The modular endoprosthesis for mandibular body replacement. Part 2: Finite element analysis of endoprosthesis reconstruction of the mandible

Computational models and their applications in biomechanical analysis of mandibular reconstruction surgery

Advanced head and neck cancers involving the mandible often require surgical removal of the diseased parts and replacement with donor bone or prosthesis to recreate the form and function of the premorbid mandible. The degree to which this reconstruction successfully replicates key geometric features of the original bone critically affects the cosmetic and functional outcomes of speaking, chewing, and breathing. With advancements in computational power, biomechanical modeling has emerged as a prevalent tool for predicting the functional outcomes of the masticatory system and evaluating the effectiveness of reconstruction procedures in patients undergoing mandibular reconstruction surgery. These models offer cost-effective and patient-specific treatment tailored to the needs of individuals. To underscore the significance of biomechanical modeling, we conducted a review of 66 studies that utilized computational models in the biomechanical analysis of mandibular reconstruction surgery. The majority of these studies employed finite element method (FEM) in their approach; therefore, a detailed investigation of FEM has also been provided. Additionally, we categorized these studies based on the main components analyzed, including bone flaps, plates/screws, and prostheses, as well as their design and material composition.

Development and validation of a digital twin of the human lower jaw under impact loading by using non-linear finite element analyses

Mandibular fractures are one of the most frequently observed injuries within craniofacial region mostly due to tumor-related problems and traumatic events, often related to non-linear effects like impact loading. Therefore, a validated digital twin of the mandible is required to develop the best possible patient-specific treatment. However, there is a need to obtain a fully compatible numerical model that can reflect the patients' characteristics, be available and accessible quickly, require an acceptable level of modeling efforts and knowledge to provide accurate, robust and fast results at the same time under highly non-linear effects. In this study, a validated simulation methodology is suggested to develop a digital twin of mandible, capable of predicting the non-linear response of the biomechanical system under impact loading, which then can be utilized to design treatment strategies even for multiple fractures of the mandibular system. Using Computed Tomography data containing cranial (skull) images of a patient, a 3-dimensional mandibular model, which consists cortical and cancellous bones, disks and fossa is obtained with high accuracy that is compatible with anatomical boundaries. A Finite Element Model (FEM) of the biomechanical system is then developed for a three-level validation procedure including (A) modal analysis, (B) dynamic loading and (C) impact loading. For the modal analysis stage: Free-free vibration modes and frequencies of the system are validated against cadaver test results. For the dynamic loading stage: Two different regions of the mandible are loaded, and maximum stress levels of the system are validated against finite element analyses (FEA) results, where the first loading condition (i) transfers a 2000 N force acting on the symphysis region and, the second loading condition (ii) transfers a 2000 N force acting on the left body region. In both cases, equivalent muscle forces dependent on time are applied. For the impact loading stage: Thirteen different human mandibular models with various tooth deficiencies are used under the effects of traumatic impact forces that are generated by using an impact hammer with different initial velocities to transfer the impulse and momentum, where contact forces and fracture patterns are validated against cadaver tests. Five different anatomical regions are selected as the impact site. The results of the analyzes (modal, dynamic and impact) performed to validate the digital twin model are compared with the similar FEA and cadaver test results published in the literature and the results are found to be compatible. It has been evaluated that the digital twin model and numerical models are quite realistic and perform well in terms of predicting the biomechanical behavior of the mandible. The three-level validation methodology that is suggested in this research by utilizing non-linear FEA has provided a reliable road map to develop a digital twin of a biomechanical system with enough confidence that it can be utilized for similar structures to offer patient-specific treatments and can help develop custom or tailor-made implants or prosthesis for best compliance with the patient even considering the most catastrophic effects of impact related trauma.

Modified internal curvilinear distraction device with a pre-embedding guide rail, drive screw, and universal joint for curvilinear lengthening of the mandible: A finite element analysis and animal experiment

OBJECTIVES

The semiburied design of the traditional internal distractor has a relatively high risk of infection and aesthetic problems. To reduce these potential risks, a modified internal distractor with design of pre-embedding curvilinear rail, drive screw, and universal joint was invented. Its stress distribution characteristics and the effect on curvilinear distraction osteogenesis (DO) in vivo were further tested.

MATERIALS AND METHODS

Finite element analysis (FEA) was performed on a model of the human mandible and distraction device to measure the stress distribution during curvilinear DO. Six beagles underwent curvilinear DO and consolidation using the new device. Radiological and histological examinations were performed on the new bone.

