Infections of Bones and Joints

Osteomyelitis is inflammation of bone tissue secondary to infection, and can be classified by duration (acute, subacute, chronic), route of infection (haematogenous or exogenous), and host response (pyogenic or granulomatous). Haematogenous osteomyelitis results from bacteraemia seeding the bone, most commonly affecting the metaphysis of long bones in children; Staphylococcus aureus accounts for 90% of cases in children and 30 to 50% of cases in adults. Exogenous osteomyelitis results from direct inoculation or spread from adjacent infection, is often polymicrobial (including gram-negatives such as Pseudomonas and E. coli), and is common in adolescents after trauma and in older adults.

The pathogenesis of untreated osteomyelitis follows a predictable sequence: infection causes inflammation in the inelastic bone tissue, elevating intramedullary pressure and causing vascular congestion and ischaemia. Suppuration follows, with pus formation that may track to form a sinus. Sequestrum (devitalised necrotic bone) forms if treatment is inadequate; if the sequestrum is not cleared, involucrum (new bone laid over the sequestrum) develops, creating a nidus for chronic recurrent infection.

Diagnosis requires aspiration of pus for gram stain and culture before commencing antimicrobials, blood cultures in suspected haematogenous cases, and imaging (X-ray, CT, MRI, bone scintigraphy). Laboratory markers include raised CRP and leukocytosis (neutrophilia). Treatment requires 4 to 6 weeks of IV antimicrobials followed by 2 to 4 weeks oral, reflecting the need to penetrate biofilm. Flucloxacillin is used for S. aureus; vancomycin for MRSA. Surgical debridement is required for chronic cases or open fractures.

Septic arthritis is direct invasion of the joint space by an infectious agent, causing acute inflammation, joint effusion, and loss of mobility. S. aureus is the most common cause; Neisseria gonorrhoeae is a significant cause in sexually active young adults; S. epidermidis causes late-onset prosthetic joint infection (3 to 24 months post-surgery) due to its superior biofilm-forming ability. Diagnosis requires urgent joint aspiration for gram stain and culture. Treatment mirrors osteomyelitis but also requires surgical drainage of infected synovial fluid (except for N. gonorrhoeae), with rapid intervention essential to prevent permanent joint destruction.