Postoperative Infections of the Spine

Noninstrumented Spinal Procedures

Noninstrumented spinal procedures include anterior and posterior decompressive surgeries and are usually confined to short stretches of the spinal column, most often with only a single level treated. Although noninstrumented procedures may include fusions, most fusion procedures are now supplemented with instrumentation.

Lumbar discectomy is one of the most common procedures performed on the spine and usually can be associated with improvement of clinical symptoms. Fortunately, rates of infection are low, with most series reporting rates of ≤1%. ^, Newer series of endoscopic minimally invasive discectomy have been reported to have even lower rates of infection, ^, with one group reporting no infections in the treatment of 262 patients. Infection may occur either superficially in the skin or muscle tissue or in the disk space itself. The former is easier to treat and often becomes obvious after 1 to 2 weeks; however, because of the avascular nature of the disk, discitis may take several weeks to months before it is diagnosed—particularly with less virulent organisms, such as Cutibacterium (formerly Propionibacterium ) acnes. The clinical symptoms of back pain may be accompanied by fever, elevation in the erythrocyte sedimentation rate (ESR) or C-reactive protein (CRP) and, in later stages, radiographic changes with disk and, in progressive cases, endplate degeneration.

Patients undergoing laminectomy without fusion also have a low incidence of infection, with rates commonly reported at around 2%. Infection may be superficial to the fascia or deep to it, with involvement around the epidural space. Laminoplasty techniques are associated with higher rates of infection but are generally not characterized as “instrumented” because they involve only the small plates used for expansion of the canal. The superficial infection rate for cervical laminoplasty ranges from 1.5% to 1.7%. ^, Wound dehiscence and deep SSI happen in <1% of cases.

Noninstrumented posterior spinal fusion is associated with a higher rate of infection than the aforementioned decompressive procedures, ^, as a result of longer operating times, greater blood loss, greater soft tissue destruction, and the placement of devascularized allograft.

In the cervical spine, single-level decompression with foraminotomy is associated with a low infection rate, with rates approaching 0% reported in the literature. More invasive posterior laminectomy can have higher infection rates than foraminotomy alone, reported at around 2%, although infection rates for laminoplasty and laminectomy and fusion are much higher. Anterior cervical surgery is usually associated with instrumentation, although on occasion anterior cervical foraminotomy or anterior cervical discectomy and fusion are performed without cervical plating. Infection rates for the anterior cervical approach, however, are extremely low with and without the use of instrumentation, making it difficult to discern any real difference between these two groups; similar findings have been shown for cervical arthroplasty, with most trials reporting a 0% infection rate.

Finally, relatively small interventional procedures such as chemonucleolysis or diskography are associated with an infection rate of up to 4% in the absence of preoperative antibiotics. Fortunately, this incidence can be dramatically decreased with the use of a two-needle technique and prophylactic antibiotics.

Instrumented Spinal Procedures

The use of instrumentation in posterior spinal procedures increases the incidence of postoperative infection to approximately 3% to 7% in many series. ^, Spinal instrumentation increases the risk of infection by acting as a locus minoris resistentiae , or a site of secondary seeding, for organisms, rather than as a source of inoculation. One study revealed that 11 of 21 patients undergoing hardware removal for noninfectious reasons had positive growth on cultures. Although most infections occur in the immediate postoperative period, a number of reports have shown that infections can occur years after surgery. Colonization of the implants is likely commonplace, and biofilm formation may offer protective effects for these organisms, predisposing patients to delayed infection.

The type of instrumentation may affect the probability of clinical infection. Older steel implants have a greater predisposition to colonization and delayed spinal infections. Furthermore, corrosion and fretting at cross-connector sites have been associated with foreign body reactions and the development of a local environment favorable for the growth of endogenous or low-virulence bacteria. ^{, , ,} This association has not been reported with newer titanium implants, which are resistant to corrosion and have a greater resistance to colonization.

Anterior instrumented spinal surgeries are associated with lower rates of infection, and when infections occur, they tend to be superficial. ^, The low incidence of infection with the anterior approach is likely due to minimal soft tissue trauma and subsequent muscle necrosis. Although the anterior approach itself is associated with a low risk of infection, the anterior-posterior approach to the spine has the highest rates of infection, a finding likely attributable to the greater length and complexity of these cases.

The indications for which minimally invasive techniques are available continue to grow, with the overarching goal of minimizing soft tissue trauma and blood loss and thereby hastening patient recovery. Results are positive for decreasing the risk of infection ^, ; using a cohort of 1442 cases of lumbar surgery, Mueller et al. found a sevenfold reduction in SSIs when comparing minimally invasive surgery with open surgery—with an overall 10-fold reduction for decompressive procedures.

Finally, the implantation of intrathecal drug delivery systems and spinal cord stimulators is associated with an approximately 5% risk of infection. ^, Infection with these devices occurs in the pump or stimulator pocket in many cases, although infection of the intraspinal component can lead to meningitis or epidural abscess. These infections tend to occur early, usually within the first 2 weeks to 2 months, and are treated by removal of the complete system.

