Figure 1.
Simulation of the field density of the used electrodes.
Using the Finite Element Method (FEM) the distributions of currents are visualized for (A) a flat gel electrode that can be used for TENS, (B) a concentric electrode, and (C) a matrix array electrode. Assuming an ohmic skin resistance and isotropic electrical properties of skin and underlying tissue, only the matrix array and concentric electrodes showed high current densities preferentially distributed across superficial layers of the skin. The gel electrode showed at least 7 times smaller maximum current densities with a much deeper current distribution. Note the different scaling of the colour codings of current densities as well as the penetration depths.
Figure 2.
Schematic drawing of stimulation sites at the volar forearm.
The concentric electrode serves as its own reference with the cathode centred as a single pin in the centre of the electrode surrounded by the annular reference (anode). The matrix array electrode is used as the cathode with a flat gel electrode that serves as the reference electrode (anode).
Figure 3.
Schematic drawing of the distribution of current density.
Both (A) a concentric electrode and (B) a matrix array electrode induce high current densities distributed within superficial layers of the skin predominately activating intracutaneous nerve fibres.
Figure 4.
Comparison between the concentric and matrix array electrode after 4 Hz stimulation.
Z values are shown according to the expression: Z = (mean baseline - mean stimulation)/SD baseline. A z-value of “0” corresponds to the mean according to the unconditioned control condition. Positive Z values indicate a functional gain, while negative Z values indicate a loss of function for the respective sensory pathway. Stars or crosses denote the level of significance with *p<0.05; **p<0.01; ***p<0.001 for the comparison to the baseline condition; ++p<0.01 for the comparison between electrode types.
Figure 5.
Comparison between the concentric and matrix array electrode after 30 Hz stimulation.
The use of this type of a higher stimulation frequency also induced increased thermal and mechanical detection and pain thresholds. Using the matrix electrode thermal detection and deep pain thresholds remained unaltered. Stars or crosses denote the level of significance with *p<0.05; **p<0.01; ***p<0.001 for the comparison to the baseline condition; ++p<0.01 for the comparison between electrode types.
Figure 6.
QST raw data compared across different stimulation frequencies.
Following conditioning stimulation with the matrix array electrode, all mechanical perception and pain thresholds (except the windup ratio) were significantly increased. This effect was pronounced after 4 Hz rather than 30 Hz stimulation, when compared to baseline condition. Stimulation using a 30 Hz frequency was an optimal sham condition, when assessing deep pain thresholds. However, this frequency still was effective in reducing superficial mechanical sensitivity. Stars or crosses denote the level of significance with *p<0.05; **p<0.01; ***p<0.001 for the comparison to the baseline condition.
Table 1.
ANOVA of QST parameters.
Figure 7.
Factor analysis of VARIMAX rotated QST data after 4 Hz matrix array stimulation.
All mechanical pain thresholds (MPT, MPS, WUR, PPT) show significant loadings on factor 1 “mechanical pain” (x-axis). Thermal pain thresholds show significant loadings on factor 2 “thermal pain” (y-axis), while thermal and mechanical detection thresholds take an intermediate position, indicating that 4 Hz stimulation differentially effects different peripheral and central sensory channels to the brain.
Table 2.
Factor analysis of QST parameters.