SPINAL FUSION

What is the spinal fusion procedure?

Spinal fusion is a surgical procedure performed to permanently immobilize a portion of the spine. It might be a primary or an additional procedure as a part of the main surgical intervention.

What is the structure of the human spine?

The spine is a mobile structure composed of vertebral bones. It carries the body weight and protects the spinal cord and nerves that run inside it. However, the spine is not a rigid structure, and its mobility is provided by joints. There are 33-34 vertebral bones comprising the spine. Sacrum and coccyx bones consist of fused vertebral bones. In the cervical, thoracic, and lumbar spine, there are three joints between each pair of vertebral bones: intervertebral disc joins and a pair of facet joints. Disc joints are the largest among them, the spine, and are located in the front part of the spine. They carry the majority of the load and provide mobility to the spine. Facet joints are relatively small, paired accessory joints located at the back of the spine. They are formed by extensions of the vertebral bones called facet processes (or articular processes). The main function of the facet joints is to restrict excessive mobility of the disc joint and prevent slippage of the vertebral bones.

What is the difference between spinal instrumentation and spinal fusion?

Fusion is a commonly performed procedure for treating various spine conditions. There is a lot of confusion about spinal instrumentation and fusion.

Spinal instrumentation is a surgical procedure that includes placing hardware and bone grafts in the spine in order to achieve spinal fusion in the future.
Spinal fusion is a biological process that includes the formation of new bone bridging two or more vertebrae together. Fusion does not develop instantly after the surgery; rather, it requires some time for bone-forming cells to multiply and make new bone.

How does the spinal fusion develop?

Successful bone fusion needs some time to develop following the surgery. The eventual goal is to have two neighbor vertebral bones to unite. There are several important factors in the surgical process:

1. One of the most important aspects of the procedure is the robustness of the hardware. Fusion does not develop if bones are mobile. It is similar to casting in order to achieve bone healing following a fracture. However, casting the spine is of limited value for achieving fusion. Doctors usually have to place metallic hardware in order to immobilize the spine robustly. There are two pain hardware systems used in the spine. The hardware may be placed in front or back of the spinal canal. Therefore, they are called anterior or posterior stabilization. The surgeons may use different types of stabilization to address specific problems. Anterior fusion with cages is usually considered stronger than posterior fusion alone. The best results, however, were achieved by combined (anterior + posterior) stabilization.

2. Another important factor is the quality of the bone graft material. Unlike fractures, spine fusion is usually performed under conditions when bones have a particular distance between each other. Therefore, in order to fill the gap, the surgeon has to place bone material into that space. The ideal bone graft should have three properties for successful fusion:

Osteoconduction is a process by which graft material provides a scaffold for newly developing bone. Bone-forming cells (osteoblasts) migrate into the graft, make a new home, and start forming new bone.
Osteoinduction is a process of stimulating osteoblasts to migrate, divide, and make new bone. This process is complex and involves multiple proteins that promote the proliferation and division of the cells.
Osteogenesis is the process in which osteoblasts located in the graft material proliferate and create new bone tissue.

There are many bone graft options available for surgeons.

Autograft– a graft obtained from the patient themselves. It is considered the best graft material since it has osteoconductive, osteoinductive, and osteogenetic properties.
Allograft– a graft material obtained from another person. It does have osteoinductive and weak osteoconductive properties. But there is no osteogenesic properties.
Xenograft- a graft material obtained from animals. It has a limited value in spinal surgery.
Artificial bone substitutes – artificially created grafts that contain calcium crystals. They have only osteoconductive properties. Sometimes, they are augmented with growth promoters to have additional osteoinductive properties. However, this feature is usually weak.

3. The patient's condition is very important for developing fusion. Advanced age, osteoporosis, poor bone quality, and bad general condition are factors impeding the fusion process. Smoking is also very detrimental to fusion, and therefore, patients undergoing fusion surgery should seriously consider smoking cessation.

What are the conditions that require treatment with spinal fusion?

