Working Diagnosis:
Acute on chronic osteomyelitis

Treatment:
Initial evaluation suggested symptomatic osteochondritis dissecans (OCD) of the lateral femoral condyle of the left knee. Advanced imaging was pursued via MRI without contrast to further characterize this lesion. The MRI revealed a lesion at the metaphyseal-physeal junction suggestive of subacute osteomyelitis, and likely normal osteochondral variant of lateral femoral condyle. Orthopedic surgery and infectious diseases were consulted, and the patient underwent CT-guided drainage and biopsy of lesion which revealed the diagnosis of S. aureus subacute bacterial osteomyelitis. He was placed on six weeks of oral antibiotics by the infectious diseases specialists.

Biopsy:
Gram Stain: No organisms.
Culture: S. aureus, oxacillin susceptible

Outcome:
While awaiting advanced imaging, the patient was placed in a knee immobilizer. After further workup revealed infectious etiology, immobilization was discontinued. At the time of biopsy, he was asymptomatic and had returned to sports. After biopsy, he rested a few days and returned to regular activity. Since initial labs were normal, he was monitored clinically. He completed his antibiotic course without complication. He will be followed by orthopedics for monitoring of the femoral ossification and growth plate changes due to the location of the infection.

Author's Comments:
Initial X-rays of the left knee were concerning for osteochondritis dissecans of the lateral femoral condyle, but follow-up with MRI revealed that this likely was a normal variant. Gebarski and Hernandez identified that aspects favoring a normal variant over the diagnosis of OCD include intact overlying articular cartilage and lack of bone-marrow edema. The MRI of our patient indeed had absence of marrow edema on T2 images. Subacute osteomyelitis, or Brodie's abscess, can present quite commonly without signs of systemic illness, and with normal laboratory findings. Most importantly in our case, the location of the lesion adjacent to the growth plate raised concerns for growth disruption if left untreated. Patients who present with infection in such a location should be followed serially to monitor for growth disruption. This case highlights the importance of maintaining a broad differential in your evaluation of a limping child. Despite normal lab findings, absence of systemic signs, and only a vaguely abnormal examination, this patient was found to have osteomyelitis.

Editor's Comments:
Several key clinical pearls to highlight in this case. First, when considering the location of OCD of the knee, the most common location is the lateral portion of the medial femoral condyle, not the lateral femoral condyle. In terms of the examination for OCD of the knee, Wilson’s test is performed by internally rotating the tibia and having the patient extend the knee from a flexed position. A positive test is pain at approximately 30 degrees from extension which is then relieved by externally rotating the tibia. There is bilateral involvement in up to 25% of cases so bilateral knee examination should be performed.

In terms of management, it is important to distinguish stable and unstable OCD lesions. Unstable lesions will be more likely to present with an effusion, mechanical symptoms or crepitus. Another key pearl from this case is that MRI imaging should be performed for a suspected OCD lesion. It allows for evaluation of the size of the lesion as well as the overlying cartilage and subchondral bone. In this case, it was pivotal in making the correct diagnosis as MRI has much greater sensitivity than plain films for osteomyelitis, especially in the early stages.

Brodie’s abscess is essentially a walled-off pocket of pus that develops within bone. Because it is surrounded by a sclerotic wall, there is little systemic inflammatory response. The most common location for a Brodie’s abscess is the tibia (48%), followed by the femur (31%). Other diseases that can present like Brodie’s abscess include osteoid osteoma, Langerhans cell histiocytosis, chondrosarcoma, eosinophilic granuloma or TB. The average length of time between presentation and diagnosis is 12 weeks. Staph aureus is found in 65% of cases. The treatment typically consists of surgery to drain the abscess in combination with antibiotics. There is no consensus on the duration of antibiotics.

References:
Gebarski, Kathleen, and Ramiro J. Hernandez. "Stage-I osteochondritis dissecans versus normal variants of ossification in the knee in children." Pediatric radiology 35.9 (2005): 880-886.

Pulido, Patricia Garcia, et al. "Brodie's Abscesses Can Stimulate the Growth Plate in Children." Journal of Bone and Joint Infection 4.6 (2019): 264-267.

van der Naald, Niels, et al. "Brodie's Abscess: A Systematic Review of Reported Cases." Journal of bone and joint infection 4.1 (2019): 33.

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