Introduction

The use of spinal instrumentation is on the rise. Various instrumentation systems have been developed in an effort to manage a myriad of spinal pathologies and trauma, and to perform deformity correction, stabilization, and fusion of the spinal column. These instrumentation systems take different forms and serve different biomechanical functions. Constructs such as pedicle screws and rods, plates and screws or hooks, and sublaminar wiring provide stability, whereas titanium mesh, expandable cages, and polyetheretherketone (PEEK) interbody fusion cages lend structural support. Artificial disks are the most complex form of spinal instrumentation in terms of their material composition and their multiple interfaces. Though various spinal instrumentation systems have helped patients achieve structural support and stability, difficulties arise when spinal infections occur.

We may classify infection of the spine, based on the site of infection, as spondylodiskitis, diskitis, epidural abscess, septic facet arthropathy, and vertebral body involvement. These entities are not mutually exclusive, and patients may present with infection at several sites. Not uncommonly, the spinal column is rendered unstable, resulting in pain, deformity, and possibly neurologic insult when significant bony destruction has occurred.

The discussion of instrumentation and spine infection can entail two clinical scenarios, namely the use of instrumentation in the vicinity of active de novo spine infection, and the use of spine instrumentation and its associated potential consequence of surgical site infection. The discussion about infection typically revolves around whether the causative organism is of tuberculous origin or is a pyogenic organism. With regard to implant retention versus implant removal in the presence of postoperative surgical site infection, there are many important clinical issues to be addressed. New research on the use of implants in the presence of active spinal infection will help improve the management of these patients.

The response of the human body to an implant is still not fully understood, and the interaction of the microorganism with both the material and the host adds another layer of complexity.

Relationship Among Implants, Host, and Microorganisms

The presence of foreign material in the human body is known to elicit a host response. Once foreign material is implanted in the body, a layer of plasma proteins, such as fibrinogen, albumin, and fibronectin, coats the material’s surface, leading to acute inflammation. The inflammation then becomes chronic, and a fibrous encapsulation of the material occurs as a result of a foreign body reaction. This fibrous capsule, being relatively avascular, confers limited access to both systemic antibiotics and the host immune defense.

The inception of biomaterial-associated infection begins with the colonization of an implant surface with bacteria, which can occur both perioperatively in the operating room as well as postoperatively as a consequence of hematogenous seeding. The race for the implant surface between the microorganism and the host immune defense commences immediately following implant placement within an area of active infection intraoperatively. In the case of postoperative contamination that occurs after spinal instrumentation, the microorganism may adhere to the protein molecules on the implant surface, forming a biofilm that is resistant to antibiotics.

The affinity of bacteria for any surface is affected by the material’s topography, especially its surface roughness, and the material’s surface chemistry, notably the wettability. ¹

The study of surface affinity of bacteria is strain specific, because the strain’s physical size, surface topography, and adhesion molecules vary. The presence of protein molecules in vivo also significantly alters bacteria adhesion to the material.

One strategy to reduce bacteria adherence to biofilm is to coat the PEEK implant with titanium dioxide and polydimethylsiloxane (PDMS), which enables the regulated release of antimicrobial silver. The hybrid coating also inhibits bacterial growth and biofilm formation.

Asymptomatic bacteria colonization of the cervical plate used in the treatment of vertebral osteomyelitis has been documented in patients whose implants were removed after healing of the infection. ² However, these patients did not exhibit active ongoing infection of the spine. It appears that the presence of microorganisms in the biofilm can be limited to only the implant surface, avoiding the adjacent tissue environment.

Thus, the fibrous capsule that renders the area relatively avascular is formed only on the implant (pedicle screw and rods) surface and does not surround the entire area of infection. Since the biofilm is formed only around the implants, an adjacent focus of osteomyelitis with adequate blood supply can progress to healing, unaffected by the implant biofilm. Infection of the artificial disk replacement, on the other hand, poses a different problem, as the focus of infection is encased within the fibrous capsule itself.

