Septic Arthritis Imaging 

In general, infectious arthritis is classified as pyogenic (septic) or nonpyogenic. Pyogenic septic arthritis is most frequently caused by Staphylococcus aureus. It also may be caused multiple other organisms, including staphylococci, Streptococcus pneumoniae, group B streptococci, Gonococcus species, Escherichia coli, Haemophilus species, Klebsiella species, Pseudomonas species, and Candida species. Infection can lead to rapid and severe joint destruction.

Nonpyogenic infective arthritis tends to be less aggressive and have a more chronic course. Causative organisms include Mycobacterium tuberculosis, fungi, and spirochetes.

Septic arthritis can be acquired through several routes of transmission. The most common cause is hematogenous spread to a joint from a distant source such as pneumonia or a remote wound infection. Direct seeding from can occur through trauma, surgery, or spread from a contiguous infection such as osteomyelitis or cellulitis.

Septic arthritis images are provided below.

Preferred examination

Imaging is not the primary means of diagnosing septic arthritis. Joint fluid aspiration and evaluation is the key to the diagnosis, and samples should be obtained in all suspected cases of septic arthritis. This sampling can usually be achieved with fine-needle aspiration performed either blindly or with fluoroscopic guidance, depending on the location. Surgical exploration may be necessary in unusual cases, such as those involving sacroiliac and sternoclavicular joint infections.

Fluid should be sent for Gram staining, culturing, glucose testing, and leukocyte count and differential determination. White blood cell counts are usually 50,000-60,000/µL, with more than 80% neutrophils. Synovial fluid glucose levels are decreased. Gram stain results are positive in 75% of patients with gram-positive cocci. Gram staining is less sensitive in cases of gonococcal infection. Only 25% of cultures of gonococcal synovial fluid are positive.

Multiple imaging modalities are available for assessing septic arthritis. Plain radiography should be used as the initial study. However, if further imaging is required, MRI is the most sensitive and specific technique. Scintigraphy, CT, and ultrasonography are also used, to a lesser extent.^{[1, 2, 3, 4]}

Limitations of techniques

Plain radiographs are not sensitive to early findings, such as joint effusion or soft tissue changes.

MRI is expensive and time consuming. It is usually unnecessary if clinical suspicion is high and if the joint is easily accessible for aspiration.

CT is similar to MRI, but it has the disadvantage of ionizing radiation. However, it can be useful for guiding the aspiration of certain joints.

Scintigraphy is extremely sensitive but extremely nonspecific.

Ultrasonographic findings can confirm a joint effusion, but it cannot be used to assess its cause. It cannot accurately depict bony or cartilaginous abnormalities. This modality also may be useful to guide joint aspiration, and it is generally less expensive than either CT or MRI.

The earliest plain film radiographic findings of septic arthritis are soft tissue swelling around the joint and a widened joint space from joint effusion. Displacement of adjacent fat pads may be present, especially in infants and children.

With progression of the disease, as shown in the images below, plain films reveal joint-space narrowing as articular cartilage is destroyed. Loss of visualization of the white cortical line over large areas of the joint surface soon ensues as bone destruction begins to develop. This is followed by marginal erosions as uncovered, intracapsular bone is destroyed. Plain film findings of superimposed osteomyelitis may develop (periosteal reaction, bone destruction, sequestrum formation).

A triad of radiographic abnormalities known as Phemister triad is characteristic of tuberculous arthritis: peripherally located bony erosions, juxta-articular osteoporosis, and gradual narrowing of the joint space.

Gas within the joint or adjacent soft tissues can sometimes be seen in infection secondary to gas forming organisms such as E coli or Clostridium perfringens. However, gas within the joint is usually secondary to prior aspiration or vacuum phenomenon, which can occur secondary to limb traction during positioning for the examination or a recent dislocation.

Plain radiographic findings in the infant hip include obliteration of soft tissue planes, swelling, displacement of the fat pads, the obturator sign, and juxta-articular osteoporosis. Subluxation or dislocation of the femoral head secondary to intra-articular fluid can occur. However, this can be difficult to identify if the femoral head is not ossified.

In children, lateral displacement of the femoral epiphysis relative to the contralateral hip signifies a joint effusion. As little as 2 mm of asymmetry in the distance measured from teardrop of the acetabulum to the medial metaphysis of the femoral neck is considered pathologic.

Degree of confidence

Initial plain radiographic findings are frequently normal. The characteristic findings are somewhat nonspecific by themselves, but they can be nearly diagnostic when correlated with the clinical picture.

False positives/negatives

Poorly defined bony erosions are characteristic of septic arthritis and a helpful feature in differentiating septic arthritis from other diseases in the differential diagnosis. Osseous erosions in gout, rheumatoid arthritis, seronegative spondyloarthropathies, pigmented villonodular synovitis, hemophilia, and synovial osteochondromatosis tend to be sharply marginated.

Although not commonly used for the evaluation of joint infections, CT scanning can be accurate in the evaluation of septic arthritis. CT scanning is very helpful when evaluating the sternoclavicular joint and the sacroiliac joint, which may be difficult to evaluate using plain films. CT scanning can depict early findings in septic arthritis, such as synovial thickening or joint effusion. Periarticular abscesses or fluid collections can also be identified.^[5]

CT findings are otherwise similar to those on plain radiographs and include joint effusion, joint-space narrowing, bone and cartilage erosions, gas within the joint, and soft tissue swelling.

