16 Postoperative Peri-Implant Spine Infections
Introduction
Postoperative spinal infections are one of the most troublesome complications. Despite advances in drug development, innovation of surgical technique, and postoperative care, wound infection is still one of the major threats in spine surgery. 1 Postoperative spinal infections are responsible for significant morbidity and mortality, 2 and are related to adverse postoperative events such as nonunion, neurologic sequelae, chronic pain, and deformity. The National Nosocomial Infections Surveillance Summary System of the Centers for Disease Control and Prevention (CDC) recently reported rates of surgical site infection (SSI) in spine surgery ranging from 0 to 20%, based on different definitions of infection. 3 These fluctuations depend on the indications for surgery, the anatomic site, the approach, the presence of instrumentation, the type of infection, and the extent of the infection. 1 Moreover, SSIs are multifactorial, and risk factors include microbial-related, patient-related, and procedure-related factors. From a financial perspective, postoperative infections in spine surgery are responsible for at least a two-fold increase in health-related and social costs. Readmission and reoperation rates have been estimated to be around 65% in a recent publication from the Surgical Infection Society. 2
An established risk factor for postoperative infection in spine surgery is the use of implants. 4 This chapter discusses the best evidence-based measures that should be adopted to identify, correctly manage, and finally minimize infection rates in instrumented spine surgery.
Some guidelines are available for antibiotic prophylaxis 5 and for antisepsis. 6 But some questions have not been adequately addressed in the literature, such as the following:
Is surgical debridement always required, and when?
When is removal of instrumentation required?
What is the ideal postoperative care and monitoring for postoperative peri-implant infection patients?
What are the main risk factors for postoperative infections?
We address these issues as well as the best evidence available in the literature. We also discuss a complex case from our practice.
Diagnosis
Early Postoperative Infection
Postoperative peri-implant infections can present in a very clear, evident way or in a more subtle, hidden setting. According to the CDC standard definition of SSI, all the peri-implant infections are “organ/space” infections 7 because they usually extend beyond the underlying fascia and muscle layers, with the involvement of the posterior bony elements of the vertebrae, of the zygapophyseal joints, and sometimes suspicion of the intervertebral disk. The clinical suspect of peri-implant infection can arise from one or more of the following observations:
• Persistent pain at the surgical incision
Fluid collection under the wound, with or without purulent drainage
Slow wound healing
Among these, pain is the most frequent clinical finding, 8 with a reported prevalence of almost 100%. Fever is not a significant sign for peri-implant infection.
The most frequent causative organism is Staphylococcus aureus (85%), and among these patients, there is a significant percentage of methicillin-resistant bacteria (30%), especially after revision surgery. Gram-negative organisms are less frequently involved. Presumably infection from gram-negative agents can be related to surgery in the lower lumbar spine and sacroiliac area.
Diagnostic workup of peri-implant infection involves laboratory investigations and imaging studies.
The white blood cell count is an unreliable indicator of infection, because altered levels could be the effect of drugs (e.g., corticosteroids) or hemodilution, for example. The acute-phase reactants C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR) are more useful for diagnosing infection but must be interpreted with respect to the time since the index surgery. ESR remains altered for up to 6 weeks after surgery, whereas CRP levels normalize within 2 weeks. Hence, abnormal CRP levels in the follow-up of a spinal instrumented surgical procedure should always be monitored and investigated, 8 and CRP is considered the more sensitive indicator of the presence of SSI.
As a consequence, imaging is usually performed to get a deeper insight, and in the vast majority of cases magnetic resonance imaging (MRI) is the first choice. Contrast-enhancement improves MRI accuracy in detecting infection. Interpretation should take into account the time from index surgery, because tissue disruption and subsequent edema and blood collection could mimic infection-related signs.
In patients with absolute contraindications for MRI, a computed tomography (CT) scan is the best option. Radiographs are not as useful as MRI or CT scan, but in some cases of delayed or chronic infections they can be informative. The presence of radiolucency around the screws, bone erosion, end-plate disruption, or implant pullout in the absence of any mechanical major issue or together with clinical features can be highly suggestive of peri-implant infection.
Delayed Postoperative Infections
Ninety percent of infections occur early in the postoperative period. Only 10% are delayed; that is, they occur 3 months or longer after surgery. 9 There are two main pathogenetic hypotheses for this delay:
Atypical agents and uncommon pathogens spawn the infection slowly and insidiously, making the diagnosis difficult and delaying it. The infection may sometimes be taking place since the early postoperative period but remains unrecognized. The most frequently responsible agent is Propionibacterium acnes. 10
The implant is colonized by hematogenous seeding, resulting from a distant invasive procedure with transient bacteriemia (dental surgery, gastroenteric, or urinary endoscopy).
There is scientific evidence of hematogenous infection in hip and knee arthroplasty. 11 The consensus guideline of the American Academy of Orthopaedic Surgeons strongly recommends performing antibiotic prophylaxis before any invasive medical procedure for patients with total joint prosthetic implants. 12 Conversely, no guidelines are available for similar procedures in patients with spinal implants. Lewkonia and colleagues 13 retrospectively reviewed patients with delayed perioperative infections, and sought a possible cause–effect relationship between invasive medical procedures and infection. At the same time, they submitted a survey to a group of selected, expert spine surgeons, asking about the administration of antibiotic prophylaxis for an invasive medical procedure in patients with spinal implants. With all the limitations due to the retrospective nature of the study, it seems unlikely that hematogenous seeding would be responsible for delayed infection. This is in line with the experts’ advising against the prophylaxis in spinal patients before invasive medical procedures.
