Failed Disc Replacement

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CHAPTER 100 Failed Disc Replacement

Paul C. McAfee, MD, David Gloystein, MD, Bryan W. Cunningham, MSc

Total disc replacement (TDR) in the cervical and lumbar spines has emerged as a safe and efficacious treatment for both lumbar discogenic back pain and cervical degenerative disc disease associated with radiculopathy and myelopathy in appropriately selected individuals. Thousands of TDR devices have been implanted internationally, and interest in “motion-preserving” alternatives to spinal fusion continues to soar for both physicians and patients. As with any emerging technology, there is a learning curve and the shortcomings and complications can be difficult to predict. Like any other new technology, analyzing and understanding the reasons for failure are critical to prevent them from occurring and manage them when they do occur.

Despite a more than 20-year history of total disc replacements worldwide, detailed analyses of results and complications from comprehensive, prospective randomized studies have only become available through the U.S. investigational device exemption (IDE) trials.¹^–⁵ Box 100–1 is a comprehensive list of reported failures to date organized by mode/type of failure. Note that Box 100–1 does not include the myriad of approach-related complications including dysphagia, esophageal perforation, vocal cord paralysis, carotid artery injuries, retrograde ejaculation, ureteral injury, or large vessel injury. These complications are not unique to total disc replacements. They are inherent to anterior spinal surgery in general and have been reviewed elsewhere.⁶^–⁸ The discussion that follows will review selected failures and focus on the prevention, evaluation, and management of these problems.

Failures of Lumbar Disc Replacement

Prevention of Failures

Benjamin Franklin’s profound words, “an ounce of prevention is worth a pound of cure,” cannot be overstated when discussing failures of the lumbar TDR. With current TDR designs, the lumbar spine is approached anteriorly for numerous reasons. These include (1) minimal morbidity allowing for short recovery times; (2) unobstructed visualization of the disc space, allowing for total discectomy and accurate implant sizing; (3) the absence of the need to enter or retract posterior neural structures in the spinal canal; and (4) the familiarity of territory for many spine and vascular surgeons. Accordingly, the approach-related risks are both predictable and relatively low.¹^–¹⁰ In contrast, revision approaches to the anterior lumbar spine have about a six times higher risk for major bleeding or thromboembolic complications due to adhesion formation, which prevents accurate identification of the great vessels.¹⁰ The anterior lumbar spine therefore remains a relatively facile approach as an index procedure but is fraught with potential complications in any revision situation. In our experience, many failures of lumbar disc replacement could have been avoided because they can be traced to surgeon-specific factors (as opposed to patient-specific factors) such as incomplete discectomy, improper device insertion, or inappropriate indications.

Total Discectomy

Rauschning has demonstrated that the bone at the periphery of the vertebral body endplate is strongest and all TDR implants have been designed to be seated along this peripheral rim of bone.¹¹ Performing a complete or radical discectomy therefore becomes paramount for accurate total disc sizing and endplate placement. Implants that are too anterior or do not achieve good “rim fit” are more prone to dislodge or subside. A fundamental impediment to complete discectomy is poor visualization of the disc space. Regan and colleagues¹² have demonstrated the importance of the “learning curve” for optimal outcomes for CHARITÉ TDR. Access surgeons and spinal surgeons who are uncomfortable or unfamiliar with the anterior approach to the lumbar spine will be less likely to retract the great vessels far enough to permit complete exposure of the lateral aspect of the disc. In contrast to lumbar interbody fusion, a relatively larger window into the intervertebral disc space is required to properly perform total disc replacement. We routinely use handheld Hohmann retractors, which pierce the vertebral bodies above and below the disc. Large Steinmann pins can serve this purpose as well. This technique of retraction prevents the iliac vessels and vena cava from gradually creeping into the surgical field, thus narrowing the exposure of the intervertebral disc. If one prefers to use fixed table–based retractor systems, then it is wise to continuously assess and reassess the field of exposure to ensure that adequate visibility is maintained to assist complete disc space preparation.

The most challenging regions to visualize when performing the discectomy are the posterolateral corners of the disc space, especially the ipsilateral side to the surgeon. Removing all of the inner annulus from these regions not only allows proper posterior positioning of the implant but also has implications in preventing displacement of disc material into the spinal canal or neural foramen as the prosthesis is being inserted. Visualizing the posterior aspects of the disc space can be especially challenging at the L5-S1 level in patients with a relatively vertical slope or slip angle of the sacrum. The use of head lights and a surgical bed that can be placed into the Trendelenburg position can assist visualization in this setting.

Sizing, Lordosis, and Placement

Subsidence, or failure of the bone-implant interface in which the disc prosthesis displaces into the vertebral body, likely has multiple etiologies. The initial SB-CHARITÉ I design consisted of a rather small stainless steel endplate that was made in one size to fit in the center of the disc space in the region of cancellous bone. Although many of these implants are still functioning, the main observed complication was subsidence. On the basis of this experience, all “modern” TDRs incorporate multiple-size options that allow size-matching with the patient’s vertebral endplate. Thus the surgeon must optimize the fit in the axial and vertical dimensions. The coverage of the endplates should be with the largest size that allows proper seating within the disc space. In the vertical dimension, one should select the largest size prosthesis that can be inserted without overdistracting the disc space.

