Unexpected positive cultures in conversion hip and knee arthroplasty.

Purpose: Total hip and knee arthroplasty in patients with previous history of periarticular surgery, such as osteosynthesis, can be surprisingly complex. This type of procedure is known as conversion arthroplasty (cTHA or cTKA) and has a higher risk of complications. The rates of unexpected positive cultures (UPC) and the risk of periprosthetic joint infection (PJI) compared to primary arthroplasty is unclear. The main purpose of this study was to evaluate rates of Unexpected Positive Cultures (UPC) in a series of conversion arthroplasty patients. The main questions to answer are: 1. Are the patients with conversion arthroplasties more susceptible to UPC than other causes of revision arthroplasties? 2. Are the conversion patients with UPC more susceptible to developing PJI?

Methods: This was a retrospective review of patients submitted to cTHA and cTKA from January 2012 to September 2018. Patients with history of previous infection or with missing intraoperative cultures were excluded. The UPC was defined as a single positive culture obtained during a procedure previously considered aseptic and PJI was defined according to the 2018 ICM criteria. After excluding 141 cases, 205 patients were analyzed, 160 hips and 45 knees.

Results: Nine (4.4%) UPC were identified, five (3.1%) in the hip group and four (8,9%) in the knee group. Staphylococcal species were the most common isolated bacteria (n = 7, 77.7%). During the study period, four (1,9%) patients were diagnosed with PJI. Only one case had an UPC and a different germ was identified during revision arthroplasty workup.

Conclusions: While UPC are more prevalent in conversion knee arthroplasties compared to conversion hip arthroplasties, the rates are similar to those observed in revision arthroplasty for other indications. Importantly, the presence of a UPC in conversion arthroplasty does not appear to elevate the risk of subsequent periprosthetic joint infection, provided a thorough PJI workup has been conducted preoperatively. Therefore, in such cases, UPCs may be safely disregarded.