between the NH reconstruction plate and mandibular contour
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by using integrated reverse engineering and 3D printing (3DP) and stamping techniques. Finite element (FE) analysis was designed for biomechanical fatigue testing and mechanical behavior
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pattern evaluation in comparison to that of a commercial straight (CS) reconstruction plate.
MATERIALS AND METHODS Design concept of the NH reconstruction plate
The NH reconstruction plate was designed to secure the transplant height consistent with the
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alveolar bone for dental implant requirement. In order to provide adjustment options for the transplant height, bending capability and structural stability, the NH reconstruction plate was
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designed to present a continuous profile with non-uniform thickness in the transplant height direction (Fig. 1). The NH reconstruction plate base was thickened to 2.4 mm to maintain structural
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strength. Triangle cross-screw patterns with locking screws at the plate base were designed to enhance the fixation stability. The upper half of the NH reconstruction plate in the transplant height direction was gradually reduced to 0.8-mm thickness for bending capability to match the curvature of the mandibular bone shape (Fig. 1).
Digital model of the segmental mandible defect mode A digital mandible model with standardized segmental defect located between the bilateral premolars, including the incisors and canines, was constructed from a normal male patient
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computed tomography (CT) scan data. All DICOM CT cross-section image data were processed to identify the contours of different hard tissues (cortical and cancellous bone), and those contours were extracted and converted to reconstruct a 3D solid model of the mandible bone with segmental defect (Fig. 2).
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The NH and CS reconstruction plates were generated based on the definition and Synthes physical specimens (Synthes, Paoli, PA, USA) in a computer-aided design (CAD) system (Creo Parametric v2.0; PTC, Needham, MA, USA). The NH and CS reconstruction plates were applied
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along the lower mandible border and fixed onto the respective models with 8 bicortical screws, 4 on each side of the fracture (Fig. 2). Screws were simulated as detail threads with diameters of 2.4 mm.
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All screws were inserted and connected fully along the plate and bone surfaces using Boolean operations to mimic locking screws. A small space was left between the plate and bone to represent the clinical situation. A bone flap was included within the simulated segmental defect models and secured along the lower border of the mandible and on the bone height for occlusal requirement using CS and NH reconstruction plates, respectively (Fig. 2). The bone flap fixed in the NH and CS
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secured models used two bicortical compressive screws. The mandibular bone, bone flap, reconstruction plates. and screw solid models were exported in ANSYS Workbench (ANSYS
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Workbench v14.5; ANSYS Inc., PA, USA) for assembly according to their position relationships.
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Finite element analysis of the segmental mandible defect mode Two segmental defect FE mesh models including NH and CS reconstruction plates (Fig. 3) were generated with quadratic 10-node tetrahedral structural solid elements after the mesh convergence test while controlling the strain energy and displacement variations to <5% for models with different element sizes. Nonlinear frictional contact elements (defined as surface-to-surface) were used to simulate the adaptation between the transplanted flap bone to the segmental defect mandible, compressive screw-to-bone and compressive screw-to-plate in all simulated models. A friction coefficient of 0.5
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was assumed for all contact surfaces. The locking screw-to-bone/plate interface assumed a full-bonded condition to allow stress transfer continuity. The numbers of total elements and nodes for the NS secured model are 521015 and 527542, and 475503 and 488828 for the CS secured model.
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The cortical bone material property was considered orthotropic in different anatomic regions of mandibular bone and was applied in association with a reference coordinate system. Cancellous bone and reconstruction plates/screws were defined as having isotropic properties, independent of
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direction. All elastic modulus and Possion’s ratio values were adopted from the relevant literature (Nagasao et al., 2010; Vajgel et al., 2013) (Table 1). The loads were applied on five principal
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muscles including deep and superficial masseter, medial pterygoid, temporalis and medial temporal, because chewing force was not considered in the patient after surgery (Table 2). The loads were a mouth opening of 5 mm on the incisive tooth, which is the condition that causes the most critical situation (tension and displacement) on the condyle (Ramos et al., 2011). Nodes on the condyle were constrained in all directions to prevent any movements as the boundary conditions. The
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recorded.
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maximum von Mises stress for the bone plate and screw and von Mises strain for the bone were
Patient-matched bending technique and biomechanical fatigue test
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A stamping manufacturing process was used to develop the patient-matched bending technique. The process places a flat reconstruction plate in blank form into a stamping press, where a tool and die surface forms the plate into a net shape. The outer surface of the native mandibular bone from the solid model was exported into the design as the stamping core die (upper die), and the tool (cavity die/lower die) was obtained using Boolean operations in the CAD system. The tool consisted of 2 parts to facilitate stamping bone plate removal (Fig. 4a). The stamping core die and the tool were duplicated as the ABS (ABS-P430, Strayasys, Ltd., Eden Prairie, MN, USA) models using a 3DP printer (Dimension 1200es SST, Strayasys, Ltd., Eden Prairie, MN, USA).
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The NH reconstruction plate and respective screws were made with Ti6Al4V for testing by the manufacturer with GMP and ISO 13485 quality management systems (BOMEi Co., Ltd., Taoyuan, Taiwan) (Fig. 4b). The NH reconstruction plate was then placed in the tool to perform the sheet-metal forming manufacturing processes. Stamping was a single-stage operation using a
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machine press in which every stroke of the press produces the desired form on a bone plate. The corresponding osteotomized mandibular models were also exported to duplicate the ABS plastic mandibular bone with segmental defect models using a 3DP printer. The plastic mandibular
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model simulated the cortical and cancellous bone structures by setting the 3DP application software (CatalystEX 4.4) to sparse−high-density function. The patient-matched bending NH and CS
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reconstruction plates were fixed with 8 bicortical screws (4 on each side) in the desired position to match the resting bone surface (Fig. 4b). The CS reconstruction plates were bent manually by our surgical clinician.
Biomechanical fatigues testing was performed on 3 samples in each NH and CS group, to simulate the segmental defect mandibular bone bridged reconstruction plate under the worst loading
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conditions. The tested samples were embedded into a condyle and clamped onto a test machine (E3000, Instron, Canton, MA, USA) designed to drive a dynamic force. The fatigue test cyclic loads
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were carried out by applying 0 N to 150 N on the reconstruction plate with 2 steel rods contacting at the plate mid-areas to simulate 1.5 times (safety factor) the muscle tensile force (100 N) (Ramos et
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al., 2011) (Fig. 5). The crosshead speed was set at 5 mm/min and the test frequency was 15 Hz. The number of cycles at each load was set at 2 x 105 because this number simulated chewing and swallowing for 1 year (Haug et al., 2001; Rosentritt et al., 2006). Fracture of any component or material yielding permanent deformation was determined as failure.