Maintaining lumbar lordosis is intuitive in terms of spinal balance in the sagittal profile; however, with lumbar disc replacement, especially L5-S1, maintaining and matching the patient’s lordosis is imperative to reduce shear and achieve good endplate contact, thereby reducing the risk of anterior dislodgement or dislocation. If the disc space lordosis measures 15 degrees, for example, but the prosthetic lordosis only reaches a maximum of 10 degrees, one or both of the prosthetic endplates will not be in complete contact with the host vertebral endplate. Assuming the prosthesis can match the 15 degrees of lordosis, it is even more favorable to have a mechanism by which the articulating portion of the prosthesis can assume a more horizontal (parallel to the floor when standing) position. One way to achieve this is to “build up” the caudal prosthetic endplate with a 10-degree or more lordotic endplate. By “horizontalizing” the disc space, the magnitude of the shear forces at the articulating surfaces is reduced but will still be significant at the caudal bone-implant interface. A note of caution might be in order for patients who have an especially steep sacral inclination of the S1 endplate. In conjunction with these concerns, L5-S1 presents with additional and theoretical risks for postoperative spondylolisthesis. The subject of facet orientation introduces the idea that if the facets are more in the sagittal plane rather than the coronal plane, there is a predisposition of future destabilization over time and increase in facet stress in flexion and extension. This has not been directly studied due to the limitation of cadaveric specimens with their respective facet orientation. Additionally, as Rousseau and colleagues showed in a comparison of constrained and semiconstrained disc arthroplasties with intact specimens, facet forces vary during flexion-extension and lateral bending. He found that one implant tended to unload the facets in extension and to a lesser degree in lateral bending, which was attributed to the implant’s instantaneous axis of rotation. The other tended to increase in these areas. Also, the flexion forces on the facets were not statistically different than for the intact specimen with a potted sacral inclination of 60 degrees.¹³

In the first U.S. IDE to reach completion, McAfee and colleagues ³ demonstrated that inaccurate positioning of the lumbar TDR was statistically correlated with reduced patient outcome.

Appropriate patient selection may be the most important factor in reducing the risk of failures. Strict adherence to the inclusion and exclusion criteria from the U.S. IDE trials such as patients with osteoporosis, metabolic bone disease, history of chronic steroid use, or age older than 60 years should reduce the risk of TDR endplate subsidence. Failure to recognize an isthmic spondylolisthesis is another known reason for failure (Fig. 100–1). In the U.S. CHARITÉ trial there were five unrecognized pars defects for fractures that became apparent at the 6-week follow-up visit.¹⁰ In four of the five cases the defect was appreciable after retrospective scrutiny of the preoperative flexion and extension images was performed. One such case demonstrated migration of the device requiring posterior pedicle screw instrumentation and fusion. In the fifth case, bilateral pedicle fractures occurred in the postoperative setting and this case was revised to an anterior interbody fusion with posterior pedicle screw-rod instrumentation and fusion. While patients with a degenerative spondylolisthesis of less than 3 mm are not excluded from U.S. IDE trials, we prefer not to perform TDR on patients with more than 2 degrees of intervertebral translation. Computed tomography (CT) is the study of choice to evaluate the presence of an isthmic spondylolisthesis when the diagnosis is in question on preoperative flexion-extension radiographs or magnetic resonance imaging (MRI). CT scans will also reveal the degree of facet arthrosis better than MRI, which might also exclude certain candidates from lumbar TDR.

FIGURE 100–1 A, Preoperative lateral radiograph demonstrating an isthmic spondylolisthesis at L5, which was not recognized before lumbar total disc replacement. B, Lateral radiograph following posterior instrumented fusion for anterior migration of the CHARITÉ Artificial Disc.

Failures (see Box 100–1)

Implant failures such as polyethylene core fractures, core dislocations, or implant breakage have been uncommon occurrences thus far. Our knowledge of them derives from case reports or small case series.¹⁴^–¹⁷ Instances of excessive polyethylene wear have been quite rare and have uniformly occurred in patients implanted before 1997. David reported a case of polyethylene failure due to oxidation 9.5 years following implantation.¹⁴ Similarly, as reported in the largest series of TDR failures reported to date, there was one case of detectable polyethylene core wear noted 12 years postoperatively.¹⁷ Since 1997, an industry-wide enhancement to the sterilization process of polyethylene, whereby irradiation occurs in an inert gas such as nitrogen rather than air, has resulted in dramatically improved wear characteristics due to a reduction in the incidence of oxidation. This corrective preventative action of avoiding polymerization and reducing the UHMWPE wear rate was born out of the total joint implant experience on partially cross-linked UHMWPE. In contrast to the previously mentioned cases, analysis of a retrieved polyethylene core from a revision surgery performed for bone-implant loosening 1.6 years postoperatively in a patient implanted post-1997 demonstrated low levels of oxidation with mechanical properties that were not substantially degraded.¹⁸ At the time of this writing, there are only two published cases of anterior dislocation of the polyethylene inlay of a ProDisc artificial disc replacement.¹⁶ A case of posterior core dislocation presented to our clinic (Fig. 100–2). There are no reported cases of metal endplate fatigue failures or deformation for modern prosthetic designs.

