Subchondral Insufficiency Fracture of the Knee Previously Diagnosed as Spontaneous Osteonecrosis: A Case Report With Histological Analysis.

Objectives: Spontaneous osteonecrosis of the knee (SPONK) shares clinical and imaging similarities with subchondral insufficiency fractures of the knee (SIFKs), leading to confusion in terminology and diagnosis. Case Presentation: We reported the case of a patient with knee pain and a presumed diagnosis of SPONK based on clinical and imaging findings. X-rays showed subchondral bone collapse and marked subchondral sclerosis. MRI showed a subchondral fracture line with extensive bone marrow edema. The lesion was classified as Stage IV according to the Koshino radiographic classification and Stage IV based on the Sayyid classification for MRI findings. The patient underwent total knee arthroplasty, during which bone samples were collected and examined. Histological analysis revealed no osteonecrosis but rather subchondral stress fractures with associated bone callus formation, proliferation of chondrocyte clusters, increased osteoblastic activity, and bone marrow edema with mild fibrosis and chronic inflammation. Conclusion: This case report highlights the discrepancies between histological findings and the current classification of osteonecrosis of the knee, suggesting that the term "osteonecrosis" may be inappropriate in some cases. Greater attention should be given to the histological substrate of these pathologies to potentially refine the nomenclature and diagnostic criteria in the future.