An open injury to the bone such as an open fracture with the bone ends piercing the skin.

An infection from elsewhere in the body such as a urinary tract infection that has spread to the bone through the blood.

A minor trauma, which can lead to a blood clot around the bone and then a secondary infection from seeding of bacteria.

Bacteria in the bloodstream, which is deposited in a focal (localized) area of the bone. This bacterial site in the bone then grows, causing destruction of bone but often time’s new bone is formed around the site.

Chronic open wounds or soft tissue localized infection can eventually extend down to the bone surface leading to a secondary bone infection.

Acute osteomyelitis may develop within a few days to a few weeks. It more commonly affects children, with up to 80 to 90 percent of children's cases due to Staphylococcus aureus bacteria, and it occurs in their arm or leg bones most often. Acute can sometimes lead to chronic, or long-term, osteomyelitis if the infection does not go away with treatment.

Chronic osteomyelitis tends to develop more gradually, usually over a few months, and can last for years. It may be caused by several kinds of bacteria. This type is more common in adults and can affect any bone, but is more likely to show up in the spine, pelvis, hips and feet. It can develop if an infection is not treated and can go undetected for months or years, sometimes with no symptoms. But usually it causes bone pain and recurring infections in the skin over the bone, often with pus draining through the skin off and on. It can take months or years to go away.

The metaphysis of the long bones (tibia, femur) are involved most frequently. In infants, medullary infection may spread to the epiphysis and joint surfaces through capillaries that cross the growth plate. In the child older than 1 year, the growth plate is avascular and the infection is confined to the metaphysis and diaphysis. The joint is spared unless the metaphysis is intracapsular. Thus, cortical perforation at the proximal radius, humerus, or femur enables the infection to migrate to the elbow, shoulder, or hip joint, regardless of the patient's age. A pathogenic species is almost always recovered from infected bone. Staphylococcus aureus is the most common organism isolated in infants, children, and adults.

Hematogenous osteomyelitis can be found in the adult population. In long bones, the infection usually begins in the diaphysis but may spread to involve the entire medullary canal. Extension into the epiphysis and joint space may occur because the growth plate has disappeared and the medullary areas are contiguous. Because the periosteum is firmly adherent to the bone, cortical penetration usually leads to a soft tissue abscess. Subperiosteal abscesses and massive cortical devitalization rarely occur. Sinus tracts connecting the sequestered nidus of infection to the skin through soft tissue extension may form. The involucrum contains the sequestered, necrotic marrow and endosteal bone. Sequestra often are found within the thickened cortex and are surrounded by reactive bone and chronic granulations. Brodie's abscess is the name given to a chronic localized bone abscess. Patients with subacute cases may have fever, pain, and periosteal elevation, whereas chronic cases are often afebrile with long-standing dull pain. The most common site of involvement is the distal part of the tibia, and the lesion typically is single and located near the metaphysis.

Vertebral osteomyelitis in the adult population usually is hematogenous in origin but may be secondary to trauma. An early involvement of the anterior-inferior edge of the vertebral body suggests spread from the bony entrance of the anterior spinal artery. Retrograde infection through Batson's venous plexus is also postulated. The segmental arteries that supply the vertebrae usually bifurcate to supply two adjacent bony segments. Therefore, the disease often involves two adjacent vertebrae and the intervertebral disk.

The lumbar region is affected in at least 50% of patients with hematogenous vertebral osteomyelitis. The thoracic spine is affected in at least 35% of patients and the cervical spine in at least 20% of patients. However, in intravenous drug abusers, the cervical vertebrae are involved much more often (27%), and the thoracic vertebrae much less frequently (4.5%).

The infection may extend into the cartilaginous end-plate, disk, and/or adjacent areas. Posterior extension of the infection may cause epidural and subdural abscesses, or even meningitis. Extension anteriorly or laterally may cause paravertebral, retropharyngeal, mediastinal, subphrenic, or retroperitoneal abscesses. Spread to adjacent vertebral bodies may occur rapidly through the rich venous networks in the spine.

The infection usually is monomicrobic when hematogenous in origin. In the normal host, S. aureus remains the most commonly isolated organism. However, aerobic gram-negative rods, including Pseudomonas aeruginosa and Serratia marcescens, are found often in intravenous drug users and patients with urinary tract infections. Positive culture results are important for diagnosis because conditions such as trauma and vertebral collapse may simulate infection.