FIGURE 100–2 A, Lateral postoperative image of CHARITÉ Artificial Disc total disc replacement at L5-S1. B, A year later the polyethylene core displaced posteriorly with neurocompression. C, Image following anterior revision anterior lumbar interbody fusion and posterolateral fusion with instrumentation.

Bone-implant failures account for the greatest number of failures of lumbar disc replacement, and the mode of failure depends on the type of device. Sagittal vertebral body fractures can occur with keeled devices because the keel slot creates a stress riser in the bone (Fig. 100–3).¹⁹ Two-level implantations in which keel slots are introduced into the superior and inferior aspects of the intercalated vertebra are at highest risk for this complication. To reduce this risk, the MAVERICK keel has been reduced from 11 mm to 7 mm.²⁰ In contrast, implants with smaller “anchors” at the bone-implant interface such as the CHARITÉ, in which six 3-mm teeth engage the vertebral endplate, exhibit a greater tendency to migrate or dislodge if improperly placed. In the CHARITÉ U.S. IDE study, there were 15 of 347 implantations that required removal and none of these had been inserted in the “ideal” position.³ Regardless of the design, TDR placement anterior to the center of rotation, especially in a hyperlordotic segment, will tend to migrate or dislocate anteriorly due to excessive shear.²¹ As previously mentioned, spondylolysis and spondylolisthesis present a biomechanically less stable environment for TDR and explain some of the early failures by migration (Fig. 100–4). Damage to the vertebral endplate during insertion, placement of a TDR in an osteoporotic spine, inaccurate positioning, and insertion of an implant that is too small are all factors that can contribute to TDR subsidence. In van Ooij and colleagues’¹⁷ report of 27 complications of the CHARITÉ device, there were 16 cases of subsidence. The cause, prevalence, or incidence of these failures could not be determined because the study was retrospective without mention of the total number of cases. Strict adherence to indications, surgeon education, and modern insertion instrumentation systems should theoretically reduce the incidence of this complication.

FIGURE 100–3 Lateral radiograph demonstrating an L5 vertebral body fracture occurring between a two-level ProDisc total disc replacement at L4/5 and L5/S1. Note the subluxation of the L5/S1 prosthesis.

FIGURE 100–4 A, Postoperative lateral radiograph demonstrating good positioning of a CHARITÉ total disc replacement at the L5/S1 level. B, Lateral radiograph demonstrating anterior prosthesis dislocation following bilateral L5 pedicle fractures. C, Axial computed tomography scan demonstrating bilateral L5 pedicle fractures. D, Lateral radiograph following anterior disc removal and 360-degree fusion with anterior titanium interbody cages and posterior pedicle screw instrumentation.

Iatrogenic deformity has a greater tendency to occur in multilevel implantations (Fig. 100–5). Preoperative scoliosis is a relative contraindication to TDR; however, subtle coronal and sagittal-plane deformities may not be taken into account. TDR insertions in these scenarios will likely exacerbate rather than reduce any deformity because stabilizing structures such as the anterior and posterior longitudinal ligament, as well as the majority of the annulus, are removed during the implantation. Even in the well-aligned spine, incomplete discectomy and “off axis” TDR placement at one level can create an “off-axis” situation at the next level (Fig. 100–6). Thus when more than one vertebral level is to be implanted, device placement errors will tend to be compounded at sequential levels of insertion, resulting in a “Z deformity.” Kyphosis is not prevalent following TDR unless the implant is inserted backwards (i.e., a lordotic device inserted in kyphosis) or severe subsidence of the implant into the posterior half of the vertebral body.

FIGURE 100–5 Anteroposterior radiograph of a three-level CHARITÉ total disc replacement at L3/4, L4/5, and L5/S1. Note the coronal plane deformity and “off-axis” loading of each prosthesis, resulting in a lateral prosthesis dislocation at the L3/4 level.

FIGURE 100–6 A and B, CHARITÉ disc replacement placed at an outside facility with off-axis placement.

Host response to particulate debris generated from virtually any articulating biomaterial is characterized by macrophage recruitment and proinflammatory cytokine release. The culmination of this cascade is matrix metalloprotease activation, which leads to bone resorption and osteolysis. Osteolysis is a well-documented complication of total joint replacement ^22,²³; however, most investigators agree that it will not be prevalent following TDR. One reason for the potentially reduced risk in the spine is the fact that the intervertebral disc lacks a surrounding synovial space, the key source for macrophages and other inflammatory cells. The second reason is the relatively reduced range of motion and hence reduced volumetric wear in TDR compared with typical diarthrodial joint replacement. With more than 20 years of global experience and more than 10,000 TDR implantations worldwide, there are only anecdotal reports of osteolysis, and to our knowledge only one has been documented histologically.¹⁰ Heterotopic ossification (HO) surrounding lumbar TDR implants has been classified previously by McAfee and colleagues²⁴ in preparation for the CHARITÉ U.S. IDE trial. In this trial, the incidence of HO was 4.3%; however, the presence of HO did not have any impact on flexion-extension range of motion or clinical outcome.²⁵ We know of no other lumbar TDR trial in which the presence of HO has been systematically evaluated; however, isolated cases of periannular ossifications have been reported. To be fair, however, the imaging characteristics of the lumbar devices, which are typically cobalt-chrome or stainless steel alloys, do not allow for identification of all but the larger amounts of periannular bone formation.

