Roles of the Fibrous Superficial Zone in the Mechanical Behavior of TMJ Condylar Cartilage

Spatial Heterogeneity Directs Energy Dissipation in Condylar Fibrocartilage

Condylar fibrocartilage with structural and compositional heterogeneity can efficiently orchestrate load-bearing and energy dissipation, making the temporomandibular joint (TMJ) survive high occlusion loads for a prolonged lifetime. How the thin condylar fibrocartilage can achieve efficient energy dissipation to cushion enormous stresses remains an open question in biology and tissue engineering. Here, three distinct zones in the condylar fibrocartilage are identified by analyzing the components and structure from the macro-and microscale to the nanoscale. Specific proteins are highly expressed in each zone related to its mechanics. The heterogeneity of condylar fibrocartilage can direct energy dissipation through the nano-micron-macro gradient spatial scale, by atomic force microscope (AFM), nanoindentation, dynamic mechanical analyzer assay (DMA), and the corresponding energy dissipation mechanisms are exclusive for each distinct zone. This study reveals the significance of the heterogeneity of condylar fibrocartilage in mechanical behavior and provides new insights into the research methods for cartilage biomechanics and the design of energy-dissipative materials.

Regenerative Engineering of a Biphasic Patient-Fitted Temporomandibular Joint Condylar Prosthesis

Regenerative medicine approaches to restore the mandibular condyle of the temporomandibular joint (TMJ) may fill an unmet patient need. In this study, a method to implant an acellular regenerative TMJ prosthesis was developed for orthotopic implantation in a pilot goat study. The scaffold incorporated a porous, polycaprolactone-hydroxyapatite (PCL-HAp, 20wt% HAp) 3D printed condyle with a cartilage-matrix-containing hydrogel. A series of material characterizations was used to determine the structure, fluid transport, and mechanical properties of 3D printed PCL-HAp. To promote marrow uptake for cell seeding, a scaffold pore size of 152 ± 68 μm resulted in a whole blood transport initial velocity of 3.7 ± 1.2 mm·s-1 transported to the full 1 cm height. The Young's modulus of PCL was increased by 67% with the addition of HAp, resulting in a stiffness of 269 ± 20 MPa for etched PCL-HAp. In addition, the bending modulus increased by 2.06-fold with the addition of HAp to 470 MPa for PCL-HAp. The prosthesis design with an integrated hydrogel was compared with unoperated contralateral control and no-hydrogel group in a goat model for 6 months. A guide was used to make the condylectomy cut, and the TMJ disc was preserved. MicroCT assessment of bone suggested variable tissue responses with some regions of bone growth and loss, although more loss may have been exhibited by the hydrogel group than the no-hydrogel group. A benchtop load transmission test suggested that the prosthesis was not shielding load to the underlying bone. Although variable, signs of neocartilage formation were exhibited by Alcian blue and collagen II staining on the anterior, functional surface of the condyle. Overall, this study demonstrated signs of functional TMJ restoration with an acellular prosthesis. There were apparent limitations to continuous, reproducible bone formation, and stratified zonal cartilage regeneration. Future work may refine the prosthesis design for a regenerative TMJ prosthesis amenable to clinical translation.

Temporomandibular disorder: A previously unreported complication of chronic suppurative otitis media

OBJECTIVE

The aim of the current clinical study was to test the hypothesis that chronic suppurative otitis media (CSOM) might be significantly associated with signs of temporomandibular joint (TMJ) internal derangement.

METHODS

The study involved 79 patients with CSOM and 79 control subjects. The TMJ was clinically examined in both groups.

RESULTS

Signs of internal derangement of the TMJ(s) were found in 67.1% of CSOM patients versus 26.6% of control subjects (p = .001).

CONCLUSION

CSOM may be associated with the extension of the inflammatory process into the TMJ, thereby predisposing to internal derangement of the joint.

Chronic craniomandibular pain after craniotomy: A long-term clinical study

OBJECTIVE

Chronic craniomandibular/cervical pain and temporomandibular disorders have not been studied in patients who had a craniotomy several years previously. The aim of the current clinical work was to address these issues.

METHODS

A total group of 150 ambulant patients who had a previous craniotomy was subclassified according to whether or not the temporalis muscle was manipulated.

RESULTS

The average incidence of multiple subsite regional head and neck pain was 69.3% a number of years after a craniotomy. Evidence of internal derangement of the temporomandibular joint was significantly higher in the group that required manipulation of the temporalis muscle during the procedure.

CONCLUSION

The pattern of chronic craniomandibular/cervical pain experienced years after a craniotomy supports the brain neuromatrix theory of pain.

Expressions of parathyroid hormone-related protein (PTHrP) and parathyroid hormone receptor-1 (PTH1R) in the condylar cartilage of temporomandibular joint modulated by occlusal elevation

Background/purpose

Parathyroid hormone-related protein (PTHrP) is an important regulatory factor in the growth, development and remodeling of bone or cartilage, and acts through its sole receptor, parathyroid hormone receptor-1 (PTH1R). The present study aimed to research the expression changes of PTHrP, PTH1R and other relevant factors in condylar cartilage during the progress of temporomandibular joint osteoarthritis (TMJOA).

Materials and methods

The animal model of TMJOA was constructed by the "resin-modified method", and Sprague Dawley (SD) rats were euthanized at 2 weeks, 4 weeks, 6 weeks and 8 weeks after occlusal elevation. The histological changes of condylar cartilage were observed by X-ray, hematoxylin-eosin (HE) and safranine O-fast green (SO-FG) staining. The expressions of PTHrP, PTH1R, Ki67, Collagen II (Col II), Collagen X (Col X) and Caspase 3 in each group were detected by quantitative real-time polymerase chain reaction (qRT-PCR) and immunohistochemistry (IHC).

