Reoperation for Failed Spinal Fusion: Augmentation of Fusion with Spinal Stimulation

Published on 27/03/2015 by admin

Filed under Neurosurgery

Last modified 22/04/2025

Print this page

This article have been viewed 2015 times

Chapter 218 Reoperation for Failed Spinal Fusion

Augmentation of Fusion with Spinal Stimulation

Argument against Bone Growth Stimulator

Jeffrey D. Gross

Failed dorsal lumbar spinal fusions create more headaches, heartaches, and backaches for patients and surgeons than most problems associated with spinal surgeries. The treatment of these pseudarthroses is more complex and difficult than many primary spinal procedures and has a high failure rate. Attempts to reduce the failure rates of spinal fusion and pseudarthrosis revision have encouraged the development of augmentative therapies such as spinal stimulation. However, clinical use of these electrical stimulating devices in the spine has remained controversial. Although supported by laboratory and animal model evidence, the application of stimulating devices for the treatment of failed spinal fusion has not been adequately proven, clinically. Furthermore, the biology of failed fusion does not parallel that of a primary fusion or of a long bone fracture or nonunion.¹^–⁴ Thus, the extrapolation of our understanding of the use of such devices for long bone and other orthopaedic nonunions to the treatment of spinal disorders is continuing to progress but is still incomplete.⁵

Electrical stimulators come in various forms. They may be implanted surgically or semi-invasively with an external generator, or applied completely noninvasively, using pulsed electromagnetic fields (PEMFs), capacitively coupled electrical stimulation, or combined magnetic fields. These variables, combined with variations in fusion approaches and techniques for multiple indications in differing patient populations, make comparison of relevant studies impractical.⁶^–¹⁴

Experimental Evidence

The biochemical basis of the cellular response to electrical fields has been elucidated and nicely summarized by Brighton.¹⁵ Cultured bone cell DNA production increases in response to capacitive coupling, inductive coupling, or combined electromagnetic fields. These electrical fields act at cell membrane receptors, causing rapid signal transduction through PI3K and mTOR proteins.^16,¹⁷ This cellular cascade activates specific protein kinases, which tend to increase intracellular calcium concentrations, increase cytoskeletal calmodulin activation, and ultimately increase cellular proliferation. This growth and differentiation of osteoblasts are the desired effects in spinal fusion. It should be noted that the mTOR cellular cascade acts through different pathways than those activated by bone morphogenetic proteins (BMPs),¹⁸ although electrical stimulation has been shown to up-regulate multiple osteoinductive BMPs.^19,²⁰

Much of the initial work in evaluating the use of external stimulation for the augmentation of a failing arthrodesis was performed on long-bone models. The relative success of these models was the impetus for the transfer of such technologies to use in the different arena of failed spinal fusions. Initial studies of spinal stimulation for lumbar fusions, however, were performed on non–weight-bearing models (i.e., quadrupeds, without axial forces experienced through the spine), undergoing primary spinal fusions, usually without instrumentation. There were no controls for these variables.

Nerubay et al.²¹ studied implanted direct current stimulation (DCS) of primary dorsal spinal fusions in 30 pigs and reported a customized nonparametric score improvement of radiographic and histologic criteria 2 months after the operation. Conversely, Kahanovitz et al.²² investigated the use of DCS in primary lumbar spinal fusions in dogs. They found no advantage for electrical stimulation in the radiologic or histologic indices for fusion until 12 weeks after the operation using an internal control. However, each facet joint involved was considered a statistical sample; when the animals were considered separately, there were no statistical differences at any time for fusion success. This same group reported the use of PEMF on a similar canine model.² They were unable to demonstrate radiologic or histologic differences between experimental and control groups at 6 and 12 weeks after operation. In another report, this group observed that by 12 and 15 weeks after surgery, there were no histologic or radiologic differences in dogs receiving primary spinal fusions using instrumentation and PEMF.²³

France et al.²⁴ showed the dose-dependent efficacy of subcutaneous DCS for primary, noninstrumented lumbar intertransverse process fusions in a controlled rabbit model. In a similar model, DCS increased fusion rates in a dose-dependent manner while employing hydroxyapatite as a bone graft substitute, as compared with nonstimulated rabbits with autologous bone.²⁵ The same team also demonstrated that in a similar model, DCS did not provide benefit to a nicotine-dosed group.²⁶ DCS also was studied in a randomized, controlled, and blinded (for outcome measure) sheep experiment, used in conjunction with titanium interbody fusion cages packed with autograft.²⁷ A dose-dependent rate of fusion was seen in this study and in another similar study in a dog model.²⁸

