Failed Total Disc Arthroplasty

Patient Selection

Proper patient selection for any spine intervention is at least as important, perhaps even more so, than perfect technical execution. Indicating patients for total disc arthroplasty is no exception. There are some conceptual differences in the treatment of cervical and lumbar disease; these differences play out when contrasting the literature for cervical and lumbar disc arthroplasty. Cervical disc arthroplasty is indicated for reconstruction following neurologic decompression. Lumbar arthroplasty is indicated for mechanical discogenic low back pain.

One inherent contrast is the fact that the majority of ventral neural compression is treated with anterior surgery in the cervical spine and posterior surgery in the lumbar spine. In the cervical spine, due to the fact that this is cord-level surgery, a great deal of radiculopathy is treated with anterior surgery, with the primary goal of surgery being neural decompression. Cervical fusion or disc arthroplasty is performed as a consequence of the subtotal discectomy necessary for adequate neural decompression. In the lumbar spine, however, neural compression in the absence of significant axial symptoms or instability is often treated with dorsal decompression alone. In the absence of instability or deformity, anterior surgery in the lumbar spine is typically reserved for treatment of axial symptoms, which, by nature, are much less predictable in terms of surgical outcome. These inherent differences between the treatment of cervical and lumbar disease are at least in part responsible for the much greater body of evidence supporting cervical disc arthroplasty outcomes. For the clinician, this means that indicating the proper patient for lumbar total disc replacement (TDR) is even more challenging.

Inclusion and exclusion criteria from US Food and Drug Administration (FDA) investigational device exemption studies identified the subsets of patients who would benefit from either fusion or TDR; 5- to 10-year follow-up of these patients has demonstrated durable and significant improvements in pain, impairment, and satisfaction scores. There are multiple patient factors considered to be relative or absolute contraindications to disc arthroplasty, which have been demonstrated in the literature to be associated with higher rates of TDR failure. Patients with very poor bone quality—certainly those diagnosed with osteoporosis (T-score <2.5) but even those with osteopenia (T-score <1.0)—are more likely to have failures of the bone-implant interface. We routinely check densitometry on potential TDR patients and utilize more stringent criteria, avoiding lumbar TDR in most patients with a T-score less than −1.0 and avoiding cervical TDR when the T-score is less than −2.5. We often discover bone density abnormalities in otherwise healthy, relatively young patients. Patients with endocrine or metabolic bone diseases that might also affect bone formation are likely to have similar consequences. On the other hand, patients with an abundance of bone formation, such as diffuse idiopathic skeletal hyperostosis or ankylosing spondylitis, are also poor candidates for arthroplasty.

Ossification of the posterior longitudinal ligament is a contraindication to cervical TDR even if bridging autofusion has not occurred, as motion preservation is not only unnecessary but potentially deleterious, given the theoretical risk of continued osteophyte growth that may perpetuate further ventral neurologic compression. Hypermobility, either through inflammatory arthritides or systemic collagen disorders, is more likely to result in instability or supraphysiologic motion after disc arthroplasty. Obesity alters the biomechanical environment, especially for lumbar disc arthroplasty; it should be considered a relative contraindication, or even an absolute contraindication, at its extremes to motion preservation. Facet arthrosis or arthropathy, if symptomatic at the time of surgery, is likely to result in continued axial pain and should be thoroughly investigated with imaging as well as the clinical history and examination. Further, many surgeons in our practice will send patients for facet injections prior to considering lumbar disc arthroplasty, a negative response to which would certainly support the absence of symptomatic facet disease.

