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Original Research |
1 Department of Radiology, Mater Misericordiae University Hospital, Eccles St.,
Dublin 7, Ireland.
2 Cappagh National Orthopaedic Hospital, Finglas, Dublin 11, Ireland.
Received June 14, 2004;
accepted after revision March 23, 2005.
Address correspondence to C. P. Cantwell
(ccanty{at}gofree.indigo.ie).
Abstract
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SUBJECTS AND METHODS. Radiofrequency ablation was performed on 11 patients with a clinical and radiologic diagnosis of osteoid osteoma. A cooled radiofrequency probe was introduced into the lesion under CT guidance. Twelve minutes of radiofrequency energy was delivered from a 200-W generator under impedance control. Postprocedural pain, function, and satisfaction were evaluated by means of an interview and questionnaire.
RESULTS. All procedures were technically successful. No serious complication occurred. Postoperative pain was scored at a mean of 6.9 ± 3.06 (95% confidence interval) on a numeric rating scale. Postoperative pain was rated as similar to night pain. By 1 week after therapy, all patients had resolution of pain and returned to normal activity. There was no recurrence during the follow-up period (range, 6-27 months; mean, 14.4 months). Patients rated their satisfaction as high.
CONCLUSION. Radiofrequency ablation of osteoid osteoma with a high-energy delivery technique is safe and has a high success rate. When compared with patients in a published series using 5-mm probes and manual energy control from lower-output generators, our cohort showed increased postoperative pain scores and an increased interval to symptom resolution.
Keywords: bone neoplasm • osteoma • radiofrequency ablation • therapy
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Noncooled 5-mm exposed-tip probes with low-output radiofrequency generators and 4- or 6-min treatment times have been used since the early 1990s for therapy of osteoid osteoma. This system was selected because, in a dog model, the treatment zone in bone was found to be similar in size to a human osteoid osteoma. Expansion in the use of radiofrequency ablation in hepatic tumor therapy has led investigators to develop methods to increase the zone of ablation. In a pig bone model, radiofrequency ablation with longer probes, higher-output generators, and longer treatment times has produced a larger thermal lesion in bone.
There is little evidence regarding the efficacy of modern radiofrequency equipment for osteoid osteoma therapy. We applied the manufacturer's recommended therapy protocol for bone, believing that a larger ablation zone would be produced and would lead to a higher clinical success rate. We looked at the pain and functional limitation before and after therapy of osteoid osteoma with modern radiofrequency equipment and compared them with data from published case series that used noncooled 5-mm probes with lower-output radiofrequency generators and shorter treatment times.
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Procedures
Informed consent was obtained. We performed all the procedures. General
anesthesia was administered. Each patient was positioned on the CT table, and
two self-adhesive grounding pads were attached to the patient's skin.
CT was performed with a 4-MDCT scanner (Somatom, Volume Zoom 4, Siemens Medical Solutions) to localize the osteoid osteoma. Using the images, we adjusted the position of the patient's limb and marked the skin at the planned access point. The skin was prepared and draped. A 20-gauge spinal needle was positioned at the mark and angled along the approximate track. Quick-check CT confirmed that the needle was positioned appropriately. A small skin incision was made. Under CT guidance, an 11-gauge Jamshidi bone biopsy set (Cardinal Health) was guided to the cortex adjacent to the nidus (Fig. 2). The central trocar was removed from the Jamshidi bone biopsy instrument. The nidus was drilled with a 4.5-French Ostycut bone biopsy needle (Angiomed) in the first seven patients and a 1.6-mm nonthreaded K-wire in the remaining four patients (MicroAire Surgical Instruments). No biopsy was performed. Through the tract track this drill hole, a 17-gauge (1.5 mm) 1- or 2-cm exposed, water perfusion-cooled radiofrequency probe (Cool-tip, Valleylab) was placed with the most proximal point of the active tip at the near edge of the nidus of the osteoid osteoma (Fig. 3). The 2-cm probe was used in only the first patient, because a postprocedural MRI study showed a large area of therapy in the bone marrow abutting the osteoid osteoma. The outer cannula of the Jamshidi was withdrawn over the probe to prevent arcing and was secured. The probe was circulated internally with saline at nearly 0°C at a rate of 80 mL/min by a mechanical pump. The radiofrequency probe was connected to a 200-W radiofrequency generator (RFG-3C, Radionics) (Fig. 4), and 12 min of impedance control radiofrequency energy was delivered at maximum output.
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Before cessation of anesthesia, 10 mg of morphine was administered IV. Nonsteroidal antiinflammatory medication was administered rectally. The patient recovered in the day ward and was discharged in the evening.