Again, CT scanning can be used to guide joint aspirations in uncommon or difficult sites such as the sacroiliac or sternoclavicular joints.

Degree of confidence

Some authors report that the sensitivity of CT is similar to that of MRI. However, joint aspiration should be performed in all cases of suspected septic arthritis, and it is the primary means of diagnosis.

False positives/negatives

Although associated findings of osteomyelitis or soft tissue abscess increase the specificity of CT, distinguishing septic arthritis from other diseases in the differential diagnosis may be difficult.

While plain radiography should be the initial imaging study in patients with a suspected septic joint, MRI, as shown in the images below, has been increasingly used to evaluate musculoskeletal infections, including septic joints. MRI is a sensitive and relatively specific imaging modality.^[6] A combination of T1-weighted, T2-weighted, short-tau inversion recovery, and postcontrast T1-weighted fat-suppressed series are most helpful.^[7]

Synovial enhancement and the presence of a joint effusion have been reported to have the highest correlation with the clinical diagnosis of a septic joint. Perisynovial soft tissue edema is also commonly seen by MRI in patients with a septic joint. However, the absence of a joint effusion, especially in the small joints of the hands and feet, does not exclude infection of the joint.

Patients with tuberculous arthritis may have more bone erosions and less marrow-signal abnormality on MRI than patients with pyogenic arthritis. Use of intravenous gadolinium contrast is also very helpful in patients with a suspected septic joint to distinguish a periarticular abscess from surrounding myositis and to evaluate the degree of synovial inflammation.^[8]

Gadolinium-based contrast agents (gadopentetate dimeglumine [Magnevist], gadobenate dimeglumine [MultiHance], gadodiamide [Omniscan], gadoversetamide [OptiMARK], gadoteridol [ProHance]) have been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). For more information, see the eMedicine topic Nephrogenic Fibrosing Dermopathy. The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans.

Degree of confidence

A clinically consistent history and the extra-articular findings of bone marrow edema or soft tissue inflammation are important in increasing the specificity of MRI for septic arthritis.

The sensitivity and specificity of MRI is increased in the setting of associated osteomyelitis. MRI is as sensitive as technetium-99m methylene diphosphonate (MDP) scintigraphy in detecting osteomyelitis, and it is more sensitive than other scintigraphic techniques. It is excellent for the evaluation of soft tissues because of its high spatial resolution and multiplanar capability.^[9]

False positives/negatives

Infected and noninfected joint effusions have the same signal intensity characteristics and cannot be distinguished by using MRI.

On T2-weighted images, high signal intensity in the adjacent bone marrow helps in differentiating septic arthritis from synovitis. However, increased signal intensity does not necessary indicate osteomyelitis. It can be secondary to hyperemia due to the nearby infection or other etiologies. Thus, MRI is sensitive but not necessarily specific.^[10]

Ultrasonography is limited in the evaluation of septic arthritis. It is a sensitive modality for the detection of joint effusions in many anatomic locations. However, it is not reliable in characterizing the effusion or its cause. The thickness of the capsule and the echogenicity of the fluid are not good predictors of infection in the joint. Occasionally, ultrasonography can be helpful for guiding needle aspiration of the affected joint.

Early-phase (blood flow) and later (blood pool) images show increased activity at the joint and on both sides of the affected area. Delayed images obtained at 4-6 hours should demonstrate continued increased activity in the bone with associated osteomyelitis.

Decreased uptake in the femoral head can be seen with decreased perfusion related to high intra-articular pressures from the joint effusion.

Degree of confidence

Scintigraphy with^99m Tc MDP is extremely sensitive, though not specific, for septic arthritis. Three-phase bone scanning has a reported sensitivity of 90-100% and a specificity of 73-79% for osteomyelitis. These values are likely decreased in septic arthritis without associated osteomyelitis.

Newer agents such as indium In–labeled autologous white blood cells, leukocytes labeled with^99m Tc hexamethyl propylamine oxime (HMPAO) labeled,^99m Tc-labeled antigranulocyte monoclonal antibodies, and gallium-67 citrate have also been used to evaluate to septic arthritis and osteomyelitis with increased specificity. However,^99m Tc MDP remains the mainstay of scintigraphic imaging.^{[11, 12]}

A positively labeled white cell scan is specific. A positive 3-phase bone scan is specific if no other factors that could cause increased bone turnover are present. Thus, bone scanning is most useful if radiographic results are normal. If other factors (eg, trauma, arthritis) that could cause a false-positive bone scan are present, results should be confirmed with another study such as white blood cell scanning.

False positives/negatives

Any process that results in increased bone turnover (eg, trauma, nonseptic arthritis) can result in a false-positive finding. Reflex sympathetic dystrophy could cause uptake on both sides of a joint, but it can be differentiated by assessing the clinical history and by finding involvement of all the joints in the affected extremity rather than a single joint.

Angiography has no practical role in the evaluation of septic arthritis.