Pediatric Annals

Special Issue Article 

Pyogenic Arthritis

Jenna E. Holmen, MD; Jumi Yi, MD


The incidence of septic arthritis among children in developed countries is estimated to be 4 to 10 cases per 100,000 children per year, peaking at about age 3 years. The most common causative organism is Staphylococcus aureus, although the microbiology varies by age. Prompt diagnosis and treatment is critical to prevent long-term sequelae. Empiric therapy should target the most likely causative organism(s) and total duration generally falls between 10 days and 4 weeks depending on clinical course, patient age, and organism. A short intravenous course is sufficient in most cases. Unusual and alternate causes of arthritis should be considered in special cases. [Pediatr Ann. 2019;48(9):e354–e359.]


The incidence of septic arthritis among children in developed countries is estimated to be 4 to 10 cases per 100,000 children per year, peaking at about age 3 years. The most common causative organism is Staphylococcus aureus, although the microbiology varies by age. Prompt diagnosis and treatment is critical to prevent long-term sequelae. Empiric therapy should target the most likely causative organism(s) and total duration generally falls between 10 days and 4 weeks depending on clinical course, patient age, and organism. A short intravenous course is sufficient in most cases. Unusual and alternate causes of arthritis should be considered in special cases. [Pediatr Ann. 2019;48(9):e354–e359.]

The incidence of atraumatic limp is unknown but has been estimated to be roughly 1.8 per 1,000 children presenting for care.1 Although the vast majority have a benign course, such as that seen with transient synovitis, prompt diagnosis of pyogenic arthritis is critical due to associated morbidity. The estimated incidence of pyogenic arthritis is 4 to 10 cases per 100,000 children per year in developed countries.2,3 All age groups can be affected, although the peak incidence is in children younger than age 5 years,4 with transient synovitis incidence peaking between age 4 and 8 years.5 Pyogenic arthritis occurs mainly via hematogenous seeding and, with the rare exception of penetrating injuries, is not correlated with preceding trauma. However, there can be clinical circumstances that may provide clues to the causative organism. Penetrating trauma is most commonly associated with Pseudomonas and anaerobes,4 upper respiratory symptoms are associated with Haemophilus influenzae and Kingella kingae,2,4,6 and gastrointestinal symptoms and aphthous ulcers are associated with K. kingae.6Staphylococcus aureus is generally described as the most common pathogen, but K. kingae is increasingly recognized as a significant causative organism.4 Use of newer culture techniques and molecular testing, namely polymerase chain reaction (PCR), have shown a much higher prevalence of K. kingae bone and joint infections than previously thought and it eclipses S. aureus as the most common causative agent in a number of studies evaluating children younger than age 4 years.7,8 Other organisms that are commonly involved include Streptococcus pyogenes, S. pneumoniae, and Salmonella. See Table 1 for a distribution of common organisms by age. Organisms such as Borellia burgdorferi, Brucella, Neisseria gonorrhea, Bartonella, and Group B Streptococcus should be considered based on patient age and exposures. In the pre-vaccine era, H. influenzae type B and S. pneumoniae were much more common. A large retrospective cohort study from 1977 to 1997 of cases of osteomyelitis and septic arthritis at a Canadian hospital showed that H. influenzae was responsible for 41% of culture-positive cases of septic arthritis prior to vaccine introduction in 1987 and decreased to none from 1991 and after.9 This decline is related to overall decrease in colonization rates of <1% in H. influenzae post-vaccine introduction.10 The incidence of invasive pneumococcal disease has decreased significantly after introduction of the pneumococcal conjugate vaccine (PCV) 7 in 2000, followed further after introduction of the PCV 13 vaccine in 2010.11 The incidence of S. pneumoniae-related osteoarticular infections specifically has decreased by about 35% since introduction of the PCV 13 vaccine with a decrease of 87% for infections secondary to strains contained in the PCV 13 vaccine.12S. pneumoniae is now responsible for about 3.6% of all osteoarticular infections.12

Organisms Associated with Pyogenic Arthritis by Age Group

Table 1:

Organisms Associated with Pyogenic Arthritis by Age Group

Clinical Presentation

The most commonly affected joints are hips (about 37% of cases), followed by knees (25%), ankles (23%), elbows (6%), shoulders (5%), sacroiliac joints (2%), and others (2%).13 As the majority of cases involve the lower extremities, limp or refusal to bear weight is the most common presenting symptom. If a hip is involved, a child is most likely to hold the affected limb in a flexed, external rotation position. The presence of fever in addition to limp or joint pain should alert the clinician to consider a diagnosis of pyogenic arthritis. A septic joint classically is warm to the touch, swollen, and red, but the absence of these do not exclude the diagnosis.3 Furthermore, the patient with a septic joint is more likely to refuse to bear weight on that joint and have more pain as opposed to the patient with transient synovitis who may favor the affected joint, but still use it.1,5 The timeline of the joint pain can also be helpful in differentiating septic arthritis from other causes including Legg-Calve-Perthes disease, slipped capital femoral epiphysis, rheumatic disease, and overuse injuries. The pain in septic arthritis is generally acute in onset as opposed to the other listed entities that tend to be more chronic.1,5


