Pre-Operative Forces Obtained from Stimulation of Intact Lower Motor Nerves for Anticipation of Nerve Transfer Outcomes in the Tetraplegic Patient

Introduction: Following complete spinal cord injury (SCI), muscles distal to the injury site lack upper motor neuron connections and may experience peripheral denervation. For patients ≥ 20 months post-injury, it is essential to assess the innervation of paralyzed target muscles before nerve transfer. Our team uses transcutaneous stimulation of target muscles and manual force grading to make this determination.

Our

Objective: To evaluate the reliability of pre-operative stimulation-induced force output in determining the reinnervation potential of target muscles more than 20 months post-injury.

Methods: We reviewed pre-operative stimulation-induced strength (Newtons), for each digit in 14 limbs from 10 patients who underwent hand reanimation surgery 20+ months post-injury. Electrodes were placed on the volar or dorsal forearm musculature to assess finger flexors and/or extensors, respectively, with stimulation of 3–10 mA at 100 Hz from a neuromuscular stimulator. Flexion and extension forces were obtained from each finger. Postoperatively, volitional force generation was assessed, typically with activation of the donor axon source (e.g., supination for extension, elbow flexion for finger flexion).
We performed paired t-tests between pre-operative stimulated force values and post-operative voluntary force measurements where a p value of ≤0.05 was considered significant.

Results: We found that pre-operative stimulated forces overestimated the outcome forces across all cases. However, this overestimation was not found to be significant (95% confidence intervals) for forces of the FPL, FDP2, FDP3, EDC3, EDC4, or EDC5 (FDP 4 & 5 not studied). We do note a significant difference in the EPLs (p=0.0006) and EDC2s (p=0.032), in which average volitional force outcomes were 45% and 62% of average initial stimulated forces, respectively.

Conclusion : These data indicate that pre-operative stimulation forces can predict outcomes following nerve transfer surgery. In cases where these metrics significantly differed, a correlation exists, rendering stimulation a useful tool in anticipating nerve-transfer outcomes.