Orthopedics

Feature Article 

Fat Thickness as a Risk Factor for Infection in Lumbar Spine Surgery

John J. Lee, MD, MS; Khalid I. Odeh, MD; Sven A. Holcombe, MS; Rakesh D. Patel, MD; Stewart C. Wang, MD, PhD; James A. Goulet, MD; Gregory P. Graziano, MD

Abstract

Body mass index does not account for body mass distribution. This study tested the hypothesis that subcutaneous fat thickness is a better indicator than body mass index of the risk of surgical site infection in lumbar spine procedures performed through a midline posterior approach. Charts were reviewed for previously identified risk factors for surgical site infection (age, diabetes, smoking, obesity, albumin level, multilevel procedures, previous surgery, and operative time) in 149 adult patients who underwent lumbar spine procedures through a midline posterior approach. Subcutaneous fat thickness was measured with a novel automated technique. Regression analysis was used to determine associations between risk factors and fat thickness with surgical site infection. In the study group, 15 surgical site infections occurred (10.1%). Bivariate analysis showed a significant association between surgical site infection and body mass index (P=.01), obesity (P=.02), and fat thickness (P=.002). With multivariate analysis, body mass index and obesity did not show significance, but fat thickness remained significant (P=.026). For every 1-mm thickness of subcutaneous fat there was a 6% (odds ratio, 1.06; 95% confidence interval, 1.02–1.10) increase in the odds of surgical site infection, and patients with fat thickness of greater than 50 mm had a 4-fold increase in the odds of surgical site infection compared with those with fat thickness of less than 50 mm. Body mass index and fat thickness were moderately correlated (r2=0.44). These results confirm the hypothesis that local subcutaneous fat thickness is a better indicator than body mass index of the risk of surgical site infection in lumbar spine procedures. [Orthopedics. 2016; 39(6):e1124–e1128.]

Abstract

Body mass index does not account for body mass distribution. This study tested the hypothesis that subcutaneous fat thickness is a better indicator than body mass index of the risk of surgical site infection in lumbar spine procedures performed through a midline posterior approach. Charts were reviewed for previously identified risk factors for surgical site infection (age, diabetes, smoking, obesity, albumin level, multilevel procedures, previous surgery, and operative time) in 149 adult patients who underwent lumbar spine procedures through a midline posterior approach. Subcutaneous fat thickness was measured with a novel automated technique. Regression analysis was used to determine associations between risk factors and fat thickness with surgical site infection. In the study group, 15 surgical site infections occurred (10.1%). Bivariate analysis showed a significant association between surgical site infection and body mass index (P=.01), obesity (P=.02), and fat thickness (P=.002). With multivariate analysis, body mass index and obesity did not show significance, but fat thickness remained significant (P=.026). For every 1-mm thickness of subcutaneous fat there was a 6% (odds ratio, 1.06; 95% confidence interval, 1.02–1.10) increase in the odds of surgical site infection, and patients with fat thickness of greater than 50 mm had a 4-fold increase in the odds of surgical site infection compared with those with fat thickness of less than 50 mm. Body mass index and fat thickness were moderately correlated (r2=0.44). These results confirm the hypothesis that local subcutaneous fat thickness is a better indicator than body mass index of the risk of surgical site infection in lumbar spine procedures. [Orthopedics. 2016; 39(6):e1124–e1128.]

The relative weights of preoperative risk assessment and the benefits of surgery influence surgical decision making. Surgical site infections, which occur in as many as 8.73% of revision spine procedures, can quadruple the cost of treatment, increase hospital stay, and increase the need for rehospitalization.1,2 Previously identified risk factors for surgical site infection in lumbar spine procedures include increasing age, diabetes, smoking, low albumin levels, obesity, multilevel procedures, previous spine surgery, and increased operative time.3–8 Preoperative counseling and intervention, along with targeted treatment strategies for a specific set of risk factors, may decrease the rate of surgical site infections.6,7,9–11 For some patients, advising against surgery when multiple risk factors may be present should be considered.

Obesity, defined by the World Health Organization as body mass index of greater than 30, is a well-described risk factor for surgical site infection in posterior lumbar spine procedures.6,7,12 However, because it is a function of height and weight, body mass index does not take into account body mass distribution, and it is not specific for surgical procedures. Its significance as a risk factor for surgical site infection may be the result of its correlation with subcutaneous fat thickness at the surgical site. Local subcutaneous fat thickness is a risk factor for surgical site infection in abdominal surgery,13–16 hysterectomy,17 stoma reversal,18 and cervical and lumbar spine surgery.19,20

This study investigated the hypothesis that subcutaneous fat thickness is a better indicator than body mass index of the risk of surgical site infection in lumbar spine procedures performed through a posterior midline approach and that the significance of body mass index as a risk factor for surgical site infection is the result of its correlation with subcutaneous fat thickness.

