Finite element analysis of the stresses around endosseous implants in various reconstructed mandibular models

Finite element analysis of low-profile reconstruction plates for atrophic mandibles: a comparison of novel 3D grid and conventional plate designs

OBJECTIVES

This study aimed to compare the reconstruction with type 2.4 and three-dimensional (3D) grid plates relating the areas of weakness to anatomical regions of force incidence in atrophic mandibles with and without segmentation using finite element analysis (FEA).

MATERIALS AND METHODS

Strength and force quality in the mandible were also determined. The atrophic mandible models with and without segmentation were divided into four groups, and the behavior of each plate was analyzed using finite element analysis. In the atrophic mandible without segmentation using the type 2.4 reconstruction plate, the highest stress concentration in the plate was observed in the posterior region of the mandibular body and in the grid type 3D reconstruction plate was observed in the mandibular angle.

RESULTS

In the segmented atrophic mandible with the 2.4 reconstruction plate, higher stress concentration was observed in the mandibular segment defect. Analysis of the 3D grid-like reconstruction plate revealed that the geometry of the plate conferred greater stiffness to the assembly, as the most significant stress concentration was observed at the mandibular angle.

CONCLUSION

The width of the plate design may influence the strength, not the thickness.

How does the Number of Implants Affect Stress Distribution in Fibula Graft at the Posterior of the Mandible? A Finite Element Analysis

Objectives

Evidence about the implant protocol and success in the osseous microvascular grafts is not sufficient. Stress distribution around dental implants is one of the important factors determining treatment success. The purpose of this study was to evaluate stress distribution in the bone supporting the implants inserted in the fibula free flap, in patients with large defects in the posterior of the mandible by finite element analysis (FEA).

Materials and Methods

The CBCT was obtained from one patient with fibula free flap in the posterior of the mandible and also from a 4.1 × 10 mm implant (Zimmer, Zimmer dental, Carlsbad, USA). Two 3D finite models were designed containing three or four implants. The implants were splinted by a suprastructure. Vertical load (300 N) and oblique load (50 N) were applied to the suprastructure. The von Mises stress distribution and the micromotion of implants were evaluated.

Results

No significant difference was observed between implants micromotion in two models. According to stress distribution analysis and determining maximum stress regions, the model with four implants imposes more stress on titanium components and surrounding bone.

Conclusion

The stress distribution around the implants of mandibular models with posterior defect that was reconstructed with fibula free flap is better in models with three fixtures versus four fixtures.

The evaluation of stress on bone level and tissue level short implants: A Finite Element Analysis (FEA) study

PURPOSE

This study aimed to evaluate the difference between the stress level and distribution around the BL and TL short implants, and their surrounding structures, using finite element analysis.

METHODS

Two different study models were constructed: BL model and TL model. Two dental implant systems (ITI (Straumann, Waldenburg, Switzerland) and NTA Short) with a diameter of 4.1 mm and 4 mm and with a length of 6 mm were used in this study. In each model, implants were placed in the mandibular 1st molar region. The von Mises stress and maximum principal (tensile) and minimum principal (compressive) stresses were evaluated.

RESULTS

The highest stress values recorded in the BL implants (von Mises: 342.77 MPa), in the peri‑implant bone around the BL implants (maximum principal stress: 114.1 MPa), as a result of oblique loading, and overall stress values were found to be higher in the BL model. However, these measured values appeared to be low to cause a fracture, when considering the yield strengths of the materials and bone.

CONCLUSIONS

The stress values were higher in the BL model, but not high enough to cause failure. Short implants could be an effective method of treatment for patients unsuitable for advanced surgical techniques.

Stability of internal fixation systems based on different subtypes of Schatzker II fracture of the tibial plateau: A finite element analysis

Background: Schaztker II fracture is the most common type of the tibial plateau fractures (TPF). There has been a large number of cadaveric biomechanical studies and finite element simulation studies to explore the most stable fixation methods for this type of fracture, which were based on a single fracture morphology. But differences among fracture morphologies could directly affect the stability of internal fixation systems. In this sense, we verified the stability of existing internal fixation modalities by simulating Schatzker II fractures with different fracture morphologies.

Objectives: To compare the stability of different filler types combined with locked compression plate/screw in different subtypes of Schatzker II TPF.

Methods: Four subtypes of Schatzker II were created based on 3D map of TPF. Each of the subtypes was fixed with LCP/screw or LCP/screw combined with different fill types. Stress distribution, displacement distribution, and the load sharing capacity of the filler were assessed by applying the maximum load during gait. In addition, repeated fracture risks of depressed fragment were evaluated regarding to the ultimate strain of bone.

Results: The stress concentration of the implant in each scenario was located on the screw at the contact site between the plate and the screw, and the filler of the defect site significantly reduced the stress concentration of the implant (Subtype A: Blank group 402.0 MPa vs. Experimental group 315.2 ± 5.5 MPa; Subtype C: Blank group 385.0 MPa vs. Experimental group 322.7 ± 12.1 MPa). Displacement field analysis showed that filler significantly reduced the reduction loss of the depressed fragment (Subtype A: Blank group 0.1949 mm vs. Experimental group 0.174 ± 0.001 mm; Subtype C: 0.264 mm vs. 0.253 ± 0.002 mm). Maximum strain was in subtype C with the value of 2.3% ± 0.1% indicating the greatest possibility of failure risk. And with the increase of its modulus, the bearing capacity of filler increased.