Instability is a contraindication to motion preservation. Pars fracture would be an extreme example of this and an obvious contraindication to disc arthroplasty. However, there were numerous failures in the early CHARITÉ trials for unrecognized pars defects. More subtle instability is less easily defined and, as such, comes with a paucity of guidelines. Patients with more than a few millimeters of static spondylolisthesis, more than a few millimeters of intervertebral translation, or facet effusion on magnetic resonance imaging (MRI) scan ought to be evaluated carefully and may be considered relative contraindications to TDR. From the standpoint of global alignment, we consider lumbar sagittal imbalance and cervical kyphosis contraindications to disc arthroplasty, and decision making is based on the extent of kyphosis or sagittal imbalance, the patient's clinical symptoms, and the number of levels being addressed at surgery. Coronal deformity is more commonly encountered in the lumbar spine and we consider greater than 11 degrees of scoliosis an absolute contraindication to disc arthroplasty. In practice, however, we typically abide by more stringent criteria and avoid motion preservation in patients with greater than 5 degrees of coronal curvature ( Fig. 104.1 ).

Discectomy and Endplate Preparation

Removal of the native disc is the initial procedural step for disc arthroplasty; performing a near-complete or radical discectomy is of paramount importance for a multitude of reasons, including neural decompression, exposure of stronger peripheral bone, and accurate implant sizing and placement. Shortcuts during discectomy will result in compounding technical challenges for the surgeon, while near-total discectomy will facilitate improved visualization, restoration of disc height and lordosis, better implant rim fit, sizing, and positioning. In the cervical spine, regional anatomic features make discectomy straightforward and near-complete removal of the disc and good visualization from uncus to uncus is rarely a technical challenge. In the cervical spine, it is our preference to remove the posterior anulus and posterior longitudinal ligament in order to confirm neural decompression, facilitate foraminotomy, and allow for parallel distraction of the endplates. This is perhaps a more technically demanding portion of the cervical discectomy and deserves careful attention as persistent foraminal compression, either static or dynamic, is the most common cause of neurologic failure after cervical disc arthroplasty. For patients with some uncovertebral spondylosis and bony foraminal narrowing, perhaps for whom a surgeon is pushing the indication for motion preservation, this portion of the procedure is even more important. Patients with significant disc space collapse and spondylosis requiring extensive bony resection are not indicated for arthroplasty. Following extensive decompression, they are at increased risk for implant subsidence and potential segmental ankylosis due to bone overgrowth with bridging osteophytes or heterotopic ossification.

Subtotal discectomy in the lumbar spine, on the other hand, is much more challenging, predominantly due to the regional vascular anatomy. Proper management of the great vessels is paramount for adequate visualization and exposure of the lateral aspects of the disc space. TDR requires even more thorough discectomy than anterior lumbar interbody fusion; thus, access surgeons and spine surgeons performing these procedures must be even more adept at the anterior approach to the lumbar spine and the mobilization of the great vessels. We prefer handheld retractors, which allow for intermittent and variable tension on the great vessels as needed during the procedure, though table-mounted retractor systems may be the only option for spine surgeons working without access assistance. In general, the most challenging portions of the disc to visualize and remove are the lateral and posterolateral portions, especially those ipsilateral to the operator. Adequate lateral discectomy is important for implant sizing and endplate coverage, while adequate posterolateral discectomy is important for preventing incomplete decompression and avoiding inadvertent displacement of disc material dorsally during prosthesis insertion. Visualization of the posterior portion of the disc space is often more challenging at L5–S1, especially in cases in which there are unfavorable pelvic parameters and greater sacral slope. Positioning the patient on an operative table with a break over the lumbosacral junction can help facilitate visualization and disc preparation. Flexing the operative table and utilizing Trendelenburg positioning can create segmental lordosis and bring the lumbosacral disc space into direct view. The posterior longitudinal ligament should be released on every case, and even resected if necessary to allow for parallel endplate distraction in patients in whom the posterior longitudinal ligament (PLL) is contracted and fibrotic.

Endplate preparation principles are similar in both the cervical and lumbar spine, with the goal being to remove the cartilaginous endplates down to the subchondral cortical bone. Violation of the cortical endplates and exposure of the cancellous bone weakens the bone-implant interface, increases the chance of subsidence, and may contribute to heterotopic ossification. There are subtle differences in technique that are implant-specific, more so than just keeled or nonkeeled; these subtleties are beyond the scope of this work. Nonetheless, the surgeon's familiarity with a particular implant and instrumentation is paramount.

