Dynamic compressive properties of articular cartilages in the porcine temporomandibular joint

Elastic, Dynamic Viscoelastic and Model-Derived Fibril-Reinforced Poroelastic Mechanical Properties of Normal and Osteoarthritic Human Femoral Condyle Cartilage

Osteoarthritis (OA) degrades articular cartilage and weakens its function. Modern fibril-reinforced poroelastic (FRPE) computational models can distinguish the mechanical properties of main cartilage constituents, namely collagen, proteoglycans, and fluid, thus, they can precisely characterize the complex mechanical behavior of the tissue. However, these properties are not known for human femoral condyle cartilage. Therefore, we aimed to characterize them from human subjects undergoing knee replacement and from deceased donors without known OA. Multi-step stress-relaxation measurements coupled with sample-specific finite element analyses were conducted to obtain the FRPE material properties. Samples were graded using OARSI scoring to determine the severity of histopathological cartilage degradation. The results suggest that alterations in the FRPE properties are not evident in the moderate stages of cartilage degradation (OARSI 2-3) as compared with normal tissue (OARSI 0-1). Drastic deterioration of the FRPE properties was observed in severely degraded cartilage (OARSI 4). We also found that the FRPE properties of femoral condyle cartilage related to the collagen network (initial fibril-network modulus) and proteoglycan matrix (non-fibrillar matrix modulus) were greater compared to tibial and patellar cartilage in OA. These findings may inform cartilage tissue-engineering efforts and help to improve the accuracy of cartilage representations in computational knee joint models.

Trueness of Fit of Biphasic Transversely Isotropic Parameters Model in the Porcine Temporomandibular Joint Disc and Mandibular Condylar Cartilage and Regional Dependence

Temporomandibular joint (TMJ) disorders (TMDs) are not well understood and the mechanical differences between the regions of the mandibular condylar cartilage (MCC) and the TMJ disc have not been thoroughly compared. As of now, there are no commercially available regenerative therapies for the TMJ. Elucidating the mechanical properties of these two structures of the articulating joint will help future efforts in developing tissue engineering treatments of the TMJ. In this study, we evaluate the compressive properties of the porcine disc and mandibular condylar cartilage by performing unconfined compression at 10% strain with 4.5%/min strain rate. Punches (4 mm biopsy) from both tissues were taken from five different regions of both the MCC and TMJ: anterior, posterior, lateral, medial, and central. Previously, theoretical models of compression in the porcine tissue did not fit the whole ramp-relaxation behavior. Thus, the data stress-relaxation was fitted to the biphasic transversely isotropic model, for both the TMJ disc and cartilage. From the results found in the disc, it was found that the posterior region had the highest values in multiple viscoelastic parameters when compared to the other regions. The mandibular condylar cartilage was only found to be significantly different in the transverse modulus between the posterior and lateral regions. Both the TMJ disc and MCC had similar magnitudes of values (for the modulus and other corresponding compressive properties) and behavior under this testing modality.

Experimental and Numerical Characterization of the Mechanical Masseter Muscle Response During Biting

Predictive simulations of the mastication system would significantly improve our understanding of temporomandibular joint (TMJ) disorders and the planning of cranio-maxillofacial surgery procedures. Respective computational models must be validated by experimental data from in vivo characterization of the mastication system's mechanical response. The present pilot-study demonstrates the feasibility of a combined experimental and numerical procedure to validate a computer model of the masseter muscle. An experimental setup is proposed that provides a simultaneous bite force measurement and ultrasound-based visualization of muscle deformation. The direct comparison of the experimentally observed and numerically predicted muscle response demonstrates the predictive capabilities of such anatomically accurate biting models. Differences between molar and incisor biting are investigated; muscle deformation is recorded for three different bite forces in order to capture the effect of increasing muscle fiber recruitment. The three-dimensional (3D) muscle deformation at each bite position and force-level is approximatively reconstructed from ultrasound measurements in five distinct cross-sectional areas (four horizontal and one vertical cross section). The experimental work is accompanied by numerical simulations to validate the predictive capabilities of a constitutive muscle model previously formulated. An anatomy-based, fully 3D model of the masseter muscle is created from magnetic resonance images (MRI) of the same subject. The direct comparison of experimental and numerical results revealed good agreement for maximum bite forces and masseter deformations in both biting positions. The present work therefore presents a feasible in vivo measurement system to validate numerically predicted masseter muscle contractions during mastication.

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.

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.