Neurologic injury has not been shown to be more prevalent in cases of lumbar TDR when compared with fusion controls in prospective randomized evaluation.²⁶ Most cases of revision surgery in the CHARITÉ U.S. IDE trial involved reoperation with laminectomy and foraminotomy for new neurologic symptoms in the lower extremities. After any anterior discectomy and instrumentation, patients should be asked if they have any “new” pain in their legs that was not present preoperatively and a thorough neurologic examination should be performed in the postanesthesia care unit. If there are any new neurologic complaints or physical examination findings (i.e., motor or sensory loss), a CT scan with myelographic contrast should be performed to assess positioning of the implant and ensure that a hematoma, bone fragment, or disc material is not present in the spinal canal. MRI is not particularly helpful postimplantation due to metal artifact obscuring the regions of greatest interest. By rapidly diagnosing the cause for new neurologic symptoms with appropriate imaging, early revision such as implant repositioning is possible. Neurologic injury without radiographic abnormalities is a complication that typically presents with left-sided leg pain in the L4 or L5 dermatome. First reported by Thalgott and colleagues, this complication has been thought by some to be due to excessive retraction of the lumbar plexus during the retroperitoneal exposure.²⁷ The lumbar plexus is at highest risk when exposing the L4-5 disc space. To mobilize the left common iliac vein to the patient’s right side, the iliolumbar vein(s) needs to be identified and ligated as it courses dorsally between the lumbar nerve roots located under the psoas muscle (Fig. 100–7). Alternatively, such symptoms have been thought to be sympathetic mediated and result from the vascular exposure because similar symptoms may be seen following nonspinal vascular dissections. In either case, whatever the etiology, the dysesthetic pain pattern typically resolves after 6 to 8 weeks; however, corticosteroid selective nerve root blocks may help minimize the symptoms. Notwithstanding, the burden of proof remains to demonstrate that the acquired symptoms are not due to new nerve root compression, which may be remedied with further surgery.

FIGURE 100–7 A, Cadaveric dissection depicting the lumbosacral plexus just lateral to the left external iliac vein and underneath the psoas muscle, which is being retracted. The labeled “DISC” is the L5/S1 intervertebral disc. B, Line drawing depicting the lumbosacral plexus in relation to the psoas muscle, which is being retracted, and the left common iliac vein and iliolumbar veins.

Deep infection surrounding the prosthesis has not been reported as a complication of total disc replacement. However, it is only a matter of time before this complication is encountered. Lessons borrowed from experience with total joint replacements suggest that early prosthesis removal and conversion to a fusion will be necessary to resolve such an infection. As the total joint literature has taught us, even in the setting of an acute infection, metallic implants can be retained with proper irrigation and débridement and polyethylene exchange. In this case, because another anterior procedure to simply exchange the poly would risk future revisions, the risks outweigh the benefits and the better part of valor would be to convert to a fusion. Additionally, attempting to treat such an infection with antibiotics alone will risk progressive periprosthetic bone destruction due to osteomyelitis, which will substantially compound the scope and morbidity of any salvage procedures.

Supraphysiologic motion is a phenomenon that is not a prevalent mode of failure for TDR but is likely underdiagnosed. It is defined as motion exceeding the natural motion arc in six degrees of freedom from the native motion in a specific patient’s functional spinal unit. This motion results in subclinical instability that the body senses and in turn attempts to combat. This can present with postoperative muscle spasms and unexplained and chronic postoperative pain. It is a diagnosis of exclusion. The TDR is typically in excellent position without signs of dislodgement, dislocation, or subsidence. The hallmark is significant hypertrophy of the facet joints (Fig. 100–8). Because the workup algorithm follows closely to ruling out spondylolysis, a CT scan can be helpful in characterizing this hypertrophy. To determine surgical candidacy following exhaustive conservative management, which typically includes long-acting narcotics, local facet blocks are the major objective tool to assess pain relief. Care must be taken to counsel the patient to give feedback following the injections to pay close attention to the mere hours of relief that may ensue corresponding to the half-life of bupivacaine hydrochloride (Marcaine). If the patient notices a significant decrease in pain, a stabilizing procedure in the form of a posterolateral fusion with instrumentation can be discussed as an option of treatment. There is presently no way of predicting which patients will be affected by this phenomenon. Some surgeons have pointed to facet orientation as a predisposition. Especially at L5-S1, more sagittally oriented facets in conjunction with sacral inclination can predispose to increased facet forces and this mode of failure. When diagnosed properly with facet blocks, patients have had resolution of pain and spasms and were likewise weaned off of narcotics completely.

FIGURE 100–8 A, Lateral postoperative image of CHARITÉ total disc replacement at L5-S1 showing borders of facet joint (radiograph is slightly rotated). B, 2.5 years later the hypertrophy of the facet joints is considerable, which was confirmed grossly intraoperatively. C, Image following posterolateral fusion with instrumentation.

Revision Strategies

Revision total disc replacement is a potentially life-threatening procedure and should be reserved for indications that justify the increased risk (Box 100–2).

