The minimally invasive anterior approach for spine fusion surgery is in many ways an ideal technique for gaining access to the disc space in the lumbar spine with minimal risks or unwanted after effects for the patient. The anterior approach (from the front—through the abdomen) allows the surgeon to:
- Completely remove the painful degenerative disc while restoring the disc to its native disc height
- Put in a bone graft in the intervertebral disc space, which allows for increase surgical area for the fusion
- Placing bone anteriorly puts it into compression and bone in compression tends to fuse better
- Indirectly decompress the nerve roots exiting the spinal canal in the foramen by distracting (opening up) the disc space
The minimally invasive retroperitoneal anterior approach has been routinely used successfully for degenerative disc disease, slipped vertebra (spondylolisthesis), scoliosis surgery and for the newer technology of total disc replacements (artificial discs). It does, however, have a few unique risks for the patient, and therefore careful consideration of the pros and cons is warranted.
Anatomy of the Anterior Approach to Spine Fusion
To understand the need for an anterior or abdominal surgical approach, one must first understand the pathology of a common back ailment, such as lumbar degenerative disc disease.
An intervertebral disc is between two vertebral bodies in each segment of the spine. Directly behind the disc and the vertebral bodies lies the spinal canal that contains the spinal nerves. With degenerative disc disease, the intervertebral disc shrinks in height, and concordantly bulges out into the surrounding spinal canal where the lumbar nerve roots are present. The alignment of the spine may decompensate, and the patient may tilt forward due to the collapsed disc, meaning he or she loses lordosis (the natural curve of the lumbar spine that allows one to look straight ahead while standing). The nerve roots can also become pinched either directly by the disc protruding and compressing the nerve roots, or indirectly due to the loss of disc height. When the disc shrinks, the foramen, which are the areas through which the nerve roots exit the spinal canal, become smaller and this can “pinch” the nerve root.
For patients who do not get adequate pain relief (or ability to function) from non-surgical treatments, the gold standard surgical procedure for pain and/or inability to function due to degenerative disc disease is a lumbar spinal fusion. To ensure a better chance for the spinal fusion to be a success, a lumbar interbody fusion may be done. An interbody fusion involves removing the worn-out disc that lies between the two vertebral bodies. This space may be replaced with a structural bone graft such as a patient’s own iliac crest bone (from the patient’s hip), or cadaver bone that has been treated to minimize any disease transmission. Metal, carbon fiber cages, or other devices may also be implanted with the graft bone based on the surgeon’s preference and experience. By placing graft bone within the disc space, there is a significantly higher rate of fusion. The intervening bone is placed in compression between the vertebral bodies, as opposed to posterior fusions (from the back) where the bone graft is placed under tension. The body can more easily fuse bone when the bone graft is in compression.
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An interbody fusion is typically done two ways. The first way is through a posterior approach (from the back) and is called either a Posterior Lumbar Interbody Fusion (PLIF) or a Transforaminal Lumbar Interbody Fusion (TLIF). The other method is through an anterior approach (from the front) and is called an Anterior Lumbar Interbody Fusion (ALIF).
- Posterior approach to spine fusion. The posterior approach normally requires removing a significant portion of bone that is needed to stabilize the spine segment (i.e. the facet joint or lamina) to gain access to the disc space. Also, retraction of the dural sac (which contains the lumbar nerves) and/or the nerve roots themselves is required and this carries the risk of leading to residual pain or nerve damage. Even when done well, it is hard to get most of the disc material and the disc space cannot be distracted to restore the normal lumbar lordosis.
- Anterior approach to spine fusion. The anterior approach allows the surgeon to have direct access to the degenerated disc without having to manipulate any nerve roots. Better correction of the collapsed disc to its native height can also be achieved by having a better leverage point to open the disc space. This can also help in restoring lordosis to the lumbar spine and to decrease fatigue of the surrounding posterior spinal muscles. No anterior or posterior muscle dissection is required to gain access to the front of the spine (unless the anterior approach is done in combination with a posterior approach for instrumentation). (See Figure 1)
As with all procedures, however, the anterior approach carries with it a few unique potential risks and complications that are not relevant to the posterior approaches. In addition, not all conditions can be successfully addressed with an anterior approach or ALIF, such as lumbar spinal stenosis, where a posterior decompression needs to be performed.
- Common conditions that may be treated with an ALIF include lumbar degenerative disc disease and lumbar foraminal stenosis.
- Common conditions that are not usually treated with an ALIF include any pathology that is mostly posterior (in the back of the spine), such as isthmic spondylolisthesis, degenerative spondylolisthesis or lumbar spinal stenosis. In cases where there is a lot of instability, such as isthmic spondylolisthesis, an ALIF may be combined with a posterior decompression fusion with instrumentation, which is called an anterior/posterior fusion. ALIF procedures are also contraindicated for anyone with thinning of the bones (osteopenic or osteoporosis). Generally, an ALIF spine surgery is inadvisable for patients older than 60-65 years.
Anterior approach for artificial disc surgery
In addition to the above examples, an anterior approach is also required for a newer procedure that is currently in clinical trials in the US for treatment of low back pain from degenerative disc disease—artificial disc for total disc replacement. The artificial disc procedure is a new technology that is designed to allow for restoration of disc motion and height at the degenerated disc level while relieving a significant portion of the patient’s low back pain. The artificial disc technologies presently under clinical trials in the US can only be implanted through the anterior approach, so many surgeons who had previously only focused on posterior spine fusions are now starting to become familiar with the anterior approach to the spine with the assistance of general and/or vascular surgeons. (See Figure 2)