RESULTS The FE simulation results indicated that the maximum von Mises stresses of the bone plate for the NH and CS secured models were 121 MPa and 546 MPa, respectively. The corresponding
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maximum von Mises stress area for these 2 secured models were both found at the right first screw hole area (Fig. 6). However, the CS secured model value was almost half the titanium alloy (Ti6A14V) ultimate strength (1100MPa) and about 4.5 times that of the NH secured model. Non-obvious variation in the maximum screw stress values was found between the NH and CS
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secured models. The maximum stress values were far from the ultimate titanium alloy strength. The maximum stress tendency of the different screws showed that high stresses were found near the defect region and that the stress value decreased with the increase in distance from the defect region
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(Fig. 7). The corresponding bone strains around the fixation screws showed that the NH secured model was significantly smaller than that of the CS secured model. Bone strain values of the CS
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secured model around the left/right screw holes and flap screw hole exceeded the bone limit value of 4000 (Fig. 8).
No fracture of any component was found in any samples during the fatigue testing. However, permanent deformation of the reconstruction plates, albeit with a non-significant difference, was
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found between the NH (0.510 ± 0.036 mm) and CS (0.527 ± 0.059 mm) secured models.
DISCUSSION
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Tumor of the mandible often requires resection part of the mandible based on the disease nature and extension, which may result in functional/esthetic impairment in patients. Although there
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are varieties of choices for mandible reconstruction, plating the defect by osteosynthesis devices, either with or without bone grafting, is fundamental. The underlying principle of the mandibular reconstruction plate is to provide rigidity to bridge the segmental defect, stabilize the mandibular stumps, maintain the occlusion and restore the facial contour. The design of CS reconstruction plate allows manual bending to adapt to the mandibular morphology. However, the graft position secured by the plate is usually not fit to the prosthetic need. Also, plate fracture resulting from material fatigue by laborious bending is another critical concern for current practice in mandible reconstruction. According to a literature review, reconstruction plate fracture and prosthetic
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restoration failure are the two major clinical complications (Esser and Wagner, 1997; Dimitroulis, 2000). Several studies reported an incidence of 2.9% to 10.7% of plate fractures (Shibahara et al., 2002; Goh et al., 2008) due to inappropriate preoperative bending to match the mandibular contour. Repeated bending causes weak spots and residual stress in the notches/grooves of the plate that are
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prone to fracture when the plate is loaded (Martola et al., 2007). Prosthetic restoration failure is usually caused by the limited vertical height of the fibular flap, resulting in an unfavorable
crown/implant length ratio, leading to implant overloading (Esser and Wagner, 1997; Dimitroulis,
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2000). Therefore, the present study proposed a new reconstruction plate design to achieve the following: 1) initial load-bearing requirement for segmental mandibular defect; 2) securing the
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transplant bone graft at the alveolar part, which is better fitted to the prosthetic need; 3) using a patient-matched bending technique to increase interfacial fit adaptation between the reconstruction plate and mandible, which may reduce mechanical failure from laborious bending procedure; and 4) maintaining the facial contour.
Prosthesis fracture is one of the major reasons for plate failure. Plate defects are usually found
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at the area of repeated unsuitable preoperative bending and are prone to fracture when the plate is loaded. In our study, a patient-matched bending technique based on a stamping manufacturing
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process was developed to form the NH reconstruction plate. The stamping core was duplicated based on the patient’s original mandibular contour. Thus, the lower part of the plate formed a good
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interfacial fit adaptation to the mandible. The stamping procedure prevents material fatigue of the plate from a laborious bending procedure. Owing to the mandibular morphology, the alveolar bone is positioned more centrally to the mandibular border. By customized stamping, the mesh structure of the plate could fix the transplanted bone graft in the position favored for dental prosthesis fabrication. This hybridization concept fits the need for facial contour maintenance and prosthesis fabrication need. Three-dimensional printing techniques have been widely used in several categories of medical practice, including customized implants/prostheses, anatomical models, and even tissue and organ
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fabrication (Ventola, 2014). In our knowledge, the present study using 3DP stamping manufacturing to pre-shape the bone plate by a patient-matched bending technique is the first to be proposed. Traditionally, the creation of stamping tool has been time consuming, with marginal cost and benefit to produce individually. Using 3DP to duplicate the stamping die/tool has the greatest advantage in
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made-to-order (custom) prostheses, and thus has a positive impact in terms of the time required for surgery and more competitive cost for small production runs. However, the 3DP material must meet the strength required for the stamping manufacturing process. A patient-specific computer-aided
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design/computer-aided manufacturing (CAD/CAM) reconstruction plate may be one of the best solutions for the current state-of-the-art mandible reconstruction. However, it is not yet foreseeable
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whether it will, in the future, become clinical routine or will be confined to selective cases (Wilde F et al., 2015). Our proposed plate provides a customized and more economic design. From the FE analysis results, the maximum von Mises stresses of the reconstruction plate for the NH and CS secured models were both found at the right first screw-hole areas. However, the NH secured model value decreased effectively when compared to the CS secured model. The
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maximum von Mises stresses of the screws decreased inversely with the distance from the defect region. The corresponding values for screw numbers 1 and 10 of NH and CS secure models were
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lower than 20 MPa (Fig. 7). It was confirmed that 3 screws on either side of a defect are the minimum number required for effective load-bearing anchorage (Bujtár et al., 2014; Ellis and Miles,
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2007). The maximum stress values were far from the ultimate titanium alloy strength in both secured models, demonstrating the benefit of the comparatively rigid fixation offered by a locking system to reduce screw loosening during cyclic loading (Bujtár et al., 2014; Koonce et al., 2012). The maximum von Mises strain for bone around the screw holes were also recorded, as the screw-loosening index owing to strain is accepted as the mechanical stimuli for bone remodeling around an implant (Frost, 1994; Cehreli et al., 2004; Yu et al., 2014). Frost (1994) suggested that bone remodeling is initiated at a critical strain level (the mechano-static theory) and that micro damage arises in normal lamellar bone when the strain exceeds 4000 micro strain. An ideal design
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to minimize the strain value in the bone surrounding the screw hole to reduce screw loosening is the most important task (Yu et al., 2014). However, the maximum bone strain values of the CS secured model around the left/right screw holes and flap screw hole fall into an unfavorable range that affects screw insertion loosening.
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Load-bearing osteosynthesis plating of the segmental mandibular defect is usually performed in a straightforward manner after tumor resection by transfacial approaches. The wide-open surgical field facilitates plate contouring and fixation. The proposed NH plate is contoured preoperatively to
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fit the individual mandibular morphology, which will further reduce the time for plate bending. It also provides a possibility to perform tumor resection, and plate fixation via a transoral approach.