Conclusion: The existence of filler at the defect site can effectively reduce the stress concentration of the implant and the reduction loss of the collapsed block, thus providing good stability for Schatzker II fracture. In subtype A fracture, the modulus of filler presented the slightest influence on the stability, followed by subtype C, while the stability of subtype B was most influenced by the modulus of filler. Therefore, it is necessary to evaluate the preoperative patient imaging data adequately to select the appropriate stiffness of the filler.

Combination of biomechanical evaluation and accurate placement of dental implants: a new concept of virtual surgery in maxillary and mandibular functional reconstruction

Biomechanics are crucial for bony regeneration and survival of implants in functional maxillary and mandibular reconstructions. However, we know of no study that has included an analysis of biomechanics to guide the optimal position of a fibular graft in virtual surgery. This study was designed to evaluate the combination of biomechanics and accurate placement of implants for virtual surgery in reconstruction of the jaw using fibular grafts. Thirty-one patients had maxillary or mandibular reconstruction with vascularised fibular grafts and the immediate placement of dental implants. Virtual studies were made preoperatively to evaluate the biomechanics and to assess the position of the fibular grafts with minimal distribution of stress. All operations proceeded accurately and with no complications with a mean (range) of 14 (6-20) months' follow-up. According to the individual biomechanical evaluations, the optimal position for the fibular graft is probably the middle of the mandibular body or below the bottom of the maxillary sinus. The combination of biomechanical evaluation and accurate placement of dental implants is a new concept that could achieve good biomechanical positioning of fibular grafts in the jaw and a desirable level of accuracy for functional reconstruction.

Finite element analysis of stress distribution on reconstructed mandibular models for autogenous bone grafts

BACKGROUND

There are several challenges in terms of mandibular reconstruction. The defect size, graft materials, and plate combinations should be taken into consideration in surgical planning.

OBJECTIVE

The aim of this study was to evaluate the effect of different reconstruction variations on the stress distribution of segmental resected mandibles with two different defect sizes using finite element analysis (FEA).

METHODS

Computerized tomography images of a human mandible, fibula, and iliac crest were used as references to build three-dimensional (3D) models on a PC. The virtual plates and screws were used to simulate reconstruction of the mandibular defects. The models were divided into two groups based on the longitudinal defect size. Different osteosynthesis variations and autogenous graft material combinations were used to reconstruct the mandibles.

RESULTS

In all models, higher von Mises stress values occurred on the mandibles reconstructed with the fibula than those with the iliac crest. Fixation of the bone grafts with a reconstruction plate for the 10-mm defects and using a mini-plate in addition to the reconstruction plate for the 30-mm defects decreased stresses on the grafted bones.

CONCLUSIONS

Mandibular reconstruction with the iliac grafts is biomechanically superior to that with the fibular grafts. In addition, osteosynthesis methods and the defect size affect the stress distribution.

Three-dimensional finite element analysis of the splinted implant prosthesis in a reconstructed mandible

PURPOSE

The purpose of this study was to analyze the effects of the splinted implant prosthesis in a reconstructed mandible using three-dimensional finite element analysis.

MATERIALS AND METHODS

Three-dimensional finite element models were generated from a patient's computed tomography data. The patient had undergone partial resection of the mandible that covered the area from the left canine to the right condyle. The mandible was reconstructed using a fibula bone graft and dental implants. The left mandibular premolars and molars remained intact. Three types of models were created. The implant-supported prosthesis was splinted and segmented into two or three pieces. Each of these models was further subcategorized into two situations to compare the stress distribution around normal teeth and implants. Oblique loading of 300 N was applied on both sides of the mandible unilaterally. The maximum von Mises stress and displacement of the models were analyzed.

RESULTS

The stress distribution of the natural mandible was more uniform than that of the reconstructed fibula. When the loading was applied to the implant prosthesis of reconstructed fibula, stress was concentrated at the cortical bone around the neck of the implants. The three-piece prosthesis model showed less uniform stress distribution compared to the others. Displacement of the components was positively correlated with the distance from areas of muscle attachment. The three-piece prosthesis model showed the greatest displacement.

CONCLUSION

The splinted implant prosthesis showed a more favorable stress distribution and less displacement than the separated models in the reconstructed mandible.

The Dynamics in Implantation for Patients with Clefts

Objective

To investigate the stresses and strains of an endosseous dental implant in patients with different types of cleft palate in a finite element model.

Materials and Methods

Seven three-dimensional (3D) maxillary models were designed on a personal computer according to computed tomography slice data obtained from seven dry skulls. Next, computer-aided modification was performed on each model to produce three other 3D models with different cleft patterns. Thus, four model types with different cleft patterns were designed and termed NORM (without cleft), ALVEOLAR (only alveolar cleft), PALATAL (only palatal cleft), and COMPLETE (complete cleft). An implant was embedded into the molar region of each model, and a 300-N vertical load and 50-N horizontal load were applied to simulate mastication. Under these conditions, the stresses occurring at the implant-bone interface were calculated by finite element analysis.

