Surprise positive culture rate in the treatment of presumed aseptic long-bone nonunion: a systematic review with meta-analysis of 2397 patients

Presence of infection is an important factor in the treatment of long-bone nonunions [1]. Infected nonunions are often managed with staged surgical treatment, whereas presumed aseptic nonunions are treated in a single stage [2, 3]. Infection can be confirmed pre- or intraoperatively in the presence of signs such as a fistula or sinus, wound breakdown, and purulent drainage or presence of pus [4]. Other factors considered are elevated serum inflammatory markers, radiological signs and suggestive local and systemic signs of infection such as erythema or fever. However, these factors are merely suggestive as they lack accuracy, specifically in detecting low-grade infections [5‐7]. Consequently, if clinical signs of infection are absent, a nonunion is often presumed aseptic and in these cases, the definitive diagnosis of infection relies on intraoperative cultures from the nonunion site.

Several studies have reported on the rate and outcomes of nonunions that exhibit no clinical and/or laboratory signs of infection but reveal positive intraoperative cultures (referred to as “surprise” positive cultures) [8‐10]. However, amongst these studies the definitions of presumed aseptic nonunions vary, as do local protocols for detection and treatment of positive cultures.

Therefore, our primary objective was to determine (1) the rate of “surprise” positive intraoperative cultures in presumed aseptic long-bone nonunion (surprise positive culture nonunion), and (2) the rate of “surprise” positive cultures that represent an infection (surprise infected nonunion) vs. a contamination (contaminated culture nonunion). Our secondary objectives were to determine the healing and secondary surgery rate for each culture result and to identify the cultured micro-organisms.

Our search yielded 14,729 articles and after duplicate removal 9354 articles remained. The full texts of 384 studies were reviewed for inclusion. A total of 21 studies were included (Fig. 1).

The surgical treatment protocol for a long-bone nonunion largely depends on the absence or presence of infection. If confirmatory clinical signs of infection are absent, a nonunion is often presumed aseptic. We established that, in these cases, surprise positive cultures occur in approximately 1 in 6, and surprise infected nonunions in 1 in 10 patients. We found that 1 in 5 patients with a surprise positive culture nonunion and 1 in 7 patients with a surprise infected nonunion required secondary revision surgery, compared to 1 in 17 patients with a negative culture nonunion. It may be possible that revision surgery was performed only because a positive culture was found. However, studies primarily initiated antibiotic treatment specific to the identified micro-organisms in case of a surprise infected nonunion that remained without clinical signs of infection. The need for additional surgery may be explained by the fact that none of the studies reported an empiric antibiotic treatment protocol until cultures return. Staphylococci – which were the most cultured micro-organisms – are able to develop a biofilm within days, which can only be eradicated by removal or exchange of implants and biofilm active antibiotic therapy (e.g., rifampicin) [35, 36]. In addition, vascularity of nonunions may be compromised, which limits local penetration of systemic antibiotics. Consequently, initiating antibiotic treatment only after cultures return might be beyond the “window of opportunity”. For confirmed FRIs, it is therefore recommended that surgical debridement should be followed by empiric broad spectrum intravenous antimicrobial therapy [37, 38]. In this systematic review, we found that final healing rates are close to 100% for presumed aseptic nonunions regardless of the culture result, and despite none of the studies reporting empiric antibiotic treatment. This would suggest that the combination of antibiotics tailored to the micro-organism(s) found with additional revision surgery is effective and that empiric broad-spectrum antibiotics may not be necessary. This is important in the context of antibiotic stewardship, cost-reduction, and prevention of side-effects. Further studies have to assess the local epidemiology, antibiotic resistance rates, and patient characteristics of surprise infected nonunions to establish tailored protocols [38].

We aimed to compare the criteria to define a presumed aseptic nonunion against the FRI criteria [4]. However, over 60% of studies did not specify these criteria. This is problematic as these descriptions (e.g., “clinical signs of infection” or “active infection”) may reflect both confirmatory or suggestive clinical signs of infection and therefore do not represent a repeatable threshold. Of the studies that did specify the criteria, most excluded patients based on suggestive clinical signs for infection. In general, a limitation of the suggestive FRI criteria is that some criteria (e.g., pain, swelling, redness, bone lysis around implants) may also be a result of the nonunion itself. Half of studies used laboratory values to rule out infection pre-operatively. The accuracy of these diagnostic test remains debatable. The study of Tosounidis et al. found 26% surprise infected nonunions, even if CRP was normal [23]. Hackl et al. found no significant differences in laboratory values between surprise infected nonunion and negative culture nonunions [26]. Others have also confirmed that laboratory values are not accurate to diagnose low-grade infection [5]. These findings show that adequate pe-operative diagnosis of infection remains difficult if confirmatory signs of infection are absent. Nonunion may even be the only symptom of the infection in these cases.

