Postoperative Spinal Infections

Introduction

Postoperative spinal wound infections are relatively frequent problems that treating spine surgeons must know how to diagnose and address. Although strategies to reduce the occurrence of infection after spine surgery have seen some success, infection rates of up to 20% continue to be reported in the literature. These infections result in significant acute and chronic morbidity to the patient and cause significant financial drain to the patient and the health care system. Because of the relatively high incidence and difficulty in diagnosis, there must be a thorough understanding of the diagnostic and management principles in order to successfully treat these patients. Current treatment strategies need to be continually revisited in order to address commonly encountered microorganisms and potential resistance patterns that may develop.

Successful infection management begins preoperatively with aseptic technique and proper antibiotic prophylaxis administration. Additionally, a thorough preoperative workup with careful attention to potential patient risk factors is essential. The astute surgeon must have keen examination skills and the clinical sense to initiate the diagnostic workup for a postoperative infection when there is any significant suspicion, particularly with deep wound infections. The results from a diagnostic workup are often vague and difficult to interpret. The decision to treat an infection solely with medical therapy versus with aggressive surgical debridement continues to be a relatively controversial topic. Prompt and successful infection eradication has significant influence on the success of the original surgery and the ultimate patient outcome.

Incidence/Epidemiology

Postoperative wound infections are among the most common complications following spinal surgery. The incidence documented in the literature has historically been quite variable, with reported ranges from 0.5% to 20%. This discrepancy is in part due to significant variation in case complexity, use of instrumentation, the definition of infection, and surgical approach. In general, increasing the complexity and invasiveness of the surgery correlates with a higher incidence of infection. More recently, however, a review of the Scoliosis Research Society Mortality and Morbidity database revealed superficial and deep infection rates to be 0.8% and 1.3%, respectively. This contrast to older studies with higher infection rates underlines improvements in surgical technique and prophylaxis protocols.

Historically, lower-risk spinal surgeries include those that do not require instrumentation. Discectomy and laminectomy have reported incidences of infection of less than 3%. With the addition of instrumentation, however, the incidence of postoperative infection increases to greater than 12% in some studies. A recent series demonstrated that infection rates in cases with implants were 28% higher than cases without implants (2.3% vs. 1.8%). Specifically, lumbar discectomy has had a reported incidence of 0.7%, and using a microscope for the procedure increases the incidence to 1.4%. In the United States, the National Nosocomial Infections Surveillance System, a voluntary performance-measurement system orchestrated by the Centers for Disease Control and Prevention, has reported a 1.25% rate of surgical site infection following laminectomy and a 2.1% rate following laminectomy with noninstrumented fusion. More recently, a large case series demonstrated an infection rate of 2.4% in decompression cases, whereas fusion cases had a significantly higher rate at 3.5%.

Consistently throughout the literature, cases that require more extensive soft tissue dissection, longer operative time, greater blood loss, more significant soft tissue devitalization, or the creation of dead space have an increased infection rate. In one study, an index of surgical invasiveness was found to be the strongest risk factor for infection, even when controlling for comorbidities and other known risk factors. Another study comparing infection rates in patients undergoing discectomy alone and those undergoing discectomy and fusion showed infection rates of 1% versus 6%, respectively. In other reports, fusion without instrumentation has been associated with an infection rate ranging from 0.4% to 4.3%. Historically, a general consensus has been that the use of devitalized bone graft material in fusions results in an infection rate from 1% to 5%. Olsen et al., however, found no increase in infection rates with the use of bone graft ; a systematic review of the literature supported their findings.

As the use of instrumentation has become more commonplace, attention must be paid to a possible associated increased infection risk. Colonization of implanted devices occurs in upward of 50% of patients, although most do not display clinical symptoms of infection. Although implants rarely act as the initial source of infection, they may become a nidus for inoculation and subclinical growth of infectious organisms. The implant provides an avascular surface upon which bacteria can create a glycocalyx, which serves as a barrier to the host immune response and antibiotic treatment. In addition, micromotion can create metallosis and subsequent granulomas, which may become a potential site for bacterial colonization. Other theories postulate that local soft tissue inflammation and postoperative seromas may serve as a potential cause for the increased infection risk seen with instrumented fusions ( Fig. 99.1 ). Historically, there has been wide variation in infection rates, but more recent literature reports elective instrumented surgical cases between 2.8% and 6%, with a recent review of retrospective studies showing a rate between 2.1% and 8.5%. Many authors feel that the actual infection risk with the use of spinal instrumentation is between 5% and 6%.

