The effect of collagen reinforcement in the behaviour of the temporomandibular joint disc

In this paper, the influence of collagen fibres in the behaviour of the temporomandibular joint disc is studied. A three-dimensional finite element model of the joint is developed from a set of medical images. The model comprises the mandible, part of the cranium and both temporomandibular joints. Joints have been considered to be composed of the articular discs and the temporomandibular ligaments. A fibre-reinforced porohyperelastic model was used to study the response under clenching of the fibrocartilage that composes the articular disc. This was divided in an intermediate zone, and two bands, an anterior and other posterior, in order to define the orientation of collagen fibres. The study demonstrates that the introduction of collagen fibres in the biphasic behaviour of the articular disc implies for a prescribed displacement not only an increase of the pressurization in the tissue, but also higher stresses in the anterior and posterior bands, as well as in the lateral zone of the disc. Thus, modelling the disc as an isotropic solid matrix leads in this case to an overestimation of the stresses in the intermediate zone, an underestimation of the pore pressure in this area, and an underestimation of the stresses in the rest of the disc.     

The temporomandibular joint disc of Asian elephant (Elephas maximus) and African elephant (Loxodonta africana)

The temporomandibular joint (TMJ) is a synovial articulation between the mandibular head of the condylar process of the mandible and the mandibular fossa of the squamous temporal bone. Extensions of the fibrous capsule into the joint space form a biconcave disc that functions as an articulating surface and divides the joint into dorsal and ventral compartments. The TMJ disc plays a crucial role in normal functioning of the joint, and differences in cranial morphology, mastication patterns, and diet are reflected in the material and biochemical properties of the disc. The purpose of the present case study was to compare the regional histologic differences between two elephant genera and quantify the biochemical and biomechanical properties of the African elephant disc. This study provides a unique insight into the elephant TMJ disc and also provides a comparison between the African and the Asian elephant genera. The results demonstrate several remarkable findings. First, structure–function relationships exist within the elephant TMJ disc. Second, regional variations exist in the elephant TMJ disc, and these are likely to correlate with its functional requirement. Additionally, it is apparent that some properties of the disc vary with the specific anatomy, diet requirement, and jaw motion. Finally, in comparison with the TMJ disc of other species, it is clear that, although the elephant disc is unique, it has properties that transcend and are preserved among the species.     

The regional contribution of glycosaminoglycans to temporomandibular joint disc compressive properties

Understanding structure-function relationships in the temporomandibular joint (TMJ) disc is a critical first step toward creating functional tissue replacements for the large population of patients suffering from TMJ disc disorders. While many of these relationships have been identified for the collagenous fraction of the disc, this same understanding is lacking for the next most abundant extracellular matrix component, sulfated glycosaminoglycans (GAGs). Though GAGs are known to play a major role in maintaining compressive integrity in GAG-rich tissues such as articular cartilage, their role in fibrocartilaginous tissues in which GAGs are much less abundant is not clearly defined. Therefore, this study investigates the contribution of GAGs to the regional viscoelastic compressive properties of the temporomandibular joint (TMJ) disc. Chondroitinase ABC (C-ABC) was used to deplete GAGs in five different disc regions, and the time course for >95% GAG removal was defined. The compressive properties of GAG depleted regional specimens were then compared to non-treated controls using an unconfined compression stress-relaxation test. Additionally, treated and non-treated specimens were assayed biochemically and histologically to confirm GAG removal. Compared to untreated controls, the only regions affected by GAG removal in terms of biomechanical properties were in the intermediate zone, the most GAG-rich portion of the disc. Without GAGs, all intermediate zone regions showed decreased tissue viscosity, and the intermediate zone lateral region also showed a 12.5% decrease in modulus of relaxation. However, in the anterior and posterior band regions, no change in compressive properties was observed following GAG depletion, though these regions showed the highest compressive properties overall. Although GAGs are not the major extracellular matrix molecule of the TMJ disc, they are responsible for some of the viscoelastic compressive properties of the tissue. Furthermore, the mechanical role of sulfated GAGs in the disc varies regionally in the tissue, and GAG abundance does not always correlate with higher compressive properties. Overall, this study found that sulfated GAGs are important to TMJ disc mechanics in the intermediate zone, an important finding for establishing design characteristics for future tissue engineering efforts.     

