Lumbar Total Disc Replacement

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CHAPTER 51 Lumbar Total Disc Replacement

Jeffrey M. Spivak, MD, Tom Stanley, MD, MPH, Richard A. Balderston, MD

Lumbar spinal fusion is a time-tested, proven successful procedure for alleviating symptoms in patients with unremitting low back pain from various causes. Immobilizing an unstable motion segment, as in a degenerative or isthmic spondylolisthesis, is the most common and least controversial reason for performing a lumbar spinal fusion. Painful disc degeneration is a more controversial indication for spinal fusion because methods for identification of an intervertebral disc as the pain generator are imperfect. Multiple studies have shown successful pain relief of symptomatic lumbar degenerative disc disease with lumbar fusion.¹^–⁴ Fusion also creates an anatomic situation with potentially negative consequences for long-term lumbar function, however. By limiting motion of the affected motion segment, additional stress is created at adjacent motion segments, leading to an increased incidence of adjacent level degeneration.⁵^–⁷ Often this degeneration becomes symptomatic and is a common cause of late failure after lumbar fusion.⁷^–¹⁰ In addition, spinal fusion fixes the sagittal alignment at that motion segment, which can affect the adjacent segments over time and be responsible for adjacent segment degeneration.¹¹ A third potential cause of adjacent segment degeneration is the potential iatrogenic injury of the adjacent level with surgery, especially injury to the cephalad facet joint with placement of pedicle screw instrumentation.

Total disc replacement (TDR) has been developed as an alternative method of stabilizing the lumbar spinal motion segment after removal of the painful disc. By maintaining mobility at the operative motion segment, patients maintain more normal overall lumbar mobility; theoretically, this may limit or eliminate the increased rate of adjacent segment degeneration over time seen with fusion surgery. Current implant designs require inherent motion segment stability before disc removal and mobilization, making significant spondylolisthesis, degenerative scoliosis, and isthmic defects contraindications to this technology. Advanced facet arthrosis is also considered a contraindication because continuing motion across arthritic facets may result in continued lumbar pain.

With these clinical and anatomic limitations on use, lumbar disc arthroplasty is appropriate only for a small percentage of overall lumbar fusion patients. Various studies have shown potential lumbar TDR candidates limited to 0% to 25% of fusion patients, depending on the specific demographics of the surgical practices being studied.^12,¹³ Many TDR implants have been shown to retain flexion-extension motion at the operative motion segment over time.¹⁴^–²⁰ This chapter addresses the general concepts of lumbar total disc arthroplasty and focuses on the specifics of lumbar TDR implants that currently have U.S. Food and Drug Administration (FDA) approval or are completing FDA-sponsored multisite investigational device exemption (IDE) trials in preparation for FDA approval.

Historical Devices

The origins of disc replacement date back to the late 1950s, with the first early attempts at disc reconstruction being accomplished by the injection of acrylic bone cement within the nucleus after discectomy.²¹^–²³ Around the same time, Fernstrom²⁴ began implanting stainless steel spheres into the intervertebral disc space through a posterior approach after laminotomy and discectomy. Low back pain was associated with progressive disc height loss, and so the goal of these early devices was to restore disc height and maintain spinal motion. Fernstrom²⁴ published his results in 1966, showing fair clinical results at 2 years postoperatively. The long-term outcomes of this device showed a high failure rate, however, secondary to subsidence of the implant with loss of motion, foraminal narrowing, and pain.

In the 1980s, artificial discs were designed using metal endplates and an intervening elastomeric rubber or plastic core, creating metal-plastic and rubber articulations among unattached components. The elastomeric discs were developed by Stefee and Lee and colleagues.²⁵^–²⁹ Only the Stefee disc (Acroflex, Acromed) was implanted clinically, but it was soon discontinued because of catastrophic failure of the rubber core.²⁵ Articulating discs include the SB CHARITÉ and the Marnay disc (later renamed the Prodisc I, precursor to the current Prodisc-L). Both discs enjoyed clinical success and are discussed later in this chapter. Another older implant design that never progressed to clinical application included the use of hinged spring articulation in all metal components. The newer, “modern,” designs that are discussed in more detail in this chapter draw upon the continued Orthopaedic experience with large joint replacement with respect to articulating materials, component geometry, and component constraint to minimize the risk of mechanical failure of the arthroplasty device or late failure of the supporting structures of the motion segment.

Biomechanical Considerations

Each lumbar vertebral motion segment consists of three articulations—the intervertebral disc and the two facet joints. Static stabilizers of the motion segment include the anterior longitudinal ligament, posterior longitudinal ligament, interspinous ligament, intertransverse ligaments, ligamentum flavum, and facet joint capsules. The normal intervertebral disc is designed to absorb compressive forces. Compressive forces are converted to tensile forces in the anulus fibrosus via the nucleus-anulus couple. The effect of degeneration of the disc space is increased forces across the facet joint, leading to joint degeneration. One goal of disc replacement surgery is to restore the biomechanical function of the disc before facet joint degeneration, preventing long-term pain and disability.

