Report

Since 1911, fusion of the spine by a variety of techniques has been used to restore stability in a number of congenital, acquired, and degenerative spinal disorders. The failure to obtain spinal fusion has persisted over the years as a relatively common problem. The documented failure of solid fusion 1 year after surgery is referred to as pseudoarthrosis. Pseudoarthrosis has been attributed to inadequate surgical technique, failure to neutralize excessive motion or shear stresses at the segment to be fused, or underlying metabolic abnormalities in the patient. (1). Some have even suggested that the responsibility for the occurrence of pseudoarthrosis may be entirely within the hands of the surgeon.(2).

Estimates of the incidents of pseudoarthrosis in lumbar spinal fusion procedures range from 0 to greater than 30%, depending on the type of procedure. Cameron and Bridges(3). reviewed the results published from 1948 to 1979 and estimated the incidence of pseudoarthrosis according to the type of procedure (Table 1).

Table

Table 1. Posterior spinal fusion: Incidence of pseudoarthrosis according to type of procedure.

Steinmann and Herkowitz(1). noted the incidence of pseudoarthrosis reported in some of the same studies and other more recent reports as follows: 4%-68% for anterior interbody fusion, 3%-25.5% for intertransverse fusion, and 6%-27% for posterior interbody fusion. They pointed out that, short of surgical exploration, the detection of pseudoarthrosis before the occurrence of late signs of fusion failure (such as pain or recurrence of spinal instability) often necessitated the use of more than one of the following procedures: conventional roentgenograms, tomography, discography, bone scintigraphy, computed tomography, or magnetic resonance imaging. They considered the following criteria as being most useful for establishing the presence of pseudoarthrosis: (1) lack of trabecular bone continuity, (2) collapse of the graft height, (3) shift in position of the graft, (4) loss of fixation, and (5) unexplained pain in the area of fusion.

The clinical significance of pseudoarthrosis is, however, questionable. For example, DePalma and Rothman(2). found that 39 (9%) of their 448 patients who had lumbar intertransverse spinal fusion had evidence of pseudoarthrosis. The 39 patients with pseudoarthrosis demonstrated the ability to return to work and resumed a level of activity similar to that of a group of 39 age-matched patients with solid fusion. Relief from back pain and sciatica was only slightly and insignificantly better in the patients with solid fusion. Other studies have also noted that approximately 40%-50% of patients with pseudoarthrosis are asymptomatic.(4,5).

Direct electrical current has been demonstrated to have stimulatory effects on bone formation in in vitro biologic models and animal models. In humans, constant, direct-current stimulation has been accepted as a standard for treating long-bone nonunion and as an adjunct in the treatment of congenital pseudoarthrosis of the tibia. (6).

The results of two studies done in animals suggest that direct-current application may have stimulatory effects on spinal fusion. Kahanovitz and Arnoczky(7). studied the effect of a direct-current electrical stimulator on the fusion of the posterior lumbar facets in dogs. They noted little or no roentgenographic or histologic differences between the control and stimulated fusion at 2, 4, and 6 weeks, but found that all of the stimulated facet joints showed solid bony fusion at 12 weeks while none of the eight control facet joints showed any osseous bridging of the fusion site. Nerubay et al(8). found evidence at 2 months, but not at 1 month, that the constant-current stimulator enhanced the fusion of the lower lumbar spine in 1-month-old pigs.

The suggestion that a substantial number of pseudoarthroses may be the result of inadequate surgical technique may be of concern as the data on the possible effects of direct-current therapy on enhancing spinal fusion are examined. For example, Steinmann and Herkowitz(1). cited a number of studies that suggest that identification of the factors that lead to nonunion, and adjustment for these factors before repair was attempted, resulted in significantly improved outcomes. For example, Brodsky et al(9). found that 95% of their patients with failed anterior cervical fusion had successful fusion after a subsequent posterior procedure, in contrast to a 47% success rate in similar patients who underwent a second anterior procedure. Edwards and Weigel(10). treated 51 low lumbar nonunions in 28 patients with reexploration, regrafting, and internal fixation with compression rods that resulted in 84 of the repairs proceeding to solid fusion.

The earliest demonstration that direct current may stimulate lumbosacral fusion was described in 1974 by Dwyer and Wickham of Australia.(11). They reported that direct-current stimulation resulted in successful fusion in 11 (91.7%) of 12 patients. In a later study, published in 1975, Dwyer(12). reported that fusion was documented radiographically in 40 (85.1%) of 47 patients, 27 of whom were at high risk for fusion failure because of the need for multiple-level fusion and previous fusion failures.

