Anterior Cervical Fusion
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.
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.
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.
Since fusion will take at least three months to set up, some type of immobilization is recommended. Often, bracing will be recommended. You may be asked to wear a neck brace for a few days to weeks after surgery. Neck bracing is often used after surgery for comfort and to decrease the motion of the neck to allow fusion.
The activity level is gradually increased. Patients are encouraged to walk as much as possible but to avoid lifting or binding early on. Strengthening and physical therapy can be started at three months post-operative if the fusion appears to be progressing well.
It should be noted that the time to fusion can vary, and usually use of the patient's own bone or use of instrumentation can result in a quicker fusion. It usually takes approximately three months, but can take up to 6 to 9 months, for the bone graft to fuse to the vertebral body bone. Heavy lifting and overhead work are usually limited until the fusion is noted to be solid.
Signs of infection like swelling, redness or draining at the incision site, and fever should be checked out by the surgeon immediately. Keep in mind, everybody is different, and therefore the amount of time it takes to return to normal activities is different for every patient. Discomfort should decrease a little each day. Most patients will benefit from a postoperative exercise program or supervised physical therapy after surgery.
The principal risk from a fusion is that it does not heal. In general, allograft bone does not heal quite as well as autograft bone, but both yield good results when used in the anterior cervical spine.
If a graft is used without instrumentation, there is a small chance (1% to 2%) of a graft dislodgment or extrusion. If this happens, another operation may be necessary to reinsert the bone graft, and instrumentation (plates) can then be used to hold it in place.
The principal disadvantage with using autograft bone is that another incision needs to be made over the hip to harvest the bone graft. Possible complications associated with taking out bone graft include:
- Graft site chronic pain (which happens 10% to 25% of the time)
- Hip fracture
- Damage to the lateral femoral cutaneous nerve (a sensory nerve that supplies sensation to the front of the thigh)
The chances of a complication increase with the size of the bone graft. The bone graft is an important part of the procedure, and many patients find the bone graft harvest site to be more painful than the cervical surgery itself.
If allograft is used, there is a theoretical risk of transmission of an infection from a donor. The risk of contracting HIV from a graft has been estimated to be between 1 in 200,000 to 1 in 1 million.
In skilled hands, this is a very safe procedure. Possible reported risks and complications of anterior cervical fusion may include:
- Damage to the spinal cord (about 1 in 10,000)
- Graft extrusion
- Continued pain
- Adjacent disc disease
- Injury to the trachea or esophagus
- Difficulty swallowing- usually transient
- Hoarseness of the voice- usually transient
- Injury to the vertebral artery or carotid