Spinal trauma patients represent a unique population that have an increased risk for developing postoperative infections. The significant soft tissue devitalization and devascularization caused by the traumatic event result in local hypoxia, leading to tissue necrosis, edema, acidosis, and hematoma formation. This combination results in a medium optimal for bacterial proliferation isolated from the host defenses. Systemically, the patient sustaining major trauma shows a hyperinflammatory state with alterations in the normally tightly controlled homeostasis of proinflammatory and antiinflammatory cytokine levels. The resultant imbalance leads to a state of immunosuppression that is thought to increase susceptibility to infection. In addition, comorbid factors such as age, medical conditions, poor nutritional status, and body habitus cannot be controlled for in the same manner that they are in elective surgery. The presence of complete neurologic injury significantly increases the risk of postoperative infection, while the presence of an infection decreases functional neurologic recovery. In the largest clinical series investigating 256 surgically treated traumatic spinal injuries, the rate of postoperative wound infections was 9.4% compared with an infection rate of 3.7% seen in patients undergoing elective spinal surgery during the same time period at the same hospital. Interestingly, one multicenter cohort study demonstrated an infection rate of 4.6%, concluding a similar rate to elective patients. Other reviews have found postoperative infection rates in spinal trauma patients ranging from 9% to 15%, which is greater than the previously discussed average infection rate seen in elective spinal surgeries. Furthermore, a systematic review demonstrated that the rate of nonsurgical infections also are higher in trauma patients compared to nonoperative controls.

Anterior spinal procedures appear to be less susceptible to infection than posterior procedures. Infection rates following anterior cervical spinal surgery have been reported in the literature to be as low as 0% to 1%. Anterior thoracic and lumbar surgery also display significantly less infection risk than their posterior counterparts, with rates 50% lower than those occurring after posterior surgery. The infection rates for anterior approaches are likely decreased by multiple factors, including better vascularity of the spinal column and decreased dead space creation. Combined anterior and posterior approaches are an independent risk factor for infection, especially in cases of staged surgeries.

With the rise in minimally invasive surgery (MIS) in spine surgery, several studies have examined its effect on surgical site infection (SSI) rates. Several studies have shown decreased rates of infections using MIS techniques. In one multicenter study reviewing 1388 cases, the infection rate was 0.74% for fusions, 0.10% for decompressions, and 0.22% overall. MIS transforaminal lumbar interbody fusion procedures also have been reported to have lower infection rates compared to open procedures. Parker et al. demonstrated an SSI rate of 0.6% in MIS compared to 4.0% with open techniques, while McGirt et al. had a 4.6% SSI rate with MIS and 7% with open surgery. Some studies, however, have shown no differences in SSI rates between MIS and open techniques. In a prospective series, there was no difference in infection rates for single-level transforaminal lumbar interbody fusion. Interestingly, in the study by Parker et al., in the subgroup of single-level fusions, there also was no difference with MIS compared to open techniques. In light of these findings, specific conclusions regarding the effects of MIS on infection risks cannot be made; for multilevel fusions, however, there appears to be a benefit.

Risk Factors

Patient risk factors play a pivotal role in influencing postoperative spinal infections. Many of these risk factors are modifiable if addressed prior to surgery. Others are nonmodifiable and have been shown to increase postoperative infection risk. Careful attention to these factors must be made in the preoperative workup because correcting them may have a significant impact on the ultimate outcome of the surgery.

Modifiable risk factors include smoking, obesity, surgery length, prolonged indwelling catheter use, length of hospital stay, and malnutrition. The patient and surgeon should work together to address these risk factors preoperatively. Poorly controlled diabetics are one of the highest-risk patient populations, with an estimated incidence of postoperative infections of 17%. A recent systematic review demonstrated that there was a statistically significant association between diabetes and SSI in five out of eight studies and in four out of five studies elevated blood glucose raised the risk of infection threefold. A level II study also showed a 4.10 adjusted relative risk of SSI in diabetics. It is thought that elevated blood glucose concentrations, particularly those above 200 mg/dL, can inhibit host immune response, including cellular chemotaxis and phagocytosis. In addition to creating a relatively immunocompromised state, poorly controlled diabetics are predisposed to chronic medical conditions, including hypertension, cardiovascular disease, and renal insufficiency. These medical issues predispose to poor tissue vascularity and can further increase postoperative infection and complication rates. Careful preoperative attention to tight blood glucose control and an assessment for other related factors may limit the risk of local infection and systemic morbidity in diabetics. In general, diabetics have increased complication rates, particularly with posterior lumbar surgery.

Malnutrition is an often underrecognized contributor to impaired healing potential of a patient. Approximately 25% of all elective lumbar fusion patients are malnourished. In this same study, 11 of 13 complications in a group of 114 patients undergoing elective lumbar fusion were in malnourished patients, making them 15 times more likely to develop an infection. Serum albumin levels less than 3.5 g/dL, arm circumference less than 80% of normal, total lymphocyte count less than 1500/mm ³ , recent weight loss greater than 10 pounds, transferrin levels less than 150 µg/dL, and abnormal skinfold measurements are all methods to evaluate nutritional status. Addressing the issue of malnourishment in the preoperative period is often underperformed and should be considered a major modifiable factor to limit poor healing.