The use of various electrical stimulating devices for spinal fusion in animal models has met with mixed results. However, no animal studies have addressed the situation of failed fusion. Furthermore, the spinal biomechanics of quadrupeds are not identical to those in upright primates, whose weight bearing has direct influence on fusion success.³ Neither invasive nor external spinal stimulation has been studied adequately in an appropriate upright animal model for failed fusion of the lumbosacral spine. The use of instrumentation is a complicating factor for which there are no controlled studies with electrical stimulation in animals. Furthermore, the application of animal data to human use is controversial, because spinal fusion rates differ among species.²⁵ Thus, there is insufficient experimental evidence in animal studies to support the use of augmentative electrical stimulation for reoperation for failed spinal fusion.²³

Clinical Evidence

Early studies concerning the use of electrical stimulation centered on nonspinal applications, both for primary surgeries and for nonunions. Earlier papers dealing with electrical stimulation for spinal fusion dealt mainly with mixed populations (e.g., primary and reoperation for pseudarthrosis). More recent trends have demonstrated the use of external electrical stimulation versus implanted stimulators. There are no published studies specifically dealing with reoperation for failed lumbar spinal fusion that prospectively control for the type of surgery, construct variety, instrumentation, and patient status (e.g., weight, smoking status, steroid therapy).^10,²⁹^–³¹ Only one referenced study indicates a statistically significant improvement in the fusion rate for failed spinal fusion with electrical stimulation.¹² The 33 patients in that study were instrumented and were not completely randomized versus controls for other factors. None of the other studies demonstrated an improvement in the fusion rate or time to fusion in the subpopulation of failed dorsal spinal fusions. In fact, one author reported that electrical stimulation had an adverse effect on the time to fusion and stated that it “is not advocated for use in any type of spine fusion.”³ Thus, although growing, the clinical evidence in favor of electrical stimulation for the augmentation of failed dorsal spinal fusions is meager and requires further controlled investigation.¹

Multiple clinical reports have indicated the efficacy of external capacitively coupled electrical stimulation (CCES) in primary spinal fusions.²⁴ In a study of 179 patients (reduced from 337 due to withdrawals and noncompliance), Goodwin et al. did not control for variables, such as a concomitant disease (e.g., diabetes), institution, bone graft materials used for fusion, and internal fixation strategies.³² There was no statistical significance for CCES efficacy in the smoking or previous surgery cohorts for this study. Jenis et al.³³ determined that neither DCS nor PEMF was able to statistically improve the fusion rate or clinical outcome of instrumented dorsolateral arthrodeses.

A retrospectively random review of patients to analyze the efficacy of PEMF to augment primary spinal fusions was undertaken to measure clinical and radiologic outcome.³⁴ A statistically significant difference was found, with 97.6% fusion success for the stimulated group, and only a 52.6% fusion rate for the control group. However, there was a bias toward using PEMF on smokers, on multilevel fusions, and in conjunction with the allograft. Surgical approaches included ventral interbody and dorsolateral approaches, or both, thereby adding further variables to the analysis of these results. The number of patients (61) probably was too small to make meaningful conclusions from the clinical outcomes. With regard to reoperation for failed spinal fusion, the review found that “the low number of patients undergoing repeat fusion precluded specific conclusions or recommendations” for the use of PEMF.

Some prospective well-designed studies in which combined magnetic fields are evaluated for primary spinal fusions have been published.³⁵ In a multicenter, randomized, double-blind, placebo-controlled trial of 201 noninstrumented fusions, Linovitz et al. demonstrated a statistically significant increase in the fusion rate at 9 months after surgery with use of a noninvasive device. When stratified by sex, this significant difference remained only for women. There were only 28 patients who smoked in the entire starting population of 243, so no conclusions can be drawn for this subgroup. Furthermore, no conclusions could be drawn with regard to reoperation for failed spinal fusion. A study by Kucharzyk dealt with a high-risk pool of patients in whom implanted fusion stimulators provided a statistically significant benefit for radiographic and clinical fusion.³⁶ Although interesting, these results could not be used to draw conclusions concerning use of electrical stimulation for failed spinal fusion. Similar findings in a retrospective study were published by Silver,³⁷ who included primary and revision fusion cases, although not as a salvage technique. Fusion results were similar in the two groups in this particular study.

