Viscoelastic shear properties of porcine temporomandibular joint disc

Abstract

Objectives: To investigate the intrinsic viscoelastic shear properties in porcine TMJ discs.

Materials and methods: Twelve fresh porcine TMJ discs from young adult pigs (6-8 months) were used. Cylindrical samples (5 mm diameter) with uniform thickness (~1.2 mm) were prepared from five regions of the TMJ disc. Torsional shear tests were performed under 10% compressive strain. Dynamic shear was applied in two methods: 1) a frequency sweep test over the frequency range of 0.1-10 rad/s with a constant shear strain amplitude of 0.05 rad and 2) a strain sweep test over the range of 0.005-0.15 rad at a constant frequency of 10 rad/s. Transient stress relaxation tests were also performed to determine the equilibrium shear properties.

Results: As the frequency increased in the frequency sweep test, the dynamic shear complex modulus increased, with values ranging from 7 to 17 kPa. The phase angle, ranging from 11 to 15 degrees, displayed no pattern of regional variation as the frequency increased. The dynamic shear modulus decreased as the shear strain increased. The equilibrium shear modulus had values ranging from 2.6 to 4 kPa. The posterior region had significantly higher values for dynamic shear modulus than those in the anterior region, while no significant regional difference was found for equilibrium shear modulus.

Conclusion: Our results suggest that the intrinsic region-dependent viscoelastic shear characteristics of TMJ disc may play a crucial role in determining the local strain of the TMJ disc under mechanical loading.

Keywords: dynamic mechanical testing; porcine animal model; shear modulus; temporomandibular joint disc.

Figure 1

A) Schematic of specimen preparation. The region and size of test specimens are shown. Shear samples from the five disc regions had an average height of 1.0mm and a diameter of 5mm. B) Schematic of rotational shear chamber.

Figure 2

Loading protocols of frequency sweep, strain sweep, and stress relaxation experiments. γ₀ represents initial amplitude of the shear strain.

Figure 3

Shear frequency sweep of porcine TMJ discs. A) Relationship of complex modulus with frequency. B) Relationship of phase angle with frequency.

Figure 4

Shear strain sweep of porcine TMJ discs. A) Relationship of complex modulus with shear strain. B) Relationship of phase angle with shear strain.

Figure 5

A) Typical stress relaxation curve for disc sample from anterior region. The multi-strain levels included five steps (0.005, 0.01, 0.05, 0.1, 0.15). As shown in the inset plot, the shear equilibrium modulus was calculated from the slope of equilibrium stress versus strain curve generated from all five stress relaxation steps. B) Average ± standard deviation of regional shear equilibrium modulus.