Pyogenic Osteomyelitis Pathologic Changes

• Osteomyelitis is the inflammation of bone and marrow, usually secondary to infection.
• Viruses, parasites, fungi, and bacteria are all organisms able to produce osteomyelitis.
• Osteomyelitis can be acute, subacute, or chronic, depending on its duration.

Pyogenic Osteomyelitis Pathologic Changes

• Suppuration

• Ischaemic necrosis (sequestra)

• Healing by fibrosis and bony repair (involucrum)

• Infection often starts in the metaphyseal end of the marrow cavity, which is filled with pus.

• Microscopy reveals congestion, oedema, and an exudate of neutrophils at this stage.

• Increased tension from pus results in spread of the infection into the haversian/Volkmann's canal, endosteum thus casing periosteitis.

• Infection can form subperiosteal abscesses in the subperiosteal space, and can penetrate through the cortex, creating skin sinus tracts.

• Sequestrum is caused by a combination of suppuration and impaired blood supply, which results in the erosion, thinning, and infarction necrosis of the cortex.

• Involucrum is the new bone forms beneath the periosteum over the infected bone over time.

• Chronic sclerosing nonsuppurative osteomyelitis of Garré is a dense sclerotic pattern of osteomyelitis from long-term neo-osteogenesis.

• Brodie's abscess is termed when acute osteomyelitis is contained to a localised area and walled off by fibrous/granulation tissue.

• Vertebral pyogenic osteomyelitis begins from the disc (discitis), and involves the vertebral bodies.

Complications of Osteomyelitis

• Septicaemia
• Acute bacterial arthritis
• Pathologic fractures
• Development of squamous cell carcinoma
• Secondary amyloidosis (long-standing cases)
• Vertebral osteomyelitis may cause vertebral collapse with paravertebral/epidural abscess, cord compression and neurologic deficits

Tuberculous Osteomyelitis

• Tubercle bacilli (M. tuberculosis) often reach the bone marrow/synovium mostly by haematogenous dissemination travelling from the lungs.
• It affects adolescents and young adults more often.
• Spine and bones of extremities are most frequently involved.
• Tuberculous lesions appear as a focus of bone destruction and replacement of affected tissue by caseous material thus including multiple discharging sinuses through the soft tissues and skin.
• Pott's disease is tuberculosis of the spine.

Tuberculous Osteomyelitis Complications

• Compression fractures and destruction of intervertebral discs
• Paraplegia
• Psoas abscess is the extension of caseous material/pus from the lumbar vertebrae to the sheaths of the psoas muscle, also known as lumbar cold abscess.
• Cold abscess may form a sinus by bursting through the skin.
• Systemic amyloidosis can develop in long-standing cases

Fracture Healing Stages

• Procallus formation
• Osseous callus formation
• Remodelling

Procallus Formation

• Haematoma forms due to bleeding from torn blood vessels.
• Fibrin, polymorphs, and macrophages cause local inflammatory response at the injury site.
• Granulation tissue begins with neovascularisation and proliferation of mesenchymal cells from periosteum/endosteum.
• A soft tissue callus is formed, joining the ends of the fractured bone but without much strength.
• After the initial few days, a callus of woven bone/cartilage starts to come into play.
• Cells of inner layer of the periosteum have osteogenic potential and lay down collagen/osteoid matrix in the granulation tissue.
• Woven bone callus happens when osteoid undergoes calcification.
• A wider zone over the cortex that is on either side of fractured ends covered by the woven bone bridge the gap between the ends that gives it a spindle-shaped appearance to the union thus called provisional callus/procallus and it has external, intermediate and internal parts.

Osseous Callus Formation

• Procallus acts as scaffolding for osseous callus composed of lamellar bone.
• Osteoclasts clear away woven bone and calcified cartilage disintegrates.
• Newly formed blood vessels and osteoblasts invade laying down osteoid and calcified lamellar bone that forms Haversian systems around them.

Remodelling

• Lamellar bone creation, osteoblastic laying, and osteoclastic removal remodel the united bone ends.
• Compact bone is formed in place of intermediate callus as bone marrow forms in the external callus.