Radiofrequency ablation of osteoid osteoma.

Osteoid osteoma is a benign bone neoplasm with a reported incidence of 2-3% among all bone primary tumors. Although it is a small and benign lesion, it is often cause of patient complaint and discomfort. It is generally characterized by a long lasting, unremitting pain that typically exacerbates at night, often leading to sleep deprivation and functional limitation of the skeletal segment involved, with a significant reduction of patient daily life activities and consequent worsening of the overall quality of life. Over decades, complete surgical resection has represented the only curative treatment for symptomatic patients. In the last years, new percutaneous ablation techniques, especially radiofrequency ablation, have been reported to be a safe and effective alternative to classical surgery, with a low complication and recurrence rate, and a significant reduction in hospitalization cost and duration. The aim of this article is to provide an overview about the radiofrequency thermal ablation procedure in the treatment of osteoid osteoma.

Osteoid osteoma (OO) is a benign bone tumor of undetermined etiology firstly described by Jaffe in 1935 (1-5). OO accounts for 2-3% of all bone primary tumors with an incidence of 10-12% among all benign skeletal neoplasms (6-10). It is more frequent in adolescents and young adults with 50% of patients being aged between 10 and 20 years and it predominates in male, with a reported male-to-female ratio of 4:1 (11-15). Clinical presentation can vary depending on the location of the lesion; patients generally complain of localized, deep and unremitting pain that increases in intensity over time, typically gets worse at night and rapidly improves after salicylates or other NSAIDs administration. Soft tissue swelling and skin erythema may be present in OO in subcutaneous location. Duration of pain can vary from weeks to years before definitive diagnosis, depending on how typical the clinical presentation is and how early the diagnostic suspect is posed.

Location

OO may occur anywhere in the axial or appendicular skeleton, but the majority of cases arises in the diaphysis or metaphysis of long bones in the lower limb (Fig. 1), with more than half involving either the femur or the tibia. Juxta-articular or intra-capsular location is generally associated with joint effusion and may clinically resemble an inflammatory arthropathy. Less commonly affected sites are the small bones of hand and feet. 10-20% of all OOs are located in the spine, with a predilection for the lumbar metamers; they almost exclusively occur in the posterior vertebral elements and they typically present with painful scoliosis (7, 11, 16-35).

Figure 1.

Osteoid osteoma of the proximal left femur. 6 year old female with left thigh pain of recent onset with no history of trauma. (A) Standard X-Ray shows wide cortical thickening of the medial portion of the proximal femoral diaphysis (arrow); radiolucent nidus is partially obscured by the surrounding reactive sclerotic bone. (B) MPR coronal view CT scan clearly demonstrates the presence of an oval shaped radiolucent nidus with some degree of calcification inside (arrowhead). (C) T2-weighted MRI image of the same lesion on the axial plane. (D) The lesion has been effectively treated with percutaneous radiofrequency thermal ablation

Pathology

OO is structurally composed of a small central area of woven bone and osteoid matter, the nidus, surrounded by a zone of reactive sclerotic bone. The nidus, round or oval in shape, rarely exceeds 1,5-2 cm in diameter and may contain a central region of variable bone mineralization. According to its site of origin within the involved bone, OO can be classified in cortical, medullary (cancellous) and subperiosteal.

Cortical OO is the most common type and it generally presents with the classic central nidus and peripheral sclerosis as described above. Medullary OO is seen in the neck of the femur, in small bones and in the spinal column, while subperiosteal type is generally located on the intra-articular surface of the involved bone. Medullary and subperiosteal OO are less frequent and often demonstrate less peripheral bone sclerosis (Fig. 2), sometimes making their diagnosis more difficult (36-45).

Figure 2.