Obese patients are also considered at high risk for developing postoperative infections. Mehta et al. found that distribution of body mass actually is even more predictive of SSI than absolute BMI, with magnetic resonance imaging (MRI) measurements of skin-to-lamina distance and thickness of subcutaneous adipose layer being significant risk factors. Overweight patients often require more extensive dissection through poorly vascularized adipose tissue. The resulting tissue devitalization and fat necrosis result in an environment favoring bacterial growth and proliferation. In addition, the increased operative time and blood loss necessary with obese patients increase their risk of infection. Obesity in itself is a risk factor for malnutrition, diabetes, and other medical comorbidities, further contributing to a poor healing environment with diminished immunogenic potential.

Smokers also have a significantly increased chance of developing postoperative infections. Smoking cessation counseling should be a routine part of the preoperative meeting between surgeons and patients.

Nonmodifiable risk factors that may increase the susceptibility to infection must also be evaluated prior to surgery. Thorough assessment of a patient's medical history during the preoperative evaluation may reveal systemic comorbidities that should be identified and optimized prior to surgery. In all patients, preoperative infections, whether in the spine or elsewhere, should be addressed and treated prior to undergoing elective surgery. Conditions such as rheumatoid arthritis, acquired immunodeficiency syndrome, adrenocortical insufficiency, long-term corticosteroid use, and malignancy may pose significant risk for developing postoperative infections. A thorough discussion of potential complications associated with these confounding medical conditions is important during preoperative counseling. Medical optimization of these conditions may limit potential postoperative complications. Although age is not considered an independent risk factor, older patients are more likely to have comorbidities associated with an increased risk of postoperative infection.

Spinal surgeries for management of tumors are associated with a significantly higher rate of postoperative infections. Patients treated with preoperative or postoperative local radiation are especially at risk. Surgery performed through previously irradiated tissue increases the risk of infection and can make the surgery technically more challenging. It is generally recommended that elective spinal surgery occur 6 to 12 weeks following radiation of the surgical bed. In patients requiring postoperative radiation around the operative site, therapy should be delayed approximately 3 weeks to allow for adequate soft tissue healing.

Microbiology

Three potential sources are hypothesized to be responsible for postoperative infections: (1) direct inoculation during the operative procedure, (2) contamination during the early postoperative period, and (3) hematogenous seeding. Of these, direct inoculation during the surgery is the most common, making aseptic technique and the appropriate use of prophylactic antibiotics of paramount importance.

Gram-positive cocci are the most common pathogens responsible for acute postoperative infections. The most commonly reported organism in the literature is Staphylococcus aureus , which causes greater than 50% of the infections in some reports. Other common gram-positive species that cause postsurgical infections include Staphylococcus epidermidis and β-hemolytic streptococci. Common gram-negative organisms cultured from infected surgical sites include Escherichia coli , Pseudomonas aeruginosa , Klebsiella pneumoniae , Enterobacter cloacae , Bacteroides , and Proteus species. In cases of patients who are immunosuppressed, the surgeon must also be aware of the possibility of fungal infections, the management of which can greatly differ from bacterial infections.

The microbiology of an infection can be influenced by the anatomic location of the surgery. Fecal contaminants are more likely to be involved in surgeries of the low lumbar or sacral regions. Bladder or fecal incontinence may predispose to gram-negative flora, especially with posterior lumbosacral incisions.

Infections that present greater than 1 year after surgery are generally caused by low-virulence organisms such as coagulase-negative staphylococcus, Propionibacterium acnes , and diphtheroids. These organisms can be present as normal skin flora, and it is hypothesized that prolonged surgical wound drainage and inflammation may result in these infections. These low-virulent organisms are usually rapidly cleared by the host immune response with appropriate treatment and generally do not result in a clinical sepsis. In a retrospective review of postoperative infections presenting greater than 1 year after surgery, 10 of 11 patients with cultures incubated for greater than 1 week grew low-virulence skin organisms. It is also critical to note that Proprionibacterium acnes cultures need to be kept for a minimum of 13 days.

Hematogenous spread can also cause surgical site infections. These blood-borne infections are usually due to highly virulent organisms, including gram-negative bacteria. These infections are often associated with systemic illness and sometimes have grave consequences, such as multisystem organ failure. Due to repeated cannulization of the venous system, intravenous drug users have a higher incidence of gram-negative infections, as do patients who have prolonged hospital admissions.

Diagnostic Modalities