Results

Different stress patterns were observed between the models with a palatal cleft (PALATAL and COMPLETE) and those without palatal cleft (NORM and ALVEOLAR). Regarding vertical load application, greater stresses occurred in PALATAL and COMPLETE types than in NORM and ALVEOLAR types. On application of a horizontal load, though the stresses did not show quantitative difference, their vector patterns differed.

Conclusion

In patients with palatal clefts, characteristic stress patterns occur on the bone-implant interface during mastication. This should be taken into consideration when performing an implant treatment in patients with clefts.

Influence of Implant Design (Cylindrical and Conical) in the Load Transfer Surrounding Long (13mm) and Short (7mm) Length Implants: A Photoelastic Analysis

Purpose:

This study compared the influence of implant design (cylindrical and conical) in the load transfer on bone surrounding 13mm and 7mm length implants under simulated occlusal loading, using photoelastic analysis.

Method:

Dental implants of 4mm diameter were divided into four groups, which varied in length and design: Group 1- standard (13 mm) cylindrical implant; Group 2 - standard conical implant; Group 3 – short (7 mm) cylindrical implant, and Group 4 - short conical implant. After the inclusion of the implant models in a photoelastic resin, they were subjected to a static load of 100 N. The lengths of the fringes that were generated were measured in three portions since the implants body: crestal, central and apical portion, parallel to the implant long axis. Furthermore, the entire extension area of dissipation of force was measured. Data were analyzed by one-way ANOVA (α = 0.05).

Results:

Lower stress was observed at the crestal bone in groups 2 and 4, while the stress levels in groups 1 and 3 were higher with significant differences compared to the other groups (p<0.05).

Conclusion:

The total amount of stress transmitted to the bone was not affected by implant length under axial loading condition, but changed in relation to the implant design with respect to the concentration of the fringes, which corresponded to the load distribution, with even more dissipation by conical implants.

3D Finite Element Study on: Bar Splinted Implants Supporting Partial Denture in the Reconstructed Mandible

AIM:

This study aimed to estimate the stress patterns induced by the masticatory loads on a removable prosthesis supported and retained by bar splinted implants placed in the reconstructed mandible with two different clip materials and without clip, in the fibula-jaw bone and prosthesis using finite element analysis.

METHODS:

Two 3D finite element models were constructed, that models components were modeled on commercial CAD/CAM software then assembled into finite element package. Vertical loads were applied simulating the masticatory forces unilaterally in the resected site and bilaterally in the central fossa of the lower first molar as 100N (tension and compression). Analysis was based on the assumption full osseointegration between different types of bones, and between implants and fibula while fixing the top surface of the TMJ in place.

RESULTS:

The metallic bar connecting the three implants is insensitive to the clips material. Its supporting implants showed typical behavior with maximum stress values at the neck region. Fibula and jaw bone showed stresses within physiologic, while clips material effect seems to be very small due to its relatively small size.

CONCLUSION:

Switching loading force direction from tensile to compression did-not change the stresses and deformations distribution, but reversed their sign from positive to negative.

Comparison of Stresses Around Dental Implants Placed in Normal and Fibula Reconstructed Mandibular Models using Finite Element Analysis

PURPOSE

This study was formulated to evaluate and compare stresses around endosseous implants in various normal and fibula reconstructed mandibular models using finite element analysis method.

MATERIALS AND METHODS

CT scan data of a completely edentulous patient and a fibula reconstructed patient was made and the Dicom images were used to design 3-D mandibular models using software. Based on the information from the scan data, various types of reconstructed FEA models were made. Implants (fixtures) and superstructures were then embedded in each model and Von Mises stress around the neck of each implant under a vertical loading of 200 N and Horizontal load of 50 N was calculated using finite element structural analysis software. The results were compared between the reconstructed mandible and their respective normal mandible.

RESULTS

Higher stress values were obtained for all the reconstructed types under horizontal loading and in reconstructed models involving larger area of reconstruction the stress were more. In the models involving smaller area of reconstruction like symphysis alone or body alone there is no significant change in the stress values on vertical loading.

CONCLUSION

Stresses were comparatively smaller in mandibles involving a smaller area of reconstruction. Hence, fixed prosthesis could be advised and a bar retained over denture for reconstruction for larger area of reconstruction.

Finite element analysis of masticatory stress on neoformed bone tissue after distraction osteogenesis and low-level laser therapy: a pilot study

OBJECTIVE

This study aimed to understand the action of masticatory forces on an implant virtually introduced into the sheep mandible after distraction osteogenesis and low-level laser therapy (LLLT) by using finite element analysis.

BACKGROUND DATA

Distraction osteogenesis as an alternative for bone reconstruction that may be used in the treatment of deformities.

METHODS

Four ewes underwent distraction osteogenis to elongate the left mandibular body by 15 m, and three of them underwent LLLT with the purpose of improving bone properties. After death, animals were scanned by computed tomography and their mandibles were tridimensionally reconstructed by computer programs. The physical properties related to hardness and modulus of elasticity of each animal were obtained from the dissected mandibles, and data were transferred to Femap software for finite element analysis.

