Septic arthritis is inflammation of a synovial membrane with purulent effusion into the joint capsule, usually due to bacterial infection. This disease entity also is referred to in the literature as bacterial, suppurative, purulent, or infectious arthritis.
Septic arthritis is a rather rare but important disease that typically affects monoarticular joints. The age range of those affected is broad, from the neonatal period to advanced age. Treatment consists of a combined medical and surgical approach. Septic arthritis usually is divided into gonococcal and nongonococcal arthritis, as clinical and treatment regimens differ. In adults, septic arthritis most commonly affects the knee; in children, infection into the hip joint predominates.
Septic arthritis can quickly destroy a joint and can cause many complications, including osteomyelitis, bony erosions, fibrous ankylosis, sepsis, and even death.
Barriers to successful management include lack of clinical suspicion in the early phase of presentation, delay in definitive diagnostic needle aspiration, and failure to provide adequate drainage of the joint.
In addition, septic arthritis in neonates and infants can be especially treacherous as a result of blunted inflammatory signals and/or confounding infection at a distant site (eg, ear, umbilical catheter site).
Pathophysiology: Various sources of infection exist for the joint space. Bacteria may enter the joint directly as with trauma. Infection may enter hematogenously (eg, intravenous [IV] drug injection). Infection may enter from osteomyelitis that is adjacent to the capsule. Infection also may enter from soft tissue infections (eg, cellulitis, abscess, bursitis, tenosynovitis). According to Orthopaedic Pathology, the knee accounts for approximately 40-50% of infections, and the hip accounts for 20-25% of infections. However, in infants and very young children, hip involvement is the most common joint infection. Shoulders, ankles, and elbows account for approximately 10-15% of infections. Finally, septic arthritis of the wrist occurs in 10% of cases.
Imaging Studies:
- Plain radiographs are of some limited value in evaluating a joint for infection.
- Periarticular soft tissue swelling is the most common finding. Radiographs are most useful in ruling out underlying osteomyelitis or periarticular osteomyelitis resulting from the joint infection itself.
- The linear deposition of calcium pyrophosphate can be detected on a plain radiograph. The radiographic findings of reactive arthritis usually are limited to those of soft tissue swelling. Periarticular osteoporosis may be detected.
- Ultrasonography may be used to diagnose effusions in chronically distorted joints (secondary to trauma or rheumatoid arthritis).
- CT scans and MRIs are more sensitive for distinguishing osteomyelitis, periarticular abscesses, and joint effusions. The information gained usually does not justify the increased cost; however, these tests are most helpful in patients with sacroiliac or sternoclavicular joint infection to rule out extension into the mediastinum or pelvis. MRI is preferred because of its greater ability to image soft tissue.
- Radionuclide scans (ie, technetium Tc 99m, gallium Ga 67, indium In 111 leukocyte scans) are used to nonspecifically localize areas of inflammation. They cannot be used to distinguish infectious from sterile processes. However, they may be of use in diagnosing septic arthritis in relatively sequestered areas, such as the hip and sacroiliac joint.
Procedures:
- Always perform joint aspiration under the most sterile conditions possible to prevent the introduction of infection.
- Obtaining a biopsy of the synovium may be necessary to diagnose one of the many causes (ie, mycobacterial, fungal) of granulomatous synovitis.