In 2008, Foley et al. prospectively studied PEMFs for higher-risk primary cervical fusion (i.e., smoking or multilevel), showing an improvement in fusion rate with stimulation at the 6-month, but not at the 12-month, mark from surgery. Prior surgery was an exclusion criterion.³⁸

In recent years, several studies of the use of PEMFs for failed spinal fusion have been done. Simmons et al.³⁹ studied patients with pseudarthrosis 9 months after initial surgery, using PEMF as a “salvage” technique. They found that such electrical stimulation (at least 2 hours per day) was effective in these cases for both dorsolateral and interbody fusion success, without any statistical differences when comparing smoking status, allograft versus autograft, instrumentation, or multilevel fusion. They reported 67% fusion success as a salvage in 100 patients in multiple sites. Such results are not inconsistent with the orthopaedic literature for nonspinal union salvages.^5,^40,⁴¹

Thus, although there is mounting evidence to support the use of electromagnetic fields for primary lumbar fusions, no prospective controlled study of recalcitrant subgroups such as patients with pseudarthroses has been performed. There appears to be a trend toward fusion in the face of electrical stimulation, although the clinical evidence for use of stimulation as a standard for failed fusion has not yet been produced. The work of Simmons et al. shows efforts in the direction of demonstrating electrical stimulation as a standard treatment for failed dorsal lumbar fusion. However, a prospective, controlled, and blinded study of electrical stimulation for failed lumbar fusion is still needed.

Discussion

The lack of upright animal models underscores the importance of understanding the biomechanics involved in bone fusion.² The weight-bearing forces acting on spinal fusions in the upright position are not equivalent to those in other positions, as has been demonstrated in nonspine models.⁴⁰ Additionally, the pathology, fusion type, and use of instrumentation affect the biomechanics of a spinal fusion construct.^2,^6,^12,²⁹ Fixation alone has been shown to enhance spinal fusion.^2,^3,¹² These variables have not been consistently controlled in the animal and patient studies published on the use of electrical stimulation for spinal fusion.^1,^3,⁷ Furthermore, the experimental and clinical evidence supporting stimulation for bone fusion for pseudarthrosis is based mainly on long-bone and other orthopaedic data. This appendicular physiology exhibits completely different biomechanical dynamics and differs from the situation of spinal fusion, where the bones are not “normally” physiologically fused bones. In other words, it is still necessary to fuse a fracture of a long bone that has failed to heal, whereas spinal segments were not meant to be connected in an immobile fashion.

Many factors affect the spinal fusion success rate, despite the method of fusion surgery, including patient health, primary pathology, weight, weight-bearing status, steroid and nonsteroidal anti-inflammatory drug (NSAID) use, PEMF compliance, and smoking.³ None of the literature studies concerning electrical stimulation for failed spinal fusions were designed with complete control for these parameters, although multivariate analyses were performed in some.¹⁰^–¹² Many authors agree that there is a paucity of appropriate controlled patient studies for electrical stimulation in spinal fusion and that further well-controlled prospective studies are warranted.^1,^3,^7,^10,^42,⁴³ However, there is ongoing improvement in the availability of clinical studies for the use of spinal stimulation for primary lumbar fusions and one noncontrolled study concerning the use of such stimulation for failed lumbar fusion. The reader should be careful not to extrapolate those conclusions automatically to the case of spinal pseudarthrosis, which is a more difficult and recalcitrant physiologic problem.

Furthermore, patients at high risk for fusion failure may be destined for failure despite the best surgical techniques, with or without electromagnetic augmentation.¹¹ These patients’ pseudarthroses are hypovascular and fibrotic, leading to a trend toward less successful subsequent reoperation. This positive feedback cycle toward negative outcome has not been borne out in the present literature analysis. Also, time to fusion was not measured in all studies, and the natural history of spinal fusion may be different in various patient populations for which the aforementioned study variables were not completely controlled.^10,³¹ These interrelated factors must be evaluated independently along with electrical stimulation to provide adequate evidence for their effective use as an augmentative device for failed spinal fusion. One author astutely reported that “good surgical technique remains the most important influence on surgical outcome.”¹²

In addition to the paucity of animal and human evidence in favor of using electrical stimulation to augment spinal fusion for pseudarthrosis, there are numerous disadvantages and potential complications. The additional time required for surgery for implantation of invasive electrical stimulators is significant and increases perioperative morbidity. Surgical use of internal stimulators provides an additional foreign body, which may increase the incidence of postoperative infection, particularly in high-risk patients with pseudarthrosis such as smokers and steroid users. The invasive stimulators would thus potentially require removal, adding further surgical risks and costs to the patient’s burdens.¹¹ Implanted leads have been shown to break and corrode on occasion.^8,⁴⁴ Additionally, patients undergoing recurrent spinal surgery typically require frequent radiologic evaluations. Broken stimulator leads have been shown to heat local tissue abnormally during MRI scanning (as much as 14° C), making MRI a contraindication for those patients.⁴⁴ Also, electrical stimulators can cause significant distortion on MRI. Furthermore, there have been cases of overstimulation, which can be deleterious to bone formation.⁴⁵ Finally, optimal electrical stimulation profiles for electrical units used in the spine have been fully elucidated.^6,^22,^31,⁴⁶

This analysis would not be complete without a discussion of the costs for the hardware and, in the case of DCS, the additional surgery required for stimulator implantation. External electrical stimulators add nearly $6000 to the cost of reoperation for failed spinal fusion, which is a significant increase in health care expenditures. However, as a salvage procedure, it may ultimately save health care dollars (if a revision surgery can be avoided).

