1 Introduction

Metabolic syndrome (MetS) is a diagnostic criteria requiring a body mass index (BMI) ≥ 30 kg/m²s, medication, and diabetes mellitus (DM) that poses a significant public health problem.^1–3 Its prevalence is estimated at 22 %–35 % in the United States.^1,4 Likewise, the prevalence of obesity, type 2 diabetes, and hypertension has rapidly increased over the past two decades from 30.5 % to 42.4 %,⁵ 4.4 % to 11.2 %,⁶ and 24 %–45.5 %,⁷ respectively. These upward trends have substantially increased the proportion of adults at risk for MetS.

Previous research has shown that patients with MetS are more likely to have co-morbidities such as lung disease, shortness of breath, and bleeding disorders.^2,8 Furthermore, various components of MetS, such as DM and obesity, are associated with higher complication rates after spinal surgery.^1,8,9 Also, obesity and other features of MetS have predisposed patients to degenerative disc diseases such as spinal canal stenosis, cervical spondylosis, and lumbar disc herniation.^8,10–12 MetS has not only been associated with a higher rate of postoperative mortality and morbidity but is also an independent predictor for adverse outcomes.^4,13

The primary objective of Transforaminal Lumbar Interbody Fusion (TLIF) is to achieve a circumferential disc fusion from a single posterolateral approach. This minimizes any risk of collateral damage to central neural structures, such as the dura mater.^14,15 However, it may also Increase the risk of damage to the paraspinal musculoskeletal injury due to significant muscle retraction and dissection, resulting in an extended postoperative recovery period. The risk of this injury is significantly higher in obese patients, where previous research has shown that large skin incisions are necessary, along with further retraction of subcutaneous adipose tissue and more prolonged radiation exposure than non-obese patients.¹¹ Analysis of the effects of metabolic syndrome on morbidity and mortality following TLIF has received very little attention. This study is the first to use the American College of Surgeons National Surgical Quality Improvement (ACS-NSQIP) database to investigate the implications of metabolic syndrome on mortality and morbidity in patients following TLIF.

2 Methods

3 Results

54,980 patients who underwent TLIF from 2006 to 2019 were identified, 10.7 % (n-5893) of which had preoperative MetS. Conducting univariate analysis of preoperative variables, patients with MetS were significantly more likely to have a history of dyspnea (p < 2.2e-16), chronic obstructive pulmonary disease (COPD) (p = 1.17E-12), congestive heart failure (CHF) (p = 2.99E-11), and bleeding disorders (p = 1.64E-14). Patients with MetS were significantly more likely to be of older age (p = 2.20E-16). On preoperative labs, patients with MetS were substantially more likely to have hypoalbuminemia (p = 3.46E-05), leukocytosis (p = 3.40E-16), thrombocytopenia (p = 2.75E-07), uremia (p < 2.2e-16), and lower eGFR (p < 2.2e-16). Significant univariate differences were not found in gender, dialysis need, sepsis, disseminated cancer, superficial wound infection, deep wound infection, corticosteroid use, or weight loss (Table 1).

On univariate analysis of postoperative variables, patients with MetS had significantly greater wound infections (p = 8.94E-09), pulmonary events (p = 2.29E-08), MACE (p = 1.00E-06), renal events (p = 1.59E-09), 30-day mortality (p = 0.01045), sepsis (p = 128E-02), urinary tract infection (UTI) (p = 1.83E-05), need for transfusion within 72 h (p = 3.68E-03), return to operating room (OR) (p = 1.45E-07), and extended length of stay (LOS), defined as greater than ten days (p = 1.38E-05). Significant univariate differences were not found in VTE events (Table 2).

On multivariate analysis of postoperative variables, patients with MetS had increased odds of the following events when compared to patients without MetS: wound infections (aOR = 1.5889, CI 1.1952–2.112, p = 0.00144), pulmonary events (aOR = 1.5517, CI 1.1207–2.1485, p = 0.00813), and renal events (aOR = 2.8685, CI 1.5511–5.3045, p = 0.00078). Additionally, patients with MetS had increased odds of sepsis (aOR = 1.6773, CI 1.1647–2.4155, p = 5.44E-03) and return to OR (aOR = 1.4764, CI 1.2201–1.7866, p = 6.19E-05). Significant multivariate differences were not found in VTE, MACE, 30-day mortality, UTI, transfusion requirements, and extended LOS (Table 3).

4 Discussion

Previous studies have shown that independent comorbidities such as diabetes and obesity are risk factors for increased complications and hospital length of stay following spine surgery.^4,8,20 However, the effect of MetS on mortality and morbidity following TLIF has not yet been extensively studied. This study found patients with MetS had a significantly higher co-morbid rate, as outlined in Table 1. This is consistent with other studies showing MetS patients to have a higher rate of lung diseases,^2,8 shortness of breath, and bleeding disorders.^2,4,21 Memtsoudis et al. (2012) (p < 0.0001) and Chung et al. (2018) (p = 0.02) reported a higher incidence of CHF in patients with MetS undergoing elective or posterior lumber spinal surgery, while Lovecchio et al. (2018) found no significant difference (p = 0.118).^2,4,8 To control for the potential comorbid effects on MetS outcomes, we used a multivariate logistic regression model to control for the independent risk factors associated with MetS.

