What is an ACDF? Part II.
We have discussed the indications for anterior cervical discectomy and fusion (ACDF). We have also discussed the evolution of the procedure from the days of Cloward to the modern ACDF today. In this post we will discuss the steps of an ACDF. Remember that surgeons can have different training experiences and skill sets. Thus, there can be subtle variation in each surgeon’s technique. This post describes the way that I do ACDFs but the general steps are the same no matter who you choose as your surgeon. At each step I’ll discuss some of the possible risks of the procedure. I apologize for the long post but I didn’t want to break up the steps of the procedure into two posts. Also, instead of using still images of an ACDF, I inserted a video at the end of the post of a one-level ACDF that I did recently.
After intubation, the patient is positioned supine (on their back) with a roll under their shoulders so that the neck is extended (tilted back.) Some surgeons would work with a neuromonitoring technician who at this point would connect wires to the patient’s arms, legs and scalp to monitor somatosensory and motor evoked potentials (SSEP and MEP). I won’t get into the details of SSEP and MEP but basically this allows the surgeon to monitor spinal cord function during the case; if there were any injury to the spinal cord during the case the monitoring technician could alert the surgeon. This is not standard of care and is only an option for the surgeon to employ. I typically do not use neuromonitoring on simple ACDFs as I think they add unnecessary cost and time to the procedure. Also, I find the data very nonspecific and therefore not always that useful.
I plan a small incision on the front of the left side of the neck, typically in a skin crease so that when the incision heals it’s barely noticeable (see figure 1). I almost always make the incision on the left side of the neck in order to avoid the recurrent laryngeal nerve. This nerve innervates some of the muscles of the vocal cords and if injured can cause hoarseness (about a 3% risk, and the hoarseness almost always resolves in a few months.) The nerve takes a more unpredictable course on the right side so in my opinion it’s safer via a left-sided incision. After the incision is I separate the platysma muscle (this is the only muscle that is cut during the procedure.) Next, I gently dissect the natural tissue plane between the carotid artery and jugular vein laterally and the trachea and esophagus medially (see figure 2). Injury to these structures is very rare (around 1/500) but patients can experience dysphagia (problems swallowing) because of trauma to the esophagus. This occurs in 5-10% of cases (more common in multi-level ACDF) and, again, usually resolves in a few days. After a bit of dissection I arrive at the front of the spine.
Figure 1: ACDF incision on left side of front of neck.
Figure 2: Cross-section MRI of the cervical spine indicating ACDF approach. The approach exploits a natural plane between the carotid artery (red) and jugular vein (blue) laterally and the trachea (yellow) and esophagus (green) medially.
Once at the spine I mark the disc space with a needle to confirm that I’m at the correct level. I’m careful to not disturb normal levels of the spine in order to avoid possible adjacent segment degneration later (the risk of which I quote at 10-15% after ACDF). I then dissect the longus colli muscle off of each side of the spine. This allows me to place a retractor along the spine that holds the soft tissue out of the working area. At this point in the operation I will bring the operating microscope into the field (not all surgeons use the microscope but I find it useful.) I then incise the annulus (outer casing) of the disc and begin removing the disc. This is the disc material that is still in the disc space, not the material that is herniated or bulging. Surgeons will use a few different types of instruments to remove the disc; I typically use a drill. Eventually, after removing the bulk of the disc, I will arrive at the posterior longitudinal ligament (PLL) which is then removed. At this point the dura (covering of the spinal cord) will be visible with its characteristic blue hue. We can then see and remove any herniated disc fragments or bone spurs that are compressing the spinal cord or nerve roots. It is during this decompression (or during placement of the spacer later) that more dreaded complications like neurological injury or paralysis can occur (very rare, occurring in less than 1/500 cases.)
Once the spinal cord and nerve root are decompressed I then scrape any remaining disc material off of the endplate of the vertebral body above and below the disc space. This is an important step that will allow for the growth of new bone (i.e. the fusion). I then insert a spacer into the disc space. These spacers come in many different types of materials, shapes and sizes but I’ll spare you the details on why one surgeon might choose one type of spacer versus another. I prefer to use a spacer made out of synthetic materials such as PEEK or silicon nitride (see figure 3). Prior to insertion I pack this spacer with graft material. It’s this graft material that acts as the “seed bone” to promote the bony fusion across the disc space. Some surgeons use the patient’s own bone (autograft) taken from the iliac crest (“the hip”). While autograft is the best graft material, many patients complain of significant posteroperative pain at the donor site at the hip (more pain than the surgical site in the neck!). Given this, I don’t use autograft from the iliac crest and instead use either cadaver bone or a stem cell product (allograft). The spacer is carefully inserted using fluoroscopic guidance to ensure that it’s at the appropriate depth.
Figure 3: ACDF spacer (source: Amedica Corp.)
With the spacer inserted I move to the final step of placement of the plate. In cases of one-level ACDF I don’t use a plate and instead use a spacer that allows me to secure the spacer in place using screws that pass through the spacer, a so-called standalone device (see figure 4). Again, this is a matter of preference. In multi-level ACDF I will use a titanium plate. This is secured in place using small titanium screws (see figure 5).
Figure 4: Standalone ACDF spacer (source: Globus Corp.)
FIgure 5: Titanium plate fastened to front of vertebral bodies with titanium screws (notice spacer in disc space). (source: Aesculap Corp.)
After all of the implants are in place I confirm that they are in good position with final xrays of the spine (see figure 6). I then spend some time making sure that any points of bleeding are coagulated (another complication after ACDF is a hematoma in the neck causing airway compression and thereby requiring emergent re-operation, seen in less than1% of cases.) I then close the platysma and the skin with suture (under the skin so that they don’t have to be removed) to complete the case.
Figure 6: Final X-rays after 3-level ACDF.
Despite the variations in technique and in the types of implants used, ACDF is almost uniformly successful with success rates exceeding 95%.
Phew, that was a long post. Thanks for hanging in there.
J. Alex Thomas, M.D.
See all of the steps of an ACDF that you just read about here in this video:
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