How Does Accounting for Worker Productivity Affect the Measured Cost-Effectiveness of Lumbar Discectomy?
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- Koenig, L., Dall, T.M., Gu, Q. et al. Clin Orthop Relat Res (2014) 472: 1069. doi:10.1007/s11999-013-3440-6
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Back pain attributable to lumbar disc herniation is a substantial cause of reduced workplace productivity. Disc herniation surgery is effective in reducing pain and improving function. However, few studies have examined the effects of surgery on worker productivity.
We wished to determine the effect of disc herniation surgery on workers’ earnings and missed workdays and how accounting for this effect influences the cost-effectiveness of surgery?
Regression models were estimated using data from the National Health Interview Survey to assess the effects of lower back pain caused by disc herniation on earnings and missed workdays. The results were incorporated into Markov models to compare societal costs associated with surgical and nonsurgical treatments for privately insured, working patients. Clinical outcomes and utilities were based on results from the Spine Patient Outcomes Research Trial and additional clinical literature.
We estimate average annual earnings of $47,619 with surgery and $45,694 with nonsurgical treatment. The increased earnings for patients receiving surgery as compared with nonsurgical treatment is equal to $1925 (95% CI, $1121–$2728). After surgery, we also estimate that workers receiving surgery miss, on average, 3 fewer days per year than if workers had received nonsurgical treatment (95% CI, 2.4–3.7 days). However, these fewer missed work days only partially offset the assumed 20 workdays missed to recover from surgery. More fully accounting for the effects of disc herniation surgery on productivity reduced the cost of surgery per quality-adjusted life year (QALY) from $52,416 to $35,146 using a 4-year time horizon and from $27,359 to $4186 using an 8-year time horizon. According to a sensitivity analysis, the 4-year cost per QALY varies between $27,921 and $49,787 depending on model assumptions.
Increased worker earnings resulting from disc herniation surgery may offset the increased direct medical costs associated with surgery. After accounting for the effects on productivity, disc herniation surgery was found to be a highly cost-effective surgery and may yield net societal savings if the benefits of outpatient and inpatient surgery persist beyond 6 and 12 years, respectively.
Level of Evidence
Level II, economic and decision analysis. See the Instructions for Authors for a complete description of levels of evidence.
With the majority of inpatient disc herniation surgeries performed on working-aged individuals, successful treatment of lumbar disc herniation has the potential to yield substantial benefits in terms of employee productivity . In the 2008 National Health Interview Survey (NHIS) , an estimated 10.5 million people reported having back problems with radiating leg pain. On average, this population reported spending 34 days in bed, and missing 26 workdays in the prior 12 months (among respondents with a work history) . In another study, back pain was estimated to cause an average of 5.3 hours of lost productive time at work per week with most of that lost time the result of reduced performance .
Surgical treatment of lumbar disc herniation has been shown to be cost-effective. For example, the Spine Patient Outcomes Research Trial (SPORT), a prospective multicenter study, showed improved clinical outcomes (pain, physical function, and disability) for patients who had surgery for lumbar disc herniation relative to nonsurgical treatment [26, 27]. During a 4-year period, lumbar disc herniation was found to be cost-effective in the pooled randomized and observational cohorts at an incremental cost of $43,800 per additional quality-adjusted life-year (QALY) with surgical treatment priced at estimated private-payer levels . The SPORT findings are consistent with those of other studies that have shown that surgery for disc herniation produces better outcomes than nonsurgical treatment [3, 9]. However, prior studies have not taken into account improvements in worker productivity as a result of surgery for lumbar disc herniation, and as such, likely have underestimated the cost-effectiveness of this intervention.
The purpose of our study was to assess the cost-effectiveness of lumbar disc herniation surgery after accounting for its affect on worker productivity. The research addressed two questions. First, we examined the affect of surgery on workers’ productivity using data from SPORT  (based on the pooled randomized and observation cohorts analyzed on an as-treated basis) and the NHIS . Second, we assessed how the inclusion of these factors influenced the cost-effectiveness of surgical treatment for lumbar disc herniation.
