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Modeling of Spinal Cord Stimulation for the PENTA Paddle Lead with Various Stimulation Configurations

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Spinal Cord Stimulation

Modeling of Dermatome Selectivity of Multiple- and Single-Current Source Spinal Cord Stimulation Systems
Xiaoyi Min, Stuart Rosenberg and Tim Fayram
From St Jude Medical, Inc., Sylmar, CA  
Introduction:   Computational modeling of spinal cord stimulation (SCS) has been well established in understanding lead designs and stimulation programming. Recent published work(Lee et al, 2011) predicted that a multiple current source (MCS) system can target a greater number of central points of stimulation in the dorsal column (DC) than a single current source (1CS) system.  However the clinical relevance of the number of central points on improvement in paresthesia coverage in patients has not been established.  This analysis compares the dermatome region selectivity of MCS and 1CS systems.
Methods:   A finite element model was built consisting of an inhomogeneous volume conductor with representations at T8 for the white and gray matter, DC, cerebrospinal fluid with a thickness of 3.2 mm, dura, epidural fat, vertebral bone, (Holsheimer and Barolat, 1998)  and a 2x8 paddle electrode array.  Two aligned tripoles were studied using incremental current changes between the two cathodes, modeling the MCS system.  For the 1CS system, the current was delivered to either cathode or both cathodes.  The activation regions were defined by a threshold from the second derivative of the voltage along axon fibers, and axon coverage was further mapped to the dermatomes by an established template (Feirabend et al., 2002).  
For both MCS and 1CS with 100% of the current to the right cathode, dermatomes covered were right L4, L5, and S1-S5 and left S2-S5.  When injecting current to both right and left cathodes in 1CS, only 1 dermatomal region on each side was changed (right L4 was no longer covered and left S1 was covered).  Incrementally splitting the current between the cathodes of the MCS system showed that these changes in coverage occurred with a minimum step size of 12.5%, so smaller steps were unnecessary.  Further, the 1CS system achieved the same changes in coverage by adjusting the current amplitude.  Finally, the wide overlap of activated regions suggests that using central points of activation as proxy for dermatome coverage is not clinically relevant.  It also showed with a 2x8 paddle array, back fibers in the lateral region of the DC ( i.e. L1&L2 for back pain) seemed harder to be reached.
Conclusions:  Computer modeling shows no difference in activated dermatomes between multiple- and single-current source SCS systems.  The higher resolution of central points provided by a multiple current-source system has little impact on dermatome coverage, so it may not provide incremental therapeutic benefit.