The Heterogeneity of Pediatric Knee Infections: A Retrospective Analysis.

Background: Musculoskeletal infection is a major cause of morbidity in the pediatric population. Despite the canonical teaching that an irritable joint and signs of infection likely represent an infected joint space, recent evidence in the pediatric hip has demonstrated that alternative diagnoses are equally or more likely and that combinations of pathologies are common. The knee is the second most commonly infected joint in children, yet there remains a paucity of available data regarding the epidemiology and workup of the infected pediatric knee. The authors hypothesize that there is heterogeneity of pathologies, including combinations of pathologies, that presents as a potentially infected knee in a child. The authors aim to show the utility of magnetic resonance imaging and epidemiologic and laboratory markers in the workup of these patients.

Methods: A retrospective review of all consults made to the pediatric orthopaedic surgery team at a single tertiary care center from September 2009 through December 2015 regarding a concern for potential knee infection was performed. Excluded from the study were patients with penetrating trauma, postoperative infection, open fracture, no C-reactive protein (CRP) within 24 hours of admission, sickle cell disease, an immunocompromised state, or chronic osteomyelitis.

Results: A total of 120 patients were analyzed in this study. There was marked variability in pathologies. Patients with isolated osteomyelitis or osteomyelitis+septic arthritis were older, had an increased admission CRP, were more likely to be infected with Staphylococcus aureus, required an increased duration of antibiotics, and had an increased incidence of musculoskeletal complications than patients with isolated septic arthritis.

Conclusions: When considering a child with an irritable knee, a heterogeneity of potential underlying pathologies and combinations of pathologies are possible. Importantly, the age of the patient and CRP can guide a clinician when considering further workup. Older patients with a higher admission CRP value warrant an immediate magnetic resonance imaging, as they are likely to have osteomyelitis, which was associated with worse outcomes when compared with patients with isolated septic arthritis.

Level Of Evidence: Level III-retrospective research study.