Effect of the type of electrical stimulation on spinal fusion in a rat posterolateral spinal fusion model

Abstract

BACKGROUND CONTEXT: Posterolateral fusion (PLF) with autogenous iliac bone graft is one of the most common surgical procedures for lumbar spinal disease. However, its limited success demands new biologically competent graft enhancers or substitutes. Although the use of direct current (DC) electrical stimulation has been shown to increase rate of successful spinal fusions, little is known about the effect of the type of current in DC stimulation.

PURPOSE: To evaluate the effects of various DC stimulators on the strength and success rate of posterolateral fusion facilitated by using a nitinol mesh container, in rats.

STUDY DESIGN: This was an experimental animal study.

METHODS: A conductive, tubular nitinol mesh container was used to carry small pieces of bone grafts. The nitinol mesh container received electrical stimulation via a lead that connected the container to different types of DC stimulators. Sixty male Sprague-Dawley rats were divided into three groups (N=20 in each): a control group that underwent PLF with a nitinol container filled with autograft, a constant DC group that received a nitinol container and constant DC (100 μA), and a pulsed DC group that received a nitinol container and pulsed DC (100 μA, 100 Hz, 200 μs). The rats underwent PLF between L4 and L5, and transverse processes were grafted with bilateral iliac grafts. A stimulator was implanted subcutaneously. The rats were sacrificed 8 weeks postsurgery, and lumbar spines were removed. Spinal fusion was evaluated by microcomputed tomography, manual testing, biomechanical testing, histologic examination, and molecular analysis.

RESULTS: All animals in the DC stimulation groups displayed solid fusion, whereas only 70% of control animals showed solid fusion. Radiographic images, biomechanical testing, histologic examination, and molecular analysis revealed improved fusion in the order control group<constant DC group<pulsed DC group. The volume of new bone mass was significantly higher in the pulsed DC group (p<.05). Fusion was more solid in the pulsed DC group than in control group (p<.05). The pulsed DC group displayed the lowest inflammatory responses.

CONCLUSIONS: Pulsed DC electrical stimulation is efficacious in improving both strength and fusion rate in a rat spinal fusion model. In addition, tubular nitinol mesh, made of conductive suture, appears useful for holding small pieces of bone grafts and maintaining a good environment for bone fusion.

CLINICAL RELEVANCE: Pulsed DC electrical stimulation may be potentially useful to increase the fusion rate after spinal fusion in humans. Future research is required to evaluate the safety and efficacy of tubular nitinol mesh and pulsed DC electrical stimulation in humans.