A very unusual location of the osteoid osteoma, the patella

Abstract text (incl. references and figure legends)

Case history

A 17-year-old woman with a 1-year history of left knee pain that woke her up at night and prevented her from running.

Clinical findings

The patient presented patellar pain, swelling and flexion antialgic contracture.

Investigation/Results

In the x-ray we found a lytic lesion. MRI described a 7 mm lesion in the patellar subcortical region with sclerotic halo, bone edema and peripatellar inflammatory changes, CT showed the same lesion with central nidus and scintigraphy revealed diffuse uptake compatible with bone edema.

Diagnosis

Osteoid osteoma vs chondroblastoma.

Therapy and Progressions

Analgesia with acetylsalicylic acid was initiated with clinical improvement. Core needle biopsy and radiofrequency ablation were performed, both CT-guided. The diagnosis obtained was osteoid osteoma.

Pain improved the same night after the thermoablation. At 3 years, the patient shows no pain or functional limitations.

Comments

Osteoid osteoma in the patella is an unusual cause of knee pain in young adults. Diagnosis is challenging due to its atypical presentation and the multiple differential diagnoses (chondromalacia, tendinitis, osteomyelitis, osteochondritis dissecans...).

Typical nocturnal pain relieved by salicylates may be absent. In addition, radiographs are not diagnostic in 80% of cases. MRI can be confusing by bone edema, and on scintigraphy, the typical double-density is often absent. Thus, CT and biopsy are the gold standard.

This presentation and the absence of characteristic images delay diagnosis 1 to 3 years and lead to wrong treatments (diagnostic arthroscopy). The treatment of choice is image-guided radiofrequency ablation, which must be done carefully in subchondral lesions so as not to injure the cartilage.

References

Sharma S , Rajnish RK , Prakash M, Agarwal S, Dhillon MS. Patellar Osteoid Osteoma as a Cause of Intractable Anterior Knee Pain - A Case Report and Systematic Review of Literature. J Orthop Case Rep. 2020;10(2):29-34

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