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Schiel et al. proposed a transoral procedure to perform extended load-bearing oteosynthesis with a preformed mandible reconstruction plate (Schiel et al., 2013). We also use the same technique in selective benign mandibular tumor resection and reconstruction. In our experience, the wound healing is not affected even by additional periosteal detachment that may need better exposure. However, plate prebending to accommodate the mandibular morphology is a prerequisite to reduce
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the trial-and-error adjustment of the prosthesis. The NH plate is pre-shaped, which facilitates insertion and fixation even in the more limit transoral route. The overall operation time is estimated
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to be further shortened if combined with CAD with a cutting jig. The concept of the NH plate is to secure the bone graft in the prosthodontic need position
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while preserving the facial contour by the plate per se. From the surgical view, the design will leave a dead space below the bone graft, which may hamper wound healing and increase the complication rate, especially in non-vascularized bone grafts. The surgeon should be alerted to the condition before wound closure. In the case of a small defect, the flexibility of the mouth floor tissue may obliterate the space, whereas in the case of a large defect, a soft tissue flap should be carefully designed to diminish the generation of dead space. One of the limitations in this study is the lack of comparison to customized preformed plate. The customized plate, for example, a MatrixMANDIBLE Preformed Reconstruction Plate (Synthes,
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Paoli, PA, USA), which is fabricated based on morphometric shape analysis of various ethnic populations, provides biomechanical benefits by reducing the bending procedures (Metzger et al., 2011). Although the design of a preformed plate is more customized to CS plates by offering anatomic preshaping to an anterior-lateral mandible, the flexibility to contour the center
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nonbendible part is relative limited. The center-overcontoured plate, even adapted well to the proximal and distal mandibular stumps, still has the chance to develop cutaneous fistulae from the sequelae of severe scleroderma of the corresponding facial region, especially in patients with a lack
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of facial soft tissue thickness after resection of buccogingival cancer and those undergoing
postoperative radiation therapy. In a case cohort study report, the most frequent complication of
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preformed plate reconstruction is estimated to be 16% (Probst et al., 2012). The present study proposed an NH plate using a patient-matched bending technique, providing a more patient-specific solution. Furthermore, the mesh shape design of the center-piece may disperse the stress concentration more than the common bar shape design of preformed/CS plates. This may further prevent the development of orocutaneous fistulae from severe soft tissue contracture.
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The simulated bite force used in this study was set at 150 N, which is much higher than the maximal bite force measured in patients with mandibular resection (Ramos et al., 2011; Maurer et
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al., 2006). Wiskott et al. (1994) pointed out chewing and swallowing contacts equal to 1800 per day, and Delong et al. also reported chewing tooth contacts equal to 240,000 in 1 year (1985). Based on
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these data, we applied a 2 x 105 cyclic load to simulate clinical chewing in our biomechanical fatigue testing. In our results, none of the NH plate was fractured in the fatigue test. In addition, the difference in permanent deformation between CS and NH plate was non-significant, meaning that the strength of the NH plate was sufficient to withstand the bite force and chewing load in the scenario of mandible resection. Although it is not comparable to a customized preformed plate, the results showed that the NH plate can meet the clinical need. Although FE analysis is generally accepted as a complementary tool and is widely applied to study the effect of surgical procedures (Szucs et al., 2010), osteotomy designs (Bujtár et al., 2012),
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fixation methods and facial reconstructive surgery (Huang et al., 2016), to construct an accurate model with detailed bone anatomy, realistic material properties and boundary conditions are necessary to ensure that the simulation results are clinically convincing (Lin et al., 2010). The precise geometry of the 3D model provides accurate anatomical volumetrics for generating a fine
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meshing process and non-linear contact analysis to mimic the interfacial condition. This approach is more realistic and descriptive than previously reported in the medical literature and could ensure the integrity of the simulations. However, the FE analysis result is limited by the theoretical
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assumptions, including the load conditions and material properties. The load condition considered only the condition that causes the most critical situation on the condyle and does not consider
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chewing force after surgery. Linear elastic (homogeneous and isotropic) properties were adopted for all materials due to numerical convergence considerations. Therefore, the simulated and experimental results provided in this study must be confirmed in further, well-controlled clinical trials.
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CONCLUSION
The present study proposed a new reconstruction plate design that hybridize the concept for
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strength, contour, and function. The NH plate provides the following benefits: 1) initial load-bearing requirement for segmental mandibular defect; 2) securing the transplant bone graft at the alveolar
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part, which is better fitted to the prosthetic need; 3) using patient-matched bending technique to increase interfacial fit adaptation between the reconstruction plate and mandible, which may reduce mechanical failure from a laborious bending procedure; and 4) maintaining facial contour. The biomechanical evaluation addressed the fact that maximum bone plate stress and bone strain around the fixation screws were decreased significantly over those in the CS secured model, indicating that the strength was sufficient to meet the clinical need. In the context of customization, the NH plate provides a more economical design than a patient-specific CAD/CAM plate. Further testing in the clinical scenario is required to clarify the procedure-related issues.
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Contributions C.H. Wu and C.L. Lin conceived and designed the experiments; Y.S. Liu performed the simulation and experiment; Y.S. Lin analyzed the data; C.H. Wu and C.L. Lin wrote the manuscript; and all
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authors read and approved the final version of manuscript.
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Competing interests None declared.
Source of support
This study is part of the Master’s Thesis of Y.S. Yu (2015) at the Department of Biomedical
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Engineering at National Yang-Ming University, Taiwan, and supported in part by MOST project
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103-2221-E-010 -01-MY3 of the Ministry of Science and Technology, Taiwan.
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Clin Biomech 27(7): 697-701, 2012
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precontoured locking plates for distal humerus fracture fixation? A biomechanics cadaver study.
