Cervical Disc Replacement

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CHAPTER 43 Cervical Disc Replacement

Joseph D. Smucker, MD, Rick C. Sasso, MD

Cervical arthroplasty has undergone a dramatic evolution since the development of the original Bristol/Cummins device. Metal-on-metal implants have evolved in parallel with the development of novel bearing concepts incorporating metal alloys, polyethylene, and ceramics. This chapter presents the current state of this technique, including the results of early outcomes of more recently developed devices. Although early data from clinical trials are encouraging, the viability of such techniques needs to be shown in the long-term.

Background

The cervical spine consists of seven vertebral bodies with intervening discs. These discs function in load bearing and motion transfer. In addition to its biomechanical functions in motion, the cervical spine serves as the protective passage for the spinal cord and vertebral arteries. Much is known about the macrobiology of the intervertebral disc. Disc degeneration and the subsequent processes that ensue in the cervical spine are also well documented as the transition from mild degenerative disc disease to cervical spondylosis progresses. For many years, the surgical treatment for pathology in the cervical intervertebral disc has been limited to procedures that remove pathologic disc material and address the bony and neurologic pathology in the region of the excised disc.

Anterior cervical discectomy and fusion (ACDF) is a proven intervention for patients with radiculopathy and myelopathy.¹ It has served as the standard by which other cervical and spinal disorders may be judged as the result of its high rate of success. The success of this technique is often judged based on its consistent ability to relieve symptoms related to neurologic dysfunction. In this sense, the clinical results with regard to the patient’s index complaint are outstanding. The radiographic results of this technique are also initially predictable with a high rate of fusion. Plating techniques have diminished the need for postoperative immobilization or eliminated them entirely.² Because of limitations specific to this procedure, investigators have developed surgical alternatives to fusion that attempt to address the kinematic and biomechanical issues inherent in it.

A major concern related to the treatment of cervical degenerative disc disease and spondylosis with ACDF is the issue of adjacent segment degeneration. This event is manifest as the radiographic appearance of degenerative change at a level directly above or below a level treated with a surgical intervention—typically being associated with degeneration of a level adjacent to a fused level. The incidence of this phenomenon has been reported to be 92% by Goffin and colleagues,³ who wrote a long-term follow-up on patients after treatment with anterior interbody fusion.

Although some debate remains regarding the causation of adjacent segment degeneration—with a mix of postsurgical and naturally determined aging cited as root causes—there is little debate regarding the existence of this phenomenon. It is also relevant to note the clinical distinction between adjacent segment degeneration and adjacent segment disease. Adjacent segment disease is defined as adjacent segment degeneration that causes clinical symptoms (pain or neurologic disorders or both) severe enough to lead to patient complaint or require operative intervention.⁴ Although this distinction has not remained consistent in published literature, it is an important consideration with regard to the phenomenon that occurs in discs adjacent to discs that have undergone a surgical intervention. Numerous studies have made a consistent point of distinguishing between radiographic degeneration and symptomatic disease.^3,⁵

There is clinical evidence to support the postsurgical nature of adjacent segment disease. In patients previously treated with fusion, adjacent segment disease has been documented at a rate of 2.9% of patients per annum by Hilibrand and colleagues,⁴ and 25% of patients undergoing cervical fusion have new onset of symptoms within 10 years of fusion. This study has received a great deal of attention and has led to further investigations into biomechanical causation. Other reports have focused on the recurrence of neurologic symptoms and degenerative changes adjacent to fused cervical levels.^3,⁶ The concept that adjacent levels need to compensate for loss of motion in the fused segment may also be valid. Segments adjacent to a fusion have an increased range of motion and increased intradiscal pressures.^7,⁸

Bone graft materials used in traditional ACDF procedures have also been a source of controversy. Current ACDF techniques make use of allograft bone, premanufactured allograft bone, and autologous iliac crest. Complications associated with autologous iliac crest harvest used as a fusion graft in ACDF are well documented. Sandhu and colleagues ⁹ reported a complication rate of 1% to 25% with such procedures. Complications such as acute and chronic pain, infection, meralgia paresthetica, and pelvic fracture are known to occur at harvest donor sites.^10,¹¹

Although allograft removes the risks associated with the harvest of autograft, it has the detriment of having the theoretical risk of disease transmission. In practice, this risk is believed to be extremely minute, although the U.S. Food and Drug Administration (FDA) has taken this issue seriously. The issues of disease transmission and contaminated graft materials have been highlighted by allograft tissue recalls by the FDA in recent years.¹² Although bone graft substitutes may play a role in the future practice of ACDF, this continues to be a minority stake in the overall graft selection of modern surgeons.

Pseudarthrosis is another complication encountered with anterior cervical fusion procedures. Pseudarthrosis is the failure of bony bridging or nonunion of a segment that has previously been treated with a bone graft or a bone graft substitute—an attempt at fusion has been made. In multilevel ACDF procedures, there is a relationship between the rate of pseudarthrosis and the number of levels fused. Brodke and Zdeblick ¹³ reported a 97% fusion rate in single-level ACDF, which decreased to 83% with fusion at three levels. Bohlman and colleagues¹ reported an 11% pseudarthrosis rate in single-level fusions that increased to 27% with multilevel fusions.

In recent years, the use of bone morphogenetic proteins has been proposed as an alternative or adjunct to traditional bone grafting techniques to combat the pseudarthrosis issue in patients deemed to be at higher risk for this complication.^14,¹⁵ This off-label use has been associated with an increased incidence of swelling complications and concerns for graft resorption and migration of interbody implants.¹⁶^–¹⁹ These issues serve to strengthen the argument for fusion alternatives in the treatment of discogenic pathology in the anterior cervical spine.

Total intervertebral disc replacement (TDR) is designed to preserve motion, avoid limitations of fusion, and allow patients to return quickly to routine activities. The primary goals of the procedure in the cervical spine are to restore disc height and segmental motion after removing local pathology that is deemed to be the source of a patient’s index complaint. A secondary intention is to preserve normal motion at adjacent cervical levels, which may be theorized to prevent later adjacent level degeneration. Cervical TDR avoids the morbidity of bone graft harvest.^20,²¹ It also avoids complications such as pseudarthrosis, issues caused by anterior cervical plating, and cervical immobilization side effects.

History of Disc Arthroplasty and Device Design

An understanding of the evolution of cervical TDR serves as an important lesson in the concepts of device design, TDR bearing and wear characteristics, and articular constraint. In the late 1980s, Cummins and colleagues ²² developed a metal-on-metal ball-and-socket cervical disc replacement composed of 316L stainless steel. With the acquisition of this technology and the later development of new metal-on-metal devices came a rapid transition from this device to the most recent device, the PRESTIGE LP (Medtronic Sofamor Danek, Memphis, TN). A predecessor of this device, the PRESTIGE ST (Medtronic Sofamor Danek, Memphis, TN) is currently approved by the FDA for human use in the United States (Figs. 43-1 and 43-2). More recent additions to the metal-on-metal category of arthroplasty devices include the Kineflex-C disc (Spinal Motion, Mountain View, CA) and the CerviCore intervertebral disc (Stryker Spine, Allendale, NJ) (Figs. 43-3 and 43-4), which are in the process of U.S. FDA investigational device exemption (IDE) trials.

