Artificial disc replacement has become increasingly established as a viable and promising option for treatment of the cervical degenerative disc. Recent studies have shown noninferiority or even superiority of the procedure compared with traditional anterior cervical decompression and fusion. These results were not readily apparent in one-level studies, but became more evident in two-level studies.
Artificial disc replacement (ADR) indications and techniques are more nuanced and restrictive than those for anterior cervical decompression and fusion (ACDF). A solid understanding of performing ACDF is necessary to optimally perform ADR.
Most critically, not all patients qualify for ADR when they may be candidates for ACDF. Some patients are not adequate candidates when they present with documented instability (risk of postoperative dislodgement), severe osteoporosis (risk of intraoperative fracture and secondary subsidence), severe disc collapse (risk of excessive compression on the device resulting in the absence of motion) and severe facet arthritis (risk of postoperative facet syndrome). ADR requires a more extensive preoperative consideration than ACDF. In some cases of multilevel disease, one level may qualify for treatment with ADR while another level may not. In this setting, hybrid constructs may be offered.
Several types of ADR devices have been developed with various modifications and attributes. These vary according the material used, such as chromium or titanium alloy, or according to the type of fixation used, such as screws or teeth or keels that are press-fit. Another way in which ADR devices vary is in the motion mechanism, such as traditional ball-and-socket or, more recently, shock absorption designs.
The M6-C Artificial Cervical Disc (Orthofix) is a shock-absorbing device made of two titanium endplates with a core made of polyethylene fibers and polycarbonate urethane polymer assembled into a shock absorbing structure. Three rows of small, 2-mm fins ensure fixation proximally and distally. These small fins have the advantage of clearing the distraction pins and allowing multilevel constructs to be used without conflict. They also have the theoretical advantage of reducing the risk of vertebral body splitting, especially in double-level ADR, although this complication is rare and has never been observed by the authors during their extensive experience using a larger keeled device. The downside of these small keels is inferior stability of the device during insertion. It is likely easier for the fins to displace from their grooves during insertion. It is therefore critical to release any distraction before insertion of the implant.
A 29-year-old patient was involved in a motor vehicle accident. Her chief medical complaint included severe neck pain, a left C6 radiculopathy and severe cervical headaches. She was found to have a cervical disc herniation at the C5-6 level (Figure 1). After failure of conservative treatment, she was indicated for ACDF by another physician. She presented to us for a second opinion. It was suggested that, given her young age and the absence of contraindication, cervical ADR was a viable alternative.
The procedure is performed under general anesthesia with prophylactic antibiotics and spinal cord monitoring. We prefer to keep the patient under muscle relaxation throughout the procedure and record only somatosensory-evoked potentials. This limits the force of retraction on the trachea and esophagus. This further limits the danger during bony work near the spinal cord, which is risky if an unexpected motion of the patient secondary to light anesthesia or unplanned motor stimulation occurs.
Proper positioning is key to ensure the cervical spine is straight and stable. We prefer to tape the patient’s head to the table. The shoulders should be pulled down when a lower level is operated. If the disc cannot be visualized under fluoroscopy intraoperatively, then a fusion should be considered. A small bolster is placed under the neck and neck extension is adjusted with an axillary roll placed under the shoulders. A slight, but not excessive extension is recommended (Figure 2).
Using a standard Smith-Robinson approach, access to the cervical disc is obtained. To prevent unnecessary damage of the annulus, we use a small clamp for confirmation of the disc level, instead of a needle (Figure 3). Distraction pins are then inserted, and a discectomy is performed. Of note:
The footprint template, for which four sizes are available, should cover as much endplate as possible. It should go laterally from one uncus to the other, extend as far posteriorly as possible while remaining in the disc space, and anterior should be 1-mm to 2-mm recessed from the anterior cortex. The template should sit even on the endplates as confirmed visually and on fluoroscopy (Figure 4).
Sizing should be upsized sequentially, starting from the shortest height (6 mm). The trial provides a midline marking guide which must be aligned with the midline of the vertebral body. Rotation is controlled by the Center Alignment Port which should be a perfect circle on the lateral view.
An adequate height is achieved only after release of distraction as noted before. It can be judged by the manual resistance of the trial, visual comparison to the adjacent disc heights, and by ensuring the endplates remain parallel. On the anteroposterior view, perfect centering is confirmed through a perfect circle after removal of the handle (Figure 5).
The fins are fixed at 2 mm in height. Distraction pins should not be in conflict. The cutter must be centered on the marks made on the bone and malleted gently under fluoroscopy. Once initiated into the space, all distraction must be released. Careful advancement under imaging should be performed until the final position of the trial is achieved. When found satisfactory, the trial should be removed and the final implant inserted following the fin tracks. After removal of the holder, final imaging is performed (Figure 6).
ADR is an attractive modality for the treatment of cervical disc degeneration, offering a more physiological intervertebral motion. The main advantage appears to be limiting facet displacement during motion. This could lead to reducing the risk of long-term facet arthritis. The M6-C instrumentation is simple and it is an exciting new technology that offers a shock-absorption mechanism. It does, however, present some challenges of which the surgeon should be cognizant, including the following:
The midline marking guide could be easily mistaken with a stop, leading to a catastrophic introduction of the trial into the spinal canal. This post ideally should be modified to serve a dual purpose of also acting as a stop; and
The fin cutter is independent from the trial, requiring removal of a trial after positioning to make suboptimal cuts. Ideally, the fin cutter should fit over the trial.
In conclusion, the M6 is an interesting new device, combining the advantages of the keel systems to provide immediate stability and of the non-keel systems (non-interference with the instrumentation). Long-term follow-up will tell if it stands the test of time.
Disclosures: Bitan and Mo report no relevant financial disclosures.
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