You are in: eMedicine Specialties > Radiology > MUSCULOSKELETAL Septic ArthritisArticle Last Updated: Mar 22, 2007AUTHOR AND EDITOR INFORMATIONSection 1 of 12
  Author: Larry Holder, MD, Residency Director, Department of Radiology, Virginia Mason Medical Center Coauthor(s): Matthew Studley, MD, MPH, Consulting Staff, Department of Radiology, Virginia Mason Medical Center Editors: Giuseppe Guglielmi, MD, Associate Professor of Radiology, Department of Radiology, Scientific Institute Hospital; Bernard D Coombs, MB, ChB, PhD, Consulting Staff, Department of Specialist Rehabilitation Services, Hutt Valley District Health Board, New Zealand; William R Reinus, MD, MBA, FACR, Professor of Radiology, Temple University; Chief of Musculoskeletal and Trauma Radiology, Vice Chair, Department of Radiology, Temple University Hospital; Robert M Krasny, MD, Consulting Staff, Department of Radiology, The Angeles Clinic and Research Institute; Felix S Chew, MD, EdM, MBA, Professor, Department of Radiology, Section Head of Musculoskeletal Radiology, Vice Chairman for Radiology Informatics, University of Washington Author and Editor Disclosure Synonyms and related keywords: infectious arthritis, pyogenic arthritis, pyogenic joint infections, nonpyogenic arthritis, non-pyogenic arthritis, nonpyogenic joint infections, granulomatous joint infections, bacterial arthritis, tuberculous arthritis INTRODUCTIONSection 2 of 12
  BackgroundIn 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. PathophysiologyInfection of the synovial membrane precedes contamination of the synovial fluid in most cases. This lack of involvement of the synovial fluid is the proposed explanation for negative Gram stains and cultures that commonly occur in early infections. In response to the bacterial infection, the synovial membrane becomes edematous and hypertrophied. The synovium produces increased amounts of exudative fluid, and eventually, frank pus accumulates within the joint. Rarely, gas accumulates within the joint or adjacent soft tissues secondary to the presence of gas-forming organisms such as E coli. The destruction of joint cartilage occurs secondary to the release of proteolytic enzymes in the synovial exudate. Abnormal fibrin deposition on the articular cartilage disrupts the nutrient supply, leading to further chondrolysis. Rapid destruction of bone and cartilage is less characteristic of nonpyogenic mycobacterial and fungal infections probably because of lower concentrations of proteolytic enzymes in the joint exudate. However, tuberculous granulation tissue containing large numbers of leukocytes and macrophages can erode directly into cartilage or insinuate between the joint cartilage and subchondral bone, leading to detached cartilage and exposed subchondral bone. Abnormalities appear at the joint margins or centrally due to overgrowth and hypertrophy of the synovial pannus and granulation tissue. Eventually, this process may extend into the underlying bone, leading to erosions and osteomyelitis. Rupture of the synovium may lead to extracapsular infectious complications such as myositis and abscess formation in adjacent soft tissues. In infants, septic arthritis is usually due to the hematogenous spread of disease, and it most commonly affects the hip. It may stem from perinatal infection of the umbilicus with hematogenous seeding of the femoral metaphysis. This leads to direct intra-articular extension due to the intracapsular location of the metaphysis. Increased intracapsular pressure reduces blood flow to the epiphysis, which can lead to ischemia compounding the damaging affects of the infection. FrequencyUnited StatesThe incidence of septic arthritis varies dramatically among age groups and among groups with risk factors for the disease. A number of risk factors are associated with septic arthritis. These include chronic illness, rheumatoid arthritis, intravenous drug use, steroids, artificial joint prosthesis, recent joint trauma or surgery, and impaired cellular immunity. Approximately 50% of cases of septic arthritis are due to gonococcal infection in the United States. InternationalAustralian epidemiologists have reported an overall prevalence of 9.2 cases per 100,000 population. Nonpyogenic (granulomatous) joint infections occur less frequently in developed countries than in underdeveloped nations, where the prevalence of tuberculosis is higher. Mortality/MorbidityIn the preantibiotic era, death was a common complication of septic arthritis. Those who survived the acute infection often had severe joint destruction with complete bony or fibrous ankylosis. Tuberculous infection is more likely to result in fibrous ankylosis than pyogenic infections.
SexReports about the sex predilection of septic arthritis have varied with the type of organism and with the geographic location of the study. Results have indicated either no difference or a slight male predominance. AgeSeptic arthritis can occur in those of any age, but it predominantly affects young children.