Osteoid osteoma of the right femoral neck. 12 year old male suffering from right hip pain irradiated to the omolateral thigh for several months was referred to our center for percutaneous treatment of an osteoid osteoma of the femoral neck; note that in this location osteoid osteoma generally shows less or absent peripheral sclerosis. (A) Radiofrequency ablation was achieved via a posterior trans-gluteal approach. (B) Post-procedural control CT scan. (C) 30 days post-treatment MRI showing regular outcomes of intervention (arrow)

Imaging

X-ray is the initial imaging modality in case OO is suspected. Plain radiograph typically shows a round or oval radiolucent nidus, usually smaller than 1,5 cm in diameter, surrounded by a variable but regular fusiform area of bone sclerosis (Fig. 1). In larger lesions, a central nucleus of bone mineralization within the nidus can also be observed. Sometimes the peripheral rim of reactive bone sclerosis is so extensive that the underlying radiolucent nidus may result obscured (46-61).

Computed Tomography is the imaging technique of choice for OO, being advantageous for detection and characterization of both the nidus and the peripheral sclerosis (Fig. 3). It is particularly helpful for small OOs, for those lesions which show less peripheral bone sclerosis (especially medullary and subperiosteal types), and in all those cases where standard radiographs are not clearly conclusive. On CT scan the nidus appears as a small well-defined radiolucent area, with soft-tissue attenuation values, which shows early vascular contrast enhancement (even if intravenous contrast administration is generally not necessary for definitive diagnosis). A variable degree of central nidus mineralization, sometimes punctate, is often seen (Fig. 4) (7, 46). Moreover, the presence of thin vascular grooves surrounding the lesion has been demonstrated to be highly specific for distinguishing OOs from other radiolucent bone tumors on CT (29, 62-70).

Figure 3.

Osteoid osteoma of the femur in elderly patient. 71 year old male with unremittent left thigh pain; osteoid osteoma was not clinically suspected, since it is quite uncommon in this age range. (A) CT scanogram demonstrating the presence of focal cortical thickening of the left femoral diaphysis. (B) Classical CT appearance of osteoid osteoma: central radiolucent nidus surrounded by sclerotic reactive bone. (C) Radiofrequency ablation was effective in achieving complete resolution of the symptoms

Figure 4.

Osteoid Osteoma of the tibia. 20 year old male with long-standing left knee pain. (A, B) Axial and coronal plane images showing an osteoid osteoma near the posterior surface of the left tibia. (C) Percutaneous radiofrequency thermal ablation of the lesion. (D) Volume-rendering reconstruction of the positioning of the inserting cannula

MRI is inferior to CT in diagnosing OO, although it is very sensitive in demonstrating bone marrow and perilesional soft tissues edema and inflammatory reaction. The nidus generally shows low to intermediate signal intensity on T1-weighted images and variable intensity on T2-weighted images. Peripheral reactive sclerotic bone and central nidus calcifications are hypointense on both T1- and T2- weighted images (11, 36, 71-75). However, MRI findings should always be correlated to standard radiographs or CT since, when used alone, MRI can lead to a wrong diagnosis in a significant amount of cases (76-81).

Technetium-99-labeled bone scintigraphy typically reveals a central focus of intense radionuclide uptake (representing the nidus) surrounded by a larger region of less intense signal (corresponding to the reactive perilesional bone), the characteristic so-called “double-density” sign; it is seldom used today, due to concerns related to the isotope radioactivity (82-86).

Treatment

Although OOs can spontaneously regress in a period of time ranging from two to six years (87-90), over decades complete surgical excision has been considered the classical treatment for all those patients who had unremitting pain despite conservative treatment or those who could not tolerate long-term NSAIDs therapy. En bloc surgical resection has a reported success rate between 88% and 100% but it carries several significant disadvantages. Firstly, for the orthopedic surgeon it may be difficult to identify the precise location of the lesion and the exact amount of bone to resect; secondly, to be sure all the nidus is removed, a substantial volume of bone must be excised, potentially leading to bone weakness and consequent necessity of bone grafts, internal fixation and postoperative immobilization to prevent subsequent fractures. Nonetheless, recurrence rate remains significantly high (reported in literature to range from 4,5% to 25%) due to incomplete nidus excision (Fig. 5) (91-95).

