Localization of Viscous Behavior and Shear Energy Dissipation in Articular Cartilage Under Dynamic Shear Loading

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 steadystate 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 highspeed confocal imaging techniques with three approaches for analyzing timeand locationdependent mechanical data to measure the depthdependent dynamic modulus and phase angles of articular cartilage. For sinusoidal shear at frequencies f = 0.01 to 1 Hz with no strain offset, the dynamic shear modulus |G*| and phase angle خ´ reached their minimum and maximum values (respectively) approximately 100 خ¼m 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 خ¼m of the articular surface, |G*| increased from 0.32 آ±â€‰0.08 to 0.42 آ±â€‰0.08 MPa across the five frequencies tested, while خ´ decreased from 12 deg آ±â€‰1 deg to 9.1 deg آ±â€‰0.5 deg. Deeper into the tissue, |G*| increased from 1.5 آ±â€‰0.4 MPa to 2.1 آ±â€‰0.6 MPa and خ´ decreased from 13 deg آ±â€‰1 deg to 5.5 deg آ±â€‰0.2 deg. Viscoelastic properties were also straindependent, 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.