Interbody Fusion
INTRODUCTION:
Spinal fusion is a technique used to stabilize the spine bones or vertebrae. The goal of a lumbar fusion is to create a solid fusion between two or more vertebrae. Solid fusion occurs when two bones mend or weld together, much like a broken bone healing in a cast. Spinal fusion may be recommended for conditions such as spondylolisthesis (forward slippage of one vertebra on top of another), degenerative disc disease, recurrent disc herniation or failed previous surgery.
ANATOMY:
The spine is composed of individual bones called vertebra. Lumbar vertebra are stacked one on top of another and act as a cushion in between by the intervertebral disc. A circle of bone extends from the vertebral body back around the sac of nerves and spinal cord. This circular protective tube of bone called the spinal canal protects the spinal cord and spinal nerves. In the lumbar spine the spinal nerves exit the spinal canal between the vertebrae at each level. Two nerves exit each level, one on the left and one on the right. These nerves exit through openings called foramina [Figure 1].
Figure 1
The interbody space is the disc space that is located between the vertebral bones. An intervertebral fusion is a tool used by the spine surgeon to help a variety of spinal conditions. Interbody fusion involves placement of fusion cages and bone graft into the area between two vertebral bodies [Figure 2]. Intervertebral fusion is shown to be effective for achieving fusion. The intervertebral cage may be made from a variety of material including metal, carbon fiber or bone. Bone graft and bone healing protein are also placed within the cage and within the interbody space to allow for bony healing to occur between two vertebrae forming a solid fusion.
Figure 2
The cage helps in several ways. First it separates and holds the two vertebrae apart. This makes the opening around the nerve roots at that area larger, relieving pressure on the nerves. The intervertebral cage and fusion also can be used to correct spinal deformity and restore proper alignment. Intervertebral cages can be implanted from the front or back part of the spine. The location of the surgery is dependent on the specific anatomy of each patient, as well as the location, and amount, of pressure that may be occurring around the nerves in the spinal canal.
In some cases, an interbody fusion can be performed without the need for additional instrumentation or stabilization. For patients undergoing interbody fusion in the lumbar spine, anterior plating placed on the lumbar spine can be used for additional stability. An interbody fusion may also be used as part of a posterior instrumented fusion procedure using pedicle screws and instrumentation.
ANTERIOR LUMBAR INTERBODY FUSION:
Anterior lumbar interbody fusion is generally used to treat back or leg pain caused by degenerative disc disease. The surgeon will stabilize the spine by fusing vertebrae together with bone graft material. The procedure is performed through a three to five inch procedure on the stomach. Two common approaches are over the center of the stomach or slightly to the side. The damaged disc is partially removed. Some of the disc wall is left behind to help contain the bone graft material. A metal cage implant filled with bone graft is placed in the empty disc space. This realigns the vertebral bones lifting the pressure off the pinched nerve roots. In some patients this will be enough to secure the vertebrae [Figure 3]. In other cases, the surgeon may need to implant a series of screws and rods along the back of the spine for additional support. Over time the bone graft will grow through and around the implants forming a bone bridge that connects the vertebra below and above. This solid bone bridge is called a fusion.
Figure 3
TRANSFORAMINAL LUMBAR INTERBODY FUSION:
A transforaminal lumbar interbody fusion (TLIF) involves placement of posterior instrumentation (screws and rods) to the spine. In addition, a fusion cage is inserted into the disc space from one side of the spine. Bone graft material is placed into the interbody space as well as on the back of the vertebra to allow for fusion [Figure 4]. Transforaminal lumbar interbody fusion provides improved results for fusion because fusion is achieved in the front and back parts of the lumbar spine. The interbody fusion occurs in the anterior portion of the spine. Benefits of fusion in the anterior interbody space include an increased area for bone to heal, as well as dispersing the force through this area when the patient stands and walks [Figure 5].
Figure 4: X-rays of the lumbar spine before and after Transforaminal Lumbar Interbody Fusion (TLIF). Titanium screws and rods support the bones and maintain proper alignment in the back of the spine, while a titanium cage has been placed in the disc space between the bones for support in the front portion of the spine. Fusion occurs in both the front and back of the spine.
Figure 5: Diagram showing TLIF. Pedicle screws achieve a solid fixation onto the spine and are connected together by a rod. Bone graft bridges the bones in this area to achieve a spine fusion.
ADVANTAGES OF TRANSFORAMINAL LUMBAR INTERBODY FUSION VERSUS
STANDARD POSTEROLATERAL FUSION:
Transforaminal lumbar interbody fusion allows the surgeon to obtain a solid fusion on both the anterior and posterior portions of the spine, all through a single surgical approach. The chance of a successful fusion is increased with the TLIF procedure because a larger surface area is available for bone graft to heal into a fusion. Transforaminal lumbar interbody fusion also allows for better restoration of space between the vertebral bones and the space around the exiting nerves in the neural foramen. For patients with spinal curvature or instability, the spacer that is placed between the vertebral bodies at the time of surgery helps restore the space and corrects spinal deformity [Figure 6].
Figure 6: X-rays before and after TLIF show correction of scoliosis and restoration of normal alignment.
A lumbar fusion may be indicated for patients with low back pain or pinched nerve. The specific decision to perform a lumbar interbody fusion depends on a number of factors that will be discussed with you by your doctor.
