Objective. Although spinal cord stimulation (SCS) is an established therapy for treating neuropathic chronic pain, in tonic stimulation, postural changes, electrode migration or badly-positioned electrodes can produce annoying stimulation (intercostal neuralgia) in about 35% of the patients. SCS models are used to study the effect of electrical stimulation to better manage the stimulation parameters and electrode position. The goal of this work was to develop a realistic 3D patient-specific spinal cord model from a real patient and develop a future clinical application that would help physicians to optimize paresthesia coverage in SCS therapy. Approach. We developed two 3D patient-specific models from a high-resolution MRI of two patients undergoing SCS treatment. The model consisted of a finite element model of the spinal cord and a sensory myelinated nerve fiber model. The same simulations were performed with a generalized spinal cord model and we compared the results with the clinical data to evaluate the advantages of a patient-specific model. To identify the geometrical parameters that most influence the stimulation predictions, a sensitivity analysis was conducted. We used the patient-specific model to perform a clinical application involving the pre-implantation selection of electrode polarity and study the effect of electrode offset. Main results. The patient-specific model correlated better with clinical data than the generalized model. Electrode-dura mater distance, dorsal cerebrospinal fluid (CSF) thickness, and CSF diameter are the geometrical parameters that caused significant changes in the stimulation predictions. Electrode polarity could be planned and optimized to stimulate the patient’s painful dermatomes. The addition of offset in parallel electrodes would not have been beneficial for one of the patients of this study because they reduce neural activation displacement. Significance. This is the first study to relate the activation area model prediction in dorsal columns with the clinical effect on paresthesia coverage. The outcomes show that 3D patient-specific models would help physicians to choose the best stimulation parameters to optimize neural activation and SCS therapy in tonic stimulation.
The investigation of the effect of the stimulation parameters by computational modeling helps to understand the electrical response of specific neural elements in Spinal Cord Stimulation (SCS) therapy for chronic pain treatment. While the effect of the amplitude, the pulse width, and the electrode configuration on neural activation has been widely studied and is well-established in tonic stimulation, how frequency influences neural activation remains unclear. Thus, the aim of this work is to study the effect of frequency on the electrical response of sensory Aβ neurons in tonic stimulation. Our approach consisted of the development of a new nerve fiber model from the combination of two previous models used in SCS modeling (the Wesselink-Holsheimer-Boom model and the Richardson-McIntyre-Grill model B). We simulate the action potential and the gates probabilities evolution of a 12.8 µm fiber diameter at different pulse frequencies (50, 350, 600, 800, and 1000 Hz). We also simulated the firing rate of two nerve fiber diameters (5.7 and 12.8 µm) in function of pulse frequency (from 1 to 1400 Hz) at different pulse widths (100, 300, and 500 µs). In the range of 2-1000 Hz, the firing rate of a 12.8 µm-diameter nerve fiber can be maximized by utilizing a 350 Hz, 300 µs-stimulus. Frequencies above 350 Hz reduce half to one-third the firing rate, and 1000 Hz-stimulus overrides the electrical activity of the sensory nerve fiber. Small fibers (5.7 µm-diameter) present lower firing rate values than large fibers (12.8 µm-diameter). High values of pulse width decrease the firing rate of the nerve fibers as well as the range of frequencies that could be used to stimulate. According to the results, the frequency could have a considerable implication on the modulation of the firing rate of a nerve fiber. Thus, the frequency could play an important role to select and increase the activity of specific neural elements of the spinal cord in SCS therapy.
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