All patients were allowed to bear weight fully and without crutches after recovery. The patients were advised to avoid contact sports and heavy lifting for 6 weeks. Antiinflammatory medications were administered as required for 1-3 days by the patient or guardian.
Some patients, as part of a parallel study, underwent MRI of the treated limb in the postprocedural period (Fig. 5).
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Preoperative day pain, night pain, and postoperative pain at discharge and at 24 hr were rated on a visual analog scale from 0 to 10, where 0 represents no pain and 10 represents the most severe pain imaginable. Patients were asked to record the frequency and type of analgesia they used preoperatively and the period between radiofrequency therapy and cessation of pain.
Pain immediately after and 24 hr after surgery was rated using a 5-point verbal score as worse than night or day pain, similar to night pain, similar to day pain, less than both night and day pain, or absent.
Persistent functional limitation was determined on the basis of the extent of residual interference with employment or school. The number of school or work days missed before and after therapy was recorded.
Finally, the patient was asked to rate satisfaction with the treatment on a scale from 0 to 10, where 10 represents complete satisfaction and 0 represents complete dissatisfaction.
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The ablation was performed as day procedures except in the youngest patient, aged 11 years, who was discharged the next day as a precautionary measure to ensure adequate analgesia.
The weighted CT dose index for the procedure was available for only three patients (range, 79.06-92.08 mGy; mean, 87.613 mGy).
Preoperative Assessment
The mean symptomatic period before radiofrequency therapy was 19.5 months
(range, 6-48 months). Night pain was the predominant feature of the condition;
the mean pain score was 7.7 (range, 3.5-10) on a numeric rating scale. Day
pain was scored at a mean of 4.4 (range, 1-7).
All patients took analgesic medication. Eight of the 11 took nonsteroidal antiinflammatory medication, and the remainder took primarily paracetamol-based products. Six of the 11 had mild residual discomfort despite regular analgesia, and the pain from the lesion affected sleep in eight of the 11.
On average, 14.3 days of school or work were missed as a result of the osteoid osteoma (range, 0-70 days).
Postoperative Assessment
The mean pain score at recovery was 6.7 ± 3.06 (95% confidence
interval). On the verbal scale, this pain was described as similar to night
pain in all respondents. The mean pain score at 24 hr was 4.8 (range, 2.5-10).
On the verbal scale, this pain was described as similar to day pain in most
cases.
On review, no significant relationship was found between nidus size and preoperative day pain, preoperative night pain, time off work, postprocedural pain, pain at 24 hr, or the interval to cessation of pain. No significant correlation (p < 0.05) was found between pain immediately after the procedure, pain at 24 hr after the procedure, or the interval to cessation of pain and the distance from the nidus to the marrow space or periosteum or the cortical thickness.
No patient had resolution of pain by 24 hr after the procedure, three had resolution by the third day, four had resolution by the fourth day, and all had resolution by the seventh day.
The patients returned to work or school between 5 and 7 days after the procedure (average, 7 days).
All patients rated their satisfaction with the procedure at the maximum on the numeric rating scale, reflecting collective satisfaction with their radiofrequency ablation.
No patient had recurrent symptoms attributable to an osteoid osteoma at the maximum follow-up interval (range, 6-27 months; mean, 14.4 months).
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The interval to being symptom-free after therapy was prolonged in our group. No patient was symptom-free at 24 hr—it took 3 days for our first three patients to be symptom-free. The remainder were not symptom-free until 1 week. The largest group of patients, five, did not become asymptomatic until the seventh day. This result can be compared with the results of two published series of patients treated with a 5-mm noncooled radiofrequency probe over a 4-min delivery of energy at 90°C. In the first series (18 patients), 10 patients were symptom-free by 24 hr; 14 were symptom-free by 3 days; and, finally, 16 were symptom-free by 1 week [3]. In the second series (13 patients), the number of symptom-free patients at 24 hr was not mentioned. By 3 days, 10 were symptom-free and one further patient was symptom-free at 1 week [2].
Postoperative pain in the group we treated with radiofrequency ablation was scored at a mean of 6.9 ± 3.06 (95% confidence interval) on the numeric rating scale. In another series—13 patients with similar follow-up who scored their postprocedural pain on the same scale—pain was scored at a mean of 3.3 [2]. Our group experienced significantly more postprocedural pain, possibly because of larger zones of ablation involving the periosteum, cortex, and marrow.
As a verbal guide, patients rated their pain immediately after the procedure as being equivalent to their night pain. At 24 hr, most patients rated their pain as similar to their day pain. This information can be a useful guide when one is advising patients as to their expected postprocedural course.
