Concentric Versus Delta Bipolar Probes for Intraneural Fascicle Selection: A Rabbit Model Study

Abstract

Background: Bipolar stimulating probes are widely used during intraoperative monitoring in nerve surgery. However, there are no reports comparing different types of bipolar probes. We present the differences between 2 different bipolar probes (concentric and Delta) in a rabbit animal model for brachial plexus injury, focusing on their applicability for intraoperative neuromonitoring in peripheral nerve surgery. Methods: Using a previously described rabbit animal model, the biceps brachii muscle's compound muscle action potentials (CMAPs) were recorded after stimulation with increasing intensities (ranging from 0.2 to 3 mV) of the middle and lower trunks using both concentric and Delta probes. The correlation between stimulus intensity and CMAP amplitude was studied, as well as interstimulus amplitude variability. Results: Five rabbit brachial plexuses were studied. CMAP amplitude ranged from 0 to 9 mV. No correlation was found between stimulus intensity and CMAP for the Delta probe (Spearman rank test R = 0.181; P = 0.264). A strong correlation was found between stimulus intensity and CMAP using the concentric probe (Spearman rank test R = 0.74; P = 0.001). The Delta probe showed less variability (coefficient of variation 0.01 versus 0.1). Conclusions: Delta probes elicit highly reproducible, high-CMAP amplitude responses even with low-intensity stimuli. Concentric probes produce slightly less reproducible, progressively higher CMAP amplitudes with increasing stimulus intensities