Comparison of Therapeutic Magnetic Stimulation With Electric Stimulation of Spinal Column Vertebrae

Electrical bone growth stimulators improve patient outcomes following the spinal fusion surgery, particularly important in high-risk patient populations. The therapeutic electromagnetic field (EMF) effects in tissue were modeled for two Food and Drug Administration (FDA) approved magnetic stimulation devices: 1) Coil A (SpinaLogic, DJO LLC, Vista, CA) and 2) Coil B system (Spinal-Stim, Orthofix Inc., Lewisville, TX). The electric current density induced in the target tissue (spinal vertebra) was compared with that of an FDA approved electric stimulation device (SpinalPak, Biomet, Warsaw, IN). For Coil A and Coil B, the EMF source was current-carrying coils that differed in the design (single coil versus two coaxial coil system) and the current characteristics (continuous 76.6 Hz sinusoid, biased versus intermittent 3850 Hz rectangular pulse bursts repeated at 1.5 Hz, unbiased). The local maximum induced electric field and current density were over 700-fold higher for Coil B system than for Coil A but only half of the values calculated for the reference electric stimulation device. The interaction of the magnetic field and the induced current in the tissue gives rise to local Lorentz force effects expressed as magnetoacoustic pressure (MAP) that for Coil B system reached 5 dB relative to 1 μPa of sound pressure level in bone (SPL) (compared with -45 dB for Coil A and absent for capacitive coupling reference device). The analysis broadens the range of possible mechanisms of therapeutic EMF action to include the MAP.