Complex regional pain syndrome (CRPS) is a debilitating disease characterized by severe, persistent pain, and patients with CRPS endure significant physical, psychological, and socioeconomic challenges [1]. In the management of CRPS, spinal cord stimulator (SCS) implantation can be a viable treatment option when conservative approaches fail to adequately control pain, although SCSs are more expensive and invasive than other treatment modalities [2]. Previous studies have shown that SCSs can significantly reduce pain and enhance the quality of life in patients severely affected by CRPS [3–5].

Some patients fully recover from CRPS after SCS implantation [6]; however, some physicians and patients have observed that the effectiveness of SCSs diminishes over time in clinical settings. These observations are supported by a literature review demonstrating that the effect of SCSs decreased over three years, with no additional benefit compared to physical therapy [7]. Furthermore, only 35% of patients reported SCS to be effective for five years after implantation [8].

Meanwhile, researchers continue to debate the effectiveness and cost-efficiency of SCSs for pain treatment [9]. In particular, due to the high costs of SCSs, there have been substantial disagreements regarding the compensation for SCS insertion and maintenance costs in lawsuits related to CRPS [10]. Additionally, SCS is an invasive procedure with a high complication rate. Hardware-related complications were found to occur in 38% of cases, with lead migration occurring in 22.6%, connection failure in 9.5%, and lead breakage in 6.0% [11]. Therefore, patients who undergo SCS implantation with high expectations for long-term effectiveness could eventually experience reduced effectiveness, various complications during the implantation period, and procedure-related pain during removal.

Despite these unresolved issues regarding SCSs, no nationwide epidemiological study has been conducted on CRPS patients who received SCS implantation. The only available studies with long-term follow-up had small sample sizes, did not include a control group, and advocated SCSs for CRPS [12,13]. Therefore, those studies cannot be interpreted as adequately representing the characteristics of CRPS patients with SCS implantation.

In this context, the aim was to determine the actual duration of SCS use in patients with CRPS through an analysis of claim data from South Korea’s National Health Insurance Database. The objective of this study was to estimate the actual duration of SCS use in patients with CRPS using nationwide data, and the authors additionally aimed to evaluate the impacts of age, sex, hospital type, and insurance type on the duration of SCS use.

1. Study design

This is a retrospective cohort study using the National Health Insurance Database. The Institutional Review Board of Seoul National University Bundang Hospital approved this study (approval No. X-2022-737-901), and the National Health Insurance Review and Assessment Service (HIRA) in South Korea approved this study protocol (protocol No. M20220203804). Given the retrospective nature of this study and the use of anonymized patient data, the requirement for informed consent was waived. All procedures strictly complied with the seventh revision of the Helsinki Declaration of 2013 [14].

2. Patients and data search strategy

The HIRA is a governmental institution established under the Korean Ministry of Health and Welfare, responsible for evaluating the appropriateness of claims and reimbursements for medical expenses. Each patient’s medical events, such as diagnoses, treatments, prescriptions, and hospitalizations, are accurately recorded in a nationwide database, linked to their unique national registration number. The database is updated as new medical events occur, and the saved data are available for analysis.

The inclusion criteria for this study were adult patients (age ≥ 18) with CRPS who received permanent implantation of an SCS between January 2014 and December 2021. During this period octa-leads were commonly used, as octa-leads were approved for use in Korea since November 2007. The exclusion criteria were patients with CRPS who received paddle-type surgical electrodes because the process of inserting and removing paddle-type surgical electrodes is quite different from that of percutaneous electrodes, which could be a confounding factor. The patients whose data were duplicate or incomplete were excluded from this study. Additionally, the patients covered by traffic-accident and industrial accident insurance systems were not included in this study because these two insurance systems exist as separate entities in Korea. The HIRA database includes information on the patients covered by National Health Insurance, Medical Aid, and healthcare for veterans but does not include information on patients covered by traffic accidents and industrial accidents.