Morphology of the meniscotemporal part of the temporomandibular joint and its biomechanical implications

The temporomandibular joint is a diarthrosis. It has classically been referred to as being a condylar, and functionally as a double condylar joint. We have used the moiré topographical technique to study the morphology of the pars suprameniscalis or meniscotemporalis of the joint. The findings show that this pars suprameniscalis could be classified as a saddle joint. The biomechanical implications are also discussed.     

Tensile properties of the porcine temporomandibular joint disc

Despite the significant morbidity associated with the temporomandibular joint (TMJ), little is known about the pathophysiology of this complex joint. TMJ disc degeneration plays a central role in the progression of TMJ disorders, and therefore disc regeneration would be a crucial treatment modality. Unfortunately, scarce information about the structural and functional characteristics of the TMJ disc is available. The current study aims to provide a standard for the biomechanical behavior of the TMJ disc for future tissue engineering studies. The disc was loaded under uniaxial tension in two directions, mediolateral and anteroposterior, and in three locations per direction. In the mediolateral direction, the posterior band was 2.5 times stiffer, 2.4 times tougher (energy to maximum stress), and 2.2 times stronger than the anterior band, which was in turn 16 times stiffer and 5.7 times stronger than the intermediate zone. In the anteroposterior direction, the central and medial regions were 74% and 35% stiffer and 56% and 59% stronger than the lateral region, respectively, although similar to each other in strength and stiffness. There was no significant difference in toughness between regions in the anteroposterior direction. These results correlated qualitatively with collagen fiber orientation and fiber size obtained using polarized light microscopy.     

Tensile properties of the mandibular condylar cartilage

Mandibular condylar cartilage plays a crucial role in temporomandibular joint (TMJ) function, which includes facilitating articulation with the temporomandibular joint disc and reducing loads on the underlying bone. The cartilage experiences considerable tensile forces due to direct compression and shear. However, only scarce information is available about its tensile properties. The present study aims to quantify the biomechanical characteristics of the mandibular condylar cartilage to aid future three-dimensional finite element modeling and tissue engineering studies. Porcine condylar cartilage was tested under uniaxial tension in two directions, anteroposterior and mediolateral, with three regions per direction. Stress relaxation behavior was modeled using the Kelvin model and a second-order generalized Kelvin model, and collagen fiber orientation was determined by polarized light microscopy. The stress relaxation behavior of the tissue was biexponential in nature. The tissue exhibited greater stiffness in the anteroposterior direction than in the mediolateral direction as reflected by higher Young's (2.4 times), instantaneous (1.9 times), and relaxed (1.9 times) moduli. No significant differences were observed among the regional properties in either direction. The predominantly anteroposterior macroscopic fiber orientation in the fibrous zone of condylar cartilage correlated well with the biomechanical findings. The condylar cartilage appears to be less stiff and less anisotropic under tension than the anatomically and functionally related TMJ disc. The anisotropy of the condylar cartilage, as evidenced by tensile behavior and collagen fiber orientation, suggests that the shear environment of the TMJ exposes the condylar cartilage to predominantly but not exclusively anteroposterior loading.     

A Comparative Study between Use of Arthroscopic Lavage and Arthrocentesis of Temporomandibular Joint Based on Computational Fluid Dynamics Analysis

Arthroscopic lavage and arthrocentesis, performed with different inner-diameter lavage needles, are the current minimally invasive techniques used in temporomandibular joint disc displacement (TMJ-DD) for pain reduction and functional improvement. In the current study, we aimed to explore the biomechanical influence and explain the diverse clinical outcomes of these two approaches with computational fluid dynamics. Data was retrospectively analyzed from 78 cases that had undergone arthroscopic lavage or arthrocentesis for TMJ-DD from 2002 to 2010. Four types of finite volume models, featuring irrigation needles of different diameters, were constructed based on computed tomography images. We investigated the flow pattern and pressure distribution of lavage fluid secondary to caliber-varying needles. Our results demonstrated that the size of outflow portal was the critical factor in determining irrigated flow rate, with a larger inflow portal and a smaller outflow portal leading to higher intra-articular pressure. This was consistent with clinical data suggesting that increasing the mouth opening and maximal contra-lateral movement led to better outcomes following arthroscopic lavage. The findings of this study could be useful for choosing the lavage apparatus according to the main complaint of pain, or limited mouth opening, and examination of joint movements.     

