UNIVERSITY OF CINCINNATI

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1 UNIVERSITY OF CINCINNATI Date: I,, hereby submit this work as part of the requirements for the degree of: in: It is entitled: This work and its defense approved by: Chair:

2 Differences in Outcomes after Spinal Cord Stimulator Device Placement in the Ohio Board of Workers Compensation A thesis submitted to the Division of Occupational & Environmental Medicine of the University of Cincinnati In partial fulfillment of the Requirements for the degree of MASTER OF SCIENCE In the Department of Environmental Health of the College of Medicine 2008 By Arthur E. Rabenhorst BS, Canisius College, 1995 MSc., Barry University, 1997 MD, St. George s University School of Medicine 2002 Committee Chair: Clara Sue Ross, MD, JD The findings and conclusions in this report are those of the author and do not necessarily represent the views of the Ohio Bureau of Workers Compensation, University of Cincinnati or Spinal Cord Stimulator Device Production Companies The third party author does not hold any financial interest in spinal cord stimulation companies and or ancillary pain control clinics/facilities utilized by the OBWC

3 Abstract Spinal Cord Stimulation (SCS) is used to treat certain types of chronic pain. It involves an electrical generator delivering pulses to a targeted spinal cord area. Leads are implanted by laminectomy, subcutaneously or percutaneously, and power is supplied by implanted battery or external transmitter. One possible mechanism of SCS is Melzack and Wall s Gate Control Theory describing how stimulation of large fibers closes the gate to stimuli reception by small pain fibers. Complications are relatively common after stimulator placement and most commonly include technical failures (battery failure, device failure, electrode failure, electrode slip, generator failure, and hardware malfunction). Biological complications include infection, spinal fluid leakage, headaches, bladder problems, and psychological intolerance. Rarely, allergic response to the implant occurs. A few studies have investigated complication types and predictors, but none involved a workers compensation claimant cohort. This analysis was undertaken to determine the complication rates and predictors using such a cohort. iii

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5 Acknowledgments I wish to thank the source of my strength and motivation in my life: my wife, Erica, my son Jack and my daughter Ellie. I love you all. I am also greatly appreciative to Sue Ross MD, JD and James Lockey MD, MS for their support and wise counsel. A very special thanks to Kari Dunning, PhD and Amy Rohs, MD, MS for their help with the statistical analysis and tireless thesis revisions. Without their help this project would not have been possible. v

6 Table of Contents Abstract..iii Acknowledgments..v List of Tables..vii Introduction/Background...1 The Current Study...8 Methods and Materials 8 Results 12 Discussion..14 Summary 19 Tables.20 Literature Citations...22 vi

7 List of Table s Table 1. Characteristics of claimants by whether or not they experienced a complication Table 2. Characteristics of claimants by whether or not they experienced a major complication (defined as spinal leaks or infection) Table 3. Category of complications Table 4. Reasons for SCS revision: biological complications Table 5. Reasons for SCS revision: technical complications Table 6. Complications by reason for implant (ICD-9 diagnosis) Table 7. Major complications (i.e. infection or spinal leak) by reason for implant (ICD-9 diagnosis) vii

8 Introduction/Background Spinal Cord Stimulation (SCS) is a procedure that uses an electrical current to treat chronic pain. Spinal cord stimulators are cardiac pacemakers-like devices used to deliver electrical impulses to the central or peripheral nervous system. To treat chronic back, leg or arm pain this small pulse generator is implanted in the back and transmits electrical impulses to the spinal cord. Stimulation of the spinal cord is done via an implantable battery and lead. The treatment may be done for people with severe, chronic pain who may have: failed back syndrome (FBS), chronic pain syndromes, or severe nerve related pain or numbness such as that caused by sciatica, spinal cord inflammation (arachnoiditis) andspinal cord scar tissue. These impulses interfere with transmission of the nerve impulses responsible for the source of the pain. Spinal cord stimulation is strategically aimed to reduce the unpleasant sensory experience of pain and the consequent modification of pain experience and behavior. The Gate Control Theory of pain developed by researchers Ronald Melzack and Patrick Wall in 1965, theorizes that electrical neurostimulation activates the body s pain inhibitory system (1). This theory, although 40 years old, remains the most plausible explanation used for the perception of pain and pain response. Pain and SCS Theories The exact mechanism for the transmission and perception of pain is poorly understood. One theory, which is generally well accepted, is called The Gate Control Theory. 1 Based upon the work by Melzack and Wall electrical impulses generated by the SCS are thought to inhibit the conduction of pain signals to the brain. The Gate Control Theory postulates that electrical neurostimulation activates the body s pain inhibitory system. Therefore, it proposes that there is a gate in the spinal cord that controls the transmission of pain signals to the brain which results in 1

