Electrical Stimulation in Spinal Fusion
by Hunter Pharis
Each year there are approximately 400,000 spinal fusion surgeries that account for $13 billion in aggregate hospital costs, the highest of any procedure in the United States [1, 2]. Diseases that often lead to fusion include degenerative disc disease, cervical spondylotic myelopathy, scoliosis, and spondylolisthesis. Posterior spinal fusion surgeries generally do well with reported fusion rates of 98.25%, 91%, and 94% in cervical, thoracic, and lumbar fusions respectively [3–5]. Despite this, nonunion, or pseudoarthrosis, remains a concern for surgeons and patients alike. Those who experience pseudoarthrosis often have debilitating axial or radicular pain requiring further surgical treatment and increased healthcare spending. In order to alleviate this burden, electrical stimulation therapies (EST) have been developed with the goal of improving fusion rates and patient outcomes .
EST has been proposed with several different modalities of action. The first type of EST is direct current stimulation (DCS). DCS works through a subcutaneous cathode implant into to the prospective fusion mass and an anode implant in the adjacent soft tissue . This continuous stimulation aims to promote bone formation by reducing the oxygen tension, raising the pH, and upregulating osteoinductive growth factors[7, 8]. Secondly, capacitive coupling stimulation (CCS) has been used to induce fusion. This noninvasive wearable device is active 24 hours per day and attempts to increase fusion rates by activating osteoblast activity and upregulating oseoinductive growth factors [9, 10]. The last type of EST is inductive coupling stimulation (ICS). This device is wearable as well, but only requires 30 minutes to 2 hours of use per day . The mechanism of action includes increased calcium release inducing bone formation and upregulation of osteoinductive growth factors [11, 12].
Each type of EST has been tested in the preclinical (animal) and clinical setting with varying results. DCS has the benefit of 100% compliance due to its invasive nature; however, it comes with increased rates of discomfort, infection, and immune response . Despite the possible side effects, DCS has shown to be the most effective implant resulting in consistent improvement in fusion rates in both preclinical and clinical trials . ICS and CCS both appear to be a favorable choice for patients since they can be worn externally. Because these devices are worn externally, they require active patient participation. ICS was shown to be effective in clinical trials only and CCS has not displayed any significant effect on spinal arthrodesis .
Overall estimates on the effectiveness of EST appear to be promising, with rates of pseudoarthrosis in those receiving the treatment being almost half that as those who did not . Even though results point to decreased rates of pseudoarthrosis, the question of cost effectiveness remains, as no high-level studies have analyzed this to date. Current Medicare coverage of EST after fusion only includes those with multilevel fusions or those with a history of failed fusion . Further studies analyzing the cost-effectiveness of EST for those at increased risk of pseudoarthrosis may aid the physician in the decision-making process and may help to improve the patient experience by reducing healthcare expenditure.
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