Influence of different mucosal resiliency and denture reline on stress distribution in peri-implant bone tissue during osseointegration. A three-dimensional finite element analysis

doi: 10.1111/j.1741‐2358.2011.00569.x Influence of different mucosal resiliency and denture reline on stress distribution in peri‐implant bone tissue during osseointegration. A three‐dimensional finite element analysis Objective: The aim of this study was to evaluate the influence of mucosal properties and relining material on the stress distribution in peri‐implant bone tissue during masticatory function with a conventional complete denture during the healing period through finite element analysis. Materials and Methods: Three‐dimensional models of a severely resorbed mandible with two recently placed implants in the anterior region were created and divided into the following situations: (i) conventional complete denture and (ii) relined denture with soft lining material. The mucosal tissue properties were divided into soft, resilient and hard. The models were exported to mechanical simulation software; two simulations were carried out with a load at the lower right canine (35 N) and the lower right first molar (50 N). Data were qualitatively evaluated using Maximum Principal Stress, in MPa, given by the software. Results: All models showed stress concentrations in the cortical bone corresponding to the cervical part of the implant. The mucosal properties influenced the stress in peri‐implant bone tissue showing a different performance according to the denture base material. The simulations with relined dentures showed lower values of stress concentration than conventional ones. Conclusions: It seems that the mucosal properties and denture reline have a high influence on the stress distribution in the peri‐implant bone during the healing period.     

Evaluation of threshold stress for bone resorption around screws based on in vivo strain measurement of miniplate

The purpose of this study is to investigate the critical threshold stress causing bone resorption evaluated from strain measurement in vivo, comparing the various finite element models. In this study strains of miniplates used for mandibular fractures were measured once a week until the strains reduced. The maximum bite force for each patient was applied in the incisal, right molar and left molar region. The strains increased and reached a peak level at 2-4 weeks, whereas the bite forces increased during the period of measurements. A 3-D osteosynthesis model using finite element method showed that the compressive stresses of the bone surrounding screws ranged within approximately -40 MPa under the condition generating the same amounts of strains measured in the miniplates. Furthermore, various finite element models simulating mandibular reconstruction using the fibular graft were constructed. The models for reconstruction using single strut fibula showed distinct stress concentration in the cortical bone surrounding screws, and the peak stress levels were 2 to 3 times as strong as that of the fracture model. We conclude that critical threshold for bone resorption should be approximately -50 MPa (3600 micro strain).     

Treatment of mandible fractures using low-profile titanium miniplates: preliminary study.

This study evaluated the short-term results of patients treated with low-profile titanium miniplates for fractures of the mandible. Thirty-one fractures of the mandible in 23 patients were treated by open reduction and internal fixation using thin, low-profile miniplates and 1.3-mm self-threading screws. Duration of intermaxillary fixation ranged from 0 to 25 days. Patients were evaluated for complications during a follow-up period ranging from 6 to 24 months. Seven patients (30.4 percent) experienced complications. These included infection (n = 1), premature occlusal contact (n = 1), wound dehiscence (n = 1), temporomandibular joint disorder (n = 1), and paresthesia (n = 3). All complications were minor and adequately managed with incision and drainage, medication, and elastic traction. Low-profile titanium miniplates can be adequately used for internal fixation in selective mandibular fractures. Advantages of these types of plates include comfort due to the thinness of miniplates and ease of application.     

On the influence of mechanical conditions in osteochondral defect healing

Despite the introduction of new surgical techniques, the treatment of cartilage defects remains challenging. Delay or complete failure of cartilage healing is associated with problems in biological regeneration. The influence of mechanical conditions on this process, however, remains unevaluated. Osteochondral defects were generated on the left femoral condyle in 18 Yucatan minipigs. After 4, 6 and 12 weeks the defect filling, trabecular orientation and bone density were compared to the intact contralateral side. The mechanical straining during this period was then analyzed using an adaptive finite element technique. Histologically, the osteochondral defects showed bone resorption at the base and bone formation from the circumference. At 12 weeks, the macroscopically healed specimens showed fibrous cartilage formation, a minimally organized trabecular structure and increased trabecular volume fraction compared to the controls (p < 0.002). The amount of cancellous, cartilagineous, and fibrous tissue and the defect size as measured in histomorphometric analysis for the three time points (4, 6 and 12 weeks) was comparable in magnitude to that predicted by finite element analysis. The simulated osteochondral healing process was not fully capable of re-establishing a hyaline-like cartilage layer. The correlation between simulation and histology allows identification of mechanical factors that appear to have a larger impact on the healing of osteochondral defects than previously considered.     

