Teaching Case

A 28-year-old male presented with a history of pain for six months in the middle third of his right lower leg. The pain was focal, atraumatic and not load-related. It worsened at night and was relieved by non-steroidal anti-inflammatory drugs (NSAIDs). Clinical examination was normal, besides morbid obesity.

Ultrasound and electromyography of the right lower leg were normal. Magnetic resonance imaging (MRI) of the lumbar spine could exclude a right-sided discoradicular conflict.

Non-contrast computed tomography (CT) of the right lower leg showed a small rounded cortical-based lytic lesion (four millimetres in diameter) with marked surrounding sclerotic cortical thickening at the posterolateral side of the middle third of the right tibia (Figure 1a, 1b). There was a small sclerotic focus within the lytic lesion, representing central mineralization. A technetium-99m bone scintigraphy (Figure 2) showed one focal area of increased radiotracer uptake in the middle third of the right tibia. These findings were highly suggestive for osteoid osteoma (OO).

Percutaneous CT-guided biopsy and radiofrequency ablation (RFA) with an anterolateral approach were performed (Figure 3), preceded by an ultrasound-guided peripheral nerve block of the tibial nerve and common peroneal nerve. Post-procedural X-ray (Figure 4a, 4b) after one month demonstrated residual cortical thickening and the RFA needle trajectory.

Background

OO is a benign painful osteoblastic bone tumour that occurs most frequently in young male patients [1]. OO is histologically composed of a nidus, which refers to the neoplastic process itself, surrounded by reactive sclerosis [2,3]. The most recent classification scheme for OO based on the localisation on CT and MRI categorizes OOs as subperiosteal, intracortical, endosteal, or intramedullary [3]. The majority of OOs are located extra-articular in the long bones, most frequently involving the femur and the tibia [2].

Clinical perspective

The most common clinical presentation is local nocturnal pain, typically relieved by salicylates and NSAIDs [4]. However, OO sometimes presents with atypical symptoms that can make the diagnosis challenging. Examples are limping, referred pain, localized swelling, painful scoliosis, or growth disturbance [5].

Imaging perspective

The typical radiographic image comprises an intracortical nidus, usually smaller than two centimetres, surrounded by cortical thickening and reactive sclerosis in a long bone shaft [3].  CT is highly specific in detecting the nidus, showing a well-defined round or oval hypoattenuating lesion, which may have a variable amount of mineralization [6]. MRI is of limited value in depicting the nidus because, in small nidi, the signal is often similar to cortical bone [3]. The added value of MRI consists of detecting the associated surrounding bone marrow oedema and soft tissue changes [6]. Bone scintigraphy is very sensitive in detecting the osteoblastic activity of OO [5]. The very specific ‘double density’ sign shows a central focus of very high radiotracer uptake (representing the nidus) in a larger area of less intense uptake and allows differentiation from osteomyelitis, stress reaction, and metastasis [5,7].

Outcome

Conservative treatment with NSAIDs can be considered as OO tends to heal spontaneously [4]. Surgical excision was the standard of care until Rosenthal et al. [8] introduced percutaneous CT-guided RFA as a new gold standard in 1992, in which thermal necrosis of the lesion is induced by ablation at 90°C for 6 minutes [9]. CT is essential for therapy planning and localisation of the nidus. RFA is safe and results in significant and lasting pain reduction [10]. Our patient reported near-complete relief of symptoms after treatment. The histopathology report from the biopsy in our case was inconclusive. Biopsy only yields reliable results in less than 50% of cases [2, 10]. Possible adverse effects of the RFA procedure comprise thermal skin burns, tissue necrosis, osteomyelitis, soft tissue infection, neuropathy, and hematoma [11]. To avoid thermal skin burns and heat-associated tissue necrosis during RFA of OO in the tibia of a thin patient, a cooling technique with sterile draping of commercially available cool packs around the needle shaft at the entry point can be applied [12].

Teaching points 

In a young male patient with local nocturnal pain in a long bone or vertebra relieved by NSAIDs, OO should be excluded. CT has the highest specificity to detect the nidus and is used for treatment planning. RFA is the gold standard for treatment.

Written informed patient consent for publication has been obtained.