The lifetime prevalence of disk degeneration and low back pain (LBP) is approximately 80% in the general population.1 Since 1990, the number of lumbar fusions for degenerative disk disease (DDD) and diskogenic LBP have risen approximately 220% in the United States.2 Therefore, it is not surprising that lumbar diskography (LD), an injection procedure used in the evaluation of patients with chronic diskogenic LBP, is frequently performed. Diskography's purpose is to determine which intervertebral disks are pain generators and may respond well to surgical intervention, most commonly lumbar fusion. Derby et al3 reported that if particular attention is paid to injection pressures, LD can be associated with false-positive rates no higher than 10%. Multiple other studies also support the role of LD in evaluating patients with diskogenic LBP.4–8
In contrast, some consider LD controversial because a growing number of reports have questioned its safety and diagnostic accuracy.9–17 Carragee et al10 found that pain provocation during LD was common in patients without LBP and that pain reproduction in many patients with chronic LBP was inaccurate. In a separate study, Carragee et al14 reported similar provoked pain responses when injecting symptomatic and asymptomatic disks, raising the concern that LD could lead to inappropriately high surgical rates. One 10-year study associated LD with accelerated disk degeneration, disk herniation, and loss of disk height.11
Furthermore, the workers' compensation (WC) population tends to have worse lumbar fusion outcomes than the general population.18–21 A 2011 cohort study reported that undergoing lumbar fusion for DDD, disk herniation, and/or radiculopathy in a WC setting is associated with increased disability, opiate use, prolonged work loss, and a poor return to work status.21
Sufficiently poor outcomes following back surgery can be characterized by the biopsychosocial phenomenon known as failed back surgery syndrome (FBSS). These patients experience decreased functional capacity, morale, and productivity as well as opioid addiction. Approximately two-thirds of all patients enrolled in chronic pain centers experience FBSS.22 Several pathways to FBSS exist, the most common of which is poor patient selection for surgery.23,24
Lumbar diskography is a diagnostic tool meant to help clinicians select which patients are good candidates for lumbar fusion, which should ideally help lower postoperative rates of FBSS. However, if LD is not associated with better postoperative clinical outcomes, then questions may be raised regarding its usefulness. To the current authors' knowledge, this study is the first to directly evaluate the efficacy of LD in the WC population. Relatively few studies have been published evaluating risk factors for poor lumbar fusion outcomes in WC patients.25–27
Therefore, the authors wanted to compare clinical outcomes following diskogenic fusion between WC patients who underwent LD before fusion and WC patients who did not. The primary objective of this study was to determine how undergoing diskography before diskogenic fusion affects rates of postoperative FBSS. Can LD, already considered controversial by some, effectively identify which WC patients with diskogenic LBP and DDD are ideal candidates for lumbar fusion, resulting in lower rates of post-fusion FBSS? Because FBSS is a chronic pain syndrome, the authors also wished to determine whether there is a difference in postoperative opioid use between patients who underwent LD before fusion and those who did not.
Materials and Methods
The authors initially identified 14,640 WC patients from the Ohio Bureau of Workers' Compensation (BWC) who were diagnosed with lumbar comorbidities after sustaining a workplace injury between 1993 and 2013 using ICD-9 diagnosis codes. Next, CPT procedural codes were used to identify 5990 of these patients who underwent lumbar fusion surgery with a minimum of 3 years of postoperative follow-up. The authors characterized the type of fusion each patient underwent using a CPT coding schema published by Nguyen et al21 in 2011. Because smoking is a known risk factor for worse outcomes, patients with a positive smoking history were excluded, as identified with ICD-9 codes and/or the use of prescription smoking deterrents.28–30 Patients diagnosed with FBSS before fusion were also excluded, as identified by the ICD-9 code 722.83.
