Patient Positioning
At the time of cervical disc arthroplasty, the patient is positioned supine on a radiolucent table to allow imaging in both the anteroposterior and lateral planes in order to localize the affected levels and to provide guidance throughout the procedure (Fig. 1).
In order to optimize end plate preparation and the placement of the arthroplasty device, neutral positioning of the neck is critical when cervical disc arthroplasty is performed. If the neck is hyperextended, an excessive amount of the posterior end plate needs to be removed to produce parallel surfaces during end plate preparation. This can result in a malpositioned, kyphotic prosthesis, especially in patients who are managed with an unconstrained device (Figs. 2 and 3). Conversely, if the neck is placed in a kyphotic position, too much of the anterior end plate is removed to produce parallel surfaces during end plate preparation and the prosthesis assumes an excessively lordotic position1,13—16. In order to prevent placing the disc in a scoliotic alignment, the neck should also be oriented squarely so that the spinous processes lie equidistant between the pedicles on anteroposterior images.
Exposure
The anterior part of the cervical spine is exposed with use of a standard Smith-Robinson approach. A retractor system is selected according to surgeon preference and, after the spine is exposed and the level of the abnormality is identified, the longus colli is elevated bilaterally. The midline is marked with unipolar electrocautery, which is then used to separate the muscle from its attachment to bone and the disc. A Penfield number-2 dissector is then used to elevate the longus colli further laterally in order to prevent inadvertent injury to an aberrant vertebral artery, which can occur if unipolar electrocautery is used in this area. The longus colli is elevated past the uncinate process and over the foramen transversarium. This allows for thorough decompression of the uncinate region as well as accurate confirmation of the midline and the lateral extent of the vertebral body and the uncinate processes 1,13—16.
Osteophyte Removal
Once the level of interest has been thoroughly exposed, all anterior osteophytes are removed with a burr or a Leksell rongeur such that the anterior margin of the disc space is flush with the rest of the vertebral body (Fig. 4). Careful removal of these osteophytes is necessary as any remaining osteophytic lipping at the disc space may lead to errors in placement of the arthroplasty device. Palpation of the disc space is often the best way to determine if osteophyte removal is adequate1.
Initial Disc Space Preparation
The anterior anulus is incised with use of a number-15 scalpel; pituitary rongeurs and small curettes (typically 3 or 4-mm up-angled curettes) are used to remove the initial disc material and fragments. While the disc material is being removed, the surgeon should be continuously mindful of the lateral border of the vertebral body in order to avoid inadvertently damaging the vertebral artery. Once the initial discectomy has been performed, the lateral borders of the uncinate processes are carefully identified. A small, 2-mm up-angled curette is used to initially identify the uncinate processes and their lateral extent and to denude the uncovertebral joints of any disc and cartilaginous material. When this maneuver is performed, care should be taken to prevent accidental injury to the vertebral artery, which can occur if the curette is used to scrape in a lateral direction1,13—16. The extent of the initial disc space preparation is shown in Figure 5.
Identification of the Midline
Confirmation of the midline location can be accomplished at this juncture by looking at the two denuded uncinate processes (Fig. 5).
End Plate Preparation
At this point, either end plate preparation or decompression is performed. For devices that allow the decompression to take place before end plate preparation is performed, pins are placed into the vertebral bodies in the midline to facilitate distraction across the disc space. Once the pins have been inserted and distraction has been applied, the burr is then used to flatten the end plates. This is done by first removing any remaining cartilage and by lightly decorticating the inferior end plate of the cephalad vertebra. The end plate initially is decorticated anteriorly and then is decorticated progressively more posteriorly down to the posterior longitudinal ligament, with removal of any remaining disc material, including the posterior anulus, during the process. A similar technique is then used to remove any remaining cartilage and to minimally decorticate the superior end plate of the caudad vertebra1,13—16.