Most of the forces seen across the motion segment are transmitted through the disc space. Force distribution across the disc and facet joint is posture dependent, however. Ledet and colleagues ³⁰ performed real-time measurements of forces across the disc space using an interbody force transducer. In this study, forces across the intervertebral disc are highest when the center of gravity is shifted forward and during twisting motions. Forces across the intervertebral disc decrease when the center of gravity is shifted posteriorly. The corresponding postures show high forces across the disc space in sitting positions with the highest forces seen during sitting with the torso flexed and twisted. Standing and lying in the supine position resulted in progressively lower forces at the disc space. A fused segment has no effect on the force transmission to the adjacent level but increases the range of motion of the adjacent level.³¹ The body attempts to preserve the total range of motion of the lumbar spine by compensating through the adjacent levels.

The vertebral motion segment is generally described as having six degrees of freedom. It can rotate (in two directions) along three axes, resulting in flexion-extension (rotation around the left-right axis or X-axis), left and right lateral bending (around the sagittal axis or Z-axis), and left and right axial rotation (around the coronal axis or Y-axis). Translational motion is also possible in the anteroposterior, left-right lateral, and up-down axial directions. Although distraction and compression can occur across the disc space, this motion has not been taken into consideration for the current first-generation articulating implant designs.

The motion segment does not act as a pure hinge in that the axis of rotation changes slightly with flexion and extension.^32,³³ Each lumbar vertebral level has a different location of the instantaneous axis of rotation for a given movement. For the L1-5 disc spaces, the average instantaneous axis of rotation is slightly posterior to midline and just below the inferior endplate on a mid-sagittal image. At L5-S1, the instantaneous axis of rotation is within the disc space.³⁴ Range of motion and type of motion vary by level. Flexion-extension ranges from 12.7 to 19.6 degrees with the highest motion occurring at the L5-S1 segment (Table 51–1).³² At the L4-5 level, the motion is mostly translational, whereas the motion is rotational at the L5-S1 segment. Most studies comparing disc arthroplasty with the normal motion segment focus on flexion and extension. The motion of each vertebral segment is not only multidimensional, but also rotation and translation are coupled to varying degrees by level.

TABLE 51–1 Flexion-Extension of Lumbar Spine

L1-2	12.7 degrees ± 2.47
L2-3	16.6 degrees ± 2.43
L3-4	16.7 degrees ± 2.36
L4-5	18.3 degrees ± 2.67
L5-S1	19.6 degrees ± 4.71

Implant Design Considerations

Any design for a lumbar TDR implant should take into account the overall goals of the procedure, which include safe surgical implantation and long-lasting preservation of spinal motion. It is estimated that the lumbar spine undergoes 125,000 bending movements per year and is involved in 2 million strides per year. Over a 40-year life expectancy for a TDR implant, one might expect the implant to see 85 million cycles of motion. Implant endurance depends on its material characteristics, the nature and biomechanics of the articulation allowing motion, and the early and long-term fixation of the implant within the disc space. The range of motion an implant allows depends on the nature of the articulation or flexible materials and the geometry of the implant design.

Constrained versus Unconstrained

The concept of constraint in TDR has evolved over time and is not the same as in other articulating joint replacements of the appendicular skeleton. Kostuik ³⁵ described constraint in terms of the implant’s potential to limit the natural range of motion of the vertebral motion segment for each of the various rotational and translational motions. An implant unconstrained for a certain motion would provide no limitation to that motion well beyond the normal physiologic range. Examples are the CHARITÉ, Prodisc-L, and Maverick discs, which all are ball-and-socket articulations that provide no inherent limitation of axial rotation. These implants do have differing constraint, however, for the other motions of the lumbar motion segment.

An implant is considered overconstrained for a given motion if it limits that motion range to less than the physiologic range. The same three designs just mentioned are examples; all are overconstrained for motion in the compression-distraction plane. An implant is considered underconstrained for a given motion if it provides a limitation to that motion at or just outside of the extreme of the normal physiologic range. An example is the CHARITÉ prosthesis with a mobile core articulation, which is underconstrained for sagittal and lateral translation motions, whereas the ball-and-socket articulations of the Prodisc-L and Maverick are considered overconstrained with respect to these translational motions. These differences in constraint between the implant designs result in differing stresses seen within the supporting soft tissue structures and the facet articulations within the treated lumbar motion segment over time.

