Subacute Osteomyelitis as the Predominant Presentation of Patellar Bone Infection in Children: A Case Series of 3 Patients

To the Editors: Primary hematogeneous osteomyelitis of the patella is rare in children, estimated at ~1.1% of pediatric bone infections, and remains easily overlooked in its subacute form.1 Fewer than 40 pediatric cases have been published to date, underscoring both its scarcity and diagnostic difficulty.2 We report 3 cases of subacute patellar osteomyelitis in otherwise healthy girls aged 14 months, 5.5 years and 6 years. All were afebrile, with mild prepatellar swelling, no clinical knee effusion and nonspecific laboratory findings. Magnetic resonance imaging demonstrated lytic patellar lesions in all patients. Microbiology identified Moraxella lacunata (Fig. 1) in 1 case and Kingella kingae in 2. All patients achieved full clinical recovery with appropriate antibiotics, accompanied by radiologic normalization.FIGURE 1.: Three-dimensional inversion recovery T2-weighted sagittal image of a 5.5-month-old girl with Moraxella lacunata osteomyelitis of the patella. The bone lesion in the upper part of the patella. Subcartilaginous lytic lesion. MRI showing a small subcartilaginous lytic lesion of the patella associated with Moraxella infection.Patellar osteomyelitis in children is frequently diagnosed late, which can defer treatment and increase the risk of complications such as abscess formation, cartilage damage and septic arthritis from intra-articular spread.3 When acute infection is missed, evolution to a sequestering stage may yield the characteristic radiographic lucencies of subacute osteomyelitis, a presentation well recognized for its muted symptoms, limited systemic inflammation and often inconclusive routine labs.4 Anatomical and physiologic features may contribute to the patella’s relative resistance to hematogeneous seeding. The pediatric patella is a sesamoid bone with a rich anastomotic arterial network, potentially enhancing local immune surveillance, and it lacks the sluggish metaphyseal flow seen beneath active physes elsewhere, factors that may limit both establishment and severity of infection.3 These attributes help explain the rarity of acute presentations and the predominance of subacute disease. The organisms identified here M. lacunata and K. kingae share low virulence and fastidious growth characteristics, aligning with the typically mild clinical phenotype and laboratory profile observed in primary subacute osteomyelitis.4,5 Although Staphylococcus aureus accounts for many reported cases in the literature,2 the contribution of host defenses likely interacts with pathogen virulence to shape the subacute presentation. Early magnetic resonance imaging is pivotal when patellar osteomyelitis is suspected, as clinical and routine laboratory features alone are insufficiently discriminatory. ACKNOWLEDGMENTS This work was conducted as part of the PREM (Programme de Recherche pour Etudiant·es en Médecine) in progress at the Faculty of Medicine, University of Geneva. The program provided valuable academic guidance and institutional resources that contributed to the completion of this study and promoting early involvement of medical students in academic research.