SPINAL FUSION

Definition

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

Anatomy

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, spine is not a rigid structure, and its mobility is provided by joints. There are 33-34 vertebral bones comprising the spine. In sacrum and coccyx bones do not have joints between them. In cervical, thoracic and lumbar spine there are   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 of 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 restricting excessive mobility of the disc joint and prevent slippage of the vertebral bones.  

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 surgical procedure which includes placing hardware and bone graft to the spine in order to achieve spinal fusion in future. 

Spinal fusion is a biological process which includes 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. 

Spinal fusion process

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 for surgical process:

1. One of the most important aspects of the procedure is robustness of the hardware. Fusion does not develop if bones are mobile. It is the similar to casting in order to achieve bone healing following fracture. However casting the spine is of limited value for achieving fusion. Doctors usually have to place metallic hardware in order to robustly immobilize the spine. 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 problem. Anterior fusion with cages is usually considered stronger than posterior fusion alone. The best results however, achieved by combined (anterior + posterior) stabilization. 

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

Osteoconduction is a process by which graft material provides 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 proliferation and division of the cells.  

Osteogenesis is a process when osteoblasts located in the graft material proliferate and make new bone tissue. 

There are many bone graft options available for surgeons. 

Autograft – a graft obtained from the patient itself. It is considered the best graft material since it has osteoconductive, osteoinductive and osteogenetic properties. 

Allograft – a graft material obtained from other 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 – an artificially created grafts that contain calcium crystals. They have only onsteoconductive properties. Sometimes they are augmented with growth promoters to have additional osteoinductive properties. However, this feature is usually weak. 

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

Indications

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

Instability. It is a condition when spine's integrity is  compromised leading to abnormal motion. Instability is not a diagnosis itself, rather its a condition seen with other disorders (trauma, tumor etc). Instability causes significant pain since spine's load carrying capacity is compromised. Additionally it is 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 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. In this case scenario the surgeon has to restore the 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 condition when one vertebral body is slipped either to the front (anterolisthesis) or to the back (retrolisthesis) regarding to the vertebra below. There are may reasons for spondylolisthesis such as trauma, degeneration, 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 spinal cord. Therefore, stabilization must be performed in these cases to restore normal structures and function of the spine.
Deformity is a condition when a part of the spine has lost its normal appearance. Scoliosis and kyphosis are two main deformity types . If surgery is performed an affected segment is corrected and hardware is places to hold that part until the fusion develops.  

Classification of spinal fusion

There are several types of spinal stabilization and fusion used for various reasons. Generally spine is divided to anterior and posterior in regard to the spinal canal. Spinal fusion may be in 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 as a variant of anterior surgery. 

Spinal fusion is also classified according to the segment of spine involved. Cervical, thoracic, lumbar, cervico-thoracic, thoraco-lumbar, cranio-cervical fusion types are examples of this 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 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 lumbar spine there are several interbody fusion options:

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

Failed fusion

In some cases, fusion fails to develop after 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, 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, cages become loose in case of failed fusion and move. Loose screws usually pull our from 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 highly experienced surgeon.