After conducting a careful history and physical examination, most experts recommend additional imaging and laboratory testing.4 Initial laboratory studies should include a complete blood count with differential, C-reactive protein (CRP), erythrocyte sedimentation rate (ESR), and blood cultures.4 Direct inoculation of blood culture bottles for culturing synovial fluid or bone can increase the sensitivity of detection of K. kingae when clinically suspected4 as can the use of various PCR methods.8,14 For example, one center compared K. kingae detection 6 years prior to the regular use of K. kingae PCR testing on joint fluid to the 4 years after universally instituting its use, and saw that K. kingae comprised 48% of their positive isolates in cases of septic arthritis, whereas they previously had a detection rate of zero for K. kingae.14K. kingae osteoarticular infections are typically associated with less fever, less leukocytosis, and less elevation of inflammatory markers.7

The Kocher criteria were established in 1999 using laboratory and clinical criteria to help differentiate between septic arthritis and transient synovitis.15 The criteria initially consisted of four parameters: fever of at least 38.5°C, inability to bear weight, a white blood cell count >12,000 cells/L, and an ESR ≥40 mm/hr,15 and were later modified to include a CRP ≥20 mg/L.16 Although initial studies found these criteria to have a high positive predictive value of 93% to 99% if maximal criteria were met,15 recent studies have found the predicted probability of septic arthritis in patients meeting all criteria to be closer to 60%.17 The Kocher/Caird criteria remain a useful clinical tool and should be interpreted in conjunction with clinical judgment to help guide decisions about whether to aspirate a joint. Aspiration should not be withheld based on normal laboratory tests if there is a high clinical suspicion of septic arthritis. Joint aspiration fluid analysis and culture is considered the gold standard for diagnosis. The joint fluid in cases of bacterial arthritis generally has >50,000 white blood cells (WBCs)/mm, although some studies use >20,000 WBCs/mm as a cut-off. Joint fluid cultures are positive in 90% of cases of nongonoccocal septic arthritis, although Gram stains suggest a pathogen about 50% of the time.18

Imaging is another important evaluation tool. Plain film radiographs are of low diagnostic utility for identification of pyogenic arthritis but may be helpful to exclude other causes of joint pain. An ultrasound can be useful to help identify a joint effusion or to guide aspiration. Magnetic resonance imaging (MRI) is important if there is concern for an adjacent osteomyelitis, which could affect treatment duration.3 The range of concomitant septic arthritis and osteomyelitis has been reported to be 4.9% to 55.5%.19 However, a recent large retrospective review of 162 children with bone and joint infections who had MRIs found that 68% of them met criteria for both osteomyelitis and septic arthritis,20 suggesting that the overlap of these diseases may be more common than previously thought and raising the question of whether MRIs should be performed more routinely for cases of septic arthritis.


Antimicrobial therapy should ultimately be based on culture results, but initial empiric therapy should target the most common causative organism, S. aureus, in addition to any other organisms that are thought to be of high probability based on history. Common empiric therapies include either clindamycin or a first-generation cephalosporin, although vancomycin may be preferred for the toxic-appearing child. Local antimicrobial susceptibility trends should help guide empiric therapy. If there is a high prevalence of methicillin-resistant S. aureus (MRSA) in the community (generally considered to be more than 10% to 20% of isolates), clindamycin may be preferred. Clindamycin has excellent bone penetration; however, it is generally less palatable and associated with more diarrhea as compared to first-generation cephalosporins. Clindamycin should also be used with caution if there is a high suspicion for Kingella, as neither it nor vancomycin provides coverage for this organism. First-generation cephalosporins, which do treat Kingella, are generally preferred if the community MRSA prevalence is less than 10% to 20%. Either antibiotic should initially be given via the intravenous route and continued via the intravenous route until the patient is afebrile and has significant improvement in pain (generally 2–4 days).4,13,21 Studies have shown that extended periods of intravenous antibiotics are not beneficial and are associated with increased complications.4,21,22 However, longer courses may be required for patients with more complicated infections.