FIGURE 43–1 PRESTIGE ST cervical disc prosthesis is currently approved by the FDA for use in the United States. This stainless steel uniarticulating device attains primary fixation to the vertebral bodies via use of locked screws.

(Implant representations courtesy Medtronic Sofamor Danek, Memphis, TN; with permission.)

FIGURE 43–2 PRESTIGE ST prosthesis in C5-6 arthroplasty. Lateral flexion and extension radiographs show motion through arthroplasty device in this postoperative patient.

(Courtesy Medtronic Sofamor Danek, Memphis, TN; with permission.)

FIGURE 43–3 CerviCore cervical disc prosthesis on lateral and expanded anteroposterior and lateral views. Initial fixation is obtained via vertical rails on the endplates.

(Courtesy Stryker Spine, Allendale, NJ; with permission.)

FIGURE 43–4 CerviCore cervical disc prosthesis is shown ex vivo and on T2-weighted MRI. MRI may show some artifact in the region of an arthroplasty device.

(Courtesy Stryker Spine, Allendale, NJ; with permission.)

Numerous devices have evolved in parallel to the metal-on-metal implants, including the BRYAN cervical disc (Medtronic Sofamor Danek, Memphis, TN) (Figs. 43-5 through 43-7), the PCM (CerviTech, Rockaway, NJ), the DISCOVER (DePuy Spine, Raynham, MA), and the MOBI-C (LDR, Austin, TX). Each of these devices is in the process of limited human trials or U.S. FDA IDE submission and represents an alternative to metal-on-metal bearing surfaces, which have the potential for metal debris and systemic concentration of metal ions. To date, one such device, the Prodisc-C (Synthes Spine, Paoli, PA) (Figs. 43-8 and 43-9), has obtained approval for use in the United States.

FIGURE 43–5 BRYAN cervical disc prosthesis is shown ex vivo and in unassembled form. The endplates of this device are unique in their design and promote ingrowth of bone into metallic surface. At the time of this publication, the BRYAN device remains under FDA review.

(Courtesy Medtronic Sofamor Danek, Memphis, TN; with permission.)

FIGURE 43–6 BRYAN cervical disc prosthesis is visualized on postoperative MRI. Titanium alloy devices such as the BRYAN device may have less MRI artifact than similar devices constructed with cobalt-chromium or stainless steel. These images show the imaging characteristics of this device at index and adjacent surgical levels.

(Courtesy Rick Sasso, Indianapolis, IN.)

FIGURE 43–7 Upright lateral view of a patient who underwent successful cervical arthroplasty at C5-6 with a BRYAN device.

(Courtesy Medtronic Sofamor Danek, Memphis, TN; with permission.)

FIGURE 43–8 Ex vivo image of Prodisc-C. This device is approved by the FDA in the United States for cervical arthroplasty and obtains initial fixation via a central keel. Bone ingrowth is promoted via the surface alterations of the superior and inferior endplates of this device.

(Courtesy Synthes Spine, Paoli, PA; with permission.)

FIGURE 43–9 Prodisc-C is visualized on lateral flexion-extension radiographs. The device retains motion at index surgical level in this patient successfully treated with arthroplasty at C5-6.

(Courtesy Synthes Spine, Paoli, PA; with permission.)

A summary of the design characteristics of each of these devices is presented in Table 43–1. Although the ideas of bearing surface, wear debris, and constraint are not new to discussions with regard to arthroplasty in general, they are relatively new in regard to the spine. A full understanding of the term constraint has not been agreed on because constraint may arise within the device or as a result of the local anatomy (e.g., facets, posterior longitudinal ligament). As the knowledge base in spine TDR increases, intelligent investigations and discussions are sure to include many of these concepts and may redefine understanding of them.

TABLE 43–1 Design Characteristics of Past and Present Cervical Arthroplasty Devices

It is relevant to understand the fact that the load borne by devices in the cervical spine is dissimilar to the load borne in the lumbar spine. The biomechanical environment of the cervical spine has been taken into account in the design of the current generation of these devices. As intermediate-term and long-term studies on individual devices become available, the design concepts of these initial devices will have the opportunity for continued examination in their in vitro environment.

Indications for Use, Contraindications, and Complications

Cervical disc arthroplasty trials have included patients refractory to nonoperative treatment modalities with and without radiculopathy or myelopathy or both and with one-level and two-level degenerative disc disease or spondylosis.²³^–²⁵ These indications have been retained throughout the FDA approval process. At the time of this writing, two devices, the PRESTIGE ST and the Prodisc-C, have achieved FDA approval for single-level use in the United States. Other devices are in various stages of the IDE and approval process (Table 43–2). ACDF may be discussed as part of the indication process for an arthroplasty procedure. The historical challenges associated with ACDF presented in this chapter may be weighed against the early nature of data with respect to cervical arthroplasty in a patient’s informed consent discussion.

TABLE 43–2 Current Status of Cervical Arthroplasty Devices in the United States *

Device	Manufacturer	U.S. FDA Status
PRESTIGE-ST	Medtronic Sofamor Danek	Approved
Prodisc-C	Synthes Spine	Approved
BRYAN disc	Medtronic Sofamor Danek	IDE data submitted, approval pending
CerviCore disc	Stryker Spine	IDE in progress
DISCOVER disc	DePuy Spine	IDE in progress
Kineflex-C disc	Spinal Motion	IDE in progress
MOBI-C disc	LDR	IDE in progress
PCM disc	Cervitech	IDE in progress
PRESTIGE-LP	Medtronic Sofamor Danek	IDE in progress
SECURE-C	Globus Medical	IDE in progress

IDE, investigational device exemption.

* Table current as of April 1, 2009.

In determining indications for cervical arthroplasty of any type in a patient, it is relevant and appropriate to discuss verbally and obtain written consent for an intraoperative alternative to arthroplasty. In current practice, ACDF with plating and anterior corpectomy and fusion with plating remain options when it becomes clear to the operating surgeon that placement of an arthroplasty device may be compromised. This judgment to proceed with a fusion may occur as the result of endplate defects; arthroplasty sizing and fixation issues; or other bony, vascular, or neurologic issues that would prevent the appropriate placement of the device.