9 pain relief. It explains that the body can inhibit these pain signals by closing the gate and activating certain non-noxious touch nerve fibers in the dorsal horn of the spinal cord. Sensory nerve fibers transmit to both the substantia gelatinosa (SG) found in lamina 1 in the spinal cord, and transmission cells (T), found in lamina V of the spinal cord. 1 The T cells are directly activated by all sensory nerve fibers. 1 The SG cells are the spinal gate keeper.the touch nerve fibers activate these SG cells, while the pain nerve fibers inhibit the SG cells. 1 Based on this most accepted theorem the SCS implanted electrodes are used to stimulate the dorsal horns for the treatment of chronic pain. SCS delivers low voltage electrical impulses to a specific target nerve in the spinal cord or peripheral nerve to block the perception of pain. When touch nerve fibers activate the SG cells they essentially close the gate because the SG cells pre-synaptically inhibit the information traveling from the touch nerve fibers to the T cell. 1 Since the pain nerve fibers inhibit the SG, the SG cells can not affect the T cells allowing pain nerve fibers to open up the gate because they can directly activate the T cells which cannot be inhibited by the SG cells. For example, this theory explains why rubbing an injured area decreases the amount of pain perceived. Rubbing an injured area activates the large diameter axon which closes the gate thus decreasing the amount of pain perceived. 1 SCS Device Hardware There are two kinds of SCS systems available. The more commonly used system is a fully implanted unit that utilizes a pulse generator and non rechargeable battery that must be replaced over time. This is the type of SCS that we will be studying in our study. The second system relies on a radio frequency and includes a transmitter and an antenna, which are carried outside the body (much like a pager or a cell phone). 2

10 The SCS system is composed of an electrical pulse generator and electrodes implanted in the epidural space, activating pain inhibiting fibers, suppressing the pain sensation and replacing it with a tingling sensation in the targeted pain area. A small wire and electrode apparatus (called a lead) is connected to a power source implanted under the skin. Low-level electrical signals are then transmitted through the lead to the spinal cord or to specific nerves to block pain signals from reaching the brain. Using a remote control, one can adjust the current on and off, or adjust the intensity. The sensations derived from the stimulator are different for everyone; however, most patients describe it as a pleasant tingling feeling. Based upon the work by Melzack and Wall, electrical impulses generated by the SCS are thought to inhibit the conduction of pain signals to the brain. The nature of SCS entails invasive modalities with inherent risks. 1 SCS Implantation Implantation of the stimulator is an invasive procedure, typically done by using conscious sedation. The treating physician first implants a trial stimulator under the skin (referred to percutaneous implantation). If successful, the surgeon implants a more permanent stimulator device (referred to as subcutaneous implantation). The stimulator itself is implanted under the skin of the abdomen and the small coated wire leads are inserted and tunneled under the skin to the insertion site within the spinal canal. 2 This placement in the abdomen is more stable for the SCS device, but implantation sites for the SCS can vary widely. However, all require the placement of electrodes and attachment to the SCS. Subcutaneous wire electrodes have power sources including radio-frequency receivers and off label implantable lithium ion batterypacks and Federal Drug Administration (FDA) approved implantable power sources. 2 SCS is applied through an electrical generator that delivers pulses by means of electrodes placed in the epidural 3

