Osteomyelitis: Types, Causes, Pathogenesis

Osteomyelitis

• Osteomyelitis is inflammation of the bone and bone marrow

Classification of Osteomyelitis

• Broadly classified as either acute or chronic
• Acute osteomyelitis includes both hematogenous and non-hematogenous types
• Chronic osteomyelitis involves chronic suppurative osteomyelitis

Acute Hematogenous Osteomyelitis

• It is a blood-borne infection of the metaphysis, typically affecting long bones in children and young adults

Causative Organisms

• Most commonly caused by Staphylococcus aureus, accounting for approximately 90% of cases
• Other causative agents include Escherichia coli, Pseudomonas, and Klebsiella

Pathogenesis

• The primary infection source is often an abscess in the upper respiratory or urinary tract
• Bacteremia ensues, leading to the localization of organisms in the metaphysis of long bones, favored by frequent hematoma and slow blood flow
• This localization results in bone inflammation, necrosis, bacterial toxins and ischemia leading to suppuration
• Osteoclasts separate necrotic bone pieces, forming sequestrum
• Pus spreads towards the cortex and periosteum
• New periosteal bone formation, known as involucrum, encircles the necrotic bone, featuring cloacae for pus drainage
• A subperiosteal abscess may form, spreading to the skin and manifesting as a pus-discharging sinus

Complications

• Spread of infection can occur, including:
  • Direct spread to muscles, tendons, and nerves
  • Involvement of the joint cavity, resulting in sympathetic effusion
  • Spread to blood can cause toxemia, pyemia, and septicemia
• Pathological fractures
• Development of chronic suppurative osteomyelitis
• Altered bone growth
• Secondary amyloidosis
• Malignancy may arise from squamous metaplasia of sinus tracts

Acute Non-Hematogenous Osteomyelitis

• Acute suppurative inflammation of bones that commonly occurs in adults
• Common sites of infection include the skull and long bones
• Caused by mixed bacterial infections

Risk factors

• Direct spread from an infection

Chronic Suppurative Osteomyelitis

• Commonly caused by Staphylococcus aureus
• Typically follows acute suppurative osteomyelitis
• Begins as chronic suppurative inflammation that forms "Brodie's Abscess"

Pathology

• Affects the metaphysis of long bones in children and young adults.
• Presents as a circumscribed area of suppurative inflammation forming a pus cavity surrounded by sclerotic bone

Complications

• Pathological fracture
• Effusion in a nearby joint
• Secondary Amyloidosis

Tuberculous Osteomyelitis

• Occurs in 1% to 3% of individuals with pulmonary or extra-pulmonary tuberculosis

Routes of Infection:

• Blood borne from a focus of active disease
• Direct extension
• Pulmonary focus into a rib or tracheobronchial nodes into adjacent vertebrae
• Lymphatics

Common Sites

• Spine: Affecting thoracic and lumbar vertebrae in 40% of cases.
• Knees
• Hips

Complications

• Resistance to control compared to pyogenic osteomyelitis
• Tuberculous arthritis
• Sinus tract formation
• Amyloidosis

Skeletal Syphilis

• A chronic venereal disease with multiple presentations

Causative Agent

• Spirochete known as Treponema pallidum sub sp. Pallidum
• This is a Gram-negative, slender, corkscrew-shaped bacterium

Types

• Congenital: Osteochondritis and Periostitis
• Acquired

Acquired Syphilis

• Begins early in the tertiary stage, 2 to 5 years after initial infection
• Characterized by "Gummas"
  • These are soft
  • Non-neoplastic
  • Single or multiple
  • Variable-sized lesions

Common Sites

• Nose
• Palate
• Skull
• Long tubular bones, especially the tibia

Syphilitic Saber Shin

• Massive reactive periosteal bone deposition on the medial and anterior surfaces of the tibia

Microscopic Features of "Gummas"

• Centers of necrotic material
• Margins composed of macrophages and fibroblasts
• Surrounded by many plasma cells