The Role of Minimally Invasive Techniques in Scoliosis Correction Surgery

Objective Recently, minimally invasive surgery (MIS) has been included among the treatment modalities for scoliosis. However, literature comparing MIS to open surgery for scoliosis correction is limited. The objective of this study was to compare outcomes for scoliosis correction patients undergoing MIS versus open approach. Methods We retrospectively collected data on demographics, procedure characteristics, and outcomes for 207 consecutive scoliosis correction surgeries at our institution between 2009 and 2015. Results MIS patients had lower number of levels fused (p < 0.0001), shorter surgeries (p = 0.0023), and shorter overall lengths of stay (p < 0.0001), were less likely to be admitted to the ICU (p < 0.0001), and had shorter ICU stays (p = 0.0015). On multivariable regression, number of levels fused predicted selection for MIS procedure (p = 0.004), and multiple other variables showed trends toward significance. Age predicted ICU admission and VTE. BMI predicted any VTE, and DVT specifically. Comorbid disease burden predicted readmission, need for transfusion, and ICU admission. Number of levels fused predicted prolonged surgery, need for transfusion, and ICU admission. Conclusions Patients undergoing MIS correction had shorter surgeries, shorter lengths of stay, and shorter and fewer ICU stays, but there was a significant selection effect. Accounting for other variables, MIS did not independently predict any of the outcomes.


Introduction
Adult scoliosis is a spinal deformity typically caused by asymmetrical disc degeneration, osteoporosis, and vertebral body compression fractures [1]. When nonsurgical treatment fails, there are multiple surgical techniques that can be used [2]. The goals of surgery are to improve functionality, relieve pain, improve cosmesis, and prevent curve progression [3]. Whether performed posteriorly or anteriorly, open techniques are associated with large blood loss, muscle injury and denervation, significant postoperative pain, and other complications [4,5].
Minimally invasive surgery (MIS) potentially avoids or lessens these complications due to its ability to reduce intraoperative blood loss, soft tissue damage, infection, postoperative pain, and recovery time [6]. The safety and feasibility of MIS for adult degenerative scoliosis have already been established [7]. Also, results have previously been reported that showed similar clinical improvement for patients who underwent open surgery versus MIS [8]. Furthermore, the patients who underwent MIS had lower morbidity and complication rates and significantly shorter hospital stays [8].
While these initial results are promising, these studies were on small subsets of patients with many confounding variables. The literature comparing open surgery versus MIS for scoliosis correction is limited; therefore, the need exists for further investigation to determine the efficacy of MIS. Here, we compared MIS and open scoliosis surgery with respect to selection for surgical technique and outcomes, including readmission rates, reoperation rates, bleeding, and clotting complications.   for candidate variables on single-variable logistic regression. A value of < 0.05 was considered statistically significant.  Table 2).  Table 3).