RESULTS

On FEA, the stress was concentrated in the condyle (128.6 MPa) and curved guide rails (324.8 MPa). Four of the six animals completed the DO period and were consolidated for 12 weeks. Secondary infections were not observed. Radiography showed that a new fan-shaped bone-15.5 ± 5.5 mm in length and 4.6 ± 1.6 mm in height-was formed in the bone gap. Micro-computed tomography and histological examinations of specimens indicated that the structure of the new bone was similar to that of the normal bone.

CONCLUSIONS

The modified internal curvilinear distraction device meets the mechanical strength requirement and achieve curvilinear DO in animal experiments.

Variation in stress distribution modified by mandibular material property: a 3D finite element analysis

BACKGROUND

Temporomandibular joint disorder (TMD) is a common oral and maxillary facial disease. Finite element method (FEM) has been widely used in TMD studies. Material assignment significantly affects FEM results. The differences in the methods of material assignment used in previous studies have not been comprehensively assessed for further calculations.

METHODS

The mandible material modelling approaches were of four types, namely: uniform modelling with (A) cortical bone; and (B) cancellous bone; (C) semi-uniform modelling with division of cortical and cancellous bone; and (D) non-uniform modelling with Computed tomography (CT) gray value related modulus. Meanwhile, the Young's modulus of values ranging from 20 to 300 GPa were considered for the teeth. Ten modellings were used to analyze and discuss the differences in contact pressure and contact force.

RESULTS

(1) The increase in teeth elastic modulus increased the maximum contact pressure on the alveolar bone and contact force on teeth, but induced insignificant stress variation on the temporomandibular joint; (2) The location of the maximum contact pressure was steady for all four modelling approaches of the mandibular material. However, the maximum contact pressure and contact force exhibited an insignificant difference.

CONCLUSIONS

Teeth with a higher elastic modulus significantly enhanced the stress concentration in the alveolar bone; in contrast, it induced minor variations in the temporomandibular joint stress states. The extreme stress regions predicted by the four mandibular models were consistent with the actual damaged regions. However, non-uniform modellings based on CT values could better describe the mechanical properties of the human bone, which should be primarily considered.

[Study of properties of three-dimensional matrices manufactured by antisolvent 3D printing for reconstruction of extensive bone defects in dentistry and maxillofacial surgery]

OBJECTIVE

The aim of the study to study of physical, mechanical and biocompatible properties of the matrices manufactured by antisolvent 3D printing from the solutions of polylactide-co-glycolide in tetraglycol.

MATERIAL AND METHODS

Three-dimensional scaffolds were made from a solution of polylactide-co-glycolide mixed with tetraglycol using antisolvent 3D printing. The elastic properties and the structure of the obtained matrices were studied. MTT-test and staining with PKH-26, Calcein-AM, DAPI with subsequent fluorescence microscopy were used to study biological properties.

RESULTS

The three-dimensional scaffolds had good mechanical properties. Young's modulus value was 18±2 MPa, tensile strength was 0.43±0.05 MPa. The relative survival rate of cells after the first day was 99.58±2.28%, on the 14th day - 98.14±2.22%. The structure of the scaffold promoted cell adhesion and spreading on its surface.

CONCLUSION

The polylactide-co-glycolide matrices produced by antisolvent printing have high porosity, biocompatibility and good mechanical properties. It is allowed to use them in the future as a basis for personalized constructions for the replacement of extensive bone defects of the maxillofacial region.

Optimal placement of fixation system for scaffold-based mandibular reconstruction

