Radiologically, the earliest sign of osteomyelitis is -
Osteomyelitis is the infection of bone by microorganisms. It can be acute or chronic. 
Acute osteomyelitis: This can be primary (haematogenous)or secondary(following open fracture or bone operation). Staphylococcus aureus is the commonest causative organism. C/f: Child presents with an acute onset of pain and swelling at end of a bone, associated with systemic features of infection like a fever. 
Investigations: Blood- polymorphonuclear leucocytosis and an elevated ESR. X-rays - The earliest sign to appear in X-ray is a periosteal new bone deposition at metaphysics. This is called periosteal reaction. It takes about 7-10 days to appear. Bone scan- bone scan using Tc 99 may show increased uptake by bone in the metaphysis.
Loss of muscle and fat planes
Presence of sequestrum
Callus formation
Periosteal reaction