Materials and Methods

This study was approved by the institutional review board at the University of Michigan. A retrospective review was performed of 149 adult (≥18 years) patients who underwent lumbar spine surgery with a midline posterior approach between 2003 and 2012 at the University of Michigan and had a computed tomography (CT) scan of the abdomen or pelvis within 6 months of surgery. Patients were identified with a concurrent search of billing codes for posterior lumbar spine procedures and abdominal or pelvic CT scan. The search years were limited to 2003 to 2012 because CT scans before 2003 were not available for review in the institution's picture archiving and communication system. The search initially identified 226 patients, and of these, 77 were excluded because they had primarily thoracic spine procedures.

Electronic medical charts were reviewed for surgical site infection as well as demographic and previously identified risk factors for surgical site infection (age, diabetes, documented smoking within 1 year of the procedure, preoperative albumin level, body mass index, number of levels involved in the procedure, previous lumbar spine procedure, and operative time). Superficial, deep, and organ space surgical site infection, as defined by the Centers for Disease Control and Prevention, within 30 days of the procedure were included. The World Health Organization obesity classification system was used in the analysis. Abdominal or pelvic CT scans obtained within 6 months of the procedure were imported from the institution's picture archiving and communication system to an internal de-identified research image and archive system. Novel semi-automated analytic morphomic techniques developed at the study institution with MATLAB software (MathWorks, Natick, Massachusetts) were used to determine the distance from the spinous processes of T12-L5 to the back skin in a standardized fashion with axial and sagittal reformats. Each measurement was manually verified for accuracy by 2 orthopedic researchers (J.J.L., K.I.O.) (Figure 1). Maximum fat thickness at T12-L5, maximum fat thickness at the operated levels, and fat thickness at L4 were used in the analysis.


Posterior midline subcutaneous fat measurements at T12-L5 in an obese woman (body mass index, 53.2 kg/m2) (A) and a nonobese woman (body mass index, 21.3 kg/m2) (B).

Figure 1:

Posterior midline subcutaneous fat measurements at T12-L5 in an obese woman (body mass index, 53.2 kg/m2) (A) and a nonobese woman (body mass index, 21.3 kg/m2) (B).

Bivariate and multivariate logistic regression analyses were used to determine the significance of the correlations between age, diabetes, smoking, preoperative albumin level, obesity, body mass index, number of levels involved in the procedure, previous lumbar spine procedure, and operative time. Student's t test was used for normally distributed continuous variables, the Wilcoxon signed-rank test was used for continuous variables that are not normally distributed, and the chi-square test was used for categorical variables. Multivariate logistic regression analyses were performed, including all potential risk factors, either obesity or body mass index, and 1 of the 3 measures of fat thickness. Pearson's correlation analysis was used to determine the correlation between body mass index and subcutaneous fat thickness, and 2-tailed P<.05 was used to determine significance for all tests. Statistical analyses were performed with SPSS version 20 software (IBM Corp, Armonk, New York).

Results

Average age of the study cohort was 53.5 years (±15.8), with 53% (79 of 149) male patients and 47% (70 of 149) female patients. Of the study group, 30% (45 of 149) were obese (body mass index, 30–40 kg/m2) and 6% (9 of 149) were morbidly obese (body mass index, >40 kg/m2), in line with national averages. There were 15 (10.1%) surgical site infections. Of the procedures, 17% (25 of 149) were decompressive and 83% (124 of 149) involved arthrodeses with instrumentation. A significant association was noted between body mass index and surgical site infection (P=.031), but no differences were found between World Health Organization Obesity classifications (Table 1). In 2 patients, albumin level was not available within 30 days of the procedure, and anesthetic time could not be obtained for another 4 patients.


Surgical Site Infection Rates by World Health Organization Obesity Classificationa

Table 1:

Surgical Site Infection Rates by World Health Organization Obesity Classification

Among previously identified risk factors for surgical site infection, bivariate analysis showed significance for body mass index (P=.014), obesity (P=.015), maximum fat thickness at T12-L5 (P=.003), maximum fat thickness at the operated levels (P=.002), and fat thickness at L4 (P=.002). Multivariate analysis did not show significance for body mass index and obesity, but all 3 measures of fat thickness remained significant (Table 2). Previous lumbar spine surgery was associated with surgical site infection in bivariate (P=.062) and multivariate analyses (P=.056). Albumin level of less than 3.5 g/dL was not significantly associated with surgical site infection.


