The skeletal system, particularly the human spine, is a complex structure designed to achieve a myriad of tasks, many of them of a complex kinematic nature. While performing its function, the spine must move into flexion (bending forward) and extension (bending backward). For example, the vertebrae that make up the lumbar region of the human spine move through roughly an arc of 15° relative to its neighbor vertebrae. Vertebrae of other regions of the human spine (e.g., the thoracic and cervical regions) have different ranges of movement. Thus, if one were to view the posterior edge of a healthy vertebrae one would observe that the edge moves through an arc of some degree (e.g., of about 15° if in the lumbar region) centered around an elliptical center of rotation. The inter-vertebral spacing in a healthy spine is maintained by a compressible disc which serves to allow the spine to move through this arc.
In situations (based upon injury or otherwise) where a disc is not functioning properly, the inter-vertebral disc tends to compress, and in doing so pressure is exerted on nerves extending from the spinal cord by this reduced inter-vertebral spacing. Various other types of nerve problems may be experienced in the spine, such as exiting nerve root compression in neural foramen, passing nerve root compression, and ennervated annulus (where nerves grow into a cracked/compromised annulus, causing pain every time the disc/annulus is compressed), as examples. Many medical procedures have been devised to alleviate such nerve compression and the pain that results from nerve pressure. Many of these procedures revolve around attempts to prevent the vertebrae from moving too close to each other thereby maintaining space for the nerves to exit without being impinged upon by movements of the spine.
One such attempt is shown in U.S. Pat. No. 6,048,342 (hereinafter “the '342 patent”) in which screws are embedded in adjacent vertebrae pedicles and rigid spacers are then sewed between the screws. In such a situation, the pedicle screws (which are in effect extensions of the vertebrae) then press against the rigid spacer which serves to distract the degenerated disc space so as to prevent the vertebrae from compressing the nerves. This works for preventing nerve pressure due to extension of the spine, however when the patient then tries to bend forward (putting the spine in flexion) the posterior portions of at least two vertebrae are effectively tied together and thus can not move through any arc, let alone through 15° of motion desired for some regions of the spine. This not only limits the patient's movements but also places additional stress on other portions of the spine (typically, the stress placed on adjacent vertebrae being the worse), often leading to further complications at a later date.
In some approaches, such as shown in European Patent Publication 01/45,576 A1, a “stop” is placed between spinous processes and the spinous processes are then banded together. This procedure has the same limitations and drawbacks as discussed above for the '342 patent.
In still another attempt to solve the compression problem, a lever arm approach has been attempted, as shown in U.S. Pat. No. 6,290,700, again resulting in the same problems, namely, an effective “welding” of two vertebrae together.
U.S. Patent Application Publication No. US/2004/002708A1 (hereafter “the '708 publication”) with a Publication Date of Jan. 1, 2004 is entitled, “DYNAMIC FIXATION DEVICE AND METHOD OF USE” shows a dynamic fixation device that allows flexion. The device and method of the '708 publication uses a geometric shape to allow flexion but makes no provision for preventing or reducing disc compression during such flexion.