A numerical approach to the biomechanical analysis of bone fracture healing

Investigations are reported in the literature, by means of experimental, analytical and numerical methods, concerning the biomechanical properties of bone. However, the evolutionary phenomena of bone fracture healing does not have a large reference literature. This work investigates and describes the behaviour of inclined human femur fractures with external fixation up to complete healing. A numerical formulation based on the finite element method has been adopted. Geometric configuration is defined using data from a magnetic resonance process applied to a femur in vivo. A three dimensional model has been developed by adopting an orthotropic material law for cortical bone and an isotropic law for the fracture gap zone. Stress and strain reponses of the bone and fixation device are investigated with reference to the evolutionary behaviour of the healing tissue.     

Determination of stress levels and profiles in the periodontal ligament by means of an improved three-dimensional finite element model for various types of orthodontic and natural force systems

This study was undertaken to investigate the stress-strain levels and distribution within the periodontal ligament for various types of physiological and orthodontic force systems, assuming that the bone resorption process, leading to tooth movements, is partly controlled by those conditions. Two finite element models were developed, simulating a full and partial mandibular morphology, respectively. Both models were based on morphology and physical parameters of human autopsy material. The effect of changing material parameters and structure, type of boundary conditions, calculation method and fineness of the model on the stress levels and profiles in the periodontal ligament was evaluated by a series of tests. A structure optimization technique was used to investigate the load bearing characteristics of the mandible and the influence of the anisotropic material properties of both the mandible and the segment. A ‘multiple modelling’ technique based on both the mandible and the segment was developed to test various types of boundary conditions in the analysis of the segment. Results presented as ‘stress profiles’ showing the correlation between the applied force system and the stress distribution in the periodontal ligament, based on the improved finite element models, were established.     

Comparison between complete denture and implant‐retained overdenture: effect of different mucosa thickness and resiliency on stress distribution

Background: The effect of different mucosa characteristics on stress distribution of complete dentures and overdentures remains unknown. Objective: The aim of this study was to evaluate the effect of different mucosa thickness and resiliency on the stress distribution of complete dentures and implant‐retained overdentures using a two‐dimensional finite element analysis. Material and methods: Representative models of the edentulous mandible were constructed on AutoCAD software according to the groups’ characteristics. In group CD, a model of the edentulous mandible supporting a complete denture was obtained while in group IO, a model of edentulous mandible supporting an overdenture over two unsplinted implants with an o’ ring system was constructed. In each group, mucosa assumed three characteristics of thickness (1, 3 and 5 mm) corresponding to the resiliencies hard, resilient and soft respectively. Evaluation was performed on Ansys software with 100N vertical load applied on central incisor teeth. The principal stress was used as analysis criteria. Results: Group IO showed higher stress values than group CD regardless of mucosal thickness and resiliency. Stress decreased at the supporting tissues in both groups as the thickness and resiliency of mucosa increased. In relation to the supporting tissues, cortical bone showed the highest stress values. Conclusion: It was concluded that the use of an attachment system increases stress values and the thickness and resiliency of mucosa influence more on these values.     

Numerical investigation of mechanical effects caused by various fixation positions on a new radius intramedullary nail

Fracture of the radius diaphysis is an unusual injury in adults. Open reduction and plate osteosynthesis has been recommended by most of the authors. However, this trend has started to change with the recent introduction of newly designed interlocking intramedullary (IM) nails. New generation of IM nails are developed in order to utilise the advantages of IM nails against plates. Because of its anatomical structure, the radius bone has a complex geometry. Therefore, the callus structure, which forms during the healing period, should not be affected from external effects, such as excessive loads or motion. In this study, effects of radial styloid process (RSP), dorsal side and ulnar notch edge fixations of a new design radius IM nail on the healing period were numerically investigated. A three-dimensional solid model of radius was obtained from computed tomography images of a volunteer and callus structure model, was placed accordingly and different fixations of implants were performed. The models were analysed under axial loads transferring from the wrist to the radius bone using finite element method. As a result of the analysis, fixation of IM nail from RSP was found to be beneficial on healing period in terms of both callus motion and emerging stresses.     

The use of miniplates in craniomaxillofacial surgery

Miniplates were used in craniomaxillofacial surgery for fixation in the skull, maxilla, and/or mandible in 74 patients with minimal or no intermaxillary wiring. Procedures included forehead and orbital repositioning, frontofacial advancement, Le Fort III and particularly Le Fort I osteotomies, as well as mandibular osteotomies and fracture repair. The miniplates provided stable fixation and, compared with other techniques, improved airway safety. The complication rate was low: there were no infections, but two plates (1 percent) became exposed in the buccal sulcus. Although application of miniplates lengthened surgery and increased the cost of the procedure, the savings in intensive care monitoring more than offset these costs. The stability of fixation minimizes the opportunity to reposition the fragments postoperatively with training elastics. Therefore, meticulous technique is mandatory, with particular emphasis on passive fitting of the plates and precise drilling of screw holes.     

