Spinal fusion is a surgery done to fuse two or more vertebrae. Fusing means two bones are permanently placed together so there is no longer movement between them. The term is technically similar to arthrodesis. Spinal fusion is usually done along with other surgical procedures of the spine.
Other terms for spinal fusion are spondylodesis or spondylosyndesis.
A spinal fusion surgery aims to stop the motion at a painful vertebral segment, which in turn should decrease pain generated from the joint.
In the procedure of spinal fusion, supplementary bone tissue like autograft or allograft is used to aid in fusion. Now, synthetic bone grafts are also used with promising results. The bone grafts or substitutes are used to stimulate and aid in fusion by working in conjunction with the body’s natural osteoblastic processes [New bone formation], this graft helps in creating a bony bridge across the vertebrae.
Indications for Spinal Fusion
The main aim of the spinal fusion is to block the movement to alleviate pain or to provide stability by fusing the vertebra. It is indicated in all the diseases where the motion of the vertebrae is contributory to pain occurrence or alleviation of the movement between two or more vertebrae may relieve the pain, Spinal fusion is may be indicated in following diseases.
- Degenerative disc disease
- Spinal disc herniation
- Spinal surgeries for spinal tumor
- Vertebral fracture
- Spinal Instability
The abnormal mobility of a motion segment in the spine is the commonest cause of chronic low back pain.
The basic indication for spinal fusion is instability in a motion segment of the spine causing low back pain.
The instability may be present from birth like in spondylolisthesis or acquired as in trauma, tuberculosis, metabolic, degenerative or neoplastic disorders of the spine not to forget the iatrogenic causes and failed back syndromes.
In congenital spondylolisthesis, a slippage beyond 25% is associated with back pain and needs fusion.
It has been observed that young patients presenting with symptoms and having Gr.ll or more spondylolisthesis will need fusion.
Decompression and fusion has been associated with better outcomes than decompression alone in degenerative spondylolisthesis.
Degenerative instability is not uncommon in the back pain population. Presence of a traction spur and abnormal translatory motion on dynamic X rays are criteria for the diagnosis.
The commonest cause is inadvertent cutting of pars interarticularis while doing laminectomy in advanced lumbar canal stenosis. A sacrifice of more than 50% of both facets or sacrifice of one entire facet will produce instability.
Discogenic low back pain
Discography is a reliable means to assess pain originating in the intervertebral disc. It is believed that fusion in such cases will alleviate back pain. With the introduction of techniques like micro lumbar discectomy and endoscopic discectomy, the role of fusion in a case of the prolapsed lumbar disc has changed. The use of fusion particularly in prolapsed L4/5 disc produces better results.
Multilevel spinal stenosis
In this condition, to achieve good decompression of lateral recess stenosis excision of medial one-third facets bilaterally becomes mandatory. This may cause instability and hence fusion must be done to relieve pain and achieve
Usually, more than one root is involved. Besides decompression, fusion helps to achieve correction of scoliosis and prevents stress factors producing further degeneration.
Types of Spinal Fusion
There are two main types of lumbar spinal fusion, which may be used in conjunction with each other:
There are several types of spinal fusion surgery options. The most commonly employed surgical techniques include:
Posterolateral Gutter Fusion
A gutter is created on the posterolateral aspect to place the bone graft. The procedure is done through a posterior approach.
Posterior Lumbar Interbody Fusion (PLIF)
This procedure is done through a posterior approach, the disc is removed disc between two vertebrae and graft bone is inserted between the two vertebral bodies
Anterior Lumbar Interbody Fusion
the procedure is done using the anterior approach and includes removal of the disc between two vertebrae and inserting bone graft.
Anterior/Posterior Spinal Fusion
In this procedure, the fusion is done from anterior and posterior is done from the front and the back
Transforaminal Lumbar Interbody Fusion (TLIF)
Surgeons perform lumbar fusion using several techniques. The method described here is called transforaminal lumbar interbody fusion is an adaptation of a posterior lumbar interbody fusion where there is unilateral access to the disc space through the intervertebral foramen.
The spine is approached posteriorly and graft is used for fusion.
Extreme Lateral Interbody Fusion
An interbody fusion in which the approach is from the lateral side
Multilevel Spinal Fusion
When needed, fusing two levels of the spine may be a reasonable option for treatment of pain. Fusing 3 or more levels of the spine is usually reserved for cases of scoliosis and lumbar deformity.
Mostly, the fusion is augmented by a fixation device. The aim to use fixation device is to keep holding the bones in position until the fusion occurs.
The fusion process typically takes 6–12 months after surgery. Sometimes spinal fusion fails to occur and a resurgery may be required.
Success of Spinal Fusion
The clinical success following spinal fusion is judged by the relief of pain and clinically stable spine. The stability is dependent on successful arthrodesis which can be studied by doing imaging studies of the spine, remembering the fact that not all cases of pseudoarthrosis are symptomatic.
Techniques of spinal fusion and constitutional factors like age, sex, and osteoporosis affect the outcome.
Complications of Spinal Fusion
With any type of spine fusion, there is a risk of clinical failure.
Failure of Relief
This risk occurs in a minimum of 20% of spine fusion surgeries. The risk increases as the number of levels of fusion increase.
This outcome is commonly referred to as failed back surgery syndrome
Failure of vertebrae to fuse. It occurs in approximately 5% to 10% of spine fusion surgeries. Occurs more in smokers.
The screws may break or become loose and surgery may be required for fixation again
Anterior grafts and cages can migrate or subside, which may require repeat spine surgery