The appropriate time to discuss complications related to cervical arthroplasty is at the time of informed consent. The approach-related risks of cervical arthroplasty are similar to ACDF and should be discussed as such. These risks have been adequately covered in other portions of this text. A unique risk of arthroplasty is the concern of heterotopic ossification in the region of the arthroplasty device.²⁶^–³⁰ Heterotopic ossification may result in loss of motion or frank fusion of the index level. Heterotopic ossification may be associated with the physiologic response to the implantation process, the amount of bleeding or hemostasis necessary in a particular procedure, or the amount of bone debris created at the time of preparation for implantation. Postoperative use of nonsteroidal anti-inflammatory drugs has been suggested to moderate the prevalence of this risk.

Other complications may occur with cervical arthroplasty that are independent of the anterior cervical approach, including infection, implant migration, subsidence, continued or new neurologic findings, vascular injury, dural injury or cerebrospinal fluid leak, hematoma, and reoperation for adjacent level disease. Many complications associated with placement of arthroplasty devices have come to light as the result of reporting and analysis of the prospective randomized multicenter U.S. FDA IDEs and large studies performed outside the United States.²³^–²⁵ ^,31 ^,32

Cervical arthroplasty is contraindicated in patients with active or prior infection, osteoporosis or poor host bone, segmental cervical instability or segmental kyphosis, trauma, tumor, primary axial neck pain, significant facet arthropathy, posterior neurologic compression, anterior soft tissue abnormality (e.g., tracheal or esophageal abnormality, prior radiation), allergy to any of the device materials, severe spondylosis, pediatric patients, compromised vertebral body morphology, or (presently) disease involving more than one level. Other relative contraindications are similar to ACDF with the exception of nicotine use.

Preoperative Imaging

At a baseline, preoperative imaging for cervical arthroplasty should include plain radiographs with anteroposterior, neutral lateral, odontoid, and lateral flexion-extension films. The flexion-extension lateral views serve as a preoperative assessment of normal and abnormal mobility at the index and adjacent surgical levels. An advanced imaging modality such as magnetic resonance imaging (MRI) or computed tomography (CT) with the possible addition of myelography can be crucial to evaluating the index surgical level. These studies combine to allow assessment of spondylosis, preexisting facet arthropathy, neurologic compression, and cause and may provide insight into any preoperative contraindications in candidates for cervical arthroplasty.

Technique of Implantation

The technique of anterior cervical discectomy and the anterior approach to the spine are beyond the scope of this chapter. The following description assumes a surgeon’s comfort with this technique and suggests only specific modifications to the current approach relevant to anterior cervical arthroplasty compared with ACDF.

Intraoperatively, patient position is important. A “physiologic” or slightly lordotic cervical spine position is preferred.³³ Assessment via fluoroscopy (C-arm) is crucial to patient positioning and implant insertion and fixation portions of these procedures. It is important to keep the head, neck, and shoulders in a stable and neutral position throughout this surgical procedure. A small towel roll may be placed under the neck to assist with appropriate positioning of the neck and shoulders and to keep a physiologic lordosis without creating a hyperlordosis (Fig. 43–10). This positioning technique differs from the typical placement of a roll under the shoulders or thoracic spine, which could place the cervical spine in hyperlordosis. The head is placed on a doughnut-type pillow or a folded towel to keep it from rolling during the procedure. The careful positioning of shoulders with a taping technique can also allow for less motion during this procedure and must be carefully weighed against the risk of traction to the shoulders. Taping of the shoulders differs from the commonly used wrist restraints in a typical ACDF procedure that are used to obtain additional longitudinal traction via a temporary “pull” on the arms.

FIGURE 43–10 Positioning for cervical arthroplasty is as crucial to the technique as any portion of the procedure. Correct positioning of a patient maintains physiologic lordosis without creating hyperlordosis in the cervical spine and may be facilitated through use of a towel roll placed under the cervical spine. Techniques have moved away from traction through spine (as shown in this picture). Preoperative and intraoperative use of fluoroscopy allows for confirmation of patient positioning and device alignment.

(Courtesy Rick Sasso, Indianapolis, IN.)

A standard right- or left-sided Smith-Robinson approach may proceed with appropriate localization and exposure of the index surgical level being the intent of this exposure. It is crucial to obtain a surgical exposure that allows for identification of the center of the index disc and vertebral bodies for later placement of the arthroplasty device. Disc arthroplasty is performed only after adequate decompression of the affected cervical level. At the surgeon’s discretion for treatment of the index neurologic complaint, this may involve a complete discectomy from ventral to dorsal that also allows for placement of a device of appropriate width and adequate decompression, symmetrical resection of uncovertebral osteophytes and spurs, resection of all or part of the posterior longitudinal ligament, and any resection of central spondylotic osteophytes associated with degenerative disc disease. Meticulous hemostasis is recommended throughout this procedure to diminish the blood loss and minimize the risk of heterotopic ossification. It may become clear at any point during the neurologic decompression, endplate preparation, or device trialing process that arthroplasty is contraindicated. Should this occur, the surgeon must adjust the surgical plan intraoperatively and proceed with a fusion-based alternative.

After neurologic decompression, assessment for placement of an appropriately sized disc and planning for proper orientation of the implant are crucial to successful arthroplasty. To this end, it should be the surgeon’s goal to place as large a device (with respect to diameter) as possible in the prepared space.³⁵ Device-specific tools may aid in this assessment.

Before any intervention that prepares the endplates, it is important to ensure the exact sagittal position of the vertebrae with lateral fluoroscopic imaging. Anteroposterior views are important to place the spinous processes at the target disc level between the pedicles to ensure perfect alignment and centering in the coronal plane. Sizing of a cervical arthroplasty device may be determined with a combination of preoperative templates and preoperative radiographic studies including CT. The use of intraoperative trials and fluoroscopic imaging allows for additional assessment of proper device sizing and placement in the coronal and sagittal planes.

Endplates are prepared in a manner consistent with the device to be implanted. This preparation may include milling of the endplate (as in the BRYAN technique) or creation of a bony trough to accommodate an endplate keel (as in the Prodisc-C technique). Preservation of subchondral bone is otherwise crucial to the prevention of implant subsidence. Instrumentation specific to each arthroplasty device may be of great assistance in endplate preparation and may include special endplate distracters, keel preparation mills, rasps, and endplate mills. After the endplate preparation has been completed, it is appropriate to reassess the centering of the preparation and recheck the neurologic decompression.

Insertion of the artificial disc device may proceed and is implant-specific. Common to all devices is the principle of implantation to an appropriate depth based on implant design, with a repeat assessment of implant centering and endplate coverage. After an assessment of the implant position in the coronal and sagittal planes has been done, the implant may be fixed to the spine with any implant-specific instrumentation such as screws.

