Thickness of fat, not BMI, is a significant risk factor for surgical site infection

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A new study, published in the Journal of Bone and Joint Surgery, indicated that the thickness of subcutaneous fat at the surgical site—rather than overall body mass index (BMI)—was a marker of surgical site infection in patients undergoing cervical spine fusion surgery.

Ankit Mehta, Duke University Medical Center, Durham, USA, and others reported that multiple studies have noted an association between BMI and surgical site infection. They added: “There appears to be a direct association between BMI and the thickness of subcutaneous tissue at the surgical tissue, resulting in increases in the need for retraction and the duration of surgery; thus, increasing the risk of necrosis.” Additionally, Mehta et al reported, a larger layer of subcutaneous tissue potentially leads to a larger dead tissue space after closure and, as a result, increases the risk for infection. However, they wrote that BMI “may not be the most accurate predictor of the risk of surgical site infection since body weight distribution is variable; also, muscle mass is included in the calculation of BMI.”  Therefore in their study, which was a retrospective cohort analysis, Mehta et al tested the hypothesis that subcutaneous fat at the surgical site (rather than overall BMI) was an identifiable risk factor for surgical site infection in patients undergoing posterior cervical fusion.


Of 213 patients who had undergone posterior cervical spinal fusion at the Duke University Medical Center (the authors’ institution) and who had the relevant data available, 22 developed a surgical site infection. The authors did not observe a significant difference either in BMI or the incidence of obesity (BMI ≥30kg/m2) between the patients who developed an infection and those who did not; therefore, neither of these factors could be said to be a marker for surgical site infection.


However, Mehta et al did find that the thickness of subcutaneous fat at the surgical site was a significant risk factor for infection. They wrote: “Patients who developed an infection had a mean thickness of 27mm compared with 21.4mm for those who did not (p=0.042).” They explained that the rate of surgical site infection also increased as the subcutaneous fat became thicker: patients with a fat thickness of >40mm had an 18.2% infection rate compared with a rate of 14.9% for patients with a thickness of 20–29.9mm, a rate of 5.2% for patients with a fat thickness of <20mm, and a rate of 2.3% for patients with fat thickness of <10mm.


According to the authors, the ratio of the fat thickness was also a “strong and significant risk factor” (p=0.02). They commented: “The mean ratio was 0.42 for patients who developed a surgical site infection compared with 0.35 for those who did not. The surgical site infection rate was also correlated with this ratio. The infection rate was 15.4% for patients with a rate of >0.5, 14.3% for those with a ratio of 0.4 to 0.49, 13% for those with a ratio of 0.3 to 0.39, 4.9% for those with ratio of 0.2% to 0.29%, and 0% for those with a ratio of <0.2.”


Mehta et al wrote that their study demonstrated that: “The distribution of body fat at the surgical site was an important clinical marker for the development of surgical site infection.” They added that they believed that BMI did not always correspond to the amount of fat present because it calculated muscle mass as well as fat and this explained why, in their study, the thickness of subcutaneous fat had a greater impact on the risk of infection than had BMI. The authors concluded: “We encourage patients undergoing cervical spine surgery to be informed of any increased risk of postoperative infection resulting from the thickness of their subcutaneous fat at the surgical site.”  


Study author Oren Gottfried, Duke University Medical Center, Durham, USA, told Spinal News International: “Our team observed a trend of infections after posterior cervical fusions surgeries in patients with ‘deeper necks’, prompting this study. In this study, our results suggest an independent risk of postoperative infection in patients with greater depth of fat and greater fat to total depth ratio. Our study suggests evaluation of these parameters prior to surgery may be helpful in communicating to patients the likelihood of infection along with other standard risks.”