Spinal Fusion - Types of Spinal Fusion

Types of Spinal Fusion

There are two main types of lumbar spinal fusion, which may be used in conjunction with each other:

Posterolateral fusion places the bone graft between the transverse processes in the back of the spine. These vertebrae are then fixed in place with screws and/or wire through the pedicles of each vertebra attaching to a metal rod on each side of the vertebrae.

Interbody fusion places the bone graft between the vertebra in the area usually occupied by the intervertebral disc. In preparation for the spinal fusion, the disc is removed entirely, for example in ACDF. A device may be placed between the vertebra to maintain spine alignment and disc height. The intervertebral device may be made from either plastic or titanium. The fusion then occurs between the endplates of the vertebrae. Using both types of fusion is known as 360-degree fusion. Fusion rates are higher with interbody fusion. Three types of interbody fusion are:

• Anterior lumbar interbody fusion (ALIF)- the disc is accessed from an anterior abdominal incision
• Posterior lumbar interbody fusion (PLIF) - the disc is accessed from a posterior incision
• Transforaminal lumbar interbody fusion (TLIF) - the disc is accessed from a posterior incision on one side of the spine
• Transpsoas interbody fusion (DLIF or XLIF) - the disc is accessed from an incision through the psoas muscle on one side of the spine

In most cases, the fusion is augmented by a process called fixation, meaning the placement of metallic screws (pedicle screws often made from titanium), rods or plates, or cages to stabilize the vertebra to facilitate bone fusion. The fusion process typically takes 6–12 months after surgery. During this time external bracing (orthotics) may be required. External factors such as smoking, osteoporosis, certain medications, and heavy activity can prolong or even prevent the fusion process. If fusion does not occur, patients may require reoperation.

Some newer technologies are being introduced which avoid fusion and preserve spinal motion. Such procedures, such as artificial disc replacement, are being offered as alternatives to fusion in the cervical spine, but have not yet been adopted on a widespread basis in the US. Their advantage over fusion has not been well established. Minimally invasive techniques have also been introduced to reduce complications and recovery time for lumbar spinal fusion.

In addition to lumbar fusions, cervical spinal fusions may also be performed on the neck. The purpose of a cervical spinal fusion is to join certain bones in the back. Bone, metal plates, or screws can be used to make a bridge between adjacent vertebrae. In extreme cases, whole vertebrae can be removed before the fusion occurs. In most cases, however, only the intervertebral disk is removed, and the bone or metal graft is subsequently inserted, allowing for the vertebrae to eventually heal together. Cervical spinal fusion can be performed for several reasons. Following injury, this surgery can help in stabilizing the neck and preventing fractures of the spinal column which could damage the spinal cord. It can also be used to treat misaligned vertebrae or as a follow up for other spinal injuries. Additionally, cervical spinal fusion can be used to remove or reduce pressure on nerve roots caused by bone fragments or ruptured intervertebral disks.

Pedicle Screw Insertion: In spinal fusion surgery, the accuracy with which screws are inserted in the pedicle has a direct effect on the surgical outcome. Accurate placement generally involves considerable judgmental skills that have been developed through a lengthy training process. Because the impact of misaligning one or more pedicle screws can directly affect patient safety, a number of navigational and trajectory verification approaches have been described and evaluated in the literature to provide some degree of guidance to the surgeon. For example, Manbachi A. et. al is one of the recent review articles that presents an overview of the need and the current status of the guidance methods available for improving the surgical outcomes in spinal fusion procedures. It also describes educational aids that have the potential for reducing the training process.