(D-146) EFFECTS OF AN END-LIMB, MUSCLE-ATTACHED ENDOPROSTHESIS IN RABBIT MUSCLE PROPERTIES

Friday, October 13, 2023

3:30 PM – 4:30 PM PDT

Location: Exhibit Hall - Row D - Poster # 146

Presenting Author(s)

FA

Fisher S. Adkisson

Undergraduate Researcher
University of Tennessee, Knoxville
Lenoir City, Tennessee, United States

Co-Author(s)

DC

Dustin L. Crouch

Professor
University of Tennessee Knoxville, United States
KE

Katrina L. Easton

Postdoctoral Research Fellow
University of Tennessee Knoxville, United States

Introduction::

Introduction: End-limb, MUscLe-ATtached Endoprostheses (EMULATEs) that physically attach to muscles may restore more natural sensorimotor function to people with amputation than current prostheses that are worn externally on the body. The function enabled by EMULATEs is expected to depend partly on the force-generating behavior of the attached muscles. Muscle force-generating behavior is strongly related to muscle properties, such as mass and length, according to well-characterized relationships (e.g., force-length-velocity) [1]. Amputation, muscle-EMULATE geometry (e.g., moment arms), EMULATE material properties, EMULATE limb use, and other factors may affect muscle properties and, thus, muscle force-generating behavior. The objective of this study was to quantify two muscle properties, mass, and length, of muscles in the hindlimb of rabbits with a foot-ankle EMULATE prototype. The leading hypothesis for this study was that muscles on the operated side with the EMULATE would have lesser mass and length than muscles on the contralateral intact side.

Materials and Methods::

Methods and Materials:

In four 16-week-old New Zealand white rabbits, we surgically replaced the left hindlimb with a jointed foot-ankle EMULATE prototype. The rabbits were part of an iterative, developmental, proof-of-concept study and, therefore, interventions were not the same across all rabbits. For example, the tibialis cranialis and triceps surae muscles were sutured to the endoprosthetic foot across the ankle joint with either the biological tendons (n=2) or polyester, silicone-coated artificial tendons (n=2) [2]. Also, the rabbits were euthanized at either 8 (n=1), 16 (n=2), or 24 weeks (n=1) post-surgery. For all rabbits, both hind limbs were removed at the hip, skinned, then fixed in 10% neutral buffered formalin for at least 5 days with the knee at 90° flexion and the ankle in neutral dorsi/plantar flexion. The fixed limbs were transferred to 70% ethanol for at least 2 days. From six muscles on each side (Figure 1), we measured muscle mass and length using a digital scale and caliper, respectively. For each rabbit, muscle, and variable, we computed the percent difference between sides. We then computed the mean and standard deviation of percent differences across rabbits. We performed one-tailed, one-sample Student’s t-tests to identify percent differences that were significantly different from zero (p ≤ 0.05). Finally, we calculated the ratio of mean percent difference in mass to length (M/L) for each muscle.

Results, Conclusions, and Discussions::

Results & Discussion: On average, muscle mass was less on the operated side than on the intact side for all muscles; the difference was statistically significant for LG = 36% (p=0.001), MG = 32% (p < 0.001), and Sol = 77% (p < 0.001) (Figure 1). Muscles on the operated side likely experienced disuse atrophy despite some being attached across the EMULATE ankle. This was potentially due to fibrotic encapsulation of the EMULATE, which limited its range of motion. Muscle length for all the muscles was significantly less on the operated side compared to the intact side, even for muscles that were attached to the EMULATE (Figure 1). The percent differences in muscle length between sides were LG = 18% (p=0.004), MG = 19% (p=0.004), Sol = 30% (p < 0.001), FD = 26% (p=0.002), TA = 18% (p=0.001), and ED = 21% (p=0.004). The muscles may have retracted during the surgical amputation, and, in the case of attached muscles, not stretched back to their pre-amputation lengths when attached to the EMULATE. Muscles in a chronically shortened position may become functionally shorter by losing sarcomeres in series [3], limiting the muscle’s excursion capacity. The M/L ratios for each muscle were LG = 2.0, MG = 1.7, Sol = 2.5, FD = 0.41, TA = 0.49, and ED = 0.58. Interestingly, the triceps surae muscle group had relatively higher bilaterial differences in mass than in length, whereas the opposite was observed for the other measured muscles; future studies should investigate the reason for this difference among muscles.

Conclusion: For several muscles crossing the ankle, muscle mass and length were significantly lower on the operated (EMULATE) side than on the contralateral intact side. Relative bilateral differences in mass and length varied among muscles. A loss of muscle mass and length will likely reduce the ability of the muscles attached to an EMULATE to generate force and change length during muscle contraction, limiting the overall biomechanical output of the EMULATE. Future work will investigate factors that contribute to loss of muscle mass and length, as well as potential strategies to avoid such loss.

Acknowledgements (Optional): :

Acknowledgements: UTK Office of Laboratory Animal Care. NIH R61AR078096.

References (Optional): :

References: [1] Zajac FE, 1989. Crit Rev Biomed Eng, 17(4):359-411. [2] Hall PT, et al., 2023. J Biomech (Accepted). [3] Abrams RA, et al., 2000. Musc Nerve, 23:707-714.