Final imaging of the device implantation is performed before wound closure. Hemostasis is rechecked, and the surgical wound is closed in a standard fashion. Postoperative immobilization is not required. Upright flexion-extension radiographs may be obtained before discharge from the hospital and serve as a comparison to postdischarge radiographs for the purposes of follow-up.

Postoperative Imaging

Follow-up imaging of arthroplasty devices is crucial to the assessment of motion retention, adjacent segment disease, device wear and settling, device fixation, and neurologic decompression and status. Because the current generation of disc arthroplasty devices retains metallic components either in the endplates or in the bearing mechanism, radiation-based technologies have predominated as a mechanism of assessment. The workhorse studies remain flexion-extension and lateral bending plain radiographs because they are easily accessible, require less technician and physician technique in acquisition, maintain the ability to assess motion, eliminate the concerns of claustrophobia with MRI and CT, and are associated with moderate patient risk from radiation exposure. CT and CT myelography require an increased dose of radiation and are technique driven with respect to myelography.

To moderate the risks of technique and radiation associated with CT myelography, MRI has been proposed as an alternative that allows for postoperative assessment of neurologic status adjacent to and at the level of a prior cervical arthroplasty procedure. Success with this technique has been described by Sekhon and colleagues ³⁴ in several devices, including the BRYAN and the PRESTIGE LP arthroplasty devices. Both of these devices use titanium alloy in their endplates, which was shown to produce less MRI artifact at the index and adjacent surgical levels than the metals associated with the manufacture of the Prodisc-C and PCM arthroplasty devices.³⁴ The BRYAN prosthesis has a polyurethane core, and the PRESTIGE LP had a titanium carbide alloy (as tested in the study). Investigators had difficulty assessing the neural structures at the index and adjacent levels with devices manufactured with nontitanium metals (cobalt-chromium-molybdenum) used in the Prodisc-C and PCM devices.

Although the Prodisc-C and PCM devices have nonmetallic bearing surfaces, the nontitanium nature of their metallic alloy seemed to be the major factor in the decreased ability of traditional MRI techniques to image neural structures. This information may prove to be crucial in the choice of implants for situations in which there is a need to assess the adequacy of neural decompression and monitoring of adjacent segment disease. Although this study did not examine all devices on the market, such as those made of 316 stainless steel or other cobalt-chromium devices, it makes a strong case for careful consideration of device materials to be used in the future manufacturing processes of cervical arthroplasty devices.

Clinical Studies

The long-term clinical benefit of maintenance of motion is postulated to be delay or avoidance of adjacent level degeneration. All of the arthroplasty systems discussed herein are being investigated for use in the cervical spine. Although some of these systems have early published outcome data or have obtained U.S. FDA approval for use in single-level arthroplasty, long-term outcome studies are still pending.

PRESTIGE Disc

Wigfield and colleagues ³⁵ reported favorable results on a 2-year pilot study of the first-generation PRESTIGE disc designed to address the safety of the technique and to assess the stability of, and motion allowed by, the device. The investigators tried to target patients most at risk for adjacent segment disease. Inclusion in the study required radiculopathy or myelopathy secondary to herniated disc or uncovertebral osteophytes confirmed on CT or MRI adjacent to a surgically or congenitally fused segment. An additional inclusion category was patients with asymptomatic disc degeneration adjacent to the symptomatic level without presence of a fusion. The study enrolled 15 patients. Wigfield and colleagues³⁵ concluded that the technique is safe because procedural complications were limited to two cases of transient hoarseness that resolved.

Motion was successfully preserved because all patients radiographically showed motion within an appropriate physiologic range. At 2 years, the mean motion in flexion-extension was 6.5 degrees (range 1 to 15 degrees). Anterior-posterior translation up to 2 mm was obtained. Device stability was concluded because no devices dislocated. Two of the 60 screws inserted broke mid-shaft at 6 months allowing settling of the caudal component. No other cases of settling were noted. The locking screws worked well, and no screw backout occurred.

A concerning finding is a lucent line that developed at the junction of the vertebral endplate and the anterior vertebral border suggestive of stress shielding. This lucency did not progress after 12 months. One patient required removal of the device and conversion to fusion for continued neck pain in extension. That device was found to be loose with surrounding fibrous tissue; however, there was no histologic evidence of infection, inflammation, or wear debris. Functional improvement was documented by improvements in visual analog scale (VAS) arm and neck scores, Oswestry Neck Disability Index (NDI) scores, and Short Form-36 (SF-36) scores at 2 years compared with preoperative scores. Statistical significance was not obtained because of the small number of patients in the study.

In a separate prospective nonrandomized study, Wigfield and colleagues ³⁶ compared the effects of the PRESTIGE disc and one-level anterior fusion on adjacent segment motion. No significant difference in adjacent segment motion was measured between the two groups preoperatively. Postoperatively, there was a significant increase in adjacent segment motion in the fusion group (mean increase of 9 degrees) compared with a slight reduction in adjacent level motion noted in the TDR group. In the fusion group, adjacent segment motion increased 5% at 6 months and 15% at 1 year. Subgroup analysis showed that increased motion occurs predominantly in normal rather than degenerative adjacent discs.

A prospective, randomized clinical trial was conducted by Porchet and Metcalf ³⁷ to compare the PRESTIGE II cervical disc with ACDF for the treatment of single-level degenerative disease. Standardized clinical outcome measures (NDI and SF-36) and radiographic examinations were used at prescribed postoperative intervals to compare the treatment groups. Of the 55 patients enrolled in the study, several had reached the final 24-month follow-up interval at the time of publication. Mean angular motion was 5.9 degrees at the disc level. Radiographic results showed that the PRESTIGE II disc maintained motion at the treated level without compromise of an adjacent segment. The authors concluded that use of the PRESTIGE II disc is as safe and as efficacious as standard ACDF at 24 months.

The most extensive report to date shows the results of the PRESTIGE ST device in a prospective randomized multicenter clinical trial.²³ Data from this report have served as the basis for the current FDA approval of the PRESTIGE ST device in the United States. In this one-to-one randomization protocol, patients underwent either single-level arthroplasty or single-level ACDF. Data were reported up to and including 24 months and showed that the PRESTIGE ST device compared favorably with ACDF with regard to the study’s primary and secondary outcome measures. In addition, the device maintained an average of 7 degrees of angular motion and had no device failures or migration.

The study reported statistically significant differences in neurologic improvement, NDI, secondary surgical interventions, SF-36 scores, and neck pain in the investigational group compared with the control group at follow-up intervals of 12 and 24 months. Return to work was shortened in the investigational group 16 days sooner than the control group. Adjacent segment disease reoperations over the interval were decreased in the investigational group. The study concluded that arthroplasty with this device was safe and efficacious.

