Cervical Arthroplasty

K. Daniel Riew and Kevin R. O’Neill

Abstract

Compared with traditional fusion operations, cervical disc arthroplasty allows the same benefits of neural decompression while preserving motion and reducing stress on adjacent disc levels. Long-term studies now support the efficacy of cervical disc arthroplasty and have demonstrated reduced rates of breakdown of adjacent levels. This chapter reviews the evidence along with indications and contraindications of its use. A detailed and illustrated description of the procedure is provided along with several technical tips. A case example is then used to demonstrate a common clinical application.

Keywords: cervical disc arthroplasty, cervical disc replacement, anterior cervical discectomy and fusion, cervical disc herniation, cervical radiculopathy, cervical myelopathy

21.1 Introduction

Anterior cervical discectomy and fusion (ACDF) has proven to be a highly successful operation for the treatment of radiculopathy and myelopathy associated with anteriorly based nerve root or spinal cord compression.¹^,²^,³^,⁴^,⁵^,⁶^,⁷^,⁸ However, as with any surgical procedure, complications may occur. Symptomatic nonunion or pseudarthrosis occurs in approximately 5% and may require revision surgery.⁴^,⁸^,⁹ In addition, there are biomechanical data suggesting increased stress on motion segments adjacent to fusions, raising the concern for accelerated degeneration.¹⁰ Whether this process is indeed accelerated when next to a fusion or is the natural result of continued degeneration, adjacent segment pathology (ASP) following anterior fusion has been found to occur in 9 to 16% of patients with an incidence of 26% predicted at 10 years postoperatively.⁴^,¹¹^,¹²^,¹³^,¹⁴ It has been shown that 61 to 82% of patients with ASP then undergo subsequent surgery.⁴^,⁷^,¹¹ A more recent study has validated these estimates, finding that 18% of patients undergo surgery for ASP following anterior fusion by 10 years postoperatively.¹⁵

21.1.1 Evolution of Technique and Advantages and Disadvantages

Anterior cervical discectomy with arthroplasty (ACDA) has been introduced as a method to provide the same successful decompression as an ACDF, but with retention of segmental motion across the disc space. With the intent of retained motion, this eliminates the concern over pseudarthrosis. In addition, this theoretically avoids the increased stress at adjacent levels caused by fusion, and therefore may reduce the incidence of ASP. To date, there have been several studies investigating ACDA versus ACDF, and several meta-analyses of those studies.¹⁶^,¹⁷^,¹⁸^,¹⁹^,²⁰^,²¹^,²²^,²³^,²⁴^,²⁵^,²⁶^,²⁷^,²⁸^,²⁹^,³⁰ The two procedures share the same surgical approach and similar decompression techniques. Some studies suggest statistically increased blood loss and operative time with ACDA, though the actual differences are not likely clinically significant.²⁴^,²⁵^,²⁶ Neurologic success, defined as maintained or improved motor or sensory function, is similar or slightly more favorable in the arthroplasty group.²⁰^,²⁴^,²⁶^,³¹ Not surprisingly, motion across the index surgical level was lower in the fusion group.²⁶^,³² Patient-reported outcomes, including visual analog scale (VAS) arm pain and neck pain and Neck Disability Index (NDI) scores, were also found to generally favor arthroplasty.¹⁶^,¹⁹^,²⁸ Further, the compensatory increased motion at adjacent segments following fusion was not found to be present following ACDA, and several studies with follow-up of 2 to 5 years have demonstrated reduced rates of reoperation in those undergoing arthroplasty.¹⁶^,¹⁹^,²²^,³²^,³³ With the longest follow-up period to date of 7 years, a recent study has demonstrated a significantly lower rate of ASP following ACDA (5%) compared with ACDF (12%).³⁴

