Posterior Lumbar Interbody Fusion

OVERVIEW

Posterior lumbar interbody fusion, commonly referred as PLIF, utilizes a posterior approach to fuse the lumbar spine bones together using an interbody fusion technique. The incision is made in the lower back and is usually 3 to 6 inches. Interbody fusion means the intervertebral disc is removed and replaced with a bone spacer known as bone graft. The posterior technique is preferably used when one or two spinal levels are being fused in conjunction with a posterior decompression, referred as laminectomy, and instrumentation, such as metal screws and rods. The function of the instrumentation is to stabilize the spine, while the bone graft is growing across the disc space. By immobilizing the motion segment, the bone graft is more likely to create a successful fusion, and by taking away patients’ instability, pain is relieved.

WHY PLIF?

PLIF is commonly performed for a variety of painful spinal conditions, such as:

SURGICAL PROCEDURE -STEP BY STEP-

INCISION MADE

                                                                                                

A 3-6 inch incision is made in the lower back.

(The size of the incision varies depending on the number of affected discs.)

                                                                                                                                   

DISC ACCESSED

                                                                                                                                   

Once the incision is made, the spine is then exposed.

Subsequently, parts of the vertebral bone need to be removed to gain access to the disc. The spinous process is removed along with the lamina to allow full visual of the spinal cord and disc, which is located underneath the spinal nerves.

                                                                                                                                   
                                                                                                                                   

DISC REMOVED

The damaged disc is often completely removed. In instances where part of the disc is still healthy, however, only a small portion of the disc may be removed (discectomy), and fusion is then not necessary. If the disc is completely removed, structural bone graft is placed inside the empty disc space, realigning the vertebral bones, and restoring height in the nerve tunnels. The graft thereby also relieves the preddure on the exiting nerver roots.

There are also several types of bone grafts:

  • Autograft - bone obtained from the same individual receiving the graft. Bone can be harvested from non-essential bones (spinous process), or from the iliac crest (pelvis).
  • Allograft - bone is derived from humans, but is not harvested from the individual receiving the graft. Allograft bone is taken from cadavers that have donated their bones to a source called the bone bank. Allograft is a safe option as there are strict rules and regulations preventing any potential risk of diseasr transmission. Additionally, the bone banks are also regulated by the U.S. Food and Drug Administration (FDA). There are typically no live cells or tissue associated with the allograft bone: it merely functions as a scaffold that gets replaced by the patient's own bone over time.
  • Synthetic graft - bone that is artificially created from ceramics such as calcium phosphates. The material can be dipped in growth factors or patient's bone marrow to promote biological activity and improve healing (fusion).
                                                                                                                                   

The disc space is typically also filled with morselized bone, which is obtained by processing the previously removed spinous process and lamina into small pieces of bone. Morselized bone is fundamental component and possesses three important characteristics:

  • Osteogenesis - occurs when vital osteoblasts (cells responsible for bone fromation) originating from the bone material contribute to new bone growth.
  • Osteoinduction - osteoprogenitor cells are stimulated to differentiate into osteoblasts that then begin new bone formation.
  • Osteopromotion - promotes the enhancement of osteoinduction.
  • Bone graft also possesses important characteristics, depending on it's type.
  • Autograft
    • Osteogenic - osteoblasts contribute to new bone growth.
    • Osteoinductive - osteoprogenitor cells are stimulated to differentiate into osteoblasts to begin new bone formation.
  • Allograft
    • Osteoconductive - the bone material serves as a scaffold for new bone growth that is perpetuated by the native bone. Osteoblasts from the margin of the defect that is being grafted utilize the bone graft material as a framework upon which to spread and generate new bone.
  • Synthetic graft
    • Typically just osteoconductive, but can be laden with osteo-inductive growth factors to promote bone healing/fusion.
                                                                                                                                   

The role of the morselized bone is to promote arthrodesis, also known as the artificial induction of joint ossification between two bones through surgery (like spinal fusion).

ADDITIONAL SUPPORT ADDED

After the structural bone graft and morselized bone are place inside the disc space, a series of screws and rods are implanted for additional support.

                                                                                                                                   

Morselized bone is also placed along the sides of the spine. The morselized bone will grow alongside the implants forming a bony bridge that connects the vertebral bodies above and below. The solid bone bridge formed is called a fusion.

                                                                                                                                   

BONE GRAFT CHOICE - What is best?

Autograft - however, there is pain and morbidity associated with harvesting bone from the iliac crest (pelvis). Local morselized bone is the next best thing, often coming from the spinous processes or the laminectomy, when performed.

The next best thing is the Allograft because:

  • No morbidity (e.g. wound infection) or pain associated with autograft harvesting.
  • Allograft eventually gets substituted by patient's own bone.
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