Fusion is the process of joining bones with bone grafts, adding bone graft or bone graft substitute to an area of the spine to set up a biological response that causes the bone to grow between the two vertebral elements and thereby stop the motion at that segment. The fusion process essentially "tricks" the body into thinking it has a fracture. Anterior cervical fusion is performed from the front of the neck region and commonly performed in conjunction with an anterior cervical discectomy.

Theoretically, fusing the two vertebral segments together after removing the disc prevents the spine from falling into a collapsed deformity (kyphosis), and also provides for a shorter post-operative rehabilitation period. Additionally, anterior cervical fusions are also done to treat cervical instability due to tumor, infection, or trauma.

The General Procedure:

1. Surgical approach
• The skin incision is about one inch and horizontal and can be made on the left or right hand side of the neck
• The thin platysma muscle is then split in line with the skin incision and the plane between the sternocleidomastoid muscle and the strap muscles is then entered
• Next, a plane between the trachea/esophagus and the carotid sheath can be entered
• A thin layer of fibrous tissue that covers the spine can easily be dissected away from the disc space
  
  
2. Disc removal
• A needle is then inserted into the disc space and an x-ray is done to confirm that the surgeon is at the correct level of the spine
• After the correct disc space has been identified on x-ray, the disc is then removed by first cutting the outer annulus fibrosis (fibrous ring around the disc) and removing the nucleus pulposus (the soft inner core of the disc)
• The dissection is often performed using an operating microscope to aid with visualization of the canal.
• Dissection is carried out from the front to back to a ligament called the posterior longitudinal ligament. This ligament can be gently removed to allow access to the spinal canal to remove any osteophytes (bone spurs) or disc material that may have extruded through the ligament.
  
  
3. Preparation of the fusion bed
• After the disc is removed for an anterior cervical disc herniation, a space remains between the vertebral bodies.
• Disc space shavers and spacers may be used to template the height, width, and depth of bone graft that is needed.
• The surfaces of the vertebral bodies are meticulously prepared for bone graft by burring any irregularities. This allows surface area for the ingrowth of bone.
  
  
4. Bone graft and fusion
• A structural, weight-bearing component is necessary to maintain the disc height and lordosis. An autograft or allograft strut structural bone graft is often employed.
• Additionally, allograft and/or bone graft substitutes and extenders can be used.
• Next, the bone graft material is press-fit into the disc space.

If only a small amount of disc is removed, the surgeon may select not to fuse. When no fusion is performed, the disc space may collapse resulting in deformity and later increased risk of neck pain and/or pinching of the nerves. However, it remains somewhat controversial and some surgeons do not do this. There is no definitive well-controlled study that supports either doing or not doing a fusion after a discectomy, although a preponderance of medical literature indicates discectomy patients do better with a fusion.

Bone replacement
The residual disc space is usually replaced with bone, either the patients own bone or bone from a bone bank (called allograft). There are several techniques to harvest the bone graft:

1. Autograft bone

Autograft bone (patient’s own bone) is harvested from the iliac crest (hip). This technique is the gold standard and has been done since Cloward, Smith and Robinson, described their respective procedures in the 1950’s. If their own bone is used, 90%-95% of patients will achieve a fusion.


2. Allograft bone

Allograft bone (donor bone from a cadaver) eliminates the need to harvest the patient’s own bone. Basically, the donor bone graft acts as a calcium scaffolding in which the patient’s own bone grows and eventually replaces. There are no cells in the bone graft, so there is no chance of a graft rejection. This process, called "creeping substitution", is slower than an autograft bone fusion.
• In one-level fusions, it yields equivalent fusion rates as autograft bone.
• If more than one level is fused, it does not heal as well as autograft bone. To enhance the healing rate ­ especially if more than one level is fused ­ many surgeons combine allograft with anterior plating of the spine.
• If plating plus allograft bone is used for a multi-level fusion, the fusion rate is equivalent to autograft bone.

Instrumentation
The surgeon may chose to use instrumentation (plate and screws) to hold this construct together until it heals. However, a solid fusion is not always achieved. There are a few factors that patients can control that are important in determining whether or not a fusion grows in solidly, including:

• Smoking cessation. It is generally advisable to quit smoking prior to a spinal fusion procedure, as nicotine is a direct toxin to bone graft and will prevent the bone from forming.
• Limited motion. Bone forms better if motion is limited, so patients are often advised to avoid bending, lifting, and twisting for three months after spinal fusion surgery.