Generally, the effect of increasing constraint is to increase the overall stability of the motion segment. A consequence of increasing constraint may be to decrease range of motion and change the stresses seen at the surrounding structures. Increasing constraint has been shown to increase the forces seen at the endplates.^34,^36,³⁷ Increased stress at the endplate can have an effect on loosening of the component over time, as is seen in large joint arthroplasty. The effect of increasing constraint on the facet joints is unclear. Finite element models have shown increasing stress on the facet joints with increasing constraint.^36,³⁸ In vitro analyses have shown increased facet joint stresses with decreased constraint.³⁹ Although it is logical to think that decreased constraint would increase the stress at the surrounding structures, the inconsistency in the data suggest that other factors may also play a role. Increased stress at the facet joints may lead to degenerative changes and pain over time. Implant design must balance between stability of the implant and restoring normal biomechanics.

Bearing Surfaces

Although many potential bearing surfaces are available, the most commonly used are cobalt-chromium alloy on ultrahigh molecular weight polyethylene (UHMWPE) and cobalt-chromium alloy on cobalt-chromium alloy. Both bearing surfaces are well established in orthopaedics for their biocompatibility and good wear resistance. Both bearing surfaces are known, however, to have long-term failures. Experience with large joint arthroplasty has shown that wear particles can lead to osteolysis, subsidence, migration, and need for revision surgery.⁴⁰^–⁴² Nonetheless, the effect of wear particles within a joint have a different local effect compared with the epidural space.⁴³ To date, significant problems with osteolysis owing to polyethylene debris have not been seen clinically with the metal-polyethylene articulating implants with more than 20 years of use. This may be due to the lack of true synovium surrounding the disc arthroplasty, which has been implicated in the osteolysis seen associated with loosening of hip and knee implants.

Wear patterns observed in retrieval analysis of cobalt-chromium alloy with UHMWPE disc arthroplasty designs show the same failure mechanisms that are seen in hip and knee arthroplasty.^44,⁴⁵ Cobalt-chromium alloy with cobalt-chromium alloy articulations also include the risk of systemic release of cobalt and chromium ions. Although the blood serum ion concentrations are similar to total hip arthroplasty, the long-term effect is unknown.^46,⁴⁷ Industrial exposure to heavy metals such as hexavalent chromium is known to increase systemic inflammation and carcinoma.⁴⁸ The types of ions that are released from an implant are different and may not have the same effect. Sarcoma adjacent to metal implants is reported in the literature, but it is very rare.⁴⁹ Metal hypersensitivity is a potential cause for persistent pain with this bearing surface.

Implant Stabilization

Any TDR implant must be stable where placed intraoperatively and maintain that position over time with the forces placed on the motion segment. An unstable implant can migrate sagittally or coronally or allow motion to occur at the bone-implant interface rather than the intended bearing surface. Subsidence of the implant into the bony endplates over time is effectively a vertical instability of the implant and can result in disc height loss, motion loss, and pain. Implant stability can be divided into early and late phases. Early stability refers to initial stability at the end of the surgical procedure and in the perioperative healing period. Late stability refers to long-term maintenance of implant position and attachment to the bony endplate surfaces.

Material and implant design considerations are factors in implant stability. Early stability is generally provided for by implant design, including roughened surface coatings, spikes, keels, or screw fixation.⁵⁰ Late stability is obtained through additional direct bony attachment to the implant via ongrowth or ingrowth into the surface coating. In addition, implants designed to maximize endplate surface area contact and promote force transfer to the peripheral aspect of the endplate, where the bone is strongest, minimize the risk of implant subsidence. Biologic surfaces have become the standard for noncemented acetabular fixation in total hip arthroplasty. Attempts at biologic coatings for total knee arthroplasty have failed secondary to loosening of the components.⁵¹ This high failure rate is likely due to micromotion at the bone-implant interface. Similar problems must be monitored for in the spine.

Clinical Indications for Lumbar Total Disc Replacement

In the United States, lumbar disc replacement is currently approved for the treatment of single-level lumbar degenerative disc disease in skeletally mature individuals with no more than grade I spondylolisthesis having failed at least 6 months of nonoperative treatment. Currently available implants are approved only for use from L3 to S1 (Prodisc L3-S1, CHARITÉ L4-S1). The contraindications for TDR are extensive (Table 51–2). A relative contraindication is significant facet joint degeneration, although this is difficult to grade.⁵² There are many reports of multilevel disc arthroplasty, including level 1 evidence in prospectively randomized clinical trials, but this is currently considered off-label use in the United States based on strict FDA-approved criteria.^39,⁵³

TABLE 51–2 Contraindications to Lumbar Total Disc Arthroplasty

Active systemic infection or infection localized to site of implantation

Osteopenia or osteoporosis defined as dual-energy x-ray absorptiometry scan bone density measured T-score ≤−1.0

Bony lumbar spinal stenosis

Allergy or sensitivity to implant materials (cobalt, chromium, molybdenum, polyethylene, titanium)

Isolated radicular compression syndromes, especially caused by disc herniation

Pars defect

Involved vertebral endplate dimensionally smaller than implant used

Clinically compromised vertebral bodies at affected level owing to current or past trauma

