Abstract
Purpose: The child with a painful swollen knee must be worked-up for possible septic arthritis; the classic clinical prediction algorithms for septic arthritis of the hip may not be the best models to apply to the knee.Materials and methods: This was a retrospective case-control study of 17 years of children presenting to one hospital with a chief complaint of a painful swollen knee, to evaluate the appropriateness of applying a previously described clinical practice algorithm for the hip in differentiating between the septic and aseptic causes of the painful knee effusions. The diagnoses of true septic arthritis, presumed septic arthritis, and aseptic effusion were established, based upon the cultures of synovial fluid, blood cultures, synovial cell counts, and clinical course. Using a logistic regression model, the disease status was regressed on both the demographic and clinical variables.Results: In the study, 122 patients were included: 51 with true septic arthritis, 37 with presumed septic arthritis, and 34 with aseptic knee effusion. After applying a backward elimination, age <5 years and C-reactive protein (CRP) >2.0 mg/dl remained in the model, and predicted probabilities of having septic knee arthritis ranged from 15% for the lowest risk to 95% for the highest risk. Adding a knee aspiration including percent polymorphonucleocytes (%PMN) substantially improved the overall model performance, lowering the lowest risk to 11% while raising the highest risk to 96%.Conclusions: This predictive model suggests that the likelihood of pediatric septic arthritis of the knee is >90% when both “age <5 years” and “CRP > 2.0 mg/dl” are present in a child with a painful swollen knee, though, in the absence of these factors, the risk of septic arthritis remains over 15%. Aspiration of the knee for those patients would be the best next step.
Highlights
A painful swollen knee is a common diagnostic dilemma for the providers treating children in an emergency room, urgent care, primary care, or specialty setting
This study aimed to examine whether children with septic arthritis of the knee may be distinguished from the noninfectious causes of a painful knee effusion using the same predictive variables as described for the hip, as well as to suggest a new clinical prediction algorithm for distinguishing the septic knee arthritis from aseptic knee effusion, incorporating the data gained during the work-up from a knee joint aspiration [10]
The exclusion criteria for the study were the absence of a synovial fluid aspirate or culture of the aspirate, diagnoses related to postoperative infections, hip transient synovitis, septic hip arthritis, ankle septic arthritis, or multiple septic joints
Summary
A painful swollen knee is a common diagnostic dilemma for the providers treating children in an emergency room, urgent care, primary care, or specialty setting. There are reliable clinical predictors for determining the presence of septic arthritis in the hip in children, previously described by Kocher and modified by Caird [4,5,6] These criteria include elevated serum white blood cell count (WBC) above 12,000 cells/mm, erythrocyte sedimentation rate (ESR) above 40 mm/h, temperature above 38.5◦C, non-weight bearing status, and Creactive protein (CRP) level above 2.0 mg/dl. These same criteria are not verified for joints other than the hip, and no other algorithm is described to rule out septic arthritis in the knee
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