11 space adjacent to a targeted spinal cord area presumed to be causing the pain. 2 The leads which are special devices containing the set of electrodes can be implanted subcutaneously, precutaneously or by laminectomy. The number of leads and parameters of stimulation can vary depending on the pain experienced by the patient. 3 Power is supplied by an implanted battery or transcutaneously through an external radio-frequency transmitter. Both sources are equipped with a computer telemetry system that allows trancutaneous programming of the specific pattern of stimulation. The overview of the surgical procedure is generally broken down into two steps: the trial screening and permanent implantation of the SCS. 3 The trial stimulation period is usually days to weeks and is used to assess whether the SCS will mask the pain effectively, identify the level of pain relief to be expected and analyze the battery requirements. At this time, patients are taught how to activate and adjust their transmitter. There is also a period where the patient judges how well the pain is controlled during activities of daily living. 3 In the clinic, it is unusual to offer SCS until other less invasive modalities have been utilized. The placement of a SCS is usually preceded by a successful trial of stimulation with lead placement and an external stimulator. 4 This is an important step to determine the efficacy of the stimulation system prior to an implantable system. Once the patient tolerates the external system it is usually followed by an insertion of a permanent generator. The permanent implantation procedure takes place in the operating suite under anesthesia. 4 Two incisions are made, one being the back and the second over the abdominal area. This allows a tunneling effect to be made to create a path from the lead to the SCS. Once the leads are connected and recovery takes place, patients are given specific operating directions. 4 4

12 Indications and Contraindications for SCS Placement The chief contraindications for SCS placement include 5 : drug or alcohol addiction problems;psychological risk factors such as active psychosis, suicidal thoughts; homicidal thoughts, no poorly treated major depression or other mood disorder and;debilitating panic or anxiety or somatoform disorder. Common indications for SCS include: archnoiditis; failed back syndrome (FBS); radiculopathy; plexus lesions; spinal cord injury; phantom limb pain; ischemic limb pain; peripheral neuropathy/neuralgia; and chronic regional pain syndrome (CRPS). SCS Effectiveness Clinically relevant information extrapolated from the medical literature is limited. Most studies or reviews do not distinguish specific diagnoses and different follow up times for each study, making comparisons difficult. 5 In 1995, North and Roark reviewed 320 consecutive patients having SCS placement at Johns Hopkins Hospital, excluding those with significant drug habits, secondary gain, or psychological problems. 6 The article documented technical details of treatment and patient characteristics as predictors of outcome and hardware reliability.. Several patient characteristics were associated with pain relief including gender and diagnosis. Women tolerated the SCS better and experienced greater pain relief compared to males. 6 Patients with mechanical back pain (e.g. sciatica) experienced greater pain relief with SCS compared to those with CRPS Chronic Regional Pain Syndrome. 6 5

13 A 1995 review by Turner and Loeser, included two randomized controlled trials (RCT) totaling 81 patients. 7 Although each RCT study found SCS to be effective, a meta-analysis was not done because of the small number of patients and observed heterogeneity. The review concluded that there is limited evidence for the efficacy of SCS and advocated more trials to determine overall effectiveness. 7 SCS Complications Complications are relatively common after SCS placement, although most are considered minor. For research purposes, complications are typically categorized as either technical or biological. Also for research purposes, complications are typically identified by the need for a SCS revision (e.g. surgical removal or repair). The most common complications evolve from revisions of devices for technical failures. 7 The leading cause of technical failures is from fibrotic tissue accumulation around the implanted hardware. 7 Biological complications include infection (deep and superficial), leakage of spinal fluid, headaches, bladder problems, and psychological intolerance to device stimulation. 7 Rarely, complications involve allergic response to the implant material. 7 In 1995, Turner and Loeser reviewed 39 case studies and found complications were frequent,occurring in 20-75% of patients across studies with an average of 42%. 7 Most complications were technical in nature, with an average of 30% of patients having at least one complications related to a hardware system component. Electrode problems were the most frequent complication occurring, on average, 24%. The infection rates for these procedures ranged from 0-12% (mean 5%). The rate of biological complication other than infection ranged from 0-42% (mean 9%). 6

