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Complications of Cervical Disc Arthroplasty
Complications of Cervical Disc Arthroplasty Leonard K. Kibuule, MD, and Jeffrey S. Fischgrund, MD Anterior cervical discectomy and fusion (ACDF) has long been the gold standard for the treatment of cervical pathology. ACDF, when performed successfully, has shown good disease-free survival of up to 89% at 5 years for patients. However, the potential for complications has prompted clinicians to search for alternatives to cervical discectomy and fusion. Recent efforts have focused on total disc arthroplasty and interest in its application to the cervical spine. If performed successfully, total disc arthroplasty would preclude the need for graft harvest for fusion, attempt to maintain more physiological kinematics of the cervical spine and prevent/delay adjacent segment disease. Currently, at least 2 devices for cervical disc arthroplasty have been approved by the US Food and Drug Administration (FDA) for clinical use while several others are still under investigation. We hope that these new devices will have a positive effect on the treatment of cervical radiculopathy and myelopathy, but from their inception to current clinical use they too have met with difficulties and complications. Such complications include: subsidence, dislocation and heterotopic ossification. Researchers and clinicians continue to peruse the development of more robust devices to meet the growing needs of surgeons who treat such pathology of the cervical spine. Semin Spine Surg 21:185-193 © 2009 Elsevier Inc. All rights reserved. KEYWORDS cervical arthroplasty, complications, disc replacement
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nterior cervical discectomy and fusion (ACDF) has long been the gold standard for the treatment of cervical pathology. Conditions related to degenerative disc disease and radiculopathy have been treated with ACDF with predictable success for ⬎50 years.1 Biomechanical studies have shown good disease-free survival of up to 89% at 5 years for patients undergoing ACDF.2 However, ACDF has not been performed without risks and complications. The potential for pseudarthrosis, graft donor site morbidity, instrument-related complications, and future adjacent segment disease is significant.3-6 Such risks have motivated clinicians to search for alternatives to cervical discectomy and fusion. Recent efforts have focused on total disc replacement and interest in its application to the cervical spine. Cervical total disc replacement, if performed successfully, would preclude the need for graft harvest for fusion, attempt to maintain more physiological kinematics of the cervical spine and prevent/delay adjacent segment disease. Currently, there are 2 devices for cervical disc arthroplasty that have been approved by the US Food and Drug Administration (FDA) for clinical use and several oth-
Department of Orthopedic Spine Surgery, William Beaumont Hospital, Royal Oak, MI. Address reprint requests to Leonard K. Kibuule, MD, William Beaumont Hospital, 3535 West Thirteen Mile Road, Suite 744, Royal Oak, MI 48073. E-mail: [email protected]
1040-7383/09/$-see front matter © 2009 Elsevier Inc. All rights reserved. doi:10.1053/j.semss.2009.05.007
ers still under Investigational Device Exemption and in clinical trial studies. We hope that these new devices will have a positive effect on the treatment of cervical radiculopathy and myelopathy, but from their inception to current clinical use they too have met with challenges and complications.
Treatment of Cervical Disc Disease The initial treatment for cervical disease without neurologic deficit should involve nonoperative care. Many patients will respond to conservative care; including nonsteroidal antiinflammatory drugs, physical therapy, and, for some, steroid injections in and around the area of compression. When nonoperative management has been exhausted and symptoms persist or neurologic deficits exist, then operative management with decompression may be indicated. Conventional anterior cervical decompression and fusion procedures are performed through an anterior approach with a goal to decompress the neuroforamen and provide structural stability to the adjacent vertebral bodies. This procedure often involves the introduction of a structural bone graft with or without rigid fixation accomplished with an anterior plate. Fusion of this segment is intended to eliminate motion at this level. Degenerative changes at a segment adjacent to ACDF may contribute to recurrent symptoms. It is likely that these de185
186 generative changes are a result of natural progression of spondylotic changes at the unfused levels, possibly in combination with an increase in biomechanical stresses adjacent to the fusion. Hilibrand et al7 reported that 26.9% of patients had symptomatic adjacent segment disease at 10 years. Goffin et al8 followed up patients with ACDFs and also discovered 92% of them with adjacent segment disease at 5 years. Gore and Sepic9 followed up patients for a mean of 21 years after their initial ACDF and reported that 16% required additional surgery for symptomatic adjacent segment degeneration. Some biomechanical studies show increased intradiscal pressures and abnormal kinematics above and below a cervical fusion.10 The risk of pseudarthrosis following ACDF is a real complication and has been shown to increase with multiple attempted fusion levels.10 Bohlman showed a 13% pseudarthrosis rate at an average 6-year follow-up of patients and this increased to 27% for multilevel fusions.2 Although the use of iliac crest bone graft shows the most favorable rates of fusion, it is also not without its own risks. Such donor site complications have caused many surgeons to use alternative grafting techniques. Risks of neurovascular injury, fracture, chronic pain, infection, and abdominal herniation2 have convinced many surgeons to consider allograft bone or other bone substitutes. A review of 1191 iliac crest harvest cases showed postoperative complications in 20% of patients and 55% with subsequent chronic donor-site pain at 1-year followup.11 Although some surgeons have began using allograft bone or synthetic substitutes, the cost of these alternatives will continue to pose a challenge and other alternatives, such as disc arthroplasty, will continue to be explored.
Historical Perspective on Disc Arthroplasty During the past 50 years, there have been numerous designs for a cervical disc prothesis.12 In as early as 1950, clinicians experimented with the concept of artificial disc replacement in the spine. In 1950, Nachemson performed biomechanical studies after injecting cadaveric discs with the self-hardening
L.K. Kibuule and J.S. Fischgrund
Figure 2 Prestige ST cervical disc prosthesis. (Image courtesy of Medtronic Sofamor Danek USA, Inc. [Memphis, TN].)
liquid silicone rubber.13 Later in 1973, Stubstad et al developed several designs in the shape of a disc made from elastic polymer and undertook a primate study.12 Many of the early designs have been used to recreate the fluidlike, compressible nature of the native disc and much of the focus was placed on developing various types of plastics and polymers. In 1991, Bao and Higham even created hydrogel beads enveloped by a semipermeable membrane of Dacron or nylon for disc replacement.12 Others attempted to introduce material directly into the disc while leaving the annulus relatively intact. Baumgartner first used a flexible elastic coil introduced into the annulus and a few years later attempted the use of elastic beads.12 Many of these early concepts focused on lumbar artificial disc replacements. Fernstrom was the first to implant an artificial device into the cervical spine in 1966.14 He used spherical metallic balls in an attempt to reproduce the “ball joint” mechanism of the disc. These balls were implanted in over 250 patients but the technology was later abandoned because the devices caused hypermobility and tended to erode into the vertebral end plate and body.12
The Bristol/Cummins/Prestige Disc Prosthesis
Figure 1 Bristol Cervical Disc replacement. (Image courtesy of Medtronic Sofamor Danek USA, Inc. [Memphis, TN].)