From the fusion population of 5990 patients, ICD-9 codes were used to identify 1591 patients who underwent fusion for the primary indications of DDD and diskogenic LBP. To do so, patients not diagnosed with DDD before fusion and patients diagnosed with spinal instability or deformity (ie, spondylolisthesis or scoliosis) in addition to DDD were excluded. Thus, the authors arrived at their final study population, comprising patients who underwent diskogenic fusion with at least 3 years of follow-up. Using the CPT code 62290, 682 (42.9%) of these patients who underwent LD before fusion were identified, forming the LD group. The remaining 909 patients became the control group. Because some of the patients in this study may have had multiple fusion surgeries while under WC, the authors only evaluated outcomes following each patient's index diskogenic fusion after WC-qualifying workplace injury. Figure 1 illustrates the patient selection process. Table 1 provides the coding used to identify this study population and to characterize each patient's fusion surgery.
The authors used a combination of ICD-9 diagnoses and CPT procedural codes to derive the final study population.
Coding Used for Study Population
The authors' primary outcome was a diagnosis of FBSS within 3 years after index fusion (AIF). The authors measured the following secondary outcomes at 3 years AIF: prescription opioid analgesic use, rates of permanent disability, rates of all-cause and perioperative mortality, development of psychological comorbidity, rates of pseudoarthrosis, and additional lumbar surgery.
The number of days each patient was supplied with prescription opioid analgesics before index fusion (BIF) and AIF were calculated. Each opioid prescription was converted to morphine equivalent units (MEQs), and the total MEQs supplied BIF and AIF were measured. Also, the authors determined the average daily morphine equivalents (MEDs) for each patient BIF and within 3 years AIF. Permanent disability rates, both BIF and within 3 years AIF, were determined by identifying patients awarded permanent partial or total disability. Because a biopsychosocial model of illness is used to explain the chronic pain experienced by patients with FBSS,22 the authors determined postoperative rates of depression, anxiety, and adjustment reaction disorders. Perioperative mortality was defined as death within 6 weeks of fusion. Additional lumbar surgeries included fusions and decompression procedures.
Furthermore, the authors collected data on each patient's age at index fusion, sex, approximated income, and presence of obesity. Specific incomes for each patient were unavailable; therefore, each patient's ZIP code of residence was correlated with a mean per capita income based on the 2010 United States Census. Next, comorbid lumbar conditions BIF, in addition to DDD, were characterized. The authors determined whether patients were clinically diagnosed with depression, generalized anxiety, and/or adjustment reaction disorders BIF. The authors identified which patients underwent a concurrent decompression procedure with index fusion and the number of days between each patient's injury and index fusion. The authors characterized the graft type(s) and instrumentation used at each patient's index fusion. The patients who were able to continue working within the same week as fusion and who had legal representation BIF were also determined.
To determine the effect of preoperative diskography on postoperative rates of FBSS, a multivariate logistic regression analysis was used. The dependent variable was whether FBSS was diagnosed within 3 years AIF. In the regression model, the authors corrected for the following binary covariates: lumbar diskography BIF, single- or multi-level index fusion, working within the same week as index fusion, permanent disability BIF, legal representation BIF, obesity, sex, prior lumbar surgery BIF, and whether a patient was supplied with opioid analgesics for more than 1 year BIF. Preoperative lumbar and psychological comorbidities were also included as individual binary variables. The authors corrected for the following continuous variables: age at index fusion, per capita income, and daily MEQs BIF. The authors corrected for the following categorical variables: type of fusion technique(s), instrumentation, and graft type(s) at index fusion. Baseline characteristics and secondary outcomes were compared between the LD and control groups using chi-square tests for categorical and binary variables. For continuous variables, t tests were used.
Table 2 highlights key information and preoperative differences between the LD and control groups. Table 3 characterizes fusion techniques, instrumentation, and graft types. The authors' study population spanned nearly 20 years. Figure 2 shows injury, diskography, fusion, and FBSS rates by year. A P value less than .05 was considered statistically significant. For all analyses, Statgraphics Centurion XVI statistical software (Statpoint Technologies, Inc, Warrenton, Virginia) was used. The authors did not require institutional review board or ethics committee approval because all subject data were provided in a deidentified manner. The authors worked directly with the Ohio BWC's legal department.
