Musculoskeletal Imaging

Septic arthritis

an infectious (usually bacterial) inflammation of joints. Among the clinical characteristics are a symptom-free interval between the inciting infection and the rheumatic reaction, a self-limiting course, acute migratory polyarthritis, fever, an elevated erythrocyte sedimentation rate, and a negative serological test for rheumatoid factor.

An infectious agent may trigger a sterile, or reactive, synovitis at a site distant from the primary infective focus, as is typical in the arthritis of acute rheumatic fever, Reiter's syndrome, intestinal bypass surgery and hepatitis. Infection occurs by four pathways:

  • haematogenous spread of infection;

  • spread from a contiguous source of infection;

  • direct implantation of infectious organisms; or

  • postoperative infection.

    Although numerous organisms may cause septic arthritis, some of the more common agents are Staphylococcus aureus, Streptococcus pyogenes, Streptococcus pneumoniae and Neisseria gonorrhoeae.

    Complications of septic arthritis may include epiphyseal destruction, osteonecrosis and disruption of adjacent capsular, tendinous and soft tissue structures. Tears of the rotator cuff may occur when the glenohumeral joint is involved. Partial or complete bone fusion may occur in patients with septic arthritis.

    Correlation of radiographic and pathologic findings is shown in Table 1.

    Septic arthritis, Table 1. Radiographic-pathologic correlation in septic arthritis.

    Pathological findingRadiographical finding
    Oedema and hypertrophy of synovial membrane with fluid productionJoint effusion, soft tissue swellling
    HyperaemiaOsteoporosis
    Inflammatory pannusJoint space loss with chondral destruction
    Pannus destruction of boneMarginal and central osseous erosion
    Fibrous or bony ankylosisBone ankylosis

    Septic arthritis of the hip in infancy and childhood is a common entity leading to displacement of the femoral head or metaphysis. As this displacement also occurs in infants with developmental dysplasia of the hip, neurologic deficits and traumatic epiphyseal separations, aspiration of the joint is mandatory for diagnosis of septic arthritis.

    The sacroiliac joint may become infected haematogenously or from contamination from an adjacent vaginal, uterine, ovarian, bladder or intestinal infection. Pressure sores caused by prolonged immobilization can also lead to subsequent bone and joint infection. Erosions and condensation of bone of variable degree are common. With treatment, intra-articular osseous fusion may be encountered.

    Scintigraphy may show changes at a time when findings on routine radiographs are ambiguous. CT scanning is also valuable in early diagnosis. MR imaging may reveal marrow oedema in the sacrum and ilium, irregularity of the subchondral bone, joint fluid, muscle oedema, and fluid-filled channels, sinus tracts and fistulae.

    Septic arthritis and rheumatoid arthritis may coexist, especially in patients treated with steroids and immunosuppressive agents. Multiple joints are frequently involved.

    DR