RESULTS

Animals exposed and not exposed to LLLT irradiation showed remarkably similar values for superficial hardness and modulus of elasticity, without statistically significant difference (p>0.05), between the values observed for the cortical bone and the cancellous bone among the groups. The neoformed mandible, after a brief period for bone healing, was able to promote stability for implant placement and proper distribution of masticatory forces.

CONCLUSIONS

An implant introduced virtually into the site of bone neoformation did not suffer any micromotions relevant to osteointegration. Furthermore, finite element analysis showed that the neoformed portion of the mandible was able to absorb and distribute masticatory forces throughout its structure, even after a brief period for bone maturation.

Use of stress analysis methods to evaluate the biomechanics of oral rehabilitation with implants

Because the biomechanical behavior of dental implants is different from that of natural tooth, clinical problems may occur. The mechanism of stress distribution and load transfer to the implant/bone interface is a critical issue affecting the success rate of implants. Therefore, the aim of this study was to conduct a brief literature review of the available stress analysis methods to study implant-supported prosthesis loading and to discuss their contributions in the biomechanical evaluation of oral rehabilitation with implants. Several studies have used experimental, analytical, and computational models by means of finite element models (FEM), photoelasticity, strain gauges and associations of these methods to evaluate the biomechanical behavior of dental implants. The FEM has been used to evaluate new components, configurations, materials, and shapes of implants. The greatest advantage of the photoelastic method is the ability to visualize the stresses in complex structures, such as oral structures, and to observe the stress patterns in the whole model, allowing the researcher to localize and quantify the stress magnitude. Strain gauges can be used to assess in vivo and in vitro stress in prostheses, implants, and teeth. Some authors use the strain gauge technique with photoelasticity or FEM techniques. These methodologies can be widely applied in dentistry, mainly in the research field. Therefore, they can guide further research and clinical studies by predicting some disadvantages and streamlining clinical time.

Finite element stress analysis of Ti-6Al-4V and partially stabilized zirconia dental implant during clenching

OBJECTIVE

The purpose of this paper is to compare the differences in stress between Ti-6Al-4V and PS-ZrO(2) dental implant during clenching and whether these changes are clinically significant to limit the use of zirconia in oral implantology.

MATERIALS AND METHODS

The model geometry was derived from position measurements taken from 28 diamond blade cut cross-sections of an average size human adult edentulous mandible and generated using a special sequencing method. Data on anatomical, structural, functional aspects and material properties were obtained from measurements and published data. Ti-6Al-4V and PS-ZrO(2) dental implants were modelled as cylindrical structure with a diameter of 3.26 mm and length of 12.00 mm was placed in the first molar region on the right hemimandible.

RESULTS

The analysis revealed an increase of 2-3% in the averaged tensile and compressive stress and an increase of 8% in the averaged Von Mises stress were recorded in the bone-implant interface when PS-ZrO(2) dental implant was used instead of Ti-6Al-4V dental implant. The results also revealed only relatively low levels of stresses were transferred from the implant to the surrounding cortical and cancellous bone, with the majority of the stresses transferred to the cortical bone.

CONCLUSION

Even though high magnitudes of tensile, compressive and Von Mises stresses were recorded on the Ti-6Al-4V and PS-ZrO(2) dental implants and in the surrounding osseous structures, the stresses may not be clinically critical since the mechanical properties of the implant material and the cortical and cancellous bone could withstand stress magnitudes far greater than those recorded in this analysis.

Evaluation of the structural behavior of three and four implant-supported fixed prosthetic restorations by finite element analysis

PURPOSE

There is much controversy about the minimum number of implants and maximum cantilever length in mandible prosthetic restoration. Finite elements analysis of three and four implant-supported prostheses was performed to determine the stresses in the superstructure, implants and cortical bone and, therefore, the failure prediction for each restoration.

METHODS

An edentulous mandible was modeled from CT scan images. Two finite element models of three and four implant-supported prostheses with cantilever lengths of 10 and 15 mm were created. Occlusal loads in different parts of the superstructure were applied and shear and normal stresses were calculated.

RESULTS

Two failure criteria were analyzed: the von Mises criterion for isotropic materials (superstructure and implants) and the Tsai-Wu criterion for transversely isotropic material (cortical bone). Both criteria predict failure in the three implant-supported prosthesis for all cases analyzed. The same applies for the four-implant prosthesis of 15 mm cantilever length. However, four implants and a cantilever length of 10mm passed the failure criteria and were considered safe.

CONCLUSIONS

The results from the patient analyzed showed that fixed support prostheses on three implants are not recommended from a structural point of view because they do not adequately support occlusal loads. Excessive stress in the superstructure and the cortical bone can be expected, which would anticipate the failure of the restoration. Fixed support prostheses on four implants with a cantilever length of 10mm properly resist occlusal loading.

