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Ultrasound Shear Wave Metric for Assessing Flexor Tendon Healing: A Preliminary Cadaver Study
Michael Schrader, BS1; Gilbert Smolyak, BS2; Constantinos Ketonis, MD, PhD3; Alayna E. Loiselle, PhD2; Michael S. Richards, PhD1
1Rochester Institute of Technology, Rochester, NY; 2University of Rochester, Rochester, NY; 3University of Rochester Medical Center, Rochester, NY

Introduction: Flexor tendon (FT) injuries of the hand are common and functional restoration after surgical repair represents a challenge. Currently, there are no longitudinal non-invasive metrics of tendon healing. B-mode ultrasound (US) has been proposed as a metric to quantify scar-tissue volume (STV) during tendon healing, however STV is not a functional metric. We propose using shear wave elastography to measure the in vivo force in the tendon with a direct measure of the external force at the tip of the injured digit as functional metric of the mechanical efficiency of the tendon.
Methods: A custom rig was designed to hold cadaver hands in a fixed position. A force sensor was attached to the distal phalanx of the tested finger to maintain an isometric state during testing. The flexor digitorum profundus (FDP) tendon was attached to an Instron machine (Fig. 1). A GE 9-LD linear array transducer was used to image the tendon in the palm, just distal to the transverse carpal ligament. A Verasonics Vantage 256 system was used to generate acoustic radiation pushes in the tendon and the shear wave speed (SWS) was measured at the push depth. Tendon force, SWS estimated force and fingertip force were acquired for 1 mm increments of tendon extension from rest up to a max SWS estimate of 25 m/s. SWS force measurement was compared to the Instron Force measurement using a linear regression. The baseline measurement of tendon force efficiency (EFFINS) was found by a linear regression of the Instron force versus fingertip force. The SWS estimated efficiency (EFFSWS) was similarly calculated for comparison.
Results: Linear regression of the US measured SWS estimated force versus Intron measured force resulted in slopes of 0.86 (R2=.91), 1.02 (R2=.83) and 1.28 (R2=.93). The calculated EFFSWS for these tendons were 0.09, 0.17 and 0.14 verses the EFFINS measurements of 0.08, 0.18 and 0.18 respectively.
Conclusions: Our preliminary data suggests the feasibility measuring the EFFSWS metric utilizing noninvasive SWS speed estimates to quantify the force in a FT while simultaneously measuring finger-tip force. In combination with a STV measurement for quantifying of scar tissue size, these clinical metrics have the potential to improve the longitudinal assessment of tendon healing following injury and guide patient specific therapy protocols.
Acknowledgements: This study was supported by a pilot award from the University of Rochester Medical Centerís UNYTE Translational Research Network in the Clinical & Translational Science Institute.


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