Implant-Related Management of Surgical Site Infections After Spine Surgery

Peer Reviewed

Removal or replacement of spinal implants in the management of late-onset surgical site infection is preferable over strategies to retain the implant, according to findings from a recent literature review by Aakash Agarwal, PhD, and colleagues published in Global Spine Journal.

The researchers’ review of retrospective or prospective studies published between 2000 and 2018 (N=49) showed that methicillin-resistant Staphylococcus aureus (MRSA), methicillin-resistant Staphylococcus epidermidis (MRSE), coagulase-negative staphylococci, and Staphylococcus aureus, followed by methicillin-resistant coagulase-negative staphylococci (MRCNS); Aspergillus fumigatus, and gram-negative, anaerobic bacteria such as Propionibacterium acnes were the most common organisms identified in surgical site infections. The surgical site infections reported were most commonly deep infections (relative distribution, 45%-100%), compared with superficial infections (13%-55%).

biofilm found on spinal implants under electron microscope. Image courtesy of Aakash Agarwal, PhDFigure. Biofilm found on spinal implants under electron microscope; yellow circles show individual bacteria. Image courtesy of Aakash Agarwal, PhD.Figure. Biofilm found on spinal implants under electron microscope; yellow circles show individual bacteria.Image courtesy of Aakash Agarwal, PhD.

To learn more about the rates of infection, time to onset of infection and effects of implant removal versus retention for management of infection, SpineUniverse spoke with Dr. Agarwal, who is Director of Research at Spinal Balance Inc. and an Adjunct Professor of Bioengineering at University of Toledo.

What are the key clinical pearls from your review on surgical site infections after spine surgery?

Dr. Agarwal: The first key clinical implication is that the prevalence of infection after spine surgery may be as high as >10%, and can occur up to 3 to 5 years after surgery. Most spine care providers may not be aware of this high rate because reimbursement policies and short-term studies only examine rates at 30 to 90 days after surgery. Thus, when most clinicians discuss surgical site infections, they are referring to these early-onset infections, which account for up to 50% of all reported surgical site infections. In reality, postoperative infections could be divided into early-onset, delayed-onset, and late-onset infection.

Delayed-onset infection can occur from 90 days to a year from the date of surgery, and constitutes between 15% and 35% of all reported infections. Late-onset infection (which occurs after a period of 1 year from surgery) is the least studied infection type due to lack of long-term follow-up periods. However, the few longer-term studies (>6 years) which considered late-onset infection have shown an average time to infection detection of 56 to 80 months post-surgery with a total incidence of 9.7%.

Secondly, patients may have undiagnosed infection, also called sub-chronic infection or occult infection, following spine surgery. Clinical studies from multiple groups in US and Europe, including our ongoing trial (Figure), show that a significant number of patients undergoing revision surgery for loosened implants actually have bacterial growth around the screw, with no known clinical symptoms of infection.1-3

To explain in brief, impregnated bacteria from contaminated implants can either lie dormant or results in immune response (some of which can cause early surgical site infection). If the bacteria lie dormant, they continue to hide from our immune system and thrive via biofilm formation on the implant. This biofilm can break, and the overgrown colonies of bacteria released can cause delayed or late-onset infection response (systemic in nature) at a later stage. In some cases, instead of a systemic infection response, there could be a local response where the implant and bone begin to disassociate, leading to screw loosening and failed fusion. This is what we have begun to call an occult infection phenomenon.

The review of studies concluded that removal or replacement of implants for late-onset infection is the preferred management strategy in most cases. Any attempt to retain the implant only places the patient at high risk of recurrent infection and biofilm growth. I would even recommend considering replacement of implants for delayed infection, perhaps using a risk-stratification approach based on patient comorbidities. Also, given this long-term risk of infection and biofilm susceptibility of implants in situ, repeated reprocessing and intraoperative exposure of permanent orthopaedic implants (which makes them contaminated before implantation) is deleterious and an unjustified risk to patients.

Why is there so much variability in infection rates following spine surgery reported in literature?