Results

TMJOA progression was time-dependent. In the experimental group, PTHrP expression was unimodal with a peak at 4 weeks, but PTH1R expression showed a decreasing trend. From 2 weeks to 8 weeks in the experimental group, Col X expression rather than Caspase 3 expression was negatively related to PTHrP's, which has no positive relation to Ki67 or Col II. These results demonstrated abnormal occlusal load may be an important pathogenic factor of TMJOA.

Conclusion

It may be one of the reasons of TMJOA progression that PTHrP can't play an effective role due to the low expression of PTH1R. PTHrP may be a direct factor regulating the hypertrophic differentiation of chondrocytes, but it does not directly regulate the proliferation and apoptosis of chondrocytes, and the realization of both regulatory effects may depend on the inhibition of hypertrophic differentiation.

A clinical study on the incidence of internal derangement of the temporomandibular joint following harvesting of temporalis fascia

OBJECTIVE

The aim of the current clinical study was to reveal whether harvesting of a temporalis fascia graft would be associated with a higher incidence of temporomandibular joint (TMJ) internal derangement.

METHODS

The study group involved 104 patients who had middle-ear operations, 67 of which involved harvesting of temporalis fascia and 37 that did not. The TMJs were clinically examined in each group.

RESULTS

The total incidence of internal derangement of the TMJ was significantly higher in the group that had temporalis fascia harvesting (79.1%), compared to the group that did not have temporalis fascia harvesting (29.7%), (p= 0.001).

CONCLUSION

Harvesting of temporalis fascia probably alters mandibular kinematics and predisposes to internal derangement of the TMJs.

Biomechanical evaluation of the unilateral crossbite on the asymmetrical development of the craniofacial complex. A mechano-morphological approach

BACKGROUND AND OBJECTIVE

The occlusion effect on the craniofacial development is a controversial topic that has attracted the interest of many researchers but that remains unclear, mainly due to the difficulties on measure its mechanical response experimentally. This mechano-morphological relationship of the craniofacial growth is often explained by the periosteal and capsular matrices of the functional matrix hypothesis (FMH); however, its outcomes have not been analytically demonstrated yet. This computational study aims, therefore, to analytically demonstrate the mechano-morphological relationship in the craniofacial development of children with unilateral crossbite (UXB) using the finite element (FE) method.

METHODS

The craniofacial complex asymmetry of ten children, five of whom exhibit UXB, was 3D-analysed and compared with the biomechanical response computed from a FE analysis of each patient's occlusion. Due to the complexity of the geometry and the multitude of contacts involved, the inherent limitations of the model were evaluated by comparing computed occlusal patterns with those recorded by an occlusal analysis on 3D printed copies.

RESULTS

Comparison's outcomes proved the reliability of our models with just a deviation error below 6% between both approaches. Out of validation process, computational results showed that the significant elongation of mandibular branch in the contralateral side could be related to the mandibular shift and increase of thickness on the crossed side, and particularly of the posterior region. These morphological changes could be associated with periodontal overpressure (>4.7 kPa) and mandibular over deformation (0.002 ε) in that side, in agreement with the periosteal matrix's principles. Furthermore, the maxilla's transversal narrowing and the elevation of the maxillary and zygomatic regions on the crossed side were statistically demonstrated and seem to be related with their respective micro displacements at occlusion, as accounted by their specific capsule matrices. Our results were consistent with those reported clinically and demonstrated analytically the mechano-morphological relationship of children's craniofacial development based on the FMH's functional matrices.

CONCLUSIONS

This study is a first step in the understanding of the occlusion's effect on the craniofacial development by computational methods. Our approach could help future engineers, researchers and clinicians to understand better the aetiology of some dental malocclusions and functional disorders improve the diagnosis or even predict the craniofacial development.

Depth-dependent patterns in shear modulus of temporomandibular joint cartilage correspond to tissue structure and anatomic location

To fully understand TMJ cartilage degeneration and appropriate repair mechanisms, it is critical to understand the native structure-mechanics relationships of TMJ cartilage and any local variation that may occur in the tissue. Here, we used confocal elastography and digital image correlation to measure the depth-dependent shear properties as well as the structural properties of TMJ cartilage at different anatomic locations on the condyle to identify depth-dependent changes in shear mechanics and structure. We found that samples at every anatomic location showed qualitatively similar shear modulus profiles as a function of depth. In every sample, four distinct zones of mechanical behavior were observed, with shear modulus values spanning 3-5 orders of magnitude across zones. However, quantitative characteristics of shear modulus profiles varied by anatomic location, particularly zone size and location, with the most significant variation in zonal width occurring in the fibrocartilage surface layer (zone 1). This anatomic variation suggests that different locations on the TMJ condyle may play unique mechanical roles in TMJ function. Furthermore, zones identified in the mechanical data corresponded on a sample-by-sample basis to zones identified in the structural data, indicating the known structural zones of TMJ cartilage may also play unique mechanical roles in TMJ function.