Lytic spondylolisthesis or degenerative spondylolisthesis of grade >1

Preoperative evaluation for total disc arthroplasty should include plain radiographs with flexion and extension films, magnetic resonance imaging (MRI), computed tomography (CT) scan, and dual-energy x-ray absorptiometry (DEXA) scan in patients at risk for osteopenia.³⁹ Discography is considered by many authors to be an important diagnostic criterion for lumbar TDR but was not a requirement for many of the FDA-sponsored clinical trials. In addition, more recent reports not only have questioned the efficacy of discography, but also have implicated it in accelerating the development of disc degeneration.⁵⁴^–⁵⁶

Surgical Approach and Discectomy

Implant designs of most lumbar TDR devices require a direct anterior exposure of the intervertebral space; this is true of both prostheses currently available for use in the United States. This exposure can be obtained through either a transperitoneal or a retroperitoneal approach. Currently, a retroperitoneal approach is preferred because it minimizes the risk of postoperative ileus, bowel obstruction secondary to adhesions, and retrograde ejaculation (in men).¹⁹ In most cases, the approach is performed with a vascular or general cosurgeon. The exposure must be wide enough to expose the entire anterior surface of the disc space, requiring greater mobilization of the great vessels than is necessary for fusion. The final view of the disc must be directly anterior. Other prostheses in development or available for use outside the United States (e.g., Oblique-Maverick) use an anterolateral or direct lateral approach to the disc space, minimizing or eliminating the need for mobilization of the great vessels.

After exposure of the disc space, the disc is removed in entirety, including the anterior anulus and anterior longitudinal ligament, the entire nucleus pulposus, and the posterior anulus. To restore posterior disc height in cases with significant disc collapse, the posterior longitudinal ligament must be either released or resected. The outer lateral anulus is preserved. The cartilaginous endplate is removed taking care to preserve the subchondral bone. To recreate the proper center of rotation for the lumbar motion segment, most of the devices are placed abutting the posterior edge of the vertebral body. Proper posterior placement also helps to decrease the forces seen at the facet joints.⁵⁷ Position of the implant is confirmed using intraoperative anteroposterior and lateral fluoroscopy throughout the procedure.

Specific Devices

Only two designs have currently received FDA approval for use in the United States: the CHARITÉ Artificial Disc (DePuy Spine Inc., Raynham, MA) and the Prodisc-L prosthesis (Synthes Inc., Paoli, PA). Both designs use a metal-on-polyethylene articulation. Other devices are currently in use outside the United States and are well along in their FDA IDE trials in the United States, including the Maverick lumbar prosthesis (Medtronic Sofamor Danek Inc., Memphis, TN), the FlexiCore lumbar prosthesis (Stryker Spine, Allendale, NJ), and the Kineflex lumbar prosthesis (Spinal Motion, South Africa). These three devices all use a metal-on-metal bearing surface.

CHARITÉ Artificial Disc

The CHARITÉ Artificial Disc (DePuy Spine Inc., Raynham, MA) is the first device to complete the U.S. FDA approval process and has the largest and the longest clinical experience. The device has a biconvex UHMWPE spacer that acts as a mobile core (Fig. 51–1). The two metallic endplates are made of a cobalt-chromium alloy, and each concave endplate articulates independently with the core. There are ventral and dorsal teeth on the endplates to aid in positioning and maintenance of final position, and the latest version of the device has a titanium and a hydroxyapatite coating.²¹

FIGURE 51–1 CHARITÉ lumbar disc replacement. A, Implant with three components nested as it rests in vivo, viewed from anterosuperior perspective. B and C, Anteroposterior (B) and lateral (C) radiographs of device in vivo.

The FDA IDE study gave level 1 evidence showing the CHARITÉ disc replacement as equivalent to anterior interbody fusion with a BAK cage (Zimmer Spine, Minneapolis MN) filled with iliac crest autograft.^14,¹⁵ In the study, 305 patients were randomly assigned to receive either the CHARITÉ lumbar disc replacement or anterior lumbar interbody fusion (ALIF) with a pair of cylindric threaded BAK cages and iliac crest autograft. Study levels included only L4-5 or L5-S1. For the CHARITÉ group, the Oswestry Disability Index (ODI) improved by 24.8 points from 50.6 preoperatively to 25.8 at 24 months. In the BAK fusion group, ODI improved by 22 points from 52.1 to 30.1. The visual analog score (VAS) improved from 72 points in the CHARITÉ group to 30.6 points at 24 months. For the BAK fusion group, the VAS improved from 71.8 to 36.3 points. These results were maintained at the 5-year follow-up.⁵⁸ In this study, FDA success at 24 months was defined by success in all of four major criteria: (1) a 25% improvement in the ODI, (2) no device failure, (3) no major complication, and (4) and no neurologic deterioration. In the FDA IDE study, 57% of the CHARITÉ group and 46% of the fusion group met all four criteria.

