We use general anesthesia for these procedures. Local anesthesia with or without conscious sedation is adequate for most bone biopsies. However, needle puncture of an osteoid osteoma is much more painful than biopsy of other lesions. When the needle enters the tumor, sudden marked tachycardia and tachypnea often occur, even under general anesthesia. The anesthetist should be aware of this phenomenon to prevent patient movement. An intravenous nonsteroidal anti-inflammatory drug such as ketorulac given approximately 20 minutes before completion helps with postprocedure pain.
A grounding plate is attached to the skin opposite to the site of entry. Tumor localization is performed as for a bone biopsy except that the scan thickness should not exceed 3 mm because of the small size of these lesions. Once the lesion has been demonstrated, the radiation exposure should be minimized by decreasing milliamperage (mA) to the lowest level that is adequate.
We use a drill to penetrate dense cortical bone (Bonopty Penetration Set, RADI Medical System AB, Uppsala, Sweden). This device produces a hole that admits a 16G (1.6 mm) biopsy needle. It is also possible to use a trephine needle or other bone biopsy tool. In either case, it is necessary to pause at frequent intervals while drilling through bone to clear the trephine or the threads of the drill. If this is not done, it becomes increasingly difficult to advance; indeed the biopsy device may become wedged into the bone, making it very difficult to withdraw.
For the relatively common lesions of the medial upper femur, the patient can be placed in the "frog-lateral" position once anesthetized (Fig. 1). When there is no safe direct access to the lesion a "transosseous" approach can be used, drilling through the entire thickness of the bone from the contralateral side (Fig. 2). When the drill comes into close proximity with the tumor (1 mm), but has not entered it, it is exchanged for the biopsy needle.
We most often use the Ostycut Bone Biopsy Needle (C. R. Bard Inc., Covington, Georgia, U.S.A.), because the sharp stylet exhibits little tendency to slip when used on the surface of the bone, and because it will fit through the cannula of the Bonopty drill. The external screw threads are helpful in advancing through the lesion, and it yields a biopsy sample of good quality. A 1.6 mm (16 G) needle is appropriate for the 1 mm outer diameter of the electrode.
We generally do not request "frozen section'' analysis because tissue preservation is not optimal by this method, and the histological diagnosis rarely alters the treatment. If infection is a serious diagnostic possibility, a frozen section may be helpful, as thermocoagulation could potentially worsen the condition by producing a sequestrum.
After the biopsy is taken, the RF electrode is introduced and CT imaging is repeated to confirm the placement. We use electrodes with either 5 or 8 mm exposed tip. The electrode is connected to the RF generator (RFG-3C, Radionics, Burlington, Massachusetts, U.S.A.), and the tip temperature is slowly (1° every one to two seconds) raised to 90°C. The temperature is maintained
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FIGURE 1 (A) Computed tomography scan of the left hip shows an osteoid osteoma in the posteromedial cortex of the proximal femur. (B) Images obtained with the patient in the "frog-lateral" position. This rotates the lesion anteriorly, allowing it to be safely approached medial to the femoral triangle (arrow).
FIGURE 2 (A) Computed tomography scan through the lateral malleolus demonstrates a typical lytic lesion involving the medial aspect of the fibula, adjacent to the talus. (B) An anterolateral transosseous approach has been used, to spare the ankle mortise.
for six minutes, after which the generator is turned off, and the procedure is complete. These parameters (90°C, six minutes) were selected because they consistently result in a spherical (5 mm electrode) or cylindrical (8 mm electrode) lesion with a radius of 5 to 6 mm. Therefore, the electrode placement must be such that no portion of the tumor is more than 5 to 6 mm away from its exposed tip (Fig. 3). If the tumor is large, or if electrode placement is off-center, more than one treatment must be performed to ensure that the entire tumor is treated (Fig. 4).
It is apparent that a minimum of 6 mm separation between the electrode and any vital structures is required to avoid tissue damage. For safety sake, we generally will not perform this procedure when the distance is less than 1 cm. The thermal effects do not reliably respect anatomical boundaries such as bone cortex.
In our experience, intralesional administration of anesthetics and postprocedure imaging is not helpful.
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