Osteoid osteomas represent 12% of benign bone tumors and were first described by Jaffe in 1935 [¹]. They are twice as common in males; 90% occurring between 5 and 30 years of age [²]. In over 50% of cases they are centered on the cortex of the diaphysis of the femur or tibia [¹]. Within the femur, lesions are usually found proximally, most commonly within the neck and inter-trochanteric region [¹]. It is known that location of osteoid osteomas in cancellous bone is rare and even rarer in intra-capsular locations [²]. However the exact incidence of juxta-articular osteoid osteomas in the femoral head is not known. In most cases, affected individuals complain of severe pain related to the lesion which is worse at night and relieved by ingestion of non-steroidal anti-inflammatory agents [³].

Plain radiographs demonstrate the nidus in 85% of cases. A total of 20% of cases may be intra-medullary and have less reactive sclerosis [⁴]. When intra-capsular in location, an osteoid osteoma may present with clinical features that mimic inflammatory synovitis and with atypical radiological findings such as lack of both sclerosis and periosteal reaction [⁵]. Magnetic resonance imaging (MRI) is less sensitive than computed tomography (CT) and allows detection of marrow edema and associated soft tissue edema; a nidus is identified in only 65% of cases with MRI. CT scanning improves detection of the nidus to more than 85% [⁶].

Surgery remains the standard treatment in cases where histology of the lesion is in doubt, neurovascular structures are within 1.5 cm, or in cases with repeated failure of any other minimally invasive ablative technique or percutaneous resection [⁷]. Successful surgical therapy occurs in 88% to 100% of cases. Primary radiofrequency ablation in a case series of over 200 patients has had a success rate of 76% to 100% [⁶]. In another series the primary and secondary success rates of this technique were 87% and 83%, respectively. Surgical resection and open curettage show comparable success rates, but are associated with higher complication rates [⁸].

RFA is an excellent alternative to surgical excision in the foveal region as it avoids the complications associated with surgical exposure of the femoral head, including injury to the capsular vessels and post-operative capsular laxity. It also avoids weakening of the femoral neck by large diameter drilling for surgical access and chondral or osteochondral damage from resection of the subchondral lesion. Furthermore, in this location there exists a potential risk of avascular necrosis owing to close proximity of the foveal artery in the ligamentum teres. The foveal artery is a branch of posterior division of the obturator artery, which becomes important to avoid avascular necrosis of the head of the femur when the blood supply from the medial and lateral circumflex arteries are disrupted.

In summary, the unusual finding in this index case is the relative absence of bone thickening, which could be due to the intra-capsular location. RFA is a better option than surgery in this location as it avoids injury to the articular margin, prevents capsular injury and reduces the risk of weakening the femoral neck. Injury to the foveal artery with the potential risk of avascular necrosis must be kept in mind when the lesion is close to the fovea of the femoral head.

Written informed consent was obtained from the patient for publication of this case report and accompanying images. A copy of the written consent is available for review by the Editor-in-Chief of this journal.

The authors declare that they have no competing interests.

Image interpretation and RFA was performed by SKNK and KMM. KMM was a major contributor to writing the manuscript. KMJ was the orthopedic surgeon involved in acquiring osseous access with the drill. All authors read and approved the final manuscript.

We acknowledge Dr George Koshy.