Acute infection of the joints can be caused by bacteria, fungi, and viruses. Bacterial infections occur most frequently. The terms septic arthritis, infectious arthritis, and acute suppurative pyarthrosis are used interchangeably with regard to bacterial infections.
Epidemiology
Septic arthritis occurs most frequently in childhood, with an estimated incidence of 1 to 37 cases per 100,000 individuals. Boys are affected more often than are girls by a ratio of 1.1 : 1.9. Describing the age distribution of septic arthritis is difficult because of the varying age intervals selected by authors of different studies, but most cases occur below 6 years of age, and a median age of 2 years has been reported in several series.
Pathophysiology
Synovial joints, also termed diarthrodial joints, are freely movable articulations containing synovia. Synovia is a transparent viscous fluid that lubricates the joint and nourishes the avascular articular cartilage. The synovium, a connective tissue layer interposed between the fibrous joint capsule and the fluid-filled synovial cavity, is responsible for formation of the joint fluid. The synovium contains a prominent capillary supply embedded in a connective tissue network containing at least two types of cells. One morphologic type (type A) seems to be related to mononuclear phagocytes; fibroblast-like type B cells seem to be responsible for the synthesis of hyaluronic acid. Joint fluid (synovia) is formed by filtration through the capillary network (i.e., the net balance of back-diffusion into the capillary bed and diffusion into the joint space). Diarthrodial joints normally contain small amounts of fluid (e.g., 0.5 to 3 mL in the knee), with glucose and electrolyte concentrations equal to those in plasma. An oxygen partial pressure of 60 to 70 mm Hg, an albumin concentration of 10 to 20 g/L, and an IgG content of 500 mg/L are typical. Pain fibers are present in the joint capsule, accessory ligaments (e.g., cruciate ligaments of the knee), and subchondral bone, but their relative contributions to the perception of pain in acute septic arthritis has yet to be elucidated.
Diffusion from the joint space is increased by any mechanism that increases pressure (distention with injected solution, active or passive motion, or external massage). Particulate material is removed from the joint space by synovial membrane macrophages and free monocytes (the latter usually are present in concentrations of <60 × 10 6 /L). The viscosity of joint fluid is affected by the properties of hyaluronic acid; enzymatic depolymerization produces a viscosity approximately equivalent to that of water. With the loss of hyaluronidase from the synovia, the articular cartilage, with continued use, becomes eroded and sclerotic.
Microorganisms can enter the joint space by hematogenous spread, direct inoculation, or extension from a contiguous focus of infection, including foci of adjacent osteomyelitis. Bacteria in blood are potentially delivered to the synovial membrane during transient bacteremia. A history of trauma often is cited as a predisposing factor for the development of bacterial arthritis, but the significance of such a history is unclear in view of the great frequency of minor trauma in childhood. Upper respiratory tract infections frequently precede the development of septic arthritis caused by Haemophilus influenzae type B and Kingella kingae, and oral ulcers are often noted prior to Kingella infections. Similar to mild traumatic injuries, they are presumed to increase the likelihood of bacteremia.
Coincident osteomyelitis and septic arthritis are frequently reported in studies of osteoarticular infections in children. In three recent case series, 9% to 33% of cases of acute osteomyelitis in childhood were complicated by septic arthritis. Humeral osteomyelitis seems to be of particularly high risk; it was present in 15 of 22 cases of shoulder joint arthritis (68%) in one recent series. Conversely either shoulder or elbow joint septic arthritis was present in 7 (14%) of 49 cases of humeral osteomyelitis in another case series. Aside from joint involvement during osteomyelitis, extension of a contiguous infection into the joint space seldom occurs. In one series, 10 of 77 patients with septic arthritis had disease resulting from extension from a contiguous soft tissue infection. Eight occurred before the availability of many antibiotics (1951). This high frequency of septic arthritis caused by spread of infection from a contiguous focus has not been seen in more recent studies.
Inoculation arthritis occurs after invasion of the joint by a contaminated object, such as kneeling on sewing needles or following arthroscopic surgery.
Gram-negative organisms are the most frequent pathogens when septic arthritis occurs after other types of surgeries or instrumentation of the urinary or intestinal tract. Salmonella septic arthritis may develop during the course of Salmonella bacteremia in a normal host, but it occurs with increased frequency in patients with sickle-cell disease and related hemoglobinopathies. Although septic arthritis occurs in children and adults infected with human immunodeficiency virus (HIV), as yet no data have substantiated that HIV increases the incidence of musculoskeletal infections in children. However, pneumococcal osteoarticular infections may be more common in HIV-positive children, compared with HIV-seronegative children, at least in the absence of immunization with protein-conjugate pneumococcal vacccine. Septic arthritis has been described during varicella, presumably caused by bacteremia resulting from infection of skin lesions. It must be differentiated from the apparent ability of varicella zoster virus to cause joint inflammation on its own.