Longer term retrospective level 3 clinical data are also available for TDR using the CHARITÉ device.^59,⁶⁰ Lemaire and colleagues⁵⁹ reviewed 100 patients with a minimum 10-year follow-up. Of patients, 90% had a good to excellent outcome with 63% returning to heavy labor jobs and 91.6% overall returning to work. Mean flexion-extension was 10.3 degrees with 5.4 degrees of lateral bending. Five patients (5%) required arthrodesis secondary to either implant failure or facet arthrosis. There were two neurologic injuries (2%), one L5 nerve injury and one sexual dysfunction, both of which recovered. There were two cases of spontaneous fusion (2%), and two cases of adjacent segment degeneration (2%). David⁶⁰ reviewed 106 patients implanted over a 6-year period with a minimum 10-year follow-up. Of patients, 82% had good to excellent outcomes; 89% returned to work with 77% returning to heavy labor. Eight patients (7.5%) required a posterior instrumented fusion. There were five cases (4.6%) of postoperative facet arthrosis, three cases (2.8%) of subsidence, three cases (2.8%) of adjacent level disease, and two cases (1.9%) of core subluxation.

Ross and colleagues ⁶¹ found poorer quality long-term results using the CHARITÉ device. Kaplan-Meier analysis was performed on 226 CHARITÉ implants. Radiologic failure was defined as a broken wire, subsidence, or lack of movement. At 8 years, there was a 90% survival rate. This rate declined dramatically to 35% at 13 years. At final follow-up, ODI improved by 14 points on average, 1 point below the benchmark for clinical improvement. At final follow-up, 69% of patients believed that they were better or much better than preoperatively; 20% believed that they were worse than preoperatively.

Cunningham and colleagues ⁶² showed that the CHARITÉ disc is effective in maintaining lumbar motion. They reviewed radiographs from the initial IDE study and assessed motion at the disc arthroplasty as a percentage of the total lumbar spine motion. These numbers were compared with cadaveric normal discs. At 2 years, the disc replacement maintained normal segmental lumbar motion and preserved the overall lumbar motion. Putzier and colleagues⁶³ reviewed 63 CHARITÉ discs at 17-year follow-up. At 17 years, 60% of the implanted discs had spontaneously ankylosed, and only 17% were deemed functional. In the patients in whom the disc replacement was still functional, there was no evidence of adjacent segment degeneration, suggesting that a functional implant is effective in preventing adjacent segment degeneration. Clinically, the patients with spontaneous ankylosis did better than patients with functional implants. One suggested cause of the clinical failure is hypermobility leading to early facet joint degeneration at the arthroplasty level. Hypermobility from removal of the anterior longitudinal ligament has been shown in vitro.⁶⁴ In contrast, Huang and colleagues⁶⁵ assessed 51 TDRs at 8.5 years. Range of motion greater than 5 degrees corresponded to improved clinical outcome.

Prodisc-L

The Prodisc-L (Synthes Inc., Paoli, PA) is a modular metal and UHMWPE device with three components, two of which are locked together during insertion to create a two-piece implant (Fig. 51–2). The two endplates are made of cobalt-chromium alloy. The upper endplate has a concave articular surface. The lower metallic endplate has slots to accept a convex UHMWPE articular component. Both endplates are secured to the vertebral endplates by means of a central keel, small spikes, and a titanium plasma-sprayed rough surface.²¹ The polyethylene is inserted into the disc space and locked to the caudal endplate after the endplates are in their final position, resulting in a single ball-and-socket articulation with the center of rotation in the upper aspect of the caudal vertebra.

FIGURE 51–2 Prodisc-L lumbar disc replacement. A, Three components. B, Implant with ultrahigh molecular weight polyethylene component attached to caudal endplate, nested with cephalad endplate as it rests in vivo. C and D, Anteroposterior (C) and lateral (D) radiographs of device in vivo.

(A and B, Courtesy Synthes, Inc.)

The only level 1 evidence for the Prodisc-L comes from the U.S. FDA IDE trial. In this study, the Prodisc-L was compared with circumferential fusion with femoral ring allograft anteriorly and instrumented posterolateral fusion with iliac crest autograft. The 2-year data have been published.²⁰ VAS pain assessment showed statistically significant improvement from preoperative levels regardless of treatment. The average VAS score was 37 in the Prodisc-L group for a 39-mm average reduction and was 43 in the fusion group with a 32-mm average reduction from baseline and trended toward significance (P = .08). The FDA success criteria included ODI reduction by at least 15 points; improvement in short-form health survey (SF-36); device success; no adverse events; and multiple radiographic criteria including no migration or subsidence, no loosening or osteolysis, no loss of disc height, and no decrease in range of motion. Using the FDA success definition as success in all of these end points, 53% of Prodisc-L patients and 41% of control patients were considered successful, with a statistically significant difference favoring the Prodisc-L group. A more recent report of 5-year follow-up found no statistically significant difference in the ODI or VAS compared with 2 years.⁶⁶ Successful radiographic range of motion was maintained by 93.7% of Prodisc-L implants.

