Osteomyelitis: Bone Infection

• Osteomyelitis is an infection in bone and bone marrow, often targeting the juxta-epiphyseal regions of long bones near joints.
• Infection sources include hematogenous spread, direct inoculation (trauma/surgery), and contiguous infection.
• Common organisms implicated include staphylococci, Pseudomonas, and Mycobacterium tuberculosis.
• The metaphysis of growing bones is the most common site due to rapid growth and high vascularity.
• Slowed blood flow in the metaphysis can lead to thrombosis, bone necrosis, and bacterial seeding.
• Joints are usually spared unless the metaphysis is intracapsular, such as in the proximal radius, humerus, or femur.

Age Variation

• Neonates experience extensive bone necrosis, increased absorption of large sequestra, increased remodeling ability, and epiphysio-metaphyseal vascular connection, potentially leading to septic arthritis.
• Adults do not typically develop subperiosteal abscesses due to adherent periosteum, they develop soft tissue abscess, instead, and vascular connection can lead to septic arthritis.

Occurrence and Risk Factors

• Osteomyelitis is most common in childhood and adolescence, affecting boys more than girls typically with a history of trauma.
• Risk factors include diabetes mellitus (especially foot involvement), compromised immunity (including AIDS), and sickle cell disease (increasing Salmonella infection risk).

Pathophysiology

• Infection leads to an inflammatory response, increasing intraosseous pressure.
• Untreated, it can cause osteonecrosis and sequestrum formation (necrotic bone fragment).
• Pus perforates the cortex, stimulating periosteal new bone formation (involucrum) and sinus development that discharge through the skin.

Clinical Features

• Localized bone pain and tenderness may be accompanied by malaise, night sweats, and pyrexia.
• Adjacent joints may be painful with movement and develop sterile effusion or secondary septic arthritis.

Investigations

• Inflammatory markers include leukocytosis, elevated CRP, and ESR.
• X-rays may appear normal in the first 10-14 days, and at least 50-75% of the bone matrix must be destroyed before radiographic changes are visible with evidence of osteopenia, localized osteolysis, and osteonecrosis.
• MRI is the imaging modality of choice for detecting early changes.
• Bone aspiration should be performed for culture via open or imaging-guided biopsy.
• Blood cultures may reveal the causative organism.

Lab Findings

• CRP is the best indicator of early treatment success, normalizing within a week.
• Failure of CRP to decline after 48-72 hours may indicate the need to alter treatment.
• Diagnosis requires 2 of the 4 following criteria: Purulent material on aspiration of affected bone, positive bone tissue or blood culture, localized classic physical findings of bony tenderness, with overlying soft-tissue erythema or edema, and positive radiological imaging.

Management

• Early recognition and management is critical.
• Principles mirror septic arthritis treatment, involving parenteral antibiotics for at least 2 weeks, followed by oral antibiotics for at least 4 weeks.
• Resection of infected bone and subsequent reconstruction may be necessary.

Complications

• Secondary amyloidosis.
• Skin malignancy at the margin of a discharging sinus (Marjolin’s ulcer).
• Septicemia & metastatic abscesses.
• Septic arthritis.
• Growth disturbance (children).
• Pathological fracture.
• Chronic osteomyelitis.

Subacute Osteomyelitis

• Results from a less virulent microorganism or elevated patient resistance.
• Mostly occurs at the distal femur or proximal tibia.
• On X-ray presents as Brodie’s Abscess, small and oval in shape and surrounded by sclerotic bone, and may be mistaken for osteoid osteoma.

Chronic Osteomyelitis

• Coexistence of infected, nonviable tissues and an ineffective host response leads to chronicity.

Factors Responsible for Chronicity

• Local factors: Cavity, sequestrum, sinus, foreign body, and degree of bone necrosis.
• General factors: Nutritional status of involved tissues, vascular disease, DM, and low immunity.
• Organism factors: Virulence.
• Treatment factors: Appropriateness and compliance.
• Risk factors: Penetrating trauma, prosthesis, and animal bite.

Pathologic Features of Chronic Osteomyelitis

• Sequestrum: Large areas of dead bone may be formed when both the medullary and periosteal blood supplies are compromised.
• Involucrum: New bone forms from surviving fragments of periosteum and endosteum to create an encasing sheath of live bone.
• Sinus tract: A bone cavity may persist or the space may be filled with fibrous tissue, which may connect with the skin surface via a sinus tract.

Types of Chronic Osteomyelitis

• A complication of acute osteomyelitis.
• Post-traumatic.
• Post-operative.

Symptoms and Signs

• Low grade fever, if present.
• ESR usually elevated, reflecting chronic inflammation.
• The blood leukocyte count ( WBC ) is usually normal.
• If a sinus tract becomes obstructed, the patient may present with a localized abscess and/or an acute soft-tissue infection.

Chronic Osteomyelitis Organism

• Usually mixed infection.
• Mostly staph. Aureus E. Coli. strep pyogen, and proteus.

Tuberculosis

• The spine is the most common and most dangerous site of skeletal tuberculosis.
• Pott’s abscess.

Pathology

• Blood-borne infection settles in a vertebral body adjacent to the intervertebral disc.
• Bone destruction and caseation follow, with infection spreading to the disc space and to the next vertebra.
• Vertebral body collapse.
• Sharp angulation (kyphosis) develops.

Clinical Features

• Long history of ill-health and backache
• Deformity.
• Paresthesia band weakness.
• Physical examination reveals back pain, kyphosis in the thoracic spine, and restricted movement.

Imaging

• X-ray: On early X-ray studies, there is anterior vertebral body destruction with preservation of disc space, and bone destruction with collapse of adjacent vertebrae, and obliteration of the disc space.
• Mantoux test is positive
• ESR may be raised

Treatment

• Conservative: Anti-tuberculous chemotherapy (6-12 months). Surgical indications: Abscess formation, neurologic deficit, spinal instability, and progressive kyphosis.
• Surgical treatment involves anterior debridement followed by uninstrumented autogenous graft.