Mummaneni and colleagues ³¹ published their clinical experience with the PRESTIGE LP device. This study focused on history, indications, patient positioning, surgical technique, complication avoidance, and revision strategies. As further evidence becomes available, the manufacturers of the PRESTIGE LP device hope to advance further on the early favorable results obtained by its predecessor, the PRESTIGE ST.²³

Prodisc-C

Bertagnoli and colleagues ³⁸ published 1-year follow-up data on 27 patients who had single-level Prodisc-C implantation for treatment of one-level cervical degenerative disc disease. Standard preoperative and postoperative assessments of outcome were performed with NDI and VAS scores. Patients were also followed radiographically. Clinical outcome measures showed a sustained improvement at 1-year follow-up with a decrease in the NDI and VAS scores. Range of motion improved by 240% at 1 year compared with preoperative studies. Neck pain decreased by approximately 40%, arm pain frequency and intensity resolved to less than half of the original value, and no device-related or approach-related complications were noted.

The Prodisc-C has obtained approval by the U.S. FDA for use in single-level disc arthroplasty. Data from the multicenter human IDE trial have been published and to date represent the most significant compilation of outcomes with regard to this device.²⁵ This prospective randomized multicenter study examined the results of the Prodisc-C cervical arthroplasty device compared with ACDF in patients treated for symptomatic single-level cervical degenerative disc disease. Demographic data were similar between the investigation and control treatment groups. A total of 24 months of outcome data were examined and showed that the arthroplasty device is safe and effective with regard to the outcome measures examined compared with the fusion cohort.

The study examined NDI and SF-36 scores, VAS neck pain and VAS arm pain, and neurologic success. NDI and SF-36 scores were significantly less compared with presurgery scores at the follow-up visits for the investigation and control treatment cohorts. In both groups, VAS neck and arm pain were improved at the standard follow-up intervals compared with their preoperative values but did not differ between the two groups. Neurologic success, judged by improvement or maintenance, was improved in both groups but was not different between the groups. Range of motion was maintained at the arthroplasty level in 84.4% of the investigational group at 24 months. The fusion cohort had an 8.5% rate of secondary surgical procedures with the investigational group at a rate of 1.8%, a statistically significant difference at 24 months. The Prodisc-C TDR at least showed equivalence in all measured primary and secondary outcomes compared with ACDF with a total follow-up of 96.5% at 24 months for the entire study cohort.

BRYAN Disc

Goffin and colleagues ³⁹ reported early results of a multicenter study of the BRYAN disc performed at single levels in 60 patients for the treatment of radiculopathy or myelopathy secondary to disc herniations or spondylosis failing at least 6 weeks of conservative treatment. Exclusion criteria included previous cervical spine surgery, axial neck pain as the sole symptom, significant anatomic deformity, and radiographic evidence of instability (translation >2 mm or >11 degrees of angulation compared with the adjacent level). Patient outcomes were determined by the Cervical Spine Research Society and SF-36 instruments. Clinical success rates at 6 months and 1 year were 86% and 90%, exceeding the study’s targeted success rate of 85%.

In a separate report, Goffin and colleagues ⁴⁰ published the intermediate-term results of this multicenter study. The study was expanded to include a second arm evaluating the treatment of two adjacent levels. The single-level arm had 103 patients enrolled, with 100 reaching the 1-year mark and 51 reaching 2-year follow-up. The bilevel study arm comprised 43 patients with 1-year data completed on 29 patients and 2-year data available on 1 patient. Success rates in the single-level study at 6 months, 12 months, and 24 months were 90%, 86%, and 90%. In the bilevel study, the success rate at 6 months was 82% and at 1 year was 96%. No device failures or subsidence was observed in any patient. At 1-year follow-up, flexion-extension range of motion per level averaged 7.9 degrees in the single-level arm and 7.4 degrees in the bilevel arm.

In the single-level study, three patients required subsequent surgical intervention. These procedures included the evacuation of a prevertebral hematoma, a posterior foraminotomy for residual compression, and a posterior laminectomy for residual myelopathy. Four subsequent procedures were required in the bilevel study: evacuation of a prevertebral hematoma, evacuation of an epidural hematoma, repair of a pharyngeal or esophageal injury caused by intubation, and anterior decompression owing to residual nerve root compression. Two patients developed dysphonia after second procedures. One patient initially had a device placed at a wrong level and developed temporary dysphonia after a device was placed at the appropriate level. The other patient developed a second symptomatic disc 21 months after the index procedure and developed severe dysphonia from bilateral vocal cord paralysis after a second device was placed from a contralateral approach.

Temporary anteroposterior device migration was detected in one patient and suspected in another. This migration was believed to be due to inadequate endplate milling early in the study. This issue was corrected with modification of the instrument system. Migration greater than 3.5 mm, the radiographic threshold of segmental stability, was not observed.

Sekhon ⁴¹ reported early results of nine patients with cervical spondylotic myelopathy who were treated with anterior decompression and reconstruction with the BRYAN disc. Follow-up ranged from 1 to 17 months. On average, the Nurick grade improved by 0.72 points, and NDI scores improved by 51.4 points. Improvement in cervical lordosis was noted in 29% of the patients. No complications were reported.

In another small prospective study, Duggal and colleagues ⁴² reported on 26 patients undergoing single-level or two-level implantation of the BRYAN artificial cervical disc for treatment of cervical degenerative disc disease resulting in radiculopathy or myelopathy or both. Patients were evaluated radiographically and via NDI and SF-36 at regular intervals. Segmental sagittal rotation from C2-3 to C6-7 was measured using quantitative motion analysis software. A total of 30 BRYAN discs were placed in 26 patients. Follow-up ranged from 1.5 to 27 months (mean 12.3 months). A statistically significant improvement in the mean NDI scores was seen between preoperative and late postoperative follow-up evaluations.

Anderson and colleagues ³³ described the follow-up results of 73 patients who had greater than 2-year follow-up status on a one-level BRYAN disc arthroplasty. Of these patients, 45 were rated as excellent, 7 as good, and 13 as fair. Only eight patients had a poor rating at the 2-year follow-up. SF-36 functional outcome data showed significant improvement from preoperative to 3-month postoperative time points. These outcomes remained stable 24 months after surgery. There was no radiographic evidence of subsidence of implants. 89% of all patients had at least 2 degrees of motion at 1 and 2 years. Average range of motion was 8 degrees. There was one early anterior device migration associated with a partially milled cavity.

