American Society for Peripheral Nerve

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Grip Strength Measurement with Implantable Nerve Cuff Electrodes: Evolution of a Novel Method for Functional Assessment
Nicholas von Guionneau, MBBS1; Chris Frost, MD1; Maria Lopez, BE2; Nicholas Hricz, BS1; Benjamin S Slavin, BS1; Karim A Sarhane, MD1; Connor Glass, BS1; Loren Rieth, PhD2; Sami Tuffaha, MD1
1Johns Hopkins University School of Medicine, Baltimore, MD; 2Feinstein Institute for Medical Research, Northwell Health, Manhasset, NY

BACKGROUND: Reliable measurement of functional recovery is critical in translational peripheral nerve regeneration research. Behavioral functional assessments in rodents such as volitional grip strength testing (vGST), are limited by inherent behavioral variability. Isometric tetanic force testing (ITFT) is highly reliable but precludes serial measurements. Combining elements of vGST and ITFT, stimulated grip strength testing (sGST) is a recent innovation that involves percutaneous median nerve stimulation to elicit maximal tetanic contraction of digital flexors, thereby allowing for consistent measurement of maximal grip strength. Implantable nerve cuff electrodes are an alternative to percutaneous nerve stimulation and may further improve the reliability of this novel functional test.

METHODS: An implantable electrode with high in vitro stability was adapted to interface with the rat median nerve. Platinum helical wires encapsulated in epoxy and silicone link an electrode cuff to a skull-mounted Omnetics interface connector. An external stimulator plugs into the Omnetics connector. We determined optimal in vivo stimulation parameters by measuring grip strength after median nerve stimulation with a cuff electrode at a range of voltages (1-8V) in adult Lewis rat forelimbs (n = 8). sGST accuracy using implantable cuff electrodes was determined by left-to-right equivalence of grip strength and compared to percutaneous sGST, vGST and ITFT (n = 7/group). Repeat stimulations were used to determine relative reliability.

RESULTS:

Optimal electrode stimulation parameters: grip strength in rats with intact median nerves increased with higher voltages until reaching a plateau with stimulation above 3V. Stimulation at 6V reliably induces maximal tetanic forelimb digital flexor contraction.

Left-to-right concordance: cuff electrode sGST provides equal accuracy to percutaneous sGST and ITFT and significantly greater accuracy than vGST. Mean absolute differences between left and right limbs were: cuff sGST 7.0 8%; percutaneous sGST 5.8 7%; ITFT 19.7 13%; vGST 39.5 7%.

Inter-trial reliability: cuff electrode sGST had the lowest coefficient of variation between repeat trials (0.02 0.05), and both cuff electrode sGST and percutaneous sGST (0.03 0.04) were significantly more reliable than vGST (0.29 0.04).

CONCLUSION: Implantable electrode cuffs can be used to assess forelimb function with high accuracy and reliability and may overcome limitations of currently available functional testing modalities. Future studies will determine the validity of using cuff electrodes to assess longitudinal functional recovery after nerve inj


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