Bone and Joint Infections — Osteomyelitis, Septic Arthritis and Pott Spine (Tuberculous Spondylitis)

Three conditions dominate exam questions: osteomyelitis (infection of bone), septic arthritis (infection of a joint) and tuberculous spondylitis (Pott disease) (TB of the spine). All three can spread into each other — osteomyelitis can break into a joint, and a vertebral infection can produce paraspinal and psoas abscesses — so recognise the typical site, organism and imaging pattern of each.

Healthy intact bone resists infection. It becomes vulnerable after a large inoculum from trauma, ischaemia or a foreign body, because exposed bone sites let bacteria bind. S. aureus expresses adhesins for bone matrix components (laminin, collagen, fibronectin, bone sialoglycoprotein) and can survive inside cells.

In long bones the metaphysis is the usual site of haematogenous infection: it is well perfused, has few functioning phagocytes, and slowing of blood flow in the vascular loops of the metaphysis lets microbes lodge. Phagocytes release enzymes that lyse bone, forming pus and raising intramedullary pressure; the exudate can then burst through the cortex into the subperiosteal space.

• Lew and Waldvogel classification: acute vs chronic, and haematogenous vs contiguous (contiguous further divided by presence of vascular insufficiency).
• Cierny and Mader classification guides management: anatomic stage — 1 medullary, 2 superficial, 3 localised, 4 diffuse — plus host class (A normal, B compromised systemically or locally, C when treatment is worse than the disease).
• Vertebral osteomyelitis is usually haematogenous and involves two adjacent vertebral endplates because segmental arteries supply adjacent endplates; Batson's venous plexus explains spread from a urinary focus in older men.

*Staphylococcus aureus* is the most common cause of acute and chronic haematogenous osteomyelitis in both adults and children, and MRSA now accounts for over one-third of staphylococcal isolates in some studies. Other organisms depend on the setting.

ESR and CRP are usually raised but non-specific; CRP is useful for monitoring response. Blood cultures may be positive, especially in haematogenous vertebral, clavicular or pubic disease. Bone biopsy (open preferred) gives the histological diagnosis, the organism and its sensitivities; superficial wound or sinus-tract cultures correlate poorly with bone cultures.

Treatment has two pillars: surgical containment (debridement of all diseased bone, because antibiotics penetrate poorly into abscesses and dead bone) and prolonged antibiotics. Adults usually receive 4 to 6 weeks of parenteral therapy; a shorter course is acceptable when infected bone is completely removed with clean margins. Revascularise an ischaemic limb and control diabetes where relevant; hyperbaric oxygen is not routine.

Septic arthritis is joint inflammation from infection, usually bacterial. It is classically monoarticular in a large joint, presents with acute pain, swelling, fever and refusal to move the joint, and is an orthopaedic emergency because inflammatory cytokines and proteases can rapidly destroy the joint. The knee is the commonest joint in adults, followed by hip, shoulder and ankle. The synovium is highly vascular and has no limiting basement membrane, so it is easily seeded by blood-borne bacteria; contiguous spread from adjacent osteomyelitis is also possible.

Epidemiology: incidence 2–6 per 100,000 population. In children the peak is at 2–3 years with a male predominance (2:1). Adult risk factors include age over 80, diabetes, rheumatoid arthritis, recent joint surgery, joint prosthesis and previous intra-articular injections. In-hospital mortality is quoted at 7–15%, and about one-third of patients have lasting morbidity.

The most useful test is arthrocentesis with synovial fluid analysis (Gram stain, culture, crystals, cell count and differential). A synovial fluid WBC count above 50,000 with 90% neutrophils suggests a bacterial cause, and identifying an organism confirms the diagnosis. Take blood cultures (two sets) before antibiotics. A normal plain X-ray does not exclude septic arthritis; ultrasound identifies effusions and guides aspiration, and MRI detects early fluid and soft-tissue or bone involvement.

Pott disease, named after Percivall Pott (1779), is tuberculosis of the spine. TB spondylitis is the most common form of osteoarticular tuberculosis (about 50–70% of osteoarticular TB) and children account for about half of cases. The organism reaches the vertebrae from a lung focus through the rich vertebral blood supply, through paraaortic lymphatics, or via the paravertebral venous plexus from genitourinary TB.

• Site: the thoracolumbar junction most commonly, then lumbar, and less often cervical vertebrae.
• Pathology: the paradiscal subchondral bone is involved first; the bony cortex and later the intervertebral disc are destroyed, producing anterior column collapse, wedging and a gibbus (sharp angular kyphosis).
• Spread: subligamentous, with a paravertebral, psoas or lumbar paraspinous abscess that may point in the groin, thigh or gluteal region; cervical disease may form a retropharyngeal abscess.
• Presentation: chronic local back pain and tenderness for months to years; fever and weight loss are uncommon (20–30%); paraplegia is present at presentation in 20–75% of patients.

A definitive diagnosis needs tissue: culture or a molecular probe positive for the organism. Histology may show epithelioid granulomas, lymphocytes and multinucleated giant cells; acid-fast smears are positive in fewer than 50%, so the diagnosis often rests on imaging, histology and high clinical suspicion.

Treatment: all patients receive anti-TB chemotherapy — an intensive phase of 2 months with isoniazid, rifampicin, pyrazinamide and ethambutol, then a continuation phase of isoniazid and rifampicin for 4 to 16 months (duration varies because large trials are lacking). Surgery is used for intractable pain, nerve compression, kyphosis correction, spinal instability, large abscesses or failure of medical therapy. Prognosis is excellent when treatment starts before vertebral collapse or severe neurological deficit. See also anti-tubercular drugs and the NTEP regimens.

• *S. aureus* is the commonest cause of osteomyelitis and septic arthritis; Salmonella in sickle cell disease; *Kingella kingae* in children under 2–3 years; gonococcus in sexually active young adults.
• Haematogenous osteomyelitis begins in the metaphysis of long bones in children; in adults the vertebrae are most often affected.
• Sequestrum (dead bone), involucrum and cloaca are the imaging features of chronic osteomyelitis; necrotic bone and sinus tracts are its histological hallmark.
• X-ray changes lag by ~14 days; MRI detects disease in 3–5 days; CT best shows sequestra.
• Septic arthritis: aspirate, then drain and treat; synovial WBC >50,000, 90% neutrophils.
• Pott spine: thoracolumbar junction, paradiscal, anterior collapse, gibbus, thin-walled abscess, 2 months HRZE then HR.
• Late-onset paraplegia of Pott disease results from severe deformity long after infection has healed.