14 In 2004, Turner and collegues conducted another SCS systemic review including analysis of complications. 8 The literature search yielded 583 articles, with 22 reporting criteria appropriate for SCS complication review. Technical complications consisted of: equipment failure (10.2%); stimulator revision not including battery change (23.1%) and; stimulator removal (11.0%).. 8 Although battery replacement is a valid reason for a stimulator revision, most studies in this review did not comment on battery replacement. Rates of infection, and pain in the region of device was 4.6% and 5.8%, respectively.. 8 Biological complications other than infection or local pain occurred in 2.5%. An evidence based practice review in 2003, included one RCT and 14 observational studies (two prospective and 12 retrospective). 9. Twelve of these studies reported age, with an average range of years at implantation. Among the 10 studies that included complications, 9-50% of patients had at least one complication. 9 The rate of SCS revision ranged from 11.1% to 50%. 9 Complications due to technical problems such as lead migration and equipment failure ranged from 8.3% to 42.8%. 9 The infection rate ranged from 1.4% to 11.1%. 9 This study had several limitations to this review. It primarily investigated the efficiency of pain management of SCS patients and inconsistently reported outcomes after surgical placement and follow-up. There is also a possible publication bias, leaving out studies which showed an unfavorable outcome. Furthermore, the observational studies inconsistently reported important demographic information, SCS characteristics and complications. 9 In 2007, Kumar published an article outlining international expert recommendations for avoiding complications with SCS devices. 10 This paper was unique in that if offered a review of the pertinent literature of hardware related complications and a consensus panel of experts. This 7

15 project proactively provided practical guidelines in which were aimed at helping implanters of these devices. There This article addressed facets of care that may be associated with complications including: appropriate imaging, patient positioning, insertion of leads, intraoperative stimulation, lead choice and suturing, implant positioning and infection control. 10 Research studies to date have not incorporated factors such as these in their analysis of complications. The Current Study The purpose of this current study was to determine rates and types of complications among OBWC (Ohio Bureau of Workers Compensation) claimants who received SCS. This study was in response to an OBWC observation that claimants may be experiencing a higher than average rate of complications post SCS placement. Characterizing the rate, types and potential predictors of complications may enable OBWC to better ascertain where corrections can be made to reduce adverse affects after SCS placement. The nature of SCS involves invasive modalities with inherent risks. This in and of itself makes it difficult to conduct large-scale, prospective, controlled, double blinded studies of SCS techniques and outcomes. 11 This observational study may facilitate future controlled studies that can more directly improve patient management and post SCS placement care. Methods and Materials Selection of Articles for literature review The literature review for this study included randomized controlled trials and systemic reviews for SCS use in failed back surgery syndrome (FBSS) and chronic lower back pain as identified 8

16 from searches of MEDLINE (PubMed), EMBASE and the Cochrane Library. This search was initially performed in January 2005 and updated in May Inclusion criteria for articles were: publication in English language; diagnoses including Reflex Sympathetic Dystrophy (RSD) or FBSS due to radiculopathy, arachnoiditis neuritis/neuropathy, any cause including but not limited to peripheral neuropathy; having a follow-up more than 6 months. Study Design The overall study design was an observational retrospective chart review of OBWC claimants who received a SCS during a six year period. The source of the data was the OBWC information data bank and electronic medical record system. The information from the OBWC data bank listed anonymous claim numbers. These claim numbers were used to access narrative medical record files. Medical records contained claimant name; however, no data collection of names were conducted. The cohort was evaluated for the presence of possible cofactors including age, gender and SCS implant location. Although we initially wanted to also use medical co-factors (e.g. diabetes, hypertension, obesity), the lack of consistent documentation precluded the ability to do so. Methods Workers compensation claims receiving SCS from were identified by an OBWC query of billing data (CPT codes). This de-identified query was provided in excel format to University of Cincinnati. As described earlier, there are two major procedural approaches for spinal cord stimulator insertions: percutaneous and subcutaneous implants. Percutaneous implants (CPT code 63650) are typically inserted on a trial basis (1-3 weeks) to determine if the stimulator works. If the percutaneous implant works, it is most common to then perform a 9