Type V Collagen Regulates the Structure and Biomechanics of TMJ Condylar Cartilage: A Fibrous-Hyaline Hybrid

This study queried the role of type V collagen in the post-natal growth of temporomandibular joint (TMJ) condylar cartilage, a hybrid tissue with a fibrocartilage layer covering a secondary hyaline cartilage layer. Integrating outcomes from histology, immunofluorescence imaging, electron microscopy and atomic force microscopy-based nanomechanical tests, we elucidated the impact of type V collagen reduction on TMJ condylar cartilage growth in the type V collagen haploinsufficiency and inducible knockout mice. Reduction of type V collagen led to significantly thickened collagen fibrils, decreased tissue modulus, reduced cell density and aberrant cell clustering in both the fibrous and hyaline layers. Post-natal growth of condylar cartilage involves the chondrogenesis of progenitor cells residing in the fibrous layer, which gives rise to the secondary hyaline layer. Loss of type V collagen resulted in reduced proliferation of these cells, suggesting a possible role of type V collagen in mediating the progenitor cell niche. When the knockout of type V collagen was induced in post-weaning mice after the start of physiologic TMJ loading, the hyaline layer exhibited pronounced thinning, supporting an interplay between type V collagen and occlusal loading in condylar cartilage growth. The phenotype in hyaline layer can thus be attributed to the impact of type V collagen on the mechanically regulated progenitor cell activities. In contrast, knee cartilage does not contain the progenitor cell population at post-natal stages, and develops normal structure and biomechanical properties with the loss of type V collagen. Therefore, in the TMJ, in addition to its established role in regulating the assembly of collagen I fibrils, type V collagen also impacts the mechanoregulation of progenitor cell activities in the fibrous layer. We expect such knowledge to establish a foundation for understanding condylar cartilage matrix development and regeneration, and to yield new insights into the TMJ symptoms in patients with classic Ehlers-Danlos syndrome, a genetic disease due to autosomal mutation of type V collagen.

Tissue engineered autologous cartilage-bone grafts for temporomandibular joint regeneration

Joint disorders can be detrimental to quality of life. There is an unmet need for precise functional reconstruction of native-like cartilage and bone tissues in the craniofacial space and particularly for the temporomandibular joint (TMJ). Current surgical methods suffer from lack of precision and comorbidities and frequently involve multiple operations. Studies have sought to improve craniofacial bone grafts without addressing the cartilage, which is essential to TMJ function. For the human-sized TMJ in the Yucatan minipig model, we engineered autologous, biologically, and anatomically matched cartilage-bone grafts for repairing the ramus-condyle unit (RCU), a geometrically intricate structure subjected to complex loading forces. Using image-guided micromilling, anatomically precise scaffolds were created from decellularized bone matrix and infused with autologous adipose-derived chondrogenic and osteogenic progenitor cells. The resulting constructs were cultured in a dual perfusion bioreactor for 5 weeks before implantation. Six months after implantation, the bioengineered RCUs maintained their predefined anatomical structure and regenerated full-thickness, stratified, and mechanically robust cartilage over the underlying bone, to a greater extent than either autologous bone-only engineered grafts or acellular scaffolds. Tracking of implanted cells and parallel bioreactor studies enabled additional insights into the progression of cartilage and bone regeneration. This study demonstrates the feasibility of TMJ regeneration using anatomically precise, autologous, living cartilage-bone grafts for functional, personalized total joint replacement. Inclusion of the adjacent tissues such as soft connective tissues and the TMJ disc could further extend the functional integration of engineered RCUs with the host.

Biomechanical impact of the porous-fibrous tissue behaviour in the temporomandibular joint movements. An in silico approach

The movement of the temporomandibular joint (TMJ) is a function of its complex geometry and its interaction with the surrounding soft tissues. Owing to an increase in the prevalence of temporomandibular joint disorders (TMDs), many computational studies have attempted to characterize its biomechanical behaviour in the last 2 decades. However, most such studies are based on a single computational model that markedly simplifies the complex geometry and mechanical properties of the TMJ's soft tissues. The present study aims to computationally evaluate in a wider sample the importance of considering their complex anatomy and behaviour for simulating both damping and motion responses of this joint. Hence, 6 finite element models of healthy volunteers' TMJ were developed and subjected to both conditions in two different behavioural scenarios. In one, the soft tissues' behaviour was modelled by considering the porous-fibrous properties, whereas in the other case they were simplified assuming isotropic-hyperelastic response, as had been traditionally considered. The damping analysis, which mimic the conditions of an experimental test of the literature, consisted of applying two different compressive loads to the jaw. The motion analysis evaluated the condylar path during the mandible centric depression by the action of muscular forces. From the results of both analyses, the contact pressures, intra-articular fluid pressure, path features, and stress/strain values were compared using the porous-fibrous and isotropic-hyperelastic models. Besides the great differences observed between patients due patient-specific morphology, the porous-fibrous approach yielded results closer to the reference experimental values and to the outcomes of other computational studies of the literature. Our findings underscore, therefore, the importance of considering realistic joint geometries and porous-fibrous contribution in the computational modelling of the TMJ, but also in the design of further joint replacements or in the development of new biomaterials for this joint.

The Use of Nanomaterials in Tissue Engineering for Cartilage Regeneration; Current Approaches and Future Perspectives

The repair and regeneration of articular cartilage represent important challenges for orthopedic investigators and surgeons worldwide due to its avascular, aneural structure, cellular arrangement, and dense extracellular structure. Although abundant efforts have been paid to provide tissue-engineered grafts, the use of therapeutically cell-based options for repairing cartilage remains unsolved in the clinic. Merging a clinical perspective with recent progress in nanotechnology can be helpful for developing efficient cartilage replacements. Nanomaterials, < 100 nm structural elements, can control different properties of materials by collecting them at nanometric sizes. The integration of nanomaterials holds promise in developing scaffolds that better simulate the extracellular matrix (ECM) environment of cartilage to enhance the interaction of scaffold with the cells and improve the functionality of the engineered-tissue construct. This technology not only can be used for the healing of focal defects but can also be used for extensive osteoarthritic degenerative alterations in the joint. In this review paper, we will emphasize the recent investigations of articular cartilage repair/regeneration via biomaterials. Also, the application of novel technologies and materials is discussed.

