Cervical Intervertebral Disk Arthroplasty

Mostafa H. El Dafrawy and Lee H. Riley

Despite the high success rate and long-term track record of anterior cervical decompression and fusion (ACDF) for treating degenerative cervical conditions,1 concerns persist regarding the increased biomechanical stresses at the adjacent segments.² These concerns have resulted in interest in motion-sparing alternatives. Cervical disk arthroplasty (CDA) attempts to replicate the native disk’s kinematics, decreasing stress on adjacent vertebral levels and reducing adjacent-segment degeneration. The main theoretical advantages of disk arthroplasty are maintaining range of motion and avoiding adjacent-segment degeneration by restoring disk height and spinal alignment, and thereby the cervical spine’s biomechanical characteristics. Other benefits include less surgical morbidity compared with fusion bone grafting, lack of pseudarthrosis as a complication, native avoidance of complications from instrumentation or immobilization, and early return to function.

Prosthetic Designs

Prostheses are classified as unconstrained, semiconstrained, or constrained (Table 62.1) based on the allowed motion relative to normal coupled motion (Fig. 62.1A,B).³ Unconstrained, or physiologic, designs do not constrain the center of rotation (COR) to a fixed point so that the locus of points that define the normal COR can be replicated. They simulate the six native degrees of freedom of movement. Most⁴ have a mobile polymer COR with a compressible nucleus and allow unconstrained resistance to angular motion. Semiconstrained prostheses have a mobile COR as in a ball-in-trough joint that allows five degrees of freedom of movement—three angular motions and two translations (usually in the sagittal and coronal planes)—but no compressibility. Constrained designs, with a fixed COR, have a ball-and-socket or saddle-type joint articulation and allow three angular degrees of freedom of movement without translation.5 There is no clear consensus on which design accomplishes the best results.

Fig. 62.1 Artist’s sketches of coupled cervical motion (A) in the coronal plane, showing lateral bending of the vertebral body coupled with translation to the side, and (B) in the sagittal plane, showing rotation in the upper cervical spine and translation of the lower cervical vertebrae.

Indications and Contraindications for CDA

CDA is approved by the Food and Drug Administration (FDA) for the treatment of single-level symptomatic degenerative disk disease with evidence of disk herniation between C3 and C7 causing radiculopathy and/or or myeloplathy.^6–8

Contraindications to CDA include multilevel disease, cervical instability (defined as translation of > 3 mm and/or > 11 degrees of rotational difference to that of either adjacent level), post-traumatic vertebral body deficiency/deformity, and severe spondylosis (bridging osteophytes, disk height loss of 50%, and absence of motion). CDA is not an option for neck or arm pain of unknown cause or axial neck pain as the sole presenting symptom. Known allergy to implant materials (titanium, polyethylene, cobalt, chromium, molybdenum) and active local/systemic infection are absolute contraindications to CDA. General contraindications include severe osteoporosis/osteopenia; active malignancy or systemic disease, such as acquired immune deficiency syndrome (AIDS), human immunodeficiency virus (HIV), hepatitis B, hepatitis C, or type I diabetes; metabolic bone diseases or autoimmune spondylarthropathies (rheumatoid arthritis); morbid obesity (body mass index of 40); or pregnancy or trying to become pregnant in the next 3 years. The role of CDA in the treatment of adjacent-segment disease is still controversial.9 Fusion adjacent to the level and previous surgery at the level to be treated are contraindications.

Technical Considerations

An aggressive discectomy with complete removal of all osteophytes, especially of the uncovertebral joint, is performed. As intervertebral motion is preserved, it is necessary to avoid potential dynamic compression in the foramen, which is not a consideration with ACDF. In a study of retrieved prostheses, the most frequent cause of revision was inadequate decompression and recurrence of symptoms.¹⁰

The posterior longitudinal ligament (PLL) is removed to ensure complete decompression, facilitate parallel distraction, restore intervertebral height, and mobilize the motion segment.

Although the cartilaginous endplate is removed for CDA, the bony endplate is preserved to minimize the risk of implant subsidence. To facilitate the even insertion of the device and to allow appropriate surface contact between the endplates and the device, the vertebral endplates should be burred until there are two parallel surfaces. Over-resection of the posterior vertebral body can lead to loss of contact posteriorly and subsequent retropulsion.

Avoiding kyphosis is vital to the restoration of normal cervical biomechanics. Yi et al.⁵ found that disk insertion angle and insertion depth were significantly correlated with postoperative sagittal imbalance.

