title: Muscle Activation During Peripheral Nerve Field Stimulation Occurs Due to Recruitment of Efferent Nerve Fibers, Not Direct Muscle Activation.
abstract: BACKGROUND
  Peripheral nerve field stimulation (PNFS) is a potential treatment for chronic low-back pain. Pain relief using PNFS is dependent on activation of non-nociceptive Aβ-fibers. However, PNFS may also activate muscles, causing twitches and discomfort. In this study, we developed a mathematical model, to investigate the activation of sensory and motor nerves, as well as direct muscle fiber activation.
  
  
  METHODS
  The extracellular field was estimated using a finite element model based on the geometry of CT scanned lumbar vertebrae. The electrode was modeled as being implanted to a depth of 10-15 mm. Three implant directions were modeled; horizontally, vertically, and diagonally. Both single electrode and "between-lead" stimulation between contralateral electrodes were modeled. The extracellular field was combined with models of sensory Aβ-nerves, motor neurons and muscle fibers to estimate their activation thresholds.
  
  
  RESULTS
  The model showed that sensory Aβ fibers could be activated with thresholds down to 0.563 V, and the lowest threshold for motor nerve activation was 7.19 V using between-lead stimulation with the cathode located closest to the nerves. All thresholds for direct muscle activation were above 500 V.
  
  
  CONCLUSIONS
  The results suggest that direct muscle activation does not occur during PNFS, and concomitant motor and sensory nerve fiber activation are only likely to occur when using between-lead configuration. Thus, it may be relevant to investigate the location of the innervation zone of the low-back muscles prior to electrode implantation to avoid muscle activation.
authors.affiliation: Department of Health Science & Technology, Integrative Neuroscience Group, Center for Sensory-Motor Interaction (SMI), Aalborg University, Aalborg, Denmark.
authors.forename: Ken Steffen
authors.initials: KS
authors.lastname: Frahm
authors.affiliation: Nordic NeuroSTIM Aps, Aalborg, Denmark.
authors.forename: Kristian
authors.initials: K
authors.lastname: Hennings
authors.affiliation: Neuromodulation Research, Medtronic Inc, Minneapolis, MN, USA.
authors.forename: Louis
authors.initials: L
authors.lastname: Vera-Portocarrero
authors.affiliation: Neuromodulation Research, Medtronic Inc, Minneapolis, MN, USA.
authors.forename: Paul W
authors.initials: PW
authors.lastname: Wacnik
authors.affiliation: Department of Health Science & Technology, Integrative Neuroscience Group, Center for Sensory-Motor Interaction (SMI), Aalborg University, Aalborg, Denmark.
authors.forename: Carsten Dahl
authors.initials: CD
authors.lastname: Mørch
mesh_terms: D000818:Animals; D004558:Electric Stimulation / Q000379:methods*; D004567:Electrodes, Implanted; D006801:Humans; D018482:Muscle, Skeletal / Q000502:physiology*; D009412:Nerve Fibers / Q000502:physiology*; D012684:Sensory Thresholds / Q000502:physiology; D004561:Transcutaneous Electric Nerve Stimulation
keywords: Computational modeling; low back pain; muscle activation; nerve modeling; peripheral nerve field stimulation