Spinal motion preserving technology in general, and total disc replacement (TDR) in particular, has captured the imagination of the spinal community. In the lumbar spine, this technology has been touted as an alternative to fusion surgery for the treatment of axial low back pain.1,2 Cervical disc replacement has a shorter clinical history than lumbar disc replacement and to date has been used following anterior discectomy for the treatment of radiculopathy or myelopathy. Some cervical TDR implants are “shrunk down” versions of their lumbar predecessors, whereas others have unique design characteristics. When considering cervical disc replacement, it is important to understand the specific kinematics, anatomy, disease processes, and treatment outcomes pertaining to the cervical spine.
Issues relating to cervical disc replacement are quite different from those in lumbar disc replacement surgery. Lumbar disc replacement has been considered for the treatment of degenerative disc disease, a condition for which treatment success with fusion has been quite limited.1,2 In contrast, fusion following an anterior cervical discectomy has been quite successful. Lumbar disc replacement surgery involves a specific decision to proceed with TDR surgery. In the cervical spine, the decision to proceed with anterior decompressive surgery for radiculopathy or myelopathy is made independent of the choice of reconstruction. Once decompression is accomplished, the surgeon may then decide to proceed with TDR, or placement of a structural interbody device with or without supplementary plate fixation.
Clinical trials (ongoing and planned) for cervical disc arthroplasty include patients having decompression for the treatment of cervical spondylosis giving rise to radiculopathy or myelopathy. The high success rate and long-term track record of anterior cervical decompression and fusion (ACDF) in the treatment of these conditions, raises the question as to the need for the development of alternate procedures. Proponents of artificial disc technology claim that, although cervical arthrodesis is often clinically successful in the short-term, fusion results in increased biomechanical stresses at adjacent segments that may hasten degeneration at these levels.3,4,5,6 Alternatively, artificial disc replacement maintains motion at the operated level, thereby maintaining adjacent level kinematics and reducing the rate of adjacent level degeneration when compared with fusion. The fate of segments adjacent to a fusion has indeed become the rationale for the development of TDR and merits further discussion.
Degeneration Adjacent to Fusion
Biomechanical studies have shown that cervical fusion alters adjacent level kinematics,3,4 whereas TDR leads to a normalization of load transfer and kinematics at adjacent levels when compared with fusion. DiAngelo et al have shown that, after anterior cervical fusion, the loss of motion at the index level is compensated for by an increase in motion at adjacent segments.3 In contrast, use of an artificial disc replacement did not alter motion at either the instrumented or adjacent levels. Eck et al found a 73% and 45% increase in intradiscal pressure at levels cephalad and caudad to a simulated fusion, respectively.5 In contrast after TDR (Bryan, Medtronic Sofamor Danek, Memphis, TN), Wigfield et al recorded stress profiles in the adjacent level intervertebral discs that were similar to those seen in nontreated, intact specimens.6 In the adjacent level anulus, the artificial disc led to reduced stresses when compared with spines with a simulated fusion.
Cervical spondylosis is thought to be an inevitable consequence of aging. After age 40, almost 60% of the population has radiographic evidence of cervical spine degeneration and by age 65, 95% of men and 70% of women have at least one degenerative change on roentgenograms.7–9 The rates of adjacent level degeneration after cervical fusion must therefore be compared with these natural history data.