This same report noted the results of 30 patients who had two-level disc arthroplasty and had reached the 1-year end point in follow-up. Of the patients, 21 were rated as excellent; 3, good; 5, fair; and 1, poor. A significant improvement in SF-36 functional outcome measures was noted postoperatively. There was no radiographic evidence of subsidence in the two-level patients. At 1 year, 84% of patients had at least 2 degrees of motion at both disc levels. The average amount of motion at each disc level was also 8 degrees. There was one posterior migration of a device, again associated with a partially milled cavity. Complications in the study as a whole included one cerebrospinal fluid leak, one esophageal injury, four hematoma evacuations, and three revision decompressions.³³

Sasso and colleagues ^43,⁴⁴ studied and reported on a group of patients regarding initial functional outcome results of the BRYAN artificial cervical disc replacement and compared them with fusion for patients with a cervical disc herniation or stenosis causing radiculopathy. Their prospective, three-center, randomized trial analyzed the data from three surgeons involved in the FDA IDE trial of the BRYAN cervical disc. Multiple outcome measures were used, including NDI, neck pain VAS, arm pain VAS, SF-36 physical component, SF-36 mental component, and range of motion flexion-extension.

At the more recent of the two publications, 12-month follow-up was presented for 110 patients, and 24-month follow-up was presented for 99 patients. Gender distribution was similar between the investigational and control groups. The average age was 43 years (BRYAN) and 46 years (fusion). Prospective data were collected before surgery and at 6 weeks and 3, 6, 12, and 24 months after surgery.

The average operative time for the control group was 1.1 hours and for the BRYAN group 1.7 hours. The mean NDI before surgery was not statistically different between groups: 47 (BRYAN) and 49 (control). At 12-month follow-up, NDI scores favored the arthroplasty device, a statistic that was retained at 2-year follow-up: NDI for the BRYAN group was 11 and for the control group was 20 (P = .005). Both groups improved compared with preoperative scores: at 1-year follow-up, BRYAN arm pain VAS was 12 and control was 23 (P = .031). At 2-year follow-up, the average arm pain VAS for the BRYAN group was 14 and for the control group was 28 (P = .014). Mean neck pain VAS improved in both groups after surgery, but the group-to-group comparison again favored the arthroplasty device: 1-year follow-up scores were 17 (BRYAN) and 28 (control) (P = .05) and at 2 years were 16 (BRYAN) and 32 (control) (P = .005). SF-36 physical component score before surgery for the BRYAN group was 34 and for the control group was 32; at 24 months, score for the BRYAN group was 51 and for the control group was 46 (P = .009).

More motion was retained after surgery in the disc replacement group than the plated group at the index level (P < .006 at 3, 6, 12, and 24 months). The disc replacement group retained an average of 7.9 degrees of flexion-extension at 24 months. In contrast, the average range of motion in the fusion group was 0.6 degree at 24 months. There were six additional operations in this series: four in the control group and two in the investigational group. There were no intraoperative complications, no vascular or neurologic complications, no spontaneous fusions (heterotopic ossification), and no device failures or explantations in the BRYAN cohort.

The most comprehensive data to date stem from the full multicenter cohorts of the U.S. FDA IDE trial published by Heller and colleagues.²⁴ This study presented the analysis of the multicenter groups using similar criteria and outcome measures as previously published in the studies by Sasso and colleagues.^44,⁴⁵ As shown in the prior three-cohort study, the multicenter study with 242 patients enrolled in the investigational group and 221 enrolled in the control group showed statistically greater improvement in many of the primary outcome variables. Although both groups improved compared with preoperative scores, the investigational group had statistically favorable results in several outcomes, including NDI, neck pain, and return to work. Arm pain, SF-36 physical and mental components, and rates of neurologic success, although significantly reduced in both cohorts compared with preoperative levels, did not show significant group-to-group differences.

Adverse events related to the implant or surgical procedure were recorded and included a 2.9% rate in the BRYAN group (2.5% rate of revision) compared with a 3.2% rate in the ACDF group (3.6% rate of revision) at 24 months, a statistically similar rate. Fusion was successful in 94.3% of the control patients, and range of motion was 8.1 degrees at 24 months in the BRYAN group. Overall success was judged to be 82.6% in the BRYAN group and 72.7% in the arthroplasty group, a statistically significant difference. The device was found to be safe and efficacious.

Anderson and colleagues ³² reported on a comparison of adverse events between BRYAN arthroplasty and ACDF. This report is a novel analysis of the 463-patient cohort enrolled in the BRYAN arthroplasty IDE.²⁴ Adverse events were recorded at the intervals prescribed by the IDE study and categorized by severity and as medically or surgically related. Overall, no differences in adverse medical events occurred between groups. Surgically related adverse events were present at a higher rate in the BRYAN cohort than in the control cohort, largely owing to the increased incidence of dysphagia complaints and late medical events that occurred. An increased incidence of more severe events, World Health Organization grade 3 and 4, occurred in the ACDF group, however. This increased incidence was related to the revision treatment of pseudarthrosis and persistent symptoms after the index ACDF. Statistically more operations occurred in the control group. No deaths or deep infections were recorded in either cohort.

MOBI-C Disc

Early clinical results with the MOBI-C disc prosthesis have been reported in several studies.^46,⁴⁷ Kim and colleagues⁴⁶ reported prospectively on a 23-patient cohort treated for cervical degenerative disc disease with 40 TDRs. Mean age was 43 years (range 31 to 62 years). Statistically significant improvement was observed in VAS arm and neck pain indices at the final follow-up interval of 6 months compared with the preoperative scores. Eight of the 23 patients were treated for an index complaint of cervical myelopathy and were assessed with the JOA myelopathy score. No statistically significant improvement was noted in myelopathy; however, the baseline presenting myelopathy for these 8 cases at the time of presentation was low. All patients were able to return to work within 1 month after surgery. No complications were observed in the entire cohort, and 95.6% of patients rated their outcome as either excellent or good by Odom criteria. Cervical mobility was preserved at the index and adjacent levels at the time of the final 6-month follow-up.

In a second study, Park and colleagues ⁴⁷ retrospectively evaluated 53 patients treated for cervical disc herniations. This study contained arthroplasty (21 patients) and control (ACDF; 32 patients) groups. Patient follow-up was 12 months. Mean operative time was similar in the two groups, as was patient satisfaction at the final follow-up. In both groups, NDI and VAS arm pain improved compared with the preoperative state at 12 months, but the groups showed no statistical difference compared with one another at this interval. Segmental range of motion at the index level was maintained overall at final follow-up. No complications were observed in the arthroplasty group, and no heterotopic ossification was noted.

PCM Disc

Current literature regarding the PCM arthroplasty device stems largely from two clinical publications ^48,⁴⁹ and two publications that focus on cadaveric biomechanical⁵⁰ and caprine biomechanical⁵¹ data. From a clinical standpoint, Pimenta and colleagues⁴⁸ published the first 53 patient experiences in 2004—a single-cohort case series. In this investigation, 82 arthroplasty devices were implanted in 53 patients who were followed with the VAS pain scale, the NDI, the Treatment Intensity Gradient Test (TIGT), and radiographic studies. The average patient age was 45 years. Of patients, 28 received a single-level arthroplasty, 22 received a two-level arthroplasty (6 patients with noncontiguous placement), and 3 received a three-level arthroplasty. At 1 week, 80% of patients had good or excellent results by Odom criteria, improving to 90% at 3 months. One postoperative complication, a device migration with 4 mm of anterior displacement, was noted at the 3-month follow-up interval.