Postoperative Infection Following Spinal Instrumentation Surgery

Infection following spinal instrumentation surgeries differs from instrumentation in the presence of de novo spinal infection, in several ways. The causative microorganism in the former scenario, since it is hospital-acquired, commonly involves multiresistant strains, such as the methicillin-resistant Staphylococcus aureus, or opportunistic microorganisms, such as Staphylococcus epidermidis or fungal infection. As well, the host in the former scenario patients generally are healthier than patients with a pyogenic spinal infection.

Spine surgery with instrumentation is associated with a higher risk of surgical site infection than is spine surgery without instrumentation, for several reasons. Spinal instrumentation surgeries tend to be more complex than noninstrumented spine surgeries, involving longer operating time, more soft tissue injury, greater blood loss, and more operating room personnel. The presence of foreign materials in the surgical site may dampen the host defense mechanisms. As well, the physical presence of hooks, pedicle screws, and rods in posterior spine surgery increases the dead space in the surgical wound and could thus predispose to surgical site infection.

Diagnosis of a spine surgical site infection may be challenging at times. The classical signs of increasing wound pain, wound edge redness, wound discharge, and systemic effects of infection may be absent. Laboratory results may be equivocal, making it difficult for clinicians to differentiate between postoperative wound inflammation and surgical site infection.

Imaging modality utilization in cases of suspected surgical site infection with implantation can be an arduous undertaking. In chronic infection with bony erosion, a plain computed tomography (CT) scan may be diagnostic. In early surgical site infection, however, imaging may not be very helpful. Although magnetic resonance imaging (MRI) has been used to detect early peri-implant osteomyelitis or intervertebral abscess, imaging artifacts from the implants coupled with postoperative inflammatory changes often complicate making a diagnosis, further defeating the value of imaging in the detection of early spine surgical site infection. Recently, ¹⁸F-fluorodeoxyglucose (FDG) positron emission tomography (PET) scanning has been advocated as the imaging most likely to confirm the presence of infection in the instrumented spine. ³

The general algorithm for treatment depends on a variety of factors, including the time delay from the index procedure, the infecting microorganism, the location and extent of the infection, the strength of the fusion mass, and the stability of the spine prior to the instrumentation.

Retention of spinal instrumentation or implant exchange following wound debridement for postoperative surgical site infections is generally attempted in patients who presented within 3 months of the index surgery, ⁴ because the intended fusion has yet to take place. However, the continued presence of instrumentation in these patients predisposes them to recurrent infection. Repeated wound debridement or prolonged wound suction drainage (open or closed) may thus be necessary to treat patients with recurrent infection. ⁵

Removal of instrumentation after surgical site infection of spine surgery is called for when the infection persists despite repeated serial debridement.

Late-onset surgical site infection is usually treated with wound debridement and removal of the implant because fusion has taken place. There may be possible progression of spinal deformity after removal of the implant in patients whose spine has already solidly fused.

Instrumentation in Tuberculous Infection of the Spine

After it was discovered that Mycobacterium tuberculosis cannot form a biofilm on the implant, as compared with a pyogenic micro organism, ⁶ various types of implant (stainless steel, titanium, and PEEK), including anterior interbody fusion cages and pedicle screws, have been used to treat spinal tuberculosis in order to correct the deformity, achieve spinal stabilization, and enable early mobilization of these patients.

Recently, posterior instrumentation of the spine with tuberculous infection without debridement was shown to be an effective treatment, demonstrated by the eradication of infection and solid fusion of the anterior column (tuberculous spondylodiskitis foci). ⁷ This suggests that stabilization of the spine leads to higher fusion rates and healing of the lesions. Posterior instrumentation of the spine in this regard would have to involve multiple segments so that it can provide adequate cantilever support to the deficient anterior column until solid fusion takes place.