A significant number of adults have had an episode of back pain or suffer chronic back pain emanating from a region of the spinal column. A number of spinal disorders are caused by traumatic spinal injuries, disease processes, aging processes, and congenital abnormalities that cause pain, reduce the flexibility of the spine, decrease the load bearing capability of the spine, shorten the length of the spine, and/or distort the normal curvature of the spine. Many people suffering from back pain resort to surgical intervention to alleviate their pain.
Disc degeneration can contribute to back pain. With age, the nucleus pulposus of the intervertebral discs tends to become less fluid and more viscous. Dehydration of the intervertebral disc and other degenerative effects can cause severe pain. Annular fissures also may be associated with a herniation or rupture of the annulus causing the nucleus to bulge outward or extrude out through the fissure and impinge upon the spinal column or nerves (i.e. a “ruptured” or “slipped” disc).
In addition to spinal deformities that can occur over several motion segments, spondylolisthesis (i.e. forward displacement of one vertebra over another, usually in the lumbar or cervical spine) is associated with significant axial and/or radicular pain. Patients who suffer from such conditions can experience diminished ability to bear loads, loss of mobility, extreme and debilitating pain, and oftentimes suffer neurological deficit in nerve function.
Failure of conservative therapies to treat spinal pain such as for example bed rest, pain and muscle relaxant medication, physical therapy or steroid injection often urges patients to seek spinal surgical intervention. Many surgical techniques, instruments and spinal disc implants have been described that are intended to provide less invasive, percutaneous, or minimally-invasive access to a degenerated intervertebral spinal disc. Instruments are introduced through the annulus for performing a discectomy and implanting bone growth materials or biomaterials or spinal disc implants within the annulus. One or more annular incisions are made into the disc to receive spinal disc implants or bone growth material to promote fusion, or to receive a pre-formed, artificial, functional disc replacement implant.
Extensive perineural dissection and bone preparation can be necessary for some of these techniques. In addition, the disruption of annular or periannular structures can result in loss of stability or nerve injury. As a result, the spinal column can be further weakened and/or result in surgery-induced pain syndromes.
Various posterior interbody approaches such as posterior lumbar interbody fusion (PLIF) and transforaminal lumbar interbody fusion (TLIF) necessitate placing an interbody device through a relatively small annular aperture from a posterior access dissection path. The desire to restore disc space height and create lordosis through anterior distraction is in conflict with the application of a fixed height interbody spacer through a limited posterior approach. In addition, the larger the “foot-print” of the interbody spacer, the less likely it will subside. This is not only due to reduced endplate pressure but also better load bearing bone near the perimeter of the endplate (versus the central region).