At 12-month follow-up, the VAS average was 20 points (range 50 to 0), the NDI average was 15 points (range 14 to 0), the TIGT average was 3.5 points (range 14 to 0), and 97% of patients reported excellent or good results according to Odom criteria. Although an improvement compared with preoperative scores was noted with the outcome assessment tools used, no statistical metrics were reported in this analysis. Of working patients enrolled in this cohort, 87% were able to return to their baseline level of employment within 3 weeks.

A second prospective, consecutive series of 229 PCM arthroplasty implantations in 140 patients was later reported by Pimenta and colleagues in 2007.⁴⁹ This study reported the differences between two cohorts: patients with multilevel arthroplasty versus patients with single-level arthroplasty. In the index cohort, 71 patients received single-level arthroplasty; in the investigational cohort, 69 patients underwent 158 arthroplasties (multiple levels per patient). In the multilevel group, 53 patients received two-level arthroplasties, 12 patients received three-level arthroplasties, and 4 patients received four-level arthroplasties. Of the total surgeries, 21 were performed as arthroplasty revision procedures adjacent to a prior ACDF.

Outcome measures were recorded with NDI, VAS, and Odom criteria. Patient age and demographics were similar between the two cohorts. Only one case of heterotopic ossification (single-level cohort) was reported in this large series at an average follow-up of 26.8 months for both groups—and no nonsteroidal anti-inflammatory drugs were administered at any point in this study. Multilevel cases had a statistically greater estimated blood loss, length of surgery, and length of hospital stay compared with the single-level group. Multilevel cases consistently showed an increased improvement in outcome assessments at each follow-up interval compared with the single-level cohort with mean NDI improvement for single cases at 37.6% versus 52.6% in the multilevel cases (P = .021). This trend was also noted in VAS, TIGT, and Odom criteria. Reoperation rates and serious adverse events were similar between the two groups (three in single-level group and two in multilevel group), with four of five devices being converted to a more constrained version of the PCM and one converted to ACDF.

CerviCore, DISCOVER, Kineflex, and SECURE-C Devices

At the time of this writing, published clinical data are lacking on the outcomes of use of the CerviCore, DISCOVER, Kineflex, and SECURE-C cervical arthroplasty devices. Each of these devices remains in the process of human IDE trials in the United States. As data from these trials become available, it will be necessary and appropriate to compare these new data with the short-term and intermediate-term data that have been collected from devices farther along in the U.S. FDA approval process or devices with current approval. The design characteristics of these devices are summarized in Table 43–1, which serves as the basis for preliminary comparison of the devices with their peers.

Biomechanics

A primary goal of cervical disc replacement is to reproduce normal kinematics after implantation (see Figs. 43-2 and 43-9). Studies have been conducted to compare the biomechanics of cervical disc replacements on two kinematically different types of devices using identical protocols by the same laboratory.^52,⁵³ These studies may suggest which type of implant design would provide more kinematically accurate motion.

The normal cervical spine motion exhibits anterior-posterior translation during flexion and extension. The motion segments of the cervical spine are complex joints. Each joint consists of three compartments (the disc and two facets) and multiple ligamentous structures. An implant designed to replace the cervical disc should consider the effect of all three compartments and multiple ligamentous structures.

DiAngelo and colleagues ^52,⁵³ investigated two different implant designs in human cadaveric spines to compare the motion of the harvested spine with the implanted spine. One of the designs was a semiconstrained device consisting of a ball-in-trough, which allows anterior-posterior translation (PRESTIGE cervical disc), whereas the other design was a constrained device consisting of a ball-in-socket with no anterior-posterior translation allowance (Prodisc-C). The results of the studies support the design rationale behind the PRESTIGE cervical disc. The semiconstrained device (PRESTIGE cervical disc) provided for normal kinematics in all ranges of motion tested, whereas the constrained device (Prodisc-C) failed to reproduce normal motion in extension.

A ball-in-socket (constrained design) does not allow for natural translation, a motion that has been clearly documented in the literature. The complexity of the cervical spine requires a “balance” of all the significant structures, including facets and ligaments. A ball-in-socket, by its design, dictates the kinematics and eliminates the normal anterior-posterior translation that the facets provide.

The most significant effect of this change in balance is in extension. When the spine goes into flexion, the facets “unshingle” and reduce their involvement in constraining the motion of the functional spine unit. When the spine goes into extension, the facets “shingle” and become more involved in constraining the motion. With a constrained facet joint and a constrained disc joint, one would expect to see binding or limited motion (also known as kinematic conflict), as one joint works against the other. This binding would give rise to decreased motion or increased stress on the system.

Balancing the kinematics in the functional spine unit is crucial. A constrained disc replacement may be unable to provide the normal kinematics of the cervical spine. Anterior-posterior translation is anatomic and must be allowed to restore normal motion. The PRESTIGE cervical disc was shown to maintain normal kinematics through all ranges of motion. Similar results would be expected from the BRYAN cervical disc system because it is also semiconstrained and allows for anterior-posterior translation in flexion and extension.

Sasso and colleagues ⁵⁴ studied the kinematics of cervical motion from patients enrolled in a prospective, randomized, multicenter clinical trial. Radiographic data, including flexion, extension, and neutral lateral radiographs, were obtained preoperatively and at regular postoperative intervals up to 24 months. All patients had received either a single-level anterior cervical plate (Atlantis anterior cervical plate, n = 221) or a single-level artificial cervical disc (BRYAN cervical disc prosthesis, n = 242) at C3 to C7. Cervical vertebral bodies were tracked to calculate the functional spinal unit motion parameters. Parameters measured included flexion-extension range of motion and translation. These data were recorded at the index surgical level and, if visible, at the functional spinal units cranial and caudal to it.

More motion was retained in the disc replacement group than the plated group at the index level, a statistically significant finding. At 24 months, the arthroplasty group retained an average of 7.95 degrees (average 6.43 degrees preoperatively). In the control ACDF cohort, the average range of motion in the fusion group was 1.11 degrees at the 3-month follow-up and gradually decreased to 0.87 degrees at 24 months (average 8.39 degrees preoperatively). In the BRYAN cohort, index level functional spinal unit translation averaged 0.36 mm at 24 months, a data point that was statistically unchanged from the first postoperative measurement of translation at 3 months. No investigational devices showed radiographic evidence of subsidence at 24 months, and no heterotopic ossification was observed in the BRYAN cohort.