In 1998, Gill and coworkers patented the Bristol cervical disc designed by Cummins15 (Fig. 1). Development of this device began in 1989 at the Department of Mechanical Engineering at Frenchay Hospital in Bristol, United Kingdom. The second generation of the Cummins disc was a ball-and-socket-type
Complications of cervical disc arthroplasty device constructed of stainless steel. One of the final devices was designed to be secured to the vertebral bodies with screws. The developers finally settled on a two-piece, stainless steel, metal-on-metal, ball-in-socket design known as the Bristol/Cummins (now referred to as the Prestige I) disc. Since the beginning of clinical trials, several reports have detailed the results of this disc prosthesis. Cummins et al17 described 20 patients who were followed up for an average of 2.4 years. Patients with radiculopathy improved, and those with myelopathy also either improved or were stable. Only 3 experienced continued axial neck pain, but no patient required additional motion segment surgery and x-rays did not demonstrate ectopic fusion at the level of the joint in any patient. In addition, no adjacent segment degeneration was seen nor any subsidence by the prosthesis into the vertebral bodies was seen. The Prestige II was built on many of the successes of its predecessor but with further improvements, including a reduced profile, a surface that allows bone ingrowth and additional selection sizes (Fig. 2).16 This disc arthroplasty was followed by the development of the Prestige ST in 2002, Prestige STLP in 2003, and finally the Prestige LP released in 2004.17 Despite some initial good results, the Cummins prosthesis also had its share of complications. Postoperatively, 2 of the 20 patients treated had a transient hemiparesis. There were also 5 partial screw pullouts, 2 broken screws, 1 joint subluxation, and 4 cases of persistent dysphagia. Fortunately, these complications did not require removal of the implant. However, one prosthesis was found to be loose and had to be removed and a standard interbody fusion employed to relieve persistent neck pain. The failure was attributed to a manufacturing error. Further examination revealed that the balland-socket fit was asymmetric but despite this finding, no significant wear debris was found in the surrounding tissues of the patient. Joint motion was preserved in all but 2 patients whose lack of motion was thought to be the result of an overdistraction of the disc space and separation of the facet joint at the time of implantation.15 With these initial clinical experiences, a second-generation Cummins artificial cervical disc was designed. New features were implemented with hopes of tackling some of the problematic issues. The new disc would rely more on the facet joints and surrounding tissue to restrain some of its motion. The hemispherical cup was replaced with a shallow ellipsoid saucer and freedom of translation and rotation were also increased.10,15 In an attempt to create a lower profile implant with less soft-tissue irritation, a screw locking mechanism was redesigned and the device made less bulky.10 The redesigned Frenchay disc (also known as the Cummins disc) was clinically examined and results published in 2002.18 Wigfield studied 15 patients who underwent cervical disc replacement for myelopathy or radiculopathy and radiographically confirmed disc herniations or posterior vertebral osteophytes. In all cases, the artificial joint maintained motion at the operative levels and re-established intervertebral height. The procedure was considered safe for experienced spine surgeons to perform, and the device was stable, with no dislocation of components or backing out of screws. Despite
187
Figure 3 Bryan cervical disc prosthesis. (Image courtesy of Medtronic Sofamor Danek USA, Inc. [Memphis, TN]. Bryan Cervical Disc System incorporates technology developed by Gary K. Michelson, MD.) (Color version of figure is available online.)
the overall reported success, there were 2 incidences of screw breakage and another patient had to undergo removal of the device for persistent neck pain while in extension. The implant was found to be loose with surrounding fibrous tissue. Wigfield attributed this failure to aggressive bone removal at the time of implantation, with resultant excessive load transfer to the facet joints.18 Further investigational studies have been performed of the Frenchay disc, including a European study in August 2000 and later in the United States in 2002 (now known under the name of Prestige ST). The Prestige ST cervical disc (Medtronic Sofamor Danek, Memphis, TN) was approved by the US FDA in July 2007. The major difference between the Prestige II and the Prestige ST was a 2-mm reduction in the height of the anterior flange.19 Prior to its approval, a limited number of published studies showed good short-term and intermediate-term results for this cervical disc replacement design.16,18 Riew et al20 recently performed an analysis of pooled data from an FDA Investigational Device Exemption study comparing cervical disc arthroplasty to ACDF for the treatment of myelopathy. This study showed an improvement in neurological status and gait function in patients treated with a Prestige cervical disc when examined at 2 years after surgery. During the Prestige ST trial, there was a single possibly implant-related adverse event in 1 (1.7%) of 59 patients, which necessitated removal of the prosthesis. In 2004, Robertson21 reported on the long-term 4-year results of 12 patients treated with the Prestige I disc. The 1 patient who required removal of the prosthesis at 12 months with subsequent fusion, as described in the original report, was not included in this follow-up study. A second patient suffered
188 progression of myelopathy and subluxation at an adjacent level due to advanced degenerative disc disease below the implant. This patient required posterior fusion adjacent and below the level of the implant. As a result, a lack of motion of the implant could be appreciated after 12 months following fusion. This illustrates how adjacent segment disease can still occur despite cervical disc arthroplasty.
Bryan Cervical Disc Prosthesis The Bryan artificial disc (Medtronic Sofamor Danek, Memphis, TN), developed in 1992, is a metal-on-plastic design based on a proprietary, low-friction, wear-resistant elastic nucleus. This nucleus is a polyurethane core located between and articulates with anatomically shaped titanium shells with porous surfaces that are fitted to the vertebral body endplates and allow for bone in-growth (Fig. 3). The disc also contains a polyurethane sheath surrounding the nucleus and is filled with saline, which acts as synovial fluid. The sheath is also intended to keep any potential wear debris from escaping into surrounding tissue. In vivo testing has revealed satisfactory wear characteristics without any significant inflammatory reaction.18 European clinical trials were initiated in 2000 and made available for general use in the same market in 2002. A randomized, prospective US investigational device exemption trial was initiated in May 2002. Goffin et al22 published data on a European multicenter trial of patients treated with a Bryan disc for single-level cervical disease. The authors reported that 44 of 49 patients (90%) had excellent, good, or fair outcomes at 2-year followup. A concurrent study involving bilevel cervical disc replacement with the Bryan prosthesis reported 25 of 26 patients (96%) having excellent, good, or fair outcomes at 1 year. These results are greater than the targeted success rate of 85% excellent, good, or fair, based on a meta-analysis of anterior cervical discectomy and fusion published data. One study reports maintenance of relatively normal kinematics of the motion segment in the short term after implantation of the Bryan disc.23 Many studies report good outcomes using the Byran cervical disc replacement for treatment of single-level cervical disease.1,24-27 Since its inception, few complications have been reported with this device. The initial studies, however, noted 30% of patients had paravertebral ossifications. Following this report, most patients began receiving a 2-week course of nonsteroidal anti-inflammatory drugs after surgery.28 Leung et al29 studied the clinical significance of heterotopic ossification (HO) in cervical disc replacement and found a strong association between HO and a loss of movement of the implanted cervical disc prosthesis. They analyzed the relationship of HO development with age, sex, duration of symptoms, and diagnosis and discovered that age and male gender were significant factors. Approach-related complications have also been acknowledged and have included hematoma, dysphonia from vocal cord paralysis, and cerebral spinal fluid leak.22,30 Incomplete decompression with persistent disease-related symptoms have also been described.22,31 Duggal described a patient who experienced increased radicular
L.K. Kibuule and J.S. Fischgrund pain and transient arm pain after Bryan cervical disc replacement despite adequate decompression seen on magnetic resonance imaging (MRI).32 In a study of 96 Bryan disc replacements in 74 patients, Pickett et al33 also discovered complications, including intraoperative prosthesis migration, HO with spontaneous fusion, partial prosthesis dislocation, and retropharyngeal hematoma formation. Device-related issues have included sporadic case reports of anterior or posterior migration22,32 or excess motion at the index level leading to explantation of the device and interbody fusion.27 Postoperative kyphosis occurring early on at the target level has also been reported in a number of patients.22,34 Pickett et al34 showed no improvement in kyphosis of the prosthesis level at late follow-up in 12 of 14 patients who underwent Bryan cervical disc replacement. However, despite kyphosis at the treated level, the overall alignment of the cervical spine remained relatively unchanged indicating that adjacent segments compensated for the focal kyphosis.32 Some of these issues were addressed during the redesign of the Bryan disc prosthesis and the associated instruments used to implant the device. For instance, the milling guide was altered to use a dual track system with a lordotic angle 7°, resulting in a more consistent shell alignment.31 More attention has been drawn to the insertion angle of the prosthesis and the preparation of the vertebral endplates and there is speculation that these may play a role in postcervical kyphosis.30,35 Others have advocated for more lordosis to be built into the prosthesis to help prevent late kyphosis.30 Hacker36 reported on vertebral bone loss in 32 patients treated with Bryan cervical disc arthroplasty. Bone loss was discovered in 3 patients as early as 6 weeks and as late as 48 months after implantation. The authors concluded that osteolysis, though not proven, was a strong consideration and a concern for device dislodgement and advocated routine annual cervical radiographs during the first 3-5 years of implantation.
