Postoperative Infection of the Spine: Management and Outcomes at a Single Institution. A Retrospective Study

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SSI is one of the most important complications of spinal surgery in terms of clinical outcomes and patients’ health status.¹ It is a major reason for readmission after lumbar spine surgery.^2-5 The infection rate after lumbar spine surgery has been reported to range from 1% to 9%.^6-12 The rate of infection after instrumented spine surgery ranges from 0.7% to 12%.^2-17 Factors that increase the risk of SSI developing include type and duration of surgery, as well as several comorbidities: diabetes, smoking, obesity, malnutrition, and poor general condition.^7-18 Risks of infection depend not only on the factors mentioned above, but also on perioperative and postoperative conditions.^{2-5,8,12-15,19-23} Failure of the operating room ventilation system, the length of the surgery, staff coming and going in the operating room, postoperative dressings protocols, and scar length have been identified as risks for infection^{.3-5,7-11,15-18,23,24}

This retrospective observational study aimed to collect and analyze data on the incidence of postoperative SSI following a consecutive series of spine operations to evaluate the associated risk factors for such infections.

Study population and setting

Between March 2015 and March 2023, a total of 2559 patients underwent surgery for degenerative diseases, spine tumors, and traumatic spine injuries at a single clinical center. All patients provided informed consent for their surgeries.

Outcome definition

The diagnosis of SSI was made between the fourth and 30th postoperative days. SSI was defined using ICD-9 codes and clinical criteria, including elevated white blood cell count, high C-reactive protein levels, and signs of infection such as pain, fever, chills, tenderness, wound dehiscence, redness, swelling, and purulent discharge (Figure).

Predictor variables

The ASA score was used to assess perioperative risks and the risk of SSI. Standard institutional antibiotic prophylaxis was administered to all patients, with an enhanced protocol introduced in March 2019.

Ethical considerations

This was an observational study. The Research Ethics Committee of the Clinical Centre in Soyaux, France, confirmed that no ethical approval was required from an IRB and that written consent from participants was not necessary.

Statistical analysis 

Data were analyzed using SPSS version 22 (IBM Corporation). Analysis of variance testing with the Tukey honestly significant difference as the post hoc test was used to compare infection ratios across different surgical and ASA score groups. 

To ensure objectivity throughout the analysis, patient names and identifying information were kept confidential. No biases or conflicts arose during surgeries, data collection, or outcome analysis. This study adhered to a double-masked review process as per the IRB guidelines, in which the identities of the authors were concealed from the IRB members and the authors remained unaware of the IRB members’ identities.

Patient characteristics

Out of 2559 operated patients, 54 (2.11%) required readmission due to infections; all 54 patients underwent reoperation. The majority of infections were caused by gram-positive bacteria, with Staphylococcus aureus being the most frequent bacterium (56.82%). The most common isolated microorganisms were gram-positive bacteria, with S. aureus being the most frequent bacterium, in 31 of the 54 cases (56.82%). The next most common organism was Staphylococcus epidermidis, in 13 cases (24.1%). In 12 cases (22.2%), 2 or more bacteria were identified. Table 1 presents the isolated microorganisms.

All of the operated patients underwent revision surgery with the aim of obtaining sampling for bacteriological detection of microorganisms and cleaning the surgical site. Among the infected patients, 28 had undergone instrumentation surgery for the first procedure, while 26 had undergone surgery without instrumentation (Table 2). The infection rate was 1.9% for instrumentation surgery and 2.3% for non-instrumentation surgery, which was a nonsignificant difference. Among the 28 infected patients who had previously undergone instrumentation surgery, 10 had the material removed and 18 had the material left in place.

Predictor analysis

The risk of SSI increased with rising ASA score. The infection rates were 0.78% for ASA I, 2.22% for ASA II, and 2.88% for ASA III patients (Table 3). The difference between ASA I with ASA II and ASA III were statistically significant (P > .001). The likelihood of SSI escalated in correlation with an increasing ASA score. Infection rates were recorded at 0.78% for patients classified as ASA I, 2.22% for those categorized as ASA II, and 2.88% for ASA III patients (Table 3). The disparities observed between ASA I and both ASA II and ASA III were statistically significant. Therefore, it can be concluded that a higher ASA score corresponds to an elevated risk of infection.