There are several reasons for which surgeon may be willing to perform a fusion procedure on a patient:

Instability. It is a condition when the spine's integrity is compromised, leading to abnormal motion. Instability is not a diagnosis itself; rather, it's a condition seen with other disorders (trauma, tumor, etc.). Instability causes significant pain since the spine's load-carrying capacity is compromised. Additionally, it may cause abnormal displacement of vertebral bones with spinal cord compression and lead to paralysis.
Tumor. In some cases, tumors growing in the spine may cause bone destruction and cause instability. Sometimes, in order to reach and eliminate the tumor, a surgeon has to remove a part of the spine. This procedure may further compromise the spine's stability, and stabilization should be performed.
Degeneration. Disc degeneration may cause loosening of the supportive system and cause instability. This effect is frequently seen in lumbar and cervical disc herniations. In this case scenario, the surgeon has to restore stability during surgery. In other cases, stabilization is performed to achieve permanent fusion and avoid recurrence of symptoms in the future.
Infection. Infections of the spine may destroy bone and ligaments and cause instability.
Spondylolisthesis, or simply spinal slippage, is a condition when one vertebral body is slipped either to the front (anterolisthesis) or to the back (retrolisthesis) regarding the vertebra below. There are many reasons for spondylolisthesis, such as trauma, degeneration, and congenital disorders.
Trauma. Trauma is one of the most common reasons for fusion. Vertebral bone fractures and soft tissue disruption may cause instability and compromise the spinal cord. Therefore, stabilization must be performed in these cases to restore normal structures and function of the spine.
The deformity is a condition when a part of the spine has lost its normal appearance. Scoliosis and kyphosis are the two main deformity types. If surgery is performed, an affected segment is corrected, and hardware is placed to hold that part until the fusion develops.

What are the types of spinal fusion?

There are several types of spinal stabilization and fusion used for various reasons. Generally, the spine is divided into anterior and posterior in regard to the spinal canal. Spinal fusion may be anterior, posterior, or combined. Sometimes, spinal fusion is performed from the side, and the procedure is called lateral. However, strictly speaking, even though the procedure is performed from the side, stabilization is done anterior to the spinal canal, and therefore, it is considered a variant of anterior surgery.

Spinal fusion is also classified according to the segment of the spine involved. Cervical, thoracic, lumbar, cervicothoracic, thoracolumbar, and craniocervical fusion types are examples of these fusion types.

There are several common types of spinal fusion:

Anterior cervical discectomy and fusion (ACDF). The surgery is performed for cervical disc herniation from the front of the neck. Following the removal of the disc herniation, the surgeon performs stabilization by using cages, plates, and screws. Usually, fusion develops within several weeks following the surgery.

In the lumbar spine, there are several interbody fusion options:

Posterior Lateral Interbody Fusion (PLIF) is performed from the back. Following discectomy, stabilization is performed by inserting a straight cage and placing bone pedicle screws
Transforaminal Lateral Interbody Fusion (TLIF) is performed from the back slightly lateral than PLIF by inserting a bigger, curved cage and bone pedicle screws
eXtreme Lateral Interbody Fusion (XLIF)is performed from the side through the psoas muscle, and a big cage is placed. It may require the placement of additional bone pedicle screws.
Anterior Lateral Interbody Fusion (ALIF)is performed from the front, and a big cage is placed. It may require the placement of additional bone pedicle screws.

What is the failed fusion syndrome?

In some cases, fusion fails to develop after the stabilization procedure. This condition is called nonunion or pseudofusion. It is one of the most common reasons for failed back syndrome. In these cases, adjacent bones become connected by soft tissue instead of solid bone. There are several reasons for nonunion. These factors are patient or technique-related. Patient-related factors are advanced age, bad general condition, smoking, and poor bone quality. Technique-related factors are lack of robust stabilization and/or lack of adequate grafting.

There are two possible scenarios for failed fusion: hardware loosening or breakage. Screws, plates, and cages become loose in case of failed fusion and move. Loose screws usually pull out from the original location. Loose cages erode and dig into nearby bones – a condition called subsidence. In some cases, hardware may not tolerate constant bending forces and break down.

Failed fusion is one of the most common reasons for spinal revision. However, this type of revision surgery is very complex and, therefore, should be performed by a highly experienced surgeon.

Tags: spinal fusion procedure, spinal fusion surgery, lumbar fusion, cervical fusion, types of spinal fusion

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SPINAL FUSION

What is the spinal fusion procedure?

What is the structure of the human spine?

What is the difference between spinal instrumentation and spinal fusion?

How does the spinal fusion develop?

What are the conditions that require treatment with spinal fusion?

What are the types of spinal fusion?

What is the failed fusion syndrome?

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SPINAL FUSION

What is the spinal fusion procedure?

What is the structure of the human spine?

What is the difference between spinal instrumentation and spinal fusion?

How does the spinal fusion develop?

What are the conditions that require treatment with spinal fusion?

What are the types of spinal fusion?

What is the failed fusion syndrome?

Aska Question