Total disc replacement (TDR) surgery began in Europe over 20 years ago and migrated to the United States in 2000 with the first TDR Food and Drug Administration (FDA) Investigational Device Exemption (IDE) trial of the Charité III disc. Results based on the long-term follow-up from European literature have been promising so far, as have the early results from our U.S. experience.¹^–⁵ Some of the first 5-year follow-up data from the FDA IDE trial comparing the Charité disc to lumbar fusion are now available.⁶ The results show no statistically significant differences in outcome measures (visual analog scales (VAS) assessing pain and the Oswestry Disability Index) between the groups at the five-year mark, substantiating the noninferiority of the disc arthroplasty group. Additionally, the Charité patients had a statistically greater rate of part-time and full-time employment and a lower rate of long-term disability at 5years. Furthermore, the range of motion of the prosthesis, as evaluated by radiographic criteria, remained the same at 5-years compared with the 2-year data, showing preservation of motion at the surgical level.

At the time of writing of this chapter, a recently published systematic review analyzing the association of symptomatic adjacent segment disease (as distinguished from asymptomatic adjacent segment degeneration) in lumbar arthroplasty compared to arthrodesis showed that 14% of arthrodesis patients developed adjacent segment disease, compared with 1% of arthroplasty patients.⁷ Given this positive trend for arthroplasty, it is reasonable to assume that disc replacement surgery will remain a valid tool in the spine surgeon’s armamentarium and will likely become more prevalent in years to come.

There are a number of factors that influence the outcome following total disc replacement, including meticulous surgical technique and appropriate implant selection. However, as with any surgical procedure, patient selection is of utmost importance for ultimate success and reproducible results. A thorough knowledge of appropriate indications and contraindications for spinal disc arthroplasty is the key to maximizing patient safety and surgical outcome. It is the goal of this chapter to discuss these various indications and contraindications as they pertain to disc arthroplasty in the lumbar spine, with particular emphasis on older patients. It is not our intention to simply reiterate the various indications and contraindications for the multiple devices as previously published for the FDA studies. Rather, we will examine this topic from a practical standpoint, considering some of the intrinsic patient factors, surgical factors, radiographic factors, and device-specific factors that would make a patient either a favorable candidate for total disc replacement or an unfavorable one. It is still important to realize that the strict criteria set forth by the FDA for the initial studies on disc replacement were intended to maximize the expected benefit from the procedure and minimize any possible complications. Additionally, one should keep in mind that the participating surgeons for the trials were chosen in large part because of their many years of surgical experience. These surgeons were poised to climb the necessary learning curve more readily than many other surgeons who were still early in their training. Therefore, the aforementioned strict inclusion and exclusion criteria for the clinical trials should be viewed as a beacon for those surgeons embarking on the first part of the learning curve with artificial disc replacement.

When we focus our attention specifically on the aging spine, we must consider a few different scenarios. The first scenario is that of the chronologically-young patient with a physiologically aged and degenerative disc, who meets the requirements for disc arthroplasty. In this case, the prosthesis will be subjected to the normal physiologic aging process and it will be the longevity of the prosthesis that poses the ultimate challenge. One must be aware of the likely need for some type of revision procedure in the future and weigh this risk with the anticipated benefit from the surgery. Furthermore, the question must be asked whether a motion-preserving implant such as a disc arthroplasty will still provide any motion in 20 years, and what the relevance of this might be. These are questions that should be answered once more long-term data are available. The second scenario is that of the patient of more advanced age who presents for consideration of a disc arthroplasty. This scenario, as would be expected, poses a completely different set of diagnostic and treatment challenges for the spine surgeon. With advanced age, the likelihood of medical comorbidities or other physiologic contraindications to disc replacement is increased. Moreover, what happens when a disc arthroplasty is implanted in an older patient who meets all the inclusion criteria at the time of surgery, but in the years to follow develops osteoporosis or significant osteopenia? Will this have an impact on the performance of the prosthesis or dramatically increase the risk of subsidence? In this chapter, we will make a specific effort to touch upon some of the special challenges of the aging spine. It is becoming more prevalent to have patients that remain highly active well into their sixth and seventh decades and who want to be evaluated as candidates for the new technology of motion preservation.

