Probably the mass proliferation of leukaemic cells is responsible for synovial effusion.
Answers
Almost any tumour arising from epi-metaphyses of long bones or juxta-articular soft tissues may mimic a joint swelling. Examples of bone tumours that commonly arise in this location include chondroblastoma and aneurysmal bone cyst in the younger population and giant cell tumour, clear cell chondrosarcoma and malignant fibrous histiocytoma in adults Extra-osseous soft tissue involvement by malignant tumours and synovial effusion resulting from 2014intra-articular tumour extension or sympathetic response may also result in peri-articular and/or joint swelling
Lipoma
True intra-articular lipomas are extremely rare lesions.Unlike lipoma arborescens, a true lipoma is a solitary round or oval lesion and is composed of mature adipose tissue enclosed by a thin fibrous capsule It may be covered by synovial tissue and may have a vascular pedicle. Villous proliferation of the synovial membrane is absent. True lipomas of the knee joint are generally located in relation to the subsynovial fat on either side of the patellar ligament or over the anterior surface of the femur (5–8). Intraarticular lipomas are usually non-calcified masses that are predominantly hyperintense on both T1 and T2-weighted images