Elements of a spine fusion

Understanding spine fusion

The most common reason for performing a spinal fusion is low back pain caused by painful motion of the vertebrae. The goal of a spine fusion is to eliminate the motion at a painful motion segment, thus reducing the pain caused by the motion. This abnormal and painful motion can be caused by painful discs (discogenic pain or degenerative disc disease), abnormal slippage and motion of the vertebra (spondylolisthesis or spondylolysis), or other degenerative spinal conditions, including but not limited to facet joint degeneration. In addition, a spine fusion may be indicated for any condition that causes excessive instability of the spine, such as certain fractures, infections, tumors, and spinal deformity (such as scoliosis).

Ingredients of a spine fusion

In order to obtain a fusion certain basic criteria must be present.

• A suitable graft must be available to serve as the bridge to connect the vertebra.

• An appropriate location must be present to lay the bone graft and allow it to heal to each vertebral segment on either end.

• The bed which this bone graft is being laid in must be prepared correctly and the patient must have the appropriate biology for the graft to fuse. This bed can be prepared by removing the outside covering of the bone (the cortex) and exposing a bed with better blood supply (decortication). It is important that there is contact between the bone and the bone graft and no soft tissue (e.g. muscles, ligaments) in the way.

• There must be adequate fixation to immobilize this area while the bone graft heals to the vertebral segments. This immobilization is usually provided by internal fixation with metallic screws and rods and/or interbody devices such as cages.

Internal fixation of the spine (usually with metallic screws and rods/plates, or interbody cages) serves to immobilize the spine, while the bony bridge heals across the two vertebrae. The degree of immobilization afforded to the spine by internal fixation will not change when the bone graft matures and heals across the two vertebrae. However, if the fusion (the healing of the bone) does not occur, over time the implants will loosen, break or pull out of the bone. This occurs despite the strength of the metallic constructs which are being used today. The term used to describe the lack of fusion after a spine fusion surgery is pseudoarthrosis.

Types of spine fusion

In general, there are two main approaches to spine fusion. One of the main differences between these two approaches is where the bone graft is laid in the spine to form the fusion.

• Posterolateral fusion. The graft to form the bony bridge can be placed between the transverse processes (shaded area in Figure 1) in the back of the spine. This will allow the bone to heal from the transverse process of one vertebra to the transverse process of the next vertebra. This type of spine fusion is called a posterolateral fusion.
  
  The most common fixation technique employed in a posterolateral fusion is pedicle screw fixation. This refers to placing screws within the pedicles (Figure 2, Figure 3) of each vertebral segment (bilaterally—on both sides of the spine) and connecting them to each other with a metal rod. A one level fusion would fuse two vertebrae and usually uses four screws and two rods. A two level fusion fuses three vertebrae and uses six screws and two rods (Figure 4, Figure 5).
  

• Interbody fusion. In an interbody spine fusion, the bone graft is placed in between the vertebral bodies where the disc usually lies. The disc has to be completely removed and endplates cleaned prior to placement of the graft. This will allow the fusion to occur from one vertebral body to the other through their endplates (red lines on Figure 2). The graft can be placed in between the vertebral bodies into an interbody position through an anterior approach (from the front) with an incision in the abdomen. This approach is called an Anterior Lumbar Interbody Fusion, or ALIF. The graft can also be placed from a posterior approach through the back. This approach is called a Posterior Lumbar Interbody Fusion or PLIF, or Transforaminal Lumbar Interbody Fusion or TLIF. One difference between a TLIF and PLIF is the angle at which the disc is approached, but both procedures are done through an incision in the patient’s back.
  
  Bone grafts are commonly placed within cages that hold the graft and resist the compressive forces of the vertebrae.

• Material: These cages are made of different materials, such as Titanium, Poly Ethyl Ethyl Ketone (PEEK) which resembles a very tough plastic, or Carbon Fiber (similar to PEEK). The latter two have the advantage of being able to show the status of the bone inside them on X-ray, facilitating the assessment of whether a fusion has occurred. Titanium cages are stronger, but make the radiographic assessment more difficult. In addition to synthetic cages, Cortical bone cages, which are stronger than the graft but not as strong as the synthetic cages, can also be used as a cage. The advantage of cortical bone cages is that eventually the cage itself can incorporate into the fusion as well.

• Shape: The shape of the cage is a function of the approach that it is used for. The cages for ALIF’s are generally larger and more round shaped. The cages for a PLIF are generally more rectangular shaped. The cages for TLIF’s are most commonly boomerang shaped or rectangular shaped.

The advantage to an interbody fusion over a posterolateral fusion is the increased surface area for bone contact and the ability of the graft to share the load on the anterior (front) portion of the spine (anterior column support). These factors usually translate to a more favorable fusion rate. The application of both techniques, an interbody fusion in addition to a posterolateral fusion, theoretically affords the highest chances for a fusion (similar to the use of belt and suspenders). This type of surgery is commonly referred to as a 360-degree fusion.

By: Ali Araghi, DO
November 27, 2006 (Original publication July 21, 2004)