Angular range of motion at adjacent levels cranial and caudal to the index surgical functional spinal unit was measured and was not statistically different between the two cohorts preoperatively. At 24 months, these differences were unchanged in both groups (statistically similar). There was no consistent correlation between angular range of motion at adjacent or index levels and standard outcome measures of success in the investigational group. These data were not analyzed in the control. This study showed that the BRYAN disc maintains mobility at the level of the prosthesis over 24 months. No conclusions with regard to the adjacent segments could be statistically reached at this early follow-up interval.

The PCM disc has also undergone bench-top and nonhuman testing published in peer-reviewed literature. McAfee and colleagues ⁵⁰ established that the posterior longitudinal ligament provides a stabilizing influence to the cervical spinal segment. Biomechanical testing was performed using human cadaveric spines and a 6-degree-of-freedom spine simulator with additional optoelectronic motion measurement. The major finding was that biomechanical stability may be restored after complete anterior cervical discectomy with resection of the posterior longitudinal ligament via implantation of an arthroplasty device.

Wear Analysis

Failure of total joint arthroplasty has been largely attributed to wear debris from polymer-bearing surfaces. Wear debris induces an intense cellular inflammatory response, which results in the production of cytokines that cause resorption of bone.^55,⁵⁶ This process, known as aseptic loosening, leads to destabilization and mechanical failure of the joint. Factors that influence the loosening process include particle size, shape, number, material surface chemistry, and concentration and duration of exposure.⁵⁷^–⁶¹ The possibility of a similar process occurring after cervical disc replacement is concerning especially given the polymeric bearing surfaces of the several disc designs. In addition, the unknown effects of wear debris and inflammatory response in the spinal canal around neural structures are worrisome.

Anderson and colleagues ^33,⁵⁷ published two pivotal studies that reported on the wear characteristics of the BRYAN disc in vitro in a cervical spine simulator and in vivo in goat and chimpanzee models. In the first report, the in vitro results of six discs were tested to 10 million or 40 million cycles in a cervical spine simulator by load and motion, and an additional three assemblies were tested only to load. Wear debris was reportedly produced at a rate of 1.2 mg/1 million cycles. Device heights decreased 0.02 mm/1 million cycles. Debris particles averaged 3.9 µ in diameter, larger than the particles typically observed in hip and knee arthroplasty.

The local biologic response was reported in two chimpanzees, and local and distant tissue biologic response were reported in nine goats. Three additional goats had anterior fusion and plating. Local wear debris was found in one chimpanzee and four goats; however, no inflammatory response was seen in any animal. Some animals had wear debris in loose connective tissue and in the epidural space without evidence of inflammation. One goat had debris particles in the lumbar spine; however, they were at the extreme margins and not incorporated in the tissue, indicating they may have been artifactual. The plated fusion group exhibited greater metal debris and inflammatory response than the artificial disc group. Given their findings, the authors conclude that the low in vitro wear rate and lack of inflammatory response in vivo predict satisfactory long-term performance.

In the second report, Anderson and colleagues ³³ examined the wear properties of BRYAN disc components in a custom cervical spine simulator and a goat model. The in vitro model involved simultaneous load and motion in six separate device assemblies with three control devices and was similar in design to the prior study.⁵⁷ Nuclei were measured before and after the test for weight, height, and diameter. Wear debris was assessed with a standardized system. The caprine model was undertaken in a certified animal care facility with anterior discectomy and placement of the BRYAN cervical disc in 11 goats. Four goats underwent a similar discectomy with fusion using allograft and plating with a titanium cervical plate.

The in vitro wear testing revealed a mean mass loss of 1.76% and a mean height loss of 0.75% after fatigue of 10 million cycles. Particulate debris had a mean diameter of 3.89 µ with a distribution histogram similar to total hip and knee replacement. The particles were elliptical in shape and larger in size, however, than particles associated with knee and hip arthroplasty. There was no dysfunction in the test or load assemblies at 10 million cycles, and the polymeric nuclei showed uniform wear on the load-bearing surface with the exception of one sample. Three assemblies were tested to the point of end plate–to–end plate contact, which was observed at 37.7 million, 39.7 million, and 40 million cycles. Loss of prosthetic height was linear in relationship to the number of cycles in this model.

The in vivo caprine model was analyzed in five predesignated groups after the sacrifice of: a baseline control goat on the day of surgery (group 1), 3 BRYAN disc goats at 3 months (group 2), 3 BRYAN disc goats at 6 months (group 3), 3 BRYAN disc goats at 12 months (group 4), and 3 goats fused with allograft and a plate at 12 months (group 5). Two goats were excluded from the study and analysis, including a goat originally assigned to group 2 who died on postoperative day 1 from a gastrointestinal obstruction and a goat originally assigned to group 5 who died as a result of a cervical fracture through a screw hole.

Tissues analyzed by an independent, blinded veterinary pathologist in the in vivo group included periprosthetic tissue, draining cervical lymph nodes, spinal canal tissues adjacent to the disc and at the rostral and caudal levels, liver, and spleen. A trend of increasing extracellular wear particles was noted with time with no appreciable inflammatory response seen at 3, 6, or 12 months. The three animals in the plate/control group all had particulate titanium debris in the periprosthetic tissues that was much greater in volume than that observed in the BRYAN prosthesis animals.

Hu and colleagues ⁵¹ examined the in vivo results of the PCM device at 6 and 12 months in a caprine model. Each goat underwent single-level arthroplasty at C3-4 and was followed thereafter for the prescribed time. At both intervals, the device had no evidence of loosening, subluxation, or inflammatory reaction. CT did not significantly obscure visualization of the spinal canal. Preservation of segmental spinal motion was noted in all specimens in biomechanical testing under load. The bone-prosthesis interface was found to be osteointegrated. Histopathologic review showed no significant wear debris of a particulate nature in local and systemic tissues—no significant pathologic changes were noted in the analyzed tissues.

Conclusion

Although far from being an accepted standard, the concept of artificial disc replacement is becoming a reality. The possibility of being able to minimize adjacent segment degeneration is exciting; however, much more intermediate-term and long-term outcome data are needed to prove that this technology supersedes the current “gold standard” of anterior fusion in ACDF. Biomechanical studies show that disc replacement creates less adjacent level strain than fusion; it is hoped that long-term studies will prove that this correlates to a lower incidence of adjacent level degeneration.

More recent clinical reports show promising early data suggesting that artificial disc replacement is comparable to fusion at least in the short-term.²³^–²⁵ ^,31 ^,45 Wear studies suggest that there may be less potential for aseptic loosening than in large joint arthroplasty, although the reality of this will be borne out only with more follow-up time. Although early reports of success in the United States with the TDR have allowed several devices to gain FDA approval^23,²⁵ and suggest that the intended effects are being achieved, the final results of arthroplasty with these and other devices are pending the outcomes of long-term studies.