Tripolar-cuff deviation from ideal model: assessment by bioelectric field simulations and saline-bath experiments

Ideally, interference in neural measurements due to signals from nearby muscles can be completely eliminated with the use of tripolar cuffs, in combination with appropriate amplifier configurations, such as the quasi-tripole (QT) and the true-tripole (TT). The operation of these amplifiers, is based on the theoretical property of the nerve cuff to produce a linear relationship of potential versus distance along its length, internally, when external potentials appear between its ends. Thus, in principle, electroneurogram (ENG) recordings from an ideal tripolar cuff would be free from electromyogram (EMG) interference generated by nearby muscles. However, in practice the cuff exhibits non-ideal behaviour leading to "cuff imbalance". The main focus of this paper is to investigate the causes of cuff imbalance, to demonstrate that it should be incorporated as a main parameter in the theoretical ENG-recording cuff electrode model. In addition to cuff asymmetry and tissue growth, the proximity of the interference source to the cuff is shown to result in cuff imbalance. The influence of proximity imbalance on the performance of the QT and TT amplifiers is also considered. Proximity imbalance is studied using bioelectric field simulations and saline-bath experiments. Variation is observed with both distance (40 mm and 70 mm was examined) and orientation (0-180 degrees), with the latter causing a more severe effect especially when the source dipole and the cuff are vertical to each other. The simulations and measurements are in close agreement. Tissue growth imbalance and asymmetry imbalance are also investigated in vitro. Finally, the signal-to-interference ratio (SIR; ENG/EMG) of the QT and TT amplifiers is examined in the presence of cuff imbalance. It is shown that proximity imbalance results in their SIR to peak only at certain cuff orientation values. This important finding offers an insight as to why in practice ENG recordings using these amplifiers have been widely reported to be degraded by EMG interference.