Unexpected anisotropic behavior of ultrasound attenuation after collagen cross-linking in porcine tendons

Progressive collagen cross-linking is a feature of many cardiovascular conditions (e.g., aging, diabetes, remodeling) and may contribute to impaired ventricular relaxation. However, it is not known whether cross-linking affects anisotropic tissue material properties more in one direction than another. We hypothesized that biological tissue comprising a highly ordered collagen matrix (porcine tendon) would manifest a directional dependence of the physical effects of cross-links that could be quantitatively delineated with ultrasound. Fresh porcine flexor tendons were sliced in 2-mm planes parallel (n=7) and perpendicular (n=7) to the tissue fiber axis for attenuation measurements (1-7 MHz). Tendons subjected to cross-linking treatment were immersed in 10% formalin, and control tendons in normal saline solution. Attenuation measurements were made immediately after immersion for both tendon groups at numerous intervals for 1 hr. The attenuation coefficient of the samples insonified perpendicular to the collagen fibers increased after formalin fixation over 1-hr period, while attenuation for parallel measurements decreased at selected frequencies during the first 20 min and subsequently increased modestly by 1 hr. Although the molecular mechanism responsible for the unexpected decrease in attenuation in the parallel direction as compared with the increase in the perpendicular direction is not clear, it may reflect rapid alterations in directionally dependent viscous and/or elastic components that contribute to material properties of well-ordered biological tissues