A current challenge in bone tissue engineering is to create favourable biomechanical conditions conducive to tissue regeneration for a scaffold implanted in a segmental defect. This is particularly the case immediately following surgical implantation when a firm mechanical union between the scaffold and host bone is yet to be established via osseointegration. For mandibular reconstruction of a large segmental defect, the position of the fixation system is shown here to have a profound effect on the mechanical stimulus (for tissue regeneration within the scaffold), structural strength, and structural stiffness of the tissue scaffold-host bone construct under physiological load. This research combines computer tomography (CT)-based finite element (FE) modelling with multiobjective optimisation to determine the optimal height and angle to place a titanium fixation plate on a reconstructed mandible so as to enhance tissue ingrowth, structural strength and structural stiffness of the scaffold-host bone construct. To this end, the respective design criteria for fixation plate placement are to: (i) maximise the volume of the tissue scaffold experiencing levels of mechanical stimulus sufficient to initiate bone apposition, (ii) minimise peak stress in the scaffold so that it remains intact with a diminished risk of failure and, (iii) minimise scaffold ridge displacement so that the reconstructed jawbone resists deformation under physiological load. First, a CT-based FE model of a reconstructed human mandible implanted with a bioceramic tissue scaffold is developed to visualise and quantify changes in the biomechanical responses as the fixation plate's height and/or angle are varied. The volume of the scaffold experiencing appositional mechanical stimulus is observed to increase with the height of the fixation plate. Also, as the principal load-transfer mechanism to the scaffold is via the fixation system, there is a significant ingress of appositional stimulus from the buccal side towards the centre of the scaffold, notably in the region bounded by the screws. Next, surrogate modelling is implemented to generate bivariate cubic polynomial functions of the three biomechanical responses with respect to the two design variables (height and angle). Finally, as the three design objectives are found to be competing, bi- and tri-objective particle swarm optimisation algorithms are invoked to determine the most optimal Pareto solution, which represents the best possible trade-off between the competing design objectives. It is recommended that consideration be given to placing the fixation system along the upper boundary of the mandible with a small clockwise rotation about its posterior end. The methodology developed here forms a useful decision aid for optimal surgical planning.

Third-generation modular mandible endoprosthesis in Macaca fascicularis

The aim of this study was to develop a third-generation modular mandible endoprosthesis that would experience less stress concentration at its stems compared to earlier generations, thereby minimizing micromotion and achieving long-term stability. In this three-piece modular design, different degrees of movement were incorporated between the endoprosthesis module interfaces. It was hypothesized that this unique feature would minimize stress concentration at the stems and hence promote osseointegration during the early phase of implantation. The endoprosthesis system was made of commercially pure grade 4 titanium, machined and surface-treated, then sterilized and implanted in segmental mandible defects of nine Macaca fascicularis. Clinical, radiological, histological, and histomorphometric evaluations were performed 4 months post-implantation. The endoprosthesis systems with a degree of movement incorporated, exhibited superior performance compared to the rigid system: 30.9-34.8 times higher percentage bone-implant contact (P< 0.0001) and 3.4-4.1 times higher percentage bone area (P<0.0001), with osseointegration noted at the posterior stems. However, fibrous tissue encapsulation was noted around the majority of the anterior stems in all groups. Although the degree of movement was favourable for improving bone healing and stability of the endoprosthesis system, more work needs to be done to investigate other strategies to further reduce loading on the endoprosthesis to achieve predictable osseointegration at the stems.

The Potential of Magnesium Based Materials in Mandibular Reconstruction

The future of biomaterial design will rely on development of bioresorbable implant materials that completely and safely degrade in vivo after the tissues grow, without generating harmful degradation products at the targeted anatomic site. Permanent biomaterials such as Ti6Al4V alloy, 316L stainless steel, and Co-based alloys currently used in mandibular reconstruction often result in stress shielding effects due to mismatch in the Young’s modulus values between the bone and the implant, resulting in implant loosening. Also, allergic responses due to metal ion releases necessitates revision surgery to prevent long term exposure of the body to toxic implant contents. Bioresorbable metals are perceived as revolutionary biomaterials that have transformed the nature of metallic biomaterials from bioinert to bioactive and multi-bio functional (anti-bacterial, anti-proliferation, and anti-cancer). In this aspect, magnesium (Mg)-based materials have recently been explored by the biomedical community as potential materials for mandibular reconstruction, as they exhibit favorable mechanical properties, adequate biocompatibility, and degradability. This article reviews the recent progress that has led to advances in developing Mg-based materials for mandibular reconstruction; correlating with the biomechanics of mandible and types of mandibular defects. Mg-based materials are discussed regarding their mechanical properties, corrosion characteristics, and in vivo performance. Finally, the paper summarizes findings from this review, together with a proposed scope for advancing the knowledge in Mg-based materials for mandibular reconstruction.

Mechanical stress in plates for bridging reconstruction mandibular defects and purposes of double plate reinforcement

PURPOSE

To evaluate the biomechanical performance of a commercially available bridging plate (2.4) as well as screws and bone simulating the reconstruction of hemimandibular defects and to indicate alternatives of reinforcement to prevent plate fractures either by strength or fatigue.