Porous Coated Motion Disc Prosthesis The porous coated motion (PCM) (Cervitech, Rockaway, NJ) cervical disc replacement was designed by McAfee and modified by Helmut Link and Arnold Keller, two of the developers of the lumbar Charité disc prosthesis.37 The PCM disc replacement is made of a cobalt-chrome alloy with an ultrahigh molecular weight polyethylene (UHMWPE) core, and was designed to mimic the natural shape and size of the disc space. The upper (cobalt-chrome alloy) component is smooth where it meets the inner UHMWPE component, and is intended to slide around and across the core as the neck bends and twists. The outside of the components feature a TiCaP (titanium/calcium phosphate) double coating attached to serrated surfaces that interface with the vertebral end plate. This surface was modeled after the coating used with the SB Charité disc replacement, and allows for bone in-growth, whereas initial fixation relies on the press-fit engagement of the serrated edges. Two different designs are available, one of which relies on anchoring screws and is intended to address any losses in integrity of the posterior longitudinal ligament during cervical decompression. The prosthesis is meant to be
Complications of cervical disc arthroplasty
Figure 4 ProDisc-C disc prosthesis. (Courtesy of Synthes, Oberdorf, Switzerland. ©2008 Synthes, Inc. or its affiliates. All rights reserved.) (Color version of figure is available online.)
load bearing, minimally constrained, and uses surrounding muscles and ligaments to limit its motion.38 The first human clinical trial of the PCM cervical disc took place outside the United States in December 2002. Short-term results of the PCM have shown promising results. At 1-year follow-up, Pimenta et al37 showed that 53 patients who underwent cervical disc replacement with the PCM had significant improvements in visual analog scale (VAS) and neck disability index (NDI) scores. Excellent to good results were reported in 97% of patients. There was 1 postoperative complication reported; a 4-mm anterior displacement of the prosthesis. This was diagnosed 3 months after the operation without any adverse clinical symptoms. There was 1 case of Grade 1 HO (McAfee classification), during the ninth-month follow-up. Other clinical studies of this device are underway.
189 1 year. Pain intensity and frequency also improved by 40%. No device-related complications were reported in this shortterm study. Cases of loosening, subsidence, migration, metallic or polyethylene failure, allergic rejection and/or reaction, visceral or neurologic injuries caused by the implant components, or infection were also not reported. No approach-related complications were noted and no spontaneous fusions discovered in this study. More recently, Murrey et al41 compared the safety and efficacy of ProDisc-C to ACDF for the treatment of single-level cervical disc disease. This prospective, randomized controlled study showed functional improvements in both groups when comparing the VAS, NDI, and SF-36 scores. There was also a statistically significant reduction in medication usage (narcotics and muscle relaxants) and number of secondary surgeries in the disc replacement group. Published reports show good clinical success of ProDisc-C with few complications. In 2007, Mehren et al39 published results of a prospective clinical study evaluating rates of HO in patients treated with the ProDisc-C. HO that led to restrictions in the range of motion was present in 8 cases (10.4%) and spontaneous fusion in 7 cases (9.7%) at 1 year after surgery (Fig. 5). The authors found significantly higher rates of ossification in multilevel arthroplasties than in single-level cases. Despite this, there was still an improvement in the VAS and NDI scores for patients at the final follow-up, indicating a poor correlation of HO with these clinical parameters. Shim et al42 reported a case of fracture of the posterior central vertebral bodies of C6 and C7 with avulsed bony fragments compressing the spinal cord. The fragments were removed and the device implanted without neurologic deficit. Shim highlighted the importance of surgeon awareness of possible avulsion fracture while using the chisel to create a path for the keel fixation points of the prosthesis. Murrey41 reported 3 complications in 103 implanted discs with 1 patient electing
ProDisc-C Disc Prosthesis The ProDisc-C is manufactured by Synthes (Oberdorf, Switzerland). It obtained approval from the US FDA in December 2007 after an Investigational Device Exemption clinical study. It is composed of 2 cobalt-chrome metal endplates with a fixed UHMWPE core (Fig. 4).39 The metal end plates have a keel design for enhanced primary stability and fixation, whereas the end plate is covered with a titanium plasma spray coating that allows bony ingrowth for long-term fixation.39,40 The polyethylene center of the device determines the height of the implant, available in 3 different sizes and various footprints. Studies of the ProDisc-C show promising short-term outcomes. Patients assessed in a study showed postoperative improvement in functional outcomes measured by VAS and the NDI at 1-year follow-up.40 There was also an improvement in range of motion, with a return to normal function by
Figure 5 ProDisc-C with HO.
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removal of the implant due to persistent pain. Another patient sustained a dural tear related to device implantation.
Devices Still Under Development As of January of 2008, multiple other cervical disc replacement devices were confirmed as either under development or undergoing clinical trials. Bartles et al17 identified at least 10 cervical disc devices either currently on the market or in active development by an implant company. The success of total joint arthroplasty for the hip and knee and the limited success of current spinal arthroplasty have ignited a new excitement for further development of this technology. Cervical total discs, such as NeoDisc (Nuvasive, San Diego, CA), Cervicore (Stryker, Kalamazoo, MI), and Discocerv (Scientix, Maitland, FL), may, in future, become available for general use by surgeons.
Challenges of Selection, Design, and Implantation As artificial cervical discs continue to emerge, complications related to this new technology will continue to challenge designers and users to develop better, more reliable implants. In an effort to minimize some of these complications, it is important for users of these devices to recognize their various limitations. Not every device is designed to be used for all patients. Proper patient selection is the first key to a successful treatment of cervical disease with disc arthroplasty. Patients with radiculopathy or myelopathy without axial neck pain have conventionally been candidates for such a procedure.2,43,44 Most of the device manufacturers under the direction of the FDA have developed guidelines pertaining to the use of these devices and when their application is contrain-
Figure 6 Cervical disc arthroplasty shown with subsidence.
Figure 7 Reveals an extruded cervical disc arthroplasty.
dicated. Patients with significant facet arthrosis, preoperative instability on flexion-extension images, or concurrent systemic illness (ie, osteoporosis, rheumatoid arthritis, metabolic bone diseases) are just a few conditions cited as contraindications to cervical disc arthroplasty by many authors and manufacturers.2,31,43 Significant facet arthrosis may limit the intended motion of the cervical arthroplasty and promote adjacent segment degeneration. Inherent instability of the cervical spine at the time of implantation may expose the device to unintended stresses and lead to early loosening or even catastrophic failure. Patients with poor bone quality may predispose the implant to risks of subsidence and settling (Fig. 6). Patients with evidence of excessive ligamentous laxity should also be treated with caution. Proper balancing of the prosthesis may be an additional challenge in these patients. Suboptimal tissues may adversely affect the prosthesis outcome or even predispose the implant to migration, instability, or frank extrusion (Fig. 7). Appropriate preoperative evaluation of patients needs to be instituted to recognize these contraindications, avoid improper implantation, and give patients the best chance of a successful outcome. Cervical disc arthroplasty sets itself apart from anterior cervical discectomy and fusion by recreating a native intervertebral disc height and attempting to preserve cervical range of motion. The healthy native cervical disc provides support for physiological loads, allows for fluid range of motion, and provides adequate space for exiting nerve roots. Native discs also have some inherent shock absorption properties. Traynelis and Haid45 commented that ideally an artificial disc should recreate these properties and last the lifetime of the patient. This device should maintain the proper intervertebral spacing while providing stability for the adjacent structures. Replacement of the native disc with a pros-
Complications of cervical disc arthroplasty
191
Figure 8 (A) Sagittal image of PCM with 1.5-Telsa MRI magnet showing greater artifact adjacent levels. (B) Sagittal image of PCM with 0.3-Tesla MRI magnet showing less artifact at adjacent levels. (Courtesy of John DeVine, MD, Madigan Army Medical Center, Tocoma, WA.)