We find an elevated risk of renal events (aOR = 2.8685, p = 0.00078), defined as the need for dialysis or AKI and reoperation (aOR = 1.4764, p = 6.19E-05). Chung et al. also found an increased risk of AKI in patients with MetS following posterior lumbar fusion surgery. In contrast, Ye et al. found a higher risk of repeat operation in patients with MetS following adult spinal deformity fusion surgery.²¹ For this reason, we strongly caution surgeons performing TLIFs on patients with MetS to be aware and understand that MetS patients are more likely to have renal complications and an increased risk of returning to the operating room within 30 days.

Patients with MetS are also at an increased risk of pulmonary complications following TLIF (aOR = 1.5517, p = 0.00813), defined as pneumonia, prolonged intubation, and reintubation. Our results are consistent with previous studies that found patients with MetS have an increased risk of pulmonary complications following lumbar surgery.^3,4,8,21 However, these studies attribute some of this increased risk directly to the co-morbidities associated with MetS since they did not control for preoperative co-morbidities such as history of COPD, dyspnea, smoking status,^3,21 or procedural complexity.⁴ The increased risk for a pulmonary complication following TLIF was statistically significant even after adjusted co-morbidities, as presented in Table 3. Therefore, MetS is an independent risk for pulmonary complications following TLIF.

In our study, patients with MetS following TLIF had a higher risk of wound complications (aOR = 1.5889, p = 0.00144), defined as deep or superficial SSI, organ space infection, and dehiscence. Likewise, the risk of sepsis was also increased (aOR = 1.6773, p = 5.44E-03) in patients with MetS after TLIF. Lovecchio et al., consistent with our results, used multivariable analysis to conclude that patients with MetS are 1.5-fold more likely to develop wound complications after posterior spinal fusion surgeries (p = 0.04).² Memtsoudis et al. and Ye et al. also found an elevated risk of sepsis in patients with MetS following posterior lumbar spine fusion surgery or adult spinal deformity fusion surgery, respectively.^8,21 Wound complications are likely associated with sepsis, although the NSQIP datasets do not provide information regarding the etiology of reported cases of sepsis. In contrast to previous studies, our study is uniquely specific to TLIF, and we find that MetS is a self-limiting risk factor for developing surgical wound complications or cases of sepsis.

Our study was based on retrospective data sets; therefore, limitations in controlling the reporting of variables and collection of data exist. Other confounding variables include uncontrollable pre-operative variables, such as insurance status, clinical presentation, number of operative levels, intraoperative blood loss, and duration of operative time. However, these limitations are largely accepted for studies utilizing the NSQIP dataset in favor of a large sample size that is powered to answer the initial research question. We believe that our conclusions remain valid regardless of surgical invasiveness given that we do control for a wide array of preoperative variables. Another limitation is that although we used all available datasets from 2006 to 2019, the incidence and understanding of metabolic syndrome have drastically evolved over the past decade. Treatment and management guidelines constantly change and may impact postoperative morbidity and mortality. We found that including a large sample size to adequately answer our research question and power our study to >0.9 for most postoperative outcomes was of upmost importance. We hope that this becomes the standard for studies that utilize the NSQIP datasets. While implants and surgical techniques have evolved over the 13-year period, we were unable to identify any specific treatment guidelines for MetS patients undergoing TLIF that were nationally recommended during the study period. Another limitation of our study is that we do not control for varying postoperative recovery pathways at different institutions. This is however not unique to our study, and we believe the impact of this confounding variable is limited as most institutions continue a patient's home medications on postoperative day zero following a TLIF. We hope however that future studies will study the impact of evolving surgical techniques, implants, and enhanced recovery pathways on TLIF postoperative morbidity and mortality due to metabolic syndrome.

Patients with MetS have a higher risk of complications following TLIF. However, we report for the first time in the literature that MetS is an independent determinant risk factor for renal events, return to OR, pulmonary events, wound infections, and sepsis after receiving a TLIF. We urge further studies to investigate the pathophysiology and genetic predispositions of patients with MetS that increase their postoperative morbidity and mortality. Spine surgeons should be aware of and consider these results in selecting patients for TLIFs. We also urge future studies to prospectively enroll metabolic syndrome TLIF patients in enhanced preoperative medical management clinics to optimize their status and postoperative enhanced recovery pathways to avoid adverse outcomes.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Guardian/patient's consent

No guardian/patient consent was necessary for our research in “Impact of metabolic syndrome on morbidity and mortality following Transforaminal Interbody Fusion (TLIF)”. All data was received as de-identified from national databases.

Ethical statement

The data was de-identified by the database used in the study (ACS-NSQIP); An IRB number is not provided for exempt studies such as ACS-NSQIP studies which is standard for the USA IRB protocols.

Declaration of interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

CRediT authorship contribution statement

George Thomas: Conceptualization, Visualization, Methodology, Investigation, Writing – original draft, preparation, Writing – review & editing. John Thomas: Writing – original draft, preparation, Writing – review & editing. Sandra Tambi: Investigation, Writing – original draft, preparation, Writing – review & editing. Taimur Chaudhry: Software, Data curation, Formal analysis. Neil D. Almeida: Supervision, Writing – review & editing. Jonathan H. Sherman: Supervision, Writing – review & editing.