MATERIAL AND METHODS

Two common hemimandibular defects are investigated using computed finite element analysis (FEA) approach. Simplified and refined computational models are developed for the geometry of the screw. Conditions of non-locking and locking plate-screw interfaces are considered. Static loads of 120 N are applied. Von Mises stresses and fatigue are calculated. As reinforcement, a second complete or partial plate is placed onto the original plate.

RESULTS

Results demonstrate that reconstruction plates are often subjected to excessive stress that may lead to fracture either by strength or by fatigue. An attached complete or partial second plate is able to reduce stress in the plate, in screws and bone so that stress remains below the allowable limit of the materials.

CONCLUSION

A simplified technique of attaching a whole or sectioned second plate onto the original plate can reduce the stress calculated and may reduce the frequency of plate fractures for the patient's comfort, security and financial savings.

Finite-element design and optimization of a three-dimensional tetrahedral porous titanium scaffold for the reconstruction of mandibular defects

Reconstruction of segmental defects in the mandible remains a challenge for maxillofacial surgery. The use of porous scaffolds is a potential method for repairing these defects. Now, additive manufacturing techniques provide a solution for the fabrication of porous scaffolds with specific geometrical shapes and complex structures. The goal of this study was to design and optimize a three-dimensional tetrahedral titanium scaffold for the reconstruction of mandibular defects. With a fixed strut diameter of 0.45mm and a mean cell size of 2.2mm, a tetrahedral structural porous scaffold was designed for a simulated anatomical defect derived from computed tomography (CT) data of a human mandible. An optimization method based on the concept of uniform stress was performed on the initial scaffold to realize a minimal-weight design. Geometric and mechanical comparisons between the initial and optimized scaffold show that the optimized scaffold exhibits a larger porosity, 81.90%, as well as a more homogeneous stress distribution. These results demonstrate that tetrahedral structural titanium scaffolds are feasible structures for repairing mandibular defects, and that the proposed optimization scheme has the ability to produce superior scaffolds for mandibular reconstruction with better stability, higher porosity, and less weight.

Mandibular Reconstruction: Overview

INTRODUCTION

Mandibular reconstruction has changed significantly over the years and continues to evolve with the introduction of newer technologies and techniques.

PURPOSE

This article reviews the history of oromandibular reconstruction, biomechanics of mandible, summarizes the reconstruction options available for mandible with defect classification, goals in reconstruction, the various donor sites, current reconstructive options, dental rehabilitation and persistent associated problems.

SUMMARY

Oromandibular reconstruction, although a challenge for the head and neck reconstructive surgeon, is now reliable and highly successful with excellent long-term functional and aesthetic outcomes with the use of autogenous bone grafts and current reconstructive options. The ideal reconstruction would provide a solid arch to articulate with the upper jaw, restoring swallowing speech, mastication, and esthetics. Autogenous vascularized bone grafts in combination with microsurgical techniques have revolutionized mandibular reconstruction in oral cancer surgery. Current trends in mandibular reconstruction aim to achieve reestablishment of a viable mandible of proper form and maxillary mandibular relationship while decreasing the need for invasive autogenous graft procurement. However the optimal reconstruction of mandibular defects is still controversial in regards to reconstructive options which include the donor site selection, timing of surgery and method of reconstruction.

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.

The feasibility of a custom-made endoprosthesis in mandibular reconstruction: Implant design and finite element analysis

This work studies the feasibility of custom-made endoprosthesis in the reconstruction of major mandibular defects. The natural anatomical and occlusal relations are used to accurately reconstruct a mandibular defect. The customized implant allows the accurate restoration of the facial profile and aesthetics. The biomechanical behaviour of mandibular endoprosthesis was validated with Finite Element Analysis for three masticatory tasks, namely incisal, right molar and left group clenching. The implanted mandible shows displacement fields and stress distributions very similar to the intact mandible. The strain fields observed along the bone-implant interface may promote bone maintenance and ingrowth. The preliminary results show that this implant may be a reliable alternative to other prosthetic mandibular reconstruction approaches.

