Relationship Between Ex Vivo Tissue Diffusion Tensor Indexes and Material Properties in Posterior Tibialis Tendon
Authors: Zellers Jennifer A. , Edalati Masoud , Eekhoff Jeremy , Lake Spencer P. , Zheng Jie , Hastings Mary K. 
Published: October 2019

Category: Ankle, tendon imaging Introduction/Purpose: Posterior tibialis tendon is of clinical importance in the development and progression of acquired flatfoot and other midfoot deformity. The ability to quantitatively evaluate tendon tissue non-invasively would enable assessment of tendon health status and tracking of recovery from injury. Magnetic resonance diffusion tensor imaging (DTI) has been used to examine tendon tissue organization in healing tendon tissue. However, the relationship of DTI-based measures to tendon mechanical function has not been established. The purpose of this pilot study was to quantitatively evaluate posterior tibialis tendon using DTI and determine the relationship of these parameters to tendon function assessed via ex vivo mechanical testing. Methods: Posterior tibialis tendons from individuals undergoing amputation were positioned vertically in an agarose mold filled with saline for imaging. High resolution diffusion imaging parameters were optimized for tendon on a 3 T MRI to acquire 13 6-mm transverse slices covering the length: 1mm2 isotropic resolution, 2 signal averaging, repetition/echo times of 5000/58 ms, diffusion strength of 500s/mm2 with 30 gradient directions, scan time 5 min. Diffusion images had sufficient quality and were corrected for motion and image distortion. DTI parametric maps including fractional anisotropy (FA), mean, axial, and radial diffusivities (MD, AD, and RD; mm2/s) were calculated along with fiber tracking indexes of fiber length (mm) and density. After imaging, specimens were preloaded to 10 Newtons, preconditioned 10 cycles at 6% strain, subjected to stress-relaxation at 6% strain (10 minutes), then loaded to a maximum of 10% strain. Relationships between DTI indexes and mechanical properties (stiffness and hysteresis) were evaluated using Spearman correlation. Results: Six individuals (4 male, mean(SD) age: 56(5)years, body mass index: 30(6) kg/m2) were included. Reason for amputation was diabetes-related complications in 5 participants and failed orthopaedic surgery in 1 participant. In DTI (Figure 1A), tendons had a tract length of 11.5(11.3)mm and tract density of 23.9(2.4) per ROI. FA, MD, AD, and RD quantify how freely a water molecule is able to move within the tissue, and the directionality of that movement. Tendons had an FA of 0.26(0.25), MD of 1.25(1.28), AD of 1.54(1.57), and RD of 1.11(1.14). Tract length was positively related to linear stiffness (rho=0.829, p=0.04) and hysteresis at 10% strain (rho=0.886, p=0.019) (Figure 1B-D). AD was positively related to hysteresis at 10% strain (rho=0.812, p=0.05). Conclusion: This is the first study to describe posterior tibialis tendon appearance on DTI. Tract length and AD are both related to tendon mechanics. Tract length is based on quantity and directionality of water displacement and may indicate degree of collagen organization given its relationship to stiffness. Tract length and AD related positively to hysteresis, which will require additional research to identify the mechanisms behind this relationship. This study is limited by sample size and specimens that likely do not represent healthy tissue. Regardless, these findings support continued investigation into in vivo imaging of tendon with DTI for quantitative tendon assessment.