Variability in palatal shape and size in patients with bilateral complete cleft lip and palate assessed using dense surface model construction and 3D geometric morphometrics

Bilateral complete cleft lip and palate (BCLP) is the most severe of the common orofacial clefts and is associated with the greatest deformity during development. The aim of this study was to use geometric morphometrics to evaluate palatal shape and size variability in patients with BCLP in comparison to nonclefted Czech boys. The variability of palatal size and shape in BCLP patients was greater when compared with the nonclefted population. Though palate variability in BCLP was wide, nonclefted palatal shapes were generally different (a high, wide and vaulted palate) and fell almost outside the range of BCLP variability. The palatal size and shape of BCLP patients (range from 12.1 to 16.5 years) was not correlated with age. A comparison of the mean shapes of the clefted and nonclefted groups showed that the BCLP palate is flatter and narrower. The most notable size difference was found in the area between the maxilla and premaxilla. This phenomenon is associated with the persisting separation of the premaxilla from the rest of the palate. The shape of the palatal configuration of the premaxilla and adjacent area was concave in the nonclefted group and convex in BCLP patients.

Interaction of hydraulic and buckling mechanisms in blowout fractures

The etiology of blowout fractures is generally attributed to 2 mechanisms--increase in the pressure of the orbital contents (the hydraulic mechanism) and direct transmission of impacts on the orbital walls (the buckling mechanism). The present study aims to elucidate whether or not an interaction exists between these 2 mechanisms. We performed a simulation experiment using 10 Computer-Aided-Design skull models. We applied destructive energy to the orbits of the 10 models in 3 different ways. First, to simulate pure hydraulic mechanism, energy was applied solely on the internal walls of the orbit. Second, to simulate pure buckling mechanism, energy was applied solely on the inferior rim of the orbit. Third, to simulate the combined effect of the hydraulic and buckling mechanisms, energy was applied both on the internal wall of the orbit and inferior rim of the orbit. After applying the energy, we calculated the areas of the regions where fracture occurred in the models. Thereafter, we compared the areas among the 3 energy application patterns. When the hydraulic and buckling mechanisms work simultaneously, fracture occurs on wider areas of the orbital walls than when each of these mechanisms works separately. The hydraulic and buckling mechanisms interact, enhancing each other's effect. This information should be taken into consideration when we examine patients in whom blowout fracture is suspected.

Combined fixation with plates and transmalar Kirschner wires for zygomatic fractures

After repair of a fractured zygoma, the fixed zygoma occasionally becomes displaced. This phenomenon--generally called "relapse"--is a complication that can be prevented by fixing plural sites with plates. However, this impairs blood supply to the bone, which causes atrophy. To solve this dilemma, we developed a new concept for fixing the zygoma. Fractured zygomas are fixed by combining plate fixation at a single site with transmalar fixation with Kirschner wires. This secures stability of the fixed zygoma without impairing its blood supply. We evaluated the stability of fractured zygomas fixed by our method by doing dynamic experiments using simulation models. The stresses at the screw-bone interfaces were significantly reduced by the additional transmalar fixation with wire, indicating that zygomas fixed by our method are less likely to relapse than zygomas fixed with a single plate. We also reviewed the outcomes of patients treated by our method, which indicates its clinical usefulness.

Use of finite element analysis in presurgical planning: treatment of mandibular fractures

BACKGROUND

The current clinical procedure for mandible fracture fixation is plate application. 3D reconstructions are used to validate procedures numerically preceding experimental analysis. This study outlines the methods used to reconstruct a numerical model of the mandible.

METHODS

A CT scan from a 22-year-old male patient with a healthy unfractured mandible was obtained. A 3D reconstruction was carried out using Mimics via thresholding and segmentation techniques. Boundary conditions and muscle forces were applied, and simulations were performed using ABAQUS.

RESULTS

3D reconstruction allows for precise anatomical dimensions, which can be used for further engineering analysis. Using the surgical Champy technique as an example, results showed that the mandible model returned to normal function post-plating.

CONCLUSIONS

The study shows the clinical relevance of 3D reconstructions to plan surgical procedures. Results illustrate the benefit of carrying out numerical validations as a prerequisite to experimental modelling and as a method of pre-validating surgical procedures.

Stability of Zygomatic Plate-Screw Orthodontic Anchorage System

Objective: To evaluate the biomechanical properties of a standard and a newly designed plate-screw orthodontic anchorage system.

Materials and Methods: A three-dimensional model of the posterior maxilla, including the zygomatic buttress region, was prepared. Insertion of standard and newly designed plates was simulated on the three-dimensional model. The effect of 200 g of orthodontic force on the plate, screws, and zygomatic bone was evaluated in three-dimensional models by finite element analysis. To determine the force distribution, Von Mises stress, principal maximum and minimum stress, and principal maximum and minimum elastic strain values were evaluated.

Results: In all plate models the highest stresses occurred on the threaded bone site where the force application unit was attached.

Conclusion: Changing the plate configuration did not affect the stress distribution in the newly designed plates. To equalize the force distribution, new plate designs that change the location of the force application unit are required.

Experimental evaluation of relapse-risks in operated zygoma fractures

OBJECTIVES

Prevention of relapse, or postoperative dislocation, of the fixed zygoma is necessary to achieve optimal results in the treatment of zygoma fractures. Assuming that the occurrence of intensified stresses on mastication at the screw-bone interface (SBI) constitutes the essential cause of the relapse, we evaluated the stresses for three different fixation methods-fixation at the frontal process (FP), inferior orbital rim (IOR), and zygomatico-maxillary buttress (ZMB).