Li P, Shen L, Li J, Liang R, Tian W, Tang W: Optimal design of an individual endoprosthesis for the reconstruction of extensive mandibular defects with finite element analysis. J Cranio Maxillofac Surg 42(1): 73-78, 2014
Lin CL, Wang JC, Chang SH, Chen ST: Evaluation of stress induced by implant type, number
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of splinted teeth, and variations in periodontal support in tooth-implant-supported fixed partial dentures: a non-linear finite element analysis. J Periodontol 81(1): 121-130, 2010 Martola M, Lindqvist C, Hänninen H, Al‐Sukhun J: Fracture of titanium plates used for
80(2): 345-352, 2007
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mandibular reconstruction following ablative tumor surgery. J Biomed Mater Res B Appl Biomater
Maurer P, Pistner H, Schubert J: Computer assisted chewing power in patients with resection
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of the mandible. Mund Kiefer Gesichtschir 10(1): 37-41, 2006 Metzger MC, Vogel M, Hohlweg-Majert B, et al: Anatomical shape analysis of the mandible in Caucasians and Chinese for the production of preformed mandible reconstruction plates. J Craniomaxillofac Surg 39:393, 2011 Nagasao T, Miyamoto J, Tamaki T, Kawana H: A comparison of stresses in implantation for grafted and plate-and-screw mandible reconstruction. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 109(3): 346-356, 2010 Probst FA, Mast G, Ermer M, Gutwald R, et al: MatrixMANDIBLE preformed reconstruction plates—a two-year two-institution experience in 71 patients. J Oral Maxillofac Surg 70(11): e657-66, 2012 Ramos A, Completo A, Relvas C, Mesnard M, Simões J: Straight, semi-anatomic and anatomic 15
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TMJ implants: the influence of condylar geometry and bone fixation screws. J Cranio Maxill Surg 39(5): 343-350, 2011 Rosentritt M, Behr M, Gebhard R, Handel G: Influence of stress simulation parameters on the fracture strength of all-ceramic fixed-partial dentures. Dent Mater 22(2): 176-182, 2006 Schiel S, Otto S, Pautke C, Cornelius CP, Probst FA: Simplified transoral load-bearing
6(3): 211-214, 2013
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osteosynthesis with preformed mandible reconstruction plates. Craniomaxillofac Trauma Reconstr
Shibahara T, Noma H, Furuya Y, Takaki R: Fracture of mandibular reconstruction plates used after tumor resection. J Oral Maxillofac Surg 60(2): 182-185, 2002
Szucs A, Bujtar P, Sándor G, Barabas J: Finite element analysis of the human mandible to
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assess the effect of removing an impacted third molar. J Can Dent Assoc 76 a72, 2009
Vajgel A, Camargo IB, Willmersdorf RB, De Melo TM, Laureano Filho JR, De Holanda
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Vasconcellos RJ: Comparative finite element analysis of the biomechanical stability of 2.0 fixation plates in atrophic mandibular fractures. J Oral Maxillofac Surg 71(2): 335-342, 2013 Ventola CL: Medical applications for 3D printing: current and projected uses. Pharm Ther (P T) 39(10): 704, 2014
Wilde F, Hanken H, Probst F, Schramm A, Heiland M, Cornelius CP: Multicenter study on the use of patient-specific CAD/CAM reconstruction plate for mandibular reconstruction. Int J Comput
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Assist Radiol Surg 10(12): 2035-2051, 2015
Wiskott HA, Nicholls JI, Belser UC: Fatigue resistance of soldered joints: a methodological study. Dent Mater 10(3): 215-220, 1994
15(4): 245-249, 1994
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Yim KK, Wei FC: Fibula osteoseptocutaneous flap for mandible reconstruction. Microsurgery
Yu JH, Lin YS, Chang WJ, Chang YZ, Lin CL: Mechanical effects of micro-thread orthodontic
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mini-screw design on artificial cortical bone. J Med Biol Eng 34(1): 49-55, 2014 Zhang C, Ruan M, Xu L, Hu Y, Yang W, Ji T, et al: Dental implant distractor combined with free fibular flap: a new design for simultaneous functional mandibular reconstruction. J Oral Maxillofac Surg 70(11): 2687-2700, 2012 Zhao L, Shang H, Chen X, Liu Y: Biomechanical analysis of a curvilinear distractor device for correcting mandibular symphyseal defects. J Oral Maxillofac Surg 72(6): 1158-1167, 2014
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Table 1. Material properties of bone and titanium
Cancellou s
Cortical bone Material property Body
Angle
Bone
Ex (MPa)
20,492
21,728
24,607
1,500
Ey (MPa)
12,092
12,700
12,971
Ez (MPa)
16,350
17,828
18,357
0.43
0.45
Poisson’s ratio (Pyz) 0.22
11,000
1,500
11,000
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1,500
0.3
0.34
0.23
0.3
0.34
0.34
0.28
0.3
0.34
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Based on Vajgel et al. (2013).
11,000
0.2
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Poisson’s ratio (Pxz) 0.34
0.38
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Poisson’s ratio (Pxy)
alloy
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Symphysis
Titanium
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Load (N) Muscle actions
Reference Y
Z
Deep masseter
7.776
127.23
22.68
M1,2
Superficial masseter
12.873
183.5
12.11
M3,4
Medial pterygoid
140.38
237.8
-77.3
M5,6
Temporalis
0.064
0.37
Medial temporal
0.97
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Based on Romas et al. (2011).
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X
18
-7.44
M7,8
M9,10
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Fig. 1. Schematic diagram of the design concept for the novel hybrid (NH) reconstruction plate.
Fig. 2. Solid computer-aided design models of the (a) commercial straight (CS) and (b) novel hybrid (NH) secured models, (a) also represented loading and boundary conditions of the finite
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element simulations and muscular actions. Five muscle pairs: M1, M2 deep masseter; M3, M4 superficial masseter; M5, M6 medial pterygoid; M7, M8 temporal; and M9, M10 medial temporal.
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Fig. 3. Finite element mesh models of the (a) commercial straight (CS) and (b) novel hybrid (NH)
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secured models.
Fig. 4. (a) Schematic diagram of the stamping press. Stamping core die (upper die) was acquired from the native mandibular bone, and tool (cavity die/lower die) was obtained using Boolean operations in the computer-aided design system and consisted of 2 parts to facilitate stamping bone plate removal. (b) The novel hybrid (NH) reconstruction plate and respective screws were made
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with Ti6Al4V for testing, and the corresponding osteotomized mandibular models were exported to duplicate the ABS plastic mandibular bone with segmental defect models using a 3-dimensional
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printer. The NH reconstruction plate was placed in the tool to perform the sheet metal−forming manufacturing processes and was fixed with 8 bicortical screws (4 on each side) in the desired
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positions to match the resting bone surface.
Fig. 5. Schematic diagram of the biomechanical fatigue testing, showing that the tested samples were embedded into condyle and clamped onto a test machine designed to drive a dynamic force.
Fig. 6. The von Mises stress distribution of the bone plate for the (a) commercial straight (CS) and (b) novel hybrid (NH) secured models.
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Fig. 7. The von Mises stress distribution of the fixation screws for the commercial straight (CS; upper) and the novel hybrid (NH; middle) secured models.
Fig. 8. The von Mises bone strains distribution around the fixation screws for the commercial
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straight (CS; left) and the novel hybrid (NH; right) secured models.