17 subcutaneous implant (CPT code 63685) for more permanent pain relief. Subcutaneous procedures were chosen for this current analysis due to this more permanent placement. Online medical record reviews were conducted for claims using a secure OBWC provider website for patient claim documents. Due to the time needed to review these records, two UC persons with medical experience reviewed the claims, the primary investigator reviewed 453 and a secondary investigator reviewed 121. Documents reviewed included all medical office/hospital notes, imaging, physical therapy and vocational rehab notes, IME s (Independent Medical Exams) and surgical notes. Complications from SCS implants were identified using the OBWC billing data indicating implant revision (CPT code or 63660). Using the medical record review, type of complication (i.e. reason for revision) was categorized trichotomously: biological, technical, or both. In order to better understand reasons for revision, subcategories were assigned: battery failure, device failure, electrode failure, electrode slip, generator failure, hardware malfunction, headaches, dural puncture, and infection. These categories were not mutually exclusive (e.g. one claim could be assigned both headaches and device failure). Additionally, major complications were defined as those claimants who experienced infections, dural puncture or spinal leaks. There were two primary outcome variables: complication (yes, no) and major complications (yes, no). The analysis began with descriptive statistics of rates and types of complications. In order to determine predictors of complication and major complication, bivariate and logistic regression analysis was conducted. Possible predictors were: age, gender, and location of implant (cervical or lumbar). 10

18 This analysis used the OBWC billing query (CPT codes) to identify claims receiving subcutaneous implants and to identify claims that had complications. The accuracy of this billing query was critical in interpreting results. Therefore, three small pilot studies were conducted using randomly selected claims to validate the OBWC billing query data using medical record reviews. First, to determine the accuracy of the date of original implant insertion, 20 subcutaneous claims (CPT code 63685) were reviewed. Second, to determine potential bias of using the CPT codes to identify subcutaneous implants, 32 percutaneous claims (CPT code 63650) were reviewed. Third, to determine potential bias of using revision CPT codes to identify complications, 17 subcutaneous claims that did not have a revision CPT code were reviewed. Data Collection and Verification of Removal or Revision Claim numbers were used to access narrative medical record files and to retrieve needed information. Medical records were reviewed for all claimants with SCS placement (whether or not they had a CPT complication code) as described below. These record reviews included a screen of allowable OBWC claims, reason for SCS placement and revision. Office notes, progress notes or other medically related notes were reviewed. Upon logging onto the secure OBWC web site electronic medical record, there was a set of tabs on the format screen. The accident/injury tab was searched revealing the appropriate diagnostic information. This information listed all the allowable diagnoses for each claimant coded by The International Classification of Diseases- Ninth Revision, (ICD-9). For verification purposes, the date of injury was also checked. 11

19 Subsequently, the claim reference tab showed clinical, legal and other ancillary medical information. The crux of the medical follow up revolved around orthopedic, musculoskeletal, rehabilitation (physical or occupational therapy) and/or operative notes. Other data available in this section related to legal hearings, case decisions, bill inquiries, psychiatric notes/assessments and case management issues. The OBWC data query included dates of removal or revision which made the search for the complication easier to locate. The clinic notes were also reviewed to verify reason for revision, diagnoses (ICD-9 codes), and anatomical loci for SCS placement. Results Population According to the OBWC billing query, there were 574 claimants who received subcutaneous spinal cord stimulator implants from Of those 574, 38.3% (220) had a revision CPT code (i.e. complication). Of those 574, 5.2% (30) had a major complication including infection, dural puncture or spinal fluid leak. Characteristics by whether or not claimants experienced a complication or a major complication are shown in Tables 1 and 2, respectively. Predictors of Complications As shown in Tables 1, there were no statistically significant differences in predictors (age, gender and SCS location) between claimants with and without complications. Although claimants with complication tended to be older (38.9 years) compared to claimants without complications (37.6 years), this difference was not statistically significant (p=0.10). Of the claims with complication, 60.5% were male compared to 64.7% among claims without complication (p=0.31). Similarly, there was no difference in predictors between claims with and 12