thetic device should be accomplished while limiting any physiological mismatch. If there is any significant mismatch present at the bone-implant interface, bone resorption with stress shielding may occur or even end the end plate or implant failure. The interaction of the disc arthroplasty with surrounding structures is crucial to its design. Any alteration in load may affect the facets and theoretically lead to facet arthrosis. Pimenta46 recently presented a study revealing that 8% of facets showed signs of arthrosis by computed tomography scan in patients implanted with a PCM disc arthroplasty at 5-year follow-up. Placement of the device should also be accomplished in such a way as to avoid the overdistraction of these important adjacent tissues. The importance of these structures cannot be overemphasized. Facets not only contribute strength and stability to the spine but they can also be a source of pain. The surrounding soft tissues should be manipulated carefully to minimize trauma to these areas and the development of any pathologic process, such as HO. Cervical arthroplasty should also exhibit tremendous endurance and longevity. The average age of a patient needing a lumbar disc replacement has been estimated to be 35 years and may be similar for cervical disc arthroplasty.47 To minimize the need for revision surgery the prosthesis must last some 30 to 40 years. A number of factors must be considered when choosing the materials that will bear these characteristics. The materials must be biocompatible with the surrounding tissues and display little to no corrosion or wear. Ideally, this material should not incite any significant inflammatory response, particularly given its close proximity to adjacent roots and the spinal cord. The fatigue strength must be high and the wear debris minimal while exhibiting low friction.48 There are limited published data on the issue of wear and
wear debris in cervical spine arthroplasty. Although, much of these data has not shown any significant immunologic response to this debris, Bartels et al state that this is an important issue that should be closely investigated and monitored to avoid any repetition of the less than favorable historical experiences encountered with hip an knee prostheses.17 Particles produced from the articulation of components can cause an osteolytic response48 and affect implant fixation, as well as lead to implant migration and gross prosthesis failure. Much of this response is dependent on the particle size, number and concentration, and tissues in contact with the debris. This contact can trigger the release of inflammatory cytokines that can lead to an osteolytic phenomenon. Younger patients who may be reasonable candidates for cervical disc arthroplasty may place greater demands on their prosthesis for longer periods and may, in theory, be more likely to display changes due to inflammatory debris. Fortunately, to date, early clinical results have not shown widespread problems with osteolysis but close long-term follow-up is needed to avoid any future problems. The implant selected should ideally create minimal distortion and allow continued use of imaging modalities, such as computed tomography scan and MRI, to evaluate adjacent segments for future disease. Titanium and its alloys lead to less imaging artifact than other metals alloys, such as stainless steel and cobalt-chrome.48 Although titanium exhibits excellent biocompatibility with human tissues, some alloys are sensitive to stress concentrations, such as notching, and vigorous testing is necessary before widespread use of a new alloy is instituted. Other materials, such as stainless steal polyethylene, polyurethane, and ceramics, used in many orthopedic implants and biomedical devices at present may also have an important role or collaborative effort in future
192 cervical arthroplasty designs with continued research. The use of modified imaging modalities also needs to be explored. DeVine49 in a recent case series evaluated cervical MR images of patients who had undergone PCM cervical arthroplasty using a 1.5- vs a 0.3-Telsa magnet. His study revealed that the strength of the magnet might affect the artifact image produced by the material used in a cervical arthroplasty. He showed that using a 0.3-Tesla magnet allowed for better MRI (Fig. 8) of adjacent levels in patients treated with disc arthroplasty. Finally, the disc prosthesis must have immediate and longterm fixation to bone. Immediate fixation may be accomplished with screws, rails, or “teeth,” which are integral to the implant. However, fixation that requires any significant removal of native bone may place the vertebral body at risk for fracture.42 Although these techniques may offer immediate stability, other options, including porous or macrotexture surfaces,48 which allow for bone in-growth should also be explored. History has shown that fixation of components may hold the key to the long-term success of implanted artificial devices.50-52 A review of published data on total hip arthroplasty has shown that an optimal pore size may need to be established to have successful bone ingrowth.52 For instance, in the acetabulum, animal studies have revealed optimal size to be approximately 100-400 m.53 Further investigation and testing are required to establish that size for cervical disc replacements. Regardless of how fixation is achieved, there must be the capability for revision. Disc arthroplasty should be designed and constructed so that failure of individual components will not result in a catastrophic event and spinal stability will be maintained while protecting neural tissue.
Conclusion The release of the ideal cervical disc arthroplasty is yet to come. Current disc arthroplasties that are available for use or are in clinical trial show great promise to becoming an adjunctive tool in the armamentarium of a surgeon treating cervical disc disease. Complications of present disc arthroplasty devices continue to challenge device makers to create a disc that recreates the native cervical motion, treats the index level disease, and yet will last the lifetime of the recipient.
References 1. Yang S, Wu X, Hu Y, et al: Early and intermediate follow-up results after treatment of degenerative disc disease with the Bryan cervical disc prosthesis: single- and multiple-level. Spine 33:E371-E377, 2008 2. Auerbach JD, Jones KJ, Fras CI, et al: The prevalence of indications and contraindications to cervical total disc replacement. Spine J 8:711-716, 2008 3. Baba H, Furusawa N, Imura S, et al: Late radiographic findings after anterior cervical fusion for spondylotic myeloradiculopathy. Spine 18: 2167-2173, 1993 4. DePalma AF, Rothman RH, Lewinnek GE, et al: Anterior interbody fusion for severe cervical disc degeneration. Surg Gynecol Obstet 134: 755-758, 1972 5. Gore DR, Sepic SB: Anterior cervical fusion for degenerated or protruded discs. A review of one hundred forty-six patients. Spine 9:667671, 1984
L.K. Kibuule and J.S. Fischgrund 6. Wigfield C, Gill S, Nelson R, et al: Influence of an artificial cervical joint compared with fusion on adjacent-level motion in the treatment of degenerative cervical disc disease. J Neurosurg 96(suppl 1):17-21, 2002 7. Hilibrand AS, Carlson GD, Palumbo MA, et al: Radiculopathy and myelopathy at segments adjacent to the site of a previous anterior cervical arthrodesis. J Bone Joint Surg Am 81:519-528, 1999 8. Goffin J, Geusens E, Vantomme N, et al: Long-term follow-up after interbody fusion of the cervical spine. J Spinal Disord Tech 17:79-85, 2004 9. Gore DR, Sepic SB: Anterior discectomy and fusion for painful cervical disc disease. A report of 50 patients with an average follow-up of 21 years. Spine 23:2047-2051, 1998 10. Le H, Thongtrangan I, Kim DH: Historical review of cervical arthroplasty. Neurosurg Focus 17:E1, 2004 11. Wippermann BW, Schratt HE, Steeg S, et al: [Complications of spongiosa harvesting of the ilial crest. A retrospective analysis of 1191 cases]. Chirurg 68:1286-1291, 1997 12. Szpalski M, Gunzburg R, Mayer M: Spine arthroplasty: a historical review. Eur Spine J 11(suppl 2):S65-S84, 2002 13. Huang RC, Girardi FP, Lim MR, et al: Advantages and disadvantages of nonfusion technology in spine surgery. Orthop Clin North Am 