Mechanical Characterization of 3D-Printed Individualized Ti-Mesh (Membrane) for Alveolar Bone Defects

Individualized titanium mesh holds many advantages over conventional mesh. There are few reports in the literature about the effect of mesh pore size and mesh thickness on the mechanical properties of titanium mesh. This study is designed to develop an individualized titanium mesh using computer-assisted design and additive manufacturing technology. This study will also explore the effect of different thicknesses and pore sizes of titanium mesh on its mechanical properties through 3D FEA. According to this study, the mechanical properties of titanium mesh increased when the thickness decreased (0.5 mm to 0.3 mm). With an increase in mesh diameter (3 mm to 5 mm), the mechanical properties of mesh decreased. The diameter of titanium mesh has less influence on its mechanical properties than does the thickness of the mesh. Titanium mesh with a thickness of 0.4 mm is strong enough and causes less stimulation to mucosa; therefore, it is more suitable for clinical use. In addition, parameters of titanium mesh should be decided clinically according to bone defect size, defect location, and force situation.


Introduction
Adequate bone volume in both horizontal and vertical dimensions plays a vital role in achieving long-term aesthetic and functional results in implant dentistry [1]. However, in our daily work, many patients suffer from horizontal or vertical bone deficiency, especially in cases of long lasting edentulous ridges or bone defects caused by trauma. Resorbed alveolar bone is often not sufficient to place dental implants during a prosthetic-driven procedure and frequently jeopardizes the successful outcome of an ideal implant placement. To achieve appropriate positioning of dental implants, a number of augmentation strategies have been developed to augment new bone growth, including distraction osteogenesis, block bone graft, and GBR. e process of distraction osteogenesis is complex, and compensatory bone resorption after the surgery cannot be avoided because it frequently leaves undesirable tissue scarring [2]. Autogenous onlay bone grafting is an advantageous technique for alveolar reconstruction because of its ability to provide sufficient bone volume and because of its biocompatibility. erefore, it still remains the gold standard in reconstructive surgery. Nevertheless, the preference for this method is lessened due to the need for a donor site to harvest an autogenous bone graft, surgical complications, the need for delayed implant placement, and compensatory bone resorption after surgery [3]. GBR is one of the most utilized methods, which uses a barrier membrane to isolate the growth of soft tissue while promoting the priority of bone tissue growth [4]. e area with the bone defect is filled with an autologous bone or bone compensatory material. e bone graft material may not be close to the bone defect, and the defective anatomy and macrostructure of the biomaterial has a substantial influence on new bone formation. In order for augmentation procedures to be successful, it is imperative to stabilize the graft during healing, support the osteogenic potential of the graft materials, and close the primary soft tissue. A series of animal and clinical studies have shown that GBR can predictably facilitate bone regeneration in critical-sized bone defects [5][6][7]. However, it is hard to maintain a desired bone shape and volume during the entire healing period for GBR especially for large bone defects. Displacement and compression of the graft material have been cited as relevant during the postoperation period [8].
Since the introduction of Ti-mesh in 1969, titanium mesh has received close attention and has been used extensively in the reconstruction of oral and maxillofacial bone defects [9,10]. Several advantages of titanium mesh have been suggested. Titanium mesh is rigid enough to maintain bone shape and volume, which is a basic prerequisite for any bone regeneration process. In addition, the pores of titanium mesh play a significant role both in maintaining vascular supply to a grafted defect from the overlaying periosteum and in improving tissue integrity [11]. However, manual shaping and bending, trimming, and fixation are required to apply conventional titanium mesh according to the individual defect. ese processes are manually challenging, time-consuming, and highly influential with respect to the regenerative outcomes [12,13]. Furthermore, the corners and edges of these cut and bended meshes can cause damage to the gingiva and mesh exposure site [14].
During the past decade, rapid developments have been made in CBCT technology and in additive manufacturing technology.
ese developments have accelerated the manufacture of complicated three-dimensional (3D) metal devices [15,16]. Personalized titanium mesh manufactured by digital modelling and 3D printing technology can enable optimal fit between the mesh and the anatomical shape of the alveolar bone. It can also reconstruct the 3D volume and position of the bone accurately and allow the operation to be planned in advance. By avoiding manual shaping and pruning during the operation, the duration of the operation can be greatly shortened.
An important criterion for the success of titanium mesh seems to be its mechanical strength that can withstand the desired load and space maintenance which is affected by its thickness and pore size properties. However, the stiffness of Ti-mesh may cause irritation to mucosa, resulting in membrane exposure. Accordingly, a proper thickness and pore size must be balanced with the likelihood of irritation when using Ti-mesh. e usual available thicknesses of titanium mesh range from 0.1 to 0.6 mm and have no uniform specifications, neither does the pore size [17][18][19][20][21][22][23]. e thickness and pore size of titanium mesh could influence the amount of new bone generated beneath the mesh; therefore, careful selection of the appropriate pore size and thickness can play a critical role in isolating the growth of soft tissue and promoting bone tissue growth. e purpose of this study was to design an individualized titanium mesh and to explore the effect of titanium mesh with different thicknesses and pore sizes on its mechanical properties. In the present study, we have designed and manufactured a customized titanium mesh based on computerized tomography (CT) of individual patients. e designed titanium mesh with different diameters and thicknesses were explored based on their mechanical strength through a three-point bending test and FEA.

