LONGITUDINAL PLATE ASSEMBLY HAVING AN
CROSS REFERENCE TO RELATED
 The present application is a continuing application of U.S. Ser. No. 09/789,938 entitled "Longitudinal Plate Assembly Having An Adjustable Length".
FIELD OF THE INVENTION
 The invention relates generally to a spinal implant assembly for holding vertebral bones fixed relative to one another. More particularly, the invention relates to a longitudinal plate assembly having an adjustable length and two ends that each can be coupled to a body structure, such as a vertebral bone, for use in surgical procedures for stabilizing the relative motion of, or permanently immobilizing, the body structures.
BACKGROUND OF THE INVENTION
 The bones and connective tissue of an adult human spinal column consists of more than twenty discrete bones coupled sequentially to one another by a tri-joint complex which consist of an anterior disc and the two posterior facet joints, the anterior discs of adjacent bones being cushioned by cartilage spacers referred to as intervertebral discs. These more than twenty bones are anatomically categorized as being members of one of four classifications: cervical, thoracic, lumbar, or sacral. The cervical portion of the spine, which comprises the top of the spine, up to the base of the skull, includes the first seven vertebrae. The intermediate twelve bones are the thoracic vertebrae, and connect to the lower spine comprising the five lumbar vertebrae. The base of the spine includes the sacral bones (including the coccyx). The component bones of the cervical spine are generally smaller than those of the thoracic spine, which are in turn smaller than those of the lumbar region. The sacral region connects laterally to the pelvis. While the sacral region is an integral part of the spine, for the purposes of fusion surgeries and for this disclosure, the word spine shall refer only to the cervical, thoracic, and lumbar regions.
 Referring now to FIGS. 1 and 2, a typical vertebral body is shown in a top view and a side view. The spinal cord is housed in the central canal 10, protected from the posterior side by a shell of bone called the lamina 12. The lamina 12 has three large protrusions, two of which extend laterally from the shell and are referred to as the transverse process 14. The third extends back and down from the lamina and is called the spinous process 16. The anterior portion of the spine comprises a set of generally cylindrically shaped bones which are stacked one on top of the other. These portions of the vertebrae are referred to as the vertebral bodies 20, and are each separated from the other by the intervertebral discs 22. Pedicles 24 are bone bridges which couple the anterior vertebral body 20 to the corresponding lamina 12 and posterior elements 14, 16.
 The spinal column of bones is highly complex in that it includes over twenty bones coupled to one another, housing and protecting critical elements of the nervous system which have innumerable peripheral nerves and circulatory bodies in close proximity. In spite of these com
plications, the spine is a highly flexible structure, capable of a high degree of curvature and twist in nearly every direction.
 Genetic or developmental irregularities, trauma, chronic stress, tumors, and disease are a few of the causes which can result in spinal pathologies for which permanent immobilization of multiple vertebral bodies may be necessary. A variety of systems have been disclosed in the art which achieve this immobilization by implanting artificial assemblies in or on the spinal column. These assemblies may be classified as anterior, posterior, or lateral implants. As the classification suggests, posterior implants are attached to the back of the spinal column, generally hooking under the lamina and entering into the central canal, attaching to the transverse process, or coupling through the pedicle bone. Lateral and anterior assemblies are coupled to the vertebral bodies.
 The region of the back which needs to be immobilized, as well as the individual variations in anatomy, determine the appropriate surgical protocol and implantation assembly. The use of screw plate assemblies for stabilization and immobilization via lateral or anterior entrance is, however, common.
 Because spinal injuries vary with regard to the number of vertebral bodies affected, the proximity of the affected vertebral bodies with respect to one another, and the proximity of the unaffected or stable vertebral bodies with respect to one another, it is necessary for the treatment of a given spinal injury to use a plate assembly having a length that can be used effectively to immobilize, with respect to one another, those vertebral bodies that must be so immobilized to achieve clinically desirable results. For example, depending on the spinal injury, it may be necessary to immobilize two adjacent vertebral bodies. Or, for example, it may be necessary to immobilize two vertebral bodies on either side of one or more unstable or damaged vertebral bodies. Potentially, each spinal injury therefore requires a plate assembly having a different length.
 In addition, the vertebral bodies of the spine are not all equal in length or identical in shape. Some are smaller than others, and are therefore shorter and, for example, have smaller transverse processes, spinous processes, and/or smaller pedicles. Therefore, depending on the location of the spinal injury along the spine, it is again necessary to select a plate assembly having a length that can be used effectively to immobilize, with respect to one another, those vertebral bodies that must be so immobilized to achieve clinically desirable results. For example, in the cervical portion of the spine, the immobilization of two adjacent vertebral bodies will require a plate assembly of a given length, while the immobilization of two adjacent vertebral bodies in the lumbar region will typically require a plate assembly that is longer. And, of course, the selection of the plate assembly of appropriate length must take into account the specific location of the bone structures to which the a plate assembly will be coupled, as these specific locations vary depending on the spinal injury and the damage caused thereby.
 Further, because the spine is routinely subject to high loads which cycle during movement, one of the primary concerns of physicians performing spinal implantation surgeries, as well as of the patients in whom the implants are placed, is the risk of screw pull-out. Screw pull-out occurs