Biomechanical properties of murine TMJ articular disc and condyle cartilage via AFM-nanoindentation

This study aims to quantify the biomechanical properties of murine temporomandibular joint (TMJ) articular disc and condyle cartilage using AFM-nanoindentation. For skeletally mature, 3-month old mice, the surface of condyle cartilage was found to be significantly stiffer (306 ± 84 kPa, mean ± 95% CI) than those of the superior (85 ± 23 kPa) and inferior (45 ± 12 kPa) sides of the articular disc. On the disc surface, significant heterogeneity was also detected across multiple anatomical sites, with the posterior end being the stiffest and central region being the softest. Using SEM, this study also found that the surfaces of disc are composed of anteroposteriorly oriented collagen fibers, which are sporadically covered by thinner random fibrils. Such fibrous nature results in both an F-D^3/2 indentation response, which is a typical Hertzian response for soft continuum tissue under a spherical tip, and a linear F-D response, which is typical for fibrous tissues, further signifying the high degree of tissue heterogeneity. In comparison, the surface of condyle cartilage is dominated by thinner, randomly oriented collagen fibrils, leading to Hertzian-dominated indentation responses. As the first biomechanical study of murine TMJ, this work will provide a basis for future investigations of TMJ tissue development and osteoarthritis in various murine TMJ models.     

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.     

Biomechanical properties of the mandibular condylar cartilage and their relevance to the TMJ disc

Mandibular condylar cartilage plays a crucial role in temporomandibular joint (TMJ) function, which includes facilitating articulation with the TMJ disc, reducing loads on the underlying bone, and contributing to bone remodeling. To improve our understanding of the TMJ function in normal and pathological situations, accurate and validated three-dimensional (3-D) finite element models (FEMs) of the human TMJ may serve as valuable diagnostic tools as well as predictors of thresholds for tissue damage resulting from parafunctional activities and trauma. In this context, development of reliable biomechanical standards for condylar cartilage is crucial. Moreover, biomechanical characteristics of the native tissue are important design parameters for creating functional tissue-engineered replacements. Towards these goals, biomechanical characteristics of the condylar cartilage have been reviewed here, highlighting the structure–function correlations. Structurally, condylar cartilage, like the TMJ disc, exhibits zonal and topographical heterogeneity. Early structural investigations of the condylar cartilage have suggested that the tissue possesses a somewhat transversely isotropic orientation of collagen fibers in the fibrous zone. However, recent tensile and shear evaluations have reported a higher stiffness of the tissue in the anteroposterior direction than in the mediolateral direction, corresponding to an anisotropic fiber orientation comparable to the TMJ disc. In a few investigations, condylar cartilage under compression was found to be stiffer anteriorly than posteriorly. As with the TMJ disc, further compressive characterization is warranted. To draw inferences for human tissue using animal models, establishing stiffness–thickness correlations and regional evaluation of proteoglycan/glycosaminoglycan content may be essential. Efforts directed from the biomechanics community for the characterization of TMJ tissues will facilitate the development of reliable and accurate 3-D FEMs of the human TMJ.     

Quantitative analysis and comparative regional investigation of the extracellular matrix of the porcine temporomandibular joint disc.

Characterization of the extracellular matrix of the temporomandibular joint (TMJ) disc is crucial to advancing efforts in tissue engineering the disc. However, the current literature is incomplete and often contradictory in its attempts to describe the nature of the TMJ disc matrix. The aim of this study was to identify the variation of key matrix components along the three axes of the porcine disc using ELISAs to quantify these matrix components, immunohistochemistry to identify their regional distribution, and SEM to examine collagen fiber diameter and orientation. The overall GAG content of the TMJ disc (including the dermatan sulfate proteoglycans) was 5.3+/-1.2% of the dry weight. Chondroitin sulfate, which comprised 74% of this total GAG content, was 4.4, 8.2, and 164 times more abundant than dermatan sulfate proteoglycan, keratan sulfate, and hyaluronic acid, respectively. In general, these GAGs were most concentrated in the intermediate zone of the TMJ disc, appearing in dense clusters, and least concentrated in the posterior band. Additionally, chondroitin sulfate was more abundant medially than laterally. Collagen II was discovered in trace amounts, with higher relative amounts in the intermediate zone. Collagen fibers were observed to run primarily in a ring-like fashion around the periphery of the disc and anteroposteriorly through the intermediate zone, with a mean fiber diameter of 18+/-9 mum. Characterization studies of the TMJ disc, including prior biomechanical and cell studies along with the current study of the extracellular matrix, collectively reveal a distinct character of the intermediate zone of the disc compared to its anterior and posterior bands.     