36:263269, 2005 14. Fernstrom U: Arthroplasty with intercorporal endoprothesis in herniated disc and in painful disc. Acta Chir Scand Suppl 357:154-159, 1966 15. Cummins BH, Robertson JT, Gill SS: Surgical experience with an implanted artificial cervical joint. J Neurosurg 88:943-948, 1998 16. Porchet F, Metcalf NH: Clinical outcomes with the prestige II cervical disc: preliminary results from a prospective randomized clinical trial. Neurosurg Focus 17:E6, 2004 17. Bartels RH, Donk RD, Pavlov P, et al: Comparison of biomechanical properties of cervical artificial disc prosthesis: a review. Clin Neurol Neurosurg 110:963-967, 2008 18. Wigfield CC, Gill SS, Nelson RJ, et al: The new Frenchay artificial cervical joint: results from a two-year pilot study. Spine 27:2446-2452, 2002 19. Traynelis VC: The prestige cervical disc replacement. Spine J 4(suppl 6):310S-314S, 2004 20. Riew KD, Buchowski JM, Sasso R, et al: Cervical disc arthroplasty compared with arthrodesis for the treatment of myelopathy. J Bone Joint Surg Am 90:2354-2364, 2008 21. Robertson JT, Metcalf NH: Long-term outcome after implantation of the prestige I disc in an end-stage indication: 4-year results from a pilot study. Neurosurg Focus 17:E10, 2004 22. Goffin J, Van Calenbergh F, van Loon J, et al: Intermediate follow-up after treatment of degenerative disc disease with the Bryan cervical disc prosthesis: single-level and bi-level. Spine 28:2673-2678, 2003 23. Sasso RC, Best NM: Cervical kinematics after fusion and Bryan disc arthroplasty. J Spinal Disord Tech 21:19-22, 2008 24. Anderson PA, Sasso RC, Riew KD: Comparison of adverse events between the Bryan artificial cervical disc and anterior cervical arthrodesis. Spine 33:1305-1312, 2008 25. Yi S, Shin HC, Kim KN, et al: Modified techniques to prevent sagittal imbalance after cervical arthroplasty. Spine 32:1986-1991, 2007 26. Sasso RC, Smucker JD, Hacker RJ, et al: Artificial disc versus fusion: a prospective, randomized study with 2-year follow-up on 99 patients. Spine 32:2933-2940, 2007; discussion 2941-2932 27. Sekhon LH: Reversal of anterior cervical fusion with a cervical arthroplasty prosthesis. J Spinal Disord Tech 18(suppl 18):S125-S128, 2005 28. Bryan VE Jr: Cervical motion segment replacement. Eur Spine J 11(suppl 2):S92-S97, 2002 29. Leung C, Casey AT, Goffin J, et al: Clinical significance of heterotopic ossification in cervical disc replacement: a prospective multicenter clinical trial. Neurosurgery 57:759-763, 2005; discussion 759-763 30. Yoondo H: Clinical and radiographical results following cervical arthroplasty. Acta Neurochir (Wien) 148:943-950, 2006 31. Goffin J: Complications of cervical disc arthroplasty. Semin Spine Surg 18:87-98, 2006
Complications of cervical disc arthroplasty 32. Duggal N, Pickett GE, Mitsis DK, et al: Early clinical and biomechanical results following cervical arthroplasty. Neurosurg Focus 17:E9, 2004 33. Pickett GE, Sekhon LH, Sears WR, et al: Complications with cervical arthroplasty. J Neurosurg Spine 4:98-105, 2006 34. Pickett GE, Mitsis DK, Sekhon LH, et al: Effects of a cervical disc prosthesis on segmental and cervical spine alignment. Neurosurg Focus 17:E5, 2004 35. Fong SY, DuPlessis SJ, Casha S, et al: Design limitations of Bryan disc arthroplasty. Spine J 6:233-241, 2006 36. Hacker RJ, Babcock R: Bone loss in patients treated with Bryan cervical arthroplasty: results of a randomized prospective study. Spine J 8:84s85s, 2008 37. Pimenta L, McAfee PC, Cappuccino A, et al: Clinical experience with the new artificial cervical PCM (Cervitech) disc. Spine J 4(suppl 6): 315S-321S, 2004 38. Link HD, McAfee PC, Pimenta L: Choosing a cervical disc replacement. Spine J 4(suppl 6):294S-302S, 2004 39. Mehren C, Suchomel P, Grochulla F, et al: Heterotopic ossification in total cervical artificial disc replacement. Spine 31:2802-2806, 2006 40. Bertagnoli R, Duggal N, Pickett GE, et al: Cervical total disc replacement. Part 2: clinical results. Orthop Clin North Am 36:355-362, 2005 41. Murrey D, Janssen M, Delamarter R, et al: Results of the prospective, randomized, controlled multicenter FDA investigational device exemption study of the ProDisc-C total disc replacement versus anterior discectomy and fusion for the treatment of 1-level symptomatic cervical disc disease. Spine J 9:275-286, 2009 42. Shim CS, Shin HD, Lee SH: Posterior avulsion fracture at adjacent vertebral body during cervical disc replacement with ProDisc-C: a case report. J Spinal Disord Tech 20:468-472, 2007 43. McAfee PC: The indications for lumbar and cervical disc replacement. Spine J 4(suppl 6):177S-181S, 2004
193 44. Durbhakula MM, Ghiselli G: Cervical total disc replacement. Part 1: rationale, biomechanics, and implant types. Orthop Clin North Am 36:349-354, 2005 45. Traynelis VC, Haid RW: Spinal disc replacement: the development of artificial discs. SpineUniverse Available at: http://www.spineuniverse.com/ displayarticle.php/article1245.html. Accessed October 10, 2008. 46. Pimenta L: Cervical facet degeneration after total disc replacement 272 levels in 158 patients: 5 years follow up. Presented at Cervical Spine Research Society 36th Annual Meeting, 2008, Austin, TX 47. Yue JJ, McAfee PC, An HS: Motion Preservation Surgery of the Spine: Advanced Techniques and Controversies. Amsterdam, NY, Elsevier Health Sciences, 2008:816 48. Taksali S, Grauer JN, Vaccaro AR: Material considerations for intervertebral disc replacement implants. Spine J 4(suppl 6):231S-238S, 2004 49. DeVine JG: Magnetic resonance imaging evaluation of adjacent segments after cervical disc arthroplasty: magnetic strength and Tis effect on image quality. Presented at Thirty-Sixth Annual Meeting of the Cervical Spine Research Society, 2008, Austin, TX, pp 119-120 50. Taunton MJ, McIntosh AL, Sperling JW, et al: Total shoulder arthroplasty with a metal-backed, bone-ingrowth glenoid component. Medium to long-term results. J Bone Joint Surg Am 90:2180-2188, 2008 51. Madey SM, Callaghan JJ, Olejniczak JP, et al: Charnley total hip arthroplasty with use of improved techniques of cementing. The results after a minimum of fifteen years of follow-up. J Bone Joint Surg Am 79:5364, 1997 52. Illgen R II, Rubash HE: The optimal fixation of the cementless acetabular component in primary total hip arthroplasty. J Am Acad Orthop Surg 10:43-56, 2002 53. Kienapfel H, Sprey C, Wilke A, et al: Implant fixation by bone ingrowth. J Arthroplasty 14:355-368, 1999
A
nterior cervical discectomy and fusion (ACDF) has long been the gold standard for the treatment of cervical pathology. Conditions related to degenerative disc disease and radiculopathy have been treated with ACDF with predictable success for ⬎50 years.1 Biomechanical studies have shown good disease-free survival of up to 89% at 5 years for patients undergoing ACDF.2 However, ACDF has not been performed without risks and complications. The potential for pseudarthrosis, graft donor site morbidity, instrument-related complications, and future adjacent segment disease is significant.3-6 Such risks have motivated clinicians to search for alternatives to cervical discectomy and fusion. Recent efforts have focused on total disc replacement and interest in its application to the cervical spine. Cervical total disc replacement, if performed successfully, would preclude the need for graft harvest for fusion, attempt to maintain more physiological kinematics of the cervical spine and prevent/delay adjacent segment disease. Currently, there are 2 devices for cervical disc arthroplasty that have been approved by the US Food and Drug Administration (FDA) for clinical use and several oth-
Department of Orthopedic Spine Surgery, William Beaumont Hospital, Royal Oak, MI. Address reprint requests to Leonard K. Kibuule, MD, William Beaumont Hospital, 3535 West Thirteen Mile Road, Suite 744, Royal Oak, MI 48073. E-mail: [email protected]
1040-7383/09/$-see front matter © 2009 Elsevier Inc. All rights reserved. doi:10.1053/j.semss.2009.05.007
ers still under Investigational Device Exemption and in clinical trial studies. We hope that these new devices will have a positive effect on the treatment of cervical radiculopathy and myelopathy, but from their inception to current clinical use they too have met with challenges and complications.
