The Microbe Matters More Than the Months: New Study Rethinks Antibiotic Duration for Spine Implant Infections

Deep infections that take hold around the metal rods, screws, and cages surgeons implant to stabilize the spine are among the most feared complications in modern spinal surgery. They carry mortality rates exceeding 10 percent, force patients into prolonged hospitalizations, and often demand multiple rounds of surgical revision. Now a large observational study from Italy, known as the DeFEndO study, has delivered a finding that challenges one of the field’s most persistent assumptions: that longer courses of antibiotics automatically translate into better outcomes. According to the research published in the Journal of Bone and Joint Infection, whether patients received roughly eight or twelve weeks of antimicrobial therapy made no significant difference to their chances of cure. What mattered instead was which microorganism had colonized the implant and which antibiotics were deployed against it.

The study, led by Renato Pascale and Sara Tedeschi of the University of Bologna together with colleagues at the IRCCS Istituto Ortopedico Rizzoli and Maria Cecilia Hospital, followed 161 adults diagnosed with deep surgical-site infections following spinal instrumentation between January 2018 and June 2022. These infections, abbreviated DSII, involve tissues beneath the fascia surrounding implanted hardware, and their management is uniquely difficult. The implants themselves provide a surface on which bacteria can form biofilms, those resilient, glue-like microbial communities that shield pathogens from both antibiotics and immune cells. Standard care therefore combines surgical debridement, the physical removal of infected tissue, with prolonged antibiotic therapy, and the choice between retaining, replacing, or removing the hardware is individualized according to spinal stability and the severity of infection.

The patient population reflected the demanding reality of complex spine surgery. Participants had a median age of 62 years, a median Charlson comorbidity index of 3, and more than half were men. Degenerative disease and scoliosis each accounted for about 29 percent of the original operations, but a striking 19.3 percent had undergone instrumentation because of cancer. Infections typically declared themselves early: 70.2 percent were diagnosed within the first three months after surgery, with a median interval of 27 days between the primary operation and the diagnosis of infection. Wound dehiscence, the reopening of the surgical incision, was the most common warning sign, occurring in 59 percent of cases. Polymicrobial infections, in which several bacterial species are present at once, made up just over a quarter of the cohort.

Despite meticulous care at two referral centers for bone and joint infection, treatment failure was common. Fifty-one patients, or 31.7 percent, ultimately failed treatment, an outcome defined by wound reopening, the appearance or recurrence of a fistula, new pain or functional decline after apparent resolution, or death attributable to the infection. Among the failures, wound reopening dominated, occurring in nearly two-thirds of cases, while 11 patients developed fistulas and three died. The figures are a sobering reminder that even in high-volume centers with dedicated infectious disease consultants, roughly one in three patients with an infected spinal construct will not be cured by the first combined medical and surgical assault.

When the researchers dug into what separated the failures from the cures, one microbial distinction stood out with statistical force. Gram-positive bacteria, the traditional culprits in implant infections, caused 60.3 percent of cases, but the 33.8 percent of infections driven by Gram-negative rods, including Pseudomonas aeruginosa and Escherichia coli, were disproportionately linked to bad outcomes. Gram-negative etiology proved to be an independent risk factor for treatment failure in two separate multivariable analyses: the odds of failure were roughly four times higher for these infections whether the treatment window was framed as eight weeks (odds ratio 4.20) or twelve weeks (odds ratio 4.46). This aligns with a growing body of evidence from prosthetic joint infections suggesting that Gram-negative pathogens are inherently harder to eradicate from foreign material, a fact that current understanding of spinal implant infections, still surprisingly thin, has only begun to absorb.