Clinical Practice Guidelines

Indications

The established indication initially set forth by the FDA IDE studies for lumbar disc arthroplasty is severe unremitting low back pain resulting from single-level degenerative disc disease that has failed to respond to a prolonged course of conservative measures. The trial period for conservative treatment is usually defined as a minimum of 6 months, although the exact time frame itself is less important than the extent to which nonoperative management has been attempted. Nonoperative treatments should incorporate the use of various antiinflammatory, nonnarcotic, and even narcotic medications if necessary; physical therapy, including active exercise and core stabilization; chiropractic modalities; and a trial of spinal injections, including epidural injections and facet injections as appropriate. The purpose of these conservative efforts is to ensure that the patient has been afforded every opportunity to obtain a satisfactory result without surgery. We know that MRI alone is not a reliable indicator in predicting whether a degenerative-appearing disc is truly symptomatic.^8,⁹ In some cases, discography may be helpful in delineating whether the disc is responsible for the patient’s clinical symptoms. The exact role for discography as part of the clinical work-up for potential surgical candidates is not clearly established, and is still an issue of significant controversy. That being said, at our institution we incorporate the use of discography in all patients that we feel might be appropriate candidates for total disc replacement. Because the interpretation of results from discography can heavily depend on the skill of the technician performing the procedure, the established relationship between the surgeon and the physician performing the test cannot be overemphasized. In our opinion, a poorly-done discogram is worse than no discogram at all.

A patient must be skeletally mature to undergo disc replacement. While many of the studies have cited an age range from 18 to 60 years of age in the inclusion criteria, the actual chronological age itself is simply a number for reference. It must be taken into account with more relevant factors such as appropriate vertebral body size to accommodate the prosthesis and adequate bone quality to support the implant, which often, but not always, can be correlated with the patient’s age. It is also the authors’ belief that disc arthroplasty technology should be reserved for those patients over the age of 25 years until further long-term data becomes available. We have been proponents of recommending bone density scans in all females older than 40 years of age and all males over 50 years of age unless other risk factors are present. The acceptable bone quality is to have a T-score of more than −1.0, based on World Health Organization criteria, meaning that there is no evidence of osteopenia. Some patients with an advanced chronological age remain physiologically young and could still be a legitimate candidate for disc arthroplasty. In fact, these are important patients to identify when we are considering the issue of the “aging spine.’ On the contrary, some patients with a younger chronological age can have medical comorbidities or poor bone quality that effectively removes them from surgical consideration. It also becomes important, particularly in the elderly population, to carefully evaluate whether motion preservation is justified compared to the alternative of fusion surgery. We do not yet have enough long-term data to make resolute conclusions regarding the proposed advantages of motion preservation in comparison with fusion as it pertains to adjacent segment disease; however, with the knowledge currently available, we can feel fairly justified in our desire to preserve motion in young patients with isolated discogenic pain. The more elderly candidates may not ultimately see the benefit of disc

Clinical Case Examples

Case 1

A 54-year-old woman presents to your clinic with complaints of severe low back pain that has progressed over the past 2 years. She has been through physical therapy focusing on core muscle strengthening and has had a trial of epidural steroid injections and various facet injections that did not improve her symptoms. She denies any radicular leg pain, but states that her low back pain is constant and is aggravated by prolonged standing, sitting, or walking. She has been on NSAIDs for the past few years, and has recently started to take narcotics because of increased pain. A complete neurological examination reveals no motor or sensory deficits. She has pain and limited motion with forward flexion of her lumbar spine. Her radiographs (Figures 55-1A and B) show normal spinal alignment with decreased disc space height at the L4-5 and L5-S1 level. No instability is present on flexion-extension views. A T2-weighted MRI (Figures 55-2A and B) reveals a dessicated L4-5 disc with preservation of hydration in the remaining lumbar discs. Given this scenario, what treatment option(s) would you discuss with this patient?