Treatment of Cervical Disc Disease The initial treatment for cervical disease without neurologic deficit should involve nonoperative care. Many patients will respond to conservative care; including nonsteroidal antiinflammatory drugs, physical therapy, and, for some, steroid injections in and around the area of compression. When nonoperative management has been exhausted and symptoms persist or neurologic deficits exist, then operative management with decompression may be indicated. Conventional anterior cervical decompression and fusion procedures are performed through an anterior approach with a goal to decompress the neuroforamen and provide structural stability to the adjacent vertebral bodies. This procedure often involves the introduction of a structural bone graft with or without rigid fixation accomplished with an anterior plate. Fusion of this segment is intended to eliminate motion at this level. Degenerative changes at a segment adjacent to ACDF may contribute to recurrent symptoms. It is likely that these de185
186 generative changes are a result of natural progression of spondylotic changes at the unfused levels, possibly in combination with an increase in biomechanical stresses adjacent to the fusion. Hilibrand et al7 reported that 26.9% of patients had symptomatic adjacent segment disease at 10 years. Goffin et al8 followed up patients with ACDFs and also discovered 92% of them with adjacent segment disease at 5 years. Gore and Sepic9 followed up patients for a mean of 21 years after their initial ACDF and reported that 16% required additional surgery for symptomatic adjacent segment degeneration. Some biomechanical studies show increased intradiscal pressures and abnormal kinematics above and below a cervical fusion.10 The risk of pseudarthrosis following ACDF is a real complication and has been shown to increase with multiple attempted fusion levels.10 Bohlman showed a 13% pseudarthrosis rate at an average 6-year follow-up of patients and this increased to 27% for multilevel fusions.2 Although the use of iliac crest bone graft shows the most favorable rates of fusion, it is also not without its own risks. Such donor site complications have caused many surgeons to use alternative grafting techniques. Risks of neurovascular injury, fracture, chronic pain, infection, and abdominal herniation2 have convinced many surgeons to consider allograft bone or other bone substitutes. A review of 1191 iliac crest harvest cases showed postoperative complications in 20% of patients and 55% with subsequent chronic donor-site pain at 1-year followup.11 Although some surgeons have began using allograft bone or synthetic substitutes, the cost of these alternatives will continue to pose a challenge and other alternatives, such as disc arthroplasty, will continue to be explored.
Historical Perspective on Disc Arthroplasty During the past 50 years, there have been numerous designs for a cervical disc prothesis.12 In as early as 1950, clinicians experimented with the concept of artificial disc replacement in the spine. In 1950, Nachemson performed biomechanical studies after injecting cadaveric discs with the self-hardening
L.K. Kibuule and J.S. Fischgrund
Figure 2 Prestige ST cervical disc prosthesis. (Image courtesy of Medtronic Sofamor Danek USA, Inc. [Memphis, TN].)
liquid silicone rubber.13 Later in 1973, Stubstad et al developed several designs in the shape of a disc made from elastic polymer and undertook a primate study.12 Many of the early designs have been used to recreate the fluidlike, compressible nature of the native disc and much of the focus was placed on developing various types of plastics and polymers. In 1991, Bao and Higham even created hydrogel beads enveloped by a semipermeable membrane of Dacron or nylon for disc replacement.12 Others attempted to introduce material directly into the disc while leaving the annulus relatively intact. Baumgartner first used a flexible elastic coil introduced into the annulus and a few years later attempted the use of elastic beads.12 Many of these early concepts focused on lumbar artificial disc replacements. Fernstrom was the first to implant an artificial device into the cervical spine in 1966.14 He used spherical metallic balls in an attempt to reproduce the “ball joint” mechanism of the disc. These balls were implanted in over 250 patients but the technology was later abandoned because the devices caused hypermobility and tended to erode into the vertebral end plate and body.12
The Bristol/Cummins/Prestige Disc Prosthesis
Figure 1 Bristol Cervical Disc replacement. (Image courtesy of Medtronic Sofamor Danek USA, Inc. [Memphis, TN].)
In 1998, Gill and coworkers patented the Bristol cervical disc designed by Cummins15 (Fig. 1). Development of this device began in 1989 at the Department of Mechanical Engineering at Frenchay Hospital in Bristol, United Kingdom. The second generation of the Cummins disc was a ball-and-socket-type
Complications of cervical disc arthroplasty device constructed of stainless steel. One of the final devices was designed to be secured to the vertebral bodies with screws. The developers finally settled on a two-piece, stainless steel, metal-on-metal, ball-in-socket design known as the Bristol/Cummins (now referred to as the Prestige I) disc. Since the beginning of clinical trials, several reports have detailed the results of this disc prosthesis. Cummins et al17 described 20 patients who were followed up for an average of 2.4 years. Patients with radiculopathy improved, and those with myelopathy also either improved or were stable. Only 3 experienced continued axial neck pain, but no patient required additional motion segment surgery and x-rays did not demonstrate ectopic fusion at the level of the joint in any patient. In addition, no adjacent segment degeneration was seen nor any subsidence by the prosthesis into the vertebral bodies was seen. The Prestige II was built on many of the successes of its predecessor but with further improvements, including a reduced profile, a surface that allows bone ingrowth and additional selection sizes (Fig. 2).16 This disc arthroplasty was followed by the development of the Prestige ST in 2002, Prestige STLP in 2003, and finally the Prestige LP released in 2004.17 Despite some initial good results, the Cummins prosthesis also had its share of complications. Postoperatively, 2 of the 20 patients treated had a transient hemiparesis. There were also 5 partial screw pullouts, 2 broken screws, 1 joint subluxation, and 4 cases of persistent dysphagia. Fortunately, these complications did not require removal of the implant. However, one prosthesis was found to be loose and had to be removed and a standard interbody fusion employed to relieve persistent neck pain. The failure was attributed to a manufacturing error. Further examination revealed that the balland-socket fit was asymmetric but despite this finding, no significant wear debris was found in the surrounding tissues of the patient. Joint motion was preserved in all but 2 patients whose lack of motion was thought to be the result of an overdistraction of the disc space and separation of the facet joint at the time of implantation.15 With these initial clinical experiences, a second-generation Cummins artificial cervical disc was designed. New features were implemented with hopes of tackling some of the problematic issues. The new disc would rely more on the facet joints and surrounding tissue to restrain some of its motion. The hemispherical cup was replaced with a shallow ellipsoid saucer and freedom of translation and rotation were also increased.10,15 In an attempt to create a lower profile implant with less soft-tissue irritation, a screw locking mechanism was redesigned and the device made less bulky.10 The redesigned Frenchay disc (also known as the Cummins disc) was clinically examined and results published in 2002.18 Wigfield studied 15 patients who underwent cervical disc replacement for myelopathy or radiculopathy and radiographically confirmed disc herniations or posterior vertebral osteophytes. In all cases, the artificial joint maintained motion at the operative levels and re-established intervertebral height. The procedure was considered safe for experienced spine surgeons to perform, and the device was stable, with no dislocation of components or backing out of screws. Despite
187
Figure 3 Bryan cervical disc prosthesis. (Image courtesy of Medtronic Sofamor Danek USA, Inc. [Memphis, TN]. Bryan Cervical Disc System incorporates technology developed by Gary K. Michelson, MD.) (Color version of figure is available online.)
the overall reported success, there were 2 incidences of screw breakage and another patient had to undergo removal of the device for persistent neck pain while in extension. The implant was found to be loose with surrounding fibrous tissue. Wigfield attributed this failure to aggressive bone removal at the time of implantation, with resultant excessive load transfer to the facet joints.18 Further investigational studies have been performed of the Frenchay disc, including a European study in August 2000 and later in the United States in 2002 (now known under the name of Prestige ST). The Prestige ST cervical disc (Medtronic Sofamor Danek, Memphis, TN) was approved by the US FDA in July 2007. The major difference between the Prestige II and the Prestige ST was a 2-mm reduction in the height of the anterior flange.19 Prior to its approval, a limited number of published studies showed good short-term and intermediate-term results for this cervical disc replacement design.16,18 Riew et al20 recently performed an analysis of pooled data from an FDA Investigational Device Exemption study comparing cervical disc arthroplasty to ACDF for the treatment of myelopathy. This study showed an improvement in neurological status and gait function in patients treated with a Prestige cervical disc when examined at 2 years after surgery. During the Prestige ST trial, there was a single possibly implant-related adverse event in 1 (1.7%) of 59 patients, which necessitated removal of the prosthesis. In 2004, Robertson21 reported on the long-term 4-year results of 12 patients treated with the Prestige I disc. The 1 patient who required removal of the prosthesis at 12 months with subsequent fusion, as described in the original report, was not included in this follow-up study. A second patient suffered
188 progression of myelopathy and subluxation at an adjacent level due to advanced degenerative disc disease below the implant. This patient required posterior fusion adjacent and below the level of the implant. As a result, a lack of motion of the implant could be appreciated after 12 months following fusion. This illustrates how adjacent segment disease can still occur despite cervical disc arthroplasty.
