Outcomes of Allogenic Cages in ALIF and PLIF: History
As we reflect on past centuries and where science has led us in the study of the spine, we can begin to envision what is in store for the next century. Take, for example, Mercer, who in 1936 stated, " The ideal operation for fusing the spine would be an interbody fusion, but the surgical difficulties encountered in performing such a feat would make the operation technically impossible" . Even among supporters of interbody fusion, enthusiasm for the technique remained sedate until the 1940s. Over the years, many variations of PLIF have been invented to facilitate the fusion process while maintaining stability of the spine. Capener first described ALIF for spondylolisthesis in 1932 . Today, spinal fusion can be accomplished by various techniques such as posterior procedures with and without internal fixation, anterior procedures with and without internal fixation, and combined anterior and posterior column procedures, which may include PLIF or ALIF for anterior column support .
During the last decade, an increasing number of studies have looked at the morphology, physiology, biomechanics, and immunology of the various components of the spine . Today, there are several options available to spine surgeons for correcting spinal instabilities and achieving physiological anterior column support, including autograft, allograft, synthetics, and metallic fusion cages. Fresh autologous cancellous bone is considered the best choice for osseous reconstruction because of its optimal biological behavior and histocompatibility . However, autologous bone has inadequate initial mechanical strength for interbody loading and may collapse and/or extrude [10, 13, 18]. Significant morbidity is also associated with anterior structural graft harvesting of the ilium and may result in infection, chronic pain, incisional hernias, vascular injuries, and iliac wing fractures . The use of allograft is a safe, simple, and inexpensive method of harvesting bone. Through continued clinical research, devices are being manufactured from cortical bone similar to metal fusion cages, providing built-in lordosis and endplate gripping "teeth" for additional stability. Two of these biological devices are the PLIF spacer for PLIF and the FRA spacer for ALIF.
Various aspects of Intervertebral disc disease have been proposed as definitive indications for PLIF. Collis' indications are lumbar pain with or without sciatica, a degenerative disc with or without a protrusions, a midline disc protrusion, post-lumbar laminectomy/disectomy syndrome, a recurrent soft tissue protrusion, spondylolisthesis (grade I or II), a reverse spondylolisthesis, or any combination of the preceding seven conditions . The advantages of PLIF are (a) the large surface area for fusions provided by the vertebral bodies, (b) the fusion is in compression across the vertebral bodies, and (c) the potential for partial restoration of disc space height. In addition, because the patient is already exposed for decompression, it eliminates the need for another incision . The disadvantages of PLIF include the high degree of technical demands and the possibility of extrusion of the graft. However, using internal fixation devices probably can decrease this risk. Dural tears are more likely when this extensive exposure is undertaken, and scarring of the anterior portion of the dural sac is more common.
Historically, the indications for and role of anterior spinal surgery of the lumbar spine have been controversial. At present, the specific indications of ALIF include symptomatic post-traumatic kyphosis with or without neurologic sequelae, introgenic lumbar kyphosis (flatback syndrome), and painful lumbar degenerative scoliosis with disc disease. In addition, relative indications for anterior internal fixation and fusion include repair of failed posterior fusion, instability secondary to wide laminectomy and posterior decompression, high-grade spondylolisthesis or spondyloptosis, and spinal osteotomy .
ALIF has the same potential benefits as PLF. First, the graft will be in compression, and there is a large area for fusion. In addition, more bone can be placed between the vertebral bodies in ALIF than in PLIF, and the disc height appears to be only temporarily increased. Other advantages include a reduced operative time and blood loss, non-interference with the potentially painful posterior elements of the lumbar spine, and avoidance of scarring within the spinal canal . However, ALIF does have several drawbacks. First, it requires a separate incision. With a transperitoneal approach, there is risk of abdominal adhesions and incisional hernias. Damage to the major vessels is a rare complication. In males, ALIF also carries the risk of impotence or retrograde ejaculation. Isolated ALIF has also been associated with a high pseudoarthrosis rate. For this reason, we routinely combine anterior fusion with posterior intertransverse fusion, often under the same anesthesia, and with internal fixation. This combination achieves the maximum biomechanical stability, neural element decompression, and recruitment of motion segment bone-grafting surface area.