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

Abstract

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.

Keywords: Periprosthetic joint infection (PJI); adverse local tissue reaction (ALTR); case report; dense cortex; popliteal lymph nodes.

Figure 1

Knee radiograph and knee CT of Patient #1 showing radiological findings suggestive of prosthetic infection. (A) An AP knee radiograph depicting a unicompartmental prosthesis located in the medial compartment of the knee, revealing an area of osteolysis adjacent to the tibial component (arrow). (B) A knee CT scan in bone reconstruction and coronal plane confirming the observed area of osteolysis in the X-ray (arrow). (C,D) Knee CT in soft tissue reconstruction, presented in axial sections, illustrating abundant joint effusion with synovitis (*) and the presence of a PLN (circle). This PLN exhibits signs of infection, characterized by a rounded morphology, an absent fatty hilum, and imprecise margins with slight adjacent fat trabeculation. CT, computed tomography; AP, anteroposterior; PLN, popliteal lymphadenopathy.

Figure 2

Knee CT of Patient #2 showing a periprosthetic fracture and radiological findings suggestive of metallosis. (A) Coronal CT of the knee in bone reconstruction revealing a femoral periprosthetic spiral fracture (thin arrow) and extensive areas of lysis at the cement-bone interface (thick arrow). (B,C) Axial CT of the knee in soft tissue reconstruction displaying significant joint effusion (*) and the presence of multiple dense PLNs (circles). The increase in density is observed in the cortex of the lymph node, while the fatty hilum remains preserved. CT, computed tomography; PLN, popliteal lymphadenopathy.

Figure 3

Knee radiograph and knee CT of Patient #3 showing radiological findings suggestive of metallosis and migration of cement to the popliteal recess. (A) Lateral knee X-ray revealing a total knee prosthesis with highly dense areas in the popliteal fossa (arrow). (B,C) Axial CT scans of the knee illustrating significant joint effusion (*). Highly dense areas in the popliteal fossa (arrows), which correspond to migrated cement, and dense PLNs (circles). The increase in density of the PLNs is also observed in the cortex of the lymph node, with preservation of the fatty hilum. CT, computed tomography; PLN, popliteal lymphadenopathy.

Figure 4

Progressive lateral knee radiographs of Patient #3 performed annually to evaluate prosthetic loosening demonstrate an increase in the density of the popliteal recess each year (arrows), corresponding to the migration of cement to the popliteal recess. No PLNs are observable in the radiographs. Images (A) and (B) serve as controls to evaluate prosthetic loosening, while (C) has already undergone prosthetic replacement. PLN, popliteal lymphadenopathy.

Figure 5

Histologic features from Patient #1, who underwent prosthetic replacement due to clinical suspicion of prosthetic infection. The pathological anatomy confirmed that it was an infectious process. (A) Synovial-like tissue with wear debris reaction (arrow) and numerous neutrophils (circle) indicative of infection (hematoxylin-eosin, 20× magnification). (B) Granulation tissue with numerous neutrophils (circle) indicative of infection (hematoxylin-eosin, 40× magnification).

Figure 6

Histologic features from Patient #2, obtained during the prosthetic replacement performed due to periprosthetic fracture. The pathological anatomy confirmed the presence of a metallosis-type ATLR, with numerous metal particles found within the cytoplasm of macrophages. (A) Synovial-like tissue with metallosis characterized by innumerable particles of metal alloy in the cytoplasm of macrophages (circles) (hematoxylin-eosin, 20× magnification). (B) Connective tissue with macrophages containing metal particles (circles). Note the presence of foreign body giant cells (arrows), indicative of ATLR (hematoxylin-eosin 40× magnification). ATLR, adverse tissue local reaction.

Figure 7

Histologic features from Patient #3, obtained during the prosthetic replacement performed due to prosthetic loosening, which show the abundant presence of migrated material in the popliteal recess, corresponding to the high-density areas observed in the CT scan. Additionally, some macrophages with metal in their cytoplasm can be observed, related to additional changes due to metallosis. (A) Synovium with prosthetic wear debris reaction (hematoxylin-eosin, 10× magnification). (B) Bone cement (*) surrounded by foreign body type giant cells (arrows). Note the presence of macrophages containing metal particles (circles) (hematoxylin-eosin, 40× magnification). CT, computed tomography.