Bivariate and Multivariate Logistic Regression Analyses of Previously Identified Risk Factors and Measures of Subcutaneous Fat Thickness With Surgical Site Infection

Table 2:

Bivariate and Multivariate Logistic Regression Analyses of Previously Identified Risk Factors and Measures of Subcutaneous Fat Thickness With Surgical Site Infection

Body mass index was most highly correlated with subcutaneous fat thickness at T12 (r2=0.49), followed by L3 (r2=0.47), L5 (r2=0.46), L4 (r2=0.44), L1 (r2=0.41), and L2 (r2=0.41), and each correlation was significant with body mass index (P<.001). For every 1-mm thickness of subcutaneous fat at L4, a 6% (odds ratio, 1.06; 95% confidence interval, 1.02–1.10) increase occurred in the odds of surgical site infection. Each additional increase in unit (kg/m2) of body mass index was associated with a 1.59-mm increase in sub-cutaneous fat thickness at L4 (95% confidence interval, 1.30–1.88; P<.001).

When patients were divided into tertiles based on subcutaneous fat thickness at L4 (Table 3), a significant association was found between tertiles and surgical site infection (P<.001). The tertile with the thickest subcutaneous fat, roughly those with fat thickness of more than 50 mm, had a 4-fold (95% confidence interval, 1.4–11.6) increase in the odds of surgical site infection (P=.012) compared with those with fat thickness of less than 50 mm (Figure 2).


Surgical Site Infection Rate by Tertiles of L4 Subcutaneous Fat Thickness

Table 3:

Surgical Site Infection Rate by Tertiles of L4 Subcutaneous Fat Thickness


Scatter plot distribution of L4 fat thickness and body mass index (BMI) for the study cohort. Triangles indicate those with surgical site infection (+SSI), and diamonds indicate those without surgical site infection (-SSI). Those with fat thickness greater than 50 mm at L4 had a 4-fold increase in the odds of SSI compared with those with fat thickness less than 50 mm. Moderate correlation was observed between L4 fat thickness and BMI.

Figure 2:

Scatter plot distribution of L4 fat thickness and body mass index (BMI) for the study cohort. Triangles indicate those with surgical site infection (+SSI), and diamonds indicate those without surgical site infection (-SSI). Those with fat thickness greater than 50 mm at L4 had a 4-fold increase in the odds of SSI compared with those with fat thickness less than 50 mm. Moderate correlation was observed between L4 fat thickness and BMI.

Discussion

Identifying patients who are at high risk for complications is a critical aspect of surgical decision making that ultimately affects the value of care. Surgical site infections, reported by the Centers for Disease Control and Prevention to occur in up to 8.73% of revision spine surgeries, are a relatively common complication that may be minimized with preoperative counseling and intervention as well as perioperative targeted treatment.1,6,7,9–11,16,21,22 This study showed the utility of local subcutaneous fat thickness as a simple independent risk assessment tool for surgical site infection in posterior lumbar spine surgery. In the study cohort, patients with fat thickness of more than 50 mm who underwent a lumbar spine procedure through a midline posterior approach had a 4-fold increase in the odds of surgical site infection compared with those with fat thickness of less than 50 mm.

The significance of obesity as a risk factor for surgical site infection in spine surgery may be related to its moderate correlation with subcutaneous fat thickness in the surgical region of interest (r2=0.44) (Figure 2). Although body mass index and obesity did not show significance in multivariate analysis, all 3 measures of subcutaneous fat thickness were significant for surgical site infection (Table 3). The thickness of subcutaneous fat in the surgical region of interest may increase the difficulty and duration of the procedure and may be associated with decreased blood flow and oxygen tension.16,23,24 Vigorous retraction of hypovascular fat tissue may lead to aseptic fat necrosis and dead space formation, increasing the risk of surgical site infection. A thick subcutaneous fat layer also may lead to longer periods of drainage and the formation of sinus tracts.

Limitations

Limitations of this study include the retrospective study design, a cohort limited to patients who had CT scans of the abdomen or pelvis, inclusion of all lumbar spine procedures, and a relatively small sample size. Mean preoperative albumin level in the study cohort was 4.12 g/dL (±0.57) and was less than 3.5 g/dL in 11% of patients, which is nearly identical to the level in patients in the American College of Surgeons National Surgical Quality Improvement Program database. This database showed a mean albumin level of 4.13 (±0.45) g/dL in 8648 patients who had a cervical spine procedure between 2006 and 2011 and a mean albumin level of 4.1 (±0.5) g/dL in 2150 patients who had spinal arthrodesis between 2005 and 2010, with 12% of these patients having an albumin level of less than 3.5 g/dL.25 This finding suggests that the study cohort patients are as healthy as the typical patient undergoing spine surgery. By selecting patients with CT scans of the abdomen or pelvis, the authors' goal was to eliminate selection bias against truncally obese patients in relation to the primary measure of subcutaneous fat thickness. At the study institution, in these patients, the dorsal skin is not always visualized on routine cross-sectional lumbar spine CT or magnetic resonance imaging, where the field of view is focused on the bony spinal elements.