FIGURE 55-1 A and B. AP (A) and lateral (B) radiographs showing disc space narrowing at L4-5 and L5-S1.

FIGURE 55-2 Sagittal (A) and axial (B) T2-weighted MRI views showing disc dehydration and posterior bulge at the L4-5 level.

Case 2

A 56-year-old male presents to your clinic for evaluation of debilitating low back pain over the last 6 years. He reports daily pain that radiates across his low back, worse on the right side. His primary care physician sent him to physical therapy for 6 weeks, but he did not see any symptomatic improvement. He has been taking hydrocodone for the past 6 months in order to bring his pain to a tolerable level but is frustrated by the need to be on narcotic medications and wishes for something to be done surgically to address his pain. His physical examination is fairly unremarkable and he has normal motor and sensory function. His plain radiographs reveal only minimal narrowing at the L4-5 and L5-S1 levels (Figure 55-3A and B). A T2-weighted MRI reveals pronounced desiccation with a small posterior bulge, along with minimal disc desiccation at the L4-5 level (Figure 55-4). On axial MRI views, the facet joints at both levels appear normal or with minimal degenerative changes. He asks whether he is a candidate for the disc replacement surgery. What do you tell him?

FIGURE 55-3 AP (A) and lateral (B) radiographs showing only minimal disc space narrowing at L4-5 and L5-S1.

FIGURE 55-4 Sagittal T2-weighted MRI showing significant disc dessication at L5-S1 and slight desiccation at the L4-5 level.

replacement, particularly if further studies suggest that adjacent-level disease is borne out over periods of 10 years or greater.

Bertagnoli and Kumar stratified indications for disc arthroplasty into four categories based on remaining disc height, status of the facet joints, adjacent level degeneration, and stability of the posterior elements.¹⁰ The prime candidate for a disc replacement, based on their evaluation of clinical outcome in 108 patients who underwent implantation of a ProDisc II prosthesis, had at least 4 mm of remaining disc space height, no radiographic changes suggestive of facet arthritis, no adjacent level disc degeneration, and intact posterior elements.

Certainly, having competent, nondegenerative facets and posterior element stability are important inclusion criteria for a patient to be considered appropriate to undergo TDR. We will discuss the issue of facet arthrosis further as a part of contraindications to disc arthroplasty, but as far as a clinical evaluation is concerned, facets should be assessed with direct palpation and by having the patient demonstrate whether spinal extension (i.e., facet loading) reproduces pain. Radiographically, the facets should be evaluated by examining their appearance on plain films, CT scans, and/or axial MR images. There exist a few grading systems for facet joints, although none has gained universal acceptance. The first, proposed by Pathria, assigned a grade of 0 to 3, depending on the extent of facet joint narrowing.¹¹ A “normal” facet joint is given a grade of 0, whereas a grade 1 is assigned for mild narrowing, 2 for moderate, and 3 for severe narrowing. Patients with grade 3 facets in this grading system should be excluded as candidates for disc arthroplasty. Fujiwara also proposed a grading system based on evaluation of the facet joints as they appear on axial MR images.¹² In this system, a grade of 0 is again assigned to “normal” facets, grade 1 for moderately compressed facets with small osteophytes, grade 2 for facets with subchondral sclerosis and moderate osteophytes, and grade 3 for facets lacking articular joint space and with large osteophytes. Again, patients who meet the criteria for grade 3 facets in this classification system are not indicated for total disc arthroplasty.