AnatomySimplistically, joints are composed of bone, articular cartilage, synovial lining, synovial fluid, and a fibrous capsule. Prosthetics or surgical hardware may also be present. Hematogenous infection usually begins with seeding to the synovial lining. This process leads to increased synovial fluid production and pressure within the joint. Frank pus then accumulates in the joint fluid. Edema of the adjacent bones and soft tissues ensues. Erosion of cartilage and bone at the endplates can occur, as can direct extension of infection into the periarticular soft tissues, which leads to muscle or tendon-sheath abscesses. In infant hips, the metaphysis is within the joint capsule, and hematogenous seeding of the metaphysis can lead to direct intra-articular extension. Clinical DetailsSites of involvement The site of involvement depends on the age of the patient, the organism, and the existence of underlying risk factors. In adults, the knee is the most commonly affected joint. In children, the knee and hip are the most common sites. Intravenous drug use may cause infections in unusual joints such as the sternoclavicular and sacroiliac joints. Risk factors A number of risk factors are associated with septic arthritis. These include chronic illness, rheumatoid arthritis, intravenous drug use, steroids, artificial joint prosthesis, recent joint trauma or surgery, and impaired cellular immunity. General presentation Patients usually present with generalized symptoms of an acute systemic infection such as fever, chills, and malaise. The affected joint is often painful, hot, and swollen. In 90% of cases, only 1 joint is involved. Age-related findings Gonococcal arthritis usually affects young sexually active adults, and it appears as an acute inflammatory arthritis of 1 or more joints. Two thirds of patients have an associated dermatitis, and 25% may present with coinciding genitourinary symptoms. Infants can present with irritability, loss of appetite, and fever. Children of walking age may present with a limp. Thigh swelling or erythema may accompany hip infections. Infants and young children may hold the affected hip in flexion, abduction, and external rotation (obturator sign). However, localized clinical signs and symptoms are often absent; this possibility makes the identification of septic joints difficult in children. Patients usually have and elevated erythrocyte sedimentation rate and an increased white blood cell count with a leftward shift. Blood culture results are positive in approximately 50% of patients with nongonococcal septic arthritis. Tuberculous arthritis is usually monoarticular, affecting the hip or knee. Most patients are middle aged or elderly. In contrast to pyogenic arthritis, tuberculosis may result in chronic joint pain with only minimal signs of inflammation. This condition often leads to a delay in diagnosis. Cultures of synovial fluid and histologic evaluation of synovial tissue are best for establishing the diagnosis. Preferred ExaminationImaging 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. Limitations of TechniquesPlain 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. DIFFERENTIALSSection 3 of 12
Osteoporosis, Involutional RADIOGRAPHSection 4 of 12
FindingsThe earliest plain film radiographic findings 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, 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 ConfidenceInitial 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/NegativesPoorly 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. CT SCANSection 5 of 12
FindingsAlthough 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. 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 ConfidenceSome 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/NegativesAlthough 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. MRISection 6 of 12
FindingsWhile plain radiography should be the initial imaging study in patients with a suspected septic joint, MRI has been increasingly used to evaluate musculoskeletal infections, including septic joints. MRI is a sensitive and relatively specific imaging modality. A combination of T1-weighted, T2-weighted, short-tau inversion recovery, and postcontrast T1-weighted fat-suppressed series are most helpful. 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. Gadolinium-based contrast agents (gadopentetate dimeglumine [Magnevist], gadobenate dimeglumine [MultiHance], gadodiamide [Omniscan], gadoversetamide [OptiMARK], gadoteridol [ProHance]) have recently 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. As of late December 2006, the FDA had received reports of 90 such cases. Worldwide, over 200 cases have been reported, according to the FDA. NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with trouble movingor straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness. For more information, see the FDA Public Health Advisory or Medscape. Degree of ConfidenceA 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. False Positives/NegativesInfected 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. ULTRASOUNDSection 7 of 12
FindingsUltrasonography 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. NUCLEAR MEDICINESection 8 of 12
FindingsEarly-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 ConfidenceScintigraphy with 99mTc 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 99mTc hexamethyl propylamine oxime (HMPAO) labeled, 99mTc-labeled antigranulocyte monoclonal antibodies, and gallium-67 citrate have also been used to evaluate to septic arthritis and osteomyelitis with increased specificity. However, 99mTc MDP remains the mainstay of scintigraphic imaging. 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/NegativesAny 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. ANGIOGRAPHYSection 9 of 12
FindingsAngiography has no practical role in the evaluation of septic arthritis. INTERVENTIONSection 10 of 12
Antibiotics are the mainstay of treatment. Antibiotic therapy should depend on Gram stain results and clinical suspicion. The age of the patient and presence of a prosthetic joint are also important considerations. Antibiotic therapy has greatly reduced the morbidity and mortality of the disease. Prompt treatment usually limits long-term damage. Patients who are symptomatic for more than 7 days before the disease is diagnosis tend to have more-severe joint damage. S aureus and gram-negative bacilli tend to be more destructive organisms. Symptoms of gonococcal infection should improve dramatically within 3 days and resolve completely within 10 days of the initiation of therapy. Prolonged therapy may be needed for tuberculous infections. The rapid accumulation of fluid within the affected joint may require daily needle aspiration. Open drainage or arthroscopy are necessary in severe cases and often used for hip infections in children. MULTIMEDIASection 11 of 12
REFERENCESSection 12 of 12
Article Last Updated: Mar 22, 2007 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||
