Dynamic Mechanical Properties and Viscoelastic Response Analysis of a Complex Biological Interpenetrating Network: The Temporomandibular Joint Cartilage Disc

Joseph Lomakin, Chemical and Petroleum Engineering, Kansas University, The University of Kansas, 4132 Learned Hall 1530 W. 15th Street, Lawrence, KS 66045, Michael Detamore, Dept. of Chemical & Petroleum Engineering, University of Kansas, 1530 W. 15th St., 4132 Learned Hall, Lawrence, KS 66045, and Stevin H. Gehrke, Departments of Chemical & Petroleum Engineering and Pharmaceutical Chemistry, University of Kansas, 1530 W 15th Street, Lawrence, KS 66045-7609.

The temporomandibular joint (TMJ) disc is a robust cartilaginous biogel separating the mandibular condyle and the fossa-eminence of the temporal bone. It is comprised of a highly hydrated interpenetrating network of collagen Type I and proteoglycans, combined in a ratio of approximately 10 to 1. The discs' structure and exceptional mechanical properties are of both clinical and materials research interest as they empower this complex biocomposite to withstand repeat exposure to high shear, compression and tension forces as part of normal jaw movement associated with chewing and talking. While the mechanical performance of the disc in response to these diverse forces have been characterized, comparison of data published in various venues remains difficult due to the lack of fundamental understanding surrounding the complex multivariable response of this and other biological materials.

In this study, the dynamic mechanical properties of the TMJ disc are determined as a function of initial loading (pre-tension), oscillation frequency and dynamic strain in an effort to reconcile mechanical property data on the disc presented thus far in the literature. Furthermore the results and analysis demonstrate the applicability of viscoelastic interpretation of the data, often applied to synthetic polymer materials, to a complex hierarchical biomaterial such as the temporomandibular joint disc.

The elastic modulus (E') of the TMJ disc measured over two orders of magnitude (0.01 rad/s 500 rad/s) of frequency and (0.01% - 10%) dynamic strain was found to span 10 to 60 MPa and tan δ to vary between 0.08 and 0.18 depending on the properties of the applied strain wave. With respect to oscillation frequency (ω), the E' exhibited a weak power law increase of a factor of 2 between 0.1 and 100 rad/s, an increase consistent with long time relaxation associated with movement of entangled ends within a loosely cross-linked interpenetrating polymer network. The elastic modulus is found to be independent of dynamic strain below 0.1%. Above this strain the modulus decreases sharply, demonstrating a pronounced power law decrease above 1% strain. The results from tests at various dynamic strain and frequencies are combined to separate the effects of two confounding variables relevant to the biomaterial's mechanical response: strain amplitude and strain rate. Strain-rate frequency superpositioning (SRFS) is used to determine the relative importance of these two parameters on material moduli and to project the dependence of the moduli on dynamic strain and frequency outside of the tested range. The master curve constructed from overlaying constant strain rate measurements demonstrates that strain rate amplitude is an important factor in determining TMJ disc material properties, an effect not typically seen with synthetic materials. This viscoelastic analysis of dynamic mechanical data facilitates understanding of the complex dependence of biomaterial mechanical properties on the applied strain in terms of underlying structural relaxations governed by molecular structure and suggests new avenues for biomimetic material design.