Septic arthritis is undeniably the most common form of osteoarticular disease of
K. kingae. In fact, some reports demonstrated that septic arthritis represents between 53 and 82.8% of all OAI due to this pathogen
[1][2][3].
K. kingae septic arthritis usually involves large weight-bearing articulations, such as the hip, knee, ankle, shoulder, or elbow
[4][5][2][3][6][7][8][9]. In terms of frequency, the infection tends to involve the lower extremity the most, and the knee is the joint that is the most frequently incriminated. However, atypical joints, such as the sternoclavicular, acromioclavicular, tarsal, or metacarpophalangeal/metatarsophalangeal joints, are overrepresented in
K. kingae arthritis compared with septic arthritis caused by other pathogens
[4][5][2][3][6][7][8][9]. The clinical and biological aspects of septic arthritis can, unfortunately, be truncated when
K. kingae is responsible for infection
[10]. The synovial fluid examination in children with culture-proven arthritis due to
K. kingae demonstrated low leukocyte counts in a substantial number of cases. On that point, the recognized cut-off value of 50,000 WBC/mL in synovial fluid aspirates, used as a diagnostic factor defining bacterial arthritis, may erroneously exclude the diagnosis of
K. kingae arthritis and should, therefore, be used with great caution
[10]. In fact, a few studies demonstrated that the WBC count of the synovial fluid showed less than 50,000 WBC/mL in a quarter of the cases, and that and the examination of the Gram-stain is, most of the time, negative
[11][12][13]. Thus, the afebrile presentation, the mild clinical symptoms, and the absence or the little disturbance of acute phase reactants in
K. kingae arthritis do not meet the diagnostic criteria of a septic joint
[14]. Even worse, when using the application of Kocher’s predictive algorithm
[15], it seems that three-quarters of children with culture-proven
K. kingae septic arthritis of the hip would have been considered to have transient synovitis
[14]. The clinical experience has taught us that this algorithm should not be used for children less than 4 years old. Here again, the pediatric orthopedist must be extremely vigilant in the face of clinical situations not very suggestive of arthritis.
Finally, the clinical course of septic arthritis due to
K. kingae is very different from those of arthritis caused by pyogenic microorganisms, which stimulate a huge influx of neutrophils to the site. In septic arthritis due to pyogenic pathogens, the clinical course is characterized by a potent activation of the immune response, in association with high levels of cytokines and reactive oxygen species, and an increase in the release of host matrix metalloproteinases and other collagen-degrading enzymes, which, in conjunction with bacterial toxins, lead to joint cartilage destruction
[16][17][18]. In addition, the antigen-induced inflammatory response may persist and continue to damage the joint architecture even after the infection has been cleared
[17][18]. Fortunately, the clinical experience acquired during the management of OAI with
K. kingae seems to demonstrate that arthritis due to this particular pathogen does not follow this pattern of cartilage destruction. Therefore, many cases of
K. kingae’s septic arthritis may even be treated by a simple syringe injection–aspiration procedure, and thus, especially when the leukocyte count appears low in the synovial fluid examination, the attitude will not be defensible in pyogenic septic arthritis.