Recent Advances in Bioinspired Gel Surfaces with Superwettability and Special Adhesion

Engineering surface wettability is of great importance in academic research and practical applications. The exploration of hydrogel-based natural surfaces with superior properties has revealed new design principles of surface superwettability. Gels are composed of a cross-linked polymer network that traps numerous solvents through weak interactions. The natural fluidity of the trapped solvents confers the liquid-like property to gel surfaces, making them significantly different from solid surfaces. Bioinspired gel surfaces have shown promising applications in diverse fields. This work aims to summarize the fundamental understanding and emerging applications of bioinspired gel surfaces with superwettability and special adhesion. First, several typical hydrogel-based natural surfaces with superwettability and special adhesion are briefly introduced, followed by highlighting the unique properties and design principles of gel-based surfaces. Then, the superwettability and emerging applications of bioinspired gel surfaces, including liquid/liquid separation, antiadhesion of organisms and solids, and fabrication of thin polymer films, are presented in detail. Finally, an outlook on the future development of these novel gel surfaces is also provided.

Condylotomy to Reverse Temporomandibular Joint Osteoarthritis in Rabbits

PURPOSE

Osteoarthritis (OA) of the temporomandibular joint (TMJ) is characterized by local tissue degeneration and pain. Treatments have been aimed at reducing symptoms, and only some can interfere with the progression of pathophysiologic changes caused by OA. Condylotomy is an alternative treatment for patients with OA refractory to conventional treatment. The aim of the present animal study was to investigate the influence of condylotomy on TMJ OA in New Zealand rabbits.

MATERIALS AND METHODS

The 36 rabbits were divided into 3 groups: the osteoarthritis group (OG; induced OA), treatment group (TG; induced OA plus condylotomy), and control group (CG; neither OA nor surgery). OA was induced using intra-articular monosodium iodoacetate injection for 40 days. The rabbits in the TG underwent condylotomy and were killed 20, 40, and 60 days after treatment. The rabbits in the CG and OG were killed at the same points.

RESULTS

The articular joint condition was better in the TG 60 days after surgery (P = .032). A direct comparison revealed regression of TMJ OA over time among the treated rabbits (P = .008). Surgical treatment promoted mandibular condylar remodeling in the TG, reversing the tissue degeneration caused by OA.

CONCLUSIONS

Our findings suggest that condylotomy could be an option for the treatment of OA and prevent damage to TMJ structures. This could be of particular importance for patients without satisfactory responses to more conservative treatment.

Two-photon fluorescence and second harmonic generation characterization of extracellular matrix remodeling in post-injury murine temporomandibular joint osteoarthritis

End stage temporomandibular joint osteoarthritis (TMJ-OA) is characterized by fibrillations, fissures, clefts, and erosion of the mandibular condylar cartilage. The goal of this study was to define changes in pericellular and interterritorial delineations of the extracellular matrix (ECM) that occur preceding and concurrent with the development of this end stage degeneration in a murine surgical instability model. Two-photon fluorescence (TPF) and second harmonic generation (SHG) microscopy was used to evaluate TMJ-OA mediated changes in the ECM. We illustrate that TPF/SHG microscopy reconstructs the three-dimensional network of key fibrillar and micro-fibrillar collagens altered during the progression of TMJ-OA. This method not only generates spatially distinct pericellular and interterritorial delineations of the ECM but distinguishes early and end stage TMJ-OA by signal organization, orientation, and composition. Early stage TMJ-OA at 4- and 8-weeks post-injury is characterized by two structurally distinct regions containing dense, large fiber collagens and superficial, small fiber collagens rich in types I, III, and VI collagen oriented along the mesiodistal axis of the condyle. At 8-weeks post-injury, type VI collagen is locally diminished on the central and medial condyle, but the type I/III rich superficial layer is still present. Twelve- and 16-weeks post-injury mandibular cartilage is characteristic of end-stage disease, with hypocellularity and fibrillations, fissures, and clefts in the articular layer that propagate along the mediolateral axis of the MCC. We hypothesize that the localized depletion of interterritorial and pericellular type VI collagen may signify an early marker for the transition from early to end stage TMJ-OA, influence the injury response of the tissue, and underlie patterns of degeneration that follow attritional modes of failure.

Stribeck Curve Analysis of Temporomandibular Joint Condylar Cartilage and Disc

Temporomandibular joint (TMJ) diseases such as osteoarthritis and disc displacement have no permanent treatment options, but lubrication therapies, used in other joints, could be an effective alternative. However, the healthy TMJ contains fibrocartilage, not hyaline cartilage as is found in other joints. As such, the effect of lubrication therapies in the TMJ is unknown. Additionally, only a few studies have characterized the friction coefficient of the healthy TMJ. Like other cartilaginous tissues, the TMJ condyles and discs are subject to changes in friction coefficient due to fluid pressurization. In addition, the friction coefficient of the TMJ is affected by the sliding direction and anatomic location. However, these previous findings have not been able to identify how all 3 of these parameters (anatomic location, sliding direction, and fluid pressurization) influence changes in friction coefficient. This study used Stribeck curves to identify differences in the friction coefficients of TMJ condyles and discs based on anatomic location, sliding direction, and amount of fluid pressurization (friction mode). Friction coefficients were measured using a cartilage on glass tribometer. Both TMJ condyle and disc friction coefficients were well described by Stribeck curves. These curves changed based on anatomic location, but very few differences in friction coefficients were observed based on sliding direction. TMJ condyles had similar boundary mode and elastoviscous mode friction coefficients to the TMJ disc, and both were lower than hyaline cartilage in other joints. The observed differences here indicate that the surface characteristics of each anatomic region cause differences in friction coefficients.

Aging does not change the compressive stiffness of mandibular condylar cartilage in horses

OBJECTIVE

Aging can cause an increase in the stiffness of hyaline cartilage as a consequence of increased protein crosslinks. By induction of crosslinking, a reduction in the diffusion of solutions into the hyaline cartilage has been observed. However, there is a lack of knowledge about the effects of aging on the biophysical and biochemical properties of the temporomandibular joint (TMJ) cartilage. Hence, the aim of this study was to examine the biophysical properties (thickness, stiffness, and diffusion) of the TMJ condylar cartilage of horses of different ages and their correlation with biochemical parameters.

