Technique

Most interventional techniques in the skeleton include direct percutaneous biopsy and therapeutic procedures such as percutaneous resection of osteoid osteoma, injections of ethibloc or calcitonin in bone cysts, and acrylic cement injection. Precise localization under fluoroscopic or computed tomography (CT) guidance is necessary for such procedures. In the future, magnetic resonance imaging (MRI) will undoubtedly be useful for localization and treatment of focal bone-marrow anomalies. Moreover, since the introduction of open magnets or short-bore closed magnets, MRI has become an interesting tool for guidance in interventional procedures (1).

The first step of the technique (penetration of the shaft using a drill) has been commonly used for more than 10 years for percutaneous resection of bone tumors such as osteoid osteoma. It is a safe procedure, but fluoroscopic or CT guidance is necessary to prevent sideways slipping and to make holes in the correct position with regard to the shaft. Thus, an oblique angle of 40° seems to be the best compromise to allow satisfactory progression of the catheterization instruments. The soft tissues must also be protected during the deep-bone approach because of the high-speed rotation of the motor-driven drill (2).

During catheterization, progression within the medullary canal is easy. However, progression within the cancellous bone of the metaphysis and epiphysis is more difficult and requires a more rigid device. We therefore developed a prototype device, which we tested in vitro on various animal and human long bones.

The method of catheterization (Fig. 1) can be divided into three steps: (i) penetration of the cortex from the shaft, (ii) progression of the catheter within the shaft, and (iii) drilling of a targeted lesion in the metaphysis or extremity.

First, cortical drilling is carried out with existing orthopedic instrumentation (Aesculap AG, Tuttlingen, Germany) (Fig. 2) used for CT-guided percutaneous resection of osteoid osteoma (3,4). The cortex is drilled with 6 or 9 mm drills used alone or pushed over 2 mm Kirschner wires using a coaxial Seldinger technique. The diameter of the drilling machine must be selected carefully. Biomechanical studies have shown that a hole made in cortical bone leads to a loss of mechanical strength, which does not exceed 30% as long as the size of the hole does not exceed 30% of the shaft diameter (5). Care should be taken in selecting the size of the drill, and relief from weight bearing of the involved limb should be recommended to minimize the risk of secondary fracture on the approach. As in Seldinger's technique, the use of guidewires allows catheters to advance without difficulty in the marrow cavity and they can be changed if necessary, as in vascular interventional procedures. The main requirement for the new device we developed was that it should be sufficiently rigid to progress within the medullary cavity but flexible enough to bend within the epiphysis of the bone toward a precise location. It should also be as small as possible to minimize bone weakening. A modified endoscope thus seems to be the best alternative to reach the epiphysis of a long bone by a percutaneous approach.

FIGURE 1 The prototype and detail of the drive cable and the burr at its extremity.

The position and the movement of the drill in relation to the main axis of the shaft are monitored under fluoroscopic guidance. The adjacent soft tissues are protected from the drill by a sheath in order to prevent thermal and mechanical damage (2). Oblique penetration at an angle of about 40° to the cortex toward the epiphyseal region appeared to be the best compromise to allow satisfactory motion within the medullary cavity and to prevent deviation during the percutaneous approach.

Second, progress within the medullary cavity with various available vascular catheters, for example, guidewires, dilators, or drainage catheters is performed. To allow catheterization of the bone extremity, a flexible, reinforced catheter is introduced into the shaft. Finally, the extremity is catheterized and the cancellous bone of the metaphysis and epiphysis penetrated. For drilling of spongy bone, a prototype catheterization device was developed, and the principle of the endoscope appeared to be very suitable for this purpose.

FIGURE 2 Drill-resection system: (left to right) 9-mm-diameter toothed drill, 9-mm-diameter drill, guidewire and coaxial system assembled for cortical drilling (above).

FIGURE 3 Different steps of the procedure: (A) Placement with an electric drill of the drill bit, using a Kirschner wire as a guide. Protection of soft tissues with a sheath. (B) Insertion of a dedicated sheath guiding a threaded drill through the long axis of the bone. (C) Replacement of the threaded drill by the modified endoscope, allowing drilling through the epiphysis of the bone.

FIGURE 3 Different steps of the procedure: (A) Placement with an electric drill of the drill bit, using a Kirschner wire as a guide. Protection of soft tissues with a sheath. (B) Insertion of a dedicated sheath guiding a threaded drill through the long axis of the bone. (C) Replacement of the threaded drill by the modified endoscope, allowing drilling through the epiphysis of the bone.

The new device consists of an ureteroscope, a drive cable, and a burr. The burr is fixed at the end of the ureteroscope and is driven by the drive cable passing through the instrumental canal. The prototype is illustrated in Figure 3. Like a conventional endoscope, the mobility of this prototype allows the operator to drill and reach the extremity of the bone (Fig. 4).

Figure 5 shows the gross anatomy of a sheep's bone, which is cut into two parts after catheterization. Penetration of the burr in the spongy bone is clearly seen.

Clinical studies must be awaited, but already a certain number of applications seem feasible. The indications for such a technique may increase with the development of specific catheteriza-tion instruments: for aspiration of neoplastic cells to decrease tumor volume (especially in

FIGURE 4 Tibial catheterization. The mobility of the prototype allows the operator to drill as far as the epiphysis. Note the reinforced catheter (arroW) placed within the marrow cavity and the hole made within the spongy epiphyseal bone (arrowhead).

FIGURE 5 Gross anatomy of a sheep bone after catheterization. Note the cavity made by the burr within the spongy bone (arrow).

hematology) (6), abscess drainage (7-9), bone-marrow sampling, in situ injections of antimitotic drug or bone-growth stimulation factors (10), or preventive cement injections in weakened bones. The latter is a particularly promising perspective with the development of liquid cements that can be injected into tumoral or osteoporotic lesions.

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