Dr. Agarwal: There are many factors that could lead to variability in surgical site infection rates in spine surgery reported in the literature including the following:

  • Patient demographics
    • Presence or absence of one or several comorbidities (eg, diabetes mellitus, chronic obstructive pulmonary diseases, coronary artery disease, osteoporosis, arrhythmia, chronic liver disease, autoimmune disease)
    • Poor nutrition and lifestyle choices, such as smoking and obesity
  • The number of patients enrolled in the study
  • Duration of follow-up: most studies examining infection rates immediately after surgery don’t have long-term follow-up data available
  • Type of study (retrospective vs prospective) and variable criteria for reporting surgical site infections
  • Inclusion criteria: trauma and deformity patients have a higher risk of infection than 1-2 level degenerative or spondylolisthesis cases; although, it is unknown if this difference is the result of high implant density or excess of exposure time, in either case the microbial bioload on the implant and patient increases
  • Surgical factors, including duration of surgery, approach, procedure, and invasiveness of a procedure
  • Use of preoperative radiation and postoperative blood transfusion, which have strong association with postoperative infection
  • Quality (training) of staff and experience level

What is the cost of readmission after surgical site infection?

Dr. Agarwal: The built-in cost of infection per spinal surgery in most hospitals ranges from $1100 to $4400, and each single revision surgery due to infection cost is estimated at between $22K and $63K.4 Given how often spinal fusions are performed, even a relatively low infection rate represents a large patient population. This means that with a rise in the number of spine surgeries, we have to be much more vigilant about reducing the sources of infection.

Is there anything else you would like to tell SpineUniverse readers about the review findings?

Dr. Agarwal: When we talk about the future of spine surgery, we think of artificial intelligence, navigation, low-dose radiation, less invasive surgery, less blood loss, better patient selection, ambulatory surgery centers, and so on. The common characteristics of all of these advances is our focus on patient safety, driven by either precision or customization in health care product and services.

However, what about the current impediments to patient safety including the delivery of contaminated implants, and the open-ended risk of infection (or implant failure) for years ahead after orthopaedic surgery? Ignoring these issues are a blatant oversight in the world of orthopaedics, to say the least. I notified the FDA of this hazard in 2018, not sure how long they will continue to “process” my petition for, but with every day we delay transitioning to “two-step asepsis”, we continue to put patients at risk.

Could you tell our readers more about the ongoing trial you mentioned on implant loosening and “aseptic” revision spine surgery?

Dr. Agarwal: Previous studies using laboratory culture techniques indicated the presence of bioload in “aseptic” revision surgery. These were considered “aseptic” only because common clinical signs of infection were not present on diagnostics. Our ongoing study is using electron microscopy to capture real-time architecture of the biofilm at the screw-bone interface, in patients undergoing revision surgery.

Our preliminary data shows that 77% of pseudarthrosis cases presented with loosened explants, and 72% of cases showed biofilm on explants. We also found that areas with biofilm tested negative for calcium phosphate whereas areas without biofilm tested positive for calcium phosphate. In addition, intraoperative swabs of tissue around the screws showed no infiltration of bacteria bioload, and always tested negative, thereby showing the inapplicability of swabbing in cases with intact biofilms.

Radiographically there was a halo in most cases (not always though) identifying the loosened implants, and the same implants under electron microscope (Figure) showed biofilms.

Looking at the results of this ongoing study we are realizing that many incidences of screw loosening could be a result of biofilm that failed to clinically present as late-onset infection. This highlights the importance of keeping the implant-bone interface devoid of contaminants. It also sheds light on the surreptitious dynamics of implant and impregnated microbes, thus explaining the sudden onset of delayed and late infection responses. I would direct scientifically curious readers to an article from Nature, in which the authors (using a mouse osteomyelitis model) showed that Propionibacterium acnes was absent from the control group (no implants) 6 months after bacterial inoculation, but in the implant group, the bacteria had survived as biofilm around the implant.5

We hope to present the preliminary results of this ongoing study later this year.

Dr. Agarwal is a Consultant/Independent Contractor for Spinal Balance Inc.; on the Editorial Board for Clinical Spine Surgery (Lippincott Williams & Wilkins, LLW) and Spine (LLW); and is an advisory board member for the Center for Disruptive Musculoskeletal Innovation (CDMI).

Updated on: 02/28/20
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Reprocessing of Pedicle Screws and Exposure in Sterile-Field Leads to Infection in Spinal Surgery
Aakash Agarwal, PhD
Director of Research
Spinal Balance Inc.

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