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The effect of lead placement and programming strategies on spinal cord stimulation (SCS) therapy has been widely studied; however, there is a need to optimize these parameters to favor dorsal column (DC) over dorsal root (DR) stimulation in complex pain treatment. This study aimed to determine the optimal lateral distance between two leads and the effect of transverse stimulation using a mathematical model.A three-dimensional computational SCS and a nerve fiber model were used to determine the effect of the lateral distance between two leads at the same vertebral level T8 and the effect of the addition of anodes with two parallel leads at T8 and three different lateral distances on the model-based results (perception thresholds, activated DC fiber area and depth, and position of the first stimulated fiber).With two parallel leads programmed with symmetrical polarities, the maximal DC fiber area stimulated was found for a lateral distance of 5 mm. The results also show a higher preference for DR stimulation as the lateral distance increased. The addition of positive contacts at the same level of active contacts in the second lead produces a displacement of the first stimulated fiber laterally.A lateral distance of 5 mm shows a DC stimulated fiber area greater than when leads are placed contiguously. The addition of anodes creates an effect whereby the area of paresthesia is not displaced to the midline, but in the opposite direction. This may be useful when the leads are too close and stimulation of one of the sides is compromised.

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 $\text{A}\beta $ 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 $12.8~\mu \text{m}$ 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 $12.8~\mu \text{m}$ ) in function of pulse frequency (from 1 to 1400 Hz) at different pulse widths (100, 300, and $500~\mu \text{s}$ ). In the range of 2–1000 Hz, the firing rate of a $12.8~\mu \text{m}$ -diameter nerve fiber can be maximized by utilizing a 350 Hz, $300~\mu \text{s}$ -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 $\mu \text{m}$ -diameter) present lower firing rate values than large fibers ( $12.8~\mu \text{m}$ -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.

[EN] 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 authors are grateful to Surgicen S. L. for providing financial assistance, also to thank Joaquin Bosque Hernandez (nurse from the Magnetic Resonance Unit of the Hospital Universitari i Politecnic La Fe) for readjusting the MRI acquisition protocol based on the capacity of the MR equipment, which made the study possible. Finally, the authors wish to express their gratitude to Virginie Callot for providing us with all the spinal cord measurements of her research group's study.

[EN] Objective: Using computer simulation, we investigated the effect of electrode polarity on neural activation in spinal cord stimulation and propose a new strategy to maximize the activating area in the dorsal column (DC) and, thus, paresthesia coverage in clinical practice. Materials and Methods: A new three-dimensional spinal cord model at the T10 vertebral level was developed to simulate neural activation induced by the electric field distribution produced by different typical four-contact electrode polarities in single- and dual-lead stimulation. Our approach consisted of the combination of a finite element model of the spinal cord developed in COMSOL Multiphysics and a nerve fiber model implemented in MATLAB. Five evaluation parameters were evaluated, namely, the recruitment ratio, the perception and discomfort thresholds, and the activating area and depth. The results were compared quantitatively. Results: The dual-guarded cathode presents the maximum activating area and depth in single- and dual-lead stimulation. However, the lowest value of the ratio between the perception threshold in DC and the perception threshold in the dorsal root (DR) is achieved when the guarded cathode is programmed. Although the two versions of bipolar polarity (namely bipolar 1 and bipolar 2) produce higher activating area and depth than the guarded cathode, they are suitable for producing DR stimulation. Similarly, dual-lead stimulation is likely to activate DR fibers because the electrodes are closer to these fibers. Conclusions: The results suggest that the activating area in the DC is maximized by using the dual-guarded cathode both in single- and dual-lead stimulation modes. However, DC nerve fibers are preferentially stimulated when the guarded cathode is used. According to these results, the new electrode programming strategy that we propose for clinical practice first uses the dual-guarded cathode, but, if the DR nerve fibers are activated, it then uses guarded cathode polarity., The authors thank Virginie Callot for providing us with all the spinal cord measurements from her research group’s study. The authors would like also to thank Surgicen S.L. for providing financial assistance