Acute<unk> 
Avascular necrosis
Crystal-induced (Gout, Pseudogout)
Gonococcal arthritis, arthritis-dermatitis syndrome
Nongonococcal septic arthritis
Lyme disease
Malignancy (metastases, osteochondroma, osteoid osteoma)
Reactive poststreptococcal arthritis
Trauma-induced arthritis
Fracture
Ligamentous injury
Overuse
Avascular necrosis
Decompression sickness
Spontaneous osteonecrosis
Hemorrhagic (e.g. hemophilia, systemic anticoagulation
Seronegative spondyloarthropathies (ankylosing spondylitis, IBD, psoriatic arthritis, reactive arthritis
RA, SLE
Sarcoidosis, amyloidosis
Periarticular pathology
Cellulitis
Tendonitis
Bursitis
Transient (Toxic) Synovitis (Hip)
Slipped Capital Femoral Epiphysis (SCFE)
Legg Calve Perthes Disease
Fracture
Ligamentous injury
Overuse
Cellulitis
Tendonitis
Bursitis
Fibromyalgia
Juvenile idiopathic arthritis
Lyme disease<unk> 
Psoriatic arthritis
Reactive poststreptococcal arthritis
Rheumatoid arthritis
Rheumatic fever
Serum sickness
Systemic lupus erythematosus
Serum sickness–like reactions
Viral arthritis
Osteoarthritis
Acute osteoarthritis
Rheumatoid arthritis
Avascular necrosis
Ankylosing spondylitis
Crystal-induced 
Reactive arthritis
Gonococcal arthritis, arthritis-dermatitis syndrome