Load transfer in Christensen(®) TMJ in alloplastic total joint replacement for two different mouth apertures

This study analyses load transfer in the fossa component based on two numerical models of total temporomandibular joint (TMJ) implants for two mouth openings. The TMJ articulation is a very complex system with muscles, ligaments and cartilage. Until now, studies of TMJ implants have analysed only condylar behaviour. The finite element models were constructed based on CT scans of a cadaveric mandible and cranium, considering the bone geometry and position. The influence of five principal muscle actions was simulated for two mouth positions, 5 mm and 15 mm openings at the incisive tooth support. Strain distributions into the surrounding bone tissue were analysed in both models in the condyle and fossa components. The results demonstrate that in Christensen(®) TJR of the temporomandibular joint the fossa component is the more critical part, presenting more stress near the screw holes and contact regions with the cranium. The most critical region is around the first two screws and the least critical is in the condyle component. For the mandible condyle reconstructed with a Christensen(®) prosthesis, the 15 mm mouth opening was more critical, as compression was increased, but for the fossa component the most critical situation occurred with the 5 mm opening. The micromovements observed suggest that the number of screws could be reduced to increase osteointegration of screws in the mandible condyle.

Biomechanical analysis of a curvilinear distractor device for correcting mandibular symphyseal defects

PURPOSE

The local mechanical environment is a determinant of successful transport disc distraction osteogenesis. This study assessed the biomechanics of a curvilinear distractor device for correcting mandibular symphyseal defects.

MATERIALS AND METHODS

The finite element method was used to analyze an intact mandible, mandibular distractor bodies with different rail thicknesses (4, 6, 8, and 10 mm), and mandibular distractor bodies with rails and auxiliary lingual brackets.

RESULTS

Rail thickness was positively correlated with maximum von Mises stress in the distractor and negatively correlated with maximum displacement of the mandibular distractor bodies. The maximum von Mises stress occurred at the junction of the rails and fixed arms. It also exceeded the yield strength of the titanium material. Compared with the maximum displacement of the intact mandible, that of the mandibular distractor bodies was visibly increased.

CONCLUSION

An auxiliary lingual bracket can effectively decrease stress in such devices and displacement of mandibular distractor bodies. Rail fixation alone cannot achieve stability for distraction osteogenesis. Using an auxiliary lingual bracket effectively prevents distractor breakage and exposure.

Establishment of sequential software processing for a biomechanical model of mandibular reconstruction with custom-made plate

The aim of this study is to describe the sequential software processing of computed tomography (CT) dataset for reconstructing the finite element analysis (FEA) mandibular model with custom-made plate, and to provide a theoretical basis for clinical usage of this reconstruction method. A CT scan was done on one patient who had mandibular continuity defects. This CT dataset in DICOM format was imported into Mimics 10.0 software in which a three-dimensional (3-D) model of the facial skeleton was reconstructed and the mandible was segmented out. With Geomagic Studio 11.0, one custom-made plate and nine virtual screws were designed. All parts of the reconstructed mandible were converted into NURBS and saved as IGES format for importing into pro/E 4.0. After Boolean operation and assembly, the model was switched to ANSYS Workbench 12.0. Finally, after applying the boundary conditions and material properties, an analysis was performed. As results, a 3-D FEA model was successfully developed using the softwares above. The stress-strain distribution precisely indicated biomechanical performance of the reconstructed mandible on the normal occlusion load, without stress concentrated areas. The Von-Mises stress in all parts of the model, from the maximum value of 50.9MPa to the minimum value of 0.1MPa, was lower than the ultimate tensile strength. In conclusion, the described strategy could speedily and successfully produce a biomechanical model of a reconstructed mandible with custom-made plate. Using this FEA foundation, the custom-made plate may be improved for an optimal clinical outcome.

Influences of implant condyle geometry on bone and screw strains in a temporomandibular implant

A 3D finite element model of an in vitro implanted mandible was analysed. The load point was placed on the condyle in three positions (inside the mouth, centred and outside) to simulate different contact points between the mandible condyle and the temporal bone. The strain fields in the condyle were assessed and detailed around the surgical screws. The temporomandibular implant studied here was modelled on a commercial device that uses four screws to fix it in vivo in a very similar position. The boundary conditions of the numerical model simulated a load on the incisors with a 15 mm mouth aperture. The same contact loads were applied to the two condyles. Numerical results were successfully obtained for the three different contact points: the inside contact produced lower strains on the condyle. The first screw created a critical strain distribution in the bone, just under the screw. The study shows that centred and inside contact induces lower strain distributions. This suggests that spherical condyle geometry should be applied in order to reduce the strains in fixation. As the top screw was observed to play the most critical role, the third screw is in fact unnecessary, since the lower strain distribution suggests that it will be loosened.