On examination, the patella appeared bigger and more prominent on the right side compared to the left. There was no localized warmth, but the patella was notably tender. In addition, there was a palpable bony thickening of the patella. No synovial thickening or knee joint effusion was noted. His knee range of motion was 0 to 110 degrees with pain at terminal flexion. There was no ligamentous laxity.

Laboratory studies revealed a hemoglobin of 13.5 gm/percent and white blood cell count of 13,000 cu.mm. Inflammatory markers were within the normal limits; his erythrocyte sedimentation rate was 18, C-reactive protein was negative, and alkaline phosphatase was 286 units. Mantoux test was non-reactive.

ABCs, although benign, can be locally aggressive. The etiology of this condition is not definitively known, although most believe it is a vascular malformation within the bone. The proposed theories for the origin of this malformation are that it either occurs de novo, when it is called primary ABC, or secondary to tumors or trauma, when it is called secondary ABC. Secondary ABCs can be seen associated with giant cell tumor (GCT), chondroblastoma, osteoblastoma, or osteosarcoma. Recent studies have identified chromosomal abnormalities suggesting that the tumor, once thought to be a reactive process, may actually be a neoplasm.² This case is unusual in that it identifies an ABC occurring at a rare site (the patella) and secondary to a giant cell tumor (a lesion which also occurs rarely in the patella).

ABCs are usually painful, so late presentation as a pathological fracture is less common than with unicameral bone cysts.³ Although both conventional radiology and MRI are useful for diagnosing ABCs, biopsy is often needed to confirm the diagnosis. In this patient, initial radiographs showed multiple lytic lesions in the right patella. At the time of presentation, our differential diagnoses included tuberculosis, bone cyst, or giant cell tumor. However, the ESR was normal and the Mantoux test was not reactive. We then performed a core biopsy of the patella which showed features suggestive of an aneurysmal bone cyst.

The mainstay of treatment of ABCs is intralesional curettage with locally applied adjuvants such as liquid nitrogen or phenol.⁴ Other options include enbloc dissection or selective arterial embolization. The recurrence following curettage of an ABC secondary to GCT has been reported as 2 to 25 percent. Young age and open physes are associated with an increased risk of local recurrence. ⁵