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Pitfalls of Cervical Disc Replacement

The authors of this article on the treatment of cervical myelopathy originally presented the results of a study comparing disc removal and replacement versus spinal fusion for this condition back in November 2008. Now, they offer a detailed look at the surgical technique and pitfalls of cervical disc arthroplasty (disc replacement in the neck), along with an update on this procedure.

Cervical myelopathy refers to a narrowing of the spinal canal, the opening formed by the vertebral bodies when stacked on top of one another. The spinal cord travels through the open spinal canal. Anything that narrows this opening (e.g., disc protrusion, bone spurs, hardening of the spinal ligaments) can put pressure on the spinal cord and cause serious neurologic problems.

At the time of the 2008 study, there was a question about whether disc replacement versus spinal fusion was the best treatment approach to the problem of cervical myelopathy. Disc replacement preserves motion but might cause ongoing microtrauma to the spinal cord if the implant puts any pressure on the spinal cord. Fusion eliminates any ongoing microtrauma but limits motion and may increase stress and load on the vertebral segments above or below the fused level.

There were no direct studies comparing the two methods, so the authors looked at the results of each procedure separately and compared final outcomes. The results were measured using patient reports of neck and arm pain, function, gait (walking ability), and self-reported general physical and mental health. They found that either treatment worked well and the myelopathy did not get worse after disc replacement. Treatment was restricted to one spinal segment and patients were only followed for two years.

Since that time, cervical disc arthroplasty (replacement) has continued to be used in younger patients who don't have a lot of degenerative changes seen in older adults. When pressure is placed on the spinal cord because of disc protrusion, then disc replacement is the treatment of choice. Cervical disc arthroplasty is not recommended when the patient has bone spurs, significant wear and tear on the facet (spinal) joints, or other age-related changes.

Other contraindications to cervical disc arthroplasty include fracture, unstable segments due to rheumatoid arthritis, or previous surgery to remove the lamina, a supporting column of bone that's part of the vertebra. Contraindication means reasons why something should not be done, in this case, the disc replacement. Anything that might compromise the stability of the spine is considered a contraindication. Infection, extreme obesity, osteoporosis (brittle bones), or other arthritic conditions that limit spinal motion are also contraindications to cervical disc arthroplasty.

In this article, the authors (three spinal surgeons from the Midwest) provide a detailed review of the standard technique used to perform a cervical disc arthroplasty. Patient positioning, approach and incision, and surgical instruments used for osteophyte (bone spur) removal are discussed. Photos of the proper patient position needed to ensure perfect placement and alignment of the implant are included. X-ray photos of improper implant alignment help the surgeon see what happens when this aspect of the procedure is not taken care of properly.

Intraoperative photos of the disc space and endplate preparation are also published. Results of decompression (refers to removal of osteophytes and anything else putting pressure on the spinal cord) are checked using CT scans. Additional photos of the implant in place during the operation are accompanied by a description of problems that can develop if the implant is wedged in too tightly (can limit motion) or the soft tissues are too loose (can create spinal instability). Implants no longer come in a one-size-fits-all, so the surgeon must carefully measure and choose the right sized implant for each patient.

And finally, the authors list for surgeons, the many possible pitfalls of this procedure. Most are technical details again related to patient positioning, disc space preparation, obtaining adequate decompression (but not too much!), and proper implant size selection and placement.

For example, too large of an implant will distract (separate) the two vertebral bodies too much resulting in an unstable segment. Not enough decompression may result in rubbing of the implant up against the spinal cord reproducing the same problem as when the disc was present and doing the same thing. Everything done to prepare for the implant must be symmetrical such as even and level edges of the bone all the way around. And the implant must be placed in the middle of the prepared disc space to avoid asymmetrical motion of the spinal segment.

Without direct studies comparing these two treatments for cervical myelopathy, surgeons are left to rely on case studies and personal experience when deciding the best approach to this condition. The 2008 comparative study was based on a large number of patients in both study groups but a direct study is still needed. In the meantime, the authors of this article bring their knowledge from reported studies and personal expertise to offer an up-to-date review of cervical disc arthroplasty.

References: Jacob M. Buchowski, MD, MS, et al. Cervical Disc Arthroplasty Compared with Arthrodesis for the Treatment of Myelopathy. In The Journal of Bone and Joint Surgery. October 2009. Suppl 2. (Part 2). Pp. 223-232.

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