Given these findings, the definitive diagnosis of infection still heavily relies on intraoperative cultures. We found that only half of the studies reported a culture strategy protocol, and these protocols often lacked detail. Consequently, the protocols did not meet current recommendations [1, 4, 39]. Inappropriate sampling may underestimate (e.g., inadequate or insufficient samples, or short culture duration) the rate of surprise infected nonunions. It is recommended to take at least five cultures from the bone-implant interface directly after the incision and incubate these for 10–14 days [4]. Only four studies reported taking five or more cultures and only five studies reported long-term culturing. Long-term culturing is important to detect slow growing micro-organisms, such as Cutibacterium species and Coagulase-negative staphylococci [40‐42]. Consequently, the prevalence of slow growing micro-organisms might be underestimated. Nevertheless, we found that over half of the identified micro-organisms were low virulent Coagulase-negative staphylococci [43]. Indeed, late (inherent to a nonunion) infections are most often caused by a low virulent micro-organism [35, 39]. Interestingly, 12% of infections were still caused by virulent Staphylococcus aureus. These micro-organisms may cause a low-grade infection when a low inoculum is introduced during the initial trauma or earlier surgery [35]. Although from the data of the present study it cannot be determined if a surprise infection truly causes nonunion, our findings do suggest that in order to adequately determine the etiology of a nonunion, prolonged culturing is necessary.

After cultures return positive, it is important to differentiate between contamination and infection as this has consequences for the treatment strategy. Only half of the studies explicitly reported criteria for such a differentiation. Most of these studies required two or more cultures to be positive to deem a nonunion as infected. This is in line with recent recommendations [4]. This is justifiable when comparing the secondary surgery rate for contaminated culture nonunions (4%, 95%CI 0–19%) and negative culture nonunions (6%, 95%CI: 1–13%).

Our study has several limitations. First, the major limitation of this study is that – although presented as single population – presumed aseptic nonunions are naturally a heterogenous group with varying characteristics; which is inherently demonstrated by our results. Accordingly, the included studies varied in patient history (e.g., infection or surgical history, open fractures), in definition of presumed aseptic nonunion, and in culture protocols. These differences may explain the substantial statistical heterogeneity we found in our meta-analysis of pooled rate [14]. Clinicians should therefore interpret these results in light of their own definitions for a presumed aseptic nonunion. Second, we excluded many studies because the authors did not report on intraoperative culture results. Reporting on intraoperative cultures may have been omitted if no positive cultures are found, which is considered publication bias. Similarly, publication bias may lead to an overestimation of the union rate as authors tend to publish successful treatment results. Third, we included studies that were published within a large timeframe (e.g., we also included studies published twenty years ago). Since then, treatment and diagnostic strategies have evolved and this may influence individual study results. Fourth, the majority of presumed aseptic nonunions affected the lower extremity. Upper extremity FRIs are often caused by different organisms (e.g., Cutibacterium acnes [44, 45]) and this limitation should be considered when extrapolating results to other anatomic regions. Last, we were unable to stratify the cultured micro-organisms into causative versus contaminant as this was not consistently reported by the individual studies.

We found that in presumed aseptic nonunion cases, surprise positive intraoperative cultures occur in approximately 1 in 6 patients and surprise infections in 1 in 10 patients. The cultured organisms are most often of low virulence and Coagulase-negative staphylococci account for 59% of all cultured micro-organisms. Patients with a surprise positive culture and surprise infection require secondary surgeries more often compared to patients with a negative culture nonunion, although final healing rates are comparably high. Combined, these findings suggest that surprise positive cultures play a role in the clinical course of a nonunion, that (long-term) culturing is important in determining the etiology of nonunion even if the pre-operative suspicion for infection is low, and that eventually high healing rates can be achieved in presumed aseptic nonunions, regardless of the definitive intraoperative culture result.