Preoperative Population Characteristics
Index Fusion Characterization
The rates of patient injury, diskography, index fusion, and postoperative failed back surgery syndrome (FBSS) by year.
Undergoing diskography BIF was positively associated with FBSS (P=.04; odds ratio [OR], 1.44). A total of 13.9% of the LD group and 8.8% of the control group developed FBSS within 3 years AIF. Overall, 11.0% of the entire study population developed FBSS within 3 years AIF. Figure 3 illustrates these results. A chi-square goodness-of-fit test determined that there was no reason to reject the fitted logistic regression model at the 95% confidence level. Additional predictors of FBSS included the ability to remain at work within 1 week of index fusion (P=.02; OR, 0.54), male sex (P=.03; OR, 1.51), opioid analgesic use for more than 1 year BIF (P=.02; OR, 1.53), and fusion technique (P<.01). For the latter covariate, patients who underwent anterior lumbar interbody fusion or posterior lumbar fusion plus posterior lumbar interbody fusion developed FBSS at the highest rates (10.2% and 15.7%, respectively). Patients who underwent PLF or 360° fusion developed FBSS at the lowest rates (5.4% and 6.4%, respectively) (Table 4). No other covariates significantly affected FBSS rates.
Undergoing diskography before diskogenic fusion was a positive predictor of postoperative failed back surgery syndrome (P=.04; odds ratio, 1.44). Diskography patients developed failed back surgery syndrome (FBSS) at a 5.1% higher rate when compared with controls.
Predictors of Failed Back Surgery Syndrome
Patients in the LD group were supplied with prescription opioid analgesics for an average of 129.7 additional days after fusion (P<.01). This equated to 16,390.5 additional MEQs (P<.01). A total of 64.5% of the LD group and 53.5% of the control group used opioid analgesics for more than 1 year postoperatively (P<.01). Daily opioid load was also higher among the LD group, at an average of 5.7 more MEQs per day (P=.04). No other secondary outcome measures differed significantly between the LD and control groups (Table 5).
Secondary Outcome Measures
The safety and diagnostic accuracy of LD are debated in the literature.3–17 The purpose of LD is to determine which intervertebral disks are pain generators and may subsequently respond well to surgery, most commonly lumbar fusion. Failed back surgery syndrome is a chronic pain syndrome most commonly due to poor patient selection,23,24 something that diskography should help prevent. If LD is consistently able to identify good candidates for fusion surgery, then patients selected to undergo lumbar fusion after positive provocation with LD should develop FBSS at lower rates. At the very least, a significant difference should not exist.
However, within the current study population of 1591 WC patients, the authors found that patients who underwent LD before diskogenic fusion were paradoxically associated with significantly higher rates of FBSS. The LD group developed FBSS at a 5.1% higher rate within 3 years AIF. In addition, patients in the LD group were prescribed a significantly higher daily opioid analgesic load for significantly longer periods of time after fusion. These results are in line with studies by Carragee et al9–15 that have questioned the current usefulness of LD.
The current study has some limitations. First, it is limited by its retrospective design. Although the authors controlled for many potentially confounding covariates, the 2 groups under study may have varied in unobserved ways. Second, the authors were unable to determine why the LD group underwent diskography and how many levels of degeneration they had. However, the numbers of levels fused, something the authors knew, should be roughly equivalent to this. Third, multiple studies report low false-positive results if particular attention is paid to the pressure of injection during LD.3,8 The authors were unable to control for injection pressures for each patient. Fourth, the authors were unable to determine the severity of some of the conditions under study, such as depression and DDD. However, this study included the entire population of Ohio WC patients who fit the authors' study design, which helped avoid bias and increased the chances of having a distribution of disease severity and mean diskography injection pressures that would be present in other similar populations. Fifth, the completeness of these data could have potentially been limited by the use of administrative information. However, a study of CPT and ICD-9 coding data from a large Veterans Affairs database provided accurate data with sensitivities and specificities of 95% or greater.31 Sixth, the authors were unable to assess magnetic resonance imaging or other imaging data for this study population. The authors also did not know how it was decided who would undergo LD. The lack of such data could introduce some variability to the groups under study that could weaken the conclusions. Finally, the generalizability of the authors' results to WC patients in states other than Ohio could be limited by varying legislative regulations, treatment guidelines, and plausible geographic variations in outcomes. For example, a 2014 study by Martin et al32 demonstrated significant outcome differences between California and Washington state WC patients due to policy and reimbursement differences.