Materials and Methods
We used regression modeling and decision analysis to estimate the incremental cost-effectiveness of disc herniation surgery on a working population. The overall approach followed those of Dall et al.  and Ruiz et al. . The model was estimated for different age-cohorts (younger than 40 years, 40–44, 45–49, 50–54, 55–59, and 60–64 years), and averages were estimated using a weighted mean based on the age distribution of patients receiving surgery. We included sensitivity analysis to test the model’s robustness to changes in our model assumptions. Further details on methods are provided in Appendix 1.
Estimating the Effects of Functional Limitations on Earnings and Missed Workdays for Individuals with Back Pain Radiating Down the Leg
To estimate the effects of lumbar disc herniation on worker productivity, we used information on functional limitations, missed workdays, and income from the NHIS . The relationships between functional status and earnings, and between functional status and missed workdays, respectively, were determined by least squares and negative binomial regressions. The analyses were run on a sample of NHIS respondents who reported limitations as a result of back pain that had spread down the leg and below the knee. We used the models to determine workers’ number of missed workdays and household earnings conditional on their level of functional ability in a given year.
The key explanatory variable in the regression models was a functional limitations index score. We obtained predicted values for earnings and missed workdays for the surgical and nonsurgical groups by using the average functional limitation index scores derived from the randomized and observational cohorts in SPORT (across 1, 2, and 4 years posttreatment) and the results of the regression models . We also assumed that workers lost a mean of 20 workdays to recover from lumbar disc herniation surgery based on the Official Disability Guidelines . Missed workdays were converted to a dollar value using average earnings-per-day estimates. These results were incorporated into a Markov model.
We used a Markov cohort analysis to estimate the differences in direct medical costs, indirect costs, and QALYs between surgical and nonsurgical treatments of lumbar disc herniation. We assumed that patients who are treated nonsurgically do not receive surgical treatment for disc herniation during the model time horizon. We did not distinguish between microdiscectomy and open discectomy in the model because outcomes, in terms of physical health and function, are comparable . Direct medical costs were determined from a private-payer perspective and included patient out-of-pocket expenses. To estimate direct medical costs for surgery, we analyzed Medicare claims data to determine average Medicare payments for disc herniation surgery. These payments then were adjusted to account for differences in private payer and Medicare payment levels. Indirect costs were based on worker productivity and were assessed by changes in earnings and number of missed work days.
In 2010, a systematic review of randomized clinical trials to assess the effectiveness of surgical treatment for disc herniation was published . This review identified two studies with a low risk of bias. Of these two studies, one included surgical patients in the comparison group, and thus was unsuitable for our purposes . In the other study from SPORT, significant crossover between the treatment and comparison groups compromised the study’s randomization and allowed for potential bias from carryover effects .
Parameters and utilities in base Markov model
Fraction of patients with satisfactory outcome after treatment
Fraction of patients with satisfactory outcome after revision
Annual revision rate after surgery–first year
Annual revision rate after surgery–subsequent years
Surgical mortality rate
Natural mortality rate
US life table
US life table
We defined success as patients’ satisfaction with the results after treatment. Weinstein et al.  reported that 75% of patients treated with surgery and 48.8% of patients treated nonsurgically indicated they were “very/somewhat satisfied with symptoms” at 2 years after treatment. The percentage did not change significantly at 3 and 4 years after treatment for both treatment groups (ie, 74.3% and 76.6% for the surgery group and 49.9% and 46.7% for the nonsurgery group) . Among participants of the Maine Lumbar Spine Study , 71% of patients treated with surgery and 56% of patients with nonsurgical treatment indicated they were satisfied with their current state at 10 years followup. In our model, we set the percentages of patients with satisfactory outcomes at 75.3% for the surgery group and 48.5% for the nonsurgery group, which are the averages across outcomes at 2, 3, and 4 years after treatment based on SPORT data. These success rates of surgical treatment of lumbar disc herniation are consistent with the ranges reported in the literature of 70% to greater than 90% . The percent of patients with satisfactory outcomes after the revision surgery was set to be the same as after the initial surgery (ie, 75.3%) because studies have reported comparable outcomes after revision surgery and initial surgery for lumbar disc herniation [17, 21].