METHODS

We used 10 computer-aided design (CAD) models simulating zygoma fractures in the experiment. For each CAD model, we fixed the fractured zygoma with four screws and one mini-plate at the FP, IOR, or ZMB. After applying a 5.5kg force simulating mastication, we calculated the intensity and distribution patterns of the stresses occurring at the SBIs of the fixation screws using the finite element method. Thereby, we evaluated dynamic stability of the fixed zygoma for each of the three fixation methods.

RESULTS

Greater stresses occur at the SBIs with IOR fixation than at those with FP and ZMB fixation. Although the stresses occurring at the SBIs on mastication demonstrated evenly distributed patterns with the FP and ZMB fixation, the stresses demonstrated concentration on one screw with the IOR fixation.

CONCLUSIONS

The fixed zygoma is more likely to cause relapse with the IOR fixation than with the FP or ZMB fixation. Hence, in performing zygoma fixation at the IOR, care should be taken to minimize the likelihood of postoperative relapse that is caused by skewed distribution of the stresses on the fixation screws.

Appropriate diameter for screws to fix the maxilla following Le Fort I osteotomy: An investigation utilizing finite element analysis

BACKGROUND

After Le Fort I osteotomy, there is sometimes a secondary deformity (relapse), with the lower segment deviating from the intraoperatively fixed position. It is hyopothesized that the structural stability of the reconstructed maxilla is affected by the diameter of the fixation screws. The present study aims to elucidate the relationship between the diameters of the screws and the structural stability of the maxilla after Le Fort I osteotomy.

METHODS

3D models were produced on a workstation from 20 dry skulls and a Le Fort I operation was simulated on them. The upper and lower segments of the divided maxilla in each of the 20 models were connected using four plates and 16 screws. Five different diameters of the fixation screws were tested. Thus altogether 100 models were produced. A 180N load was applied to the molar region for each model. Using finite element analysis, the resultant stresses and deviations of the lower segments were calculated. Finally, referring to these values, the relationships between screw diameters and stability of the lower segment were evaluated.

RESULT

The stability of the lower segment was greatest when the diameter of the fixation screws was equal to the thickness of the bone at each fixation site.

CONCLUSION

In performing Le Fort I osteotomy, it is recommended that bone thickness is measured at each fixation site in advance, and the diameter of the fixation screws matched accordingly; thereby optimum stability of the reconstructed maxilla can be anticipated.

Evaluation of different miniplates in fixation of fractured human mandible with the finite element method

The objective of this study was to develop a 3-dimensional finite element model (FEM) to formulate biomechanical justification of the positioning of different plates to achieve stable fixation of a fractured mandible. Miniplate systems that give acceptable levels of rigidity were investigated, and recommendations about miniplate location, orientation, and type selection are made. A fracture near the body region was bridged with a variety of commonly used plate configurations. Number, positioning and type of the plate system parameters. The results of this fracture model support the advantage of 2-plate systems. Using a longer plate in the superior position and a shorter one in the inferior position produced a more stable condition. Number of screws or length of the miniplate had no significant effect on the stability of fractured segments. The results obtained from this study offer the choice of a particular plate size, thickness, design, or configuration for application and thus provide information for clinical use.

Effectiveness of Additional Transmalar Kirschner Wire Fixation for a Zygoma Fracture

BACKGROUND

The purpose of this study was to verify the effectiveness of transmalar Kirschner wire fixation as additional fixation for the treatment of zygoma fractures.

METHODS

The authors compared two methods for zygoma fixation at the frontozygomatic suture from both theoretical and clinical viewpoints: miniplate fixation (plate fixation) and miniplate fixation with an additional transmalar Kirschner wire fixation (wire plus plate fixation). For the theoretical study, the authors produced zygoma fractures on 20 skull simulation models; these were generated on the basis of computed tomographic data of actual dry skulls. In their simulation surgery, they fixed the fractured zygoma with the above-mentioned two fixation methods, producing 20 plate fixation models and 20 wire plus plate fixation models. A 10-kgf load was then applied on the fractured zygoma in the anteroposterior and superoinferior directions. Finally, the stresses around the fixation screws and the deviation of the zygoma were calculated using finite element analysis. For the clinical study, the authors compared the postoperative zygoma alignment based on computed tomography of six patients treated with plate fixation and eight patients treated with wire plus plate fixation using a visual analogue scale.

RESULTS

In the theoretical study, the wire plus plate fixation models demonstrated a significant decrease in both the stresses around the screws and the deviation of the fractured bone compared with the plate fixation models. In the clinical study, the visual analogue scale scores for the wire plus plate fixation group were significantly higher than those for the plate fixation group.

CONCLUSION

Because the additional transmalar Kirschner wire fixation can effectively increase the stability of the fractured zygoma that has been fixed with one miniplate, it should be recommended as an effective technique for the treatment of complicated zygoma fractures.