Preparation of Test Specimen.
Nine groups of standard titanium mesh specimens with diameters of 3.0 mm, 4.0 mm, or 5.0 mm and thicknesses of 0.3 mm, 0.4 mm, or 0.5 mm were designed using 3-Matic software ( Figure 1); the length of the sample was 40 mm, and the width was 10 mm. e data were converted to a stereolithography (STL) file and were transferred to an SLM-RP molding machine Laser-CUSING ® (Concept Laser GmbH, Germany). Finally, the SLM products were obtained ( Figure 2).

Standard Specimens Mechanical Testing.
e mechanical strength of titanium mesh was analysed using threepoint bending tests on standard specimens. After the etching treatment and the polishing procedure, bending strength was tested using GB/T14452-93. Nine groups of specimens were fabricated with five specimens printed for each group (Figure 2). e three-point bending tests were done on a universal mechanical tester (C43.104.MTSLtd., China). e strided distance (L s ) was 10 mm. e compressive load was vertically applied at a rate of 1 mm/min. Constant pressure was applied until the specimen was destroyed. e peak force value of the bending or breaking of each specimen was recorded. e load displacement curve was generated, and the bending strength was calculated. e results were shown as the mean ± standard deviation. Data were analysed statistically with analysis of variance (one-way ANOVA) (SPSS 13) followed by a Tukey's post hoc analysis using a significance level of p < 0.05.

