Working Diagnosis:
Intra-articular osteoid osteoma of the right hip.

Treatment:
Interventional Radiology performed radio-ablation of the osteoid osteoma and prophylactic femoral neck screw and side plate fixation was performed by Orthopedics.

Outcome:
2-weeks post-op: Significantly improved pain. Mild soreness, full range of motion, normal gait, and normal neurovascular exam. Restricted to walking only until 6-week mark.
6-weeks: Reported feeling "110%". Cleared to begin jogging, with gradual progression to running and jumping.
3-months: Pain free. Cleared to return to contact sports.
6-months: Some restlessness at night and difficulty sleeping, potentially related to hardware. If symptoms persist, may consider hardware removal.

Author's Comments:
Osteoid osteomas are benign bone tumors often affecting male adolescents. They typically cause nighttime pain and respond to NSAIDs. They usually occur in the diaphysis or metaphysis of long bones. Intra- or peri-articular osteoid osteomas may not have worsened pain at night, and response to anti-inflammatories can be modest. When located in a joint, they can result in significant reactive inflammation due to intra-articular prostaglandins, masquerading as inflammatory arthritis. They are often missed on XR and MRI if obtained early in the course. CT or bone scintigraphy are more sensitive. Rates of misdiagnosis and delayed diagnosis are high, around 60%, and mean time to diagnosis is about 26 months compared to 9 months when extra-articular. Definitive treatment has historically been surgical excision, with radio-ablation more recently taking its place.

Editor's Comments:
Osteoid osteomas are the third most common kind of pediatric benign bone tumor (4). While they mainly occur in the diaphysis of long bones, they can be intra- or peri-articular as in this case. Osteoid osteomas can be difficult to diagnose and often require collaboration with radiology. CT is more sensitive than radiographs or MRI though bone scintography or fluorine 18-labeled sodium fluoride PET/CT may be needed in some cases to confirm diagnosis (4).

References:
1. Dookie AL, Joseph RM. Osteoid Osteoma. PubMed. Published 2021. https://www.ncbi.nlm.nih.gov/books/NBK537279/
2. Sonam Vadera, Gaillard F. Osteoid osteoma. Radiopaediaorg. Published online May 2, 2008. doi:https://doi.org/10.53347/rid-1802
3. Corrêa DG, Costa FM. Osteoid Osteoma Mimicking Arthritis. J Clin Rheumatol. 2023;29(1):e1-e2. doi:10.1097/RHU.0000000000001934
4. Bedoya MA, Iwasaka-Neder J, Tsai A, Bixby SD. Intra-articular Osteoid Osteomas: Imaging Manifestations and Mimics. Radiographics. 2024;44(7):e230208. doi:10.1148/rg.230208

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