Application of the finite element method in dental implant research

This article provides a review of the achievements and advancements in dental technology brought about by computer-aided design and the all powerful finite element method (FEM) of analysis. The scope of the review covers dental implants, jawbone surrounding the implant and the biomechanical implant and jawbone interaction. Prevailing assumptions made in the published finite element analysis (FEA) and their limitations are discussed in some detail which helps identify the gaps in research as well as future research direction.

Three-dimensional finite-element analysis investigating the biomechanical effects of human mandibular reconstruction with autogenous bone grafts

PURPOSE

To investigate the biomechanics of the mandible following reconstruction with autogenous bone grafts.

MATERIAL

Computerized tomography scan images of a human mandible, fibula and iliac crest were collected and used to build models on a PC.

METHODS

Four finite-element analysis (FEA) models of mandibles reconstructed with autogenous bone were created. The principal stresses of marked points, the Von Mises stresses at anatomical index regions, and the force values of temporo-mandibular joints and masticatory muscles were calculated.

RESULTS

Compared with the normal mandible, the one repaired with a fibula had greater Von Mises stresses on the grafted bone; the one repaired with iliac crest bone had the similar distribution of the Von Mises stresses as in the normal mandible. The principal stresses in the autograft varied between tensile and compressive stresses from the right graft/bone binding interface to the left in all reconstructed mandibles. On the whole, the maximum Von Mises stress was greater on the mandible reconstructed with fibula than that reconstructed with iliac crest.

CONCLUSION

Mandibles repaired with iliac crest grafts have more mechanical properties similar to normal than those repaired with fibula grafts.

Adaquate fixation of plates for stability during mandibular reconstruction

PURPOSE

To determine the most appropriate plate fixation for dispersing the stress around screws in mandibular reconstruction.

MATERIAL AND METHODS

Forty-eight three-dimensional reconstructed mandibular models with Central (C) or right Lateral (L) defects were created and divided into three groups, fixed with: (1) two screws on each side, (2) three screws on the left side with the third (middle one) located distally, or (3) three screws on the left side with the third (middle one) located proximally. A 300N vertical load was applied to the left molar region. The maximum stress was calculated using a finite element method and statistically evaluated.

RESULTS

Stress was concentrated around the screw at the distal end of the mandibular halves on the loaded side ('crucial screw'). For the C defect, stress concentrated around the 'crucial screw' was greater when there were only two screws for fixation (20.55MPa) than when there were three screws (16.17MPa; p=0.022). No significant difference was found between the two groups with three screws for fixation. For the L defect on the other hand, stress on the 'crucial screw' was relatively greater when there were three screws for fixation (18.9MPa) than when there were only two screws for (12.83MPa; p=0.051). No significant difference was found between the two groups with three screws for fixation. As for the reconstruction plate, the stresses on the plates were not different among the various screw groups for fixation nor among the C and L defects.

CONCLUSION

This paradoxical result is explained by plate bowing. Thus in large defects screw positions should take into consideration the potential for plate bowing.

The Effect of Striking Angle on the Buckling Mechanism in Blowout Fracture

BACKGROUND

The buckling mechanism is widely accepted as a mechanism of blowout fractures, along with the hydraulic mechanism. Although many studies have been performed related to the buckling mechanism, none of them have taken the direction of the striking force into consideration. As the orbital floor is not parallel to the horizontal plane, a difference in the striking force direction might affect resultant fracture patterns. The present study aims to investigate whether fracture patterns in the orbital floor were influenced by the striking force direction in terms of the buckling mechanism.

METHODS

The authors produced three-dimensional models on a workstation simulating eight dry skulls and applied striking forces on the orbital rim of each model from three different angles (0, 15, and 30 degrees in the upward direction). Using finite element analysis, the authors calculated the width of the area where the resultant stresses exceed the bone's yielding criterion. The width was termed the "theoretical fracture width" because, theoretically, fracture was expected to occur in the area. Then, the authors compared the theoretical fracture width in groups with the three different striking force angles. Finally, the validity of the theoretical width was verified with an experiment on actual skull models.

RESULTS

The theoretical fracture width was the greatest when the striking force was directed at 30 degrees in the upward direction.

CONCLUSIONS

For the buckling mechanism, fracture would occur in a wider area of the orbital floor when striking force was directed upward than when the force was horizontally directed. This finding would be helpful in predicting fracture width in blowout fractures.

Effects of different inter-implant distances on the stress distribution around endosseous implants in posterior mandible: A 3D finite element analysis

PURPOSE

The aim of this study was to evaluate the effects of different inter-implant distances on stress distribution in the bone around the endosseous titanium implants under vertical, oblique and horizontal loads in the posterior mandibular edentulousim by finite element analysis (3D FEA).

MATERIALS AND METHODS

3D FEA models representing mandible and ITI implant (Straumann, Waldenburg, Switzerland) were simulated. The distances in-between the units were set at 0.5, 1.0 and 2.0 cm. Vertical (V) 70 degrees N, 60 degrees oblique (BL) 35 degrees N in buccolingual direction and horizontal (MD) 14 degrees N in mesiodistal direction loads were applied to each of these designs. The principal stresses (tensile and compressive stress) on each model were calculated using MSC MARC finite element analyze solver software.

