Working Diagnosis:
Considering the clinical presentation and radiographic findings, the patient was diagnosed with right ankle septic arthritis with distal tibial osteomyelitis.

Treatment:
The patient was taken emergently to the operating room. Under general anesthesia, the ankle was tapped and the purulent fluid was sent for culture. After cultures were obtained, drainage and ankle joint washout was performed. A drain was placed in the right ankle. She was then given IV antibiotics. After two days, the final ankle culture grew out methicillin-susceptible Staphylococcus aureus. A peripherally inserted central catheter was placed, and she received six weeks of intravenous antibiotics. The drain was removed prior to discharge. The patient's sore throat resolved after five days.

Outcome:
After two weeks of antibiotics, the patient noticed significant improvement. She progressed well and resumed normal daily activities. She currently has no limitations in the affected joint. The patient has returned to her baseline level of activity and is getting ready for the next softball season.

Author's Comments:
Septic arthritis (SA) and osteomyelitis are infections that develop from abnormal bacteria or fungi in the joint and bone tissues, respectively (1). These conditions are described as orthopedic emergencies because a delay in treatment can cause irreversible damage to the tissue. Diagnosis is often difficult because many symptoms overlap with other common medical conditions. Septic arthritis is most common in the hip and knee joints (2). Often, the infection is spread through a hematogenous route, although adjacent osteomyelitis and local skin penetration are less common contributors (1). Staphylococcus aureus is the most common cause of SA, but group A streptococci, Streptococcus pneumoniae, and methicillin-resistant Staphylococcus aureus are other frequently seen microbes (3). A delay in adequate treatment is one of the greatest contributors to increased morbidity (4). Therefore, joint aspiration and culture should occur as early as possible to ensure that appropriate antibiotics are initiated in a timely manner. Clinicians must maintain a heightened awareness and low threshold to initiate aggressive treatment in suspected cases of SA. Bone and joint infections in the pediatric population are especially unique, given the open growth plate. It has been hypothesized that the epiphyseal plate provides a barrier to infection extension because of the unique blood supply pattern to the bone (5).

Septic arthritis of the ankle is a rare condition, especially in the absence of skin laceration. Interestingly, the patient had a sore throat, which self-resolved. However, this raises the question of whether an upper respiratory infection permitted hematologic seeding of the distal tibial metaphysis. The infection then likely invaded past the growth plate and into the epiphysis and ankle joint. This case does not support the current theory that the growth plates of pediatric bones provide a barrier to infection spread. This unique case emphasizes the importance of keeping a wide differential when faced with a common problem.

Editor's Comments:
Septic arthritis and osteomyelitis of the physis are extremely common in the pediatric population due to the abundant blood flow to these regions. Similar to toxic synovitis of the hip, children often present with upper respiratory infection symptoms followed by the sudden onset of arthralgia. Night pain, inability to bear weight, and limping are red flags and should not be dismissed. The most common areas for osteomyelitis in the pediatric population involve the growth plates of long bones and include the proximal femur, distal femur, proximal tibia, and distal tibia. Septic arthritis in children are most common in the large joints such as the hip and knees but may appear anywhere, especially after trauma. Diagnosis and treatment typically includes aspiration of the joint effusion, joint washout, and IV antibiotics.

References:
1. Krogstad P. Septic arthritis. In: Feigin and Cherry's Textbook of Pediatric Infectious Diseases, 7th ed. Cherry JD, Harrison GJ, Kaplan SL, et al. (Eds), Philadelphia: Elsevier Saunders; 2014. p.726-727.

2. Heberling JA. A review of two hundred and one cases of suppurative arthritis. J Bone Joint Surg Am 1941;23(4):917-921.

3. Thompson A, Mannix R, Bachur R. Acute pediatric monoarticular arthritis: distinguishing lyme arthritis from other etiologies. Pediatrics 2009;123(3):959-965.

4. Fabry G, Meire E. Septic arthritis of the hip in children: poor results after late and inadequate treatment. J Pediatr Orthop 1983;3(4):461-466.

5. Elgazzar AH. Diagnosis of Inflammatory Bone Diseases. In: Orthopedic Nuclear Medicine, 1st ed, New York: Springer-Verlag, Berlin, Heidelberg; 2004. p.37-39.

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