Timing

Revision anterior exposure within 2 weeks of TDR incurs relatively little additional morbidity because adhesion formation is minimal. For this reason, surgeons should have a low threshold for revising implants that are clearly malpositioned or exhibit early migration within this 2-week time frame. If the prosthesis can be repositioned or revised to another TDR, without damage to host bone, it seems reasonable to do so. Beyond this period of time, a revision strategy must be individualized to the particular clinical situation. A posterior instrumented fusion with or without a decompressive laminectomy is currently the most effective salvage procedure. Preoperative planning is critical to a successful anterior revision. The authors analyze the cause for failure to establish individual goals for the revision. Corrections such as polyethylene replacement, size, or position changes are relatively easy to anticipate. Appropriate patient counseling regarding the increased risks, potential for changes, or need to abort the original plan for safety considerations is also critical. Reviewing the initial operative reports, clinic notes, and original indications can be particularly enlightening if a revision to a TDR is contemplated. If a patient failed to meet indications for a TDR at the index procedure, it is unlikely that he or she will meet indications at the revision. Patients with significant host bone loss, deformity, or instability, along with any of the indications in Box 100–2, should be revised to an interbody arthrodesis.

In our experience, major vascular injury, deep venous thrombosis, and potential ureteral injury are at particularly increased risk in the anterior revision scenario, and our operating room preparation strives to mitigate these risks. Ureteral stenting is easily performed preoperatively and is valuable not only for intraoperative identification of the ureter but also for maintaining ureteral patency during the healing phase if an injury were to occur. Significant reduction in the somatosensory-evoked potentials of one leg may be the first indicator of excessive arterial retraction, and for this reason, we routinely use this form of electrophysiologic monitoring. Placing a pulse-oximeter probe on the great toes is another form of monitoring that can assist in detecting excessive arterial retraction or occlusion. Often temporary relaxation of the retractors will result in normalization of the electrophysiologic or pulse-oximeter readings. We currently insert inferior vena cava (IVC) filters preoperatively because postoperative lower extremity duplex evaluations have missed a deep venous thrombosis (DVT) in the pelvic great vessels that progressed to a postoperative pulmonary embolism (PE). Finally, we prepare the inguinal region into the operative field through which percutaneous vascular access wires are placed into the right and left femoral veins. If an injury to one of the venous great vessels occurs, balloon catheters can be inflated to tamponade and control bleeding in a timely fashion.

TDR revision approaches depend on the reason for failure and the original surgical approach; however, the currently available options include posterior decompression, posterior decompression and instrumented fusion, anterior TDR removal and fusion, or anterior TDR removal and reinsertion. For patients meeting one of the criteria in Box 100–2, anterior TDR removal and fusion is the safest salvage strategy because gaining the exposure necessary to implant a new disc replacement is rarely possible due to adhesion formation (Fig. 100–9). Anterior interbody fusion devices typically require less exposure and can be inserted obliquely across the disc space. This can be extremely handy when the only accessible area to the disc is to one side. Revision to a TDR may be considered in the early (<2-week) postoperative period when exposure is excellent and an easily correctable problem such as inaccurate placement to anterior has been identified.

FIGURE 100–9 A, Lateral fluoroscopic image immediately postimplantation of a CHARITÉ total disc replacement (TDR) at the L4/5 level. B, Lateral radiograph 6 weeks postoperatively demonstrating anterior extrusion of the prosthesis. C, Lateral radiograph after intravenous contrast injection demonstrating vascular occlusion of the inferior vena cava by the extruded TDR. D, Lateral radiograph after anterior device removal and 360-degree fusion with anterior carbon fiber cage and posterior pedicle screw instrumentation.

Revision anterior exposures should always be performed with an experienced access surgeon and gaining access via a virgin territory is desired but rarely possible. At L5-S1, if a left-sided retroperitoneal approach was used at the index procedure, then a transperitoneal or right-sided retroperitoneal approach can be considered. Conversely, if a transperitoneal approach was used at the index procedure, then a right- or left-sided retroperitoneal exposure can be considered. L4-5 and higher is always more challenging because the left-sided retroperitoneal approach is virtually always used during the initial exposure. Transperitoneal and right-sided approaches are technically far more demanding at L4-5 because of the central to right-sided position of the IVC. For this reason, anterior revisions to L4-5 should be performed via a transperitoneal or through the same left-sided retroperitoneal approach. By identifying the left psoas muscle, one can generally palpate the lateral border of the lumbar spine and, from this point, a subperiosteal dissection can assist safe exposure toward the midline and beyond. In the end, the experience and comfort level of the access surgeon will be as important, if not more so, as the type of approach used during the index procedure.

Given the aforementioned technical challenges, the most common salvage procedure for failed TDR is posterior instrumented fusion with pedicle screws. This technique essentially locks the prosthesis from any further movement and allows for bone grafting in the posterolateral intertransverse region. Cunningham and colleagues ²⁸ found that pedicle screws alone combined with a lumbar disc replacement were not found to be statistically different biomechanically from pedicle screws and a femoral ring allograft. Posterior hemilaminectomy or laminotomy without fusion is an alternative for focal disc or bone displacements into the spinal canal; however, extreme care must be taken to avoid destruction of stabilizing structures like the facet joints.

