Radiographic Assessment of the Popliteal Lymph Node to Aid the Differentiation of Canine Stifle Osteosarcoma From Moderate to Severe Stifle Degenerative Joint Disease.

Degenerative joint disease of the stifle is a common canine osseous pathology. Early degenerative changes may show mild mineral proliferation of periarticular margins with soft tissue, resulting in joint distension. If the joint is chronically unstable, osseous changes become more complex. Moderate to severe stifle degenerative disease then transitions to a combination of bony proliferation and resorption as subchondral cystic changes develop. The radiographic descriptions of moderate to severe degenerative disease overlap with those of subtle aggressive disease. If this occurs in a location prone to primary bone tumors, like the distal femur, radiographic interpretation becomes difficult. The aim of this study was to evaluate the utility of objectively assessing the popliteal lymph node size and shape on radiographs to differentiate moderate to severe stifle degenerative joint disease from distal femoral or tibial osteosarcoma. Stifle radiographs were acquired for evaluation of pelvic lameness localized to the stifle joint. Dogs with moderate to severe degenerative stifle joint disease were radiographed at 3 time points: prior to surgery for a tibial plateau leveling osteotomy (103), immediately postoperation (103), and at a recheck 6-8 weeks postsurgery (62). Dogs with osteosarcoma were radiographed only at time of diagnosis (42). Length and width gross popliteal lymph node measurements and a ratio of long axis to short axis were compared between groups using nonparametric analysis. No significant difference was determined using a long axis to short axis ratio for the popliteal lymph node to differentiate moderate to severe stifle degenerative joint disease and osteosarcoma (P = .290). The gross measurements of the long axis and short axis of the popliteal lymph node for dogs with osteosarcoma were significantly greater than the degenerative joint disease groups (P = .017, P = .000). Thus, evaluation of a long axis to short axis ratio for popliteal lymph nodes does not provide a clinically useful objective measure in differentiating peristifle osteosarcoma from moderate to severe degenerative disease of the stifle joint. The length and width of the popliteal lymph node differed significantly; however, the clinical difference is minimal.