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S1010-5182(17)30248-2
DOI:
10.1016/j.jcms.2017.07.010
Reference:
YJCMS 2733
To appear in:
Journal of Cranio-Maxillo-Facial Surgery
Received Date: 17 November 2016 Revised Date:
18 June 2017
Accepted Date: 18 July 2017
Please cite this article as: Wu C-H, Lin Y-S, Liu Y-S, Lin C-L, Biomechanical evaluation of a novel hybrid reconstruction plate for mandible segmental defects: A finite element analysis and fatigue testing, Journal of Cranio-Maxillofacial Surgery (2017), doi: 10.1016/j.jcms.2017.07.010. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Biomechanical evaluation of a novel hybrid reconstruction plate for mandible segmental defects: A finite element analysis and fatigue testing Cheng-Hsien Wu, DDS, Ph.D.1, Yang-Sung Lin, Ph.D.2, Yu-Shen Liu, MS3 and Chun-Li Lin, Ph.D.4,*
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1 Name: Cheng-Hsien Wu Highest academic degree(s): DDS, Ph.D. Institution: Attending Surgeon, Oral & Maxillofacial Surgery, Taipei Veterans General Hospital Assistant Professor, School of Dentistry, National Yang-Ming University, Taipei, Taiwan Tel: 886-2-28213792 Email: [email protected] Address：No.155, Sec.2, Linong Street, Taipei, 112
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2 Name: Yang-Sung Lin Highest academic degree(s): Ph.D. Institution: Post-doctor Researcher, Department of Biomedical Engineering, National Yang-Ming University, Taipei, Taiwan. Tel: 886-2-28213792 Email: [email protected] Address: No.155, Sec.2, Linong Street, Taipei, 112
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3 Name: Yu-Shen Liu Highest academic degree(s): MS Institution: Department of Biomedical Engineering, National Yang-Ming University, Taipei, Taiwan. Tel: 886-2-28213792 Email: [email protected] Address: No.155, Sec.2, Linong Street, Taipei, 112 4 Name: Chun-Li Lin Highest academic degree(s): Ph.D. Institution: Professor, Department of Biomedical Engineering, National Yang-Ming University, Taipei, Taiwan. Tel: 886-2-28267000 ext 7039 Fax: 886-2-28210847 Email: [email protected] Address: No.155, Sec.2, Linong Street, Taipei, 112
*Corresponding author: Chun-Li Lin, Ph.D., Professor, Department of Biomedical Engineering,
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National Yang-Ming University, Taipei, Taiwan. Tel: 886-2-28267000 ext 7039 Fax: 886-2-28210847 Email: [email protected] Address: No.155, Sec.2, Linong Street, Taipei, 112
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Sources of support in the form of grants: This study is one part of the Master Thesis of YS Yu (2015) at the Department of Biomedical Engineering at National Yang-Ming University, Taiwan and supported in part by MOST project
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103-2221-E-010-012-MY3 of the Ministry of Science and Technology, Taiwan.
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Summary
Purpose: This study develops a novel hybrid (NH) reconstruction plate that can provide load-bearing strength, secure the bone transplant at the prosthesis favored position, and also
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maintain the facial contour in a mandibular segmental defect. A new patient-match bending technique which uses a three-dimensional printing (3DP) stamping process is developed to increase the interfacial fit between the reconstruction plate and mandibular bone.
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Materials and Methods: The NH reconstruction plate was designed to produce a continuous profile with non-uniform thickness and triangular cross-screw patterns with a locking-screw feature
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at the plate base. Two mandible segmental defect finite element models including the NH reconstruction plate to secure a bone flap for occlusal requirement and the commercial straight (CS) reconstruction plate to secure a bone flap along the lower mandible border were generated for biomechanical fatigue testing.
Results: The simulated results showed that the maximum von Mises stresses of the reconstruction
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plate for CS secured model are about 4.5 times more than the NH secured model. The bone strains around the fixation screws showed that the CS secured model was meaningfully higher than that of
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the NH secured model and exceeded the bone limit value. No fracture of any component was found in any sample in the fatigue testing.
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Conclusion: In conclusion, the newly developed NH reconstruction plate can secure the transplant position in accordance to the individual occlusal requirements without sacrificing the maintenance of facial contour. Finite element−based biomechanical evaluation demonstrates superior mechanical strength compared to commercial standard plates.
Keywords: reconstruction plate, finite element, segmental defect, 3D printing, fatigue
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Large mandibular defects resulting from trauma, tumor or osteoradionecrosis are common findings in the maxillofacial area. Failure to correct such defects will jeopardize functions such as masticating, swallowing, pronounciation, and esthetics, to different degrees (Chenping et al., 2012;
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Zhao et al., 2014). Using osteosynthesis devices to bridge the defect with or without bone grafting is fundamental to mandible reconstruction. Among the techniques used, the free vascularized fibular flap has become the preferred method for mandibular reconstruction, owing to the advantages of
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adequate bone length, low donor site morbidity and an abundant periosteal blood supply (Cordeiro et al., 1999; Kim and Blackwell, 2007; He et al., 2011; Chenping et al., 2012). One of the
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disadvantages of fibular bone grafts is the insufficient bone height for implant-supported prosthesis design (Yim and Wei, 1994; He et al., 2011). Meanwhile, the conventional plate design for segmental mandibular defects places the device and secures the bone graft in the lower border of mandible, which is not suitable for prosthesis fabrication (Chiapasco et al., 2008; Chenping et al., 2012). Several options have be developed to increase the fibula vertical height, such as the
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double-barrel technique, second-stage bone graft and distraction osteogenesis (Chang et al., 2008; Goh et al., 2008; Cho-Lee et al., 2011; Chenping et al., 2012). However, these methods may
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increase the flap failure rate, the need for secondary operation, and extended time before oral rehabilitation. (Goh et al., 2008; He et al., 2011).
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The purpose of reconstruction plate is not only to bridge the mandibular stumps, it should also bear the occlusal load to maintain the fixation stability during the bone healing phase (Vajgel et al., 2013; Bujtár et al., 2014; Li et al., 2014). Two major concerns are frequent encountered when using reconstruction plate technique. Firstly, the current commercial reconstruction plates needs manual contouring to fit the individual mandibular geometry profiles. This procedure is quite experience related and technique sensitive, which means that, for inexperienced surgeons or in the case of an extensive mandibular defect, the trial-and-error plate contouring (bending) approach may generate residual stress to predispose to bone plate fracture and fatigue failure (Goh et al., 2008). Secondly,
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the poor adaptation of the reconstruction plate will impair the esthetic outcome and exert unfavorable stress concentrated around the fixation screws when occlusal load is applied. This will ultimately result in screw loosening and prosthesis failure (Kimura et al., 2006). This study develops a novel hybrid (NH) reconstruction plate that can provide load-bearing
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strength, secure the bone transplant at the prosthesis favored position and also maintain the facial contour in the mandibular segmental defect. A patient-matched bending technique was developed to increase the interfacial fit adaptation between the NH reconstruction plate and mandibular contour
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by using integrated reverse engineering and 3D printing (3DP) and stamping techniques. Finite element (FE) analysis was designed for biomechanical fatigue testing and mechanical behavior
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pattern evaluation in comparison to that of a commercial straight (CS) reconstruction plate.
MATERIALS AND METHODS Design concept of the NH reconstruction plate
The NH reconstruction plate was designed to secure the transplant height consistent with the
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alveolar bone for dental implant requirement. In order to provide adjustment options for the transplant height, bending capability and structural stability, the NH reconstruction plate was
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designed to present a continuous profile with non-uniform thickness in the transplant height direction (Fig. 1). The NH reconstruction plate base was thickened to 2.4 mm to maintain structural
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strength. Triangle cross-screw patterns with locking screws at the plate base were designed to enhance the fixation stability. The upper half of the NH reconstruction plate in the transplant height direction was gradually reduced to 0.8-mm thickness for bending capability to match the curvature of the mandibular bone shape (Fig. 1).
Digital model of the segmental mandible defect mode A digital mandible model with standardized segmental defect located between the bilateral premolars, including the incisors and canines, was constructed from a normal male patient
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computed tomography (CT) scan data. All DICOM CT cross-section image data were processed to identify the contours of different hard tissues (cortical and cancellous bone), and those contours were extracted and converted to reconstruct a 3D solid model of the mandible bone with segmental defect (Fig. 2).
