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Echogenicity and Stiffness Changes of Healing Flexor Digitorum Profundus Tendons in Zone II: a Rabbit Model
Danielle M. Stoll; Jonathan L. Tueting, MD; Sarah Duenwald-Kuehl, PhD; Ellen M. Leiferman, DVM; Ray Vanderby Jr, PhD; University Wisconsin Madison
University of Wisconsin Madison, Madison, WI, USA

Hypothesis

Acoustoelastic ultrasound can be used to detect stiffness and healing changes of repaired zone II flexor tendons in the rabbit model.

Methods

The flexor digitorum profundus (FDP) tendons for the 3rd and 4th digits (forepaw) of a female New Zealand white (NZW) rabbit (~2.5 kg) were transected in zone II. Each tendon was repaired with a modified Kessler stitch and the skin closed with single interrupted stitches. Using a GE Logiqe ultrasound (linear array transducer 12 l-RS) at 12 MHz, ultrasound videos were pre- and post-operatively obtained. With the transducer over the surgical site, each digit was passively extended from a neutral position to initiate a small tendon stretch as videos were recorded. The videos were processed in an image analysis program (EchoSoftTM, Echometrix, Madison, WI) to compute the mechanical properties in the healing tendon.

Results

Acoustoelastic ultrasound detects the changes in echo intensity of deformed elastic materials.1 The material stiffness changes with deformation altering the impedance, sounds wave speed, and reflection coefficient.2 Stiffness changes represent functional compromise in healing tissue. The echo intensity and stiffness of healing rabbit FDP varies from the normal. Figure 1(a) demonstrates the frequency of echo change, where a tall/narrow curve indicates homogeneous properties and a short/wide curve indicates inhomogeneity in the tissue. Normal tendon shows a tall narrow curve, while curves from tendons 1 and 2 weeks post-repair are much shorter and wider. Figure 1(b) shows the stiffness vs. strain curve; stiffness is reduced from normal at weeks 1 and 2 after the repair. It is probable that consecutive increases in stiffness and echo homogeneity in the FDP will result at additional time points, signifying an increase of strength of the healing FDP tendon.

Conclusions

Acoustoelastic ultrasound can detect changes in tissue stiffness from echo intensity in healing FDP's of the rabbit forepaw

Quantifying the changes in stiffness of a zone II FDP is advantageous as it could aid in early and customized rehabilitation programs for human FDP repairs

Customized rehabilitation (based on stiffness) for patients with zone II FDP repairs would aid in early mobilization by eliminating adhesions without rupturing the sutures

1(a) shows the frequency of echo changes at a specific intensity. The normal (pre-operative) has many tendon fibers display similar frequencies of echo change within a small range of echo intensity.

1(b) The FDP tendon is stiffest preoperatively compared to 1 and 2 weeks after repair.


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