The second headline finding concerns quinolones, the antibiotic class that includes ciprofloxacin and levofloxacin. Patients whose regimens included a quinolone had dramatically lower odds of treatment failure, with odds ratios of 0.11 in both the eight-week and twelve-week analyses, meaning the odds of failure were reduced by nearly 90 percent in this observational dataset. The biological rationale is plausible: quinolones penetrate bone well, are active against many Gram-negative rods, and can be administered orally for months. Yet the investigators themselves urge caution. Because quinolones were likely reserved for patients with susceptible pathogens and optimized targeted therapy, the association may reflect careful antimicrobial selection within a higher-risk subgroup rather than a genuine causal benefit. Residual confounding, they acknowledge, cannot be excluded in a retrospective design.

Perhaps the most consequential result is the one that did not appear. When the team compared treatment schedules of eight versus twelve weeks, excluding patients who failed early during therapy to isolate the effect of a completed course, they found no significant association between duration and outcome. To guard against a subtle statistical trap known as immortal time bias, in which patients must survive long enough to complete a long regimen before being counted as such, the researchers also performed prespecified landmark sensitivity analyses at eight and twelve weeks, restricting comparisons to patients still at risk at each time point. Those analyses, conducted on 90 and 75 patients respectively, confirmed the primary result: longer was not demonstrably better. The median treatment duration in the cohort was 82 days, and the question of whether some patients could safely stop earlier remains, as the authors stress, unresolved rather than settled.

The study’s methodology reveals both its strengths and its limits. The investigators applied a rigorous Delphi consensus definition of postoperative spine infection, using hierarchical criteria spanning microbiology, imaging, laboratory values, intraoperative findings, and histology, and a dedicated infectious disease team made every diagnosis. Firth penalized logistic regression was used to temper the small-sample bias that arises when only 51 events are distributed across multiple covariates. Yet the same constraints shape the conclusions. The study spans two hospitals in the same Italian region, limiting generalizability; treatment duration was chosen by individual physicians rather than randomized, inviting confounding by indication; the rate of culture-negative infections exceeded that reported in prior literature; and detailed surgical variables such as the number of instrumented levels were not consistently available for adjustment. The authors explicitly label their duration findings exploratory.

What emerges is a reframing of the clinical conversation around one of orthopedic surgery’s costliest complications. Rather than asking only how long to treat, the DeFEndO study suggests clinicians should ask what they are treating. Cancer patients, whose immunosuppression, chemotherapy-induced mucosal damage, and malnutrition impair infection control, fared worse, and so did those harboring Gram-negative rods. Rifampin, long championed for biofilm infections of prosthetic joints, showed no protective signal here, echoing the mixed results of randomized trials elsewhere. The authors call for prospective studies, particularly to define optimal therapy for Gram-negative spinal infections, where evidence remains scarcest. Until then, the message for surgeons and infectious disease physicians is quiet but profound: the calendar may matter less than the microbe, and curing a spine seeded with infection begins with knowing exactly which organism is hiding on the hardware.

Subject of Research: Risk factors for treatment failure of deep surgical-site infections following spinal instrumentation

Article Title: Risk factors for treatment failure of surgical site deep infections following spinal instrumentation: multicentre observational study (DeFEndO study)

Article References: Pascale, R., Gallo, M., Regge Gianas, N., Fornaro, G., Grassi, I., Zagarrigo, M., Grechi, A., Zuppiroli, A., Santoro, D., Tenti, E., Boriani, L., Zamparini, E., Gasbarrini, A., Viale, P., Giannella, M., Curti, S., & Tedeschi, S. (2026). Risk factors for treatment failure of surgical site deep infections following spinal instrumentation: multicentre observational study (DeFEndO study). Journal of Bone and Joint Infection, 11(5), 601-611. https://doi.org/10.5194/jbji-11-601-2026

Image Credits: AI Generated

DOI: 10.5194/jbji-11-601-2026

Keywords: spinal instrumentation, surgical site infection, deep infection, treatment failure, antibiotic duration, quinolones, Gram-negative bacteria, biofilm, rifampin, spine surgery, multicentre observational study, infectious diseases