Summary

Based on animal and patient studies, we find inadequate scientific evidence to support the use of electrical stimulation to augment a failed dorsal lumbosacral fusion. Because it appears that there is no harm in using noninvasive devices before or after attempting reoperation for failed spinal fusion (despite the lack of current prospective controlled studies), however, these devices may be considered for application. Alternatives to fusion augmentation may include: (1) attempting a more biomechanically sound fusion, with or without instrumentation; (2) having patients avoid steroids and NSAIDs; and, (3) counseling patients to limit tobacco use. Further prospective, well-controlled, randomized patient studies for the evaluation of electrical stimulation for failed fusion are still needed.

Key References

Jenis L.G., An H.S., Stein R., Young B. Prospective comparison of the effect of direct current electrical stimulation and pulsed electromagnetic fields on instrumented posterolateral lumbar arthrodesis. J Spinal Disord. 2000;4:290-296.

Meril A.J. Direct current stimulation of allograft in anterior and posterior lumbar interbody fusions. Spine (Phila Pa 1976). 1994;19:2393-2398.

Rogozinski A., Rogozinski C. Efficacy of implanted bone growth stimulation in instrumented lumbosacral spinal fusion. Spine (Phila Pa 1976). 1996;21:2479-2483.

Silver R.A. Application of pulsed electromagnetic fields (PEMF) after lumbar interbody or posterolateral spinal fusion surgery in a heterogeneous patient population. J Neurol Orthop Med Surg. 2001;21:51-62.

Simmons J.W. Treatment of failed posterior lumbar interbody fusion (PLIF) of the spine with pulsing electromagnetic fields. Clin Orthop Relat Res. 1985;193:127-132.

Simmons J.W., Mooney V., Thacker I. Pseudarthrosis after lumbar spine fusion: nonoperative salvage with pulsed electromagnetic fields. Am J Orthop (Belle Mead NJ). 2004;33:27-30.

References

1. Kahanovitz N. Spine update. The use of adjunctive electrical stimulation to enhance the healing of spine fusions. Spine (Phila Pa 1976). 1996;21:2523-2525.

2. Kahanovitz N., Arnoczky S.P., Nemzek J., Shores A. The effect of electromagnetic pulsing on posterior lumbar spinal fusions in dogs. Spine (Phila Pa 1976). 1994;19:705-709.

3. Kornblatt M.D., Casey M.P., Jacobs R.R. Internal fixation in lumbosacral spine fusion. A biomechanical and clinical study. Clin Orthop Relat Res. 1986;203:141-150.

4. Sharrard W.J.W. A double-blind trial of pulsed electromagnetic fields for delayed union of tibial fractures. J Bone Joint Surg [Br]. 1990;72:347-355.

5. Frykman G.K., Taleisnik J., Peters G., et al. Treatment of nonunited scaphoid fractures by pulsed electromagnetic field and cast. J Hand Surg. 1986;11A:344-349.

6. Bassett C.A. Why are the principles of physics and anatomy important in treating osteoporosis? [editorial]. Calcified Tissue Int. 1995;56:515-516.

7. Boden S.C., Schimandle J.H. Biologic enhancement of spinal fusion. Spine (Phila Pa 1976). 1995;20(Suppl):113S-123S.

8. Dwyer A.F., Wickham G.G. Direct current stimulation in spinal fusion. Med J Austr. 1974;1:73-75.

9. Dwyer A.F., Yau A.C., Jeffcoat K.W. Use of direct current in spine fusion. J Bone Joint Surg [Am]. 1974;56:442.

10. Meril A.J. Direct current stimulation of allograft in anterior and posterior lumbar interbody fusions. Spine (Phila Pa 1976). 1994;19:2393-2398.

11. Mooney V. A randomized double-blind prospective study of the efficacy of pulsed electromagnetic fields for interbody lumbar fusion. Spine (Phila Pa 1976). 1990;15:708-712.

12. Rogozinski A., Rogozinski C. Efficacy of implanted bone growth stimulation in instrumented lumbosacral spinal fusion. Spine (Phila Pa 1976). 1996;21:2479-2483.