Segmental Mobilization

The artificial disc implant does not confer motion. It allows and controls the segmental motion that the surgeon is able to gain at the time of discectomy and release both in the cervical and lumbar spine. In the cervical spine, a thorough discectomy and resection of the PLL along with decompression by minor symmetric uncovertebral joint resection generally mobilizes the segment adequately. Distraction pins and an intradiscal mechanical distractor also facilitate good segmental mobilization, with the TDR implant then functioning to control the motion. Failure to mobilize appropriately, “forcing” an implant into a tight disc space, or relying on bony endplate resection to accommodate an artificial disc replacement implant are all recipes for failure by lack of motion, subsidence, or bridging ossification.

The same principles apply to mobilization in the anterior lumbar spine, but the techniques are different. The spine surgeon must be comfortable working around the great vessels, there are no external distractor pins, and the visualization is more difficult due to the depths of the retroperitoneum compared to the anterior cervical spine. Mobilization is arguably the most critical step in lumbar arthroplasty technique, determining the effectiveness of motion preservation. Failure to adequately release the posterior longitudinal ligament and balance the lateral annular fibers without violating the endplates can result in early failure by implant subsidence, improper positioning, or even by anterior expulsion ( Fig. 104.2 ).

Implant Sizing, Placement, and Alignment

The principles of implant sizing are similar for both cervical and lumbar disc arthroplasty, the most important of which is to use the largest-size footprint available that fits within the disc space and to slightly undersize the implant height to allow for improved motion range. The cortical bone in the periphery of the endplate is the strongest, meaning that smaller implant footprints result in higher contact stress on weaker bone. In the cervical spine, there are instances in which a small portion of the medial aspect of the uncus must be reshaped in order to achieve appropriate fit. This occurs when one implant trial is undersized and the next-largest size impinges on the medial aspect of the uncus. In these instances, it is better to reshape the uncus than to undersize the implant. The identification of the midline is an important component of sizing and implant placement. It is important to identify the midline aspect of the disc space following appropriate-level verification. An anteroposterior fluoroscopic view is obtained early in the procedure and the midline is marked such that every part of the procedure, including the discectomy, is performed with reference to the midline. An implant that is not appropriately placed in the coronal plane can result in issues with direct nerve impingement on the ipsilateral side or perhaps underdistraction on the contralateral side and associated persistent stenosis. Sagittal plane implant placement is optimal when the posterior edge of the implant is in close continuity with the posterior aspect of the vertebral body, which typically results in proper placement of the center of rotation. Sagittal plane sizing and placement is less commonly a source of technical shortcomings, as most surgeons utilize a predominance of lateral fluoroscopy during implant placement.

Implant lordosis is another parameter that can serve as a source of technical error leading to implant failure. The L5–S1 level presents a unique challenge, especially in patients with high sacral slope, in that the articulating surfaces of the implant may experience higher shear forces, resulting in higher rates of implant failure. Patients with higher, more vertical sacral inclination should be considered to have a relative contraindication to disc arthroplasty. Direct line-of-sight visualization of the posterior portion of the disc space may not be possible if a line drawn through the disc space on a lateral radiograph intersects below the pubic symphysis. In this situation, arthroplasty at the lumbosacral disc space may not be technically feasible and a fusion to better resist shear forces may be a more thoughtful surgical choice.

Indications and Technique Issues

Poor patient selection, technical shortcomings, and device malpositioning are the most common causes of both cervical and lumbar disc arthroplasty failures. Although there is sparse evidence for the management of TDR failures, the etiology of these failures has been thoroughly investigated in the literature through numerous investigational device exemption studies and longitudinal follow-up series. It is clear that the vast majority of TDR failures are the result of surgeon decision-making and technical missteps, which are preventable. The previous section presented in detail the most important strategies for avoiding these sources of TDR failure. Poor indications, failure to adequately mobilize the segment, and suboptimal implant positioning are the prime causes for poor postoperative results following arthroplasty.