Customized Design for Titanium Mesh.
Cone-beam computerized tomography (CBCT) scans were taken of a patient, with the patient's approval. e patient had anterior teeth missing due to trauma, leaving a large bony defect. Based on Digital Imaging and Communications in Medicine (DICOM) data of the CBCT scan, a model of the patient's anatomy was established. e software package Mimics 17.0 (Materialise, Leuven, Belgium) was utilized to convert the slice data into a 3D model of the patient's bone tissue, using the system's built-in threshold function. e 3D model of the patient's mandible anatomy generated in the Mimics ® software is demonstrated in Figure 3. e 3D model created in Mimics was saved as an STL (standard tessellation language) file for further processing of the mandible and to construct the titanium mesh model (Figure 4(a)). Steps involved in customized titanium mesh design were as follows: First, the missing teeth were arranged based on the patient's adjacent and opposite teeth (Figure 4(b)). Second, to determine the required bone volume (horizontally and vertically), the virtual surgical procedure placed the implants in the atrophic edentulous area. Two implants (diameter of 3.4 mm and length of 10 mm) were arranged on the basis of the planned position of the missing teeth where insufficient bone was available (Figure 4(c)). ird, after the virtual positioning of the dental implant, virtual bone augmentation (horizontally and vertically) guided by the three-dimensional ideal 2 Journal of Healthcare Engineering location of the implant was taken (Figure 4(d)). To inhibit the biological formation of pseudoperiosteum, an overcontouring of 1.5 mm of bone volume was devised in order to cover the titanium mesh. Finally, the titanium mesh was designed using Geomagic Studio software (Geomagic Company, NC, USA) and the 3-Matic software (Figures 4(e) and 4(f)). e shape of the mesh weave was made as a triangle in order to increase the contact area between the titanium mesh and the bone tissue.
To remove the possibility of creating a pathway for the penetration of bacteria, the mesiodistal end of the titanium mesh was separated from the adjacent tooth by at least 2 mm. In addition, the edge of the mesh was designed as smoothly as possible to avoid postoperative mucosal exposure due to the tension of the flap. e screw holes for fixing the device were also designed in the same manner. e diameter of the holes was set to 1.7 mm. Two holes on the buccal side and one on the lingual side were made for inserting the miniscrews with which to affix the titanium mesh to the underlying cortical bone and cancellous bone. Nine groups of individualized titanium mesh with diameters of 3.0 mm, 4.0 mm, or 5.0 mm and thicknesses of 0.3 mm, 0.4 mm, or 0.5 mm were designed using 3-Matic software and Geomagic Studio software in the same manner ( Figure 5).

Standard Specimens FEA.
e 3D geometries of the standard specimens were imputed to ANSYS software to simulate the three-point bending test. Nine groups of 3D FE  models of standard titanium mesh of different diameters and thicknesses were built ( Figure 6). e applied force was 30 N in an axial direction. e data regarding stress, strain, and displacement of the titanium mesh were outputted for further analysis.  material that filled in bone defects was generated using 3-Matic software. e mucosa covering the mandible with an average thickness of 2 mm was also generated using Geomagic Studio software. e titanium screws used to affix the titanium mesh to the underlying cortical bone were designed by SolidWorks ® 12.0 (SolidWorks Corporation, Velizy-Villacoublay, France). e model of titanium mesh and bone graft was meshed in 3-Matic and formatted with a .cdb file for further processing. e geometries of the cortical bone, cancellous bone of mandible, mucosa, and titanium screws were then calculated using the FE software ANSYS (Swanson Analysis System Co., Houston, TX, USA). Nine groups of 3D FE models of individualized titanium mesh with different diameters and thicknesses were built (Figure 7). e total number of elements and nodes is listed in Table 1. e titanium mesh and titanium screws were made of Ti6Al4V titanium alloy. e material properties of cortical bone, cancellous bone, mucosa, soft callus, and titanium alloy were taken from the literature and are listed in Table 2  Journal of Healthcare Engineering

Contact Management and Loading Conditions.
To simulate the intraoral situation precisely, the cortical bone was bonded to the cancellous bone, and the mucosa and the bone were considered fixed together. e titanium mesh was affixed to the underlying cortical bone using titanium screws, and the interface between bone and titanium screws was presumed fixed. However, the interface between titanium mesh and screws was not presumed fixed, nor was the junction of titanium mesh and cortical bone, in order to simulate the early stage of healing. e mandibles were constrained by the temporomandibular joint (TMJ) and masticatory muscles. ese muscles include the temporalis and masseter muscle attachments at the lateral surface, and attachments for the digastric, temporalis, lateral pterygoid, and medial pterygoid muscles on the medial surface [24]. A functional loading force of 100 N in an axial direction was applied. e data regarding stress and displacement of the titanium mesh were outputted for further analysis.