SPORT data suggested that the revision rate in the first year after surgery is higher than in subsequent years . Specifically, the weighted cumulative revision rates combining the randomized and observational cohorts were 6% 1 year after surgery, 8% 2 years after surgery, 9% 3 years after surgery, and 10% 4 years after surgery. This translates to an approximate annual revision rate of 2.5%. Data from the Maine Lumbar Spine Study  revealed that 64 of the 217 patients (ie, 29%) treated with surgery and who were still alive by the 10-year followup had a reoperation. Osterman et al.  reported that, among patients with one revision after lumbar discectomy, 25.1% experienced additional spinal surgery before the 10-year followup, which indicated an approximate annual second reoperation rate of 2.5%. In our model, the first-year revision rate was set at 6% and the annual reoperation rate in the subsequent years was set at 3% per year. Patients were allowed to have up to two revisions in the Markov model. Surgical mortality was set to 0.14% . Natural mortality comes from the age- and sex-specific mortality in the US life tables .
Two cost-effectiveness studies using SPORT data reported the average EuroQol-5 dimensions (EQ-5D) posttreatment utility level of patients treated either surgically or nonsurgically [22, 23]. The average utility of patients after treatment was approximately 0.8 for surgical treatment and 0.7 for nonsurgical treatment (where utility of 1.0 indicates no decline in quality of life associated with health problems and utility of 0.8 indicates a 20% decline in quality of life). However, neither study reported the average utility separately for patients satisfied and dissatisfied with the symptoms. Based on data from the Beaver Dam Health Outcome Study, Malter et al.  reported a time tradeoff utility level of 0.89 for patients with satisfactory outcomes and 0.56 for patients with unsatisfactory outcomes after treatment of lumbar disc herniation. Using these utility values and success rates of 75.3% and 48.5% for surgical and nonsurgical treatments, respectively, the estimated utility is 0.81 for patients treated with surgery and 0.72 for patients treated nonsurgically, which is consistent with the EQ-5D utility from SPORT. We used the utility values reported by Malter et al.  in the Markov model (0.89 and 0.56 for satisfactory and unsatisfactory outcomes, respectively). The utility value of the temporary revision state was set at 0.69 (the midpoint between utility of surgery treatment and utility of unsatisfactory outcomes) until the revision reached full benefit in the next cycle. A utility of zero was assigned to the dead state.
We used the 2009 5% Medicare claims  to estimate the surgery payments and select 2009 State Ambulatory Surgery Databases  (Colorado, New Jersey, Florida, and Wisconsin) to estimate the percent of surgeries done in inpatient and outpatient surgical settings. For both settings, surgery cost includes payments to facilities and physicians.
Direct medical cost estimates of treatment of lumbar disc herniation
For the sensitivity analysis, we used at least a 10% range around the base estimates (ie, 10% below and above the base estimates) or ranges suggested in the literature for most parameters . For missed work and earnings, we used lower and upper bounds based on a 95% CI.
Estimates of functional limitations index scores, household earnings, and value of missed workdays by age group
Age group (years)
Percentage of patients
Average functional limitations score (SPORT)
Value of missed workdays (excludes surgery recovery)
Costs and additional QALYs from surgical treatment of lumbar disc herniation (4-year time horizon)
Total direct cost (A)
Earnings offsets (B)
Value of missed workday offsets (C)
Net costs (D = A–B–C)
Total direct cost (F)
Younger than 40 years
Sensitivity analysis of key parameter assumptions (based on 40-year-old patient)
Value in base model
Value range tested
Incremental cost effectiveness range
First year revision rate after surgical treatment
Revision rate in subsequent years after surgical treatment
Fraction of surgical patients with satisfactory outcome
Fraction of non-surgical patients with satisfactory outcome
Utility of satisfactory outcome
Utility of unsatisfactory outcome
Annual increase in earnings after surgical treatment, compared with nonsurgical treatment
Annual reduction in missed workdays after surgical treatment, compared with nonsurgical treatment
Annual medical spending after surgery for disc herniation
Annual medical spending after nonsurgical treatment for disc herniation
Missed workdays recovering from disc herniation surgery
Effective treatment for disc herniation could yield benefits to individuals and employers. Although research has shown benefits associated with surgery for disc herniation [3, 4, 27], neither the SPORT nor the Maine Lumbar Spine Study found statistically significant differences in the probability of being employed between patients receiving surgery and patients treated nonsurgically. We found that during a 4-year period, surgical treatment for disc herniation increased earnings by $7154 and increased missed work days by 8.9 days. In total, the net effect of changes in earnings and missed work days resulted in a surgical cost offset of $5603. The inclusion of indirect costs improved the incremental cost-effectiveness ratio of surgery for disc herniation by 34% from $52,416 to $35,146.
