Common causes		Less common causes	Rare causes
Avascular necrosis of bone		Bone malignancies	Amyloidosis
Crystals		Bowel-disease–associated arthritis	Behçet’s syndrome
	Monosodium urate	Hemoglobinopathies	Familial Mediterranean fever
	Calcium pyrophosphate dihydrate	Juvenile rheumatoid arthritis	Foreign-body synovitis
	Apatite	Loose body	Hypertrophic pulmonary
	Calcium oxalate	Psoriatic arthritis	osteoarthropathy
Hemarthrosis		Rheumatoid arthritis	Intermittent hydrarthrosis
Infectious arthritis		Reactive arthritis	Pigmented villonodular synovitis
	Bacteria	Sarcoidosis	Relapsing polychondritis
	Fungi		Still’s disease
	Mycobacteria		Synovioma
	Viruses		Synovial metastasis
	Lyme disease		Vasculitic syndromes
Internal derangement
Osteoarthritis
Osteomyelitis
Overuse
Trauma