Importance of ASA score and type of surgery in infection

Analysis of these data suggests that the risk of SSI increases with rising ASA score. As noted above, the SSI rate was less than 1% (0.78%) in ASA I patients and was 2.22% in ASA II patients. The rate rose to 2.88% in ASA III patients (Table 3). The ASA score could be considered a pivotal factor in identifying patients at high risk for SSI. This finding suggests that more stringent preventive care should be implemented for these patients postoperatively. These findings are statistically significant and are consistent with the literature.^25-27

The highest number of infections was observed in patients who had interlaminar devices plus diskectomies (12 cases), followed by interlaminar devices plus recalibrations (11 cases), bilateral recalibrations without instrumentation (9 cases), and unilateral recalibrations without instrumentation (8 cases) (Table 2). However, statistical analysis shows a lower infection percentage in patients operated on with interlaminar devices plus diskectomies (1.54%) and interlaminar devices plus recalibrations (1.93%). The highest infection percentage was observed in patients operated on with lumbar interbody cages (7 cases, with 1 infection [14.29%]), followed by posterior osteosynthesis plus interbody cages (19 cases, with 2 infections [10.53%]). The infection rate varied markedly among different surgical groups, with the exception of oblique lateral interbody fusion, transforaminal lumbar interbody fusion, anterior lumbar interbody fusion, and surgeries for lumbar or thoracic spine tumors, where the ratio was 0 and the sample size was small. The low case numbers in these groups may introduce bias in the percentage calculation. Among the 1128 patients operated on without instrumentation, there were 26 infections (2.30%). Among the 1431 patients operated on with instrumentation, 28 patients had infection (1.96%). There were no significant differences between the 2 groups. However, many studies show that spinal instrumentation is associated with an increased rate of SSI.^5,10,28-30 

Pull ter Gunne and Cohen⁵ reported an SSI rate of 1% in spine surgery without instrumentation and 9% with instrumentation. Gerometta et al reported a 1% SSI rate after simple diskectomy, increasing to 2% to 5% when there is a posterior spinal fusion.³⁰ Di Martino et al reported an SSI incidence ranging from 0.26% to 2.75% for spinal surgery without instrumentation and 2.1% to 8.5% for instrumented surgery.³⁹ Reoperated patients were followed for 1 year postoperatively and were evaluated at 1 month, 3 months, 6 months, and 1 year. There were no cases of relapse or discitis.

Microorganisms and antibiotic therapy

As soon as patients were operated on and samples were obtained, empiric antibiotic therapy was initiated for coverage of staphylococci, streptococci, enterococci, and gram-negative bacilli. In the first 34 cases, after sampling the patient received empiric dual antibiotic therapy consisting of gentamicin 1.5 mg/kg/12 hours plus vancomycin 15 mg/kg/12 hours. For the last 20 patients, this dual therapy consisted of piperacillin-tazobactam 4 g/8 hours and linezolid 600 mg/12 hours. Linezolid is a relatively new synthetic antibiotic with an effect on methicillin-resistant S. aureus.³² The authors of the current study found no specific advice on the use of linezolid in the literature, but its addition to piperacillin-tazobactam was effective and showed a reduction in C-reactive protein and other inflammatory signs. The use of linezolid has been reported in the literature without any real efficacy.^32-34 As soon as the antibiogram has been performed, the patient should receive empiric dual antibiotic therapy for 6 weeks. In the immediate postoperative period, the patient should undergo blood tests twice a week, a cardiac ultrasound, and lumbar imaging (ie, magnetic resonance imaging or, if that is contraindicated, computed tomography). Thereafter, biological monitoring should be performed weekly. However, Palmowski et al³⁵ recommend antibiotic therapy for 10 weeks. If the implant needs to be retained, a 2-week course of IV antibiotic treatment should be followed by 10 weeks of oral antibiotic therapy.

