Scott I. Lee, MD
Could Cervical Disc Replacement Be An Option For You?
Cervical Disc Replacement is an exciting new surgical option for many cervical spine conditions. Compared to cervical fusion — the tried and true gold-standard procedure for many cervical spine conditions — Cervical Disc Replacement can preserve neck motion. There is increasing evidence that Cervical Disc Replacement provides potentially superior patient outcomes compared to cervical fusion. Is Cervical Disc Replacement an option for you?
What Conditions Are Treated With Cervical Spine Surgery?
Disc degeneration is the common theme linking most cervical spine conditions that require surgery. The cervical discs are located between vertebrae and are critical in maintaining neck alignment and motion (Figure 1). Over time, the discs can degenerate, leading to disc settling and arthritis (Figure 2), disc herniations, and potential compression of the cervical nerve roots and spinal cord (Figure 3).
Cervical disc degeneration can lead to a combination of the following symptoms:
Neck stiffness and painful neck motion
Forward-shifted neck posture
Radiating arm pain, numbness, and weakness (due to cervical nerve root irritation)
Difficulty with balance and dexterity (e.g. fastening buttons, handwriting, utensils) (due to cervical spinal cord compression)
How Does Cervical Spine Surgery Help These Symptoms?
The tried and true gold-standard procedure for many cervical spine conditions is an Anterior Cervical Discectomy and Fusion (ACDF). This procedure is performed through a minimally-invasive, cosmetic neck incision. The overlying soft-tissue structures are gently moved to the side in order to access the cervical disc and bones. Using special instruments and a surgical microscope, the degenerated disc is carefully removed (discectomy) and lifted off of the cervical nerve roots and spinal cord. This removes compression and alleviates irritation of the nerve structures — the root cause of many of the above-mentioned symptoms.
Once the disc is removed, the disc material must be replaced with an implant for the following reasons:
Restore stability across the involved segment
Restore alignment and disc height
Help prevent future neurologic compression
Options for implants include bone graft plugs harvested from cadavers, bone graft plugs harvested from your own body, or cages composed of ceramic or metal. The implant is often protected and stabilized by a metal plate secured to the bone with metal screws.
(Click here for a video illustration of the ACDF procedure)
For years, ACDF has been the gold-standard for the treatment of most cervical spine conditions. The procedure has proven results, is very successful, and is associated with good patient outcomes and low complication rates. So why should we mess with a good thing?
What Are The Issues With Anterior Cervical Discectomy and Fusion?
Despite the overall favorable outcomes with Anterior Cervical Discectomy and Fusion (ACDF), some of the issues are due to the fusion aspect of the procedure. As mentioned previously, once the disc material is removed, it must be replaced by an implant to restore stability and alignment, while also helping to prevent neurologic compression. The available implants restrict motion across the involved cervical segments, allowing your own body to heal across the disc space and form a bony fusion. Thus, the ultimate goal is to eliminate motion across the involved cervical segments.
The fusion aspect of ACDF is associated with the following potential issues:
Restricted neck motion
ACDF leads to decreased motion in flexion-extension (motion of nodding your head “Yes”), axial rotation (motion of shaking your head “No”), and lateral bending (bringing your ear to your shoulder). The exact reduction in motion is debatable and difficult to study, but current evidence suggests a reduction of approximately 5-10 percent per level for each motion when compared to healthy controls. There is the potential to regain some overall neck motion with time due to compensation by the other cervical segments. But this leads to the next point…
Accelerated degeneration of the other cervical discs
The unfused cervical segments may see additional stress as they compensate for the fused segments in an effort to restore overall neck motion. This can potentially lead to what is termed, Adjacent Segment Degeneration (ASD). ASD can lead to pain and neurologic symptoms similar to what prompted the original surgery. So called symptomatic ASD has quoted incidence rates of between 1 - 3% per year following cervical fusion. Such cases may require revision surgery.
Risk that bone fusion does not occur
In the months following surgery, your body is attempting to replace the graft material in the discectomy site with your own bone. There are multiple factors that can disrupt this process, including infection, excess motion, smoking, and poor bone quality. If your body is unable to fuse across the discectomy site, this is known as pseudarthrosis, with quoted rates from less than 1% to as high as 20%. Pseudarthrosis can lead to neck pain and recurrent nerve compression that requires a revision operation.
What Are The Purported Advantages Of Cervical Disc Replacement?
Cervical Disc Replacement (CDR) was developed to overcome the potential issues of fusion associated with ACDF. Similar to an ACDF, CDR can be performed through the same minimally-invasive, cosmetic approach, and for some patients, can also be performed in the outpatient setting. The CDR implant removes the requirement for fusion, offers the potential to preserve segmental neck motion, and as such, theoretically reduces the risk for Adjacent Segment Degeneration.
There are multiple different CDR implants available, but they all revolve around similar design principles that include a mobile-bearing surface that is placed into the disc space and secured to the bone. An example can be found here. The mobile-bearing surface is responsible for the motion-preservation properties of the implant.
(Click here for a video illustration of the CDR procedure)
Does The Literature Support The Purported Advantages?
Over the past decade, there has been increasing focus on Cervical Disc Replacement (CDR) in the scientific literature. Does the literature support the purported advantages of CDR? Let us consider each individual point:
CDR may restore more physiologic neck motion
In a 2011 randomized controlled trial, Auerbach et al. compared radiographic neck range of motion between one-level ACDF and one-level CDR at two-years follow-up. After CDR, the operative level maintained range of motion consistent with pre-operative measurements. There was no change in range of motion of the other cervical segments. Contrast this to the ACDF group, where there was significant reduction in motion at the fused segment (makes sense). However, there was also significant compensatory increase in motion at all other cervical segments. While the overall range of motion at 2 years post-op were similar for both groups, the CDR group exhibited a more natural, physiologic distribution of motion, and this may in fact reduce the risk of Adjacent Segment Disease…
CDR may reduce the risk of Adjacent Segment Disease (ASD)
In a 2016 meta-analysis, Zhu et al. analyzed 14 large randomized controlled trials with 2 to 7 year follow-up. Based on the pooled analysis, the authors found that CDR was associated with an approximate 40-50% reduction in the rate of ASD as well as the need for re-operation when compared to ACDF.
CDR produces results that are comparable to ACDF
Multiple studies, including a 2015 meta-analysis of randomized controlled trials by Gao et al. have found no significant difference between ACDF and CDR in terms of pain scores, neurologic recovery, and rates of adverse events. As such, CDR has demonstrated that it is at least on-par with ACDF in terms of patient outcomes. There are additional studies and analyses that suggest that CDR is superior to ACDF in terms of patient outcomes, but this is currently open to debate.
What does this evidence mean when all taken together? At the very minimum, CDR is as effective as ACDF for properly selected patients with one-level or two-level cervical disease. With longer term data, CDR may in fact prove to be superior to ACDF.
Cervical Disc Replacement Is Not For Everyone
Unfortunately, Cervical Disc Replacement (CDR) is not for everyone. An appropriate candidate for CDR must have the following:
Cervical condition limited to one or two disc levels
Absence of significant arthritis and neck pain
Good bone quality
Adequate neck alignment
Even if you are not a candidate for CDR, do not fret. The alternative — an Anterior Cervical Discectomy and Fusion (ACDF) — is also an excellent option (check back soon for a blog post).
As always, the point is to ask your surgeon questions!
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