Sex dependent mechanical properties of the human mandibular condyle

The mandibular condyle consists of articular cartilage and subchondral bone that play an important role in bearing loads at the temporomandibular joint (TMJ) during static occlusion and dynamic mastication. The objective of the current study was to examine effects of sex and cartilage on 1) static and dynamic mechanical analysis (DMA) based dynamic energy storage and dissipation for the cartilage-subchondral bone construct of the human mandibular condyle, and 2) their correlations with the tissue mineral density and trabecular morphological parameters of subchondral bone. Cartilage-subchondral bone constructs were obtained from 16 individual human cadavers (9 males, 7 females, 79.00±13.10 years). After scanning with micro-computed tomography, the specimens were subjected to a non-destructive compressive static loading up to 7N and DMA using a cyclic loading profile (-5±2N at 2Hz). After removing the cartilage from the same specimen, the series of loading experiments were repeated. Static stiffness (K) and energy dissipation (W), and dynamic storage (K'), loss (K'') stiffness, and energy dissipation (tan δ) were assessed. Gray values, which are proportional to degree of bone mineralization, and trabecular morphological parameters of the subchondral bone were also measured. After removal of the cartilage, static energy dissipation significantly decreased (p<0.009) but dynamic energy dissipation was not influenced (p>0.064). Many subchondral bone properties were significantly correlated with the overall mechanical behavior of the cartilage-subchondral bone constructs for males (p<0.047) but not females (p>0.054). However, after removal of cartilage from the constructs, all of the significant correlations were no longer found (p>0.057). The current findings indicate that the subchondral bone is responsible for bearing static and dynamic loading in males but not in females. This result indicates that the female condyle may have a mechanically disadvantageous TMJ loading environment.

Ex vivo thickness measurement of cartilage covering the temporomandibular joint

Articular cartilage covers the temporomandibular joint (TMJ) and provides smooth and nearly frictionless articulation while distributing mechanical loads to the subchondral bone. The thickness of the cartilage is considered to be an indicator of the stage of development, maturation, aging, loading history, and disease. The aim of our study was to develop a method for ex vivo assessment of the thickness of the cartilage that covers the TMJ and to compare that with two other existing methods. Eight porcine TMJ condyles were used to measure cartilage thickness. Three different methods were employed: needle penetration, micro-computed tomography (micro-CT), and histology; the latter was considered the gold standard. Histology and micro-CT scanning results showed no significant differences between thicknesses throughout the condyle. Needle penetration produced significantly higher values than histology, in the lateral and anterior regions. All three methods showed the anterior region to be thinner than the other regions. We concluded that overestimated thickness by the needle penetration is caused by the penetration of the needle through the first layer of subchondral bone, in which mineralization is less than in deeper layers. Micro-CT scanning method was found to be a valid method to quantify the thickness of the cartilage, and has the advantage of being non-destructive.

Biomechanical and cellular segmental characterization of human meniscus: building the basis for Tissue Engineering therapies

OBJECTIVE

To overcome current limitations of Tissue Engineering (TE) strategies, deeper comprehension on meniscus biology is required. This study aims to combine biomechanical segmental analysis of fresh human meniscus tissues and its correlation with architectural and cellular characterization.

METHOD

Morphologically intact menisci, from 44 live donors were studied after division into three radial segments. Dynamic mechanical analysis (DMA) was performed at physiological-like conditions. Micro-computed tomography (CT) analysis of freeze-dried samples assessed micro-structure. Flow cytometry, histology and histomorphometry were used for cellular study and quantification.

RESULTS

Anterior segments present significantly higher damping properties. Mid body fresh medial meniscus presents higher values of E' compared to lateral. Cyclic loads influence the viscoelastic behavior of menisci. By increasing the frequency leads to an increase in stiffness. Conversely, with increasing frequencies, the capacity to dissipate energy and damping properties initially decrease and then rise again. Age and gender directly correlate with higher E' and tan δ. Micro-CT analysis revealed that mean porosity was 55.5 (21.2-89.8)% and 64.7 (47.7-81.8)% for freeze-dried lateral and medial meniscus, respectively. Predominant cells are positive for CD44, CD73, CD90 and CD105, and lack CD31, CD34 and CD45 (present in smaller populations). Histomorphometry revealed that cellularity decreases from vascular zone 1 to zone 3. Anterior segments of lateral and medial meniscus have inferior cellularity as compared to mid body and posterior ones.

CONCLUSION

Menisci are not uniform structures. Anterior segments have lower cellularity and higher damping. Cyclic loads influence viscoelastic characteristics. Future TE therapies should consider segmental architecture, cellularity and biomechanics of fresh tissue.

Effect of Pre-Stress on the Dynamic Tensile Behavior of the TMJ Disc

Previous dynamic analyses of the temporomandibular joint (TMJ) disc have not included a true preload, i.e., a step stress or strain beyond the initial tare load. However, due to the highly nonlinear stress-strain response of the TMJ disc, we hypothesized that the dynamic mechanical properties would greatly depend on the preload, which could then, in part, account for the large variation in the tensile stiffnesses reported for the TMJ disc in the literature. This study is the first to report the dynamic mechanical properties as a function of prestress. As hypothesized, the storage modulus (E′) of the disc varied by a factor of 25 in the mediolateral direction and a factor of 200 in the anteroposterior direction, depending on the prestress. Multiple constant strain rate sweeps were extracted and superimposed via strain-rate frequency superposition (SRFS), which demonstrated that the strain rate amplitude and strain rate were both important factors in determining the TMJ disc material properties, which is an effect not typically seen with synthetic materials. The presented analysis demonstrated, for the first time, the applicability of viscoelastic models, previously applied to synthetic polymer materials, to a complex hierarchical biomaterial such as the TMJ disc, providing a uniquely comprehensive way to capture the viscoelastic response of biological materials. Finally, we emphasize that the use of a preload, preferably which falls within the linear region of the stress-strain curve, is critical to provide reproducible results for tensile analysis of musculoskeletal tissues. Therefore, we recommend that future dynamic mechanical analyses of the TMJ disc be performed at a controlled prestress corresponding to a strain range of 5–10%.