21.2 Anterior Cervical Discectomy with Arthroplasty

21.2.1 Indications

The indications for ACDA are similar to those for ACDF, as they share the same approach and decompression maneuvers. The primary indication for both of these procedures is the treatment of radiculopathy or myelopathy caused by anterior compression at the disc level. While most studies have investigated single-level disease, arthroplasty can be considered for up to three disc levels.³⁵^,³⁶ For an acute radiculopathy, nonoperative measures should be attempted for at least 6 weeks, and in most cases 3 months, prior to consideration of surgery, as symptoms often resolve over this period. For myelopathy, surgery may be considered sooner, especially when more severe neurologic deficits are present. Cross-sectional imaging should be obtained and demonstrate focal compression behind the disc space. Magnetic resonance imaging (MRI) provides excellent visualization of both the discs and neural elements, though computed tomography (CT) myelogram can be obtained when contraindications to MRI exist. Given that there is a high rate of disc pathology demonstrated in asymptomatic patients, it is important that imaging findings correlate appropriately with both the patients’ symptoms and physical exam findings.³⁷^,³⁸

21.2.2 Contraindications

There are several contraindications to arthroplasty. Because decompression is performed at the disc level, congenital or retrovertebral stenosis will not be alleviated by ACDA. Similarly, if neutral lateral radiographs show focal kyphosis, adequate decompression cannot be achieved with ACDA. When present, these pathologies are better addressed by anterior corpectomy, posterior decompression, or a combined approach.

All patients should be evaluated with CT to determine if there is ossification of the posterior longitudinal ligament (OPLL), which is a contraindication. OPLL is often associated with retrovertebral stenosis that would not be alleviated by ACDA. In addition, continued motion following arthroplasty may lead to bone formation posterior to an arthroplasty device in patients with OPLL, which may result in worsening stenosis and neurologic deterioration. Finally, OPLL may predispose the motion segment to spontaneous fusion despite arthroplasty, eliminating the advantage of this procedure over ACDF. Similarly, the presence of diffuse idiopathic skeletal hyperostosis (DISH) and ankylosing spondylitis may predispose the level to spontaneous fusion and therefore failure of the arthroplasty, and these are therefore also contraindications.

Posterior column instability is another important contraindication. In all cases, flexion-extension lateral radiographs should be obtained to rule out instability. Because the procedure involves resection of the anterior longitudinal ligament (ALL) and potentially the posterior longitudinal ligament (PLL), the additional presence of posterior instability may result in an unstable spinal column. Prior laminectomy is therefore considered a contraindication. Previous laminoplasty may also cause posterior instability, though this has not been investigated. The presence of fracture or posterior ligamentous disruption is an absolute contraindication. Finally, rheumatoid arthritis may result in laxity of posterior facet capsules and is considered a contraindication as well, except in rare cases with minimal rheumatoid involvement.

The presence and severity of neck pain is an important consideration. A patient with predominantly axial neck pain is not an ideal candidate for arthroplasty. While a degenerative disc may be a cause of axial neck pain with the potential for pain relief from arthroplasty, this has not yet been demonstrated. Similarly, facet arthrosis with associated neck pain is considered a contraindication. Such arthritic facets will continue to experience motion and therefore generate pain following arthroplasty. In contrast, the presence of facet arthrosis alone, in the absence of significant neck pain, is not considered a contraindication.

Finally, the quality of bone and local environment at the surgical level should be considered. Certainly, an active spine infection at the index surgical level is a contraindication. Introduction of the implant into an infected environment may make eradication more difficult and may lead to loosening and failure of the implant. Severe osteoporosis is also a contraindication as this too can lead to implant subsidence and failure.