Interestingly, it has been shown that delayed infections are closely related to scoliosis surgery, but not with the number of fused levels. This could be related to such features of scoliosis surgery as the need for blood transfusion and the use of bone allograft, for example. 14 Moreover, delayed peri-implant infections are frequently culture-negative. The typical bacteriological profile of them, in fact, is made up of less virulent pathogens.
Table 16.1 shows the time to onset of infection from the index surgery in postoperative spine infections.
Treatment
Debridement
When a peri-implant infection is diagnosed, immediate washout and surgical debridement with intraoperative sampling should be performed. 15 , 16 The use of drains is supported by our personal experience and by the available literature. Some surgeons routinely use continuous irrigation to avoid repetitive surgical debridement, 17 but in our experience a single washout procedure has been shown to be effective for nearly all the early peri-implant postoperative infections. The advantages of this quick, simple procedure are as follows:
It provides the microbiological information necessary to correctly target the antibiotic therapy. Intraoperative samples are the gold standard for the laboratory identification of pathogens.
The removal of purulent material and necrotic avascular tissue facilitates drug delivery and eradication of infective foci.
The placement of a drain facilitates wound closure and healing, and reduces the fluid collection and the subsequent bacterial regrowth.
Evidence-based data support the improved outcome when surgical washout and debridement for early infections are performed immediately after an infection diagnosis. 18
For delayed infections, the same procedure has shown lower efficacy, and there is a slight need for implant removal. This is probably related to the presence of a structured, resistant biofilm.
Thus, the answer to the question about when to perform surgical debridement in cases of diagnosed postoperative peri-implant spinal infection is as soon as the diagnosis is made. The success rate is high for early (within 1 month from index surgery) postoperative infections, but even in delayed infections, it is important to obtain intraoperative samples to correctly address pathogens.
Implant Retention/Removal
It is debated whether implant retention or removal would be the better solution for postoperative peri-implant infections. There are important differences between early and delayed peri-implant infections management.
Early
The available literature favors retention of implants in early infection. 16 , 18 – 21 Some interesting data recently reported by Kanayama and colleagues 22 provide initial evidence that in cases with clear osteomyelitis or vertebral abscess, implant removal is mandatory from the very beginning. These authors, in their retrospective review on 1,445 patients, found that patients with peri-implant infection with a positive MRI for the aforementioned findings were more likely to undergo repetitive failed surgeries, with progressive bone destruction and a nonunion rate of 75%. Removing the hardware seems to be the most appropriate step for these patients, and the authors feel that it should probably have been adopted from the first surgical revision procedure. Implant exchange is a suitable option in this situation, in cases of a severe clinical presentation (i.e., acute sepsis) after a careful evaluation of the risk-benefit balance for the patient, especially in deformity correction or severe postsurgical instability (i.e., three-column osteotomies). Interbody devices are usually left in place. The need for removal of an interbody device is usually a consequence of antibiotic therapy failure, because the risk-benefit profile is, in cases of early infections, against the revision of the interbody device.
In our experience, we have successfully retained instrumentation in all the early peri-implant postoperative infections.
Delayed
The general consensus in the literature is that implants should be removed in cases of delayed infections. 20 , 23 – 25 Attempts at hardware retention lead to high infection recurrence rates. With hardware removal, successful eradication of infection is more likely to be achieved. In the majority of cases, the infection is responsible for nonunion, so implant removal is related to an increased risk of progressive deformity, with the need of further corrective surgery. This potential complication should always be considered, and patients should be informed about the possible need for a reoperation for a deformity correction in the future.
A recent retrospective case-control study by Tominaga and colleagues 26 focused on the risk factors related to unavoidable hardware removal. They found that revision surgery with multiple previous procedures, low preoperative hemoglobin (Hb) values, high preoperative serum creatinine values, and the presence of methicillin-resistant Staphylococcus aureus are statistically related to hardware removal. Another parameter known as the postoperative infection treatment score for the spine (PITSS) has been found to be related to implant removal. This scoring system tries to identify those patients at risk for multiple, repeated surgical washout and debridement procedures. The predictive model has been validated over a 4-year period, and the results have been published by DiPaola et al. 27 The scoring system in summarized in Table 16.2 . A score between 7 and 14 is defined as low risk for repeated surgical debridement; between 15 and 20 is defined as undetermined risk; 21 or above is high risk, which correlates with the likelihood of hardware removal. Patients with clearly unstable segments but with a likelihood of hardware removal can be selected for hardware exchange, as follows:
Remove the existing colonized implants with their biofilm.
Preserve neurologic function, preventing injuries secondary to instability.
This is a halfway choice in cases of delayed infections, but to be seriously considered when the neurologic risks of hardware removal are high.
An interbody device in late infections should be managed similarly. After removal of the colonized device, infected tissues can be harvested and a bone graft or trabecular tantalum cage should be used.
In our experience the only patient who has been selected for implant removal was a paraplegic teenager, whose case follows.
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