Gigli, in 1894, described a simple wire device for cutting bone. Since that time, the Gigli saw has become very popular. However, use of this device may result in cutting of the soft tissue around the bone, a serious risk. The saw also may jam because its surface is rough.
We have developed a new device for cutting bone, a threadwire saw (T-saw; Tomita and Kawahara, Kanazawa, Japan) (Fig. 1). The saw is made of stainless-steel microcable with a diameter of approximately 0.5 millimeter. It produces an extremely thin and smooth cut in the bone with use of a reciprocating motion, similar to that used with the Gigli saw. We have found it to be particularly useful for operations on the spine.
The threadwire saw has a number of advantages compared with the conventional Gigli saw. The threadwire saw is thin and flexible and can be introduced safely into a confined space, such as the epidural space or the intervertebral foramen, with use of an epidural catheter or a specially made T-saw guide. The T-saw guide is a c-shaped malleable metal tube with an inside in diameter of 0.6 millimeter and a handle. The surface of the threadwire saw is so smooth that it is possible to cut the lamina or the pedicle without injuring the dura or the spinal nerve roots; moreover, the saw does not jam in the bone during use. The cut is so thin that bone loss may be negligible, and this makes it possible to achieve exact matching if the bone is replaced or reapproximated after cutting.
Certain features of the technique required for use of the threadwire saw should be borne in mind. First, it the is necessary to cool and irrigate the threadwire saw with saline solution during the reciprocating motion of the device. Second, the threadwire saw is disposable and should be discarded, as it deforms and becomes fragile after a single use.
The threadwire saw has led to the development of several new operative techniques, including recapping laminaplasty and slide-opening arch laminaplasty3, and to the improvement of existing techniques, such as midline cervical laminaplasty (splitting of the spinous process in the sagittal plane)2,3, total en bloc spondylectomy4, and resection of the sacrum.
Recapping laminaplasty may be indicated for a lesion in the spinal canal, such as a tumor of the spinal cord or syringomyelia, in order to obtain good visualization of the operative field and to allow anatomical reconstruction of the spine. During this procedure, the threadwire saw is introduced into the vertebral foramen with use of a T-saw guide and is pulled out through the adjacent intervertebral foramen (Fig. 2-A). When both ends of the saw are pulled tightly in the posterolateral direction, the saw runs adjacent to the inner wall of the pedicle and posterior to the nerve root. With use of a reciprocating motion, the saw produces a fine cut from the pedicle through the transverse process (Fig. 2-A). This laminotomy provides an extremely wide exposure of the spinal canal, the lateral recess, and the foramen (Fig. 2-B). After the operative procedure in the canal, the excised lamina can be replaced in perfect in situ apposition in the anatomical position.
Twenty-two patients were managed with recapping laminaplasty at our institution from 1990 through 1994. One of these patients was a forty-four-year-old woman who had a large intradural extramedullary tumor of the spinal cord; the tumor extended from the twelfth thoracic to the second lumbar level. The patient was managed with laminotomies of the twelfth thoracic and the first and second lumbar vertebrae, excision of the tumor, and recapping laminaplasty (Fig. 3-A and 3-B).
Computerized tomography demonstrated primary bone-healing in all twenty-two patients by three to four months postoperatively. None of the patients had dural tears, damage to the nerve roots, or leakage of cerebrospinal fluid. Furthermore, we have not noted any other neurological complication in association with use of the threadwire saw.
No signs of aseptic necrosis, such as bone absorption or collapse or sclerotic changes in the replaced laminae, were observed on the roentgenograms of the ten patients who had more than two years of follow-up. Neurological degradation due to postoperative spinal stenosis (narrowing of the spinal canal caused by excessive bone formation) also was not noted.
In summary, the threadwire saw is so thin, flexible, and smooth that it can be introduced through any narrow space, and the bone can be cut at an optimum position with negligible bone loss. The saw can be used for a variety of operations on the spine, such as recapping laminaplasty, slide-opening arch laminaplasty3, midline cervical laminaplasty2,3, total en bloc spondylectomy4, and resection of the sacrum.
*No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. No funds were received in support of this study.
†Department of Orthopaedic Surgery, School of Medicine, Kanazawa University. 13-1, Takaramachi, Kanazawa 920, Japan. E-mail address: seikei@kenroku.ipc.kanazawa-u.ac.jp.