Conclusion

Body mass index moderately predicts subcutaneous fat thickness. Measuring subcutaneous fat thickness on preoperative imaging provides a more precise assessment of the risk of surgical site infection than body mass index. In the study cohort, patients with subcutaneous fat thickness of more than 50 mm at L4 had 4 times the odds of surgical site infection compared with those with fat thickness of less than 50 mm. This simple guideline can be used to inform patients of their risk of surgical site infection and to develop treatment strategies to minimize this risk. In some cases, the risk may be great enough that the best approach may be to advise against surgery.

References

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Surgical Site Infection Rates by World Health Organization Obesity Classificationa

Body Mass Index, kg/m2World Health Organization ClassSurgical Site Infection Rate (No./Total No.)
<18.5Underweight20% (1/5)
18.5–24.9Normal5% (2/44)
25.0–29.9Overweight4% (2/46)
30.0–39.9Obese16% (7/45)
≥40Morbidly obese33% (3/9)

Bivariate and Multivariate Logistic Regression Analyses of Previously Identified Risk Factors and Measures of Subcutaneous Fat Thickness With Surgical Site Infection

Risk FactorSurgical Site Infection (n=15)No Surgical Site Infection (n=134)BivariateMultivariate


POdds Ratio (95% Confidence Interval)POdds Ratio (95% Confidence Interval)
Patient factors
  Age, mean (SD), y51.3 (17.5)53.7 (15.6).579
  Diabetes (No./Total No.)26.7% (4/15)14.9% (20/134).249
  Smoking within 1 y (No./Total No.)33.3% (5/15)30.6% (41/134).828
  Albumin, mean (SD), g/dL3.95 (0.65)4.14 (0.56).233
  Body mass index, mean (SD), kg/m232.7 (8.0)27.9 (6.4).014a
  Obesity (No./Total No.)66.7% (10/15)32.8% (44/134).015a4.09 (1.32–12.7)
Fat thickness, mm
  Maximum (SD), T12-L557.4 (17.5)44.2 (14.5).003a1.06 (1.02–1.10).036a1.05 (1.00–1.10)
  Maximum (SD), operated levels56.0 (20.2)41.8 (15.2).002a1.06 (1.02–1.10).032a1.05 (1.00–1.10)
  At L4 (SD)54.9 (18.2)40.4 (15.2).002a1.06 (1.02–1.10).026a1.05 (1.01–1.10)
Perioperative factors
  Levels (mean), No.4.1 (2.2)3.7 (1.8).413
  Previous procedure (No./Total No.)26.7% (4/15)9.7% (13/134).0623.39 (0.94–12.2).0564.22 (0.96–18.5)
  Operative time, mean (SD), min297 (233)257 (127).309

Surgical Site Infection Rate by Tertiles of L4 Subcutaneous Fat Thickness

L4 Fat Thickness, mmaSurgical Site Infection Rate (No./Total No.)Odds Ratio (95% Confidence Interval)
7.9–33.34% (2/49)1 (Reference)
33.6–47.46% (3/50)1.47 (0.25–8.80)b
47.7–92.120% (10/50)4.90 (1.07–22.4)c
Authors

The authors are from the Division of Orthopaedic Trauma (JJL), Memorial Healthcare System, Hollywood, Florida; and the Department of Orthopaedic Surgery (KIO, RDP, JAG, GPG) and the Department of General Surgery (SAH, SCW), University of Michigan, Ann Arbor, Michigan.

Drs Lee, Odeh, Patel, Goulet, and Graziano have no relevant financial relationships to disclose. Mr Holcombe has a patent pending with Analytic Morphomics. Dr Wang has a patent pending with Analytic Morphomics.

The authors thank Lucy Hully for her assistance with using the research image and archive database.

Correspondence should be addressed to: John J. Lee, MD, MS, Division of Orthopaedic Trauma, Memorial Healthcare System, 1150 N 35th Ave, Ste 130, Hollywood, FL 33021 ( jleecal@hotmail.com).

Received: April 20, 2016
Accepted: June 21, 2016
Posted Online: August 30, 2016

10.3928/01477447-20160819-05

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