An important hypothesis in this study was that the removal of an SCS would indicate that the SCS was no longer expected to be effective. This does not mean SCS removal as a ‘failure’, instead, ‘a state where patients no longer expected benefits from SCS.’ From this hypothesis, the duration of SCS implantation was investigated by searching the insertion and removal dates of the implantable pulse generator (IPG). The patients’ diagnoses for this study included CRPS type 1 (disease code: M8900-M8909, G905, G906, G907) and type 2 (G564, G578, G588, G589, G5881, G5890). The SCS-related procedures included IPG permanent insertion (procedure code: H0792, H3690, H3691) and explantation (procedure code: H07923). Patients who underwent IPG replacement (procedure code: H07922) were considered to have continued using the SCS.

3. Statistical analysis

The primary outcome variable was the duration of SCS use. The authors defined the duration of SCS use as the period from IPG implantation to explantation, under the assumption that the patients would have their SCS removed if the benefit of SCS was no longer expected or if complications arose during the implantation period that prevented its continued use. The median duration from IPG implantation to removal was calculated using Kaplan–Mayer analysis. The impacts of age, sex, type of hospital (tertiary hospital, general hospital) insurance type (National Health Insurance, Medical Aid, Healthcare for veterans) on duration of SCS use were also evaluated using log-rank test. Continuous variables were summarized using means ± standard deviations for normally distributed variables and medians with interquartile ranges for non-normally distributed variables. Frequencies and percentages were used to summarize binary and categorical variables. SAS Enterprise Guide version 7.1 (SAS Institute Inc.) was used for the statistical analysis. P values less than 0.05 were considered statistically significant.

Out of 408 potential patients, 373 met the eligibility criteria and were included in this study. A CONSORT (Consolidated Standards of Reporting Trials) diagram is presented in Fig. 1, and the demographic data of the patients are shown in Table 1. The median period of SCS implantation was 4.4 years (95% confidence interval, 2.4–6.2 years). Male patients retained their SCSs longer than female patients (4.7 vs. 4.0 years, P = 0.014, Fig. 2). The insurance type also showed a significant association with the SCS implantation period; specifically, the longest duration of SCS use was found in beneficiaries of healthcare for veterans (P = 0.013, Fig. 3). However, age and the type of hospital did not exhibit significant associations with the SCS implantation period (P = 0.381 and P = 0.122, respectively) (Table 2).

Table 1 Characteristics of the cohort of patients with spinal cord stimulators (n = 373)

Variables	Values
Age (yr)	45 (34–57)
< 30	71 (19.0)
30–39	57 (15.3)
40–49	92 (24.7)
50–59	84 (22.5)
60–69	50 (13.4)
≥ 70	19 (5.1)
Sex Male Female	231 (61.9) 142 (38.1)
Period of SCS implantation (yr)a	4.4 (2.4–6.2)
Type of hospital
Tertiary hospital	259 (61.9)
General hospital	114 (38.1)
Type of insurance
National Health Insurance	281 (75.3)
Medical Aid	70 (18.8)
Healthcare for veterans	22 (5.9)
Type of CRPS
1	281 (75.3)
2	92 (24.7)

The numbers represent median (interquartile range) or number (percent).

SCS: spinal cord stimulator, CRPS: complex regional pain syndrome.

^aThe minimum implantation period was 71 days, and the maximum was 2,922 days.

Table 2 Impact of patient factors on the duration of spinal cord stimulator (SCS) use (n = 373)

Variables	Median (95% confidence interval, yr)	P value
Age (yr)		0.381
< 30	4.2 (3.7–5.3)
30–39	4.7 (4.0–5.7)
40–49	4.8 (3.6–5.1)
50–59	4.0 (2.8–5.0)
60–69	4.5 (2.2–5.9)
≥ 70	4.1 (2.8–4.8)
Sex		0.014
Male	4.7 (4.2–5.1)
Female	4.0 (3.3–4.5)
Type of hospital		0.122
Tertiary hospital	4.2 (3.8–4.7)
General hospital	4.9 (4.0–6.0)
Type of insurance		0.013
National Health Insurance	4.2 (3.8–4.7)
Medical Aid	4.3 (2.7–5.3)
Healthcare for veterans	6.9 (4.6–7.8)

Numbers represent median (95% confidence interval).

