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Comparisons of Rabbit Flexor Tendon Stiffness via Acoustoelastic Ultrasound during Mechanical Testing and In vivo Loading
Danielle Stoll-Tronnes, MS; Ellen M. Leiferman, DVM; Michael Stitgen, BS; Sarah Duenwald-Kuehl, PhD; Ray Vanderby Jr, PhD; Jonathan L. Tueting, MD
Orthopedics and Rehabilitation, University Wisconsin Madison, Madison, WI

Hypothesis: The purpose of this study was to investigate whether acoustoelastic (AE) ultrasound is able to detect stiffness differences between healing and normal rabbit tendons during mechanical testing.

Methods: Eighteen female New Zealand White rabbits (~2.5 kg) underwent transections and repair of left 3rd flexor digitorum profundus (FDP) tendons in zone II. 0, 3, and 7 weeks after repair, ultrasound video was recorded as the healing tendon was stretched. Each rabbit was sacrificed and the tendons harvested. The right 3rd digit served as the control. Each normal and healing tendon was submerged in a bath of saline and pulled to failure (Mark-10 ESM301 Copiague, NY) at a rate of 20mm/min with a 0.2N preload. Dynamic B mode ultrasound images of tendons were recorded during testing using a linear array transducer at 12MHz and GE LOGIQe ultrasound (General Electric, Fairfield, CT). The ultrasound videos were processed in an image analysis program (EchoSoftTM, Echometrix, Madison, WI) to compute the stiffness gradient index (SGI) of the healing and normal tendons. Additionally, ultimate load was recorded from the results of the mechanical test. Additionally we conducted a pull failure test of a transected and sutured 3rd digit FDP from cadaver New Zealand white rabbits (week 0) to serve as a sutured control.

Results: The SGI (normalized stiffness) values obtained from the ultrasound during the pull to failure tests appeared higher in control tendons than in the healing tendons (obtained through ultrasound in vivo and pull to failure; Figure 1). The stiffness obtained from the healing tendon in vivo and via ex vivo mechanical testing were generally similar. Figure 1 displays the most distinct differences between the different groups. Inconsistences obtained with AE ultrasound for other specimens could be due to the small size the of rabbit tendons. Stiffness of the healing tendons calculated from in vivo and ex vivo images appeared fairly similar, with some deviation. This deviation could be attributed to the different loading rates of the in vivo stretching and ex vivo pull to failure.

Summary: Tendon SGI quantified via AE in vivo appears similar to the stiffness quantified via AE ultrasound ex vivo (pull to failure) SGI of normal FDP tendons is greater than healing FDP tendons at week 7 obtained through AE ultrasound The ultimate load of the normal FDP tendon is higher than the healing tendon at week 3 and 7

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