Decompression
For disc arthroplasty to be successful, a thorough decompression must be performed and all posterior vertebral and uncinate osteophytes must be removed (Figs. 6-A through 7-D). We prefer to resect the osteophytes with a burr in order to avoid additional trauma to the spinal cord, which is especially important in the setting of myelopathy. A side-cutting matchstick burr is preferred so that the tip can rest lightly on the posterior longitudinal ligament (and even briefly on the dura) without damaging the soft tissues (Fig. 7-A). Once the posterior osteophytes have been removed, bone wax is applied to the bleeding cancellous bone to achieve hemostasis as well as to prevent ossification that may compromise postoperative segment motion1. If a large disc herniation is present on preoperative imaging studies, the sequestered invaginated disc fragments behind the posterior longitudinal ligament should be identified and removed (Fig. 7-D). While some surgeons prefer to resect the posterior longitudinal ligament in all cases, we usually thin it down and leave it in place unless there is a large disc fragment lying behind the ligament. In addition, the posterior longitudinal ligament may need to be preserved for stability when some disc replacement designs are used, and the surgeon should be aware of these specific requirements. If the preoperative imaging studies reveal foraminal stenosis, the stenotic foramina are decompressed once the central decompression has been completed (Fig. 7-C). First, any remaining cartilage within the uncovertebral joint is removed with use of a microcurette to determine the lateral margin of the uncinate process. A matchstick burr is then used to decompress the stenotic foramen. Although we prefer a burr for this task, a small (1-mm) Kerrison rongeur can be used instead1. The disadvantage of using a Kerrison rongeur is that the tip of the device must be placed blindly into the foramen and can inadvertently injure the vertebral artery or nerve root, especially if the foramen is already severely narrowed. Great care must be taken to avoid injury to these structures during the foraminal decompression. Occasionally, a partial or subtotal uncinate process resection is necessary to achieve a thorough foraminal decompression; however, bilateral complete resection of the uncinate processes should be avoided because of their contribution to overall spinal stability and kinematics. Biomechanical studies have shown that cervical range of motion increases substantially in all planes with progressive resection of the uncovertebral joints but, whereas resection of one uncinate process allows preservation of rotational stability with an unconstrained prosthesis, resection of both may lead to spinal instability1,23,24. Importantly, we use copious irrigation to remove any bone dust that results from burring and we apply wax to all bleeding surfaces of the uncinate process in order to prevent unintended fusion.
Implant Insertion
Once the neural elements have been thoroughly decompressed and the end plates have been prepared, sizing trials are used (with most systems) to determine the appropriate size of the arthroplasty device1,9—22. The trial prosthesis should fit snugly within the disc space; however, if it is too tight, the ligamentous structures around the motion segment may be too taut, possibly causing posterior neck pain and limiting motion. Conversely, if the device is too loose, the intervertebral height may not be adequate, resulting in foraminal stenosis and poor function of the device. Therefore, care must be taken to ensure the selection of an appropriately sized implant. In most cases, a 5 or 6-mm height is appropriate. Once the proper size has been determined, slots or mortises may need to be fashioned in the vertebral body to allow for the placement and fixation of the arthroplasty device. The definitive device is then inserted into the interspace, usually under fluoroscopic guidance (Fig. 8). Final placement of the device is then confirmed radiographically in both the anteroposterior and lateral planes (Figs. 9-A and 9-B). If required, screws are placed with use of standard techniques. The wound is then irrigated and closed in the customary fashion. Postoperative immobilization typically is not necessary.