The shape of the mandible in the domestic sheep: a biomechanical analysis using EMG as an estimator of muscle force

Analysis of the maximal loads to which a skeletal element is subjected in vivo offers attractive possibilities of explaining the shape of the element. The underlying assumption is that the element will be constructed in such a way that deformations (strains) which result from mechanical stress will not exceed certain limits and that stresses will be evenly distributed. The sheep mandible shows a number of characteristic morphological features that invite this kind of explanation. We investigated the patterns of activity of the masticatory musculature by multichannel electromyography, expressing the activity of any particular muscle during a given interval as a percentage of the highest activity recorded for the muscle in question. In combination with data on the physiological cross sections of the muscles, which provide indications of the maximal forces which can be exerted by the muscles, three-dimensional patterns of relative muscular forces acting on the mandible can be constructed for successive stages of a masticatory cycle. No absolute forces can be measured or even estimated by this technique. A two-dimensional finite element model of the mandible was designed, by means of which predictions of stress and strain resulting from the muscular loading can be made. Calculations were based on the highest loads that occurred, during the power stroke of rumination. It is concluded that mechanical loading of the mandible provides a partial explanation of the form, and that a more satisfactory model should include other than purely mechanical influences.     

Biomechanical response of the human mandible to impacts of the chin

The purpose of this study was to determine the force-time and force-displacement response of the human mandible under direct loading at the chin. Sub-fracture response of the mandible and temporomandibular joint (TMJ) were analyzed from 10 cadavers that were impacted at the chin with a 2.8-kg mass at drop heights of 300, 400 and 500 mm and a 5.2-kg mass at 500 mm. Motion of radio-opaque markers adhered to the surface of the bone was recorded at 1000 Hz by a bi-planar X-ray and converted to three-dimensional coordinates. Peak force ranged from 0.90 to 4.54 kN causing chin displacement of 1.2-4.4 mm. A bi-linear response was observed with stiffness of 475.1+/-199.8 kN/m for chin displacement resulting from loading up to 0.6 kN and 2381.6+/-495.7 kN/m for loads from 0.6 to 3.25 kN. This defines the biomechanical response of the mandible for chin motion under impact loading. The response of different segments of the mandible and TMJ are also documented. Force-time and force-displacement response corridors for the mandible can be used for finite element model and/or the development and validation of a biomechanical surrogate.     

Numerical estimation of bone density and elastic constants distribution in a human mandible

In this paper, we try to predict the distribution of bone density and elastic constants in a human mandible, based on the stress level produced by mastication loads using a mathematical model of bone remodelling. These magnitudes are needed to build finite element models for the simulation of the mandible mechanical behavior. Such a model is intended for use in future studies of the stability of implant-supported dental prostheses. Various models of internal bone remodelling, both phenomenological and more recently mechanobiological, have been developed to determine the relation between bone density and the stress level that bone supports. Among the phenomenological models, there are only a few that are also able to reproduce the level of anisotropy. These latter have been successfully applied to long bones, primarily the femur. One of these models is here applied to the human mandible, whose corpus behaves as a long bone. The results of bone density distribution and level of anisotropy in different parts of the mandible have been compared with various clinical studies, with a reasonable level of agreement.     

Mechanics of the right whale mandible: Full scale testing and finite element analysis

In an effort to better understand the mechanics of ship-whale collision and to reduce the associated mortality of the critically endangered North Atlantic right whale, a comprehensive biomechanical study has been conducted by the Woods Hole Oceanographic Institution and the University of New Hampshire. The goal of the study is to develop a numerical modeling tool to predict the forces and stresses during impact and thereby the resulting mortality risk to whales from ship strikes.Based on post-mortem examinations, jaw fracture was chosen as a fatal endpoint for the whales hit by a vessel. In this paper we investigate the overall mechanical behavior of a right whale mandible under transverse loading and develop a finite element analysis model of the bone. The equivalent elastic modulus of the cortical component of right whale mandible is found by comparing full-scale bending tests with the results of numerical modeling. The finite element model of the mandible can be used in conjunction with a vessel-whale collision event model to predict bone fracture for various ship strike scenarios.     

A method for material parameter determination for the human mandible based on simulation and experiment

In cranio-maxillofacial surgery planning and implant design, it is important to know the elastic response of the mandible to load forces as they occur, e.g., in biting. The goal of the present study is to provide a method for a quantitative determination of material parameters for the human jaw bone, whose values can, e.g., be used to devise a prototype plastic model for the mandible. Non-destructive load experiments are performed on a cadaveric mandible using a specially designed test bed. The identical physiological situation is simulated in a computer program. The underlying mathematical model is based on a two component, linear elastic material law. The numerical realization of the model, difficult due to the complex geometry and morphology of the mandible, is via the finite element (FE) method. Combining the validated simulation with the results of the tests, an inverse problem for the determination of Young's modulus and the Poisson ratio of both cortical and cancellous bone can then be solved.     

