Musculoskeletal Infection

Infection reaches a joint or bone by three main routes: through the bloodstream, known as haematogenous spread, for example from a distant infection; by direct spread from adjacent infected skin or soft tissue; or by direct inoculation through penetrating trauma, a joint injection or surgery. The commonest organism is Staphylococcus aureus, with streptococci and others also relevant, and gonococcal infection should be considered where the sexual history or features of disseminated gonococcal infection suggest it.

In septic arthritis, bacteria within the joint provoke an inflammatory response that rapidly damages cartilage, which is precisely why speed matters. Osteomyelitis may be acute or chronic; in children it is often haematogenous and affects the growing part of long bones, while in adults it more often follows contiguous spread or inoculation, with diabetes and poor circulation predisposing, particularly in the foot. Vertebral osteomyelitis and discitis affect the spine, and prosthetic joints are especially vulnerable. Recognised risk factors include pre-existing joint disease such as rheumatoid arthritis, a prosthetic joint, diabetes, immunosuppression, intravenous drug use, breaks in the skin, recent injection or surgery, and older age.

Septic arthritis presents as an acutely hot, swollen, painful joint, usually a single large joint such as the knee or hip, with severe pain and very restricted movement and an unwillingness to move or weight-bear. Osteomyelitis presents with localised bone pain, tenderness, warmth and sometimes swelling, which can be more insidious, with systemic features in acute disease. Spinal infection, whether vertebral osteomyelitis or discitis, causes persistent back pain, often with raised blood markers, and can threaten the spinal cord.

Fever is common but may be absent, particularly in older or immunosuppressed athletes and in prosthetic joint and spinal infection, so a normal temperature must not reassure. The essential discipline is to treat a hot, swollen joint as septic arthritis until proven otherwise, even though the differential includes crystal arthritis such as gout and pseudogout, reactive arthritis, a flare of inflammatory arthritis, bleeding into the joint and cellulitis. Misattributing a septic joint to a sports injury or a gout flare, and delaying, is the classic and costly error.

The key investigation in suspected septic arthritis is urgent joint aspiration, sending synovial fluid for an urgent Gram stain, culture and sensitivities, a white cell count and examination for crystals. Samples are ideally taken before antibiotics, but only where this does not delay treatment: in a septic, systemically unwell athlete, blood cultures are taken promptly and intravenous antibiotics started immediately rather than waiting. A negative Gram stain or a relatively low synovial white cell count does not exclude septic arthritis, and finding crystals does not rule out a coexisting infection.

Aspirating an accessible joint such as the knee may be within scope, but deep joints such as the hip and any prosthetic joint are aspirated through the appropriate specialist pathway rather than routinely in an SEM clinic. Blood tests include a full blood count (FBC), C-reactive protein (CRP), erythrocyte sedimentation rate (ESR) and blood cultures, though normal inflammatory markers do not by themselves exclude infection. Plain x-rays are often normal early, ultrasound can confirm an effusion and guide aspiration, and magnetic resonance imaging (MRI) is the most sensitive test for osteomyelitis and spinal infection, supported by bone biopsy and culture where needed.

Septic arthritis is an emergency needing urgent orthopaedic referral for joint drainage, whether by aspiration, arthroscopic washout or open washout, together with empirical intravenous antibiotics after appropriate samples are taken, followed by organism-directed treatment and oral step-down according to the clinical response, microbiology advice and local protocols, with the duration varying by infection and organism. Treatment must not be delayed while awaiting results. Osteomyelitis is treated with a prolonged course of antibiotics, with surgical debridement where there is dead bone or a collection, and prosthetic joint infection needs combined specialist orthopaedic and microbiology management.

Suspected musculoskeletal infection in a child needs urgent hospital assessment involving both paediatrics and orthopaedics, and septic arthritis in a child requires prompt drainage, ideally within 24 hours of diagnosis. The SEM clinician's role is recognition and urgent referral, with supportive care such as analgesia and resting the joint, and not attempting definitive treatment in the community. Prevention of iatrogenic infection sits here too: injections use strict aseptic technique, and a joint is not injected where infection is suspected. If there is any question of a septic joint, the right move is to aspirate and investigate rather than to inject a corticosteroid.

Once the infection is controlled, rehabilitation restores range of movement, strength and function, and recovery is often prolonged, particularly after osteomyelitis or where the joint has been damaged. A joint that has had septic arthritis may be left with stiffness, cartilage damage and a higher long-term risk of osteoarthritis, so structured rehabilitation and follow-up matter.

Return to sport is delayed and individualised, and completing the antibiotic course alone does not establish readiness. Practical clearance depends on the infection having clinically resolved with no fever or wound drainage, inflammatory markers improving, pain-free functional movement, restored strength, and specialist approval, decided with the treating team rather than against a fixed timeline.

• •United Kingdom guidance on the hot swollen joint and septic arthritis, from rheumatology and orthopaedic bodies, underpins the aspirate-before-antibiotics approach.
• •The current British Orthopaedic Association standards covering paediatric musculoskeletal infection, acute prosthetic joint infection and native joint infection set out the specialist pathways.
• •National guidance on related areas, such as the assessment of fever in children and of spinal infection, is also relevant.
• •Sport and exercise medicine sources emphasise not mistaking joint infection for injury and maintaining safe injection practice.

• •Septic arthritis and acute severe musculoskeletal infection are emergencies, and a septic joint can be destroyed within days.
• •A hot, swollen, painful joint is septic arthritis until proven otherwise and needs urgent aspiration and referral.
• •Aspirate before antibiotics where possible, but do not delay treatment in a septic, unwell athlete.
• •A negative Gram stain, a low synovial cell count or crystals in the fluid do not exclude septic arthritis.
• •Magnetic resonance imaging is the most sensitive test for osteomyelitis and spinal infection, fever may be absent, and early x-rays are often normal.
• •Do not inject a joint where infection is possible, and use strict aseptic technique for all injections.