Diagnosis of PJI is challenging owing to the existing biofilm around implants . Pathogens embedded in a biofilm are enclosed in a polymeric matrix and have altered their phenotype into an extremely resilient form of life, protecting them from antimicrobial and host immune responses . Biofilm bacteria exhibit a markedly higher resistance to antimicrobial killing compared with planktonic bacteria .
Sonication was popularized as a diagnostic tool for PJI by Trampuz et al. . During the process, adherent pathogens are dislodged from the surface of the implant through low-frequency ultrasound, preserving their viability . Various studies have shown an improved sensitivity following the use of sonication compared with conventional tissue culture. In a meta-analysis (16 studies) performed by Liu et al. , sonication yielded a pooled sensitivity and specificity of 79% (95% confidence interval [CI]: .76–.81) and 95% (95% CI: .94–.96), respectively. The results were similar in a meta-analysis (12 studies) conducted by Zhai et al. , showing a pooled sensitivity and specificity of 80% (95% CI: .74–.84) and 95% (95% CI: .90–.98), respectively. Furthermore, a meta-analysis of four studies investigating sonicate fluid in blood culture bottles displayed a sensitivity and specificity of 85% (95% CI: .77–.91) and 86% (95% CI .81 to .91), respectively . However, the reference standards for the diagnosis of PJI differed among the studies included in the meta-analyses.
In our study, SFC showed low sensitivity (69.0%) compared with previous data regarding sonication [12, 16]. Overall, we detected 45 false negative PJI. However, PJI was defined according to the EBJIS criteria, as having a low threshold for the detection of an infection.
When adopting the ICM criteria, the sensitivity and specificity of SFC were comparable with those reported in recent meta-analyses (87.5 and 93.5%, respectively), after the exclusion of inconclusive findings. The discrepancy between the EBJIS criteria and more commonly used criteria (i.e., MSIS) has been reproduced in a recent study performed by Renz et al.  which sought to determine the diagnostic value of the alpha defensin lateral flow test using different classification systems. The sensitivity of this test was 84% by MSIS criteria but only 54% by EBJIS criteria.
We found a significantly higher sensitivity of SFC compared with PTC (69.0% vs. 62.8%, respectively; p = 0.04). However, the specificity was similar (90.2% vs. 92.9%, respectively; p = .65). Our results were in concordance with those of recent studies, that have mostly observed a superior sensitivity of SFC compared with PTC [10, 19, 20]. Two recent studies showed that the sensitivity of SFC was lower than that of PTC [21, 22]. The discrepancy between studies can be attributed to the varying study conditions and definitions of PJI. In particular, the colony-forming unit (CFU) value required to define a positive SFC result differed greatly among studies.
Subgroup analysis showed a superior sensitivity of SFC throughout all subgroups. However, there were no significant differences between SFC and PTC among subgroups. There was a somewhat higher sensitivity for SFC of hip implants when compared with knee implants (hip: 70.4% vs. knee: 65.4%). A possible explanation may be the more frequent use of antibiotic-loaded cement in knee arthroplasty, which may disturb the structure of the biofilm . However, SFC of knee implants might provide a greater advantage against PTC when compared with SFC of hip implants (knee: p = .15 vs hip: p = .24). Previous administration of antibiotics did not influence the sensitivity of SFC, confirming the findings of comparable studies [24, 25]. However, there was a trend towards higher sensitivity of SFC against PTC (p = .12) in the antibiotics groups as opposed to when antibiotics were not administered (p = .36). It has to be noted, however, that the antibiotic group was characterized by a relatively small sample size (n = 63).
We found both SFC and PTC sensitivity to drop consecutively with delayed and late infections. Low sensitivity in late infections might be a result of EBJIS criteria. A high amount of cases in this group would have been classified as aseptic under alternative definition criteria. Furthermore, we found a strong trend towards higher sensitivity of SFC against PTC in the “late infections” group (59.5% vs. 45.9%; p = 0.06) which was in concordance with a recent study by Puig– Verdié et al. . Apparently, in early infection, a majority of microorganisms are expected to not have formed biofilms yet, leading to high detection rates both in SFC and in PTC. In late infection, meanwhile, a biofilm is expected to have formed by most microorganisms and sonication should provide an advantage against conventional culturing.
The prevalence of pathogens was in concordance with current literature. CNS, as in our study (26%), were by far the most common pathogens in various studies across the board [6, 10, 27,28,29]. The relatively high prevalence of gram-negative rods (9%), especially in early infection (9/81, 11%), is also a finding supported by literature . Moreover, the detection of six anaerobes in 77 cases (7.8%) of delayed and late infections is a finding concordant with literature . Interestingly, out of the six cases, four times anaerobes were detected by SFC alone supporting the notion that these microorganisms are especially susceptible to SFC and may be missed by PTC . With EBJIS criteria, any detection of anaerobes in SFC is a PJI, irrespective of CFU.
We found a comparably high prevalence of culture-negative PJI (31%). In a recent meta-analysis, Reisener et al.  reported that the incidence of culture-negative PJI ranged from 7 to 42%. As expected, we found that highly virulent pathogens were more prevalent in early infection than in delayed or late infection, confirming the findings of previous studies [26, 31]. The SFC and PTC results were similar, however, culture-negative results were more frequent in PTC.
The present study has several limitations. Firstly, it is limited by its retrospective nature. Not all elements of the EBJIS PJI criteria were available for each patient. PJA was available in 73 and 65% of PJI and AF cases. WBC/PMN was available in 54 and 50% of PJI and AF cases, respectively. Moreover, PM was available in only 38 and 13% of PJI and AF cases, respectively. An explanation for the lack of data may be the new introduction of standardized principles in our clinic.
Diagnosis according to the ICM criteria may have been biased owing to missing data. The rate of erythrocyte sedimentation among serum markers, as well as leukocyte esterase, alpha defensin and synovial C-reactive protein among synovial markers, were not part of the diagnostic routine. The resulting score contained only points received from available tools.
Besides cases defined as “infected” and “not infected”, the new ICM criteria allow for the detection of “inconclusive” results in patients with a score of 4 or 5. In our study, 20 patients were defined as such. These patients present a real diagnostic challenge and might benefit from molecular diagnostic testing such as next generation sequencing which- of course- was not at our disposal.
Colony counting was not performed with sonication. Especially in SFC(+)PTC(−) cases, measurement of CFU is of high importance in distinguishing between infection and contamination. According to the EBJIS criteria, a CFU count of > 50 CFU/ml is indicative of an infection. In this study, this conflict was solved by interpreting SFC as an additional PTC, and subsequently applying the respective rules. Our method of sonication does not include the step of vortexing. In a subgroup meta-analysis , cases that included vortexing within the process of sonication exhibited higher sensitivity (79% vs. 78%, respectively) and higher specificity (96% vs. 85%) versus those that did not.
The strength of this study lies in its relatively high sample size (n = 257), which allowed for a reliable comparison between SFC and PTC, and yielded large subgroups for further analyses. Furthermore, definition of PJI and aseptic failure was performed in a very meticulous manner, including all aspects of the respective definition criteria. To our knowledge, this is the first study investigating sonication using the EBJIS criteria as a reference standard for PJI, while simultaneously incorporating the most recent ICM criteria.