The median duration of SCS use was calculated using log-rank test.

Figure 1. CONSORT (Consolidated Standards of Reporting Trials) diagram.

Figure 2. Survival curves showing the difference in the duration of spinal cord stimulator (SCS) use according to sex. Male patients retained SCS longer than female patients (4.7 vs. 4.0 yr, P = 0.014, log-rank test).

Figure 3. Survival curves showing the difference in the duration of spinal cord stimulator (SCS) use according to insurance type. Beneficiaries of healthcare for veterans showed the longest duration of SCS use compared to National Health Insurance and Medical Aid beneficiaries (P = 0.013, log-rank test).

This study represents the first nationwide study using National Health Insurance Database to analyze CRPS patients with SCSs in Korea. In this analysis of nationwide healthcare data, it was found that the median duration of SCS use was 4.4 years. Similar findings have also been reported in previous studies which investigated the effect duration of SCSs [7]. The effect of an SCS diminishes after 3 years and has no additional benefit compared to physical therapy. Another retrospective study that analyzed 100 patients found that 53% of patients who underwent permanent SCS implantation eventually had their implant removed, and the median time to removal was 39 months post-implantation [15].

The limited duration of the pain relief effect of SCSs could be attributed to several mechanisms. First, SCS tolerance could account for the decreasing effect of SCSs. Tolerance, which refers to the gradual decline in pain control after initially satisfactory pain relief, results from gross fibrosis around the stimulation tip of the electrode that interferes with the electrical current and alteration of the ascending afferent nociceptive pathway via neuronal plasticity [16]. However, recent technical advances in SCSs (e.g., high-frequency and burst stimulation) have helped overcome SCS tolerance [17], and the effects of an SCS can be expected to last longer if this advanced equipment is used. Second, some patients might have overestimated the effects of the SCSs during the trial period, possibly due to the placebo effect of the SCS. Finally, some patients might have fully recovered from CRPS and thus no longer needed the SCS, although the proportion of patients who fully recover from CRPS is likely to be low [6].

We observed that sex and insurance type were significantly associated with the SCS implantation period. Both these findings are thought to have stemmed from a single origin; namely, many male patients were treated at veterans’ hospitals due to military injuries during military service and underwent SCS implantation. Men constitute a substantially higher proportion of patients treated at veterans’ hospitals.

It remains unclear why beneficiaries of healthcare for veterans maintained SCSs longer than those covered by other insurance types. One possibility is that this finding arose due to a higher prevalence of severe injuries among these patients. Alternatively, it could be attributed to potential secondary benefit issues, such as the assessment of disabled veterans, if these patients experienced a similar degree of injury or pain as patients with other insurance types.

SCS implantation is an invasive procedure, and patients who undergo SCS implantation experience a high rate of device-related complications. A meta-analysis reported that the overall average complication rate after SCS implantation was 21.1% [18]. Furthermore, the proportion of patients who received reoperation due to complications was 38% within two years after SCS implantation and 42% within five years [8].

Patients generally expect long-term use of their SCS, considering the device-related burdens such as cost, invasiveness, and complications. The findings of this study suggest that there may be a disparity between patients’ expectations and the observed duration of SCS use. Although it is acknowledged that SCSs are effective in some CRPS patients, many studies have not emphasized that many patients undergo SCS explantation due to reduced efficacy after implantation. Considering that patients may experience difficulties during SCS implantation, subsequent device-related complications, and the need for device removal due to a diminishing effect, a more cautious approach is needed regarding SCS implantation for patients with CRPS.