13. Savini R., DeSilvestre M., Garguilo G., Bettini N. The use of pulsing electromagnetic fields in posterolateral lumbosacral spinal fusion. J Bioelect. 1990;9:9-17.

14. Tejano N.A., Puno R., Ignacio J.M. The use of implantable direct current stimulation in multilevel spinal fusion without instrumentation. A prospective clinical and radiographic evaluation with long-term follow-up. Spine (Phila Pa 1976). 1996;21:1904-1908.

15. Brighton C.T., Wang W., Seldes R., et al. Signal transduction in electrically stimulated bone cells. J Bone Joint Surg [Am]. 2001;83:1514-1523.

16. Patterson T.E., Sakai Y., Grabiner M.D., et al. Exposure of mouse pre-osteoblast cells to pulsed electro-magnetic fields rapidly activates the mTOR signaling pathway. Bioelectromagnetics. 2006;72:535-544.

17. Schnoke M., Midura R. Pulsed electromagnetic fields rapidly modulate intracellular signaling events in osteoblastic cells: comparison to parathyroid hormone and insulin. J Orthop Res. 2007;25:933-940.

18. Selvamurugan N., Kowk S., Vasilov A., et al. Effects of BMP-2 and pulsed electromagnetic field (PEMF) on rate primary osteoblastic cell proliferation and gene expression. J Orthop Res. 2007;25:1213-1220.

19. Fredericks D.C., Smucker J., Petersen E.B., et al. Effects of direct current electrical stimulation on gene expression of osteopromotive factors in a posterolateral spinal fusion model. Spine (Phila Pa 1976). 2007;32:174-181.

20. Wang Z., Clark C., Brighton C. Up-regulation of bone morphogenetic proteins in cultured murine bone cells with use of specific electric fields. J Bone Joint Surg [Am]. 2006;88:1053-1065.

21. Nerubay J., Marganit B., Bubis J.J., et al. Stimulation of bone formation by electrical current on spinal fusion. Spine (Phila Pa 1976). 1986;11:167-169.

22. Kahanovitz N., Arnoczky S.P. The efficacy of direct current electrical stimulation to enhance canine spinal fusion. Clin Orthop Relat Res. 1990;251:295-299.

23. Kahanovitz N., Arnoczky S.P., Julse D., Shires P.K. The effect of postoperative electromagnetic pulsing on canine posterior spinal fusions. Spine (Phila Pa 1976). 1984;9:273-278.

24. France J.C., Norman T.L., Santrock R.D., et al. The efficacy of direct current stimulation for lumbar intertransverse process fusions in and animal model. Spine (Phila Pa 1976). 2001;26:1002-1008.

25. Bozic K.J., Glazer P.A., Zurakowski D., et al. In vivo evaluation of coralline hydroxyapatite and direct current electrical stimulation in lumbar spinal fusion. Spine (Phila Pa 1976). 1999;24:2127-2133.

26. France J.C., Norman T.L., Buchanan M.M., et al. Direct current stimulation for spine fusion in a nicotine exposure model. Spine J. 2006;6:7-13.

27. Toth J.M., Seim H.B., Schwardt J.D., et al. Direct current electrical stimulation increases the fusion rate of spinal fusion cages. Spine (Phila Pa 1976). 2000;25:2580-2587.

28. Dejardin L.M., Kahanovitz N., Arnoczky A.P., Simon B.J. The effect of varied electrical current densities on lumbar spinal fusions in dogs. Spine J. 2001;1:341-347.

29. Benzel E.C. Spinal fusion. In: Benzel E.C., editor. Biomechanics of spine stabilization: principles and practice. New York: McGraw-Hill, 1994., pp. 103–110

30. Nerubay J., Katznelson A. Clinical evaluation of an electrical current stimulator in spinal fusions. Int Orthop. 1984;7:239-242.

31. Simmons J.W. Treatment of failed posterior lumbar interbody fusion (PLIF) of the spine with pulsing electromagnetic fields. Clin Orthop Relat Res. 1985;193:127-132.

32. Goodwin C.B., Brighton C.T., Guyer R.D., et al. A double-blind study of capacitively coupled electrical stimulation as an adjunct to lumbar spinal fusions. Spine (Phila Pa 1976). 1999;24:1349-1357.

33. Jenis L.G., An H.S., Stein R., Young B. Prospective comparison of the effect of direct current electrical stimulation and pulsed electromagnetic fields on instrumented posterolateral lumbar arthrodesis. J Spinal Disord. 2000;4:290-296.