Bryan Cervical Disc Prosthesis The Bryan artificial disc (Medtronic Sofamor Danek, Memphis, TN), developed in 1992, is a metal-on-plastic design based on a proprietary, low-friction, wear-resistant elastic nucleus. This nucleus is a polyurethane core located between and articulates with anatomically shaped titanium shells with porous surfaces that are fitted to the vertebral body endplates and allow for bone in-growth (Fig. 3). The disc also contains a polyurethane sheath surrounding the nucleus and is filled with saline, which acts as synovial fluid. The sheath is also intended to keep any potential wear debris from escaping into surrounding tissue. In vivo testing has revealed satisfactory wear characteristics without any significant inflammatory reaction.18 European clinical trials were initiated in 2000 and made available for general use in the same market in 2002. A randomized, prospective US investigational device exemption trial was initiated in May 2002. Goffin et al22 published data on a European multicenter trial of patients treated with a Bryan disc for single-level cervical disease. The authors reported that 44 of 49 patients (90%) had excellent, good, or fair outcomes at 2-year followup. A concurrent study involving bilevel cervical disc replacement with the Bryan prosthesis reported 25 of 26 patients (96%) having excellent, good, or fair outcomes at 1 year. These results are greater than the targeted success rate of 85% excellent, good, or fair, based on a meta-analysis of anterior cervical discectomy and fusion published data. One study reports maintenance of relatively normal kinematics of the motion segment in the short term after implantation of the Bryan disc.23 Many studies report good outcomes using the Byran cervical disc replacement for treatment of single-level cervical disease.1,24-27 Since its inception, few complications have been reported with this device. The initial studies, however, noted 30% of patients had paravertebral ossifications. Following this report, most patients began receiving a 2-week course of nonsteroidal anti-inflammatory drugs after surgery.28 Leung et al29 studied the clinical significance of heterotopic ossification (HO) in cervical disc replacement and found a strong association between HO and a loss of movement of the implanted cervical disc prosthesis. They analyzed the relationship of HO development with age, sex, duration of symptoms, and diagnosis and discovered that age and male gender were significant factors. Approach-related complications have also been acknowledged and have included hematoma, dysphonia from vocal cord paralysis, and cerebral spinal fluid leak.22,30 Incomplete decompression with persistent disease-related symptoms have also been described.22,31 Duggal described a patient who experienced increased radicular
L.K. Kibuule and J.S. Fischgrund pain and transient arm pain after Bryan cervical disc replacement despite adequate decompression seen on magnetic resonance imaging (MRI).32 In a study of 96 Bryan disc replacements in 74 patients, Pickett et al33 also discovered complications, including intraoperative prosthesis migration, HO with spontaneous fusion, partial prosthesis dislocation, and retropharyngeal hematoma formation. Device-related issues have included sporadic case reports of anterior or posterior migration22,32 or excess motion at the index level leading to explantation of the device and interbody fusion.27 Postoperative kyphosis occurring early on at the target level has also been reported in a number of patients.22,34 Pickett et al34 showed no improvement in kyphosis of the prosthesis level at late follow-up in 12 of 14 patients who underwent Bryan cervical disc replacement. However, despite kyphosis at the treated level, the overall alignment of the cervical spine remained relatively unchanged indicating that adjacent segments compensated for the focal kyphosis.32 Some of these issues were addressed during the redesign of the Bryan disc prosthesis and the associated instruments used to implant the device. For instance, the milling guide was altered to use a dual track system with a lordotic angle 7°, resulting in a more consistent shell alignment.31 More attention has been drawn to the insertion angle of the prosthesis and the preparation of the vertebral endplates and there is speculation that these may play a role in postcervical kyphosis.30,35 Others have advocated for more lordosis to be built into the prosthesis to help prevent late kyphosis.30 Hacker36 reported on vertebral bone loss in 32 patients treated with Bryan cervical disc arthroplasty. Bone loss was discovered in 3 patients as early as 6 weeks and as late as 48 months after implantation. The authors concluded that osteolysis, though not proven, was a strong consideration and a concern for device dislodgement and advocated routine annual cervical radiographs during the first 3-5 years of implantation.
Porous Coated Motion Disc Prosthesis The porous coated motion (PCM) (Cervitech, Rockaway, NJ) cervical disc replacement was designed by McAfee and modified by Helmut Link and Arnold Keller, two of the developers of the lumbar Charité disc prosthesis.37 The PCM disc replacement is made of a cobalt-chrome alloy with an ultrahigh molecular weight polyethylene (UHMWPE) core, and was designed to mimic the natural shape and size of the disc space. The upper (cobalt-chrome alloy) component is smooth where it meets the inner UHMWPE component, and is intended to slide around and across the core as the neck bends and twists. The outside of the components feature a TiCaP (titanium/calcium phosphate) double coating attached to serrated surfaces that interface with the vertebral end plate. This surface was modeled after the coating used with the SB Charité disc replacement, and allows for bone in-growth, whereas initial fixation relies on the press-fit engagement of the serrated edges. Two different designs are available, one of which relies on anchoring screws and is intended to address any losses in integrity of the posterior longitudinal ligament during cervical decompression. The prosthesis is meant to be
Complications of cervical disc arthroplasty
Figure 4 ProDisc-C disc prosthesis. (Courtesy of Synthes, Oberdorf, Switzerland. ©2008 Synthes, Inc. or its affiliates. All rights reserved.) (Color version of figure is available online.)
load bearing, minimally constrained, and uses surrounding muscles and ligaments to limit its motion.38 The first human clinical trial of the PCM cervical disc took place outside the United States in December 2002. Short-term results of the PCM have shown promising results. At 1-year follow-up, Pimenta et al37 showed that 53 patients who underwent cervical disc replacement with the PCM had significant improvements in visual analog scale (VAS) and neck disability index (NDI) scores. Excellent to good results were reported in 97% of patients. There was 1 postoperative complication reported; a 4-mm anterior displacement of the prosthesis. This was diagnosed 3 months after the operation without any adverse clinical symptoms. There was 1 case of Grade 1 HO (McAfee classification), during the ninth-month follow-up. Other clinical studies of this device are underway.
189 1 year. Pain intensity and frequency also improved by 40%. No device-related complications were reported in this shortterm study. Cases of loosening, subsidence, migration, metallic or polyethylene failure, allergic rejection and/or reaction, visceral or neurologic injuries caused by the implant components, or infection were also not reported. No approach-related complications were noted and no spontaneous fusions discovered in this study. More recently, Murrey et al41 compared the safety and efficacy of ProDisc-C to ACDF for the treatment of single-level cervical disc disease. This prospective, randomized controlled study showed functional improvements in both groups when comparing the VAS, NDI, and SF-36 scores. There was also a statistically significant reduction in medication usage (narcotics and muscle relaxants) and number of secondary surgeries in the disc replacement group. Published reports show good clinical success of ProDisc-C with few complications. In 2007, Mehren et al39 published results of a prospective clinical study evaluating rates of HO in patients treated with the ProDisc-C. HO that led to restrictions in the range of motion was present in 8 cases (10.4%) and spontaneous fusion in 7 cases (9.7%) at 1 year after surgery (Fig. 5). The authors found significantly higher rates of ossification in multilevel arthroplasties than in single-level cases. Despite this, there was still an improvement in the VAS and NDI scores for patients at the final follow-up, indicating a poor correlation of HO with these clinical parameters. Shim et al42 reported a case of fracture of the posterior central vertebral bodies of C6 and C7 with avulsed bony fragments compressing the spinal cord. The fragments were removed and the device implanted without neurologic deficit. Shim highlighted the importance of surgeon awareness of possible avulsion fracture while using the chisel to create a path for the keel fixation points of the prosthesis. Murrey41 reported 3 complications in 103 implanted discs with 1 patient electing
ProDisc-C Disc Prosthesis The ProDisc-C is manufactured by Synthes (Oberdorf, Switzerland). It obtained approval from the US FDA in December 2007 after an Investigational Device Exemption clinical study. It is composed of 2 cobalt-chrome metal endplates with a fixed UHMWPE core (Fig. 4).39 The metal end plates have a keel design for enhanced primary stability and fixation, whereas the end plate is covered with a titanium plasma spray coating that allows bony ingrowth for long-term fixation.39,40 The polyethylene center of the device determines the height of the implant, available in 3 different sizes and various footprints. Studies of the ProDisc-C show promising short-term outcomes. Patients assessed in a study showed postoperative improvement in functional outcomes measured by VAS and the NDI at 1-year follow-up.40 There was also an improvement in range of motion, with a return to normal function by
Figure 5 ProDisc-C with HO.