Cakir and colleagues ⁶⁷ examined the clinical effect of TDR range of motion on clinical outcome at a minimum of 3 years after Prodisc-L implantation. They found no statistical difference in the preoperative and postoperative range of motion, with an increase in motion in 40% and a decrease in 30% of patients. The postoperative range of motion had no effect on clinical outcome. Bae and colleagues⁶⁸ reviewed 219 patients from the IDE trial at 2-year follow-up. In contrast, these authors found that greater average postoperative range of motion correlated significantly with less VAS pain at 6 weeks at all time points. They also found that a greater body mass index correlated to a decrease in preoperative and postoperative range of motion at 1 and 2 years after arthroplasty.

Although Prodisc-L TDR is FDA approved only for single-level use in the United States, the results of multilevel surgeries have been reported. The U.S. FDA IDE study included a two-level arm, with adjacent two-level painful degenerative disc diseases cases prospectively randomized (in a similar 2 : 1 fashion as the one-level arm) to either a two-level Prodisc-L or circumferential fusion. These data were submitted to the FDA well after the one-level approval and have not yet been approved as a new amended indication for the use of Prodisc-L by the FDA as of the writing of this chapter. Results from the IDE trial for two-level Prodisc-L have been presented.⁶⁹ In the trial, 237 patients were randomly assigned to receive either two-level Prodisc-L or circumferential fusion at two levels. Intraoperative data showed that Prodisc-L cases had a significantly shorter operative time, estimated blood loss, and length of hospital stay (P < .0001, P = .0006, P < .0001). At 24 months, 90% of Prodisc-L patients and 86.7% of fusion patients reported improvement in ODI from preoperative levels; 73.3% of Prodisc-L patients and 55.9% of fusion patients met the defined 15-point ODI improvement criteria. Overall neurologic success in Prodisc-L patients was superior to fusion patients (Prodisc-L 89.2%, fusion 77.9%; P = .0260). At all follow-up points, Prodisc-L patients recorded SF-36 scores significantly higher than fusion patients (P = .0523). VAS pain scores were significantly improved from preoperative scores regardless of treatment (P < .0001); at 24 months, the Prodisc-L group showed significantly higher pain reduction than the fusion group (P = .0466). VAS satisfaction at 24 months significantly favored Prodisc-L patients over fusion patients. At 24 months, a significant difference favored Prodisc-L patients over fusion patients (Prodisc-L 97.6%, fusion 91.8%, P = .0497) in device success—absence of any reoperation required to modify or remove implants and no need for supplemental fixation. Radiographic range of motion of patients was maintained within a normal functional range.

Siepe and colleagues ⁷⁰ reviewed the results of 218 patients implanted with the Prodisc II device. The patients were divided into groups by the number of levels that the device was implanted. They found that implanting more than one level leads to worsening results compared with a single level. The results of Siepe and colleagues⁷⁰ are contradicted by another published study using the same device. Hannibal and colleagues⁵³ compared 27 patients receiving Prodisc II at one level with 32 patients receiving Prodisc II at two levels. The number of patients in each group was small, but the investigators found no statistical difference between the two groups at 2 years. These results are consistent with other published series.^71,⁷²

Other Devices

The Maverick lumbar TDR prosthesis (Medtronic Sofamor Danek Inc., Memphis, TN) is an all-metal two-piece design that uses a highly polished cobalt-chromium alloy ball-and-socket metal-on-metal articulation. The device has a central fin on both endplates and hydroxyapatite coating of all bony contact surfaces for increased stability after insertion (Fig. 51–3). The center of rotation is fixed posteriorly to the mid-portion of the prosthesis, in the upper aspect of the caudal vertebrae. The U.S. FDA IDE trial is currently ongoing; in this study, the Maverick TDR is compared with an anterior-only fusion using an LT cage with Infuse-BMP (Medtronic, Memphis, TN).

FIGURE 51–3 A, Two-piece all-metal Maverick lumbar disc replacement. B and C, Anteroposterior (B) and lateral (C) radiographs of device in vivo.

Published prospective level 2 evidence is available after European use. Le Huec and colleagues ⁷³ reported the results of 64 implanted devices with a minimum of 2-year follow-up. ODI scores preoperatively and at 2-year follow-up were 43.8 and 23.1 (P < .05). Low back pain improved from a mean VAS score of 7.7 preoperatively to 3.2 at 2 years. Le Huec and colleagues⁷³ found that grade I and II facet joint degeneration did not influence the clinical outcome. They also found that paraspinal muscle atrophy resulted in a poorer outcome. The Maverick metal-on-metal articulation introduces the possibility of metal ion release into the bloodstream. Zeh and colleagues^47,⁷⁴ found increased levels of cobalt and chromium at implantation of the Maverick prosthesis. The increased levels were persistent at 3-year follow-up and were similar to levels with a metal-on-metal total hip prosthesis.