Fabrication of Designed Individualized Titanium Mesh.
e data regarding customized mesh in a stereolithography (STL) file for bone augmentation were directly fabricated using a LaserCUSING ® (Concept Laser GmbH, Germany).
Finally, the SLM products were obtained (Figure 8).

ree-Point Bending Test.
e peak force value (N) and three-point bending strength (MPa) of the standard specimens are shown in Table 3. As the thickness of the titanium mesh increased, the maximum force value and the bending strength of the titanium mesh increased. When the diameter increased, the maximum force value and bending strength of titanium mesh decreased (Figure 9). e bending strength of the 0.5 mm group was highest, and the 0.3 mm group was lowest. ere was a significant difference in bending strength among the different mesh thickness groups (p < 0.05). e bending strength of the 3 mm group was highest, and the 5 mm group was lowest. ere was also a significant difference in bending strength among the different mesh diameter groups (p < 0.05).    Figure 10. Among the nine models evaluated in this study, the maximum stress value, deformation value, and the maximum strain value were observed in specimens 5 mm in diameter and 0.3 mm in thickness. e minimum stress value, deformation value, and the minimum strain value were observed in specimens 3 mm in diameter and 0.5 mm in thickness. e results showed that the maximum value of stress and strain as well as the deformation distributions had increased when the thickness decreased (0.5 to 0.3 mm). In addition, the level of stress, strain, and the deformation distributions increased with the increase in mesh diameter (3 to 5 mm).

Customized Titanium Mesh FEA.
e pattern of stress distribution was similar for all models (Figure 11). e von Mises criteria showed stress concentration of the titanium mesh on the crest of the alveolar ridge and at the labial side near the top of the alveolar ridge. However, the increase of mesh thickness generated less concentrated stress, and the maximum stress level decreased. All models presented a similar pattern of strain distribution with contour lines of different colours representing mesh strain ( Figure 12). e results showed that the high strain value increased when the thickness decreased. Furthermore, the high level of strain increased with the increase in mesh diameter. As shown in Figure 13, the displacement distribution patterns are shown as contour lines of different colours to represent mesh displacement with the bite force of 100 N. e increase of the mesh diameter (3 to 5 mm) tends to increase the displacement tendency. However, the increase of mesh thickness (0.3 to 0.5 mm) tends to decrease the displacement tendency. Among the nine models evaluated in this study, the maximum stress value and the maximum strain value were observed in specimens 5 mm in diameter and 0.3 mm in thickness. e minimum stress value and the minimum strain value were observed in specimens 3 mm in diameter and 0.5 mm in thickness. Compared with change in thickness, the stress, strain, and displacement of titanium mesh did not increase much with the change of the pore size of the mesh (Figure 14).