34. Marks R.A. Spine fusion for discogenic back pain: outcomes in patients treated with or without pulsed electromagnetic field stimulation. Adv Ther. 2000;17:57-67.

35. Linovitz R.J., Pathria M., Bernhardt M., et al. Combined magnetic fields accelerate and increase spine fusion. Spine (Phila Pa 1976). 2002;27:1383-1389.

36. Kucharzyk D. A controlled prospective outcome study of implantable electrical stimulation with spinal instrumentation in a high-risk spinal fusion population. Spine (Phila Pa 1976). 1999;24:465-469.

37. Silver R.A. Application of pulsed electromagnetic fields (PEMF) after lumbar interbody or posterolateral spinal fusion surgery in a heterogeneous patient population. J Neurol Orthop Med Surg. 2001;21:51-62.

38. Foley K.T., Mroz T.E., Arnold P.M., et al. Randomized, prospective, and controlled clinical trial of pulsed electromagnetic field stimulation for cervical fusion. Spine J. 2008;8:436-442.

39. Simmons J.W., Mooney V., Thacker I. Pseudarthrosis after lumbar spine fusion: nonoperative salvage with pulsed electromagnetic fields. Am J Orthop (Belle Mead NJ). 2004;33:27-30.

40. Garland D., Moses B., Salyer W. Long-term follow-up of fracture nonunions treated with PEMFs. Contemp Orthop. 1991;22:132-138.

41. Gossling H.R., Bernstein R.A., Abbot J. Treatment of ununited tibial fractures: a comparison of surgery and pulsed electromagnetic fields (PEMF). Orthopedics. 1992;15:711-719.

42. Kane W.J. Direct current electrical bone growth stimulation for spinal fusion. Spine (Phila Pa 1976). 1988;13:363-365.

43. Paterson D. Treatment of nonunion with a constant direct current: a totally implantable system. Orthop Clin North Am. 1984;15:47-59.

44. Chou C.K., McDougall J.A., Chan K.W. RF heating of implanted spinal fusion stimulator during magnetic resonance imaging. IEEE Trans Biomed Engin. 1997;44:367-373.

45. Bassett C.A., Pawluk R.J., Pilla A.A. Acceleration of fracture repair by electromagnetic fields. A surgically noninvasive method. Ann N Y Acad Sci. 1974;238:242-262.

46. Paterson D.C., Carter R.F., Tilbury R.F., et al. The effects of varying current levels of electrical stimulation. Clin Orthop Relat Res. 1982;169:303-312.

Argument for Bone Growth Stimulator

Ran Harel, Thomas E. Mroz

Electromagnetic therapy has a long and interesting history. In the 1st century ce, the physician Scribonius Largus prescribed standing on a wet beach near an electric fish as a treatment for headaches and gout. In the 16th century, Paracelsus was known to have used lodestone, a naturally magnetized piece of the mineral magnetite, for disorders such as epilepsy, diarrhea, and hemorrhage. In the next century, Sir Kenelm Digby reported using magnetic stimulation to treat wounds. The first reported use of electromagnetic stimulation to treat bone malunion was in 1841 by Hartshorne. Initial experiments relating the use of electricity to heal bone were performed in Japan by Yasuda, who concluded that electronegativity on the concave surface of bone was the cause of new bone formation during the remodeling process. Later, Bassett, in North America, demonstrated abundant bone formation around the negative electrode of an implanted sterilized battery pack in the bones of dogs. The observation that a small amount of electrical current stimulates osteogenesis has been confirmed experimentally and clinically by many other researchers. Contemporary clinical applications of electrical stimulation in regard to modification of bone physiology include the management of acute bony fractures, spinal pseudarthrosis, osteoporosis, osteoarthritis, and avascular necrosis of the femoral head.¹^–³

Delivery Methods

Various methods have been developed and are in use in both experimental models and clinical applications.

Direct Current

The most direct and invasive method of applying an electric current to human tissue is through electrodes surgically implanted into the targeted tissue. The electric current is produced by an implantable generator and delivered to the treated site by implantable electrodes placed on the bony surface during surgery. This method has the advantage of delivering the current directly to the fusion site, but it is an implanted device, which has obvious drawbacks.

Pulsing Electromagnetic Fields

Pulsing electromagnetic fields (PEMFs) are generated by a time-varying current applied to metallic coils at a certain duration and intensity. Application of the electromagnetic fields may be done through external electrodes or a corset-like or orthotic type of apparatus.

Capacitively Coupled Electric System

In the capacitively coupled electric (CCE) system, the energy is delivered by two charged metal plates or equivalent structures that are attached to a voltage source to produce an electric field. As with PEMFs, the magnetic energy can be delivered through external electrodes or a corset-like or orthotic-type apparatus.¹

Neither the PEMF nor the CCE method is invasive. However, neither method delivers the energy directly to the fusion site, and success is affected by patients’ compliance.