In the United States, the most commonly accepted duration of total therapy is 3 to 4 weeks. A recent randomized prospective trial provides data to suggest that a shorter course of therapy of 10 to 14 days is adequate and effective for uncomplicated cases of septic arthritis in patients who have improved examinations and a CRP <20 mg/dL at the end of therapy; however, this study did not evaluate patients with MRSA.21 This treatment strategy has been adopted into the European society guidelines for pediatric septic arthritis with a recommendation of 2 to 3 weeks for uncomplicated cases and 3 to 4 weeks if there is associated osteomyelitis.4 If there are complicating factors such as resistant or unusual pathogens (including MRSA), slow or poor response to therapy, very young age (<3 months), complicated disease (including deep venous thrombosis), involvement of the pelvis or spine, sepsis, or immunocompromise, then treatment duration should be prolonged.4,22 For cases of beta-lactam allergy, clindamycin may be preferred, or alternatively, trimethoprim-sulfamethoxazole or quinolones if there is suspicion for Kingella.4 Culture-negative cases should be treated similarly to culture positive cases.23

Early surgical aspiration is essential to both obtaining a diagnosis as well as decompressing the joint. Open joint debridement may be necessary, particularly with an infected hip or shoulder as they are tightly encapsulated. This approach is less commonly necessary with other joints13 where arthroscopic aspiration can be performed. There currently is some debate as to whether arthrocentesis alone is sufficient for the majority of all joint infections, including hips and shoulders,13 and this is an active area of research. Some patients require repeat aspirations, and this should be considered in any patient who is not improving despite appropriate antimicrobial therapy.

Potential Complications

Overall, 75% to 90% of children with septic arthritis who receive appropriate therapy will recover without sequelae. For those that do not, the most common problems are joint stiffness and osteonecrosis24 but may also include instability or abnormal bone growth. These sequelae may not be apparent initially and may not reveal themselves until years later. Patients are more likely to suffer adverse sequelae if they have a delay in diagnosis, inadequate/inappropriate therapy, sickle cell disease, are younger than age 6 months, have adjacent osteomyelitis, or have infection secondary to a Panton-Valentine leucocidin (PVL) producing strain of S. aureus.4,24

Timely therapy for a septic joint is critical as damage and joint destruction can begin as early as 8 hours after inoculation.2 However, a good outcome can be expected if surgery and antibiotics are started within 4 days of symptom onset.5,24 Inadequate joint decompression can lead to avascular necrosis or cartilage destruction.12 Likewise, a delay in appropriate antimicrobial choice or dose can prolong infection and lead to greater joint damage.

Newborns are particularly vulnerable as they have greater permeability and vascularization of the growth plate allowing for greater spread of infection. They also have thinner bone cortices, leading to an increased risk for subperiosteal abscesses. Patients with sickle-cell disease are at increased risk because their joint pain can mimic a vaso-occlusive event, so initiation of therapy is frequently delayed.24

PVL toxin, which can be secreted by strains of S. aureus and is, most commonly associated with MRSA, causes tissue necrosis and neutrophil destruction allowing for more rapid spread of disease. It has been associated with increased morbidity in S. aureus osteoarticular infections, including subperiosteal abscesses, pyomyositis, thrombosis, and necrotizing fasciitis. It has also been associated with increased surgical interventions and increased length of stay.24

Other Considerations

There are other less common organisms associated with infectious arthritis to consider based on patient age and history in addition to noninfectious causes of joint pain that must also always be considered (Table 2).

Differential Diagnosis of Hip Pain in the Pediatric Population

Table 2:

Differential Diagnosis of Hip Pain in the Pediatric Population

Lyme arthritis should be considered in any patient from or with recent travel to an endemic area, where 5% of pediatric patients presenting with acute joint pain have Lyme arthritis.25 If present, the erythema migrans rash associated with Lyme generally precedes arthritis by 1 to 2 months. These patients generally have less fever and peripheral leukocytosis/inflammation as compared to more common causes of septic arthritis; however, they will have purulent joint fluid and joint fluid will have positive Lyme PCR testing.25 Treatment is with amoxicillin or doxycycline.