It appears that, within the WC population, LD may not be as helpful in identifying ideal fusion candidates as one would hope. It is not possible to answer with this study alone why these patients developed FBSS at higher rates. This study identified an association, not causation. The 682 patients in the LD group could have been poor candidates for fusion surgery in the first place. Given such a large number of patients, this is unlikely. Undergoing LD before fusion alone could carry an increased risk of post-fusion FBSS. Complicating matters further is that this study is of WC patients, an already complicated patient population. Workers' compensation patients could exhibit amplified pain responses not directly related to disk pathology, leading to inappropriately high surgery rates. These speculations require further inquiry. Also, because WC patients tend to have worse clinical outcomes than the general population,18–21 care should be taken when applying the conclusions of the current study to non-WC patients.
A negative association exists between the early receipt of opioid pain medications and outcomes in patients with LBP,33 and patients in the LD group were prescribed narcotics for significantly longer times before fusion. The authors corrected for this possibility in their regression model. Also, because the LD group developed FBSS at significantly higher rates, higher opioid use is almost to be expected. However, this is still concerning because the safety and effectiveness of long-term opioid therapy for chronic LBP remains unproven.34
The authors showed an association between preoperative LD and higher rates of FBSS after diskogenic fusion and increased postoperative opioid use. Future studies identifying why these patients developed FBSS at such a high rate are necessary because the current study identified an association, not causation. In those who cope poorly with pain and have significant neurophysiologic changes in addition to their structural spine changes, LD has already been shown to be a poor tool.9,12,15 The authors corrected for a number of psychosocial covariates in their analysis. Yet, questions remain. Is the increased risk for FBSS inherent to LD itself? Is the issue inherent to the WC population? Or is it a combination of both? Regardless, the fact that significantly more patients in the LD group developed FBSS after fusion should raise significant concerns about the current role of LD among the WC population.
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Coding Used for Study Population
| Posterior lumbar fusion (PLF), CPT|
| Posterior lumbar interbody fusion (PLIF), CPT|
| Single-level||22630 or 22633|
| Multi-level||22630 or 22633-22632 or 22634|
| Anterior lumbar interbody fusion (ALIF), CPT|
| Lateral extracavitary lumbar fusion (LEC), CPT|
|Lumbar comorbidity, ICD-9|
| Degenerative disk disease||722.52|
| Disk herniation||722.10|
| Lumbar sprain||847.2|
| Radiculopathy||724.4 or 729.2|
| Spinal stenosis||724.02–724.03|
Preoperative Population Characteristics
|Characteristic||LD Group||Control Group||P|
|Age at index fusion, mean±SD, y||43.8±8.5||46.1±9.2||<.01|
|Sex, No. (%)||.45|
| Male||471 (69.1)||611 (67.2)|