Patient-Specific Factors

Important among patient-specific factors are medical comorbidities, including increasing age, obesity, diabetes mellitus, poor nutritional status, and alcohol and tobacco use. ^{, ,} Other factors associated with an increased risk of postoperative infection include steroid use, rheumatoid disease, and an immunocompromised state. ^{, ,}

Obesity is a frequent comorbid condition in the population of patients undergoing spine surgery and can lead to poor outcomes regardless of the indication, pathologic features, or technical aspects of the procedure. Several studies have demonstrated the increased infectious risk of obese patients undergoing spine surgery. ^{, , , ,} Obese patients are subject to longer operative times; greater amounts of retraction forces, which in turn cause increased soft tissue necrosis; greater amounts of poorly vascularized fatty tissue with decreased oxygen tension; decreased immune defense in adipose tissue; and poor tissue concentrations of prophylactic antibiotics. ^, In addition, overweight and obese patients have been found to have an increased risk of postoperative complications (e.g., superficial wound infection, pulmonary embolism) relative to patients with a normal weight. A study by Ye et al. demonstrated that patients with metabolic syndrome were more likely to have a superficial SSI and require reoperation; metabolic syndrome itself was an independent risk factor for superficial SSI (odds ratio [OR], 2.8; 95% confidence interval [CI], 1.4–5.7).

Malnutrition is a well-known factor that predisposes a patient to infection. It has been demonstrated to impair immune response and delay wound healing. Klein et al. reported that 25% of patients undergoing elective lumbar surgery had positive indices of malnutrition and that 11 of 13 infections occurred in these patients. Other authors have also reported a high rate of infection in malnourished patients undergoing spinal surgery as well as the development of malnutrition in some spinal surgery patients during their hospital stay, a particular concern for those undergoing staged procedures. Commonly used indices of malnutrition are serum albumin and total lymphocyte count, with values of less than 3.5 mg/dL and 1500/mm ³ , respectively, being considered abnormal. Malnutrition may also be related to malignancy and trauma, two conditions associated with higher rates of infection. Other indices, including skinfold thickness, serum transferrin levels, arm muscle circumference, and weight-to-height ratio, can also be used as proxy measures for nutrition.

Diabetes mellitus, associated with or without obesity, impairs wound healing and predisposes patients to SSI in spinal operations. ^{, ,} Postoperative wound infections have been reported to occur in up to 24% of diabetic patients undergoing spine surgery. ^, Proposed mechanisms by which diabetes contributes to infection risk include increased glucose concentrations in wound fluids, the presence of dysfunctional polymorphonuclear neutrophils and macrophages, impaired lymphocyte chemotaxis, and delayed wound reepithelialization. Impaired glucose tolerance without overt diabetes has additionally been correlated with wound infection complications. ^, Although studies of deep sternal SSI in cardiothoracic procedures have demonstrated an ability to reduce this risk with strict perioperative glucose control, no current study exists in the spine literature. ^,

Finally, tobacco use has been demonstrated as a risk factor for wound infection in several studies. Hypothesized mechanisms include deprivation of oxygen to tissues, impaired wound healing, and deficient neutrophil function.

Surgery-Specific Factors

Several surgical variables, other than those discussed earlier, may predispose patients to infection. Many of these variables correlate with the magnitude of the surgery itself. Therefore it is not surprising that the number of levels treated, length of surgery, procedural complexity, and amount of blood loss have all been associated with an increased risk of infection. ^a

a References 7, 33, 58, 73, 88, 89.

Operative times of greater than 5 hours and blood loss greater than 1000 mL are both associated with an increased rate of infection. ^,

The use of a Cell Saver system (Haemonetics, Braintree, MA) has been inconsistently correlated with infection risk. Although blood that has been processed by the Cell Saver system has been shown to be contaminated in 37% of various surgical procedures, no contamination was found in neurosurgical procedures. In addition, whereas the use of the Cell Saver system was correlated with infection in series of spinal patients, no increased risk has been noted in other specialties. The use of blood transfusion, however, has been correlated with infection in numerous studies, and this risk may be independent of the amount of blood loss. ^{, ,}

Other surgical risk factors include revision surgery, the use of allograft material, and surgery extending to the sacrum or pelvis, the latter of which may be attributable to urine and fecal contamination. ^{, ,} Finally, the presence of two or more resident surgeons being involved in the procedure was correlated with increased infectious risk in one study. Although it has not been completely explored, this variable is likely a reflection of the length and complexity of the procedure rather than a potentially modifiable independent risk factor.

Disease-Specific Factors

Infection rate has repeatedly been linked to the disease state of the patient. The presence of malignancy appears to be associated with the highest incidence of infection, reported to be as high as 20% in some series. ^, This rate has been reported to be even higher in patients undergoing radiation therapy prior to open surgery. The higher rate of postoperative infection in this population is multifactorial: poor nutritional status, long and complex surgical procedures necessary for spinal reconstruction, and use of adjunctive therapies including corticosteroids all contribute to the dramatically elevated risk of infection.

Traumatic spinal injury is also associated with a significantly higher risk of infection, especially in the presence of a complete neurological injury. ^, The risk in this group is multifactorial, including prolonged stay in the intensive care unit, urinary or fecal incontinence, and the need for complex instrumentation to stabilize the patient. However, early surgery and hypervigilance in these patients have reduced the rates of infection in more recent series to less than 5%.

Prolonged presurgical hospitalization and postoperative stays in the intensive care unit are also risk factors for wound infection. Blam et al. reported that patients staying in the intensive care unit for more than 1 day had a 6- to 13-fold greater risk of postoperative infection than patients who did not stay in the intensive care unit. Wimmer et al. showed that extensive presurgical hospital stay was significantly associated with infection.