Radiculopathy attributable to cervical disc degeneration at one, two, or three levels or myelopathy due to cervical discdegeneration and minimal spondylotic changes at one, two, or three levels with retrodiscal spinal cord compressionRadiographic evidence of cervical disc herniation or spondylosis at one, two, or three levelsSymptoms corresponding to anatomical findings between C3 and C7Failure of nonoperative treatment (after a minimum of six weeks, but most commonly after three months)
Radiculopathy attributable to cervical disc degeneration at one, two, or three levels or myelopathy due to cervical discdegeneration and minimal spondylotic changes at one, two, or three levels with retrodiscal spinal cord compression
Radiographic evidence of cervical disc herniation or spondylosis at one, two, or three levels
Symptoms corresponding to anatomical findings between C3 and C7
Failure of nonoperative treatment (after a minimum of six weeks, but most commonly after three months)
CONTRAINDICATIONS1,6,9—22:
Structural instability of the cervical spine, acute fracture, rheumatoid arthritis with instability, or previous cervical laminectomy, which could lead to instability after the procedureSevere spondylosis with complete loss of disc height or motion of <2°, ankylosing spondylitis, or diffuse idiopathic skeletal hyperostosis, which could limit the amount of motion following the procedureCongenital stenosis, ossification of the posterior longitudinal ligament, or myelopathy due to any other etiology that causes ret rove rte bra l compressionAxial neck pain as the solitary symptom (because axial neck pain, which is often due to facet arthropathy and/or disc degeneration, does not predictably resolve following cervical disc arthroplasty)A history of recent cervical spine infectionOsteoporosis and related metabolic bone diseases, which may preclude osseous growth into the arthroplasty device, leading to its looseningMorbid obesity that precludes an anterior cervical approachInability to visualize the treated segment radiographically during surgery
Structural instability of the cervical spine, acute fracture, rheumatoid arthritis with instability, or previous cervical laminectomy, which could lead to instability after the procedure
Severe spondylosis with complete loss of disc height or motion of <2°, ankylosing spondylitis, or diffuse idiopathic skeletal hyperostosis, which could limit the amount of motion following the procedure
Congenital stenosis, ossification of the posterior longitudinal ligament, or myelopathy due to any other etiology that causes ret rove rte bra l compression
Axial neck pain as the solitary symptom (because axial neck pain, which is often due to facet arthropathy and/or disc degeneration, does not predictably resolve following cervical disc arthroplasty)
A history of recent cervical spine infection
Osteoporosis and related metabolic bone diseases, which may preclude osseous growth into the arthroplasty device, leading to its loosening
Morbid obesity that precludes an anterior cervical approach
Inability to visualize the treated segment radiographically during surgery
PITFALLS:
Neutral positioning of the neck during the procedure is critical. If the neck is hyperextended, an excessive amount of the posterior end plate may be removed to produce parallel surfaces during end plate preparation, resulting in a prosthesis that rests in a kyphotic position. If the neck is placed in a kyphotic position, too much of the anterior end plate is removed and the prosthesis may rest in a lordotic position.
Successful disc space preparation requires only light decortication of both end plates, removal of all posterior vertebral osteophytes, and thorough foraminal decompression.
Accurate identification of the midline is required. Furthermore, midline placement of distraction pins that are used during the decompression is critical in order to avoid asymmetric distraction across the disc space, with resultant asymmetric decompression and end plate preparation, the potential development of uneven loads across the segment, and, in severe cases, a scoliotic deformity.
End plate preparation must be performed carefully and judiciously in order to minimize excessive end plate resection because the majority of the end plate should be preserved to support the stresses associated with motion across the disc space and to prevent end plate subsidence. Furthermore, the various arthroplasty systems have different requirements for end plate removal, with which the surgeon should be familiar.
A wider and more thorough uncinate process and osteophyte resection is necessary when a cervical disc replacement is performed in patients with myelopathy (and even in patients with spondylotic radiculopathy) than is the case when a fusion is performed in such patients. If the decompression is inadequate, continued motion across the segment may lead to recurrence of symptomatic spondylosis; in contrast, after cervical fusion procedures, osteophytes often regress once a solid fusion has been obtained.
Sizing should be critically assessed to allow as large an end plate footprint as possible. Overdistraction should be avoided.
AUTHOR UPDATE:
Cervical disc arthroplasty continues to be a successful procedure for the treatment of myelopathy when spinal cord compression is localized to the disc space (i.e., when compression is retrodiscal in nature) and mild spondylotic changes (or no such changes) are present. For these reasons, in general, the procedure is most successful for patients younger than those with cervical spondylotic myelopathy (who frequently present with substantial degenerative changes such as facet arthropathy, which is a contraindication to cervical disc arthroplasty).