Discussion
Minimally invasive surgical techniques could potentially reduce the morbidity associated with traditional open surgical techniques in scoliosis correction [9][10][11]. Currently, the literature on MIS for scoliosis correction is limited. Many of the studies performed to date observed relatively few patients, and multiple systematic reviews have concluded that more research is needed [9,12]. Our study examined 207 scoliosis correction surgeries and identified selection factors for MIS, how MIS outcomes compare to open surgery outcomes, and predictors of outcomes for each technique.
Importantly, the median number of levels fused predicted selection for MIS technique with MIS patients having fewer levels fused (4 versus 9, < 0.0001). Many previous studies did not search for a selection effect for MIS versus open surgery selection. One meta-analysis on scoliosis correction, by Dangelmajer et al., did examine selection bias and found that both older patients and patients with less severe deformities were more likely to be selected for an MIS technique [13]. Our analysis agrees with this finding.
Furthermore, having private insurance ( = 0.068), undergoing a posterior approach ( = 0.087), and not requiring surgical decompression ( = 0.070) each showed a trend toward selection for MIS. Similarly to our finding that private insurance was an important determinant of surgery choice, a study by Park [14]. This trend of selection biases between the two groups was consistent across most of the studies that reported patient selection information, limiting the ability to conclude a true difference in outcomes between the MIS and open techniques.
In our study, there was significantly more variance in the number of levels fused among patients undergoing open surgery ( < 0.0001). Dangelmajer et al. came to the same conclusion in their systematic review and attributed it to the fact that patients undergoing open procedure had a larger preoperative scoliosis [13]. This result shows that an open technique can be used for a broader range of spinal levels than MIS.
MIS surgery was significantly shorter (287.0 minutes versus 433.0 minutes, = 0.0023, Figure 2) and patients undergoing MIS were less likely to have surgery last >6 hours ( = 0.0051) based on single-variable analysis. Anand et al. noted that their surgical outcomes data for MIS scoliosis correction was similar to open correction outcomes data when compared to the literature [8]. However, a metaanalysis on MIS versus open approach in degenerative lumbar disease revealed significant variability in operating times [15]. For example, one study found an average operating time of 161 minutes for the MIS approach compared to 375 minutes for the open approach [16]. In contrast, a study in the same meta-analysis found an average operating time of 159.2 minutes for the MIS approach versus 113.06 minutes for the open approach [17]. We suspect that confounding variables have an important impact on operating time, which would explain the significant variability between studies.
MIS patients also had shorter overall lengths of stay based on single-variable analysis (4.5 days versus 8.0 days, < 0.0001, Figure 3). While it seems promising that MIS patients typically had a shorter length of stay, the results from other studies are variable, potentially indicating a selection effect [6,14,18]. Our length of stay results were consistent with a meta-analysis by Phan [13,19]. However, similarly to our study, these groups also noted the presence of confounding variables such as age and preoperation severity of deformity that could have attributed to these results. In fact, in our analysis, open surgery did not predict ICU admission on multivariable regression.
While MIS or open correction was not independently associated with ICU admission, the number of levels fused did independently predict ICU admission ( < 0.001). As the number of levels fused was also independently associated with the selection of technique, it may be a confounding factor that accounts for the significant difference in likelihood of ICU admission on single-variable analysis. Similarly, age was likely a confounding factor, as age showed a trend toward significance in predicting technique, and was a significant predictor of ICU admission ( = 0.031).
Our finding that comorbid disease burden independently predicted ICU admission ( = 0.012) and readmission within 30 days ( = 0.003) is consistent with the existing spine surgery literature [20,21]. Cardiac, GI, and respiratory issues that were present before the operation are frequent causes of ICU admission and readmission and appear to be an important factor when comparing MIS versus open technique for scoliosis correction as well.
On multivariable regression analysis, age ( = 0.049) and BMI ( = 0.007) predicted VTE within 30 days postop. These variables are typically found to be strongly associated with such outcomes in spinal surgery, as evidenced in numerous previous studies [22][23][24]. Importantly, MIS surgery was not found to be an independent predictor of any outcome analyzed during multivariable regression. So, although we found that the typical outcome predictors (age, BMI, and comorbid disease burden) were significant in this study, we did not find any significant difference in patient outcome based on MIS versus open technique alone.
Our study has a number of important limitations. The study was conducted retrospectively and is subject to the biases inherent to this study design. A prospective study would enable us to further understand if the trends we discovered (private insurance, number of levels fused showing selection for MIS) were actively affecting the surgeon's decision whether to use an MIS or open approach. The operations we collected data on varied in the minimally invasive technique and approach used, which made it a less homogenous population to draw conclusions from. As a single-institution study, it only reflects the clinical decision-making of our spine surgeons with respect to patient selection and management. Our series is limited by its size, and a larger series would allow for a more thorough comparison between MIS and open surgery. Our study does not provide radiographic comparisons of corrections, as is common in the scoliosis literature. However, multiple prior studies have compared radiographic outcome for MIS and open scoliosis correction, the results of which have been meta-analyzed [2,5,7,13,[25][26][27]. Despite its limitations, our study contributes to the existing literature on scoliosis correction by examining selection factors for MIS versus open surgery, as well as a variety of perioperative outcomes.

Conclusion
Patients undergoing MIS scoliosis correction had shorter surgeries, shorter lengths of stay, and shorter and fewer ICU stays, but there was a significant selection effect. Accounting for other clinical variables, undergoing MIS surgery did not independently predict any of the outcomes analyzed.

Conflicts of Interest
Dr. Koski is a consultant at NuVasive.