Cost-utility analysis of posterior minimally invasive fusion compared with conventional open fusion for lumbar spondylolisthesis

Outcomes

In general terms, patient-reported clinical outcomes at 1 year are consistent with the respective literature for both MIS posterior interbody fusion and conventional open decompression and fusion for low-grade spondylolisthesis. ^{5–7, 11, 16–18} More specific to posterior MIS fusion, the results of our study regarding the impact of the MIS technique (transforaminal lumbar interbody fusion) on acute short-term outcomes, such as blood loss, transfusion rates, and shorter hospital stay, are consistent with the results of 8 recently published studies comparing MIS posterior fusion with open fusion.^5–12 Data from these studies (with total of 296 MIS patients vs 408 controls undergoing open technique) showed that MIS patients had fewer postoperative complications than open controls (7.6% vs 16.6%), less blood loss (192 mL vs 455 mL), shorter operating room time (167 minutes vs 184 minutes), and shorter LOS (3.4 days vs 5.2 days). Several articles reported no significant between-group differences in clinical outcomes at 6, 12, and 24 months’ follow-up.^{5, 6, 11} With regard to patient-reported clinical outcome, our study is also consistent with those comparative studies reporting clinical outcomes at or beyond 1 year.^{5, 6, 11}

Cost factors

The mean and median difference in direct cost (public health care system perspective) between the MIS and open groups were Can $4,461 and Can $3,333, respectively. Given the heterogeneity (Table 1) within these 2 small cohorts, the relatively small (statistically significant) difference in cost could be easily affected by several factors. Multivariate regression analysis showed that the majority of cost savings was achieved through reduction in LOS, with the only independent factors affecting LOS being MIS technique and age. As expected, the number of levels also affected cost, with a 2-level fusion patient's cost averaging Can $2,777 greater than a single-level patient's cost. This was independent of the MIS or open technique (ie, the cost is higher for a 2-level fusion regardless of technique). In addition, the number of levels fused did not affect LOS. In our health care system, it would appear that LOS is a modifiable but dominant factor that influences direct cost for lumbar fusion in this population. The 2-day reduction in LOS in this study is consistent with the available comparative literature.^5–12 It should be noted that reported LOS for the same diagnoses is variable from one health care system/country/region to another and typically depends on whether there is a direct cost disadvantage to the patient. Thus comparisons must always be made using the relative change in LOS within the same health care system and not the absolute mean LOS for a given study. Although age was also shown to be an independent predictor of LOS, the mean age was essentially equal between groups. Given the small sample size, our multivariate models were limited to 4 predictor variables and therefore confounding effects cannot be ruled out. Several areas of heterogeneity in our cohorts could have affected cost and are worthy of discussion. First, there were more 2-level fusions performed in the open group (P = .01); although this was not a predictor of LOS, it is likely associated with a greater LOS because of greater morbidity. The mean LOS was 8.0 days for 1-level open fusions and 8.9 days for 2-level open fusions (P = .584). Comparison of the mean cost for the 2-level MIS technique (Can $15,410; SD, 2,279) and 2-level open technique (Can $20,370; SD, 6,897) showed a statistically significant difference (P = .045). Second, the MIS cohort used 45 interbody cages, whereas the open cohort used only 14 (P < .01). At our institution, each cage is approximately Can $1,000 to Can $1,250, and hence this would add greater cost to the MIS group. Although this favors the cost utility of the open group, we believed that the variations in surgical technique of the 3 open surgeons were reflective of current clinical practice and hence the cost was more generalizable. A third issue is the use of recombinant human bone morphogenetic protein 2. Given the lack of evidence of superior clinical outcomes in non–high-risk patients, the cost of bone morphogenetic protein (BMP) is not paid for at our institution and hence represents an out-of-pocket expense to the patient.¹⁹ Patients may purchase BMP from the manufacturer at a significantly discounted price if they choose. In the MIS group, 65% of patients chose to use BMP for their fusion. Because this is an out-of-pocket expense, it is not counted in the direct institution cost or the cost-utility analysis (see below). Obviously, if the institution (public health care system) was to cover the routine use of BMP, then the total cost for the MIS group would increase. For this cohort, if every patient in the MIS cohort used BMP and none in the open group did so, the mean cost difference would essentially become neutral. However, as evident in the United States, if BMP was readily available and paid for, the use of BMP would be increased in both groups.²⁰ One final issue that may have increased cost for the open group is the fact these patients were generally more disabled than those in the MIS group, a fact that may have increased the mean LOS. However, adjusted analysis did show that in addition to baseline ODI, MIS technique also significantly impacted the final ODI scores. Recently, Carreon et al.,²¹ confirming the findings of other authors, showed that a worse preoperative ODI (as in the open cohort) predicts a greater improvement in ODI after lumbar fusion. This was also shown in our study; however, the MIS group had a comparable improvement, which is consistent with the results of the regression model showing a positive effect on final outcome by the baseline score and the MIS technique. Clearly, any or all of these factors could influence the LOS and cost in favor of either technique. Given the standard deviations, a much larger sample size would have been necessary to truly determine a cost advantage of either technique. Though not studied, a 2-day difference in LOS could also be impacted in either group by institution of a more aggressive postoperative care pathway. Although all patients were cared for on the same spinal ward, caregiver biases and/or patient placebo effect of the MIS technique may have also influenced LOS.

