Elements of a spine fusion

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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 patient¹s back.

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.

 

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