MATERIALS AND METHODS

We measured the compressive stiffness of the condyles, after which the diffusion of two contrast agents into cartilage was measured using Contrast Enhanced Computed Tomography technique. Furthermore, the content of water, collagen, GAG, and pentosidine was analyzed.

RESULTS

Contrary to our expectations, the stiffness of the cartilage did not change with age (modulus remained around 0.7 MPa). The diffusion of the negatively charged contrast agent (Hexabrix) also did not alter. However, the diffusion of the uncharged contrast agent (Visipaque) decreased with aging. The flux was negatively correlated with the amount of collagen and crosslink level which increased with aging. Pentosidine, collagen, and GAG were positively correlated with age whereas thickness and water content showed negative correlations.

CONCLUSION

Our data demonstrated that aging was not necessarily reflected in the biophysical properties of TMJ condylar cartilage. The combination of the changes happening due to aging resulted in different diffusive properties, depending on the nature of the solution.

Shock absorbing ability in healthy and damaged cartilage-bone under high-rate compression

Articular cartilage is a soft tissue that distributes the loads in joints and transfers the compressive load to the underlying bone. At high rate and magnitudes of mechanical loading, cartilage and subchondral bone together are susceptible to damage. In addition, any disruption to the cartilage's structure, caused by injury, trauma or disorder such as osteoarthritis (OA), can alter the mechanism of load transfer from the cartilage to the underlying bone. Changes in the cartilage structure can also alter the ability of cartilage-bone to absorb and dissipate the impact energy. To investigate the effects of cartilage degradation on cartilage-bone shock absorption ability, the top 50% of the cartilage thickness was removed (modified cartilage) to mimic the cartilage thickness reduction in Grade III cartilage lesion and the remaining cartilage-bone unit (modified cartilage-bone) was compressed at high-rate (4% strain at 5 Hz). High-speed camera and microscope were used to capture microscopic deformation, and digital image correlation technique (DIC) employed to quantify the deformation of cartilage and bone. The mechanical properties (i.e. stiffness, strain, absorbed and dissipated energies) of cartilage and bone were calculated before and after the removal of the top 50% of the cartilage thickness, consisting of both the superficial tangential zone (STZ) and part of the middle zone of the cartilage. The results showed a significant degradation in the mechanical properties of the cartilage-bone unit after the removal of the top 50% cartilage thickness. The stiffness of the modified cartilage reduced significantly (by ~39%) and energy absorption in underlying bone increased by 32%, which can make the bone more vulnerable to damage in the modified cartilage-bone unit. In addition, the energy dissipation in the modified cartilage-bone unit was also increased by approximately 14%. These changes in mechanical properties suggest a crucial role of the STZ and middle zone (within the top 50% cartilage thickness) in protecting the underlying bone from the severe compressive impact loading. Results also indicated that under physiological contact stress of 7 MPa, strain in damaged cartilage was increased by 3.22% without affecting the mechanical behaviour of the underlying bone.

Physiology and Engineering of the Graded Interfaces of Musculoskeletal Junctions

The connective tissues of the musculoskeletal system can be grouped into fibrous, cartilaginous, and calcified tissues. While each tissue type has a distinct composition and function, the intersections between these tissues result in the formation of complex, composite, and graded junctions. The complexity of these interfaces is a critical aspect of their healthy function, but poses a significant challenge for their repair. In this review, we describe the organization and structure of complex musculoskeletal interfaces, identify emerging technologies for engineering such structures, and outline the requirements for assessing the complex nature of these tissues in the context of recapitulating their function through tissue engineering.

Properties of the Temporomandibular Joint in Growing Pigs

A subset of temporomandibular joint (TMJ) disorders are attributed to joint degeneration. The pig has been considered the preferred in-vivo model for the evaluation of potential therapies for TMJ disorders, and practical considerations such as cost and husbandry issues have favored the use of young, skeletally immature animals. However, the effect of growth on the biochemical and biomechanical properties of the TMJ disc and articulating cartilage has not been examined. The present study investigates the effect of age on the biochemical and biomechanical properties of healthy porcine TMJs at 3, 6, and 9 months of age. DNA , hyrdroxyproline, and glycosaminoglycan (GAG) content were determined and the discs and condyles were tested in uniaxial unconfined stress relaxation compression from 10% - 30% strain. TMJ discs were further assessed with a tensile test to failure technique, which included the ability to test multiple samples from the same region of an individual disc to minimize the intra-specimen variation. No differences in biochemical properties for the disc or compressive properties at 30% stress relaxation in the disc and condylar cartilage were found. In tension, no differences were observed for peak stress and tensile modulus. The collagen content of the condyle were higher at 9 months than 3 months (p<0.05), and the GAG content was higher at 9 months than 6 months (p<0.05). There was a trend of increased compressive instantaneous modulus with age. As such, age matched controls for growing pigs are probably appropriate for most parameters measured.

