Osteomyelitis and Septic Arthritis

• Osteomyelitis and septic arthritis are infections of the bone and joint, respectively

Definitions

• Osteon: bone
• Myelo: marrow
• "itis": inflammation
• Osteomyelitis is inflammation of one or multiple parts of the bone (periosteum, medullary cavity, and cortical bone) by infecting microorganisms.
• Osteomyelitis can be localized or spread to the soft tissue surrounding the bone.

Classification of Osteomyelitis

• Classification is based on duration and source of infection
• Acute osteomyelitis lasts less than 2 weeks.
• Subacute osteomyelitis lasts for 2-6 weeks.
• Chronic osteomyelitis lasts more than 6 weeks.

Source of Infection

• Exogenous source: Post-trauma or surgical contamination.
• Hematogenous dissemination (endogenous osteomyelitis):
  • In infants and children, it involves the metaphysis of long bones.
  • In adults, it involves vertebral bodies.
• Contiguous sources:
  • from joints infection
  • Soft tissue
  • Common in diabetic patients and patients with severe vascular disease.
  • Multiple organisms involved: aerobic and anaerobic.

Causative Microorganisms

• Pyogenic (suppurative) osteomyelitis
• Tuberculous osteomyelitis: M. tuberculosis

Causative Organisms of Pyogenic Osteomyelitis

• Staphylococcus aureus is the most common cause of all types of osteomyelitis in adults and children, mainly through hematogenous spread.

Bacterial Causes

• Infants <1 year: Staphylococcus aureus, Streptococcus agalactiae, E.coli.
• Children > 1 year: S. aureus, Streptococcus pyogenes, Haemophilus influenza, Kingella kingae.
• Adults: S. aureus
• Other bacterial causes:
  • S. epidermidis: foreign-body-associated infection (prosthesis).
  • Pseudomonas aeruginosa: complication of localized infections.
  • Anaerobic bacteria: diabetic foot lesions.
  • Salmonella spp., and Streptococcus pneumonia: splenectomy and sickle cell disease.
  • Treponema pallidum: congenital syphilis.
  • Bartonella henselae: HIV.

Risk Factors of Osteomyelitis: Host Factors

• Normal bone is resistant to infection
• Occurrence of bone infection depends on host factors
  • Malnutrition
  • Renal or hepatic failure
  • Diabetes mellitus
  • Malignancy
  • Chronic hypoxia (sickle cell disease)
  • Immune suppression
  • Splenectomy
  • Defected bone vascularity (arteritis, venous stasis)
  • Radiation fibrosis
  • Neuropathy
  • HIV/AIDS

Microbial Virulence Factors

• Staphylococcus aureus:
  • Adhesins: fibronectin-binding protein, collagen-binding protein, clumping factor-adherence
  • Staphylococcal surface protein A: prevent phagocytosis.
  • Exotoxins: kill the phagocytes and inhibit B cells differentiation.
  • Biofilm formation.
• Streptococcus species:
  • Protein M
  • Fibronectin-binding protein.
  • Teichoic acid.
• Gram-negative bacteria:
  • Pili (adherence)

Pathogenesis

• Hematogenous spread
• Exogenous source: post-trauma or surgical contamination
• Contiguous focus: mostly in patients with vascular disease
• Microorganisms cause bacterial proteolytic enzymes & toxins
• Inflammation and ischemia occur
• Bone necrosis and destruction lead to sequestrum formation.

Osteomyelitis in Infants

• Bones are soft, and the periosteum is loosely attached to the cortex
• Infection spreads into the subperiosteal region, resulting in a sub-periosteal abscess.
• Transphyseal vessels spread infection from the metaphysis to the epiphysis and adjacent joint space.

Osteomyelitis in Older Children and Adolescents

• Transphyseal capillaries are no longer present
• Infection is confined to the metaphysis and the thicker cortex
• Dense periosteum limits the spread to soft tissues.
• Possible bone necrosis leads to Brodie abscess, sequestrum, and involucrum formation.

Osteomyelitis in Adults

• Mostly occurs in the vertebrae
• Adult vertebral bone is highly vascular
• Aging causes corkscrew anatomy in vessels, predisposing to bacterial hematogenous seeding
• The sternoclavicular joint, pelvic bones, and long bones are primarily affected in intravenous drug users.

Clinical Features

• Acute:
  • Sudden onset
  • High fever, chills, localized pain, restriction of movement, swelling, redness, and tenderness.
• Subacute and chronic:
  • Develops slowly
  • Brodie abscess, sequestrum formation, and involucrum
  • Mild pain, low-grade fever, swelling, redness
  • Bone deformity, local sinus draining pus.

