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.
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.
Fusion is a commonly performed procedure for treating various spine conditions. There is a lot of confusion about spinal instrumentation and fusion.
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:
There are many bone graft options available for surgeons.
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.
There are several reasons for which surgeon may be willing to perform a fusion procedure on a patient:
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:
In the lumbar spine, there are several interbody fusion options:
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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