Lytic Patella.

Lytic patellar lesions may occur in a variety of conditions, from commonly seen developmental, degenerative, traumatic, metabolic, infectious, iatrogenic to uncommon benign and malignant neoplasms. Patellar metastasis is rare. Due to the low incidence and the non-specific symptoms of metastatic anterior knee pain, delayed diagnosis is common and the prognosis is poor. It is worthwhile to remind clinicians and radiologists the possibility of patellar metastasis in patients with a history of previous malignancy presenting with an intractable anterior knee pain. We proposed the diagnosis and differential diagnosis for patellar metastasis. The diagnosis may be challenging in patellar giant cell tumor, chondroblastoma, aneurysmal bone cyst, metastasis, and gout. MRI is the most sensitive in detecting bone metastasis, delineating intraarticular invasion and planning surgical boundary. Arthrocentesis fluid cytological examination is useful as a rapid and simple diagnostic method. Multimodal imaging analysis may help make a definite diagnosis.