Increased cortical density in popliteal lymphadenopathy as a promising radiological sign to help differentiate adverse local tissue reaction from infections in complications following a knee arthroplasty-three case reports.

Background: Total knee arthroplasty (TKA) is an effective surgical procedure for managing advanced osteoarthritis of the knee, significantly reducing pain and improving function. However, some patients experience complications leading to revision surgery, often caused by periprosthetic joint infection (PJI) in early failures and adverse local tissue reactions (ALTR) or aseptic loosening in late failures. Differentiating between PJI and ALTR is crucial because their clinical presentations can overlap, yet their treatments are distinct. While traditional imaging like radiography is useful for assessing alignment and detecting osteolysis, it may miss subtle pathological changes. Computed tomography (CT) has been increasingly utilized to provide additional diagnostic detail, especially regarding lymphadenopathy, which has been linked to septic complications in hip prostheses. However, the role of popliteal lymphadenopathy (PLN) in knee prosthesis complications remains unexplored. Case Description: We present three cases of knee prosthesis complications, diagnosed as either septic or aseptic, where CT imaging revealed distinct patterns of PLN. In the first case, which involved septic loosening, three enlarged PLNs with rounded morphology, normal density, and an absent fatty hilum were observed. The second case, complicated by ALTR and a periprosthetic fracture, showed six PLNs with increased cortical density but a preserved fatty hilum. The third and final case of aseptic loosening revealed three PLNs with increased cortical density and prosthetic debris in the popliteal recess. These findings suggest a range of PLN characteristics depending on the underlying complication, with distinct differences in morphology and cortical density observed between septic and aseptic cases. Conclusions: The presence and characteristics of PLN may serve as a valuable imaging biomarker for diagnosing and differentiating knee prosthesis complications. CT evaluation of PLNs could enhance diagnostic accuracy, particularly in distinguishing between PJI and ALTR, prompting further research to validate these findings and explore their diagnostic potential.