Osteomyelitis

• Inflammation of the bone and marrow

 

Pyogenic osteomyelitis

 

• Almost always cause by bacteria. Organisms can reach the bones via;
  • Haematological spread
  • Extension from a continguous site
  • Direct implantation
• Most cases are haematogenous anddevelop in the long bones or vertebral bodies
• Initial bacteraemia can be due to;
  • Injury to the intestinal muscosa
  • Vigorous chewing of hard food
  • Minor infection of skin
• Most commonly caused by Staph aureus
• E.coli, pseudomonas and klebsiella are more frequently isolated from patients with genital tract infections and those who are I.V drug abusers
• Mixed bacterial infections are seen in the setting of surgery or open fractures
• In the neonatal period, patients are most susceptible to Haemophilus and Group B strep
• Patients with Sickle cell have an increased predisposition for salmonella infection
• In almost 50% of cases no organism can be detected
• The location of the lesion depends upon the vascular circulation which varies with age
• In the neonate the metaphyseal vessels penetrate the growth plate resulting in frequent infection of the metaphysis, epiphysis or both
• In children infection of the metaphysis is common
• After growth plate closure the metaphyseal vessels unite with their epiphyseal vessels and provide a route for bacteria to seed the epiphyses and the subchondral regions

 

Morphology

 

• Once localised in the bone, the bacteria proliferate and induce necrosis of the bone ]the bacterial and inflammation can then spread throughout the shaft and may percolate throughout the Haversian system to reach the periosteum
• Subperiosteal abscesses may form which can track for long distances along the bone
• Lifting of the periosteum impairs the blood supply to the affected region increasing necrosis
• The dead piece of bone is known as the sequestrum
• Rupture of the periosteum leads to an abscess in the surrounding tissue and the formation of a draining sinus
• In infants but less commonly adults, epiphyseal infection spreads through the articular surface to reach the joint and produces a septic arthritis
• Over time, a chronic inflammatory response develops. The release of cytokines stimulates osteoclast reabsorption, growth of fibrous tissue and deposition of reactive bone in the periphery.
• When a sleeve of living tissue forms around a segment of devitalised bone it is known an involucrum

 

Clinical course

 

• Acute systemic illness with malaise, fever, chills, leukocytosis and throbbing pain over affected area
• Characteristic x-ray findings of lytic focus of bone destruction surrounded by a zone of sclerosis
• Blood cultures are generally positive
• Treatment is via antibiotics and surgical drainage
• In 5-25% may develop into a chronic infection where acute flare-ups mark the clinical course
• Other complications of chronic osteomyelitis are;
  • Pathological fracture
  • Secondary amyloidosis
  • Endocarditis
  • Sepsis
  • Development of squamous cell carcinoma in the sinus tract
  • Rarely sarcoma of the infected bone