Conclusions

Lumbar TDR is a safe and effective treatment option for appropriately selected patients with lumbar degenerative disc disease. Bone-implant failures can be prevented by strict adherence to Food and Drug Administration indications and accurate placement of the device. For at least one device, suboptimal positioning has been correlated with worse patient outcome, a finding that will likely be borne out for other TDR designs. Device failures for current designs are rare and are characterized by polyethylene fracture or dislocation. Anterior revision surgery with an experienced access surgeon and preoperative placement of ureteral stents, vena cava filters, percutaneous vascular access wires, and spinal cord monitoring can reduce the risks of major vessel injury or thrombosis.

Cervical Failures

Recently in the literature, anterior cervical discectomy and fusion (ACDF) and cervical arthroplasty were found to be slightly superior to the results of total hip and total knee arthroplasty with respect to patient perceived outcomes.²⁹ Considering that total joint arthroplasty has provided the standard by which most orthopedic procedures are compared, this contradicts previous stereotypes of mediocre outcomes in spine surgery. In addition, the complication rate of the anterior approach to the cervical spine is much lower than other orthopedic procedures, especially infection.³⁰

Since the introduction of the Bryan Total Cervical Disc Prosthesis in 2000, many other discs have entered the market as viable options and are in widespread use today. There are two FDA-approved implants presently on the market with 19 others under study. Although ACDF continues to be the gold standard for treatment of cervical radiculopathy and myelopathy, cervical disc arthroplasty has proven to match the goals of ACDF including decompression of the spinal canal, disc height and alignment restoration, and cessation of neurologic compression. Because of this parallelism in goals, complications are similar to a degree—namely, approach-related complications, recurrence of symptoms, instrumentation malposition, fixation related, alignment related, or stability related. Conversely, with the introduction of a motion segment rather than fusion, there is a divergence of complications. Problems identified with ACDF include the incidence of adjacent segment disease, the need for perioperative immobilization, bone graft site morbidity (if used), and pseudoarthrosis. With arthroplasty, the obvious reasons are due to motion including supraphysiologic motion or hypermobility, traumatic instability, heterotopic ossification, and dynamic neural compression.³¹ The primary reason to choose cervical disc arthroplasty over fusion in the properly selected patient is maintenance of motion and avoidance of adjacent segment disease, which has been studied and reported, but long-term studies are pending.³²

Prevention of Failures

Complications in cervical disc replacements are far from numerous. With most of our knowledge gleaned from prospective randomized IDE studies for new implants, long-term outcomes are lacking. Success of arthroplasty is truly related to the proper patient selection. A close second is technique of placement. The importance of the latter is confirmed by most arthroplasty implant companies requiring formal attendance to implant methodology and intraoperative placement training. The complication of infection and others related to the approach to the anterior cervical spine is not discussed in this manuscript. In U.S. IDE studies of the Bryan (Medtronic Sofamor Danek, Memphis, Tenn.); Prestige (Medtronic Sofamor Danek); and Prodisc-C (Synthes, Paoli, Pa.) cervical disc replacements, investigators have reported no surgical site infections in either the control (fusion) or investigational (disc replacement) groups.³³^–³⁵ All three of these studies revealed that the most common reason for removal of cervical disc arthroplasty was for recurrent or new radiculopathy related to incomplete decompression.

The discussion of complications in any surgery is followed closely by a review of revision techniques for these diagnoses. Unlike anterior lumbar procedure revisions, cervical revisions have much less risk through the same incision.³⁶ As would be thought, any revision approach increases the rate of complications. In the anterior cervical spine, this rate is already low for the index procedure and thus subsequent revision has been found to be 10% including infection.³⁷ In our experience, many of the failures of anterior cervical disc replacement can be linked to iatrogenic factors like improper implant placement, insertion or sizing, and inappropriate indications.

Patient Selection

Proper patient selection cannot be overstated. A decision to operate can be just as detrimental to outcome as performing poor technique during device insertion. Absolute contraindications include present or previous trauma, morbid obesity, hypermobility, endocrine/autoimmune or metabolic bone abnormalities or related medications known to be detrimental to bone formation, severe facet arthrosis, metal allergy, DISH/ankylosing spondylitis, absence of motion, ossification of the posterior longitudinal ligament (OPLL), osteoporosis diagnosed by DEXA (T ≤ −2.5), and active malignancy (see Box 100–2). Additionally, as mentioned previously, this procedure is not intended for axial neck pain. The presence of radiculopathy and/or myelopathy must exist.

Endplate Preparation

As noted in the discussion of the lumbar spine, the strongest bone on an endplate of a vertebral body is at the periphery. Performing a complete discectomy is not an issue seen in the cervical spine contrary to the lumbar spine. Visualization of the disc space from uncus to uncus is straightforward in cervical discectomy. Removal of the cartilaginous endplates to subchondral cortical bone with avoidance of cancellous bone exposure is paramount. The problem lies in incomplete removal of the posterior annulus and subsequent take down of the posterior longitudinal ligament (PLL). To obtain parallel distraction of the endplates, this is an important step. Complete removal of the PLL is not mandatory, but it is the authors’ opinion that this is the only true way to ensure no disc fragments are left behind because of herniation through a rent in the PLL and thus not visible unless taken down. Some implant companies recommend not removing any bone either at the anterior osteophyte or the posterior osteophyte for reasons of potential subsidence due to loss of peripheral bone. When this is done, visibility is significantly decreased. We recommend removing the least amount but enough to visualize the posterior structures.