Since the facets transmit nearly 20% of the load-bearing forces in the lumbar spine in the normal state, but can increase this number to 50% in the degenerative state when a patient is standing, they must not be a contributing pain generator if a patient is to expect maximal benefit from a motion-preserving procedure. As for the importance of the posterior elements, a prosthetic disc alone cannot substitute for lack of stability in a given spinal motion segment. This is certainly true in the case of the less constrained prostheses. A patient must also have no more than 3 mm of anterolisthesis at the level under consideration to be considered appropriate for disc replacement.

Concern over the relationship between preoperative disc height and clinical outcome in disc arthroplasty with severely collapsed disc space (i.e., less than 4 mm) is somewhat controversial. Despite speculation that TDR is not appropriate for severely collapsed discs, there exist few data to support or refute this. At our institution, we set out to determine if there was a relationship between preoperative and/or postoperative disc height and clinical outcome at 2 years (Li, Guyer, et al., International Meeting on Advanced Spinal Techniques, 2008).¹³ For 117 patients (42 Charité and 75 ProDisc-L) undergoing a single-level TDR, we recorded disc height as a ratio of vertebral body height, thereby accounting for variation in individual size and radiographic magnification. Patients were categorized into four groups based on these ratios (most collapsed, second most collapsed, second least collapsed, least collapsed). For all groups, the mean VAS pain score improved significantly from preoperative values, but there were no statistically significant differences among the groups. We therefore concluded that there is no relationship between preoperative disc height and clinical outcome. If patients with severely collapsed discs otherwise meet the strict selection criteria for TDR, they can expect as favorable an outcome from the surgery as patients with discs that are not as collapsed.

In addition to the aforementioned clinical criteria, it is also important to ensure that the patient is capable of completely understanding the various risks related to the surgery itself and the realistic expectations following the procedure. Based on the accumulated VAS and Oswestry scores from the various IDE studies of disc arthroplasty, including data from the Charité, ProDisc-L, Maverick, Flexicore, and Kineflex studies, patients can be counseled that 80% of people undergoing lumbar TDR can expect to achieve a 50% reduction in their pain and a 50% improvement in their functional ability. They must be willing to comply with any postoperative restrictions imposed on them by the surgical procedure and must also be willing participants in the postoperative rehabilitation protocol.

Contraindications

It is often easier to define patients who are not good candidates for a given procedure than to accurately define those who would be suitable. To some extent, the same can be said for lumbar disc arthroplasty. In recent years, much attention has turned toward defining and understanding the established contraindications for TDR, and this, in turn, has generated some controversy. In an epidemiological study to investigate the contraindications to lumbar total disc arthroplasty in their patient population (that of an academic medical center), Huang et al reported that 95% of patients had at least one of ten contraindications to surgery.¹³ This finding was further substantiated in a recent publication by Wong et al,¹⁴ who retrospectively reviewed 100 consecutive lumbar spine surgery patients with specific analysis of facet arthrosis and noted that all patients had one or more of the aforementioned ten contraindications to the procedure. In their population (a private medical center), they found that 97% of patients had facet arthrosis as the contraindication against TDR, followed by spondylolisthesis (75%), and central spinal stenosis (72%).

For the purpose of our discussion, we will group contraindications into two categories: absolute or “hard” contraindications, and relative or “soft” contraindications. Absolute contraindications include osteopenia and osteoporosis, history of previous disc infection or ongoing infection, prior fusion at the level of consideration, severe posterior element pathology, instability at the operative segment, vertebral fracture, malignancy, curves of greater than 11 degrees, metal allergy, and a psychosocial state that places a given patient at increased risk for poor surgical outcome. Additionally, as this is entirely an elective procedure, pregnancy should be viewed as an absolute contraindication. Relative contraindications include history of prior abdominal surgery, and obesity. To better appreciate the reasons behind the various contraindications, we will discuss a number of them in further detail. The reader may also refer to Table 55-1, which gives a summary of the contraindications that will be discussed in the chapter.

