Spine surgeons have long known that obesity is a risk factor for surgical site infection. Body mass index (BMI) is one measure of obesity, though in studies of perioperative spine infection it has not always been a strong predictor of infection, likely due to the fact that it does not measure the amount of fat at the surgical site and is also increased by non-adipose mass (i.e. higher muscle mass). In an attempt to find a better measure of the effect of adipose on infection rate, Dr. Mehta and his colleagues from Duke measured the thickness of subcutaneous back fat and the skin to lamina distance in lumbar fusion patients and found that these measures were more strongly associated with lumbar surgical site infection than BMI. They performed a case-control study in which they compared baseline characteristics of the 24 patients who had a surgical site infection and the 274 patients who did not. BMI over 30, number of levels fused, lamina to skin distance, subcutaneous back fat thickness, and length of anesthesia were all significantly associated with surgical site infection. Average BMI was only 2 points higher in the infection group (30.9 vs. 28.9, p=0.12), indicating that it is a less sensitive predictor of infection than back fat thickness. This is a somewhat intuitive finding given that wound healing problems and infection are more likely influenced by the local environment than by adipose elsewhere. However, this finding does have implications for predicting infections and counseling patients pre-operatively. Some patients with relatively high BMI can have high muscle mass and abdominal obesity without much back fat, and this paper suggests that these patients might not be at increased risk of infection provided they don’t have other risk factors (i.e. diabetes, immunosuppression, etc.). On the other hand, some patients with low muscle mass and low amounts of abdominal fat but a relatively thick layer of back fat may have only mildly elevated BMIs but could be at increased risk of infection.
The authors of this paper should be applauded for quantifying the effect of back fat thickness on infection rate, a factor that surgeons have intuitively felt is associated with infection but had yet to be formally confirmed. However, the results of this paper should be viewed as preliminary given the relatively small number of patients involved and the fact that the paper was underpowered to detect certain associations that would have likely been significant in a larger cohort (i.e. diabetes and infection). The authors appropriately acknowledge this limitation. In the future, an interesting multivariate analysis would be to determine which factors are the strongest independent predictors of infection. Such an analysis would likely require more patients in order for it to be sufficiently powered. Given that subcutaneous back fat thickness, skin to lamina distance, and BMI are all related, determining the strongest predictor of infection would be helpful in creating prediction models to stratify risk preoperatively. As shared decision making becomes more comprehensive, the ability to individualize the likely risks and benefits of surgery will become more important. Hopefully this paper will give surgeons a more accurate measure of obesity that they can use to help inform patients about their likely risk of surgical site infection and allow future investigators to evaluate these measures as potentially better predictors of infection than traditional measures like BMI.
Please read Dr. Mehta’s paper and the accompanying commentary on The Spine Blog. Will this paper change how you counsel patients about their risks of surgical site infection? Let us know by leaving a comment on The Spine Blog.
Adam Pearson, MD, MS
Associate Web Editor