Every year there are a significant number of people who suffer severe neck injuries from trauma. These injuries include cervical fractures, fracture dislocations combined with retropulsion of the disc and other major injuries. Also, each year many people undergo cervical spine surgery for degenerative diseases, especially degenerative spinal stenosis, which involves the removal of much of the bone which supports the cervical region of the spine resulting in instability of the cervical region.
Treatment of these conditions has included traction either with a halter or with Crutchfield type tongs followed by application of a cast or brace. If surgery is necessary, the area of injury is often fixed with wire to allow fusion of the vertebrae in the affected region of the vertebral column. Often the treatment includes anterior decompression and fusion, or more recently, plates and screws have been used to immobilize the unstable region. Such plates may be used either anteriorly or posteriorly, or in a few cases, both anteriorly and posteriorly.
While these fixation devices have been used effectively, they have a number of serious disadvantages. For example, the use of wires prevents flexion, as required for effective fixation, but does not prevent rotation nor extension of the cervical region, which leads to poor fixation of the vertebrae in this region. Currently, wires are nearly always used in combination with bone grafts. Sometimes the grafts are placed between the posterior elements of the vertebrae in such a fashion as to prevent extension, and since the wires prevent flexion, some stabilization is obtained. However, the stabilization is usually insufficient to avoid the necessity for the additional stability conferred by the use of halo devices, large casts or braces.
In the case where posterior plates and lateral bone mass screws are used, the stability is increased by simply screwing the plate into the bone. However, since the bone in the cervical area is relatively soft, the screws of such devices easily pull or cut out of the bone and, therefore, do not immobilize well. Even if there is no major failure of the device, such as the screws pulling out of the bone, the screws eventually work loose and plates, that initially prevented all motion, tend to loosen as the patient recovers from the surgery and becomes more active. As a result, halos, casts and braces are often used in conjunction with screw and plate fixation.
Numerous fixation devices have been described, such as those described in U.S. Pat. Nos. 5,030,220 and 5,034,011, for use in the lumbar-sacrum regions of the spine. While these fixation devices have proven successful for use in the lumbar region, the physiology and structure of the cervical spine, C1 to C7, is very different from the lumbar-sacrum regions. For example, the bone screws, when placed in the lateral masses of the cervical vertebra, are usually placed at an angle from horizontal, whereas in the lumbar region they are placed in a horizontal plane. However, for proper stabilization and fixation, the hardware carried by the bone screws ought be such as to provide support and stabilization, generally along the axis of the cervical portion of the spine, rather than at an angle to it. It is desirable that a cervical fixation device be developed which would preferably take into account these considerations. In addition, it is desirable that a cervical fixation device be relatively compact and easy to install.