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Non-Invasive Isometric Muscle Force Measurement of Plantar Flexion in the Rat Sciatic Nerve Injury Model
Aditya S. Chawla, BS; Robert J. Spinner, MD; Michael J. Yaszemski, MD, PhD; Anthony J. Windebank, MD; Huan Wang, MD, PhD
Mayo Clinic, Rochester, MN, USA
Background: Muscle force restoration is a practical and functional outcome capable of tracking peripheral nerve recovery. Presently, in the rat sciatic nerve injury model maximal isometric force is measured by detaching tendons and fixing them to load cells. As this method is terminal it prevents longitudinal data collection. This study aims to validate a minimally invasive method of measuring changes in isometric plantar flexion force in rats over the course of nerve regeneration.
Methods: Seven female Lewis rats weighing 200-210 grams were used. A nerve injury was created by transecting the sciatic nerve at mid-thigh level. An immediate repair was performed using four circumferentially placed 10-0 sutures. Force measurements were obtained using a custom dynamometer. A leg restraint maintained the rat's hind limb in 90° of hip flexion, 90° of knee flexion, 20° of ankle dorsiflexion, and prevented movements outside the sagittal plane. A load cell was positioned against the ball of the foot. Plantar flexion was then evoked either directly or indirectly by delivering a square pulse current via needle electrodes, stereotactically, inserted into the gastrocnemius muscle or near the sciatic nerve, respectively. Measured torque values were used to determine the force generated by muscles of plantar flexion. Nerve conduction study was also carried out to provide a secondary index of nerve function by which to gauge the sensitivity of our force data. Data were collected at baseline and then 7, 14, 28, 42, 56, and 84 days after the date of injury.
Results: During the first 28 days after nerve injury, tibial nerve conduction studies demonstrated compound muscle action potential (CMAP) amplitudes of 0 mV. During this same period direct muscle stimulation measurements consistently decreased, reflecting muscle atrophy. With one outlier, nerve stimulation elicited forces approaching zero Newtons. From day 42 onwards nerve conduction studies reported continually increasing CMAP amplitudes, indicating the beginning of muscle reinnervation. Simultaneously, both direct and indirect stimulation protocols elicited continually increasing force values.
Conclusion: The custom made rat dynamometer coupled with direct or indirect muscle stimulation offers a minimally invasive method of measuring maximal isometric force of plantar flexion to longitudinally track peripheral nerve regeneration. While subject to further refinement, these techniques are sensitive enough to detect the initial stages of muscle reinnervation.
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