Basic Science

As early as 1920, Ingvar reported that fibrocytes grown in vitro could be induced to grow along a vectorial gradient of low-density electric current.¹ In the 1950s and 1960s, Yasuda, Bassett, and Becker showed that when bone is mechanically strained, electrical potentials are generated; electronegative potentials are found in areas of compression and electropositive potentials in areas of tension. This observation led to the assumption that remodeling of bone in reaction to stress (Wolf’s law) is mediated thorough electromagnetic fields.³ When direct current (DC) electrodes are used, the Faradic reaction at the cathode lowers oxygen concentration, increases pH, and produces hydrogen peroxide. A decrease in oxygen concentration has been found to enhance osteoblastic activity, whereas an increase in pH increases osteoblastic activity and decreases osteoclastic activity.^4,⁵ Cho et al.⁶ showed that macrophages exposed to hydrogen peroxide express increased levels of vascular endothelial growth factor (VEGF) mRNA and elevated levels of VEGF in the medium. Fredericks et al.⁷ examined the role of DC implantable electrodes in the lumbar fusion model in rabbits. The study group had higher expression of mRNA of bone morphogenetic protein (BMP)-2, BMP-6, and BMP-7 compared with the control group.

Lohmann et al.⁸ found that enhanced cellular differentiation was the end result of PEMF on osteoblast metabolism, as evidenced by increased alkaline phosphatase activity, osteocalcin synthesis, and collagen production. PEMF appears to promote the production of matrix vesicles at 4 days, as evidenced by increased levels of alkaline phosphatase in exposed cells as compared with controls. The polyamine compounds c-myc and c-fos play a relevant role in both protein synthesis processes and cell differentiation. De et al.⁹ demonstrated an increased production of c-myc and c-fos mRNA in response to PEMF, concluding that PEMF affects the mechanisms involved in cell proliferation and differentiation. Schwartz et al.¹⁰ studied the effects of PEMF on human mesenchymal stem cells cultured in osteogenic media with and without BMP-2. PEMF had no effect on cell cultures without BMP-2, but synergistically increased alkaline phosphatase and osteocalcin, and enhanced the effects of BMP-2 on PGE2, latent and active TGF-b1, and osteoprotegerin. The greatest effect was recorded on days 12 to 20.

CCE acting on bone cell cultures was shown to increase mRNA expression of BMP-2 through BMP-8 and increase BMP-2 protein production and alkaline phosphatase activity.¹¹

Electrical Stimulation

Animal Studies

Although controversy remains regarding which method is most effective, clinical evidence supports the use of direct current electrical stimulation, PEMF, and capacitive coupled electrical stimulation in increasing fusion rates in either a posterolateral or interbody lumbar spinal fusion. Additionally, all three techniques have demonstrated improved fusion healing potential in instrumented and noninstrumented fusions. Various animal studies have contributed to our current understanding of electrical stimulation as a supplement to spinal fusion, most of which were done utilizing the DC stimulators.¹² Bozic et al.¹³ evaluated various current densities supplied with DC stimulation combined with an osteoconductive bone graft substitute (coralline hydroxyapatite) in a rabbit model. They found that increasing current up to 100 μÅ improved the quality and success rate of posterolateral fusion results over lower current values, and concluded that the utilization of DC stimulator avoids the morbidity of autologous bone graft harvesting. Neruby et al.¹⁴ examined DC stimulators in a posterolateral fusion model in pigs. They blinded the study by implanting 15 working stimulators and 15 stimulators without an active power source, combined with iliac autograft in both groups. The active group had better fusion results than the nonactive group. France et al.¹⁵ demonstrated a trend of increasing radiographic fusion success in a rabbit posterolateral fusion model as the current density was incrementally increased (sham, 20 μÅ, 60 μÅ). Statistically significant differences were shown in peak load-to-failure ratios, stiffness, and in the host tissue response to autograft bone. Reparative granulation tissue and osteoid surrounded the autograft bone in the high-current spines, whereas necrotic fibrous tissue surrounded the autograft bone in the sham group.