Roles of Ihh signaling in chondroprogenitor function in postnatal condylar cartilage

Condylar articular cartilage in mouse temporomandibular joint develops from progenitor cells near the articulating surface that proliferate, undergo chondrogenesis and mature into hypertrophic chondrocytes. However, it remains unclear how these processes are regulated, particularly postnatally. Here we focused on the apical polymorphic layer rich in progenitors and asked whether the phenotype and fate of the cells require signaling by Indian hedgehog (Ihh) previously studied in developing long bones. In condyles in newborn mice, the apical polymorphic/progenitor cell layer was ~10 cell layer-thick and expressed the articular matrix marker Tenascin-C (Tn-C), and the underlying thick cell layer expressed Tn-C as well as the chondrogenic master regulator Sox9. By 1 month, condylar cartilage had gained its full width, but became thinner along its main longitudinal axis and displayed hypertrophic chondrocytes. By 3 months, articular cartilage consisted of a 2-3 cell layer-thick zone of superficial cells and chondroprogenitors expressing both Tn-C and Sox9 and a bottom zone of chondrocytes displaying vertical matrix septa. EdU cell tracing in juvenile mice revealed that conversion of chondroprogenitors into chondrocytes and hypertrophic chondrocytes required about 48 and 72 h, respectively. Notably, EdU injection in 3 month-old mice labeled both progenitors and maturing chondrocytes by 96 h. Conditional ablation of Ihh in juvenile/early adult mice compromised chondroprogenitor organization and function and led to reduced chondroprogenitor and chondrocyte proliferation. The phenotype of mutant condyles worsened over time as indicated by apoptotic chondrocyte incidence, ectopic chondrocyte hypertrophy, chondrocyte column derangement and subchondral bone deterioration. In micromass cultures of condylar apical cells, hedgehog (Hh) treatment stimulated chondrogenesis and alkaline phosphatase (APase) activity, while treatment with HhAntag inhibited both. Our findings indicate that the chondroprogenitor layer is continuously engaged in condylar growth postnatally and its organization and functioning depend on hedgehog signaling.

Osteochondral Interface Stiffening in Mandibular Condylar Osteoarthritis

Osteoarthritis (OA) of the temporomandibular joint (TMJ) is associated with dental biomechanics. A major change during OA progression is the ossification of the osteochondral interface. This study investigated the formation, radiological detectability, and mechanical property of the osteochondral interface at an early stage, the pathogenesis significance of which in OA progression is of clinical interest and remains elusive for the TMJ. Unilateral anterior crossbite (UAC) was performed on 6-wk-old rats as we previously reported. TMJs were harvested at 4, 12, and 20 wk. The progression of TMJ OA was evaluated using a modified Osteoarthritis Research Society International (OARSI) score system. Osteochondral interface was investigated by quantifying the thickness via von Kossa staining of histological slices and in vivo calcium deposition by calcein injection. Tissue ossification was imaged by micro-computed tomography (CT). Mechanical properties were measured at nanoscale using dynamic indentation. Time-dependent TMJ cartilage lesions were elicited by UAC treatment. Geometric change of the condyle head and increased value of the OARSI score were evident in UAC TMJs. At the osteochondral interface, there was not only enhanced deep-zone cartilage calcification but also calcium deposition at the osseous boundary. The thickness, density, and stiffness of the osteochondral interface were all significantly increased. The enhanced ossification of the osteochondral interface is a joint outcome of the aberrant deeper cartilage calcification at the superior region and promoted formation of subchondral cortical bone at the inferior region. The micro-CT detectable ossification from an early stage thus is of diagnostic significance. Although the environment of the cartilage and subchondral bone could be changed due to the stiffness of the interface, whether or not the stiffened interface would accelerate OA progress remains to be confirmed. With that evidence, the osteochondral interface could be a new diagnostic and therapeutic target of the mechanically initiated OA in the TMJ.

Mechanical stiffness of TMJ condylar cartilage increases after artificial aging by ribose

OBJECTIVE

Aging is accompanied by a series of changes in mature tissues that influence their properties and functions. Collagen, as one of the main extracellular components of cartilage, becomes highly crosslinked during aging. In this study, the aim was to examine whether a correlation exists between collagen crosslinking induced by artificial aging and mechanical properties of the temporomandibular joint (TMJ) condyle. To evaluate this hypothesis, collagen crosslinks were induced using ribose incubation.

METHODS

Porcine TMJ condyles were incubated for 7 days with different concentrations of ribose. The compressive modulus and stiffness ratio (incubated versus control) was determined after loading. Glycosaminoglycan and collagen content, and the number of crosslinks were analyzed. Tissue structure was visualized by microscopy using different staining methods.

RESULTS

Concomitant with an increasing concentration of ribose, an increase of collagen crosslinks was found. The number of crosslinks increased almost 50 fold after incubation with the highest concentration of ribose. Simultaneously, the stiffness ratio of the samples showed a significant increase after incubation with the ribose. Pearson correlation analyses showed a significant positive correlation between the overall stiffness ratio and the crosslink level; the higher the number of crosslinks the higher the stiffness.

CONCLUSION

The present model, in which ribose was used to mimic certain aspects of age-related changes, can be employed as an in vitro model to study age-related mechanical changes in the TMJ condyle.

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±84kPa, mean±95% CI) than those of the superior (85±23kPa) and inferior (45±12kPa) 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.

Osteoarthritis year in review 2016: mechanics

Inappropriate biomechanics, namely wear-and-tear, has been long believed to be a main cause of osteoarthritis (OA). However, this view is now being re-evaluated, especially when examined alongside mechanobiology and new biomechanical studies. These are multiscale experimental and computational studies focussing on cell- and tissue-level mechanobiology through to organ- and whole-body-level biomechanics, which focuses on the biomechanical and biochemical environment of the joint tissues. This review examined papers from April 2015 to April 2016, with a focus on multiscale experimental and computational biomechanical studies of OA. Assessing the onset or progression of OA at organ- and whole-body-levels, gait analysis, medical imaging and neuromusculoskeletal modelling revealed the extent to which tissue damage changes the view of inappropriate biomechanics. Traditional gait analyses studies reported that conservative treatments can alter joint biomechanics, thereby improving pain and function experienced by those with OA. Results of animal models of OA were consistent with these human studies, showing interactions among bone, cartilage and meniscus biomechanics and the onset and/or progression OA. Going down size scales, experimental and computational studies probed the nanosize biomechanics of molecules, cells and extracellular matrix, and demonstrated how the interactions between biomechanics and morphology affect cartilage dynamic poroelastic behaviour and pathways to OA. Finally, integration of multiscale experimental data and computational models were proposed to predict cartilage extracellular matrix remodelling and the development of OA. Summarising, experimental and computational methods provided a nuanced biomechanical understanding of the sub-cellular, cellular, tissue, organ and whole-body mechanisms involved in OA.