This study has several limitations. First, our estimates of indirect costs are based on a representative population of people with back pain that had spread to the leg and below the knee. We inferred the effects of discectomy on productivity by linking back pain, functional limitations, and earnings. Thus, the reliability of these findings is sensitive to the validity of the model’s assumptions. We included a sensitivity analysis to test the robustness of our findings, but it is impossible to eliminate all uncertainty in a study of this nature. Further research is needed to understand the extent to which our model accurately reflects the relationship between functional limitations and earnings for patients who undergo disc herniation surgery. Second, statistical error was present in the estimation of the relationship between treatment approach and functional status, and the estimation of the relationship between functional status and economic outcomes. Although we have provided confidence intervals for each estimation stage, owing to data limitations, we are unable to provide a measure for how this joint uncertainty affects our final results. Third, the NHIS patient sample was limited to patients with back pain with radiating leg pain. Although these are characteristic symptoms of disc herniation, our sample might have included some patients with other conditions. We assume that the relationship effect of back pain on economic outcomes is independent of the cause of the back pain. Fourth, our information for clinical outcomes differences came from an observational study. Although the results were risk-adjusted, there may be remaining differences in baseline health status that bias the findings. Finally, reimbursement for disc herniation surgery varies among payers and insurance markets. Our estimated costs of surgery to payers and patients are based on average payments, and thus might not always be reflective of costs in all circumstances.
Significant geographic variation exists in the rates of back surgery . This suggests that in some areas, discectomy may be either under- or overused. Our estimates of benefits from discectomy are based on average indirect cost reductions for patients who underwent the procedure, but not all patients are equally good candidates for surgery. Careful consideration of individual patient needs and alternatives to surgical treatment may further increase the societal value of lumbar disc herniation surgery.
Our study showed that functional limitations resulting from lumbar disc herniation were associated with lower earnings and an increased number of missed workdays. The level of improvement in functioning for patients undergoing disc herniation surgery suggests material offsets to the cost of surgery in terms of higher earnings and fewer missed workdays. After incorporating estimates of the effects of surgery on earnings and missed workdays, we found that patients treated surgically gain an additional 0.31 QALYs during a 4-year period at an additional cost of $10,895. We believe that this is the first study to estimate the effects of disc herniation surgery on productivity as captured in earnings. Earnings can be affected by disc herniation in numerous ways. First, employees may not be able to work as many hours (and may need to work part-time instead of full-time). Second, the human capital approach postulates that wages are set equal to a worker’s marginal product. Thus, less productive workers are paid less. Prior research has focused on the effects of disc herniation surgery on employment without consideration for the effect of hours worked or productivity changes on earnings.
These findings imply an incremental cost-effectiveness ratio of $35,146 per QALY gained. By comparison, Tosteson et al.  reported an incremental cost-effectiveness ratio of $43,800 per additional QALY for surgery priced at private-payer amounts. The difference between our incremental cost-effectiveness ratio and that reported by Tosteson et al. is explained primarily by our inclusion of offsets from increased earnings.
Disc herniation surgery is measured as more cost-effective when the benefits of surgery in terms of earnings and missed workdays are factored in and when the procedure is performed on an outpatient basis. The value of discectomy may be further enhanced by shifting more clinically appropriate patients to an outpatient setting.
We thank the AAOS Value Project Team for their valuable comments on earlier drafts of the article. We are grateful to the Spine Patient Outcomes Research Trial (SPORT) for providing patient outcome data. We also recognize the contributions of the Rothman Institute and Alexa Narzikul, in particular, for providing patient outcome data. Finally, we thank Andrea Cornejo, Paul Gallo, Jennifer Nguyen, and Sheila Sankaran for research assistance and for help in preparing the manuscript.
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