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The NH and CS reconstruction plates were generated based on the definition and Synthes physical specimens (Synthes, Paoli, PA, USA) in a computer-aided design (CAD) system (Creo Parametric v2.0; PTC, Needham, MA, USA). The NH and CS reconstruction plates were applied
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along the lower mandible border and fixed onto the respective models with 8 bicortical screws, 4 on each side of the fracture (Fig. 2). Screws were simulated as detail threads with diameters of 2.4 mm.
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All screws were inserted and connected fully along the plate and bone surfaces using Boolean operations to mimic locking screws. A small space was left between the plate and bone to represent the clinical situation. A bone flap was included within the simulated segmental defect models and secured along the lower border of the mandible and on the bone height for occlusal requirement using CS and NH reconstruction plates, respectively (Fig. 2). The bone flap fixed in the NH and CS
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secured models used two bicortical compressive screws. The mandibular bone, bone flap, reconstruction plates. and screw solid models were exported in ANSYS Workbench (ANSYS
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Workbench v14.5; ANSYS Inc., PA, USA) for assembly according to their position relationships.
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Finite element analysis of the segmental mandible defect mode Two segmental defect FE mesh models including NH and CS reconstruction plates (Fig. 3) were generated with quadratic 10-node tetrahedral structural solid elements after the mesh convergence test while controlling the strain energy and displacement variations to <5% for models with different element sizes. Nonlinear frictional contact elements (defined as surface-to-surface) were used to simulate the adaptation between the transplanted flap bone to the segmental defect mandible, compressive screw-to-bone and compressive screw-to-plate in all simulated models. A friction coefficient of 0.5
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was assumed for all contact surfaces. The locking screw-to-bone/plate interface assumed a full-bonded condition to allow stress transfer continuity. The numbers of total elements and nodes for the NS secured model are 521015 and 527542, and 475503 and 488828 for the CS secured model.
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The cortical bone material property was considered orthotropic in different anatomic regions of mandibular bone and was applied in association with a reference coordinate system. Cancellous bone and reconstruction plates/screws were defined as having isotropic properties, independent of
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direction. All elastic modulus and Possion’s ratio values were adopted from the relevant literature (Nagasao et al., 2010; Vajgel et al., 2013) (Table 1). The loads were applied on five principal
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muscles including deep and superficial masseter, medial pterygoid, temporalis and medial temporal, because chewing force was not considered in the patient after surgery (Table 2). The loads were a mouth opening of 5 mm on the incisive tooth, which is the condition that causes the most critical situation (tension and displacement) on the condyle (Ramos et al., 2011). Nodes on the condyle were constrained in all directions to prevent any movements as the boundary conditions. The
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recorded.
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maximum von Mises stress for the bone plate and screw and von Mises strain for the bone were
Patient-matched bending technique and biomechanical fatigue test
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A stamping manufacturing process was used to develop the patient-matched bending technique. The process places a flat reconstruction plate in blank form into a stamping press, where a tool and die surface forms the plate into a net shape. The outer surface of the native mandibular bone from the solid model was exported into the design as the stamping core die (upper die), and the tool (cavity die/lower die) was obtained using Boolean operations in the CAD system. The tool consisted of 2 parts to facilitate stamping bone plate removal (Fig. 4a). The stamping core die and the tool were duplicated as the ABS (ABS-P430, Strayasys, Ltd., Eden Prairie, MN, USA) models using a 3DP printer (Dimension 1200es SST, Strayasys, Ltd., Eden Prairie, MN, USA).
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The NH reconstruction plate and respective screws were made with Ti6Al4V for testing by the manufacturer with GMP and ISO 13485 quality management systems (BOMEi Co., Ltd., Taoyuan, Taiwan) (Fig. 4b). The NH reconstruction plate was then placed in the tool to perform the sheet-metal forming manufacturing processes. Stamping was a single-stage operation using a
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machine press in which every stroke of the press produces the desired form on a bone plate. The corresponding osteotomized mandibular models were also exported to duplicate the ABS plastic mandibular bone with segmental defect models using a 3DP printer. The plastic mandibular
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model simulated the cortical and cancellous bone structures by setting the 3DP application software (CatalystEX 4.4) to sparse−high-density function. The patient-matched bending NH and CS
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reconstruction plates were fixed with 8 bicortical screws (4 on each side) in the desired position to match the resting bone surface (Fig. 4b). The CS reconstruction plates were bent manually by our surgical clinician.
Biomechanical fatigues testing was performed on 3 samples in each NH and CS group, to simulate the segmental defect mandibular bone bridged reconstruction plate under the worst loading
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conditions. The tested samples were embedded into a condyle and clamped onto a test machine (E3000, Instron, Canton, MA, USA) designed to drive a dynamic force. The fatigue test cyclic loads
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were carried out by applying 0 N to 150 N on the reconstruction plate with 2 steel rods contacting at the plate mid-areas to simulate 1.5 times (safety factor) the muscle tensile force (100 N) (Ramos et
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al., 2011) (Fig. 5). The crosshead speed was set at 5 mm/min and the test frequency was 15 Hz. The number of cycles at each load was set at 2 x 105 because this number simulated chewing and swallowing for 1 year (Haug et al., 2001; Rosentritt et al., 2006). Fracture of any component or material yielding permanent deformation was determined as failure.
RESULTS The FE simulation results indicated that the maximum von Mises stresses of the bone plate for the NH and CS secured models were 121 MPa and 546 MPa, respectively. The corresponding
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maximum von Mises stress area for these 2 secured models were both found at the right first screw hole area (Fig. 6). However, the CS secured model value was almost half the titanium alloy (Ti6A14V) ultimate strength (1100MPa) and about 4.5 times that of the NH secured model. Non-obvious variation in the maximum screw stress values was found between the NH and CS
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secured models. The maximum stress values were far from the ultimate titanium alloy strength. The maximum stress tendency of the different screws showed that high stresses were found near the defect region and that the stress value decreased with the increase in distance from the defect region
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(Fig. 7). The corresponding bone strains around the fixation screws showed that the NH secured model was significantly smaller than that of the CS secured model. Bone strain values of the CS
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secured model around the left/right screw holes and flap screw hole exceeded the bone limit value of 4000 (Fig. 8).
No fracture of any component was found in any samples during the fatigue testing. However, permanent deformation of the reconstruction plates, albeit with a non-significant difference, was
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found between the NH (0.510 ± 0.036 mm) and CS (0.527 ± 0.059 mm) secured models.