Etiology
In most case series published since the mid-20th century, Staphylococcus aureus has been the most common cause of culture-positive septic arthritis, including methicillin-susceptible (MSSA) and methicillin-resistant S. aureus (MRSA). Streptococci (especially group A β-hemolytic organisms and pneumococci) have been responsible for most other gram-positive infections ( Table 56.1 ). K. kingae has been recognized increasingly as a cause of septic arthritis, perhaps because of improvements in culture methods and the use of polymerase chain reaction (PCR) detection of bacterial nucleic acids. In one series from Israel, Kingella was the most common bacterial isolate (48% of cases), and S. aureus was not found in children less than 24 months of age. In another recent report from western Europe, Kingella was the most common cause of both osteomyelitis and septic arthritis in children under 4 years of age.
| Year of Report | GRAM-POSITIVE BACTERIA | GRAM-NEGATIVE BACTERIA | Other | Total Cases | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| S. aureus | Streptococci | S. pneumoniae | CNS | H. influenzae | K. kingae | N. meningitidis | Salmonella | Non- Salmonella Enterobacteriaceae | N. gonorrhoeae | |||
| 1941 | 50 | 45 | 2 | 10 | 0 | 0 | 0 | 0 | 0 | 14 | 0 | 121 |
| 1958 | 18 | 8 | 5 | 2 | 3 | 0 | 0 | 0 | 2 | 0 | 1 | 38 a |
| 1972 | 40 | 20 | 8 | 5 | 37 | 0 | 4 | 2 | 7 | 13 | 11 | 146 a |
| 1975 | 37 | 10 | 0 | 5 | 14 | 0 | 0 | 0 | 2 | 0 | 7 | 75 |
| 1987 | 40 | 8 | 5 | 0 | 20 | 0 | 4 | 0 | 2 | 2 | 9 | 90 |
| 1995 | 0 | 2 | 3 | 0 | 8 | 19 | 1 | 1 | 1 | 0 | 5 | 40 |
| 1999 | 10 | 5 | 2 | 2 | 1 | 3 | 3 | 0 | 4 | 2 | 1 | 33 |
| 2008 | 19 | 8 | 5 | 3 | 1 | 0 | 0 | 0 | 2 | 2 | 4 | |
| Total | 195 | 98 | 25 | 24 | 83 | 22 | 12 | 3 | 18 | 31 | 34 | 549 |
| % of all isolates | 36 | 18 | 5 | 4 | 15 | 4 | 2 | 1 | 3 | 6 | 6 | |
H. influenzae type B historically has been an important cause of septic arthritis in children younger than 2 years old but now is seen only rarely in areas with widespread immunization. Arthritis caused by Streptococcus pneumoniae also generally occurs in children younger than 2 years old and may diminish in frequency in the era of protein conjugate vaccines. In a few cases, septic arthritis is seen with systemic Neisseria meningitidis infection. In neonates and sexually active adolescents with suspected septic arthritis, Neisseria gonorrhoeae should be considered.
Salmonella spp. cause approximately 1% of the total cases of septic arthritis. Beyond the neonatal period, infections with other enteric gram-negative bacteria are rare occurrences in pediatric septic arthritis and often are associated with inoculation, instrumentation, or an immunocompromised state. Infections with Serratia, Aeromonas, Enterobacter, Bacteroides, and Campylobacter generally occur in patients with malignancy who are immunosuppressed. Pseudomonas aeruginosa infections are associated with arthritis in infants, with infection of puncture wounds, or with injectable drug use. Discussion of Lyme arthritis is beyond the scope of this chapter, but intermittent, inflammatory arthritis is seen in many patients after Borrelia infection is transmitted by a tick bite.
Unusual causes of septic arthritis include Propionibacterium acnes, Actinomyces (formerly Corynebacterium ) pyogenes , Pasteurella multocida , and Yersinia pestis (personal observation). Streptobacillus moniliformis infection of joints may become evident 2 to 3 days after a rat bite occurs; a macular rash commonly is present at initial evaluation. Brucella, mycobacteria ( Mycobacterium tuberculosis and other species), and Nocardia asteroides may cause a chronic monarticular arthritis with granulomatous histopathology.