A Method for Material Parameter Determination for the Human Mandible Based on Simulation and Experiment

In cranio-maxillofacial surgery planning and implant design, it is important to know the elastic response of the mandible to load forces as they occur, e.g., in biting. The goal of the present study is to provide a method for a quantitative determination of material parameters for the human jaw bone, whose values can, e.g., be used to devise a prototype plastic model for the mandible. Non-destructive load experiments are performed on a cadaveric mandible using a specially designed test bed. The identical physiological situation is simulated in a computer program. The underlying mathematical model is based on a two component, linear elastic material law. The numerical realization of the model, difficult due to the complex geometry and morphology of the mandible, is via the finite element (FE) method. Combining the validated simulation with the results of the tests, an inverse problem for the determination of Young's modulus and the Poisson ratio of both cortical and cancellous bone can then be solved.     

A 3d-fem Simulation of Highest Stress Lines in Mandible Fractures by Elastic Impact

In this article, the more usual mandible fracture areas are located by identifying the highest stress lines using a three-dimensional (tetrahedral) finite element method. By taking into account the temporomandibular contact and the inertia effects, the mathematical model is considered to be a dynamic Signorini's problem, that is, a dynamic variational inequality which is discretized in time following Newmark's method. So, in each time step a stationary variational inequality is solved by a penalty-duality algorithm. Finally, some numerical results obtained by simulating the more usual fractures in the human mandible are presented and compared with clinical experimental information.     

Fibula free flap: a new method of mandible reconstruction

The fibula was investigated as a donor site for free-flap mandible reconstruction. It has the advantages of consistent shape, ample length, distant location to allow a two-team approach, and low donor-site morbidity. It can be raised with a skin island for composite-tissue reconstruction. Twelve segmental mandibular defects (average 13.5 cm) were reconstructed following resection for tumor, most commonly epidermoid carcinoma. Five defects consisted of bone alone, and four others had only a small amount of associated intraoral soft-tissue loss. Eleven patients underwent primary reconstructions. At least two osteotomies were performed on each graft, and miniplates were used for fixation in 11 patients. Six patients received postoperative radiation, and two patients received postoperative chemotherapy. The flaps survived in all patients. All osteotomies healed primarily. The septocutaneous blood supply was generally not adequate to support a skin island for intraoral soft-tissue replacement. The aesthetic result of the reconstruction was excellent in most patients, particularly in "bone only" defects. There was no long-term donor-site morbidity.     

Mandibular stiffness in humans: numerical predictions

The chin is a feature unique to humans. This study evaluates the effect of mandibular symphyseal design on biomechanical masticatory effectiveness as determined by structural stiffness and stress developed under flexural and torsional loading. A simple model of three symphyseal shapes (chin, flat symphysis and lingual buttress), was built to represent human, Neanderthal and higher primate symphyses and these were subjected to wishboning and torsional forces. Additionally, an anatomically detailed reconstruction was made of the CT scan of an actual human mandible, which was then also morphed into a chinless model. The results of a 3-D finite element analysis show firstly, that none of the three different symphyseal shapes is biomechanically more advantageous than the others for the given loading condition. Secondly, we show in a CT-derived model, that the presence of a chin does not confer significantly improved stiffness to torsional or flexural loading. These results indicate that the acquisition of a chin in modern humans is not related to the functional demands placed upon the mandible during mastication, but suggest that it may have developed in response to other biomechanical demands.     

Determination of expander apparatus displacements and contact pressures on the mucosa using FEM modelling considering mandibular asymmetries

This paper presents a method for prediction of forces and displacements in the expansion screw of a modified mandibular Schwarz appliance and the contact pressure distributions on the mucosa during malocclusions treatment. A 3D finite element biomechanical model of the complete mandible–mucosa–apparatus set was built using computerised tomographic images of a patient's mandible and constructive solid geometry by computer software. An iterative procedure was developed to handle a boundary condition that takes into account the mandibular asymmetries. The results showed asymmetries in the contact pressure distributions that indicated with precision the patient's malocclusion diagnosis. In vivo measurements of contact pressure using piezoelectric sensors agreed with the computational results. It was shown that the left and right ends of the expansion screw move differently with respect to the patient mandible, even though the expansion screw has an opening mechanism to ensure equal stretching at both ends. The contact pressures between the apparatus and the mucosa vary linearly with applied forces, which can simplify the analysis of the biomechanical behaviour of the expander mandible apparatus. The biomechanical modelling proposed in this paper can be a useful tool to improve malocclusions treatment, safely avoiding the use of forces acting on live structures beyond the biological tolerance, which could result in traumatic effects.