Baba et al reported that, at an average of 8 years after ACDF, 25% of patients developed new onset spinal stenosis adjacent to the previously fused segments.10 Gore and Sepic observed new spondylosis in 25% of 121 patients and progression of preexisting spondylosis in another 25% of patients who had undergone prior ACDFs with a mean follow-up of 5 years.11 They noted no correlation between these new radiographic findings and the development of clinical symptoms. In a follow-up study of 50 patients, Gore and Sepic described that 14% of patients underwent additional surgery for adjacent level disease after ACDF.12 It remains challenging to determine whether the reports of degeneration adjacent to cervical fusion reflect only the consequence of altered biomechanics resulting from the fusion or represent to some degree the natural tendency toward degeneration of the cervical spine with aging, particularly in the group of individuals who have had clinical symptoms and signs leading to surgery. Hillibrand et al reported on the long-term follow-up of 409 patients that had anterior cervical decompression and fusion procedures.13 They reported that 14% of patients had additional neck surgery over a 21-year period with an average annual incidence of development of adjacent level disease of 3%. The authors noted that anterior cervical fusion performed at more than one level had a significantly lower rate of development of adjacent level disease than those fusions performed at a single level. This seems counterintuitive, as one might have predicted that the longer fusion constructs would result in greater adjacent level stresses than single level fusion. Hillibrand et al, however, noted “the results of this study suggest that adjacent segment degeneration was a common problem, but may reflect the natural history of the underlying cervical spondylosis.”14 In conclusion, review of the literature suggests that, although biomechanical studies have proven the deleterious effects of cervical fusion on adjacent level kinematics, the clinical relevance is not clearly established.
Other rationales for the use of cervical TDR relate to the complications or morbidity associated with ACDF. Anterior cervical arthrodesis heals gradually, and not always. In many cases, surgeons impose limitations on patient activities in an effort to enhance fusion. This may slow the patient’s ability to return to normal employment and lifestyle. After disc replacement, patients will likely be able to more rapidly resume unlimited activities. In a small percentage of ACDFs, pseudarthrosis develops, which may compromise the ultimate clinical results, and leads to revision surgery.15,16 This complication would obviously be eliminated by TDR. In addition, TDR would eliminate the risks and morbidities associated with bone graft procurement for arthrodesis. However, before embracing TDR technology, we must ensure that a new set of more significant complications and morbidities are not created by this intervention (Table 1).
Implant design characteristics are important for functioning and longevity of TDR. The articulating surfaces must be able to tolerate anticipated load without fatigue or failure, while minimizing friction, and should have superior wear characteristics with minimal debris generation. In addition, the implants must remain permanently affixed to the adjacent vertebral bodies (Table 2).
Although the stated goal of all cervical prosthesis designs is to restore, or maintain, normal cervical disc motion after discectomy, the kinematics and biomechanics of cervical TDRs have not been widely reported. Cervical range of motion involves complex coupled motions that may be difficult to reproduce artificially. Puttlitz et al have recently reported that in a cadaveric model, a ball-and-socket design disc prosthesis (Prodisc C, Synthes, Paoli, PA) produced normal physiologic motion and maintained coupled motion patterns.17 The importance of restoring physiologic motion in a degenerating motion segment that is naturally tending toward decreased range of motion is unclear, as are the consequences of that. In addition, the increased motion after TDR may allow for ongoing nerve irritation unless the neural elements are adequately decompressed. It is therefore imperative to directly decompress the neural elements during TDR surgery and probably desirable that the prosthesis provides foraminal distraction to effect, and maintain, decompression of the nerve roots.
Most TDR articulations have either single-gliding or double-gliding interfaces. The geometry of articulations in current double-gliding designs of cervical TDR include ball-and-socket and saddle designs that permit rotation and in some instances translation. In order to protect the facet joints from abnormal stresses, the implant should have an axis of rotation (AOR) that mimics that seen in the normal spine. In certain implant designs the AOR remains fixed, whereas in others it is dynamic.
Implants may be constrained or relatively unconstrained, in which case they are reliant on surrounding soft tissues to provide restraint to extremes of range of motion. With unconstrained implants, appropriate soft tissue tensioning is important for stability. Retention of the posterior longitudinal ligament has been shown to enhance stability after discectomy when placing a cervical prosthesis.18 Generally, unconstrained devices allow translation and diminish stress concentration at specific points on the articulating surfaces. The lack of constraint may, however, subject the facet joints to greater shear and torsional loads.19,20 These unconstrained devices have a mobile AOR, so that they may be more forgiving of small errors in implant placement. More constrained devices achieve greater stability; however, they create greater stresses at the implant-bone interfaces. Constrained devices generally have a fixed axis of rotation and in theory minimize shear at the facet joint.19 However, devices with a fixed AOR may be less forgiving and require precise anatomic placement so that the TDR axis of rotation mimics the natural posterior AOR of the motion segment.
