There are two main forms of osteomyelitis:

• Acute osteomyelitis. An infection that involves the bone’s periosteum (dense fibrous membrane covering the surface of bones) or cortical bone and medullary cavity (marrow cavity in the shaft of a long bone). This disease is more common in children than adults, and more common in boys than girls. 
  
  
• Chronic osteomyelitis. A case of acute osteomyelitis that becomes recurring or that is not resolved. Chronic osteomyelitis can spread to other parts of the body if left untreated.
  
  Types of chronic osteomyelitis include:
  
  
• • Medullary. The endosteum (thin layer of cells lining the medullary cavity) is infected.
    
    
  • Superficial. Surrounding soft tissue that does not heal. Cortex (outer layer of the bone) and periosteum are infected.
    
    
  • Combined localized. Both the medulla (inner core of the bone) and the cortex are infected. A fistula (abnormal passage resulting from an injury) may be present.
    
    
  • Combined diffuse. Similar to combined localized, with the addition that the bone and limb are both unstable.