CHAPTER 69 Electrical Stimulation for Spinal Fusion
Electrical stimulation therapies have been used for the past 35 years to enhance spinal fusions. Three types of electrical stimulation are currently employed. These three types are direct current (DC), capacitive coupling (CC), and inductive coupling. Inductive coupling includes the mechanisms of pulsed electromagnetic fields (PEMFs) and combined magnetic fields (CMFs).1–3 Clinical data demonstrated an enhancement in fusion rates with electrical stimulation even before the true mechanisms of action of these technologies were well understood. Recent studies have provided insight into the distinct mechanisms of action of these different electrical therapies and support the validity of the clinical data previously published. This chapter reviews the history of electrical stimulation and provides a review of the current concepts of mechanism of action including the upregulation of several growth factors.
History of Electrical Stimulation
The earliest use of electricity was in 1841 for the purpose of healing long bone fractures. Hartshorne reported on a patient with a tibial nonunion.4 In 1850 Lente reported successful use of galvanic current in the treatment of patients with delayed union or nonunions.5 The observations of Wolff in 1892 are the basis for the modern theories of electrical stimulation. He described the phenomenon that bone is formed in response to stress.6
Yasuda, Bassett, and Becker in the 1950s further characterized the electrical events occurring in healing bone callus and described the electrical potentials arising in long bones from mechanical stress.7–9 Those authors reported that the areas of bone under compression were electronegative and those under tension were electropositive. These electrical potentials were subsequently found to be related to stress and strain rates. The authors proposed and demonstrated that electricity applied to a fractured bone could impart healing.7–9 These observations were further confirmed by the work of Shamos, Friedenburg, and Brighton, who characterized the bioelectric or steady-state potential of living bone.10–12
Methods of Electrical Stimulation
Direct Current Electrical Stimulation
DC stimulation involves the surgical implantation of electrodes connected to a battery (Fig. 69–1). The cathodes are in direct contact with the exposed fusion bed. The cathode’s effective stimulation distance is 5 to 8 mm. The batteries deliver a constant DC for 6 to 9 months. The surgical implantation of the device obviates the need for patient compliance. However, there are disadvantages to the placement of the device because the manufacturer recommends removing the battery in 6 to 9 months, which entails a second surgical procedure. Also, there is a rare but reported risk of seeding the battery from systemic infection.
Basic Science Research Using Direct Current Stimulation
Recent studies have demonstrated that DC application enhances the production of a number of osteoinductive factors that are normal regulators of bone matrix formation. This mechanism of action was discovered using Boden’s animal model of spinal fusion.13 This model uses a New Zealand White rabbit intertransverse process fusion model for spinal fusion using autogenous bone graft. The model involves a surgical procedure similar to that performed in humans and has a similar nonunion rate to that of autograft.13 Morone and colleagues14 studied this model and showed that within the developing fusion mass, there is a distinct temporal and anatomic location for the production of bone morphogenic proteins (BMPs) and other factors that are necessary to achieve a solid fusion.
Fredericks and colleagues15 used this animal model to study the effects of DC on the temporal expression of growth factors in the developing fusion mass. They demonstrated that there was an upregulation of the production of BMP 2, 6, and 7 relative to controls. In addition, the use of DC stimulation avoided the potential complications seen with the application of a single high dose of growth factor to achieve fusion. Complications of ectopic bone formation, bone resorption, or antibody formation against the single growth factor have been reported with the application of a single growth factor.15
Clinical Studies of Direct Current Electrical Stimulation
The clinical use of DC current stimulation began as early as 1974 by Dwyer, who demonstrated clinical success in 11 out of 12 patients who received an implanted bone stimulator.16–17 Kane reported the results of a multicenter trial involving 84 patients who had an implantable DC current stimulator. This group was compared with a historical control group of 159 patients.18 The experimental group using the DC current stimulator had a higher percentage of patients who had previous surgery and nonunion. Despite this bias favoring the control group, there was a significant increase in successful fusion in the DC stimulated group: 91% versus 83% in controls. In 1988 Kane published an additional study of “difficult” patients undergoing posterior spinal fusion.18 These patients were deemed difficult because they (1) had one or more previous failed spinal fusions, (2) had grade II or worse spondylolisthesis, (3) required extensive bone grafting necessary for a multilevel fusion, or (4) had other risk factors such as obesity. He found that there was a statistically significant improvement in fusion rate of 81% in the electrically stimulated group versus 54% of controls.
In 1994 Meril reported a 93% fusion rate in patients who had undergone anterior lumbar or posterior lumbar interbody fusion with DC current stimulation compared with a 75% fusion rate in the control group.19