After all, the primary reason for surgery is radiculopathy and/or myelopathy and without removal of the compressive disc/osteophyte complex, the surgery is already a failure. The endplate is also typically concave at the cranial portion of the disc space and convex at the caudal limit. Implant designs are noted to be both flat and convex. It is important to know your device and use this information accordingly in preparation. The keeled implants are typically flat because they rely more on the keel cut as a barrier to posterior migration, whereas nonkeeled implants rely on a friction fit of the bone-implant interface to avoid migration until appropriate ingrowth occurs. In the case of the keeled implant, the authors have noted that proper placement of the implant is hindered if the posterior lip of the cranial endplate of the disc space is not taken down. No matter which device is used, familiarity and experience are keys for success.

Verification of Midline, Sizing/Lordosis, and Placement

The ultimate goal of any arthroplasty prosthesis is to reproduce exactly the motion of the native joint. The functional unit of the spine is no different. This linkage with the posterior ligamentous complex and facet joints posteriorly and disc space anteriorly provide for a complex kinematic loading system that includes coupled motion. Motion is typically a combination of two planes. It has been documented that the center of rotation is not a fixed point but rather follows an elliptical path in the sagittal plane.³⁸ These facts stress the importance of placing the implant in the midline of the endplates. Verification of midline is not a common practice when placing interbody fusion devices or grafts except when trying to make an x-ray appearance symmetric. Frankly, this can be an added step in ACDF surgery that is time consuming and unnecessary. Additionally, a device placed too far off of the midline can cause problems with the exiting nerve root or vertebral artery. This could introduce overdistraction on one side, causing a traction phenomenon, and stenosis on the other, away from the prosthesis, causing recurrent or new radicular symptoms. To add insult to injury, this could be a dynamic phenomenon because of the preservation of motion.

When sizing the implant, your goal is to use the largest footprint available. This occasionally requires slight squaring of the uncus-to-uncus distance when your initial trial has too much room on the lateral borders of the prosthesis and appears undersized. Sagittal plane sizing is not much of a problem due to judicious use of intraoperative fluoroscopy. The goal is to arrive at a placement of the device’s posterior edge that corresponds to the posterior edge of the vertebral body. This placement correlates with the proper placement of the center of rotation, which allows for coupled motion to recreate normal physiologic motion.³⁸ As more cervical discs are placed and more reports are published, subsidence has become more of a theoretical issue. Lin and colleagues³⁹ showed in a finite element study that implants with a keel or flanges may increase the propensity to subsidence due to high-contact stresses that could induce resorption of bone. This study also showed the importance of avoiding an undersized implant on the basis of increased stress concentrations per unit area. Lordosis preservation is important as well for maintenance of normal kinematics of the cervical spine. Implants come with differing degrees of lordosis to match the patient’s anatomy. Studies have shown that single-level segmental kyphosis can cause accelerated adjacent segment degeneration in fusion,⁴⁰ but others have shown that it does not affect it. Pickett and colleagues⁴¹ reported 12 cases of preoperative lordosis with postoperative disc arthroplasty kyphosis but no long-term follow-up or short-term complications from this. The remainder of the cervical spine compensated for this single kyphotic segment and maintained an overall lordotic alignment.

Failures

The focus of this review is to point out implant-related failures of total disc arthroplasty. Complications that can occur commonly with ACDF procedures are not covered here because they are patient selection, neurologic recovery, as well as approach related. Because long-term outcome studies are lacking, there is a paucity of literature showing common methods of failure. Articles in the past have categorized the complications into time dependence with early and late.^41,⁴² The most obvious failures are migration and loosening because they are found on routine follow-up imaging (Fig. 100–10). The predominance of these cases is asymptomatic. The exact cause is rarely known. Theoretical reasons include poor ingrowth, poor sizing, shear force of a whiplash injury, or endplate resorption coupled with subsidence. Most common out of these is likely poor sizing. Implants now have the importance of a press-fit size that is usually difficult to remove during the procedure. If implants are easily adjustable in the sagittal plane intraoperatively, they are likely undersized and have an increased chance of displacement in the future. Keels, teeth, and screw fixation of implants have all been improvements in this initial fit and subsequent risk of displacement. Cunningham and colleagues⁴³ conducted a biomechanical pull-out strength test of multiple constructs that were compared with anterior plate and tricortical iliac crest. No correlation was noted with total surface area of the implant itself, but rather the presence of screws, which was strongest, as well as toothed ridges and keels, which were next in strength.

FIGURE 100–10 Failed PCM disc before V-teeth. A, Patient, 38 years old, treated with single-level disc arthroplasty. B, Six months later the patient presented for routine follow-up with this radiograph. History of MVA. Denied ever having new-onset dysphagia. C, Revised to ACDF.