Centralization of the prosthesis is important to the success of a cervical total disk arthroplasty. Proper midline identification and placement of the device are critical; in contrast, a graft for ACDF can be placed eccentrically. For CDA, careful fluoroscopic evaluation in the anteroposterior and lateral planes to ensure proper visualization of the disk space in the lateral view is crucial.⁴ If body habitus precludes clear visualization of the interspace, fusion should be strongly considered.

Proper sizing of the implant involves as much coverage of the endplate as possible. A small disk will increase contact forces on the endplate, leading to subsidence. A too-large prosthesis might become extruded anteriorly.

Overdistraction of the interbody space should be avoided because it may lead to nerve root stretch, facet joint overload, and loss of motion.

Outcome

The highly predictable and favorable outcome of ACDF in cervical disease has always posed a strong argument against the need for CDA.2 For the past two decades, many spine surgeons have compared the efficacy and safety of CDA and fusion. A recent systematic literature review¹¹ reported on 13 case series presenting clinical data on the effectiveness of CDA (single- and multiple-level), four randomized controlled trials comparing single-level fusion to arthroplasty, and one nonrandomized comparative study.^6–8,12,13 Three randomized controlled trials represented FDA-regulated investigational device exemption (IDE) studies that evaluated the safety and effectiveness of the Prestige ST Cervical Disk System (Medtronic, Memphis, TN),¹³ Bryan Disk (Medtronic Sofamor Danek, Memphis, TN),⁶ and ProDisc-C (Synthes Spine, West Chester, PA).⁷ The Prestige System study concluded that it maintained physiologic segmental motion at 24 months after implantation and provided improved neurologic success, improved clinical outcomes, and a reduced rate of secondary surgeries compared with ACDF.¹³ The Bryan disk study showed there was no statistical difference between the ACDF and CDA groups with regard to the rate of secondary surgical procedures performed.⁶ Patients who received the artificial cervical disk returned to work nearly 2 weeks earlier than did those in the ACDF group.⁶ The ProDisc-C study showed no difference in visual analog scale scores for neck and arm pain between the CDA and ACDF groups, but ProDisc-C patients achieved equal to or greater than 4 degrees of motion or maintained motion relative to preoperative baseline at the operated level.⁷ There was a statistically significant difference in the number of secondary surgeries, with 8.5% of ACDF patients needing a reoperation, revision, or supplemental fixation within the 24-month postoperative period compared with 1.8% of ProDisc-C patients.⁷

In summary, there was level I evidence of significant improvement on the Neck Disability Index and neck and arm pain visual analog scale scores for the CDA and ACDF groups, similar mean angular motion values before and after surgery for the CDA group, faster return to work for the CDA group, and lower reoperation rates for the patients in the CDA group.

Complications

Approach-Related

Complications of the anterior approach to the cervical spine include dysphagia, hoarseness, and unilateral vocal fold paralysis, which are not specific to arthroplasty.^12,14

Specific to Arthroplasty

Infection

Of more than 5500 Bryan cervical disks implanted worldwide, 11 are known to have been explanted, with four revised because of infection.10 Three of the ~ 300 implanted Prestige disks were retrieved, with one disk revised because of infection.¹⁰

Prosthesis Migration

Extrusion, retropulsion, and subsidence are well-known complications of CDA.

Wear

Wear is affected by the type of material, duration of exposure to debris, prosthetic load and kinematics, specific joint, lubrication, host responsiveness, and particle size, shape, and concentration. The nature of fibrocartilaginous joints may mean that there is a less inflammatory response in the cervical spine than in relatively proinflammatory synovial joints. Accelerated wear and particle debris formation leading to device loosening do not appear to be an issue.¹⁰

Heterotopic Ossification

Motion preservation after CDA is guaranteed only if spontaneous fusion can be prevented. According to McAfee et al.,¹⁵ heterotopic ossification (HO) after CDA can be classified as grades 0 to 4. Reports of HO are variable among different prostheses, ranging from 21% to 71.4%.¹⁶ One study reported 49.4% of the patients with grade 2 to 3 ossification after 1 year.¹⁷ Early HO after arthroplasty can potentially be avoided by administering nonsteroidal anti-inflammatory drugs (NSAIDs) in the immediate postoperative period.¹⁸

Conclusion

CDA is a reasonable alternative to ACDF for one-level disease in patients with soft disk herniations and minimal bony degenerative changes. Complete discectomy and osteophyte removal, preservation of the bony endplate, and proper positioning and sizing of the implant are critical for preservation of motion. The indications and contraindications of the procedure are currently evolving with wider use and longer patient follow-up.