SNS electrode; adjacent tissues; adverse stimulation effects; bioelectric phenomena; bioelectric potentials; biological tissues; biomechanics; biomedical electrodes; biophysical models; bipolar stimulation; computational modelling; contact configuration; current flow; electric current; electrical stimulus; electrode contacts; electrode model choice; established treatment; finite element analysis; finite element model; induced spread; monopolar stimulation; nerve fibres; neural fibre stimulation; neuromuscular stimulation; neurophysiology; patient treatment; physiological models; prosthetics; quadripolar electrode; sacral foramen; sacral nerve root; sacral nerve stimulation; similar effect; stimulation parameters; therapeutic stimulation effects