Cervical Disc Disease Cost-Effectiveness Analysis
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Appendix:
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Clinical trials reviewed:
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Table S1 lists the articles with data used in our analysis, arranged in alphabetical order. It
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also lists for each study the number of operated cases, the study type and the surgical
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procedure(s) performed. It should be noted that various aspects of a number of multicenter
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RCTs, most done for FDA exemption of implant devices, were variously reported by different
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trial participants. To avoid data duplication, we reported composite data under the device name.
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Trial participants, though listed in the table, did not have entries for number of cases unless there
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was a separate analysis of some aspect of their own patient population not studied by other trial
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participants.
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Late reoperations:
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An inverse variance-weighted, random effects model was used to calculate temporal
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trends in reoperations for each of the surgical strategies. Separate meta-analyses were done for
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reoperations at the index and adjacent levels. The results are illustrated in Figures S1a and S1b.
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The reoperation rates at 60 months are reported in the Results section, Table 2. Because
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of the large 95% confidence intervals, the increase in reoperation rates over time was significant
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for the index level only for ACDF with autograft (p = 0.005) and approached significance only
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for CDR (p = 0.055). For the adjacent level, there was a significant correlation between length of
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follow-up and reoperation rate only for CDR (p = 0.027).
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Five-year effectiveness calculations:
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Comparative effectiveness research uses utility and other measures of quality of life to
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assess global effectiveness of treatment. To assess the time dimension of effectiveness, we
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multiplied the utility of the outcome by the duration (years) of response. The product in QALYs
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Cervical Disc Disease Cost-Effectiveness Analysis
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is the standard indicator of effectiveness used in most cost-effectiveness studies. For example, a
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patient whose fusion failure requires reoperation after three years of follow-up accrues fewer
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QALYs than one who goes 10 years before reoperation21. We used the relative number of
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QALYs as our measure of effectiveness. Standard scales of disability or pain such as JOA, NDI,
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ODI and VAS or time to reoperation are valuable indicators of success but are not useful in cost-
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effectiveness studies because they only measure one aspect of surgical outcome.
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Ideally, we would calculate QALYs by comparing changes in utility from preoperatively
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to various times following surgery. Unfortunately, none of the publications on the subject
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addressed serial utilities changes. Instead of looking for changes in overall utility, our model
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assumed that successful surgery results in a utility score of 1. Complications and surgical failures
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at the index and adjacent levels were used to calculate the effects of adverse outcomes on utility.
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The resulting QALY scores represent a valid comparison of operative approaches.
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Since the utility of an operation without any adverse events (perioperative complications
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or reoperations) equals 1, these cases would be assigned a maximum value of five QALYs over
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the five years of follow-up. Cases who suffered perioperative complications but underwent no
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reoperation would be assigned five QALYs multiplied by the utility associated with the
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complications of their particular procedures (see Table 2). We assumed that reoperations were
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performed midway during follow-up. Such cases would have their initial postoperative utilities
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for 2.5 years and their new utilities following reoperation for the remaining 2.5 years. For
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example, a patient undergoing CDR and experiencing perioperative complications would have a
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utility of 0.842 (± 0.043) and thus 2.5 * 0.842 or 2.105 QALYs after 2.5 years. If this patient
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required reoperation and there were no perioperative complications with the second procedure,
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the new utility would be 0.915 (see Table 2, reoperation) * 0.842 (original utility), or 0.770. For
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Cervical Disc Disease Cost-Effectiveness Analysis
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the remaining 2.5 years in follow-up, the utility would be 1.926 QALYs. Thus, the five-year
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utility for this case would be 2.105 + 1.926 or 4.031 QALYs.
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Cost calculations:
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Base costs are shown in Table 3 of the main manuscript. The following formulas were
used to calculate costs:
1. Surgeon-related: Medicare reimbursement for primary procedure + 0.5 * sum of
component procedures
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2. Hospital-related: DRG reimbursement for facility, adjusted for complications.
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3. Implant-related: hospital costs for devices, constructs and allograft bone where
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appropriate.
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4. Outpatient-related: cost of one year of follow-up, adjusted for complications.
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Hence, costs for the first procedure equaled the sums of each cost component above,
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adjusted for the individual procedure and whether or not complications occurred. Costs for same-
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level reoperations were calculated with procedure-specific surgeon-related values, whereas
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adjacent-level surgeries were calculated the same, as were initial surgeries. Table S2 reports
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costs for each outcome category.
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We assumed that 90% of ACDF procedures employed internal fixation, follow-up costs
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were limited to one year, hospital-related costs for reoperation were the same as for the initial
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procedure and reoperations for failed CDR were evenly divided between replacement and
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revision. We further assumed that no perioperative complications were considered severe for
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DRG purposes and that all adjacent-level surgeries employed ACDF with allograft. Lastly, we
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assumed all delayed surgeries involved one year of outpatient follow-up.
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