Nanoindentation modulus of murine cartilage: a sensitive indicator of the initiation and progression of post-traumatic osteoarthritis

OBJECTIVE

This study aims to demonstrate that cartilage nanoindentation modulus is a highly sensitive indicator of the onset and spatiotemporal progression of post-traumatic osteoarthritis (PTOA) in murine models.

DESIGN

Destabilization of the medial meniscus (DMM) surgery was performed on the right knees of 12-week old male, wild-type C57BL/6 mice, with Sham control on contralateral left knees. Atomic force microscopy (AFM)-based nanoindentation was applied to quantify the nanoindentation modulus, Eind, of femoral condyle cartilage at 3 days to 12 weeks after surgery. The modulus changes were compared against the timeline of histological OA signs. Meanwhile, at 8 weeks after surgery, changes in meniscus, synovium and subchondral bone were evaluated to reveal the spatial progression of PTOA.

RESULTS

The modulus of medial condyle cartilage was significantly reduced at 1 week after DMM, preceding the histological OA signs, which only became detectable at 4-8 weeks after. This reduction is likely due to concomitantly elevated proteolytic activities, as blocking enzymatic activities in mice can attenuate this modulus reduction. In later OA, lateral condyle cartilage and medial meniscus also started to be weakened, illustrating the whole-organ nature of PTOA.

CONCLUSIONS

This study underscores the high sensitivity of nanoindentation in examining the initiation, attenuation and progression of PTOA in murine models. Meanwhile, modulus changes highlight concomitant changes in lateral cartilage and meniscus during the advancement of OA.

Articular Cartilage Inspired Bilayer Tough Hydrogel Prepared by Interfacial Modulated Polymerization Showing Excellent Combination of High Load-Bearing and Low Friction Performance

Articular cartilage is a load-bearing and lubricious tissue covering the ends of articulating bones in synovial joints to reduce friction and wear. It ideally combines the high mechanical property and the ultralow friction performance as a result of biphasic structure and lubricious biomolecules. A biomimicking hydrogel with bilayer structure of thin porous top layer covering a compact and tough hydrogel bulk is fabricated with interfacial modulated polymerization. The top porous layer ensures the ultralow friction toward its contact pairs, while the bottom renders the high load-bearing property. Therefore, with bilayer architecture, hydrogel achieves an outstanding combination of low friction and high load bearing performance with long wear life when sliding against either steel or silicone elastomer counterpair.

Activation of α2A-adrenergic signal transduction in chondrocytes promotes degenerative remodelling of temporomandibular joint

This study tested whether activation of adrenoreceptors in chondrocytes has roles in degenerative remodelling of temporomandibular joint (TMJ) and to determine associated mechanisms. Unilateral anterior crossbite (UAC) was established to induce TMJ degeneration in rats. Saline vehicle, α2- and β-adrenoreceptor antagonists or agonists were injected locally into the TMJ area of UAC rats. Cartilage degeneration, subchondral bone microarchitecture and the expression of adrenoreceptors, aggrecans, matrix metalloproteinases (MMPs) and RANKL by chondrocytes were evaluated. Chondrocytes were stimulated by norepinephrine to investigate signal transduction of adrenoreceptors. Increased α2A-adrenoreceptor expression was observed in condylar cartilage of UAC rats, together with cartilage degeneration and subchondral bone loss. Norepinephrine depresses aggrecans expression but stimulates MMP-3, MMP-13 and RANKL production by chondrocytes through ERK1/2 and PKA pathway; these effects were abolished by an α2A-adrenoreceptor antagonist. Furthermore, inhibition of α2A-adrenoreceptor attenuated degenerative remodelling in the condylar cartilage and subchondral bone, as revealed by increased cartilage thickness, proteoglycans and aggrecan expression, and decreased MMP-3, MMP-13 and RANKL expressions in cartilage, increased BMD, BV/TV, and decreased Tb.Sp in subchondral bone. Conversely, activation of α2A-adrenoreceptor intensified aforementioned degenerative changes in UAC rats. It is concluded that activation of α2A-adrenergic signal in chondrocytes promotes TMJ degenerative remodelling by chondrocyte-mediated pro-catabolic activities.

Conditional Deletion of Fgfr3 in Chondrocytes leads to Osteoarthritis-like Defects in Temporomandibular Joint of Adult Mice

Osteoarthritis (OA) in the temporomandibular joint (TMJ) is a common degenerative disease in adult, which is characterized by progressive destruction of the articular cartilage. To investigate the role of FGFR3 in the homeostasis of TMJ cartilage during adult stage, we generated Fgfr3^f/f; Col2a1-CreER^T2 (Fgfr3 cKO) mice, in which Fgfr3 was deleted in chondrocytes at 2 months of age. OA-like defects were observed in Fgfr3 cKO TMJ cartilage. Immunohistochemical staining and quantitative real-time PCR analyses revealed a significant increase in expressions of COL10, MMP13 and AMAMTS5. In addition, there was a sharp increase in chondrocyte apoptosis at the Fgfr3 cKO articular surface, which was accompanied by a down-regulation of lubricin expression. Importantly, the expressions of RUNX2 and Indian hedgehog (IHH) were up-regulated in Fgfr3 cKO TMJ. Primary Fgfr3 cKO chondrocytes were treated with IHH signaling inhibitor, which significantly reduced expressions of Runx2, Col10, Mmp13 and Adamts5. Furthermore, the IHH signaling inhibitor partially alleviated OA-like defects in the TMJ of Fgfr3 cKO mice, including restoration of lubricin expression and improvement of the integrity of the articular surface. In conclusion, our study proposes that FGFR3/IHH signaling pathway plays a critical role in maintaining the homeostasis of TMJ articular cartilage during adult stage.