Neisseria gonorrhea should be considered in an adolescent who is sexually active. Unlike the previously discussed forms of pyogenic arthritis, which are generally monoarticular, gonococcal arthritis tends to be polyarticular and migratory. There is an associated dermatitis in 40% to 70% of patients. Joint fluid is positive in only about one-half of patients, but testing of other sites including urine, cervix, vagina, and rectum may be performed. Treatment is generally with a parenteral third-generation cephalosporin. Azithromycin is also commonly coadministered given the risk of concomitant infection with Chlamydia trachomatis.26

Tuberculous arthritis should be considered in any patient from an endemic area, and musculoskeletal forms of tuberculosis account for 1% to 3% of all cases of tuberculosis. Often, these infections go undiagnosed for a long time and many patients present with significant joint stiffness and even limb shortening. The synovial fluid in these patients generally has less leukocytosis, and joint fluid cultures fail to grow organisms in a number of cases, so a synovial biopsy is recommended if there is concern. Treatment is prolonged with multidrug therapy for tuberculosis.27

Fungal arthritis is rare in the child who is immunocompetent except for certain endemic mycoses. Sporothrix schenckii, Coccidioides immitis, Histoplasma capsulatum, Blastomyces dermatidis, Paracoccidioides brasiliensis, and Penicillium marneffei should be considered in patients with appropriate exposures. Other known sites of dissemination are identified in many of these cases.28

Viral arthritis can occur either by direct invasion or immune complex deposition and is most commonly associated with rubella, parvovirus B19, several herpes viruses (herpes simplex virus, varicella-zoster, Epstein-Barr, and cytomegalovirus), hepatitis B, and hepatitis C. Cases related to rubella used to be associated with active disease, but are now more commonly seen after receiving the vaccine. Viral arthritis tends to be polyarticular.29

Immune-complex deposition from distal infections can lead to reactive arthritis that may mimic septic arthritis; however, the joint itself is not directly infected. Reactive arthritis is most commonly associated with gastrointestinal and genitourinary infections in addition to poststreptococcal infections and rheumatic fever. More recently, it has also been associated with atypical bacteria. Reactive arthritis is generally responsive to nonsteroidal anti-inflammatory drugs and, with the exception of rheumatic fever, short-lived and self-limited.29

Rheumatologic disease should also be considered in any patient with joint pain, particularly if they have other symptoms or history to suggest a rheumatologic etiology or if multiple joints are involved. These patients generally have more chronic symptoms, joint swelling, warmth, and tenderness, although frequently, they are still able to bear weight. They may have other systemic symptoms including rash, fatigue, weight loss, anemia, and anorexia.30


The presentation of hip pain in children is not uncommon, and pyogenic arthritis is an entity that should not be missed given the risks for long-term sequelae with delay in treatment. Laboratory examination can help guide decision-making, but any child with a high clinical suspicion should have a joint aspiration for diagnostic and therapeutic purposes with subsequent initiation of intravenous antibiotics. For uncomplicated cases, antibiotic courses may be as short as 2 to 4 days of intravenous therapy and 10 to 14 days of total therapy provided the patient shows signs of near full recovery by the end of therapy. History, examination, and laboratory testing can help the clinician determine causes of pyogenic arthritis and other causes of arthritis.


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Organisms Associated with Pyogenic Arthritis by Age Group

Age Group Associated Organisms
Neonates (<3 months) Staphylococcus aureus, group B streptococcus, Gram-negative rods (eg, Escherichia coli), Candida albicans
Children (<5 years) S. aureus, Haemophilus influenzae, Kingella kingae, Streptococcus pyogenes, S. pneumoniae
Children (>5 years) S. aureus, S. pyogenes
Adolescents (sexually active) S. aureus, S. pyogenes, Neisseria gonorrhoeae

Differential Diagnosis of Hip Pain in the Pediatric Population

Alternate Entities to Consider Special Considerations
Lyme arthritis25 If in endemic area, monoarticular, knee most common, recurrent if untreated
Tuberculous arthritis27 If risk factors, check PPD (purified protein derivative)/interferon gamma release assay
Viral arthritis29 Based on symptoms/exposures. Common viral causes include herpes viruses, Rubella, parvovirus B19, arboviruses, hepatitis B
Fungal arthritis28 If in an endemic area, Sporothrix schenckii, Coccidioides immitis, Histoplasma capsulatum, Blastomyces dermatidis, Paracoccidioides brasiliensis, and Penicillium marneffei. Opportunistic fungi in immunocompromised
Reactive arthritis29 Associated with gastrointestinal/genitourinary organisms and poststreptococcal infections
Rheumatologic disease30 Often polyarticular, may have family history, rash, and other systemic signs
Transient synovitis5 Fever less common, pain less severe
Legg-Calve Perthes disease5 Chronic, no fever
Slipped capital femoral epiphyses1 Obese adolescent with limp, no fever

Jenna E. Holmen, MD, is a Pediatric Infectious Disease Fellow. Jumi Yi, MD, is a Pediatric Infectious Disease Physician. Both authors are affiliated with the UCSF Benioff Children's Hospital Oakland.

Disclosure: The authors have no relevant financial relationships to disclose.

Address correspondence to Jumi Yi, MD, 747 52nd Street, Infectious Diseases, UCSF Benioff Children's Hospital Oakland, Oakland, CA 94609; email:


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