The primary hypothesis of this study was that patients would want to maintain implanted SCSs if their effects persisted, keeping in mind that patients could continue to use their SCSs even after the battery wears out by having the battery replaced. In contrast, they would want to have their SCSs removed if the device could no longer be expected to exert pain-relieving effects. The evaluation method may have overestimated the duration of SCS use (i.e., favorable results for SCS) in some patients who only maintained SCS implantation without actively using it because such patients maintained SCS implantation status without actual use, which increased the implantation period. Since some patients might not actively use their SCSs, even while they remained implanted, the evaluation method may have overestimated the duration of SCS use (i.e., yielding findings favorable for SCSs). Conversely, some patients might have wanted to continue using their SCSs, but were required to undergo SCS removal due to complications (i.e., infection). If such patients want to use an SCS again, they will undergo SCS re-implantation. It is highly likely that this study overestimated, rather than underestimated, the duration of SCS use, as SCS explantation would often probably have occurred after some time had elapsed since the device had become ineffective. Therefore, it is very unlikely that the actual effect duration of SCS in patients with CRPS would exceed the estimated value of 4.4 years. This finding highlights the need to refine patient selection to prevent potentially unnecessary SCS implantations, which are associated with significant health and financial burdens.

Some limitations of this study should be kept in mind. First, the authors did not differentiate the various reasons for SCS removal. While the National Health Insurance Database provides a large sample size and comprehensive coverage of medical events, it lacks specific clinical details regarding the reasons for SCS removal. Due to the inherent limitations of this database, it could not be determined whether SCSs were removed due to symptom improvement, lack of effectiveness, or complications. This limitation prevents us from identifying the precise causes of SCS removal. Given that CRPS is an intractable disease, it is widely understood among pain physicians that SCS removal is more likely due to diminished effects or device-related complications than symptom improvement. Collecting data from multiple individual hospitals may enable us to distinguish the reason for SCS removal more accurately, therefore, a multicenter study with a large sample would be needed to clarify these causes. Second, this study did not include patients covered by traffic-accident and industrial accident insurance, which exist as separate systems in Korea. It is unfortunate that these patients were not included, as their inclusion could have increased the number of subjects in this study. It was not possible to include these patients because their information is not open to the public for research purposes. Most national health insurance systems do not comprehensively manage general public patients, traffic accident patients, and industrial accident patients within a single insurance system. As such, analyzing all these groups under one study proves impractical. Further study is needed to include these patients for evaluating the SCS effect for CRPS. Given that there is no evidence that the severity, course, or treatment method of CRPS differs depending on whether the cause is a traffic accident or an industrial accident, it is assumed that the overall results would not change significantly, even if these patients were included in the analysis. Third, this study did not consider patients’ symptoms, the stimulation mode, or the SCS model in each patient. Recent advances in SCS technology have enabled more effective pain reduction with fewer device-related complications; therefore, the effect duration of more recent SCSs is likely to be longer. Nevertheless, a major strength of this study is that it is the first nationwide study based on large-scale population data to evaluate the duration of SCS use in patients with CRPS. For this reason, the study provides a general overview of this issue, rather than focusing on the characteristics of individual hospitals or individual physicians.

In conclusion, the median duration of SCS use in patients with CRPS was 4.4 years. Given the invasiveness of this procedure and the high rate of complications, patients should be thoroughly informed about the expected duration of SCS use, as well as the associated risks and benefits.

The authors are grateful to Ms. Mihee Park of Seoul National University Bundang Hospital for her help in preparing the illustrations.

The datasets supporting the findings of this study are not publicly available due to personal information protective policy and technical limitations in National Health Insurance Review and Assessment Service in South Korea.

Francis S. Nahm is the editor-in-chief of the Korean Journal of Pain; however, he has not been involved in the peer reviewer selection, evaluation, or decision process of this article. No other potential conflicts of interest relevant to this article were reported.

This work was supported by a research grant of Seoul National University Bundang Hospital (06-2022-0063).

In-Ae Song: Formal analysis; Joon Hee Lee: Writing/manuscript preparation; Woong Ki Han: Writing/manuscript preparation; Francis Sahngun Nahm: Writing/manuscript preparation.

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