Osteophyte formation and matrix mineralization in a TMJ osteoarthritis mouse model are associated with ectopic hedgehog signaling

The temporomandibular joint (TMJ) is a diarthrodial joint that relies on lubricants for frictionless movement and long-term function. It remains unclear what temporal and causal relationships may exist between compromised lubrication and onset and progression of TMJ disease. Here we report that Proteoglycan 4 (Prg4)-null TMJs exhibit irreversible osteoarthritis-like changes over time and are linked to formation of ectopic mineralized tissues and osteophytes in articular disc, mandibular condyle and glenoid fossa. In the presumptive layer of mutant glenoid fossa's articulating surface, numerous chondrogenic cells and/or chondrocytes emerged ectopically within the type I collagen-expressing cell population, underwent endochondral bone formation accompanied by enhanced Ihh expression, became entrapped into temporal bone mineralized matrix, and thereby elicited excessive chondroid bone formation. As the osteophytes grew, the roof of the glenoid fossa/eminence became significantly thicker and flatter, resulting in loss of its characteristic concave shape for accommodation of condyle and disc. Concurrently, the condyles became flatter and larger and exhibited ectopic bone along their neck, likely supporting the enlarged condylar heads. Articular discs lost their concave configuration, and ectopic cartilage developed and articulated with osteophytes. In glenoid fossa cells in culture, hedgehog signaling stimulated chondrocyte maturation and mineralization including alkaline phosphatase, while treatment with hedgehog inhibitor HhAntag prevented such maturation process. In sum, our data indicate that Prg4 is needed for TMJ integrity and long-term postnatal function. In its absence, progenitor cells near presumptive articular layer and disc undergo ectopic chondrogenesis and generate ectopic cartilage, possibly driven by aberrant activation of Hh signaling. The data suggest also that the Prg4-null mice represent a useful model to study TMJ osteoarthritis-like degeneration and clarify its pathogenesis.

TNF Accelerates Death of Mandibular Condyle Chondrocytes in Rats with Biomechanical Stimulation-Induced Temporomandibular Joint Disease

Objective

To determine if temporomandibular joint chondrocyte apoptosis is induced in rats with dental biomechanical stimulation and what a role TNF takes.

Methods

Thirty-two rats were divided into 4 groups (n = 8/group) and exposed to incisor mal-occlusion induced by unilateral anterior crossbite biomechanical stimulation. Two groups were sampled at 2 or 4 weeks. The other two groups were treated with local injections of a TNF inhibitor or PBS into the temporomandibular joints area at 2 weeks and then sampled at 4 weeks. Twenty-four rats either served as unilateral anterior crossbite mock operation controls (n = 8/group) with sampling at 2 or 4 weeks or received a local injection of the TNF inhibitor at 2 weeks with sampling at 4 weeks. Chondrocytes were isolated from the temporomandibular joints of 6 additional rats and treated with TNF in vitro. Joint samples were assessed using Hematoxylin&eosin, Safranin O, TUNEL and immunohistochemistry staining, real-time PCR, fluorogenic activity assays and Western blot analyses. The isolated chondrocytes were also analyzed by flow cytometry.

Results

Unilateral anterior crossbite stimulation led to temporomandibular joint cartilage degradation, associated with an increase in TUNEL-positive chondrocytes number, caspase-9 expression levels, and the release of cytochrome c from mitochondria at 2 weeks without changes in TNF and caspase-8 levels until after 4 weeks. TNF stimulated apoptosis of the isolated chondrocytes and up-regulated caspase-8 expression, but did not change caspase-9 expression levels. Local injection of TNF inhibitor down-regulated caspase-8 expression and reduced TUNEL-positive cell number, but did not reverse cartilage thickness reduction, caspase-9 up-regulation or cytochrome c release.

Conclusions

Unilateral anterior crossbite stimulation induces mitochondrion-mediated apoptosis of articular chondrocytes. TNF accelerated the unilateral anterior crossbite induced chondrocytes apoptosis via death-receptor pathway. However, anti-TNF therapy does not prevent cartilage loss in this model of temporomandibular joint.

Determining Tension-Compression Nonlinear Mechanical Properties of Articular Cartilage from Indentation Testing

The indentation test is widely used to determine the in situ biomechanical properties of articular cartilage. The mechanical parameters estimated from the test depend on the constitutive model adopted to analyze the data. Similar to most connective tissues, the solid matrix of cartilage displays different mechanical properties under tension and compression, termed tension-compression nonlinearity (TCN). In this study, cartilage was modeled as a porous elastic material with either a conewise linear elastic matrix with cubic symmetry or a solid matrix reinforced by a continuous fiber distribution. Both models are commonly used to describe the TCN of cartilage. The roles of each mechanical property in determining the indentation response of cartilage were identified by finite element simulation. Under constant loading, the equilibrium deformation of cartilage is mainly dependent on the compressive modulus, while the initial transient creep behavior is largely regulated by the tensile stiffness. More importantly, altering the permeability does not change the shape of the indentation creep curves, but introduces a parallel shift along the horizontal direction on a logarithmic time scale. Based on these findings, a highly efficient curve-fitting algorithm was designed, which can uniquely determine the three major mechanical properties of cartilage (compressive modulus, tensile modulus, and permeability) from a single indentation test. The new technique was tested on adult bovine knee cartilage and compared with results from the classic biphasic linear elastic curve-fitting program.