Vertebral body fracture is the most extreme case of loosening. Presently there is only one case report and many anecdotal reports of these in the cervical spine with multilevel adjacent implants. This complication is at most risk when a keel cut is required in the same vertebral body. Datta and colleagues ⁴⁴ presented a case with an intraoperative fracture of the intervening vertebral body in a two-level case. Postoperative CT showed a fracture in the superior-most vertebral body, as well as the middle vertebral body with two keel cuts. The authors cited dull chisels as the reason for fracture because there was at least 5 mm of space between the two keel cuts. This complication presumably gave birth to the side-cutting milling drill bit, which requires no mallet or force to produce a keel cut. The lumbar spine has a report of this complication by Shim and colleagues.¹⁹ Another type of fracture includes a posterior vertebral body fracture, perhaps referred to as a reverse tear drop because it is of the triangular piece of bone at the posterior edge of vertebral body. The mechanism is provided by too shallow keel cuts. In this situation, attempts are made at placing the implant in the optimal position under fluoroscopy, but the keel is bottomed out. Harder strikes of the mallet cause this fracture, which can displace fragments into the canal, causing compression. Shim and colleagues⁴⁵ described this as an avulsion fracture, but this is a misnomer. This is a direct fracture of the vertebral body by the implant or use of keel chisel rather than a ligament pulling off the piece of bone. The use of side-cutting milling drill bits has decreased the risk of this complication, as well as finesse with implant placement. If the implant does not want to arrive at the optimum position, attempt to recut the keel.

Heterotopic ossification (HO) is another failure of cervical disc replacement. This complication, although not catastrophic, defeats the purpose of placing a motion-preserving device. The goal is to be able to predict who will develop it and perform an ACDF instead. Parkinson and colleagues ⁴⁶ presented the first case report on spontaneous fusion of cervical arthroplasty. A nonkeeled device was used. Lack of motion was noted immediately postoperatively on the basis of flexion-extension radiographs. The authors speculated that the cause was not directly known but because of the acute decrease in preoperative and postoperative motion that it was a device malfunction versus overdistraction and overstuffing the disc space. Mehren and colleagues found an incidence of 66% with any HO present following cervical disc arthroplasty in a prospective series of 77 keeled implants. Grade of HO was assessed at 1 year postoperatively on the basis of McAfee and colleagues’²⁴ classification system described for the lumbar spine. Complete fusion (grade 4) was noted in 9%. Loss of motion and fusion is described as either 2 degrees or less of dynamic motion or change in the interspinous distance of 2 mm or less.^48,⁴⁹ Heller and colleagues⁵⁰ found that not only did the ossification typically occur in the first 100 days of surgery but it was typically lateral near the uncovertebral joints. The authors recommended judicious use of nonsteroidal anti-inflammatories (NSAIDs) in high-risk patients. What is a high-risk patient? No evidence in the literature points to a specific set of variables that will predict HO. Perhaps in the older patient with mild global spondylosis, NSAIDs are indicated, but this introduces the potential for poor ingrowth into these implants, although this is theoretical. As the indication for disc replacement expands, implants are being placed in older, “borderline” patients who perhaps require an ACDF instead. More long-term studies that compare age with onset, as well as the use of a high-speed burr, need to be conducted.

Inflammation or hyaline cartilage formation after placement of a total cervical disc arthroplasty refers to a host response complication. The most obvious reason is inflammation from particulate debris. Wear characteristics of cervical disc replacements have been studied. Several studies have reported on the wear debris production and inflammatory effects of disc replacements. They all showed minimal debris and no inflammatory response after 40 million cycles in one.^51,⁵² So although theoretical, wear debris has not been a problem to date in the literature. A 37-year-old woman treated with a disc replacement with a polyethylene articular component presented to our facility with increasing pain 9 months postoperatively. Her radiculopathy had subsided, but axial neck pain was increased from her preoperative state and required increasing amounts of narcotics. Her examination revealed lower extremity hyperreflexia and relative weakness 4/5 throughout the lower extremities. Imaging studies including a CT myelogram revealed no canal encroachment or myelomalacia. Infection indices were negative. The patient was convinced that the implant was causing the problems. After considerable counseling of the patient on risks of revision and encouragement to avoid another surgery, she wished to proceed with conversion to a fusion. During the case, neuromonitoring was used. Baseline EMGs revealed poor amplitudes throughout. Interestingly and surprisingly, at the moment the total disc replacement was removed, amplitudes increased instantaneously in all leads. Her postoperative course was characterized by improvement of her strength and reduced pain. In a similar case, Cavanaugh and colleagues⁵³ reported a case characterized by radiographic evidence of a soft tissue mass 9 months after cervical disc placement with recurrence of radicular symptoms. Gross exploration revealed a thick layer of hyaline cartilage with histologic analysis revealing chronic inflammatory debris. Both cases display a possible metal hypersensitivity reaction rather than wear debris due to the short time course of the symptom onset.

Dynamic radiculopathy may present another complication. Jan Goffin ⁴² addresses this type of complication when referencing malposition of implants in his review of cervical disc arthroplasty complications. With an undersized implant that is not placed in the midline, preferential loading may cause a coronal plan abnormality that causes the contralateral foramen to decrease in size, effectively causing a bony stenosis of the foramen or possible impingement on the vertebral artery. Symptoms would include recurrence of radiculopathy if on the same side as the previous disc herniation or new contralateral radiculopathy. A reduction of symptoms may be seen with lateral bending toward the offset side of the implant. This requires revision.