DISCUSSION
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Tumor of the mandible often requires resection part of the mandible based on the disease nature and extension, which may result in functional/esthetic impairment in patients. Although there
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are varieties of choices for mandible reconstruction, plating the defect by osteosynthesis devices, either with or without bone grafting, is fundamental. The underlying principle of the mandibular reconstruction plate is to provide rigidity to bridge the segmental defect, stabilize the mandibular stumps, maintain the occlusion and restore the facial contour. The design of CS reconstruction plate allows manual bending to adapt to the mandibular morphology. However, the graft position secured by the plate is usually not fit to the prosthetic need. Also, plate fracture resulting from material fatigue by laborious bending is another critical concern for current practice in mandible reconstruction. According to a literature review, reconstruction plate fracture and prosthetic
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restoration failure are the two major clinical complications (Esser and Wagner, 1997; Dimitroulis, 2000). Several studies reported an incidence of 2.9% to 10.7% of plate fractures (Shibahara et al., 2002; Goh et al., 2008) due to inappropriate preoperative bending to match the mandibular contour. Repeated bending causes weak spots and residual stress in the notches/grooves of the plate that are
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prone to fracture when the plate is loaded (Martola et al., 2007). Prosthetic restoration failure is usually caused by the limited vertical height of the fibular flap, resulting in an unfavorable
crown/implant length ratio, leading to implant overloading (Esser and Wagner, 1997; Dimitroulis,
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2000). Therefore, the present study proposed a new reconstruction plate design to achieve the following: 1) initial load-bearing requirement for segmental mandibular defect; 2) securing the
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transplant bone graft at the alveolar part, which is better fitted to the prosthetic need; 3) using a patient-matched bending technique to increase interfacial fit adaptation between the reconstruction plate and mandible, which may reduce mechanical failure from laborious bending procedure; and 4) maintaining the facial contour.
Prosthesis fracture is one of the major reasons for plate failure. Plate defects are usually found
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at the area of repeated unsuitable preoperative bending and are prone to fracture when the plate is loaded. In our study, a patient-matched bending technique based on a stamping manufacturing
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process was developed to form the NH reconstruction plate. The stamping core was duplicated based on the patient’s original mandibular contour. Thus, the lower part of the plate formed a good
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interfacial fit adaptation to the mandible. The stamping procedure prevents material fatigue of the plate from a laborious bending procedure. Owing to the mandibular morphology, the alveolar bone is positioned more centrally to the mandibular border. By customized stamping, the mesh structure of the plate could fix the transplanted bone graft in the position favored for dental prosthesis fabrication. This hybridization concept fits the need for facial contour maintenance and prosthesis fabrication need. Three-dimensional printing techniques have been widely used in several categories of medical practice, including customized implants/prostheses, anatomical models, and even tissue and organ
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fabrication (Ventola, 2014). In our knowledge, the present study using 3DP stamping manufacturing to pre-shape the bone plate by a patient-matched bending technique is the first to be proposed. Traditionally, the creation of stamping tool has been time consuming, with marginal cost and benefit to produce individually. Using 3DP to duplicate the stamping die/tool has the greatest advantage in
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made-to-order (custom) prostheses, and thus has a positive impact in terms of the time required for surgery and more competitive cost for small production runs. However, the 3DP material must meet the strength required for the stamping manufacturing process. A patient-specific computer-aided
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design/computer-aided manufacturing (CAD/CAM) reconstruction plate may be one of the best solutions for the current state-of-the-art mandible reconstruction. However, it is not yet foreseeable
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whether it will, in the future, become clinical routine or will be confined to selective cases (Wilde F et al., 2015). Our proposed plate provides a customized and more economic design. From the FE analysis results, the maximum von Mises stresses of the reconstruction plate for the NH and CS secured models were both found at the right first screw-hole areas. However, the NH secured model value decreased effectively when compared to the CS secured model. The
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maximum von Mises stresses of the screws decreased inversely with the distance from the defect region. The corresponding values for screw numbers 1 and 10 of NH and CS secure models were
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lower than 20 MPa (Fig. 7). It was confirmed that 3 screws on either side of a defect are the minimum number required for effective load-bearing anchorage (Bujtár et al., 2014; Ellis and Miles,
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2007). The maximum stress values were far from the ultimate titanium alloy strength in both secured models, demonstrating the benefit of the comparatively rigid fixation offered by a locking system to reduce screw loosening during cyclic loading (Bujtár et al., 2014; Koonce et al., 2012). The maximum von Mises strain for bone around the screw holes were also recorded, as the screw-loosening index owing to strain is accepted as the mechanical stimuli for bone remodeling around an implant (Frost, 1994; Cehreli et al., 2004; Yu et al., 2014). Frost (1994) suggested that bone remodeling is initiated at a critical strain level (the mechano-static theory) and that micro damage arises in normal lamellar bone when the strain exceeds 4000 micro strain. An ideal design
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to minimize the strain value in the bone surrounding the screw hole to reduce screw loosening is the most important task (Yu et al., 2014). However, the maximum bone strain values of the CS secured model around the left/right screw holes and flap screw hole fall into an unfavorable range that affects screw insertion loosening.
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Load-bearing osteosynthesis plating of the segmental mandibular defect is usually performed in a straightforward manner after tumor resection by transfacial approaches. The wide-open surgical field facilitates plate contouring and fixation. The proposed NH plate is contoured preoperatively to
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fit the individual mandibular morphology, which will further reduce the time for plate bending. It also provides a possibility to perform tumor resection, and plate fixation via a transoral approach.
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Schiel et al. proposed a transoral procedure to perform extended load-bearing oteosynthesis with a preformed mandible reconstruction plate (Schiel et al., 2013). We also use the same technique in selective benign mandibular tumor resection and reconstruction. In our experience, the wound healing is not affected even by additional periosteal detachment that may need better exposure. However, plate prebending to accommodate the mandibular morphology is a prerequisite to reduce
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the trial-and-error adjustment of the prosthesis. The NH plate is pre-shaped, which facilitates insertion and fixation even in the more limit transoral route. The overall operation time is estimated
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to be further shortened if combined with CAD with a cutting jig. The concept of the NH plate is to secure the bone graft in the prosthodontic need position
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while preserving the facial contour by the plate per se. From the surgical view, the design will leave a dead space below the bone graft, which may hamper wound healing and increase the complication rate, especially in non-vascularized bone grafts. The surgeon should be alerted to the condition before wound closure. In the case of a small defect, the flexibility of the mouth floor tissue may obliterate the space, whereas in the case of a large defect, a soft tissue flap should be carefully designed to diminish the generation of dead space. One of the limitations in this study is the lack of comparison to customized preformed plate. The customized plate, for example, a MatrixMANDIBLE Preformed Reconstruction Plate (Synthes,
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Paoli, PA, USA), which is fabricated based on morphometric shape analysis of various ethnic populations, provides biomechanical benefits by reducing the bending procedures (Metzger et al., 2011). Although the design of a preformed plate is more customized to CS plates by offering anatomic preshaping to an anterior-lateral mandible, the flexibility to contour the center
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nonbendible part is relative limited. The center-overcontoured plate, even adapted well to the proximal and distal mandibular stumps, still has the chance to develop cutaneous fistulae from the sequelae of severe scleroderma of the corresponding facial region, especially in patients with a lack
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of facial soft tissue thickness after resection of buccogingival cancer and those undergoing
postoperative radiation therapy. In a case cohort study report, the most frequent complication of
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preformed plate reconstruction is estimated to be 16% (Probst et al., 2012). The present study proposed an NH plate using a patient-matched bending technique, providing a more patient-specific solution. Furthermore, the mesh shape design of the center-piece may disperse the stress concentration more than the common bar shape design of preformed/CS plates. This may further prevent the development of orocutaneous fistulae from severe soft tissue contracture.