Numerical simulation (FEM) of the human mandible: validation of the function of the masticatory muscles]

The article describes part of a research project aiming to develop a new modular software tool for the individual dynamic numerical simulation of the human mandible using the finite element method (FEM). Its planned use in the clinical setting makes it very important to validate the results of the simulations. Here, the function of the masticatory muscles is to be tested. On the basis of biomechanical data from the literature, standard movements, such as closing the mouth, forward movement, lateral movement or backward movement, were dynamically simulated. Apart from muscle activity, the movements of the mandible are defined by the temporomandibular joint. At present, translating the condylar dynamics to the simulation still poses problems. For this reason, therefore, simulations of the two extreme cases "fixed" and "force-free" condyles are compared. While in the case of fixed condyles, some of the movements could be reproduced either not at all or only weakly, in the case of force-free condyles, all standard movements were reproduced qualitatively, albeit without the guiding effect of the joint capsule or the articular disc.     

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.     

Finite element analysis of the temporomandibular joint during lateral excursions of the mandible

One of the most significant characteristics of the temporomandibular joint (TMJ) is that it is in fact composed of two joints. Several finite element simulations of the TMJ have been developed but none of them analysed the different responses of its two sides during nonsymmetrical movement. In this paper, a lateral excursion of the mandible was introduced and the biomechanical behaviour of both sides was studied. A three-dimensional finite element model of the joint comprising the bone components, both articular discs, and the temporomandibular ligaments was used. A fibre-reinforced porohyperelastic model was introduced to simulate the behaviour of the articular discs, taking into account the orientation of the fibres in each zone of these cartilage components. The mandible movement during its lateral excursion was introduced as the loading condition in the analysis. As a consequence of the movement asymmetry, the discs were subjected to different load distributions. It was observed that the maximal shear stresses were located in the lateral zone of both discs and that the lateral attachment of the ipsilateral condyle-disc complex suffered a large distortion, due to the compression of this disc against the inferior surface of the temporal bone. These results may be related with possible consequences of a common disorder called bruxism. Although it would be necessary to perform an exhaustive analysis of this disorder, including the contact forces between the teeth during grinding, it could be suggested that a continuous lateral movement of the jaw may lead to perforations of both discs in their lateral part and may damage the lateral attachments of the disc to the condyle.     

Numerical analysis of the influence of nucleus pulposus removal on the biomechanical behavior of a lumbar motion segment

Nucleus replacement was deemed to have therapeutic potential for patients with intervertebral disc herniation. However, whether a patient would benefit from nucleus replacement is technically unclear. This study aimed to investigate the influence of nucleus pulposus (NP) removal on the biomechanical behavior of a lumbar motion segment and to further explore a computational method of biomechanical characteristics of NP removal, which can evaluate the mechanical stability of pulposus replacement. We, respectively, reconstructed three types of models for a mildly herniated disc and three types of models for a severely herniated disc based on a L4–L5 segment finite element model with computed tomography image data from a healthy adult. First, the NP was removed from the herniated disc models, and the biomechanical behavior of NP removal was simulated. Second, the NP cavities were filled with an experimental material (Poisson's ratio = 0.3; elastic modulus = 3 MPa), and the biomechanical behavior of pulposus replacement was simulated. The simulations were carried out under the five loadings of axial compression, flexion, lateral bending, extension, and axial rotation. The changes of the four biomechanical characteristics, i.e. the rotation degree, the maximum stress in the annulus fibrosus (AF), joint facet contact forces, and the maximum disc deformation, were computed for all models. Experimental results showed that the rotation range, the maximum AF stress, and joint facet contact forces increased, and the maximum disc deformation decreased after NP removal, while they changed in the opposite way after the nucleus cavities were filled with the experimental material.     