Metal composition of the prosthesis is an important design consideration. Cobalt chrome (CoCr) alloys have been extensively used as bearing surfaces in joint arthroplasty because of their excellent wear characteristics. Stainless steel alloys have long been used as orthopaedic implants; however, they have not been widely used for arthroplasty because of inferior mechanical properties. Titanium alloys have generally not been widely used for articulating components because of their poor wear characteristics. In the cervical spine, where MRI imaging may be required after TDR, titanium with its improved MRI imaging compatibility offers distinct advantages. Surface treatment of titanium, such as coating with nitride or diamond-like carbon, may improve hardness and wear characteristics.21,22
In the appendicular skeleton generation of wear debris is the primary source of artificial joint implant degradation and the subsequent tissue, and systemic reaction to the debris is an important factor in limiting longevity of the prosthesis. Debris has been associated with osteolysis, implant loosening, and prosthesis failure.23,24 These complications are influenced by not only the number but also the size and shape of the wear particles.25 Debris may be generated by wear, fretting, or fragmentation. Polyethylene-on-metal provides a low friction articulation but generates polyethylene wear debris. Polyethylene wear is an established cause of failure of hip and knee arthroplasty. Cross-linking with gamma irradiation has been used to improve wear properties of ultra-high molecular weight polyethylene. However, this can also affect the mechanical properties.26,27 Metal-on-metal articulation has gained popularity because of dramatically lower wear rates compared with polyethylene-on-metal articulations. Although with metal-on-metal articulations the wear debris generated is markedly less volumetrically, there are greater numbers of debris particles that are smaller in size than particles generated by polyethylene-on-metal articulations. Enthusiasm for metal-on-metal articulating surfaces must be tempered by reports of systemic metal deposition after hip arthroplasty, although no adverse clinical effects have been attributed to the deposition of heavy metal.28,29,30 It is uncertain whether dissemination of metallic debris after hip arthroplasty is analogous to an arthroplasty in the relatively avascular, nonsynovial, cervical disc space. In addition metal-on-metal prostheses provide less shock absorption than does metal-on-polyethylene articulations. To date, acceptable rates of wear (typically to 10 million test cycles) have been claimed by the manufacturers of all cervical devices in their U.S. FDA Investigational Device Exemption (IDE) submissions. Manufacturers have also reported that wear particles generated have been nontoxic in rabbit studies.
Fixation to Bone
Long-term implant fixation depends on bone ingrowth into the surface of the prosthesis. Ingrowth depends on initial stability of the implant, pore size, and pore geometry. The initial stability with a TDR depends on soft tissue tensioning, implant surface geometry (corrugated or serrated) and dimensions, as well as any anchoring of the implant to the host bone using, for example, stabilizing fins or screws. Surface coatings to improve bony ingrowth include titanium wire mesh, plasma-sprayed titanium, porous CoCr, and bioactive materials such as hydroxyapatite and calcium phosphate. Although not well characterized in cervical implants, good bony ingrowth into the surface of lumbar disc implants has been demonstrated in nonhuman primates.19
The potential for implant subsidence remains a concern with total disc implants. This phenomenon has been described after lumbar TDR.31 Subsidence could lead to implant loosening, altered implant kinematics affecting TDR function, and wear characteristics. It also leads to increased stresses on the facet joints. Maintaining the harder subchondral bone during discectomy and placing a large enough implant footplate that is anchored on the peripheral apophysial ring and the harder lateral uncovertebral bone may reduce the risk of subsidence.32 With aging and development of osteoporosis, the impaired ability of the cancellous bone of the vertebral body to support the vertebral endplates may, however, place an implant at risk for late subsidence and failure.
