2 Risk Stratification and Prevention of Postoperative Spinal Infections
Introduction
The rate of surgical site infection (SSI) in spine surgery has been reported to range from 0.7 to 10.9%. 1 , 2 Despite an increased focus on risk factors for SSI, infection continues to be major challenge because it is virtually impossible to completely avoid bacterial contamination during surgery. If the host’s defense is unable to overcome the bacterial load, SSI may result. Once an infection occurs at the surgical site, treatment requires a substantial use of time and resources, and secondary problems including pseudarthrosis, nerve injury, and poor clinical outcome may result.
In the field of spine surgery, the use of metallic implants has become commonplace. Unfortunately, using instrumentation itself is a risk factor for postoperative SSI as the presence of foreign material suppresses the host’s defense locally 3 and helps bacteria in the formation of biofilm. 4 In addition, the use of surgical treatment for elderly people and compromised hosts is increasing. Therefore, risk assessment and precautionary management for postoperative SSI are particularly important in the field of spine surgery. Many risk factors for postoperative SSI have been identified in past studies. Some of these factors are modifiable before surgery and some are not. Recent efforts on the part of spine societies to promote risk stratification efforts for SSI after surgery are ongoing. Lee and colleagues 5 developed and validated a predictive model for the risk of SSI after spine surgery based on the patient’s comorbidity profile and invasiveness of surgery. We can also predict SSI by using a universal American College of Surgeons National Surgical Quality Improvement Program (ACS-NSQIP) surgical risk calculator. 6 A clearer understanding of preoperative risk for SSI is a necessary condition and first step in optimizing surgical spine care. 7 Also, it is important to mitigate as much as possible the risk factors that can be adjusted. Prevention of postoperative SSI includes both perioperative and postoperative management.
This chapter describes preoperative risk strat ification strategies and preventive measures for SSI during surgical preparation, the intraoperative procedure, and postoperative treatment. Postoperative SSI in spine surgery is considered a very serious problem, but unfortunately there are not many high-quality studies in this area. In part, this is due to concerns about the ethical limitations of randomized controlled trials in the field of spine surgery. Therefore, we focused on studies with the highest evidence level based on the appropriate methodology. In studies in which the evidence level is low, we also searched the literature in the field of general orthopedic surgery.
Risk Factors and Risk Stratification for SSI After Spine Surgery
Every operation carries the potential risk of postoperative SSI, because it is very difficult to completely avoid bacterial contamination during surgery. Whether or not the bacterial contamination leads to SSI depends on the quantity and pathogenicity of the bacteria, and on the patients’ host defense mechanisms. A relatively large number of studies have identified preoperative factors to help mitigate the risk of SSI. In a systematic review, Pull ter Gunne and colleagues 8 reported 34 variables that are significantly associated with SSI in one or more studies, of which 11 variables were confirmed in two or more studies. These variables can be categorized as patient factors and surgical factors ( Table 2.1 ).
Patient Factor | Surgical Factor |
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*These factors can be modified to mitigate the risk of SSI. | |
Patient Risk Factors
In a study of 24,774 patients enrolled in a prospective registry (Veterans Affairs National Surgical Quality Improvement Program, VA-NSQIP) who underwent spinal decompression and fusion, multivariate logistic regression identified the following risk factors of postoperative SSI: insulin-dependent diabetes; tobacco use; American Society of Anesthesiologists (ASA) class of 3, 4, or 5; a 10% preoperative weight loss in the 6 months before surgery; dependent functional status; and disseminated cancer. 9 In another systematic review, Schuster and colleagues 10 reported that age > 60 years, presence of diabetes, malnutrition, obesity, ASA score > 3, and higher serum glucose levels were associated with postoperative SSI. In addition, they also described the overall strength of the evidence defining preoperative risk factors for postoperative SSI as “moderate.” That is, further research is likely to have an important impact on our confidence in the estimate of effect and may change the estimate. In a more recent systematic review, Pull ter Gunne and colleagues 8 reported that of these variables, only two were found to be significantly related to SSI in more than half of the studies: diabetes mellitus in 11 of 18 studies (61%) and obesity in 8 of 14 studies (57%). They also reported that a history of a spinal infection and ASA risk class were confirmed more often as a significant risk factor for SSI.
