Poster BB9 - Characterization of TMJ Condylar Cartilage and Comparison to Knee Articular Cartilage

Introduction: Temporomandibular joint (TMJ, or jaw joint) pain affects 10-35 million adults in the United States according to the TMJ Association. The mandibular condyle, or TMJ condyle, is a common locus of affliction; recent studies have reported that over 80% of TMJ condyles in patients over the age of 20 have abnormal morphology, such as condylar flattening, and that condylar degeneration affects 25% of the population. Tissue-engineered implants have the potential to regenerate TMJ cartilage defects to arrest degenerative changes, but design criteria and native tissue benchmarks must be established prior to performing in vivo studies. Thus, the objective of this work is to characterize the cartilage of the TMJ condyle of the Yucatan minipig and compare it to the knee. The Yucatan minipig was chosen because it has recently become the gold standard large animal model for TMJ tissue engineering.

Materials and

Methods: Sixteen TMJs were acquired from 8 skeletally mature Yucatan minipigs (4 male, 4 female), culled for purposes unrelated to this study. Right condyles were imaged (Figure 1), then creep indentation analysis was performed on the anterior, central, posterior, medial, and lateral regions to determine aggregate modulus (HA). The same regions on the left condyles were used for tensile testing in the anterior-posterior (AP) and medial-lateral (ML) directions to determine Young’s modulus (EY) and ultimate tensile strength (UTS). The right condyles were then used for biochemical analyses; collagen quantification with hydroxyproline assay, glycosaminoglycan quantification with dimethyl-methylene blue assay, DNA quantification with PicoGreen assay, and pyridinoline collagen crosslink quantification with liquid chromatography—mass spectrometry. The left condyles were used for histology.

Results, Conclusions, and Discussions: There were no significant differences in the HA among regions of the TMJ condyle cartilage, which ranged from 26.6 to 44.1 kPa (Figure 2A). There were no significant differences in EY among regions, but the AP direction (50.3 MPa) was significantly stiffer than the ML direction (25.5 MPa, Figure 2B). A similar trend was seen in the UTS; there were no significant differences among regions, but the AP direction (9.9 MPa) was significantly stronger than the ML direction (5.6 MPa, Figure 2C). These mechanical data were compared to our group’s historical data on Yucatan minipig knee cartilage: HA of 430 kPa, AP EY of 13.3 MPa, and AP UTS of 6.9 (Figure 2, red lines). In the TMJ condyle, glycosaminoglycan content per dry weight was highest in the central region (3.9% versus 1.4% to 2.2% in other regions, Figure 3A), collagen content per dry weight ranged from 59.9% to 75.0% with no significant differences among regions (Figure 3B), and DNA content ranged from approximately 0.05%-0.10% with no significant differences among regions (Figure 3C). Pyridinoline crosslinks were highest in the posterior region (15.8 mmol/mol normalized to hydroxyproline). The histology (Figure 4) shows a distinct layer of fibrocartilage at the surface of the mandibular condyle, which is not observed in histological images of knee articular cartilage.

The properties of the TMJ condyle are substantially different from those of knee cartilage. For example, the TMJ condyle cartilage has an EY 3.8-times that of knee cartilage and the HA of knee cartilage is 12.4-times that of TMJ condylar cartilage. These differences are results of the unique fibrocartilage layer at the surface zone of the TMJ condyle. These properties are critical considerations when engineering biomaterials such as engineered neotissues to regenerate cartilage in damaged or diseased TMJ condyles.

Acknowledgements (Optional): This work was supported by the US National Institutes of Health (NIH) F32DE032896 (B.J.B.) and R01DE015038 (K.A.A.). The content is solely the responsibility of the authors and does not necessarily represent the official views of the NIH.