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The simulated bite force used in this study was set at 150 N, which is much higher than the maximal bite force measured in patients with mandibular resection (Ramos et al., 2011; Maurer et
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al., 2006). Wiskott et al. (1994) pointed out chewing and swallowing contacts equal to 1800 per day, and Delong et al. also reported chewing tooth contacts equal to 240,000 in 1 year (1985). Based on
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these data, we applied a 2 x 105 cyclic load to simulate clinical chewing in our biomechanical fatigue testing. In our results, none of the NH plate was fractured in the fatigue test. In addition, the difference in permanent deformation between CS and NH plate was non-significant, meaning that the strength of the NH plate was sufficient to withstand the bite force and chewing load in the scenario of mandible resection. Although it is not comparable to a customized preformed plate, the results showed that the NH plate can meet the clinical need. Although FE analysis is generally accepted as a complementary tool and is widely applied to study the effect of surgical procedures (Szucs et al., 2010), osteotomy designs (Bujtár et al., 2012),
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fixation methods and facial reconstructive surgery (Huang et al., 2016), to construct an accurate model with detailed bone anatomy, realistic material properties and boundary conditions are necessary to ensure that the simulation results are clinically convincing (Lin et al., 2010). The precise geometry of the 3D model provides accurate anatomical volumetrics for generating a fine
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meshing process and non-linear contact analysis to mimic the interfacial condition. This approach is more realistic and descriptive than previously reported in the medical literature and could ensure the integrity of the simulations. However, the FE analysis result is limited by the theoretical
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assumptions, including the load conditions and material properties. The load condition considered only the condition that causes the most critical situation on the condyle and does not consider
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chewing force after surgery. Linear elastic (homogeneous and isotropic) properties were adopted for all materials due to numerical convergence considerations. Therefore, the simulated and experimental results provided in this study must be confirmed in further, well-controlled clinical trials.
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CONCLUSION
The present study proposed a new reconstruction plate design that hybridize the concept for
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strength, contour, and function. The NH plate provides the following benefits: 1) initial load-bearing requirement for segmental mandibular defect; 2) securing the transplant bone graft at the alveolar
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part, which is better fitted to the prosthetic need; 3) using patient-matched bending technique to increase interfacial fit adaptation between the reconstruction plate and mandible, which may reduce mechanical failure from a laborious bending procedure; and 4) maintaining facial contour. The biomechanical evaluation addressed the fact that maximum bone plate stress and bone strain around the fixation screws were decreased significantly over those in the CS secured model, indicating that the strength was sufficient to meet the clinical need. In the context of customization, the NH plate provides a more economical design than a patient-specific CAD/CAM plate. Further testing in the clinical scenario is required to clarify the procedure-related issues.
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ACCEPTED MANUSCRIPT Ethical statement Ethical approval was not required.
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Contributions C.H. Wu and C.L. Lin conceived and designed the experiments; Y.S. Liu performed the simulation and experiment; Y.S. Lin analyzed the data; C.H. Wu and C.L. Lin wrote the manuscript; and all
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authors read and approved the final version of manuscript.
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Competing interests None declared.
Source of support
This study is part of the Master’s Thesis of Y.S. Yu (2015) at the Department of Biomedical
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Engineering at National Yang-Ming University, Taiwan, and supported in part by MOST project
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103-2221-E-010 -01-MY3 of the Ministry of Science and Technology, Taiwan.
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Table 1. Material properties of bone and titanium
Cancellou s
Cortical bone Material property Body
Angle
Bone
Ex (MPa)
20,492
21,728
24,607
1,500
Ey (MPa)
12,092
12,700
12,971
Ez (MPa)
16,350
17,828
18,357
0.43
0.45
Poisson’s ratio (Pyz) 0.22
11,000
1,500
11,000
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1,500
0.3
0.34
0.23
0.3
0.34
0.34
0.28
0.3
0.34
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Based on Vajgel et al. (2013).
11,000
0.2
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Poisson’s ratio (Pxz) 0.34
0.38
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Poisson’s ratio (Pxy)
alloy
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Symphysis
Titanium
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Load (N) Muscle actions
Reference Y
Z
Deep masseter
7.776
127.23
22.68
M1,2
Superficial masseter
12.873
183.5
12.11
M3,4
Medial pterygoid
140.38
237.8
-77.3
M5,6
Temporalis
0.064
0.37
Medial temporal
0.97
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Based on Romas et al. (2011).
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X
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-7.44
M7,8
M9,10
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Fig. 1. Schematic diagram of the design concept for the novel hybrid (NH) reconstruction plate.
Fig. 2. Solid computer-aided design models of the (a) commercial straight (CS) and (b) novel hybrid (NH) secured models, (a) also represented loading and boundary conditions of the finite
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element simulations and muscular actions. Five muscle pairs: M1, M2 deep masseter; M3, M4 superficial masseter; M5, M6 medial pterygoid; M7, M8 temporal; and M9, M10 medial temporal.
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Fig. 3. Finite element mesh models of the (a) commercial straight (CS) and (b) novel hybrid (NH)
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secured models.
Fig. 4. (a) Schematic diagram of the stamping press. Stamping core die (upper die) was acquired from the native mandibular bone, and tool (cavity die/lower die) was obtained using Boolean operations in the computer-aided design system and consisted of 2 parts to facilitate stamping bone plate removal. (b) The novel hybrid (NH) reconstruction plate and respective screws were made
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with Ti6Al4V for testing, and the corresponding osteotomized mandibular models were exported to duplicate the ABS plastic mandibular bone with segmental defect models using a 3-dimensional
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printer. The NH reconstruction plate was placed in the tool to perform the sheet metal−forming manufacturing processes and was fixed with 8 bicortical screws (4 on each side) in the desired
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positions to match the resting bone surface.
Fig. 5. Schematic diagram of the biomechanical fatigue testing, showing that the tested samples were embedded into condyle and clamped onto a test machine designed to drive a dynamic force.
Fig. 6. The von Mises stress distribution of the bone plate for the (a) commercial straight (CS) and (b) novel hybrid (NH) secured models.
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Fig. 7. The von Mises stress distribution of the fixation screws for the commercial straight (CS; upper) and the novel hybrid (NH; middle) secured models.
Fig. 8. The von Mises bone strains distribution around the fixation screws for the commercial
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straight (CS; left) and the novel hybrid (NH; right) secured models.
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