Introduction: Combined magnetic fields (CMF) have proven efficacious in the clinic as an adjunctive therapy for lumbar spinal fusion (1). A CMF bone growth stimulator generates an extremely low frequency and extremely low intensity electromagnetic field that has both AC and DC components. The purpose of this study was to model the therapeutic field associated with a bone growth stimulator utilizing CMF. The model was first exercised to test the hypothesis that CMF provide targeted and complete coverage of lumbar spinal fusion sites for both interbody and posterolateral procedures. An additional hypothesis was tested to determine whether the therapeutic effect of CMF stimulation for spinal fusions is a result of the induced electric field.
Methods: A realistic anatomical model of the vertebrae and discs of the lumbar spine was developed to represent interbody and posterolateral fusion sites. Computer simulations of the EMF generated by the transducer coil were analyzed to determine coverage of the fusion sites.
Results: For the interbody fusion model, CMF provided 100% coverage of the intervertebral fusion sites for all disc spaces from L1 to L5 (Figure 1). For the posterolateral fusion model, CMF also provided 100% coverage for all planes spanning the posterior aspect of the L1-L5 vertebrae (Figure 2). Within the vertebral column, the magnitude of the electric field reached a maximum value of 3.6*10-4 V/m.
Conclusions: Simulations of the magnetic field confirmed complete and targeted coverage of the bone graft volumes for both interbody and posterolateral spinal fusion sites. In addition, the electric field was several orders of magnitude less than any reported study demonstrating a biological effect (2). Given its clinical efficacy, a bone growth stimulator using CMF must rely on the action of its combined magnetic fields rather than its electric field for its therapeutic effect.
Patient Care: The successful repair and consolidation of a spinal fusion by an EMF device is directly correlated to the coverage of the bone graft volume by the stimulatory fields. This computational simulation demonstrates the extent of coverage by a combined magnetic field device.
Learning Objectives: 1) Discuss therapeutic coverage of interbody fusion sites by CMF.
2) Discuss therapeutic coverage of posterolateral fusion sites by CMF.
3) Identify role of electric field for a CMF device.
References: (1) Linovitz R, Pathria M, Bernhardt M, et al. Combined magnetic fields accelerate and increase spine fusion: a double-blind, randomized, placebo controlled study. Spine (Phila Pa 1976). 2002;27(13):1383-9.
(2) ICINRP. Guidelines for limiting exposure to time-varying electric and magnetic fields (1 Hz to 100 kHz). Health Phys. Dec 2010;99(6):818-36