Surgical site infections represent the most frequent and expensive infectious complication in Europe and USA, as they often require a prolonged hospitalization and sometimes a new surgery, significantly impacting on mortality and morbidity (9).
In the present study, we enrolled a relatively large cohort of patients (n=760) undergoing TJA (hip, knee and shoulder) at Santa Maria Maddalena Hospital, a third level regional reference center for orthopedic surgery.
The comparison between our study population and the Italian epidemiological data available (19) shows a lower percentage of deep SSIs and PJI compared to the national infection rates. In fact, in the Italian case series, 54% of SSI diagnosed in orthopedic surgery are classified as deep or PJI, while in our population only 40% of SSI match this classification.
It is worth noting that none of the revision surgeries carried out at our institution developed a SSI, although literature data points out that the incidence of this complication is significantly higher following a revision compared to a primary implant (20). The reason for this difference, however, could be found in the small number of revisions performed (4%), compared to primary arthroplasties (96%).
Both sex and mean age of the study population are demographically homogeneous with the results reported in the "National surveillance system of SSIs" (19); the mean BMI of the enrolled population is 29.10 kg/m2, explaining how obesity plays a key role in joint arthrosis.
In the comparison of the two study groups, none of the clinical variables considered reached statistical significance. According to literature data, some of the considered variables (eg. BMI, diabetes, steroid and anticoagulant therapy, number of surgeons, duration of surgery, staples) have been associated with an increased risk of developing SSI, an association not found in our study.
On the contrary, the analysis of the main laboratory tests brought some significant results: the total leukocytes and neutrophils count at the time of hospital discharge were significantly higher in group B (p=0,025; p=0,016, respectively). To our knowledge, the role of total leukocytes and neutrophils at discharge has never been highlighted in literature data as an independent risk factor or the development of SSI following joint replacement.
Cut-off values for total leukocytes>7860/µL and neutrophils> 5185/µL may be able to discriminate the two groups. Although the diagnostic value of these tests appears weak to recommend them as diagnostic tests for SSI, their high negative predictive value (total leukocytes: 98.03%, OR: 3.69; neutrophils: 97.85%, OR: 3.98) suggest their potential role during the post-operative screening, aimed at highlighting patients with a higher risk of developing SSI, alone or as a part of a cluster of variables or a predictive score.
The limit of these two tests lies in the very low positive predictive value (total leukocytes: 6.97%; neutrophils: 7.69%), which suggests the need to integrate further diagnostic exams in the selected population to discriminate infected patients from false positive cases. However, given the crucial one-month deadline to perform a successful DAIR, we believe that rapidly excluding patients at lower risk and concentrating the efforts on the others could be a useful (and potentially cost-effective) solution in the clinical pathway of TJA follow-ups.
By comparing the timing of SSI diagnosis to the CDC classification (7), we found an earlier SSI diagnosis for deep infections compared to the superficial ones (p=0.008) and PJI (p = 0.145), (Figure 3). Even though in the latter case statistically significance is not reached, a relationship between the time of SSI diagnosis and the tissue depth of the infection itself hasn’t been described yet in literature reports yet, except for PJI that rise later in time (7).
The outpatient examination performed by an infectious disease specialist for SSI surveillance was carried out between the 17th and 25th day post-surgery. Data from the enrolled population show that all diagnosed superficial SSI and PJI fall within this observation period. Deep SSI, on the other hand, anticipated the screening period, since many cases were diagnosed before the hospital discharge.
According to the clinical presentation, hyperpyrexia was recorded mainly in patients with deep or periprosthetic SSI.
Among SSI with microbiological diagnosis (n=9), 4/9 cases were supported by a Gram negative bacterium. This element suggests that contamination of the surgical site may have occurred at a time other than the surgical act, presumably due to wrong management of the dressing after discharge. Risk factors for the development of SSIs from Gram negative bacteria are diabetes mellitus and the presence of urinary catheter (21); none of the patients who developed SSI caused by Gram negative bacteria were diabetic, whereas data regarding the placement of a urinary catheter were not collected in this study.
All patients treated for SSIs have completed the planned therapeutic course, without any systemic or local complication. To date, no recurrence has occurred in superficial and deep SSI. The only cases of relapse involved periprosthetic infection treated with DAIR (2/4, 50%) caused by MSSA and P. mirabilis, respectively.
In literature, DAIR treatment for PJI shows success rates varying between 32 and 100% (22). DAIR represents an attractive surgical modality for treatment of PJI although protocols differ in several retrospective series and randomized controlled or prospective trials on this topic are lacking. It is well known that the increase in the chances of success of this procedure depends on a careful selection of patients associated to radicality of the surgical toilette.
In the present study, the KLIC score used to stratify the risk of relapse in patients undergoing DAIR has shown poor accuracy for outcome prediction even in patients defined at "low risk" (KLIC score <4); according to our opinion, the main limit of the score is represented by the lack of a microbiologic parameter. Relapsed periprosthetic infections, in fact, were sustained by virulent pathogens (MSSA and P.mirabilis, respectively).
Main limitation of our study is represented by the poor PPV of total leukocytes and neutrophils value at discharge that may discourage its application as possible clinical tests to discriminate patients at risk to develop future SSI. This may be partially due to the low sample size of the group of patients presenting with SSI, which unbalanced the study. However, we consider it as a good starting point for studies on larger populations to replicate our findings.