Although bacterial infections are the most common cause of septic arthritis, infectious arthritis is sometimes caused by viral pathogens (including varicella zoster, erythrovirus [parvovirus B19], rubella, dengue, chikungunya and other togaviruses, variola, vaccinia, certain enteroviruses) as well as by mycobacteria (including M. tuberculosis ). Septic arthritis may also be caused during disseminated fungal infections, including candidiasis, histoplasmosis, coccidioidomycosis, blastomycosis, and cryptococcosis. The knee joint is most often involved in coccidioidal and cryptococcal arthritis.
Diagnosis
Clinical Findings
Most children have fever or constitutional symptoms within the first few days of infection. Table 56.2 lists the frequency of specific joint involvement in hematogenous septic arthritis of childhood. Infections of the major weight-bearing joints (hip, knee, and ankle) consistently account for approximately 80% of all cases. Focal findings in the joint involved almost always are present. In infants, in whom the hip is one of the most frequent joints involved, swelling, tenderness, and heat may be absent. Most commonly, the infant with septic arthritis lies with the involved leg abducted and externally rotated, and hip joint dislocation may occur. When the capsule of the joint can be examined, swelling is noted; effusion was present in 22 of 24 cases in one series. Likely because pain fibers are located in the capsule, any maneuver that increases intracapsular pressure also produces pain. In the hip, this pain can be elicited by compression of the head of the femur into the acetabulum. A portal of entry almost never is apparent, and bilateral hip joint infection may occur. Pyogenic sacroiliitis often is accompanied by tenderness detected by pressure applied over the sacrum during a digital rectal examination and by pain experienced during simultaneous flexion, abduction, and external rotation at the hip.
| Reference | Knee | Hip | Ankle | Wrist | Elbow | Shoulder | Small Diarthrodial Joints |
|---|---|---|---|---|---|---|---|
| Heberling 1941 | 40 | 50 | 13 | 3 | 8 | 9 | 2 |
| Samilson et al. 1958 | 8 | 19 | 2 | 0 | 6 | 3 | 0 |
| Nelson 1972 | 103 | 48 | 38 | 12 | 35 | 10 | 4 |
| Gillespie 1973 | 37 | 41 | 13 | 2 | 3 | 3 | 0 |
| Yagupsky et al. 1995 | 16 | 6 | 13 | 1 | 2 | 3 | 1 |
| Goergens et al. 2005 | 15 | 15 | 4 | 2 | 4 | 1 | 3 |
| Gafur et al. 2008 | 46 | 51 | 5 | 1 | 10 | 3 | |
| Total | 265 | 230 | 88 | 21 | 68 | 32 | 10 |
| % of all cases | 37 | 32 | 12 | 3 | 10 | 4 | 1 |
Gonococcal arthritis in neonates has nonspecific prodromal symptoms, including poor feeding, irritability, and fever. The exact portal of entry is unknown, and the joints below the hip usually are involved (knee, ankle, and metatarsal). During adolescence, gonococcal arthritis occurs as a manifestation of sepsis with fever, chills, rash, and multiple small joint involvement, often with tenosynovitis. The illness frequently follows the onset of menses by a few days.
Radiologic Findings
Findings on plain radiography are due to capsular swelling. In the joints readily accessible to physical examination, radiographs add little to the diagnostic evaluation, but when septic arthritis of the hip is suspected in a child, they are a valuable adjunct and may identify other causes of hip pain, such as Legg-Calvé-Perthes disease, slipped capital femoral epiphysis, and fracture. Films of the hip should be made with the child in the frog-leg position and with the legs extended at the knee and slightly internally rotated. The early signs of septic arthritis of the hip are caused by swelling of the capsule, which shifts the fat lines. One of the oldest signs is the obturator sign: as the tendon of the obturator internus passes over the capsule of the hip joint, the margins of this muscle are displaced medially into the pelvis ( Fig. 56.1 ). With continued swelling of the hip joint capsule, the femoral head is displaced laterally and upward. One of the most consistent findings is obliteration or lateral displacement of the gluteal fat lines (see Fig. 56.1 ). Coincident with filling of the capsule with exudate, the femoral portion of the Shenton line is raised and its arc is widened. Ultrasound evaluation has proved useful in evaluation of septic arthritis of the hip. In a series of 96 patients, none of the 40 patients with normal ultrasound findings had septic arthritis. If a technetium bone scan is performed, increased uptake on either side of the joint is seen during the “blood pool” phase of the scan.
