Purpose: The purpose of this work was to define clinical and microbiological criteria allowing the distinction of postoperative infections of joint implants from secondary infections and to propose a probability score.

Material and methods: This retrospective multicentric study was conducted on 134 cases of infections of hip and knee prostheses. The first step was to distinguish three populations of infections (postoperative, secondary, unknown) based on the germ isolated at diagnosis of prosthesis infection (coagulase-negative Staphylococus, other germs, and Staphylococcus aureus respectively). The second step was to analyse clinical features in the group of postoperative infections and in the group of secondary infections in order to evaluate the predictive value of each of the following parameters: infection-free period, time to infection, distant focus.

Results: Fifty-seven cases of secondary infections were identified. There were 34 cases of postoperative infection and 43 cases of infection of unknown category (Staphylococcus aureus infections). The diagnosis of prosthesis infection was retained on the basis of intraoperative samples in 81% of the cases. The infection-free period was less than 1 year (time between implantation and first joint signs) in 69% of the cases of postoperative infection and in 36% of the cases of secondary infection. The difference was significant (p=0.003). A distant focus of infection was found in 44% of the cases (and documented by a bacteriological sample in 13%, 18/134). An invasive therapeutic procedure was identified in 12% (4/34) of the postoperative infections and in 56% (32/57) of the secondary infections. The difference was significant.

Discussion: The duration of the infection-free period and the notion of a distant focus of infection (documented or not) are significant criteria enabling a distinction between postoperative and secondary infections after prosthesis implantation. The notion of time to infection (time between the infectious episode and diagnosis of infected prosthesis) is not a significant criteria for identifying secondary infection (p > 0.005). Staphylococcus aureus infection remains difficult to classify.

Conclusion: Analysis of clinical features as a function of the microbiological findings enabled identification of significant criteria of secondary infection of joint prostheses. This enables proposing a probability score to be evaluated on a larger scale.