Acute osteoarthritis
Avascular necrosis
Crystal-induced <unk> ,<unk> 
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
Gout and pseudogout
Acute osteoarthritis
Boxer's fracture
Avascular necrosis
Colles' fracture
Crystal-induced 
Salter-Harris fractures
Gonococcal arthritis, arthritis-dermatitis syndrome