Implanting Pedicle Screws Without Bacteria: How Do We Keep Track of Invisibles?

By Aakash Agarwal PhD

The latest prospective cohort by McClelland, et al. (2016) demonstrated that surgical site infections (SSI) occur at the higher end of 2 percent to 13 percent (i.e., 12.7 percent) and are egregiously underestimated largely based on retrospective data not subjected to the inclusivity of SSI as defined by the Centers for Disease Control and Prevention (CDC).1 

Furthermore, the widespread and indiscriminate use of local vancomycin powder has raised concerns of near-term antibiotic resistance, leading to establishment of global alliances against overuse of antibiotics taskforces. Despite concerns associated with vancomycin application immediately before surgical site closure, there is still no way to irrigate the screw-bone interface post-implantation.2 Therefore, in cases where the bio-dose of natural or administrated immunity is limited, any contamination present from implantation is permanent. This leads to deep bone infection, or hardware loosening due to encapsulation of biofilm between the bone and the screw.3-7 Nevertheless, if an opportunity presents to nullify the contamination of pedicle screws altogether, before the implantation, shouldn’t become the standardized practice for the sake of patient’s safety? 

The current communication seeks to provide well-quantified results through peer-reviewed data available in the literature, for spine surgeons to make an educated and ethical choice. Recent literature review has shown that both reprocessing (preoperative) and handling (intraoperative) of implants seem to lead to contamination of sterilized implants.8 Furthermore, Agarwal, et al. demonstrated in their two-staged approach research, the pedicle screws are being contaminated in two phases: preoperative phase and intraoperative phase, both of which could be mitigated by employing newer methodologies or standard of care.9-12

In the first phase, i.e. preoperative contamination, the pedicle screws undergo repeated bulk-cleaning with dirty instruments from the operating room (OR), leading to residue build up at the interfaces and possibly on the surfaces too. This, due to its concealed nature, remains unnoticed by the sterile processing department (SPD) or other hospital staff.9 Nevertheless, this can be avoided by using single-use pre-sterilized screws, which are becoming popular these days, with many countries issuing a ban on repeated implant reprocessing.13

In the second phase, i.e., intraoperative contamination, the sterile pedicle screw shafts (in the sterile field) are directly touched by the scrub tech with soiled (assisting the surgeon dispose the tissues from the instruments in use) gloves for loading onto an insertion device/screw driver.10 It is then kept exposed on the working table (either separately or next to the used instruments as the pedicles' holes are being prepared). Their investigation shows that by the time it is implanted in the patient, it can harbor up to 10e7 bacterial colony forming units.10-12 The study also included a treatment group, which in essence is a functional impermeable sterile-sheath around the sterile-implant, which shields the pedicle screws intraoperatively until it is implanted into the patient.10-12 Use of the sheath resulted in zero contamination, thereby establishing itself as a precautionary measure against possible SSI or subclinical and chronic sepsis leading to screw loosening and pain. 

There also exists an analogous study from three years ago where the surgical team showed significant reduction in SSI rate by changing gloves every time before anyone touched the screw.14 In theory, adoption of this practice would require universal education and is dependent upon the consistency and compliance of every individual surgeon. Even then, a pedicle screw is held by a scrub tech during unwrapping and attachment to an insertion device, followed by its placement next to other dirty surgical instruments, surfaces or open-air. In contrast, the use of intraoperative-sheath provides uninterrupted protection from all the aforementioned elements.

Figure 1: Schematic showing the binary nature of bacterial dose on the pedicle screw, from us-ing or not using the intraoperative-implant-guard

Figure 2: Schematic showing the pathogenesis of SSI, highlighting that the only preventative measure in our control is dosage

Figure 1 shows a schematic of the process and the data. Figure 2 shows the schematics for pathogenesis of SSI. Factors that define the pathogenesis of SSI are the virulence, host-site immunity, and dosage.15 The virulence is the microorganism’s ability to infect the host. Although many bacterial species have been identified to cause SSI, the most common ones, Staphylococcus epidermidis and Staphylococcus aureus, are always present at the vicinity as part of a patient’s own flora. In addition, they have the potential to form biofilms, secluding itself from macrophages or other immune responses at the host site. The host sites in spine surgery are the pedicles of the vertebrae. This in combination with availability of metal surface (pedicle screws) provides a conducive environment for the bacteria to grow. Lastly, the dose dictates how much bacterial bioburden the “sterile” implant carries, after handing and at implantation. 

Among these, the only factor that we can really control at the screw-bone interface is the bacterial dosage, which essentially is at the core of aseptic surgical philosophy. Therefore, shouldn’t some form of intraoperative-implant-guard be a standard in spinal fusion surgery, to fight the one of the key pathogenesis of SSI?

Clinicians are further encouraged to reflect on the questions below in sequence:
1. What does it mean for a practicing surgeon?
2. What would your patient choose considering this information? 
3. Should “lower” infection rates be an excuse or a reason to knowingly put bacteria in patients?
4. Is the surgical site infection rate low? McClelland, et al. (2016) says 12.7 percent SSI rate in a PCT.    
5. Think about the spinal deformity surgery. Why is the SSI rate higher in them? Is it more implants or longer exposure time? May be both!
6. Do you know that bacteria at screw-bone interface causes screw loosening? Leitner by et al 2018
7. Could this help reduce antibiotic administration long-term, therefore combating the bigger war against antibiotic resistance?

Aakash Agarwal, PhD, is from the Engineering Center for Orthopedic Research Excellence (ECORE) in the Department of Bioengineering and Orthopedics Surgery, Colleges of Engineering and Medicine, at the University of Toledo.

References:
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