M. Mesnard

Straight, semi-anatomic and anatomic TMJ implants: The influence of condylar geometry and bone fixation screws

A 3D finite element model of in vitro intact and implanted mandibles with different temporomandibular joints (TMJ) was analyzed. Three TMJ implant geometries were assessed. The displacements, stress and strain fields on the condyle were obtained for both simulated cases. Strains were also assessed near the screws that fixate the implant to the mandible. The geometry of the mandible was obtained through 3D digitalization of a synthetic model. The TMJ implants studied were modelled considering a commercial implant which was also used to create semi-anatomic and anatomic implants that were analyzed and to assess the influence of the geometry. Numerical finite element models were built and the implants were positioned by an experienced orofacial surgeon. All implants were fixed by four screws which were placed in the same position on the mandible. The boundary conditions were simulated considering the support on the incisive tooth, the loads of the five most important muscular forces and a 5mm mouth aperture. This study indicates that the deformation on the intact mandible was similar when an anatomic implant was considered in the implanted mandible. However, the anatomic geometry presented some problems concerning the implant integrity due to geometric variations. The geometry of TMJ implant also played a role relatively to the screws structural integration and bone fixation. The geometry of TMJ implant defines the necessary number of screws and position in the mandible fixation.

Biomechanical Analysis Comparing Natural and Alloplastic Temporomandibular Joint Replacement Using a Finite Element Model

PURPOSE Prosthetic materials and bone present quite different mechanical properties. Consequently, mandible reconstruction with metallic materials (or a mandible condyle implant) modifies the physiologic behavior of the mandible (stress, strain patterns, and condyle displacements). The changing of bone strain distribution results in an adaptation of the temporomandibular joint, including articular contacts. MATERIALS AND METHODS Using a validated finite element model, the natural mandible strains and condyle displacements were evaluated. Modifications of strains and displacements were then assessed for 2 different temporomandibular joint implants. Because materials and geometry play important key roles, mechanical properties of cortical bone were taken into account in models used in finite element analysis. RESULTS The finite element model allowed verification of the worst loading configuration of the mandibular condyle. Replacing the natural condyle by 1 of the 2 tested implants, the results also show the importance of the implant geometry concerning biomechanical mandibular behavior. The implant geometry and stiffness influenced mainly strain distribution. CONCLUSION The different forces applied to the mandible by the elevator muscles, teeth, and joint loads indicate that the finite element model is a relevant tool to optimize implant geometry or, in a subsequent study, to choose a more suitable distribution of the screws. Bone screws (number and position) have a significant influence on mandibular behavior and on implant stress pattern. Stress concentration and implant fracture must be avoided.

Relationships between geometry and kinematic characteristics in the temporomandibular joint

Motions of the temporomandibular joint (TMJ) involve both translation and rotation; however, there may be substantial variations from one human to another, and these variations present significant difficulties when designing TMJ prostheses. The disc–condyle glides along the temporal bone and the condyle centre describe a curve that depends on the individual morphology. This study analyses disc–condyle rotatory and translatory displacements moving all along the temporal bone facets which are mainly composed of two areas: the articular tubercle slope (ATS) and the preglenoid plane separated by the articular tubercle crest. Displacements were quantified using 3D video analysis, and this technique was computer-assisted. From a population of 32 volunteers, we were able to establish a correlation between the kinematic characteristics of the joint and the disc–condyle trajectories. This study quantifies the geometrical characteristics of the ATS and their inter-individual variations, which are useful in TMJ prosthesis design.

Stress Analysis of Temporomandibular Joint Disc During Maintained Clenching Using a Viscohyperelastic Finite Element Model

PURPOSE People with bruxism exert parafunctional grinding and clenching activities. Those habits are suspected to be associated with temporomandibular disorder development. The aim of this study was to analyze the behavior of the temporomandibular joint disc under maintained clenching. MATERIALS AND METHODS For this analysis, a viscohyperelastic finite element model was used. The model included half the mandible, the left disc, and the left temporal bone and used muscular efforts as loading conditions. The viscohyperelastic properties of the disc were based on literature data from asymptomatic human cadaveric disc specimens. RESULTS Stresses in the disc decreased slightly (<15%) after 10 seconds of maintained clenching. In contrast, strains increased in nearly all disc regions, with the maximum (33%) in the lateral part of the disc. The greatest creep strain (-0.1) also was found in the lateral part. CONCLUSION Results suggest that maintained clenching leads to an increase in strains in the entire disc and to greater creep strain in the lateral part. This may be related to disc damage.

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.

Discrimination of objective kinematic characters in temporomandibular joint displacements.

