Article: Buckley MR, Bonassar LJ, Cohen I; (2013) Localization of Viscous Behavior and Shear Energy Dissipation in Articular Cartilage Under Dynamic Shear Loading. Journal of Biomechanical Engineering-Transactions of the ASME, 135 (3)
Abstract: Though remarkably robust, articular cartilage becomes susceptible to damage at high loading rates, particularly under shear. While several studies have measured the local static and steady-state shear properties of cartilage, it is the local viscoelastic properties that determine the tissue’s ability to withstand physiological loading regimens. However, measuring local viscoelastic properties requires overcoming technical challenges that include resolving strain fields in both space and time and accurately calculating their phase offsets. This study combined recently developed high-speed confocal imaging techniques with three approaches for analyzing time-and location-dependent mechanical data to measure the depth-dependent dynamic modulus and phase angles of articular cartilage. For sinusoidal shear at frequencies f = 0.01 to 1Hz with no strain offset, the dynamic shear modulus vertical bar G*vertical bar and phase angle delta reached their minimum and maximum values (respectively) approximately 100 mu m below the articular surface, resulting in a profound focusing of energy dissipation in this narrow band of tissue that increased with frequency. This region, known as the transitional zone, was previously thought to simply connect surface and deeper tissue regions. Within 250 mu m of the articular surface, vertical bar G*vertical bar increased from 0.32 +/- 0.08 to 0.42 +/-0.08 MPa across the five frequencies tested, while delta decreased from 12 deg +/- 1 deg to 9.1 deg +/- 0.5 deg. Deeper into the tissue, vertical bar G*vertical bar increased from 1.5 +/- 0.4MPa to 2.1 +/- 0.6 MPa and d decreased from 13 deg +/- 1 deg to 5.5 deg +/- 0.2 deg. Viscoelastic properties were also strain-dependent, with localized energy dissipation suppressed at higher shear strain offsets. These results suggest a critical role for the transitional zone in dissipating energy, representing a possible shift in our understanding of cartilage mechanical function. Further, they give insight into how focal degeneration and mechanical trauma could lead to sustained damage in this tissue.