Surgical site infections are among the most common complications following spine surgery and may present in as many as 20% of spine surgical procedures depending on the patient population and type of intervention. Postoperative infections can have a devastating impact on the outcome of a surgery, especially in the event of a delayed diagnosis or inadequate treatment. Surgical site infections present on a spectrum from superficial wound infection or seroma/hematoma colonized by bacteria to epidural abscess and chronic osteomyelitis. They also may occur in the acute postoperative period or present years after an intervention. Acute postoperative infections (e.g., within the first 30 days following surgery) are the most frequent but it is important to recognize that pathogenesis, etiology, presentation, and treatment approaches may be different for acute surgical site infections and those that occur several months to years following surgery. Regardless of the time period in which they present, spine surgical site infections may be associated with instrumentation failure, pathologic fracture, neurologic compromise, and even death. The prompt diagnosis and treatment of postoperative infections may obviate the possibility of long-term sequelae. The results of several studies indicate that, if treated appropriately and in an expeditious fashion, surgical site infections do not adversely impact the long-term results of spine surgical interventions.

Surgical site infections transpire as a result of direct inoculation, or hematogenous seeding of a surgical site. Direct inoculation may occur intraoperatively or in the early postoperative period. It can also arise in a delayed fashion when subsequent spinal interventions, such as epidural steroid injections or spinal cord stimulator placement, are performed on patients with prior spine surgery. In the event of direct inoculation bacterial flora, oftentimes from the patient themselves, can be transferred to the wound and preferentially seed the surgical site. Hematogenous seeding transpires in the setting of septicemia, when bacteria present in the patient’s bloodstream are sequestered in the area of the spinal surgery. Inert substances such as spinal instrumentation or unincorporated bone graft, as well as dead space and compromised blood supply engendered during the surgical procedure, may potentiate an environment suitable for bacterial growth. When bacteria are seeded, regardless of the route of transmission, they usually exist in a free or “planktonic” form. The presence of spinal instrumentation and unincorporated bone graft provide a surface for bacterial adherence and sites for the creation of a biofilm exudate. This biofilm enhances bacterial cell communication and also impairs the effectiveness of the host response. The host’s local immune response is also already compromised following spine surgery, due to diminished vascularity at the surgical site.

Exposed bone surfaces, hematoma, and pockets of serous fluid all create a nutrient rich environment in which bacterial colonies can propagate. Systemic factors, including hyperglycemia, immunosuppression, and malnutrition may accelerate, or exacerbate, the infectious process, as can organism virulence and the overall bacterial load. A combination of the mechanisms of bacterial metabolism and the host immune response lead to the symptoms of surgical site infection, including fever, pain, purulent drainage, and osseous catabolism.

The incidence of surgical site infections following spine surgery has been found to vary widely with respect to the population in which surgery is being performed, the type of surgical intervention, and other mitigating factors such as operative time, the use of allogeneic blood transfusion, and the number of personnel in the surgical suite (Table 10.1). Determinations are very sensitive to the type of infection surveillance employed, the definition of what constitutes a surgical site infection, and the size of the population under consideration. Most generic estimations regarding the rate of postoperative infection
following spine surgical procedures, as a whole, fluctuate in the range of 1% to 5%. Nonetheless, the true incidence of surgical site infections after spine surgery is poorly described and likely underestimated. The frequently cited value of 2% to 5% risk of infection is probably most reasonably applied to a middle-aged, healthy individual undergoing a primary instrumented lumbar fusion procedure. Indeed, in a large series of patients (∼6,000) who underwent spinal fusion and had their data reported to the National Surgical Quality Improvement Program, postoperative infections occurred in 2% of the population.

A number of patient-based, hospital process and surgical factors may be considered to effectively modulate the risk of postoperative infections (Table 10.2). Patients with the following characteristics are felt to be at elevated risk of surgical site infection:

The Spinal Surgical Invasiveness Index, a metric that quantifies the intensity of a spinal operation based on a number of variables, including approach, use of a fusion-based procedure and/or instrumentation, has also been found to correlate with the risk of postoperative infection.

Within the spine trauma population, the postoperative infection rate is thought to be higher, in the range of 3% to 10%, and can be elevated further if high-dose steroids are administered as a neuroprotective measure. Risk factors that increase the risk of surgical site infection after spinal trauma include:

Infection rates may also be much higher (10% to 20%) following revision surgery, after interventions for spinal tumors, and among procedures performed on patients who are immunocompromised or have chronic liver disease. Patients with HIV are no longer thought to be at a greater risk of postsurgical infection
as compared to the general population as long as they are receiving appropriate antiretroviral therapy, their viral load is negligible, and CD4 counts remain within normal limits.

Infection rates may also be substantially lower in minimally invasive (MIS) procedures, or those that do not employ spinal instrumentation. For example, in the spinal stenosis and lumbar disk herniation arms of the Spine Patient Outcomes Research Trial (SPORT), infection rates for patients were documented in the range of 2.0% to 2.5%. Similarly, in a series of more than 100,000 patients whose cases were reported to the Scoliosis Research Society, the infection rate for discectomy procedures was maintained to be only 1%. A multicenter study that included over 1,000 patients who received MIS spine interventions found that among instrumented cases the infection rate was 0.74%, while in noninstrumented procedures infections occurred in only 0.22%. The following interventions have also been found to be effective in reducing the risk of infection following spine surgery:

Gram-positive cocci are the most common organisms encountered in postoperative infections involving the spine. Depending on the patient sample, S. aureus (MSSA or MRSA) or Staphylococcus epidermidis are frequently encountered. Other species of bacteria that are cultured less often include Propionibacterium acnes, Enterococcus species, and Escherichia coli. Infections with anaeorobic bacteria may present in diabetic patients and those with surgical sites that have a compromised blood supply (e.g., a surgical wound bed that has been irradiated or otherwise is severely scarred). Intravenous drug abusers may develop infections with Pseudomonas species, E. coli, and MSSA/MRSA. Infections that occur in wounds treated with vancomycin powder at the time of the index procedure frequently occur due to gram-negative organisms or are polymicrobial.