Discussion
e current approach presented an innovative multidisciplinary protocol for designing individualized titanium mesh based on prosthetically guided bone regeneration. We have designed a customized titanium mesh from a patient's CBCT scan dependent on the desired implant positions. e four main advantages of this system were as follows. (i) Extensive indications: current commercial flexible titanium meshes  need to be fixed by implant and cannot be applied in cases of severe bone deficiency. e customized titanium could be used in various types of bone defects, especially those with large bone deficiency. (ii) e individualized titanium mesh fitted securely to the morphology of the alveolar bone, so the surgeon did not need to trim or bend the mesh during the operation. (iii) e individualized titanium mesh designed on the desired implant position allowed a definite size and volume of augmentation. is could minimize the required amount of harvested or synthetic graft material. (iv) Round and blunt edges of the customized titanium mesh were developed to prevent mucosal irritation, resulting in a reduction of mesh exposure after operation. e study explored the effect of different thicknesses and pore sizes of titanium mesh on its mechanical properties. Following surgery, numerous types of compressions may arise if the titanium mesh does not work as a stiff box, potentially caused both by the impact of an unexpected food block and muscle movement. e deformation of titanium mesh during the healing period can cause detrimental and unfavourable variations in the planned augmented bone, thus influencing the overall augmented bone quality and volume. erefore, the titanium mesh must be stiff enough to sustain pressure from the overlying flaps, muscle movement, or mastication loading until the blood clot underneath the membrane has matured enough to support it [12].
Regarding the optimal thickness of the titanium mesh, the three-point bending test and the FEA results showed that the mechanical properties were enhanced when the thickness increased, whether it was a standard sample or personalized titanium mesh. e FEA results of individualized  Journal of Healthcare Engineering titanium mesh indicate that the largest von Mises stress value of titanium mesh with a thickness of 0.3 mm is over the yield strength of titanium alloy (σ � 780-950 MPa) [27], suggesting that the titanium mesh 0.3 mm in thickness may be fractured and is not safe for bone augmentation. e FEA results of individualized titanium mesh indicate that the maximum stress on titanium mesh with the thicknesses of 0.5 and 0.4 mm are under the yield strength of titanium alloy (σ � 780-950 MPa) [27], suggesting that titanium meshes 0.5 and 0.4 mm in thickness are safe for bone augmentation. e thinner titanium mesh may result in less mucosal irritation which can lead to exposure of the mesh. erefore, the appropriate thickness of a titanium mesh must be rigid enough to maintain space for bone regeneration and should be balanced with the likelihood of irritation. e thickness should not adversely influence its ability to integrate with the surrounding tissue when using titanium mesh for GBR [28]. Titanium mesh with a thickness of 0.4 mm can not only bear enough strength but also lead to less stimulation of mucosa; therefore, it is more suitable for clinical use.
Membrane pores were considered to play an essential role in maintaining blood supply and in facilitating bone regeneration and soft tissue healing [29]. However, in conventional titanium mesh, these multiporous properties created sharp edges when the material was manually shaped or bent and may offer a simple pathway for microbial invasion into the healing site [30]. e effect of pore size on the osteogenesis of titanium mesh is controversial. One study advocated that macroporous (in the millimetre range) membranes promoted greater bone regeneration and prevented significant soft tissue ingrowth compared with microporous membranes [22]. Another study indicated that   microporous (in the micron range) titanium mesh had some potential for greater bone regeneration [19]. e three-point bending test and the FEA results demonstrated that the mechanical properties weakened with the increase in mesh diameter, whether it was a standard sample or individualized titanium mesh. In the range of 3-5 mm, the diameter of titanium mesh has less influence on its mechanical properties. Further animal or clinical research should be directed towards identifying the mesh pore size that would inhibit excessive soft tissue ingrowth and facilitate bone ingrowth in ridge augmentation procedures.
For the anterior teeth with a single tooth missing, a titanium mesh with a thickness of 0.3 mm could be chosen and patients should be told not to bite hard objects. For the patients with thin mucosa, in order to reduce the exposure of the titanium mesh, a thin titanium mesh should be selected. For multiple tooth defects, especially in the posterior teeth, a thicker titanium mesh should be chosen to prevent the titanium mesh from breaking. erefore, parameters of titanium mesh should be decided clinically according to defect size, defect location, and force situation.

Conclusion
e stiffness of Ti-mesh can maintain space better than other membrane and is conducive to the shaping of bone-grafting materials but may result in mucosal irritation that leads to exposure of the membrane. e custom-made titanium   mesh offers greater advantages over commercial meshes by shortening the duration of surgery and eliminating the risk of postoperative infections. Within the limitations of this research regarding the optimal thickness of the titanium mesh, the results indicate that the titanium mesh with a thickness of 0.4 mm is more suitable for clinical use. Regarding the optimal pore size of the titanium mesh, the diameter of titanium mesh had less influence on its mechanical properties. Further animal or clinical research should be directed towards identifying an optimal mesh pore size that would inhibit excessive soft tissue ingrowth and facilitate bone ingrowth in ridge augmentation procedures. Otherwise, the parameters of titanium mesh should be decided clinically according to defect size, defect location, and force situation.

Data Availability
All data included in this study are available upon request by contacting the corresponding author.

Conflicts of Interest
e authors declare that there are no conflicts of interest regarding the publication of this paper.