20 without major complications (Table 2). When predictor variables (age, gender and location) were tested in a logistic regression model, there were no significant findings. Reasons for Revision Of the 220 claimants with complications, 39.5% were biological, 36.8% were technical and 23.6% both (Table 3). As shown in Table 4, among these 220 claims, the common biological complications were: failure to control pain (34.1%), deep and superficial tissue infections (13.2%), patient could not tolerate the device stimulation (10.4%), headaches related to insertion (2.7%), spinal leak (0.5%) and allergy type reactions (0.5%). There were no deaths associated to SCS insertion in the review of this cohort. As shown in Table 5, the most common technical complications were: electrode migration/slippage (25.5%), broken leads (21.8%), battery failure (16.8%), generator failure (10.5%), non specific device failure (15.5%). Complication Rate by Diagnosis Among the 574 claimants, the leading reasons for implant (as identified by ICD-9 in the medical record) were: 32.4% RSD (n=186); 28.6% FBS (n=164); 23.0% disk herniation (n=132). Tables 6 and 7 show complication and major complication rates by reason for implant. Rates of complications (Table 6) ranged from 18.0% (radiculopathy) to 43.6% (RSD). Rates of major complications (Table 7) ranged from 0% (sciatica) to 8.3% (other). Validity Pilot Studies As described in the Methods Section, randomly selected claims were used to determine data quality in the OBWC billing data and determine possible bias. First, to determine the quality of the date of original implant insertion, 20 claims were randomly selected. Medical records were 13

21 reviewed to validate the OBWC query date with the medical record date. Of those 20 claims, 70% (14) were correct within 3 months but 30% (6) were incorrect by at least 6 months (ranging from a 6 month to a 4 year discrepancy). Second, to determine the accuracy of identifying claims with subcutaneous implants using CPT codes, 32 claims that had percutaneous claims (CPT code 63650) were reviewed. These claims did not have subcutaneous CPT codes, therefore according to CPT codes these claims did not have subcutaneous SCS implants. The medical record review showed of these 32 claims, 28% (9) had a subcutaneous implant (in addition to a percutaneous implant). Therefore, by using only CPT code to identify subcutaneous implants, we potentially missed a large number of claims that received subcutaneous implants after initial percutaneous implant. Third, in order to determine the accuracy of using revision CPT codes to identify complications, 17 subcutaneous claims that did not have revision CPT codes were reviewed. Therefore, according to CPT codes, these claims did not have revisions. Medical record review showed of these 17 claims, 59% (10) had a complication and a revision. Discussion Our Findings summarized Among the 574 claims with subcutaneous CPT codes, rate of overall complications was 38.3% and rate of major complications (including infection and spinal leak) was 5.2%. Age at time of injury, gender, or placement location (cervical versus lumbar) was not predictive of complication or major complication. 14

22 Our pilot validity study identified several important limitations to the current methods used. Subcutaneous claims were identified using a billing query performed by OBWC. Medical chart reviews to validate this data suggested that the results of this report may be subject to selection bias and does not include all claims receiving subcutaneous implants. Date of insertion from the OBWC billing query could not be used due to inaccuracy. Therefore, claimant age at implant and the length of time from implant to complication could not be determined. Most important, the rate of complication is underestimated based on missing or underutilized revision CPT codes. Limitations There were several limitations in this study. Workers compensation claimants may differ from the general population, thus limiting external validity. Complications among these claimants are only in the database if they are brought to the attention of the physician, otherwise complications are not documented. Documentation for this study was taken from a myriad of provider settings ranging from large multi-center facilities to small clinical suites. Physician experience and training regarding use of SCS were not available for our analysis. Other factors that may influence complication rates were not routinely found in the medical records including number of leads, device manufacturer and other hardware details (12). Hardware type and technical type complications may have played a significant role in total complications (12). Listing the types of electrodes, leads, pulse generator, neurostimulator makes and models may be valuable in auditing post surgical response and complications (13). 15