Diagnosis: Radiology

• Plain X-Ray: soft tissue swelling, periosteal thickening, cortical erosion or destruction, osteopenia, or Brodie abscess
• Magnetic resonance imaging (MRI) or CT scan

Diagnosis: Laboratory Tests

• Inflammatory markers: marked leukocytosis ≥ 20,000, high C-reactive protein, and ESR.
• Specimen collection: Pus aspiration, bone biopsy, blood culture
• Microscopy: Gram stain
• Culture and sensitivity: Aerobic and anaerobic cultures
• PCR: Nucleic acid detection

Characteristics of Common Bacteria Causing Osteomyelitis - Staphylococcus aureus

• Microscopic morphology: Gram-positive cocci in clusters
• Culture characteristics: golden yellow colonies on nutrient agar
• Beta hemolysis on blood agar
• Identification test: catalase test (positive) and coagulase test (positive)

Staphylococcus epidermidis

• Gram’s positive cocci in clusters.
• Catalase test positive.
• Coagulase test negative.

Streptococcus pyogenes

• Microscopic morphology: gram positive cocci in chains.
• Cultural characteristics: Beta hemolysis on blood agar
• Catalase negative.

Streptococcus pneumoniae

• Microscopic morphology: Gram positive diplococci (lancet shaped), capsulated.
• Culture characteristics: Alpha hemolysis on blood agar

Gram-Negative Bacilli

• Pseudomonas aeruginosa: exo-pigment
• E. coli: lactose fermenting colonies
• Haemophilus influenzae: grows on chocolate agar

Treatment

• Surgical debridement is required
• Start empirical treatment:
  • Cover gram-positive organisms: vancomycin
  • Cover gram-negative organisms: meropenem or third-generation cephalosporin or ciprofloxacin
• Shift according to culture and sensitivity.
• Antibiotics are given for 4–8 weeks.

Complications

• Sepsis, Septic arthritis
• Pathological fractures.
• Formation of a draining sinus.
• Muscle-wasting & atrophy.
• Marjolin ulcer (squamous cell carcinoma that arises from chronic, non-healing wounds).

Tuberculous Osteomyelitis

• Direct extension from a pulmonary focus into a rib or from tracheobronchial nodes into vertebrae
• Primarily affects thoracic and lumbar vertebrae
• Spinal involvement is called Pott’s disease, leading to Psoas abscesses

Diagnosis for Tuberculous Osteomyelitis

• Radiology
• Laboratory tests:
  • Inflammatory markers
  • Direct microscopy: Ziehl-Neelsen stain
  • Culture and sensitivity: Aerobic on Löwenstein-Jensen medium

Septic Arthritis

• Arthritis: Inflammation of joint.

Classification for Arthritis

• Septic arthritis.
• Aseptic arthritis.

Risk Factors for Septic Arthritis

• Age >80 years, neonates
• Presence of prosthetic joint
• Recent joint surgery
• Diabetes mellitus
• Pre-existing joint disease: Rheumatoid arthritis, osteoarthritis, gout
• Intravenous drug abuse

Causative Bacteria of Septic Arthritis

• S. aureus: most common causative organism
• In addition to the same causative agents of osteomyelitis
• Neisseria gonorrhoeae: gram-negative cocci.
  • Sexually active adults
  • Complement deficiency
  • Infants < 3 months of age

Sources of Bacteria

• Hematogenous spread
• Direct introduction of microbes from external sources: trauma, surgery
• Contiguous sources: osteomyelitis
• Pathogenesis: Bacterial toxins and enzymes cause inflammation, synovial effusion and cartilage degradation

Clinical Manifestations for Septic Arthritis

• Fever, joint pain, swelling, warmth, and restricted movement
• Usually single joint: knee (>50 %), wrists, ankles, and hips.

Laboratory Diagnosis for Septic Arthritis

• High WBCs count > 90% neutrophils, high C-reactive protein and ESR.

Microbiological Tests for Septic Arthritis

• Specimens: Synovial fluid aspiration
• Direct Microscopy:
  • Gram stain
• Culture: on blood, chocolate agar or Thayer-Martin agar
• PCR

Treatment & Management for Septic Arthritis

• Arthrocentesis: drainage of infected synovial fluid
• Antimicrobial therapy according to gram stain results:
  • Gram-positive cocci: vancomycin
  • Gram-negative bacteria: Meropenem or third-generation cephalosporin or ciprofloxacin
• Shift according to culture and sensitivity results