There are cervical disc replacement designs by a number of companies undergoing FDA IDE studies in the United States. These include the Prestige and Bryan discs (Medtronic Sofamor Danek), ProDisc Cervical (Synthes), and the PCM (Cervitech, Roundhill, NJ). At the time of this writing, the CerviCore (Stryker, Rutherford, NJ) is awaiting approval for an IDE study (Table 3).
The Prestige ST prosthesis, currently in U.S. IDE study, is a two-piece prosthesis constructed of stainless steel, employing a ball-in-groove articulation. This design permits motion in flexion, extension, lateral bending, and axial rotation about the center of rotation of the ball component of the upper base plate. The ball-socket design allows for anterior-posterior translation of the center of rotation. The endplates are roughened by grit blasting to promote bony ingrowth and are attached to the adjacent vertebral bodies with a locking screw.
Traynelis has reported the results of wear testing (presumably obtained from the implant manufacturer).33 After 10 million cycles in flexion and extension and 5 million of a coupled axial rotation and lateral bending, 0.37 to 0.42 mm3/million cycles of material were lost. This compares favorably with the 5 mm3/million cycles of debris generated by Co-Cr total hip prosthesis. No information as to the size of particles generated was published.
The design of the Prestige ST has been modified (Prestige STLP) to include rails to secure the prosthesis to the adjacent vertebral body endplates, which eliminates the anterior profile of the prosthesis that originally had been placed for screw attachment.
Robertson and Metcalf reported the 4-year results in 14 of 17 patients who had a Prestige I prosthesis implanted for “end-stage” disease, in patients who “often had a history of multiple previous fusions.”34 Radiographic analysis confirmed maintained motion at the operated segment. In 2002, Robertson et al reported on patients with single-level cervical radiculopathy or myelopathy who were decompressed and randomized to receive uninstrumented arthrodesis or the Prestige II disc device (precursor to Prestige ST).35 Twenty-seven patients were randomized to each arm. At 2 years, the arthroplasty groups had retained motion across the operated level and had improved pain and physical function scores when compared with the arthrodesis group. Porchet and Metcalf, in 2004, reported on what appears to be the same cohort (55 patients) showing clinical improvement in both the ACDF and the arthroplasty groups with radiographic results showing that the Prestige II disc maintained motion at the treated level.36
The Bryan disc is single-piece metal-on-polymer prosthesis comprised of a polycarbonate/polyurethane core between two porous coated endplate shells, encapsulated by a polymer sheath. The instantaneous axis of rotation is variable and not limited by the geometry of articulating surfaces characteristic of two-piece disc designs. The polyurethane sheath is intended to contain debris and also prevent soft tissue ingrowth, The endplates are porous-coated titanium alloy. At the time of this writing, worldwide more than 4,000 Bryan prostheses have been implanted, and a U.S. IDE is in process (Figure 1).
Anderson et al have reported the results of wear testing for the Bryan Disc.37 After 10 million cycles, the mean height loss was 0.75%. Particles generated had a mean diameter of 3.89 μm. Particle shape varied from those generated after hip and knee arthroplasty testing. After 40 million cycles, endplate-to-endplate contact was observed. The change in prosthetic height was 0.02 mm per million cycles. The authors stated they think that 100,000 to 400,000 stimulator cycles represent 1 year of clinical use. The in vivo inflammatory response of the device was studied in a caprine model at C4–C5. No inflammatory response was observed 1-year postimplantation.