Preliminary results at 2-year follow-up have been presented from the U.S. FDA IDE trial.¹⁶ In the trial, 405 investigational (TDR) patients and 172 control (ALIF) patients with single-level degenerative disc disease (L4-S1) were treated after failing conservative care for 6 months. Mean (TDR and ALIF) operative time (1.8 hours and 1.4 hours) and blood loss (240.7 mL and 95.2 mL) differences were significant (P < .001); mean hospital stay (2.2 days and 2.3 days) and operative level distribution (73.6% and 78.5% L5-S1) were not. The mean ODI score improved dramatically for both groups at 12 months and 24 months (TDR 33.9 and 33.8 points; ALIF 29 and 29.3 points). At 24 months, 82.2% of TDR patients reported an ODI improvement of at least 15 points, an FDA-defined measure of success, versus 75.2% of ALIF patients.

Statistical superiority over fusion was concluded for Maverick mean improvements from preoperatively at all follow-up intervals, including 24-month ODI, SF-36 physical component summary, and back pain scores. Both groups also showed significant improvement in leg pain scores from preoperatively. Fusion success was 100% for ALIF patients. TDR disc height and angular motion were maintained postoperatively. TDR patients returned to work 21 days sooner (P = .038) than ALIF patients. Finally, at 24 months, 86.2% of TDR patients said that they would have the surgery again versus 74.5% of ALIF patients (P = .002). Overall success rates at 24 months, an FDA composite measure of the primary study end points, showed superiority for TDR (77.2%) versus ALIF (64.2%).

The FlexiCore lumbar prosthesis (Stryker Spine, Allendale, NJ) is a cobalt-chromium alloy highly polished ball-and-socket metal-on-metal prosthesis.²¹ The device is a one-piece design, which prevents separation and migration of a single endplate. The device also has a built-in rotational stop to prevent the facets from being overstressed (Fig. 51–4). There are fixation spikes on the upper and lower base with a titanium plasma spray coating to allow for bony ongrowth. There are currently no published clinical results with this device, but preliminary results have been presented. Adult patients with single-level discogenic pain who failed 6 months of nonoperative care were randomly assigned in a 2 : 1 ratio to FlexiCore disc replacement or circumferential fusion.¹⁹ A total of 140 FlexiCore subjects and 58 fusion subjects were available for review.

FIGURE 51–4 A, All-metal FlexiCore lumbar disc replacement. B and C, Anteroposterior (B) and lateral (C) radiographs of device in vivo.

At 2-year follow-up, the overall treatment success rate was 63% for FlexiCore versus 56% for fusion. Both treatments were beneficial for most patients, with 82% versus 79% achieving more than 15 points of VAS reduction and 70% versus 71% achieving more than 15 points of ODI improvement. The mean VAS change was −48.4 versus −45.9, and the mean ODI change was −30.8 points versus −29.4 points. Disc height was increased and maintained for both groups. Despite comparable VAS and ODI improvement, FlexiCore had less narcotic use at 24 months than fusion.

The Kineflex lumbar prosthesis (Spinal Motion, South Africa) is an unconstrained device that allows for the option of a metal-on-polyethylene or metal-on-metal articulation with a mobile core. The mobile core is designed to replicate more closely the dynamic center of rotation of the normal disc (Fig. 51–5). The endplates have serrations and a keel for increased fixation. There are currently no published clinical results with this device; however, preliminary results have been presented. Hahnle and colleagues¹⁷ reported their 2-year results with the Kineflex prosthesis. Using return to work, ODI, pain scoring, and patient satisfaction as the outcome measures, 100 patients were evaluated: 39 patients underwent an isolated single-level disc replacement; 25 patients, a two-level disc replacement; 1 patient, a double-level replacement adjacent to a previous fusion; and 7 patients, a fusion of another level at the time of the index procedure (hybrid cases). For 72 of 75 patients, 2-year clinical outcome was available (44 excellent, 17 good, 7 fair, and 4 poor). ODI score improved from 47.8 preoperatively to 14.5 (P ≤ .01) at the latest follow-up questionnaire. The pain score improved from 9.1 preoperatively to 2.9 at 2 years. As a group, the hybrid cases had the poorest outcome with two poor results out of seven.

FIGURE 51–5 Kineflex lumbar disc replacement. A, Three components, with metal core resting on inferior endplate. B, Implant with three components nested as it would rest in vivo. C and D, Anteroposterior (C) and lateral (D) radiographs of device in vivo.