Human Investigations

Clinical trials of electrical stimulation in humans typically have produced higher spinal fusion success rates in controlled and noncontrolled human trials. Dwyer reported on its first use of DC electrical stimulation in humans in 1974, in which all but one patient had a solid fusion mass visible on radiography.¹²

Direct Current Stimulation

Implanted DC devices are unobtrusive, do not encumber patients in their postoperative rehabilitation, and offer no issues of patient compliance. However, their use exposes the patients to additional hardware implants and extended operative time. Typically, the DC is applied via two titanium leads placed paraspinally. In 1988, Kane et al.¹⁶ reported the results of three independent studies. In the first study, 82 patients underwent dorsal spinal fusion with DC stimulation. The results were compared with those of a historical control group of 150 patients with fusion alone without DC stimulation. The DC group was found to have a statistically significant higher fusion rate of 91.5%, compared with 80.5% in the control group. The second study was a randomized, prospective, controlled clinical study on the use of DC stimulation on high-risk patients undergoing dorsal spinal fusions. The patient population consisted of those with previous failed fusions, patients with grade 2 or worse spondylolisthesis, patients requiring multiple-level fusions, and patients with other risk factors such as obesity, smoking, and diabetes. The DC-stimulated group had an 81% fusion success rate compared to 54% in the control group. The third study evaluated 116 patients from the same “difficult-to-fuse” population in an uncontrolled clinical study with DC stimulation for dorsal spinal fusion. A 93% fusion rate was reported. Meril¹⁷ reported statistically significant increases in fusion rates in patients undergoing ventral or dorsal interbody fusion with allograft (93% vs. 75%). The significant difference was sustained even in high-risk groups such as smokers (92% vs. 71%), patients with no internal fixation (91% vs. 65%), and patients undergoing L4-5 fusion (91% vs. 59%). Tejano et al.¹⁸ assessed the role of DC stimulator in uninstrumented lumbar fusion. They prospectively followed 143 patients who had been implanted with DC stimulators during uninstrumented facet or posterolateral fusion, using iliac crest autograft, and compared the results with those in previous publications. Of those patients, 91.5% had evidence of fusion after 1 year, and the authors concluded that uninstrumented fusion with stimulation is superior to uninstrumented fusion without stimulation and is equivalent to an instrumented fusion. Rogozinski and Rogozinski¹⁹ and Kucharzyk²⁰ studied the effects of DC stimulation on posterolateral instrumented fusion using pedicle screws and autologous bone graft. Both studies showed elevated fusion rates and fusion success in the stimulated groups compared to the control groups.

Pulsing Electromagnetic Field

The use of external electrical stimulation such as PEMF depends primarily on patient compliance. Simmons et al.²¹ examined the role of PEMF in 13 patients with failed posterolateral interbody fusion, defined as nonunion at least 18 months after the fusion surgery. They found that an external orthosis containing a coil delivering a pulsed electromagnetic current centered over a failed fusion site worn 8 to 10 hours per day for 12 months achieved interbody fusion in 77% of patients evaluated by radiographs, and can provide an effective alternative to the surgical management of failed interbody fusions. In a double-blind prospective randomized study, Mooney²² reported increased fusion success rates for a PEMF-stimulated group over controls in patients undergoing either ventral or posterior lumbar interbody fusion (92% vs. 65%), including high-risk surgical groups such as smokers (89% vs. 60%) and multilevel fusions (89% vs. 54%). Bose²³ examined the effect of PEMF on 48 high-risk patients undergoing instrumented posterolateral fusion with iliac autograft augmented with allograft. He reported 97.9% radiologic success rate and 83.4% good and excellent clinical results. Marks²⁴ retrospectively analyzed a series of 61 patients who had undergone lumbar fusion—with autograft or allograft, or both—for discogenic back pain, with or without a PEMF. He reported a 97.6% successful fusion rate (defined by radiologic evidence of fusion and good functional outcome) in the stimulated group compared with 52.6% in the nonstimulated group. Foley et al.²⁵ evaluated the role of PEMF for ventral cervical fusions with allograft and a ventral plate. Either smokers or multilevel ACDF candidates were randomized to use a PEMF stimulator or be in the control group. Patients were prospectively followed. At 6 months, the treated group had a fusion rate of 83.6%, while the fusion rate for the control group was only 68.6%; however, at 12 months the difference between the groups became insignificant and all other outcome scores were similar.

Capacitively Coupled Electrical Stimulation

CCE stimulation, like PEMF, is again largely dependent on patient compliance. To our knowledge, the only successful use of CCE stimulation to augment a lumbar spinal fusion was reported by Goodwin et al. in 1999.²⁶ The study population included patients suffering from degenerative disc disease and scheduled for a lumbar fusion, with or without an interbody fusion, done either ventrally or dorsally using allograft or autograft, or both. Patients were randomized for treatment versus placebo groups. Out of 337 patients enrolled, only 179 were analyzed. The active group had an 84.7% radiologic and clinical fusion rate, whereas the placebo group had a 64.9% success rate (P = .004).