Designing a temporomandibular joint (TMJ) total prosthesis requires the assessment of joint displacements for open/close movements. Current knowledge presents disc-condyle motions as involving both translation and rotation but there may be substantial variations from human to human. The aim of this study is to discriminate objective kinematic characters amongst thirty-two volunteers. The displacements are determined using 3D video analysis. The ratio between rotation and translation can be defined by introducing a coefficient. This coefficient varies relatively to the opening amplitude and presents the same dispersion rate whatever the variations. Then it allows to discriminate amongst volunteers, regardless of any jaw opening values. Three groups can be isolated relatively to three kinematic models: a translatory preponderant group, a common group and a rotatory preponderant group. All subjects in the first group present concomitant rotatory/translatory displacements up to maximal opening. The other two groups present variations due to different quasi-pure rotation phases at the end of the opening movement. These investigations will make it possible to establish a correlation between the kinematic characters and the disc-condyle trajectories. The disc-condyle glides along the temporal facet and the condyle centre describes the tubercular morphology. The temporal facet geometry, useful for the TMJ prosthesis design, will be studied in a next paper.

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.

Prediction at long-term condyle screw fixation of temporomandibular joint implant: A numerical study

The fixation of commercial temporomandibular joint (TMJ) implant is accomplished by using screws, which, in some cases, can lead to loosening of the implant. The aim of this study was to predict the evolution of fixation success of a TMJ. Numerical models using a Christensen TMJ implant were developed to analyze strain distributions in the adjacent mandibular bone. The geometry of a human mandible was developed based on computed tomography (CT) scans from a cadaveric mandible on which a TMJ implant was subsequently placed. In this study, the five most important muscle forces acting were applied and the anatomical conditions replicated. The evolution of fixation was defined according to bone response methodology focused in strain distribution around the screws. Strain and micromotions were analyzed to evaluate implant stability, and the evolution process conduct at three different stages: start with all nine screws in place (initial stage); middle stage, with three screws removed (middle stage), and end stage, with only three screws in place (final stage). With regard to loosening, the implant success fixation changed the strains in the bone between 21% and 30%, when considering the last stage. The most important screw positions were #1, #7, and #9. It was observed that, despite the commercial Christensen TMJ implant providing nine screw positions for fixation, only three screws were necessary to ensure implant stability and fixation success.

The stock alloplastic temporomandibular joint implant can influence the behavior of the opposite native joint: A numerical study

PURPOSE The objective of the study was to investigate the effect of total stock temporomandibular implants on load mechanisms in both condyles in a specific patient. The patient presented with a disc with wear, and the introduction of a total temporomandibular prosthesis was simulated to compare the articular behavior. MATERIAL AND METHODS Based on specific patient computed tomographic images, two finite element models were created: one model with two intact temporomandibular joints (one joint with pathology), and other model with one implanted joint. The simulations considered the five most important muscles acting in the mandible, and it was possible to evaluate the biomechanical changes in the structures (skull, mandible, and articular disc). RESULTS The results revealed more load transfer in the opposite condyle than in the damaged one; the insertion of a total temporomandibular implant changed the load transfer to the opposite condyle. There was decreased stress in the disc by about 50% and increased strain distribution. In the mandibular condyle with implant, the screw fixation is critical, with minimum strain around -9430 με for first screw position. In the cranium, the implant changed the bone strains with a minimum principal strain observed around -2500 με in six screw positions. CONCLUSION This study indicates that replacing the damaged joint by an implant in an ideal position will improve joint position and consequently redistribute the loads. The study findings provide strong evidence that placing an implant on one side of the mandible will affect the load distribution on that structure and particularly on the opposite side. The temporomandibular joint changes condyle movement; with an implanted condyle, the movement is almost blocked.

Comparison of load transfers in TMJ replacement using a standard and a custom-made temporal component

PURPOSE The temporomandibular joint (TMJ) is a complex articulation and depending on the available prosthesis models, the ultimate solution for mechanical improvements is a very late total joint replacement (TJR). The objective of the present study is to analyse the importance of the geometry of the fossa component with respect to the load transfer. METHODS Two finite element models were analysed, a Christensen standard fossa component and a custom-made fossa component, using the same commercial condyle geometry and screw fixation. The biomechanical behaviour of components was analysed only for a 5 mm mouth aperture in incisive teeth. RESULTS Geometry was seen to influence strain distribution in the condyle and the fossa. Maximum strain was observed in the screw fixation in the cranium around screws for the Christensen and for the custom-made fossa but in other position. The fossa component has some rotation in commercial models, but both components revealed lower potential for bone integration with maximum micromovements of around 40 μm. CONCLUSION The study demonstrates the importance of the geometry of the fossa component as it changes the load transfer in the mandibular condyle and the strain distribution near the screws. The screw positions in the fossa component are influenced by the fossa geometry.