A recent general orthopedic study reported on the genetic similarity between nasal Staphylococcus aureus isolates and the SSI isolates. 11 Although SSI is generally thought to be caused by bacterial contamination, this study raises the possibility of an endogenous infection pathway. Previous studies reported that preoperative nasal screening test, and sterilization of both nasal cavity and skin in methicillin-resistant S. aureus (MRSA) carriers reduced the occurrence of postoperative SSI. 12 , 13 Verification of this finding in the field of spine surgery is still lacking. Considering the findings from these studies, it is reasonable to acknowledge that diabetes, obesity, ASA grade ≥ 3, history of previous SSI, elevated serum glucose levels, older age, and malnutrition are patient risk factors for SSI ( Table 2.1 ).
Surgical Risk Factors
The same VA-NSQIP study reported that fusion/instrumentation, intraoperative transfusion, and an operative duration of longer than 3 hours were significant surgical risk factors for postoperative SSI. Schuster and colleagues 10 reported that blood transfusion and a posterior approach were consistently associated with postoperative SSI. The use of instrumentation and duration of surgery has also been associated with SSI in most studies, but not uniformly. Pull ter Gunne and colleagues 8 noted that while many surgical risk factors have been demonstrated to be significantly associated with the occurrence of an SSI after spine surgery, a causal relationship has less often been confirmed ( Table 2.2 ).
Other factors that can be discussed before surgery include the type of surgical approach, the use of microscopes, the staffing requirements in the operating room, geographical variations, and the primary disease being treated.
Risk Stratification and Risk Mitigation
Lee and colleagues 5 published a regression model to predict a patient’s risk for postoperative SSI in the field of spine surgery. The postoperative SSI prediction formula is free and available online 14 ; the risk factors include heart failure, diabetes, past history of rheumatoid arthritis (RA), surgical invasiveness, older age, diagnosis, weight loss, and obesity. However, these factors are not necessarily consistent with the risk factors identified in previous systematic reviews. 8 , 10 In addition, the odd ratios for the risk factors were relatively small and may not be clinically relevant. 15 The accuracy of these predictive models may also be influenced by other factors such as the surgeon’s skill and the hospital setting. Among the many risk factors for SSI, some variables, such as older age, cannot be modified. However, other variables, such as malnutrition or elevated serum glucose levels, can theoretically be modified to mitigate the risk of SSI.
Preventive Strategies for SSI
Maintaining the surgical site sterility is an important goal in preventing SSI. The strategies to accomplish this goal can be included in the surgical preparation, the intraoperative procedure, and postoperative treatment. We discuss antimicrobial prophylaxis separately.
Surgical Preparation
Shaving
In a randomized trial, Celik and Kara 16 compared the incidence of SSI in a “shaving”cohort (N = 371) to a “no shaving” cohort (N = 418). A postoperative infection developed in four patients (1.07%) in the “shaving” group and in one patient (0.23%) in the “no shaving” group (p< 0.01). The authors concluded that shaving the patients increases the risk of SSI compared with not shaving them. In addition, current recommendations are to use a surgical clipper, as shaving with a razor with increases the risk of damaging the skin. 17 , 18
Surgical Site Scrubbing and Surgical Drape
Although there is no evidence that scrubbing of the surgical site just before surgery is beneficial, it is still recommended in the field of spine surgery. Commonly used disinfectants include chlorhexidine gluconate, povidone iodine, and alcohol. Chlorhexidine gluconate has bactericidal action due to destruction of the cell membrane and has the highest residual effect, but its bactericidal activity against tuberculosis/fungus is somewhat weak, and it takes time to develop its action. Povidone iodine has a bactericidal action due to the oxidizing action of iodine; the residual effect is relatively small, and it takes time to develop its action. Alcohol has a strong bactericidal action due to protein denaturation and is quick acting, but it has no residual effect and is resistant to spores. There is no evidence that scrubbing the surgical site before spine surgery makes a difference in the postoperative SSI incidence.
There is also no evidence that the incidence of postoperative SSI decreases with povidoneiodine–free drape. In general orthopedic surgery, the risk of postoperative SSI may decrease with povidone-iodine–containing drapes. 19 An appropriately powered randomized control study specifically for spine procedures is needed to draw valid conclusions.
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