After these demonstrations by Dwyer,(11,12). a multicenter clinical trial was initiated in the United States in 1978 to study the role of implantable bone-growth stimulators in spinal fusion surgery. Although the data did not formally appear in the literature, Kane(13). briefly reviewed the data that were available in 1981 on 84 stimulated patients from the study and on 159 historical control patients. The 84 patients were from 23 orthopaedic surgeons in 18 clinical centers in 12 States, and 82 of these patients were available for followup. The 159 historical control patients were from the University of South Carolina and Northwestern University, Chicago. The two groups were comparable in age and sex, but the incidence of previous surgery and that of pseudoarthrosis were 2-4 times higher in the stimulated group (80% and 55% vs. 28% and 20%, respectively). This difference in patient characteristics might have led to a bias against finding successful fusion in the stimulated group. Fusion was successful, however, in 75 (91%) of 82 patients in the stimulated group compared with 128 (81%) of 159 in the control group (p = .02). In a subgroup of 46 patients who had pseudoarthrosis and were included in the stimulated group, 42 (91%) achieved successful fusion. Kane noted that the results of the control group were comparable to the overall success rate of 74% that was derived from 31 papers on lumbosacral fusions in 3,383 patients, which were reviewed in a presentation "at the International Society for the Study of the Lumbar Spine in 1981," but not published, by Evans of England.

Kane(13). then directed a randomized, prospective, controlled trial that showed that the rate of radiographically successful spinal fusion was higher when the direct-current implantable stimulator was used as an adjunct to the conventional spinal fusion procedure than when the surgery was performed alone. This multi-investigator trial included only patients who were classified as "difficult" spinal fusion patients with (1) one or more previous failed spinal fusions, (2) grade II or worse spondylolisthesis, (3) extensive bone grafting necessary for multiple-level fusion, or (4) other high-risk factors for failure of fusion, including gross obesity. Randomization was in blocks of four patients per investigator, with two patients implanted with the stimulator and two without. To maintain randomization validity, investigators were required to enter at least four patients. Measurement of success was based on radiographic assessments for fusion.

Of 99 patients entered into the trial, 63 were from investigators meeting the criterion of submitting at least four patients. Of the 63 patients, 59 from seven investigators were available for followup. Thirty-one patients received stimulation therapy and 28 control patients had the usual surgical procedures. The randomization procedure succeeded in the assignment of subjects with comparable characteristics, such as age and other entry criteria, into the treatment and control groups. Eighteen months after surgery, successful fusion was achieved in 25 (81%) of 31 treated patients compared with 15 (54%) of 28 in the control group (p = .026). Kane presented a table on the rate of successful fusions obtained in the control and treatment groups to illustrate that "higher success rates were seen in the treatment groups for all entry criteria" (Table 2).

Table

Table 2. Random study results by entry criteria.

The data show that, among patients who had had failed fusion or extensive grafting (two or more levels of fusion), successful fusion occurred in more patients in the stimulated group than in the control group; however, for patients meeting the other two entry criteria, the rate of successful fusion was essentially the same in the stimulated and control groups.

Kane also reported that 13 (81%) of 16 electrically stimulated patients who were not included in the former analysis had successful fusion; this rate was comparable with that in those randomized treated group described above. Similar data on the remaining 20 of 36 excluded patients were not given.

The Food and Drug Administration's (FDA) summary of data on the safety and effectiveness of the implantable bone-growth stimulator(14). contained the data on all of the patients in Kane's randomized trial. The FDA report noted that, of the original 99 patients, 8 were lost to followup, 4 did not complete the treatment, and 2 did not meet the entry criteria. Of the 36 patients who were not included in the randomized group, 26 were available for analysis (10 controls and 16 treated patients). As Kane had noted, 13 of the 16 treated patients had successful fusions. However, 10 (100%) of the 10 untreated control patients also had successful fusions.

Kane(13). also reported briefly on a nonrandom study that took place at the same time as his randomized study described above. The number of surgeons contributing data for this study was not stated. Kane reported that 108 (93%) of 116 patients treated with the implantable stimulator achieved successful fusion. The poorest result, 26 (87%) of 30, was seen in the subgroup of patients who had the highest risk of fusion failure according to the criteria for patients selected for the randomized trial.