Cost-utility factors

From a cost-effectiveness perspective, the cost utility (cost per QALY gained) of both procedures at the 1-year mark is over the $100,000 per QALY, and hence these procedures would be considered costly (Table 4). For the reasons stated previously, the cost utilities in this study should not be considered significantly different in that the variability of the numerator (cost) is likely not reflected in this limited cohort. Similar utility outcomes (denominator) and cost utility at 2 years were noted from the degenerative spondylolisthesis SPORT (Spine Patient Outcomes Research Trial) study.²² As noted by Tosteson et al.,²² the cost utility of a procedure will improve assuming that the clinical impact is maintained over time. Furthermore, the longer the positive impact on health utility, the better the cost utility. Recently, the authors of the degenerative spondylolisthesis SPORT trial have demonstrated sustained outcomes of surgery compared with nonsurgical treatment at the 4-year mark.¹⁶ Sustained outcome is also expected for the isthmic spondylolisthesis group.¹⁷ On the basis of current evidence, it is reasonable to assume that the outcomes in our cohorts will remain relatively stable for 4 years (by use of a 5% per annum discount on the health utility score), and thus the cost utility for these 2 cohorts should significantly improve at 2 and 4 years (Table 4). This, however, does not account for any revision cost that may occur in that time period. The projected 2-year cost utility for both MIS and open fusion is favorable compared with that reported by Tosteson et al. at 2 years. This is because of the greater estimated direct cost (numerator) in the US health care system (US $31,938 per patient). This cost is essentially double that of our study and highlights the challenges of interpreting and comparing cost-utility analyses.^23–31 In addition, when interpreting cost-utility analyses, the reader must also consider the utility measure that was used. For example, the interval change of the EuroQol (ED-5Q) and SF-6D may differ within the same population, and hence these measures are not interchangeable. ^32–39 The SF-6D tends to be associated with a smaller effect and hence would result in an increased cost utility than if the ED-5Q was used instead. This effect was shown in the study by Tosteson et al., where the increase in the cost utility for surgical intervention was significantly greater (approximately US $30,000) with the SF-6D than with the ED-5Q. For our study, another specific issue to consider in interpretation of the cost-utility value is the denominator. As noted previously for ODI, the utility score was also independently affected by the MIS technique and the baseline outcome score; thus the true effect of MIS on the change in utility score versus other confounders is unknown.

Despite the limited 1-year time horizon, this study does allow a relative comparison of the cost-effectiveness of the MIS technique compared with the conventional open technique. A cost-effectiveness analysis would typically be calculated by use of an incremental cost-effectiveness ratio (ICER); in brief, ICER equals the cost of a new strategy less the cost of current practice, divided by the change in outcome of the new strategy, minus that of the current practice. ²⁶ The ICER analysis typically makes the assumption that the new strategy is likely to cost more but have a clinically greater effect and hence typically is used to determine the cost per the differences in outcome. In this case because the new strategy cost less or is at least equivalent and has a greater yet statistically insignificant difference in effect on the outcome, the MIS technique would be at least cost neutral. Consequently, an ICER analysis was not performed. ²⁶

Limitations

The major limitation of this study is the baseline differences in outcome measures, which reflects a more disabled open fusion cohort. Because this is a retrospective analysis of a prospectively collected database documenting outcomes in different practices, these differences in baseline factors are not surprising. Although selection bias may account for the more disabled open cohort, the MIS cases are consecutive, and all 1- and 2-level fusions performed by the MIS surgeon are performed using the aforementioned technique. Despite efforts to control for various factors with regression modeling, given the limited models and small sample size, there are differences in the patient populations that may have impacted key outcomes. The inherent biases of retrospective study are also shown by the fact that patientreported outcomes data (pain, ODI, SF-6D) were only available for 75.7% of the MIS group and 70.7% of the open group.

As with many health economic studies, other assumptions and limitations must be considered when one is interpreting the results. The costs in this study represent only those from an institutional perspective and hence do not reflect indirect societal or direct out-of-pocket patient costs. The micro– case costing data presented in this study are comprehensive and from a single payer system. Given the congruency of diagnosis and overall treatment, as well as similar preoperative and postoperative protocols (physician visits, imaging), no patient in either cohort underwent inpatient rehabilitation, and there were no revisions in either group at the 1-year mark. We assumed that all other direct costs would be equal between groups. Several studies have noted that the majority of cost associated with a surgical procedure typically reflects the majority of the direct cost.^{22, 29} Thus we believe that we have adequately represented the direct cost from a Canadian hospital perspective associated with fusion for low-grade spondylolisthesis. However, the relative rate of revision surgery or sustained change in utility scores (ie, QALYs gained) over time between groups will significantly impact the cost-utility analysis, and thus longer-term follow-up is paramount.

In conclusion, this observational cohort study confirms that MIS fusion for the treatment of spondylolisthesis reduces blood loss, transfusion requirements, and hospital LOS by 2 days on average. Both groups were associated with substantial clinical improvements at 1 year. Furthermore, contrary to popular belief, MIS fusion did not increase operative time or direct cost. Because of group heterogeneity and small sample size, the cost utility of the MIS technique was considered comparable to the open technique. Longer-term follow-up is required to determine the impact of revision rates and utility scores on the relative cost utility of both techniques.