Goffin et al reported similar motion after implantation of Bryan discs at C5–C6 compared with normal volunteers.38 Goffin et al reported on 60 patients that underwent single-level anterior discectomy and placement of a Bryan disc prosthesis.39 At 12 months, they reported success rates of 85% to 90%. Two patients had possible device migration. Range of motion was preserved, and no device has been revised or explanted. Duggal et al recently reported on 26 patients treated with the Bryan disc for degenerative cervical radiculopathy or myelopathy.40 A significant improvement in Neck Disability Index and a trend toward improved SF-36 scores was noted at 24 months. Motion was preserved at the treated spinal segment (mean, 7.8°). Pickett et al reported on 14 patients who received the Bryan disc prosthesis and were followed from 6 to 24 months.41 Motion at the index level was similar before surgery and at final follow-up. The treated level, however, became and remained more kyphotic after insertion of the Bryan prosthesis.
Goffin et al reported heterotopic ossification (HO) around the Bryan disc with some impact on clinical results.42 Development of HO may be the result of the extensive bone removal (milling) required for implantation of this particular prosthesis. Heller et al43 have recommended routine use of nonsteroidal anti-inflammatory medication in the perioperative period to reduce the risk of HO, and this is required in the U.S. IDE study with this prosthesis.
The Prodisc-C is constructed of two chromium-cobalt endplates with sagittal fins for fixation into the adjacent vertebral body and a fixed polyethylene core. The joint consists of a concave cephalad component that rides on a UHMWPE insert fixed to the caudad component. This articulation provides coupled motion without independent translation resulting in a fixed center of rotation in the vertebral body below the disc space. The surfaces of the prosthesis in contact with the vertebrae have a plasma-spray titanium layer to promote bony ingrowth.
Porous Coated Motion (PCM)
The PCM is a polyethylene-on-metal design with a large radius UHMWPE bearing surface attached to the caudal endplate, allowing for translational motion. The cobalt-chromium endplates are coated with titanium with electrochemically coated calcium phosphate in a 1:1 ratio. The surface encourages osseous integration. The PCM endplates are shaped to maximize support in the dense lateral bone in proximity to the unco-vertebral joints.
Pimenta in Brazil implanted 81 PCM discs in 52 patients with degenerative disc disease and radiculopathy or myelopathy.44 Surgery took approximately 50 minutes per level and estimated blood loss was 50 mL per level implanted. Significant improvement in pain intensity, disability, and analgesic use was noticed at 1-year follow-up. Complications included a single prosthesis that displaced 4 mm anteriorly and one case of mild HO.
The CerviCore prosthesis is a metal-on-metal (chrome-cobalt) design with a saddle-shaped articulation. The designers of this device assert that the articulation allows for maintaining the axis of vertebral rotation in the caudal vertebral body during flexion-extension while simultaneously maintaining the axis of rotation in the cephalad-vertebral body during lateral bending, mimicking the normal disc AOR. The designers also claim that the articulation mimics the function of the unco-vertebral articulation and promotes vertebral foraminal widening during coupled rotation and bending. These claims await verification. The base plates feature a titanium spray and three spikes. After placement of the device, bone screws are inserted through anterior flanges into the vertebral bodies. To date, there are no reports of clinical implantation of this prosthesis.
Cervical disc replacement is an exciting technology that preserves motion at the instrumented level/s and will potentially improve load transfer to the adjacent levels when compared with fusion. Clinical reports of success of cervical TDR are encouraging but are also quite preliminary. The consequences of failure of a cervical TDR implant in close proximity to the spinal cord, the esophagus, and the trachea must be considered. In addition, reconstructive strategies after device failure are likely to be complex. As the U.S. IDE studies are completed, a clearer role for the place of cervical TDR in the spine surgeon’s armamentarium should emerge. Most implant designs in trial are quite similar, and alternative design concepts will probably be developed in the future.
- The perceived risk of accelerated degeneration at the motion segments adjacent to cervical fusion provides the rationale for the development of cervical disc replacement.
- Biomechanical studies suggest that cervical fusion alters adjacent level kinematics, whereas cervical disc replacement leads to a normalization of load transfer and kinematics at adjacent levels compared with fusion.
- Preliminary short-term clinical results for cervical disc replacement are encouraging with few complications reported.
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