Dowdell et al³⁶ recommended antibiotic therapy for 3 months or more in cases of postoperative discitis/osteomyelitis. Kowalski et al³⁷ recommended treatment for 4 to 6 weeks. However, Clark and Shufflebarger⁴¹ reported successfully treating SSI with a 3-day IV course of antibiotics and 7 days of oral antibiotics, which resulted in complete resolution of infection. The authors of the current study have not found a precise protocol for empiric antibiotic therapy, but all the authors believe that IV antibiotics should be maintained until culture is obtained and that culture should begin after the debridement if the patient’s condition is stable. In the septic patient, broad-spectrum antibiotics should be started even before sampling.^{2-8,10,12-18,20,21,28-31,35-41} 

As noted above, S. epidermidis was the second most commonly identified organism in the present study, with 13 cases (19.12%), followed by Enterococcus faecium and Escherichia coli in 4 cases each (5.88%). In 53 cases (77.94%), the microorganisms found were gram-positive cocci. In 15 cases (22.06%), bacilli were responsible for the infection. The denominator represents the total count of identified germs, as there were instances where multiple germs were detected in a single patient. This indicates that the total number of germs exceeds the number of infected patients. Abdul-Jabbar et al⁴² found similar results, with S. aureus isolated in 45.2% and S. epidermidis isolated in 31.4% of 239 cases of SSI. Kobayashi et al⁴³ reported similar results, as well. 

Kobayashi et al⁴³ highlighted the climatic factor in infection, noting an SSI rate of 2.6% in the spring, 3.9% in summer, 1.3% in fall, and 1.8% in winter out of a total of 1174 patients. Durkin et al⁴⁴ also reported a significant worsening in the summer; notably, the rates of SSI due to gram-positive cocci and S. aureus were higher in summer. Analysis of the data in the present study does not confirm these findings: there were 13 infections in the spring (24.07%), 13 in summer (24.07%), 13 in fall (24.07%), and 15 in winter (27.78%). There were no yeast infections.

Several studies note the importance of prevention. Tomov et al⁴⁵ recommended application of intra-wound vancomycin powder, wound irrigation with dilute betadine solution, preoperative chlorhexidine gluconate scrubs, preoperative screening with nasal swabbing, and decolonization of S. aureus, as well as perioperative antibiotic administration. They reported an average 2% decrease in infection with these measures during the study period (2006-2016).⁴⁵ Meza et al¹⁷ reported a decrease in infection from 1.70% to 0.20% after betadine irrigation and intrawound vancomycin powder before closure became standard treatment. 

The lower back is a site of friction, characterized by thick skin and poor vascular perfusion. The feelings and experiences of the authors of the present manuscript led the authors to believe that these anatomic particularities may contribute to the onset of infection. However, there are no studies that discuss this.

This study has several limitations. Firstly, the small sample size for certain surgery types may affect the generalizability of the findings. Secondly, the research was conducted at a single center, which could limit the applicability of the results to other settings. Additionally, it was necessary to change the antibiotic regimen during management due to nonresponsiveness in some cases. Moreover, the timing of debridement was not precisely documented, and there is a lack of long-term follow-up data for the patients included in the study. These factors may affect the overall interpretation and robustness of the outcomes.

The current study aligns with other research regarding the microorganisms most commonly involved in SSIs. The authors’ protocol for managing SSI using empiric antibiotic therapy prior to organism identification proved to be effective. The ASA score can be a crucial indicator for identifying patients at high risk for SSI. Therefore, more rigorous preventive measures should be applied to these patients after surgery. Notably, no significant difference was found in infection rates between surgeries without instrumentation and those with instrumentation. This finding contrasts with the majority of the literature, which typically reports higher SSI rates following instrumentation. However, the results of this study were more favorable, indicating a lower infection rate in such cases.