Septic arthritis, also known as infectious arthritis, refers to the infection and inflammation of a joint. It can occur due to direct entry through trauma, surgery, or injections. It may also occur through haematogenous spread from another infectious site.
It occurs in about 4-6 per 100,000 (and more common in males by 2x approximately). In patients with prosthetic joints or underlying pathology of the joint it increases to 70 per 100,000. Around 1% of uncomplicated hip or knee surgeries result in septic arthritis.
Pathophysiology
Septic arthritis is caused by entry of a pathogen (most commonly bacteria) into a synovial joint.
The manner in which the pathogen entry is by 1 of 3 mechanisms:
- Direct entry - this occurs with penetrating injuries, surgery, or diagnostic/therapeutic procedures.
- Dissemination of osteomyelitis or soft tissue infection - infection of adjacent bones may result in infection. This is more common in the hip and shoulder where the ends of the metaphysis may be encapsulated in the joint capsule.
- Haematogenous spread - this is the most common route in children. Bacteraemia allows bacteria to enter the joint capsule due to their rich vascular. The bacteria then adhere to the synovial membrane and the infection spreads.
Once in the joints, the bacterial toxins and enzymes (such as chondrocyte protease) which destroy the cartilage and bone. This inflammatory response increases the pressure within the joint, leading to occlusion and compression of blood vessels. This in turn leads to necrosis of the bone and cartilage and promotes irreversible joint damage and functional impairment of the joint.
If treatment is delayed, the infection can lead to bacteraemia which can further disseminate into sepsis.
- ⭐️ Staphylococcus aureus and streptococcus pyogenes - responsible for 91% of cases. MRSA is more common with patients with indwelling catheters, recently discharged from hospital, patients with leg ulceration and nursing home residents.
- Neisseria gonorrhoeae (gonococcal arthritis) - in sexually active patients.
- Clostridium tetani and other anaerobes - are associated with penetrating trauma.
- Mycobacterium tuberculosis - in areas where TB is common.
- Salmonella enterica - in patients with sickle cell disease.
⚠️ Risk factors
- Immunosuppression - such as seen in HIV, diabetes, immunosuppressants.
- Osteoarthritis or rheumatoid arthritis and other joint pathology.
- Prosthetic joint and joint surgery
- Aged >80 years old
- IV drug use
- Cutaneous ulcers or skin infections
- Intra-articular steroid injections
- Joint surgery
😷 Presentation
It is usually a single joint (but 22% of patients have oligoarticular disease). It is most commonly the knee or hip joints but theoretically any joint may be affected. It presents acutely and patients present with a sudden onset of symptoms (<2 weeks).
- Painful joint - if it is a weight-bearing joint the patient will be unable to walk. If the patient has underlying joint pathology, the pain will be out of proportion with the normal disease activity.
- Inflamed joint - the joint looks red and is hot and swollen.
- Joint effusion
- Stiffness and reduced range of motion
- Systemic symptoms
- Fever
- Lethargy
- Sepsis
🔍 Investigations
- 🥇 Joint aspiration and culture - done prior to starting antibiotics. The joint fluid may seem purulent
- Bloods:
- CRP and ESR
- Blood cultures
- FBC - a raised white cell count may be seen
- X-ray - may help detect bony erosions and skeletal changes
- Ultrasound - identifies joint effusion and can be used for guidance of joint aspiration
- MRI - identifies joint effusion and complications such as abscess formation or osteomyelitis.
🔢 Criteria
The Kocher criteria is used to diagnose septic arthritis and differentiate it from transient synovitis:
- Fever >38.5ºC
- Inability to weight bear on affected side
- Raised ESR
- Raised WCC
Each component positive is equal to 1 point. A score ≥3 is associated with a 93% probability of septic arthritis diagnosis.
🧰 Management
- IV antibiotics guided by local guidelines and susceptibilities. They are usually continued for 4-6 weeks:
- 🥇 Flucloxacillin
- 🥈 Clindamycin - if the patient has a penicillin allergy.
- 🥇 Vancomycin or teicloplanin - if MRSA
- 🥇 Cefotaxime/ceftriaxone - if gonococcal infection or gram negatives.
- Consideration of joint lavage under general anaesthesia to remove infected material.
- Physiotherapy following the resolution of acute infection to restore joint function.
🚨 Complications
- Osteomyelitis
- Chronic osteoarthritis
- Ankylosis (joint fusion resulting in immobility)