At the maximum follow-up interval (range, 6-27 months; mean, 14.4 months) for our series of patients, none had experienced a recurrence after successful treatment. In the second largest published series, by Vanderschueren et al. [4], 76% of the 97 patients were symptom-free after one treatment. In the largest series (126 patients) with at least a 2-year follow-up, 89% remained symptom-free; this series was limited by the fact that 28% of the treatment population was lost to follow-up [5].
With time, equipment has been altered to increase the zone of ablation. In the original published work of Tillotson et al. [6], a dog bone model had been used for radiofrequency ablation. The electrode was a 5-mm exposed-tip probe that was not internally cooled. Energy was delivered from a radiofrequency generator with a low output. The energy delivered was determined by manual setting of the input current so that the probe thermostat read 80-90°C. Radiofrequency ablation of 0.9-1.3 cm of bone was possible in the dog bone model, and the size of the thermal lesion was independent of the time the probe was active (from 30 sec to 4 min). Subsequently, radiofrequency was applied in humans, but for 4-6 min, depending on the operator [5, 7].
Techniques to deliver higher energy have increased the treatment zone. Aschoff et al. [8] performed radiofrequency ablation with a 150-W maximum-output generator and a 2-cm exposed cooled probe. The ablation was manually applied to the metaphysis of the femur in pigs at 90°C for 10 min in an MRI field. This procedure produced a lesion averaging 15.4 ± 2.7 mm in diameter. Nour et al. [9] performed a similar experiment using porcine lumbar vertebrae and the same protocol as Aschoff et al., except that the generator had a maximum output of 100 W. The mean lesion diameter was 11.0 ± 1.8 mm.
The thermal profile of the lesion can be altered by internal perfusion of the probe with saline cooled to nearly 0°C, reducing charring around the probe. When the temperature of tissue about the probe increases to nearly the boiling point, impedance to current flow rises. The generator reduces current delivery until a resting period is over and then redelivers the current at the end of lesion cooling. In soft tissue, Goldberg et al. [10] reported the ability of a perfusion-cooled probe to increase the volume of ablation.
Probe cooling limits the operator's ability to monitor therapy. The cooled probe prevents the operator from monitoring temperature as an indicator of lesion therapy either during or after the procedure, but if internal cooling ceases, the longer exposed-tip probe can be used with manual control. Unlike the resterilizable 5-mm noncooled probe, the cooled probe is a single-use device, which does increase the procedure cost.
Mechanisms of bone weakening include disruption of the bone matrix by drilling of a hole for the probe and induction of cellular death in periosteal, medullary, and cortical bone by radiofrequency ablation. Large-bore drilling of osteoid osteoma has also been associated with increased postoperative morbidity and fracture. In one case series, the morbidity was 24%, including two fractures, one case of focal chronic osteomyelitis, three skin burns, two hematomas, and one case of femoral cutaneous nerve damage [11]. The access tract necessary for a 5-mm exposed-tip probe is 0.9 mm wide (20 gauge). Our probe was only marginally larger, at 1.5 mm. We reduced the access diameter in our series using a 1.6-mm nonthreaded K-wire or a 4.5-French Ostycut bone biopsy needle for drilling. Other operators have used larger access holes created with a 2.1-mm Steinmann pin (MicroAire Surgical) or a 1.7-mm Bonopty set (Radi Medical Systems) or an Ackermann 2-mm needle (Cook Medical) [4, 5]. The potential risk of increasing the treatment zone includes an unknown risk of fracture due to the weakening effect of an extended zone of cellular ablation. Despite the potential for bone weakening, we experienced no fracture in our series. Venbrux et al. [12] reported skin burns at the site of probe placement and a single case of neuropraxia in a series of nine patients with osteoid osteoma treated with cooled probes (Cool-tip) or expandable multiple electrode systems (RITA Medical Systems). This group used CT or fluoroscopy for probe localization. The number of probe placements (average, 2.6) and the number of retreatments is high in this cohort. Considerable heterogeneity in the energy-delivery protocols makes it difficult to draw conclusions from these data. We experienced only one minor complication and believe that CT guidance is preferable to ensure that the probe is more than 1.5 cm from a neurovascular structure and that the skin is more than 1 cm from the nearest margin of treatment [12].
Modern methods of radiofrequency delivery are safe and increase the efficacy of therapy of osteoid osteoma. Techniques that deliver higher energy increase pain after the procedure and prolong the interval to symptom resolution.
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  U. Albisinni, R. Ciminari, C. Malaguti, and E. Rimondi Radiofrequency Ablation of Osteoid Osteoma with Cooled Probes and Impedance-Control Energy Delivery Am. J. Roentgenol., June 1, 2007; 188(6): W578 - W578. [Full Text] [PDF]
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C. P. Cantwell, S. Eustace, and J. O'Byrne Reply Am. J. Roentgenol., June 1, 2007; 188(6): W579 - W579. [Full Text] [PDF]
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