Viscoelastic properties of the central region of porcine temporomandibular joint disc in shear stress-relaxation

In this study, shear relaxation properties of the porcine temporomandibular joint (TMJ) disc are investigated. Previous studies have shown that, in fatigue failure and damage of cartilage and fibrocartilage, shear loads could be one of the biggest contributors to the failure. The aim of the present study is to develop an evaluation method to study shear properties of the disc and to do a mathematical characterization of it. For the experiments, twelve porcine discs were used. Each disc was dissected from the TMJ and, then, static strain control tests were carried out to obtain the shear relaxation modulus for the central region of the discs. From the results, it was found that the disc presents a viscoelastic behavior under shear loads. Relaxation modulus decreased with time. Shear relaxation was 10% of the instantaneous stress, which implies that the viscous properties of the disc cannot be neglected. The present results lead to a better understanding of the discs mechanical behavior under realistic TMJ working conditions.     

TRAIL,DR5 and caspase 3-dependent apoptosis in vessels of diseased human temporomandibular joint disc. An immunohistochemical study

To evaluate the apoptosis involvement in the angiogenesis as a self-limiting process in patients with temporomandibular joint (TMJ) degenerated disc vessels, we assessed, by immunohistochemistry, the detection of TRAIL, its death receptor DR5 and caspase 3. TRAIL, its death receptor DR5 and caspase 3 expression were studied by immunohistochemistry in 15 TMJ discs displaced without reduction and in 4 unaffected discs. These apoptosis molecules were detected in the intima and media layers of newly formed vessels affected discs. In conclusion, vessels apoptosis activation in TMJ disc with ID could be regarded as a self-limiting process that try to leads to vessel regression; in this way an inhibition of angiogenic vessels may prove a key strategy in limiting pathological angiogenesis, by cutting off blood supply to tumors, or by reducing harmful inflammation.Key words: temporomandibular joint disc, vessels, apoptosis.     

Analysis of the open-closing movement of the human temporomandibular joint

By using X-ray cinemographic techniques and biomechanical methods, the open-closing movement of the human temporomandibular joint was investigated in 14 living subjects. The theory of the instantaneous center of rotation (IRC) of the mandibular movement is strongly recommended and the position and the tendency of the IRC are also roughly determined. Moreover, a two-step model of the joint is put forward.     

Growth factor effects on costal chondrocytes for tissue engineering fibrocartilage

Tissue-engineered fibrocartilage could become a feasible option for replacing tissues such as the knee meniscus or temporomandibular joint disc. This study employed five growth factors (insulin-like growth factor-I, transforming growth factor-beta1, epidermal growth factor, platelet-derived growth factor-BB, and basic fibroblast growth factor) in a scaffoldless approach with costal chondrocytes, attempting to improve biochemical and mechanical properties of engineered constructs. Samples were quantitatively assessed for total collagen, glycosaminoglycans, collagen type I, collagen type II, cells, compressive properties, and tensile properties at two time points. Most treated constructs had lower biomechanical and biochemical properties than the controls with no growth factors, suggesting a detrimental effect, but the treatment with insulin-like growth factor-I tended to improve the constructs. Additionally, the 6-week time point was consistently better than that at 3 weeks, with total collagen, glycosaminoglycans, and aggregate modulus doubling during this time. Further optimization of the time in culture and exogenous stimuli will be important in making a more functional replacement tissue.     

Effect of non-uniform thickness of samples in stress relaxation tests under unconfined compression of samples of articular discs

A precise information of the biomechanical properties of soft tissues is required to develop a suitable simulation model, with which the distribution of stress and strain in the complex structures can be estimated. Many soft tissues have been mechanically characterized by stress relaxation tests under unconfined or confined compression. In general, full-thickness samples are extracted to reduce the damage in the tissue as much as possible. However, it is not guaranteed that these samples have a uniform thickness or, in other words, planar parallel faces. In particular, in the articular disc of the temporomandibular joint, many studies can be found testing full-thickness samples for which that thickness is known to be non-uniform, while making the assumption of uniaxial stress state to extract the mechanical properties from those tests. That inaccuracy may have a strong influence in some cases and needs a profound revision. The main goal of this work is to quantify the error committed in that assumption and the influence of the variation of thickness on that error in a particular test: stress relaxation tests under unconfined compression. Based on this error and defining an allowable tolerance, a criterion is established to reject samples depending on their aspect ratio.