| Female||211 (30.9)||297 (32.7)|
|Lumbar comorbidities BIF, No. (%)|
| Disk herniation||395 (57.9)||543 (59.7)||.47|
| Spinal stenosis||62 (9.1)||153 (16.8)||.70|
| Radiculopathy||114 (16.7)||155 (17.1)||.86|
|Psychological comorbidities BIF, No. (%)|
| Depression||60 (8.8)||70 (7.7)||.43|
| Anxiety||6 (0.9)||6 (0.7)||.62|
| Adjustment reaction||5 (0.7)||12 (1.3)||.26|
|Patients using psychotherapy BIF, No. (%)||88 (12.9)||92 (10.1)||.08|
|Permanently disableda BIF, No. (%)||293 (43.0)||405 (44.6)||.53|
|Legal representation BIF, No. (%)||566 (83.0)||714 (78.5)||.03|
|Working within 1 wk of index fusion, No. (%)||104 (15.2)||174 (19.1)||.04|
|Patients with prior lumbar surgery, No. (%)||142 (20.8)||250 (27.5)||<.01|
|Time from injury to index fusion, mean±SD, d||1511.0±1078.7||1440.9±1129.7||.21|
|Narcotic use BIF|
| Time supplied, mean±SD, d||615.8±841.4||430.0±710.4||<.01|
| Patients supplied for >1 year, No. (%)||342 (50.1)||312 (34.3)||<.01|
| Net MEQs prescribed, mean±SD||38,608.0±78,393.4||25,124.6±64,632.8||<.01|
| Average MEDs, mean±SD||57.4±45.0||53.5±54.3||.13|
Index Fusion Characterization
|Procedural Variable||No. (%)||P|
|LD Group (n=682)||Control Group (n=909)|
|Type of fusion||<.01|
| ALIF||58 (8.5)||50 (5.5)|
| PLF||122 (17.9)||245 (27.0)|
| PLIF||66 (9.7)||147 (16.2)|
| PLF+PLIF||347 (50.9)||381 (41.9)|
| 360° fusion||88 (12.9)||85 (9.4)|
| LEC||1 (0.2)||1 (0.1)|
| Single||381 (55.9)||528 (58.1)|
| Multi||301 (44.1)||381 (41.9)|
| Instrumented||173 (25.4)||284 (31.2)|
| Intervertebral biomechanical device (eg, cage)||94 (13.8)||139 (15.3)|
| Instrumented+intervertebral biomechanical device||385 (56.5)||433 (47.6)|
| Uninstrumented||30 (4.4)||53 (5.8)|
| Column total||682 (100)||909 (100)|
| Allograft||47 (6.9)||62 (6.8)|
| Allograft+autograft||62 (9.1)||69 (7.6)|
| Autograft||327 (48.0)||431 (47.4)|
| BMP or other graft material||246 (36.1)||347 (38.2)|
| Column total||682 (100)||909 (100)|
Predictors of Failed Back Surgery Syndrome
|Independent Variable||Odds Ratio||95% CI||P||FBSS Ratea|
|Ability to remain working within same wk as index fusion||0.54||0.31–0.94||.02||6.5%|
|Supplied with opioid analgesics for >1 y BIF||1.53||1.07–2.18||.02||14.8%|
|Type of fusion surgeryb||<.01|
| 360° fusion||0.77||0.32–1.83||6.4%|
Secondary Outcome Measures
|Secondary Outcome||LD Group (n=682)||Control Group (n=909)||P|
|Narcotic use AIFa|
| Time supplied, mean±SD, d||778.0±661.6||648.3±670.3||<.01|
| Patients supplied for >1 y, No. (%)||440 (64.5)||486 (53.5)||<.01|
| Net mg of MEQs prescribed, mean±SD||60,607.7±92,630.3||44,217.2±70,734.9||<.01|
| Average MEDs, mean±SD||67.1±51.6||61.4±55.5||.04|
|Mortality AIF, No. (%)|
| All-cause||12 (1.8)||13 (1.4)||.60|
| Perioperative||0 (0)||2 (0.2)||.22|
|Psychological developments AIF, No. (%)|
| Depression||120 (17.6)||142 (15.6)||.29|
| Anxiety||8 (1.2)||19 (2.1)||.16|
| Adjustment reaction||11 (1.6)||14 (1.5)||.91|
|Nonunion/pseudoarthrosis AIF, No. (%)||8 (1.2)||6 (0.7)||.28|
|Permanent disabilityb AIF, No. (%)||144 (37.0)||188 (37.3)||.94|
| Additional lumbar surgery AIF, No. (%)||144 (21.1)||169 (18.6)||.21|
| Additional fusion||107 (15.7)||136 (15.0)||.69|
| Additional decompression||90 (13.2)||99 (10.9)||.16|