Septic arthritis

Last revised by Daniel J Bell on 8 May 2024

Septic arthritis is a destructive arthropathy caused by an intra-articular infection which usually manifests as severe symptoms such as pain and decreased range of motion. This condition requires prompt treatment aiming to avoid permanent damage to the joint, which may result in chronic deformity or mechanical arthritis and even death 7.

Risk factors for septic arthritis include:

  • bacteremia

  • advanced age

  • immunocompromised state

  • rheumatoid arthritis

  • intra-articular injections

  • prosthetic joints

The diagnosis of joint sepsis is often considered straightforward. Patients often present with a painful joint, fever and purulent synovial fluid.

In the absence of trauma or recent instrumentation of the joint, septic arthritis is usually secondary to hematogenous seeding. Staphylococcus aureus is the most commonly isolated agent 6 as well as Streptococci spp., Pseudomonas, Escherichia coli, and Serratus 10. 

Haemophilus influenzae was once a common causative agent for septic arthritis in children under the age of 2 but has significantly reduced in incidence due to vaccination 8,9. Gonococcal as a cause is not uncommon in young adults and adolescents 9

Large joints with abundant blood supply to the metaphyses are most prone to bacterial infection, with the most commonly affected joints theoretically being the shoulder, hip, and knee.

In intravenous drug users, the sternoclavicular and sacroiliac joints are more frequently affected.

The Hunka classification is used to grade the anatomical deformity following septic arthritis in a pediatric hip.

Imaging generally plays an adjunct role to arthrocentesis in the diagnosis of joint sepsis. If synovial fluid cannot be retrieved, however, radiologic studies become of the utmost importance.

  • may be normal in the very early stage of the disease

  • joint effusion may be seen

  • juxta-articular osteoporosis due to hyperemia 

  • narrowing of the joint space due to cartilage destruction in the acute phase

  • destruction of the subchondral bone on both sides of a joint

  • if left untreated, reactive juxta-articular sclerosis and, in severe cases, ankylosis will develop 

  • useful in superficial joints and in children

  • shows joint effusion

    • echogenic debris may be present

  • color Doppler may show increased perisynovial vascularity 

  • can be used to guide joint aspiration

  • CT features of septic arthritis are similar to those spotted on radiographs 

  • a fat-fluid level can be a specific sign in the absence of trauma

MRI is sensitive and more specific for early cartilaginous damage with joint effusion being characterization 10

  • T1: low signal within the subchondral bone

  • T2: thin rim of subchondral edema; pericapsular edema 10

  • T1 C+ (Gd): synovial enhancement reflecting synovitis; pericapsular enhancement 10

The treatment principle for septic arthritis is prompt drainage of purulent fluid and appropriate antibiotics 7.

If the patient is hemodynamically stable then attempts should be made to obtain a sample of joint fluid for microscopy and culture prior to starting treatment with antibiotics. This will allow focussed treatment of the infection 4.

With smaller joints needle drainage or aspiration to decompress the joint followed by antibiotics may be adequate. For larger joints or persistent infection, surgical debridement and washout will be required 4.   

If unrecognised and left untreated, septic arthritis can result in irreversible joint damage within 48 hours of the onset of infection due to the proteolytic enzymes of the white blood cells that flood the infected synovial space. Osteonecrosis is also an important sequela of septic arthritis due to effusion and an increase in intra-articular pressure which compromises blood circulation. 

Conversely, approximately 90% of patients with septic arthritis will recover with appropriate antibiotic treatment. Therefore, timely diagnosis and treatment are critical.

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