21.2.3 Preoperative Planning

Patients with stenosis associated with disc pathology causing radiculopathy or myelopathy are potential candidates for ACDA. It is important that physical exams findings correlate with areas of compression demonstrated on cross-sectional imaging. MRI provides excellent visualization of both the discs and neural elements and is the preferred imaging modality. If contraindications to MRI exist, CT myelogram can be obtained. Radiographic imaging should include AP, neutral lateral, flexion/extension lateral, and oblique views. A complete neurological exam should be performed, including evaluation of motor and sensory function and appropriate reflexes for each nerve root level. Additionally, tests for cervical myelopathy should be performed including evaluation of gait and tandem gait, Romberg’s sign, Hoffmann’s sign, Lhermitte’s sign, finger escape sign, grip and release test, sustained clonus, and Babinski test.

Instrumentation

Fernström ³⁹ implanted the first cervical artificial disc in 1966, which consisted of a stainless steel ball-bearing implant. Another stainlesssteel device with a ball-in-socket metal-on-metal design was introduced in 1989 by Cummins et al,⁴⁰ but had a similar high failure rate. A redesign of this implant was introduced as the Frenchay artificial disc, which later became the Prestige Disc (Medtronic, Inc.).³¹ This metal-on-metal implant utilizes screw fixation to the vertebral bodies. In 1992, Bryan²⁰ introduced another arthroplasty implant, consisting of titanium shells with a polyethylene core surrounded by a saline-filled polyurethane sheath. This utilizes a press-fit design that does not require screw fixation. The Pro-disc C (Synthes, Inc.) was introduced by Marnay,²⁴ and utilizes a cobalt-chrome shell with a polyethylene articulating surface. This design included keels that are press fit into the vertebral bodies. Another cobalt-chrome and polyethylene implant, the Porous Coated Motion Disc Prosthesis (Nuvasive, Inc.), was introduced by McAfee.⁴¹ This device has a calcium phosphate coating that allows bone ingrowth to the implant after an initial press fit. Numerous other designs have subsequently been described, with nine devices currently with FDA approval.⁴¹ It is essential that the surgeon be familiar and comfortable with the chosen arthroplasty implant system.

21.2.4 Surgical Technique

Positioning

The patient is positioned supine on the operating table. We prefer to use a Jackson table with radiolucent flat table, though any radiolucent table can be used. A shoulder roll is placed under the shoulders, and sheets and foam are placed under the head as necessary to achieve a neutral neck position. Unlike ACDF where a more extended position is often preferred, this should be avoided in ACDA. Placing the neck in an extended position can result in excessive posterior vertebral body resection while creating parallel endplates in preparation for the arthroplasty implant. In contrast, kyphotic positioning should also be avoided, as this makes exposure and decompression more difficult and can lead to excessive anterior loading on the implants. Shoulders are taped down to allow better radiographic visualization of the lower cervical levels, and the head is secured to the table using tape across the forehead to minimize neck rotation. Care is taken to insure that the anterior neck is prepped and draped, keeping the bottom of the chin and sternal notch exposed as an additional alignment reference.

Exposure

The anterior cervical spine is exposed via the same interval described originally by Smith-Robinson³ ( Fig. 21.1). After exposure, the longus colli are used to initially identify midline on the vertebral bodies adjacent to the disc space, which is then marked with electrocautery. A combination of bipolar electrocautery and a Penfield #2 dissector is then used to elevate the longus colli bilaterally, exposing over the uncinate processes and foramen transversarium. Use of the Penfield to expose laterally minimizes potential risk of injury to the vertebral artery. A hemostat is utilized to clamp a small medial portion of the longus colli at the level of the disc space. Only a small portion is clamped in order to avoid injury to the sympathetic plexus. The correct level is then confirmed using a lateral fluoroscopic or radiographic view. The radiograph is also checked to insure that the correct patient is being operated on and that the segment is in proper alignment. Any anterior osteophytes are removed using a Leksell rongeur or burr such that it is smooth with the native vertebral body, which can be verified by digital palpation ( Fig. 21.2). Self-retaining retractors are then placed, which may be specific to the arthroplasty system being utilized. The retractor blades should be tucked under the previously elevated longus colli. Adequate elevation of longus colli and resection of osteophytes are key steps in allowing the retractors to be positioned appropriately.