23 There were also documentation limitations based on the reliance on anecdotal physician operative reports. Furthermore, capturing variables on electronic medical records was also limited because of illegible penmanship or poor scanning of critical documents to the patient s record. Inadequate clinic notes including records from follow up evaluations, physical exam records and other documentation may bias the results as a result of excluding claimants who may have had a complication. Past medical histories were not consistently documented, making the use of co-morbidities as a factor in predicting complications unreliable. This may occur because of data management issues involving the physician, clinic/hospital, MCO (Managed Care Organizations) and OBWC. The pilot validity study identified several important limitations in our study methodology. Subcutaneous claims were identified using a billing query performed by OBWC. Medical chart reviews to validate this data suggested that the study results do not include all claims receiving subcutaneous implants and, therefore, subject to selection bias. Date of insertion from the OBWC billing query could not be used due to inaccuracy. Therefore, claimant age at implant and the length of time from implant to complication could not be determined. Most important, the rate of complication is underestimated. Furthermore, patients outside of the cohort time frame may have experienced future complications. Factors, such as subject motivation and compliance may have influenced biological complications and were not controlled for in this analysis. In addition, individual psychological coping strategies and social support mechanism may also be important. 16

24 Battery life is typically reported in the literature as 4-5 years (14). However, there were conflicting views as to whether to include battery depletion as a complication (15). The Turner systemic literature synthesis which spanned four years did not report on battery half life. The OBWC SCS study spanned 6 years which exceeded the average battery life. Future OBWC analysis may need to incorporate shorter time frames to compare with previous studies. The revision of a SCS may also be based on battery type or manufacturer, which as mentioned was not routinely reported in the OBWC chart. The current project analyzed two different outcomes: complication (including battery failures) and major complication. Comparisons with Published Literature The Turner 1995 literature review reported complication in 31 studies of patients with chronic low back pain with at least 30 days of follow up. 7 Mean follow up was 16 months and 86.0% were workers compensation patients. Analysis of complications was difficult given the different study designs. The overall complication rate for the Turner study was 42% 7 compared to 38.3% in our study using OBWC data. The complication rate among OBWC claimants with FBS was similar to Turner at 40.9%. Turner found the percent of complications were: infection (5%), electrode (30%), lead wire (7%), generator (2%). 7 Our study using OBWC data demonstrated similar infection (5%) and generator (4%) complication rates. By combining our results for electrode slippage (n=56) and failure (n=48), there was an 18.1% electrode failure (104/574). It is possible we may have missed complications as a result of invalid CPT codes based on our pilot validity studies. Therefore, we may have underestimated complication rates. Turner conducted another review in 2004 that included 22 studies and found an overall complication rate of 34.3%. 8 The rate of superficial infection was 4.5%. 8 Other complication 17

25 categories (e.g. lead complications) could not be directly compared to Turner s studies using our data. Grabow reviewed 10 studies of patients with Reflex Sympathetic Dystrophy who received SCS.. 9 Unfortunately this review did not combine data from different studies but listed ranges of complication rates. Among these individual studies, overall complication rates ranged from 9-50%. 9 Infection rate ranged from 1.4% to 11.1%.. 9 Technical problems including equipment failure and lead migration ranged from 8.3% to 42.8%.. 9 Our overall rate of all complication among OBWC claimants with RSD was 43.6% (81/186). The Cameron literature review included 2,753 SCS patients and found 27.2% experienced technical complications, 4% experienced biological complications, and 5% experienced other complications. 15 Specific complication rates were:lead migration 13.2%; battery failure 1.6%; infection 3.4%; allergic reaction 0.1%. 15 Comparatively, among the OBWC claimants receiving SCS, we found: lead migration 9.8%; battery failure 6.5%; infection 5.1%; allergic rash 0.2%; spinal leak 0.2%. The Cameron study reported post implant infection rates of 3.4% (100) 15 compared to our study 11.89% (29). Many of the specific complication rates found by Cameron are much lower than our results. For example, specific complication rates in Cameron s study were: pain over implant accounted for 0.9% (24), undesirable stimulation 2.4% (65), allergic reaction 0.1% (3). 15 Our study using OBWC claims found: pain over implant accounted for 16% (39), undesirable stimulation 9.4% (23), allergic reaction 0.4%. In addition, the Cameron study had lower technical complications than our study using OBWC data. Cameron specific rates of technical complications were: lead migration 13.2% (361), electrode breakage 9.1% (250), battery failure 1.6% (35) and SCS device failure 2.9% (80). 15 These percents are substantially 18