Cameron and Bridges(3). reported qualitative radiologic evidence for an increased rate of bone graft incorporation in 41 scoliosis patients who received electrical bone-growth stimulation. An intertransverse fusion procedure with the lowest known failure rate of 11.5% was accomplished with bone grafted on both sides of the spine. The cathode from an implantable stimulator was applied to one side, with the other side serving as "control." Radiologic examinations were done at 3 months, 6 months, and 1 year and then at 6-month intervals until fusion occurred and the battery was removed. Only 1 patient (2.5%) failed to achieve fusion, although the theoretically expected rate of failure was 16%. Visually significant differences between the two sides were said to have been evident in 80% of the patients at the 3-and 6-month followup examinations. The differences became less apparent with time, and "fusion" in 40 of the 41 patients was reported by the hospital radiologist 12 months after surgery.

It appears clear that direct electrical current stimulates bone formation, and it has been used as a standard of care in the treatment of long-bone fractures that have failed to fuse. Direct-current stimulation may play a similar role in spinal fusion, especially in patients who have had fusion failures or are at high risk for fusion failure. Although many of the spinal fusion failures may be the result of less-than-optimal surgical technique, the available data appear to indicate that direct-current stimulation increases the chances for obtaining solid fusion in these patients. This seemed apparent from results with high-risk patients reported by Dwyer(12). and by Kane.(13). However, the highly varied rate of unsuccessful spinal fusion that had been reported (ranging from 0 to 30% following initial surgery) makes it difficult to assess the effects, if any, of direct-current stimulation on initial spinal fusion attempts. For example, the uncontrolled study by Nerubay and Katznelson(15). that showed that a solid fusion was obtained in all five children with spondylolisthesis cannot be readily interpreted as a demonstration of the influence of the implanted electrical stimulator. Perhaps a study of electrical stimulation in smokers might yield definitive information regarding the possible benefit of electrostimulation as an adjunct for spinal fusion in high-risk patients such as smokers, who have been shown to have a five-fold increased risk for fusion failure compared with nonsmokers in a retrospective study of patients who had had spinal surgery during 1977 and 1978.(16).

Although the published data on the effects of direct-current stimulators in lumbar spinal fusion are old and involve a relatively small number of patients, they indicate that the use of a direct-current stimulators as an adjunct to spine surgery may result in a better chance for solid fusion, especially in patients at high risk for fusion failure. Most studies purporting to demonstrate the beneficial effect of direct-current stimulation on lumber spinal fusion have indicated that they did not report the fusion results separately for these patients. The only controlled study that gave data on patients identified with specific high-risk criteria(13). showed that direct-current stimulation appeared to influence spinal fusion only in patents with two of the four entry criteria for high risk.

Direct-current application has been shown to stimulate osteogenesis in spinal fusion studies with animal models(7,8). and appears to increase the rate of bone graft incorporation in humans. (3). Although these studies and reports of better fusion success rates in uncontrolled studies(11-13). imply that direct-current electrical stimulation may increase the success rate of spinal fusion, other studies that were cited by Steinmann and Herkowitz(1). indicate that improved surgical technique may obviate the need for using electrical stimulators. On the other hand, direct-current stimulation may optimize the chances for solid fusion in patients who are considered to be at high risk for fusion failure.

The available data appear to suggest that an implantable bone-growth stimulator may be a useful adjunct that could enhance the probability of fusion success in patients who have had previous fusion failure or need extensive bone grafting for multiple-level fusion. Although direct-current stimulation might also increase the likelihood for successful spinal fusion in other high-risk patients such as those who have severe spondylolisthesis, are obese, or are smokers, there are insufficient data available at the present time to include these latter patients among those who may benefit from electrical stimulation.

References

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2.

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3.

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7.

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Brodsky AE, Khalil MA, Neuman BP. Comparison of posterior vs anterior repair of pseudoarthrosis of anterior interbody fusions of the cervical spine. Orthop Trans. 1988;12:–.

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Edwards CC, Weigel MC. A prospective study of 51 low lumbar nonunions. Orthop Trans. 1988;12:–.

11.

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Dwyer AF. The use of electrical current stimulation in spinal fusion. Orthop Clin North Am. 1975;6:265–273. [PubMed: 1113970]

13.

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14.

Food and Drug Administration. Summary of safety and effectiveness data: Implantable electrical bone growth stimulator (unpublished report). 1987.

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Nerubay, J Katznelson A. Clinical evaluation of an electrical current stimulator in spinal fusions. Int Orthop. 1984;7:239–242. [PubMed: 6611315]

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Brown CW, Orme TJ, Richardson HD. The rate of pseudoarthrosis pseudarthrosis (surgical nonunion) in patients who are smokers and patients who are nonsmokers: A comparison study. Spine. 1986;11:942–943. [PubMed: 3824072]

AHCPR Pub. No. 94-0014