Results at 12 months from the ongoing IDE trial have also been presented. In the trial, 58 patients were randomly assigned in a 1 : 1 ratio to receive either the Kineflex lumbar disc replacement or the CHARITÉ lumbar disc replacement (the control group).¹⁸ Overall complication rates did not differ significantly between groups. At 1 year postoperatively, both groups showed statistically and clinically significant reductions in ODI (investigational group 50.1% and control group 56.6%) and VAS (investigational group 56.7% and control group 71.8%). There were no significant differences between the two groups. The FDA defines individual clinical success for the study at 24 months. When the criteria were applied at 12 months, the results did not differ significantly between groups (57.1% investigational and 72% control). Both groups had high patient satisfaction scores. No significant difference in time to return to work was shown between groups (investigational group 140 days and control group 113 days). Only one patient working preoperatively failed to return to work within 12 months. In the investigational group, six patients eligible but not working preoperatively returned to work.

Hybrid Procedures

A hybrid construct refers to the use of a lumbar disc replacement adjacent to a fusion. The TDR may be performed along with fusion of an adjacent level, or it may be performed adjacent to a previously fused level (Fig. 51–6). Both of these clinical scenarios are considered off-label use for the prostheses currently available for use in the United States based on the inclusion and exclusion criteria from the FDA IDE trials. The theoretical advantages of preventing adjacent segment disease may have more utility when used in hybrid constructs. The advantages would be increased in longer fusions, which have a higher incidence of adjacent segment disease.⁷⁵^–⁷⁸ Aunoble and colleagues⁷⁹ implanted hybrid constructs in 42 patients and followed them prospectively. At 2 years, ODI decreased by 53%. No prospective randomized level 1 data or long-term prospective data exist to support the regular use of these hybrid constructs, however.

FIGURE 51–6 Hybrid construct with Prodisc TDR at L4-5 and fusion at L5-S1 done for a patient with two-level painful degenerative disc disease and prior L5-S1 left discectomy with facetectomy. A and B, Anteroposterior (A) and lateral (B) views. C and D, Flexion (C) and extension (D) lateral views.

Complications and Revision Strategies

Complications after total disc arthroplasty can be early or delayed. Although there are complications associated with use of an anterior approach, these are not specifically reviewed here because they are not specific to total disc arthroplasty. The full-width, direct anterior exposure required for disc arthroplasty requires greater mobilization of the great vessels, which may increase the morbidity of the procedure compared with use of cage or bone graft, which can be inserted off midline or via an anterolateral approach if necessary. Early complications include endplate fracture, subsidence, nerve entrapment, and dislocation. Late complications include infection, osteolysis and loosening, hardware failure, facet arthrosis, fracture, and spontaneous fusion. The early complication rate ranges from 0% to 25%.^39,^60,^61,⁸⁰^–⁸³ This complication rate increases as the number of levels operated on increases.⁸⁴ The late complication rate is 21%, although a true assessment of this number requires long-term data.^59,⁸⁵

Each individual device has its own unique mechanisms of failure, so each device design should be evaluated individually. The early CHARITÉ design was associated with rim fracture of the polyethylene insert.⁸⁶ Alternative designs using a polyethylene core may not have the same problem. Implant revision is an important consideration because it is likely that many young patients require a revision in their lifetime. Prosthesis removal, when necessary, can often be accomplished via an anterolateral or direct lateral approach without full mobilization of the great vessels.

The limited life span for any motion-preserving device requires an assessment of revisability. In situ posterior fusion with retention of the components is possible for patients who experience subsidence, fracture, or facet arthrosis. It is still unclear whether this approach, with retention of the TDR implant, yields fusion or clinical results similar, better, or worse than strategies that include removal of the TDR device and interbody fusion. There are limited options for revision when removal of the components is required. Anatomic approaches include right and left retroperitoneal, transperitoneal, and direct lateral approach, either with reflection of the psoas muscle or with a trans–psoas muscle–splitting approach. Even keeled prostheses can be removed via an anterolateral or direct lateral approach.^87,⁸⁸ Ideally, a surgical approach different from that used in the primary implantation should be considered, unless revision is done for acute failure within 3 weeks of the index procedure.

A revision anterior surgical approach is associated with a significantly higher complication rate than a primary approach.^80,⁸⁹ Scar tissue complicates the anterior approach because mobilization of the great vessels becomes much more challenging and sometimes impossible. This is most problematic at the L4-5 level. Changing the approach minimizes the risk of injury to the large vessels that are encased in the scar tissue. McAfee and colleagues⁹⁰ reviewed 24 patients who underwent revision using an anterior approach. Four patients (16.7%) sustained an injury to the large vessels. Leary and colleagues⁹¹ had a large vessel injury rate of 10% (2 of 20) after revision anterior exposure. Bumpass and colleagues⁹² performed cadaveric dissections of the lumbar plexus to determine retrievability through a posterior approach. The smallest root-to-root distance measured was 9.1 mm. The smallest measured root-to-tether distance was 39.2 mm. These measurements theoretically allow for removal of small implants posteriorly. No clinical studies assess the risk of nerve injury through a posterior approach.