26 lower than our results for similar technical complications: lead migration 23.0% (56), electrode breakage 19.7% (48), battery failure 15.2% (37), and SCS device failure 5.3% (13). The difference between Cameron and our data may be due to longer follow up in our OBWC data and methods used to define and categorize complications. SUMMARY Synthesizing the current evidence regarding rate of complication after SCS is challenging due to heterogeneity between studies including: methodology (e.g. different ways to categorize and document complications); duration of follow up after SCS; different types of patients (e.g. workers compensation, different diagnoses). In addition, a recent article describing guidelines to reduce complications identifies factors that have not been considered in research studies to date (e.g. patient positioning, insertion of leads, intra-operative stimulation, lead choice and suturing, implant positioning, infection control). More research is needed to clarify these issues. 19

27 TABLES Table 1. Characteristics of claimants by whether or not they experienced a complication (n=574) Characteristic Complication Complication Total p-value Yes (n=220) No (n=354) (n=574) Age at injury: mean years (8.8) 0.10 (SD) * Gender: percent (n) 0.31 Male Female 60.5 (133) 39.6 (87) 64.7 (229) 35.3 (125) 63.1 (362) 36.9 (212) Location : percent (n) ** Cervical Lumbar 23.0 (50) 77.0 (167) 20.5 (72) 79.6 (128) 21.3 (122) 78.0 (447) 0.47 *Age at time of initial spinal cord stimulator insertion could not be determined **n=5 other or unknown Table 2. Characteristics of claimants by whether or not they experienced a major complication (defined as spinal leaks or infection) (n=574) Characteristic Major Major Total p-value Complication Yes (n=30) Complication No (n=544) (n=574) Age at injury: mean years 40.0 (10.0) 38.4 (8.8) 38.5 (8.8) 0.34 (SD) * Gender: percent (n) Male Female Location : percent (n) ** Cervical Lumbar 60.0 (18) 40.0 (12) 63.2 (344) 36.8 (200) 30.0 (9) 21.0 (113) 70.0 (21) 79.0 (426) *Age at time of initial spinal cord stimulator insertion could not be determined **n=4 Other 63.1 (362) 36.9 (212) 21.3 (122) 78.0 (447) Table 3. Category of complications (n=220) Category of Frequency Percent Complications (n) Biological % Both % Technical % 20

28 Table 4. Reasons for SCS revision: biological complications (n=220) *Not mutually exclusive Reason for revision Frequency Percent (n) Failure to control pain % Associated headaches 6 2.7% Infections (superficial and deep tissue) % Rash/allergic symptoms 1 0.5% Spinal leaks and or arachnoiditis 1 0.5% Patient couldn t tolerate SCS % Table 5. Reasons for SCS revision: Technical complications (n=220) *Not mutually exclusive Reason for revision Frequency percent (n) Battery failure % Electrode lead broken % Electrode lead migration % SCS generator failure % Migration of SCS stimulator device % Non-specific device failure not otherwise stated % Table 6. Complications by reason for implant (ICD-9 diagnosis) Reason for implant (ICD-9) Complications percent (n) Herniation (n=132) 35.6 (47) FBS (n=164) 40.9 (67) RSD (n=186) 43.6 (81) Radiculopathy (n=39) 18.0 (7) Sciatica (n=29) 31.0 (9) Other (n=24) 37.5 (9) Table 7. Major complications (i.e. infection or spinal leak) by reason for implant (ICD-9 diagnosis) Reason for implant (ICD-9) Major Complications Herniation (n=132) FBS (n=164) RSD (n=186) Radiculopathy (n=39) Sciatica (n=29) Other (n=24) percent (n) 6.1 (8) 3.1 (5) 7.0 (13) 5.1 (2) (2) 21

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