RESULTS

The tensile stress (P(max)) values have been evaluated that they rose at the cervical region of buccal side when the inter-implant distances increased under V and BL loads and they diminished while the inter-implant distances decreased. In short inter-implant distances the compressive stress (P(min)) has been presented with increased values and found at the lingual surface of the cervical region.

DISCUSSION

The results of this study indicated that the magnitude of the stress was influenced by complex factors such as the direction of loads and the distance between adjacent fixtures. The stress occurring around fixtures differs significantly with various types of inter-implant distance.

CONCLUSION

The evaluation of tensile and compressive stresses for cortical and cancellous bone under V, MD and BL loading conditions in aspect of inter-implant distance shows; the 1.0 cm of inter-implant distance is the optimum distance for two fixture implantation.

Comparison of Plate-Screw Systems Used in Mandibular Fracture Reduction: Finite Element Analysis

A finite element model of the human dentate mandible has been developed to provide a comparison of fixation systems used currently for fracture reduction. Volume domains for cortical bone, cancellous bone, and teeth were created and meshed in ANSYS 8.0 based on IGES curves created from computerized tomography data. A unilateral molar clench was loaded on the model with a fracture gap simulated along the symphysis. Results based on Von Mises stress in cortical and cancellous bone surrounding the screws, and on fracture surface spatial fixation, show some relative differences between different screw-plate systems, yet all were judged to be appropriate in their reduction potential.

A comparative study of most suitable miniplate fixation for mandibular symphysis fracture using a finite element model

The purpose of this study is to determine the most stable fixation method for mandibular symphysis fractures by comparing the mechanical characteristics of models fixed at different positions with different numbers of plates. Fractures were generated in 3-dimensional finite element models, and were fixed with a single miniplate, parallel double miniplates, or perpendicular double miniplates. A 300 N perpendicular load was then applied on the left molar region, and a finite element analysis was performed. We compared vertical gaps between the fractured surfaces, maximum stress within the screw/plating system, and maximum stress around screw holes in the bone. Compared to the single miniplate, both the parallel and perpendicular double miniplates demonstrated significantly less stress in the screw/plating system and screw holes in the bone. In addition, the perpendicular double miniplateshad significantly smaller vertical gaps between fracture surfaces when compared to the single miniplate. Comparing parallel and perpendicular double miniplate fixations, less stress was found around the screw holes of the perpendicular miniplate models than those of the parallel miniplatemodels. There were no differences in vertical gaps or maximum stress within the screw/plating systems between the 2 double miniplate fixations. These results suggest that perpendicular double miniplate fixation is more suitable for fixing mandibular symphysis fractures.

The Dynamic Role of Buttress Reconstruction after Maxillectomy

BACKGROUND

The purpose of this study was to investigate the dynamic effect of maxillary reconstruction after partial resection of the maxilla.

METHODS

On a personal computer, three-dimensional maxilla models were designed based on computed tomographic data obtained from 10 edentulous skull models. Simulation surgery was performed on each model, creating 10 pairs of half-removed maxilla models and corresponding models after reconstruction with a rib. The three different patterns seen in the 10 models were termed normal maxilla, half-removed maxilla, and reconstructed maxilla. After an implant was fixed on the molar region of each model, a 300-N vertical load and a 50-N horizontal load were applied. Using finite element analysis software, the deviation and stress on each model were calculated and compared between different model patterns.

RESULTS

Regarding deformity of the maxilla, when a vertical load was applied, no significant difference was observed among the three model patterns. However, a difference was observed in response to a horizontal load in that there was a tendency for deformation to occur, with that of half-removed maxilla being the greatest followed by reconstructed and normal maxilla. Regarding stresses around the implant, when the vertical and horizontal loads were applied, no significant difference was observed among the three model patterns in maximum stress around the implant.

CONCLUSIONS

A buttress reconstruction is effective in increasing the stability of the maxilla against a horizontal load. However, the maximum stress around the implant in the molar region is unaffected whether or not removal or reconstruction is performed.

Finite element analysis of the stresses around fixtures in various reconstructed mandibular models--part II (effect of horizontal load)

AIM OF EXPERIMENT: Having already studied the effect of vertical load (Nagasao et al., 2002), the purpose of this study was to investigate the effect of horizontal load on implants embedded into a virtually reconstructed mandible.

MATERIAL AND METHODS

Three-dimensional virtual models of various reconstructed mandibles were designed on a PC and 50 N horizontal loads were applied on the cantilever portion of the upper structure. Then, using a finite element analysis, stresses occurring around the implants were calculated, and the directions of the forces that cause the maximum von Mises stresses were evaluated. Finally, the results were compared with those from vertical loading.

RESULTS

In all models, the maximum stresses occurred around the implants embedded on the premolar region of the loaded side. In addition, the locations at which the maximum von Mises stresses occurred were greatly influenced by the structural character of the loaded side.

CONCLUSION

When horizontal loads were applied to reconstructed mandibles, the location and direction of the maximum stresses around the dental implants appeared to be influenced much more by the structural characters of the reconstructed mandibles when compared with vertical loads.