190
L.K. Kibuule and J.S. Fischgrund
removal of the implant due to persistent pain. Another patient sustained a dural tear related to device implantation.
Devices Still Under Development As of January of 2008, multiple other cervical disc replacement devices were confirmed as either under development or undergoing clinical trials. Bartles et al17 identified at least 10 cervical disc devices either currently on the market or in active development by an implant company. The success of total joint arthroplasty for the hip and knee and the limited success of current spinal arthroplasty have ignited a new excitement for further development of this technology. Cervical total discs, such as NeoDisc (Nuvasive, San Diego, CA), Cervicore (Stryker, Kalamazoo, MI), and Discocerv (Scientix, Maitland, FL), may, in future, become available for general use by surgeons.
Challenges of Selection, Design, and Implantation As artificial cervical discs continue to emerge, complications related to this new technology will continue to challenge designers and users to develop better, more reliable implants. In an effort to minimize some of these complications, it is important for users of these devices to recognize their various limitations. Not every device is designed to be used for all patients. Proper patient selection is the first key to a successful treatment of cervical disease with disc arthroplasty. Patients with radiculopathy or myelopathy without axial neck pain have conventionally been candidates for such a procedure.2,43,44 Most of the device manufacturers under the direction of the FDA have developed guidelines pertaining to the use of these devices and when their application is contrain-
Figure 6 Cervical disc arthroplasty shown with subsidence.
Figure 7 Reveals an extruded cervical disc arthroplasty.
dicated. Patients with significant facet arthrosis, preoperative instability on flexion-extension images, or concurrent systemic illness (ie, osteoporosis, rheumatoid arthritis, metabolic bone diseases) are just a few conditions cited as contraindications to cervical disc arthroplasty by many authors and manufacturers.2,31,43 Significant facet arthrosis may limit the intended motion of the cervical arthroplasty and promote adjacent segment degeneration. Inherent instability of the cervical spine at the time of implantation may expose the device to unintended stresses and lead to early loosening or even catastrophic failure. Patients with poor bone quality may predispose the implant to risks of subsidence and settling (Fig. 6). Patients with evidence of excessive ligamentous laxity should also be treated with caution. Proper balancing of the prosthesis may be an additional challenge in these patients. Suboptimal tissues may adversely affect the prosthesis outcome or even predispose the implant to migration, instability, or frank extrusion (Fig. 7). Appropriate preoperative evaluation of patients needs to be instituted to recognize these contraindications, avoid improper implantation, and give patients the best chance of a successful outcome. Cervical disc arthroplasty sets itself apart from anterior cervical discectomy and fusion by recreating a native intervertebral disc height and attempting to preserve cervical range of motion. The healthy native cervical disc provides support for physiological loads, allows for fluid range of motion, and provides adequate space for exiting nerve roots. Native discs also have some inherent shock absorption properties. Traynelis and Haid45 commented that ideally an artificial disc should recreate these properties and last the lifetime of the patient. This device should maintain the proper intervertebral spacing while providing stability for the adjacent structures. Replacement of the native disc with a pros-
Complications of cervical disc arthroplasty
191
Figure 8 (A) Sagittal image of PCM with 1.5-Telsa MRI magnet showing greater artifact adjacent levels. (B) Sagittal image of PCM with 0.3-Tesla MRI magnet showing less artifact at adjacent levels. (Courtesy of John DeVine, MD, Madigan Army Medical Center, Tocoma, WA.)
thetic device should be accomplished while limiting any physiological mismatch. If there is any significant mismatch present at the bone-implant interface, bone resorption with stress shielding may occur or even end the end plate or implant failure. The interaction of the disc arthroplasty with surrounding structures is crucial to its design. Any alteration in load may affect the facets and theoretically lead to facet arthrosis. Pimenta46 recently presented a study revealing that 8% of facets showed signs of arthrosis by computed tomography scan in patients implanted with a PCM disc arthroplasty at 5-year follow-up. Placement of the device should also be accomplished in such a way as to avoid the overdistraction of these important adjacent tissues. The importance of these structures cannot be overemphasized. Facets not only contribute strength and stability to the spine but they can also be a source of pain. The surrounding soft tissues should be manipulated carefully to minimize trauma to these areas and the development of any pathologic process, such as HO. Cervical arthroplasty should also exhibit tremendous endurance and longevity. The average age of a patient needing a lumbar disc replacement has been estimated to be 35 years and may be similar for cervical disc arthroplasty.47 To minimize the need for revision surgery the prosthesis must last some 30 to 40 years. A number of factors must be considered when choosing the materials that will bear these characteristics. The materials must be biocompatible with the surrounding tissues and display little to no corrosion or wear. Ideally, this material should not incite any significant inflammatory response, particularly given its close proximity to adjacent roots and the spinal cord. The fatigue strength must be high and the wear debris minimal while exhibiting low friction.48 There are limited published data on the issue of wear and
wear debris in cervical spine arthroplasty. Although, much of these data has not shown any significant immunologic response to this debris, Bartels et al state that this is an important issue that should be closely investigated and monitored to avoid any repetition of the less than favorable historical experiences encountered with hip an knee prostheses.17 Particles produced from the articulation of components can cause an osteolytic response48 and affect implant fixation, as well as lead to implant migration and gross prosthesis failure. Much of this response is dependent on the particle size, number and concentration, and tissues in contact with the debris. This contact can trigger the release of inflammatory cytokines that can lead to an osteolytic phenomenon. Younger patients who may be reasonable candidates for cervical disc arthroplasty may place greater demands on their prosthesis for longer periods and may, in theory, be more likely to display changes due to inflammatory debris. Fortunately, to date, early clinical results have not shown widespread problems with osteolysis but close long-term follow-up is needed to avoid any future problems. The implant selected should ideally create minimal distortion and allow continued use of imaging modalities, such as computed tomography scan and MRI, to evaluate adjacent segments for future disease. Titanium and its alloys lead to less imaging artifact than other metals alloys, such as stainless steel and cobalt-chrome.48 Although titanium exhibits excellent biocompatibility with human tissues, some alloys are sensitive to stress concentrations, such as notching, and vigorous testing is necessary before widespread use of a new alloy is instituted. Other materials, such as stainless steal polyethylene, polyurethane, and ceramics, used in many orthopedic implants and biomedical devices at present may also have an important role or collaborative effort in future
192 cervical arthroplasty designs with continued research. The use of modified imaging modalities also needs to be explored. DeVine49 in a recent case series evaluated cervical MR images of patients who had undergone PCM cervical arthroplasty using a 1.5- vs a 0.3-Telsa magnet. His study revealed that the strength of the magnet might affect the artifact image produced by the material used in a cervical arthroplasty. He showed that using a 0.3-Tesla magnet allowed for better MRI (Fig. 8) of adjacent levels in patients treated with disc arthroplasty. Finally, the disc prosthesis must have immediate and longterm fixation to bone. Immediate fixation may be accomplished with screws, rails, or “teeth,” which are integral to the implant. However, fixation that requires any significant removal of native bone may place the vertebral body at risk for fracture.42 Although these techniques may offer immediate stability, other options, including porous or macrotexture surfaces,48 which allow for bone in-growth should also be explored. History has shown that fixation of components may hold the key to the long-term success of implanted artificial devices.50-52 A review of published data on total hip arthroplasty has shown that an optimal pore size may need to be established to have successful bone ingrowth.52 For instance, in the acetabulum, animal studies have revealed optimal size to be approximately 100-400 m.53 Further investigation and testing are required to establish that size for cervical disc replacements. Regardless of how fixation is achieved, there must be the capability for revision. Disc arthroplasty should be designed and constructed so that failure of individual components will not result in a catastrophic event and spinal stability will be maintained while protecting neural tissue.
Conclusion The release of the ideal cervical disc arthroplasty is yet to come. Current disc arthroplasties that are available for use or are in clinical trial show great promise to becoming an adjunctive tool in the armamentarium of a surgeon treating cervical disc disease. Complications of present disc arthroplasty devices continue to challenge device makers to create a disc that recreates the native cervical motion, treats the index level disease, and yet will last the lifetime of the recipient.
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