Fig. 5

Recurrence of osteoid osteoma after surgical curettage. 24 year old female complaining of persistent pain after surgical curettage of an osteoid osteoma of the scaphoid treated with bone graft from the distal radius. (A) Standard radiograph taken 3 months after the curettage shows the persistence of a radiolucent area at the distal pole of the right scaphoid (black arrow); missing bone from previous bone grafting (black arrowhead). (B) MRI confirmed the diagnosis of recurrence of osteoid osteoma (white arrow); note the outcomes of bone grafting from the distal radius (white arrowhead). (C-D) Radiofrequency ablation was the definitive treatment; wrist pain disappeared and no recurrence was observed.

Over the last years, percutaneous radiofrequency ablation (RFA) has been proposed as a new interventional technique for symptomatic patients with OO. It has widely and rapidly been accepted as a valid minimally invasive alternative to traditional orthopedic surgery, so much to be considered today the treatment of choice for OOs, especially for those located in difficult areas for surgery (96-98). Percutaneous ablation is reported to be a safe and cost-effective treatment, with a superior long-term efficacy and a significant reduction in hospitalization cost and duration when compared to other surgical techniques. It has a reported success rate close to 100% and a recurrence rate of about 5% with almost negligible post-procedural complications (37, 99-106).

The procedure is performed in the CT room under aseptic conditions; patient position can be prone, supine or, more rarely, on lateral decubitus – depending on the location of the lesion. Two grounding pads are properly positioned on patient skin and connected to the RF generator. Local anesthesia is generally sufficient to control pain for all the duration of the operation, however the possibility to use general, spinal or epidural anesthesia is always discussed with the anesthesiologist (who is present in the CT room during the entire procedure), according to the patient collaboration and clinical conditions. CT scan is obtained to precisely localize the lesion and multi-planar reconstructions are used to plan the optimal trans-cutaneous approach. Close attention must be paid to adjacent blood vessels and nerves, since these structures must carefully be avoided. A penetration cannula with an inner stylet is introduced through the skin and soft tissue according to the previous established access route. New CT images are then obtained to verify the correct position of the cannula. Computed tomography has an extraordinary spatial resolution, but it carries the disadvantage that it cannot be used in real-time guidance; for this reason CT-fluoroscopy is increasingly adopted (107-113). Once ensured the distal end of the cannula is well positioned on the bone surface, the inner stylet is removed. Bone penetration is then accomplished by inserting – through the cannula – a Kirshner wire gradually tapped until the center of the nidus. Automatic orthopedic drill represents a feasible and helpful alternative for this step (114). Once within the nidus, the Kirshner wire is replaced with the ablation electrode (Fig. 6) and, after another accurate CT control of the electrode tip position, the electrode is connected to the RF generator. Whenever possible, once the nidus is accessible and before the electrode in inserted, a core biopsy of the lesion should be performed. When the electrode is documented to be exactly within the nidus, RFA starts. The lesion is generally heated to 90°C for a time of 5-6 minutes, which are widely reported to be the optimal ablation parameters. Lower temperatures would be ineffective or would significantly increase the ablation time. Higher temperatures can lead to tissue vaporization, a circumstance that would limit proper heat propagation through the lesion. Once the procedure is terminated, a last CT scan is obtained to exclude possible soft tissue damage. The electrode and the cannula are removed, local anesthetic is administered, and sterile dressing is applied to the skin entry site. The entire procedure can be performed in approximately 60 minutes. After a brief period of observation, all patients are generally discharged from hospital on the same day, with a 30 days scheduled follow-up visit and MRI control. Daily activities may immediately be resumed, with the exception for driving (101, 115-117). Post-procedural complications are rare, and may include skin burn, muscular hematoma, infection and nerve injury.

Figure 6.

Osteoid osteoma of the talus. 39 year old male with recurrent left ankle pain. (A) CT scan demonstrates the presence of an osteoid osteoma of the talus (arrow). (B) Multiplanar reconstructions are used to guide the electrode inside the lesion. (C) 30 days post-treatment MRI showing regular thermal ablation outcomes (arrowhead)

Conclusion

CT guided RFA is proven to be a safe and effective procedure for the treatment of OOs. When compared to surgical excision, it shows a significant reduction in hospitalization cost and duration. With its almost negligible post-procedural complications and a low rate of recurrence, RFA is at the moment the treatment of choice for these bone tumors.