TABLE 55-1 Contraindications to TDR

Discussion of Clinical Case Examples

Case 1

In our first case example, the patient is a middle-aged woman who has been through appropriate conservative management but has persistence of severe back pain. She has imaging studies suggestive of single-disc disease at the L4-5 level. If she is insistent upon having surgery, it is a reasonable option to discuss a one-level fusion at the suspected symptomatic L4-5 level. However, concern might exist that by fusing the L4-5 level, an increased amount of stress would be placed upon the L5-S1 level, possibly causing it to become symptomatic sooner than it otherwise would have done. She is also a very reasonable candidate to undergo total disc replacement, and given the aforementioned concern, we would likely be in favor of TDR over fusion. She ultimately decided to undergo TDR and had complete resolution of her back pain (Figure 55-5 A and B).

FIGURE 55-5 Postoperative AP (A) and lateral (B) films showing a Charité TDR at the L4-5 level.

Case 2

The gentleman presented in the second case represents more of a diagnostic and treatment challenge. He has two disc levels that could be symptomatic, with the L5-S1 level appearing the worst, based solely on imaging studies. However, knowing that imaging studies alone can be misleading, it would be helpful to ensure that the suspected level is truly the pain generator and determine whether the L4-5 level is also a contributing factor. We used discography to help with our decision-making (Figure 55-6). Both the L5-S1 and L4-5 levels reproduced concordant pain of 10/10 and 8/10 respectively, and the L3-4 level was completely asymptomatic. The options discussed with him included either fusion at both levels or consideration of fusion at the more caudal level and a disc replacement at the cranial level. At this point, two-level lumbar total disc replacement has not been FDA-approved. He decided to undergo a hybrid procedure, with L5-S1 fusion and an artificial disc at the L4-5 level (Figure 55-7 A and B). Within 6 weeks following surgery, he was able to discontinue use of all narcotic medications.

FIGURE 55-6 Discography was utilized to evaluate the L3-S1 levels. This figure shows the normal L3-4 disc and the injection at L4-5.

FIGURE 55-7 Postoperative AP (A) and lateral (B) films showing fusion at the L5-S1 level and TDR at the L4-5 level.

Another anatomical consideration that may preclude disc arthroplasty is the slight anatomical variations that exist at the more cranial lumbar levels with regard to the kidneys and the renal vasculature. At the L2-3 level, it may sometimes not be possible or safe to implant an artificial disc because of the inability to mobilize the renal artery or vein, or even the kidneys themselves.

Psychosocial Factors

Last, but certainly not least, we must discuss psychosocial factors as a potential contraindication to total disc arthroplasty. Much work has been done with regard to evaluating the effect of a patient’s psychological state in relation to predicting surgical outcome. Even the most perfectly executed surgical procedure will fail to alleviate pain in patients with serious psychological overlay. The concept of presurgical psychological screening (PPS) has been advocated to objectively identify psychosocial risk factors that can lead to poor results from surgery, even when the physical pathology causing pain has been eliminated. The screening process takes many things into consideration, including personality and emotional factors, behavioral and environmental factors, and even historical factors for a given patient.

One of the strongest risk factors for poor surgical outcome relates to excessive pain sensitivity as assessed by the hysteria and hypochondriasis scales of the MMPI. Elevations in these scales have been shown to be associated with poor spine surgery outcome in numerous studies.¹⁶ Other studies have determined that patients who abuse narcotic medications and/or alcohol also have a high failure rate following spine surgery. Recently the use of PPS, specifically in lumbar TDR patients, was reported (Block et al, North American Spine Society, 2008). The authors found that the results of screening were significantly related to clinical outcome. In cases where the spine surgeon has suspicion that multiple psychosocial factors may exist and compromise outcome from the proposed surgery, it can be quite helpful to incorporate the use of PPS as part of the preoperative work-up.

Conclusions

Technological advances in materials and design, coupled with a greater biomechanical appreciation for motion, have spawned a new age in spine surgery. Total disc replacement has yet to see the pinnacle of its day, but has generated enough press that patients often present to clinics with the expectation and desire to be recipients of this procedure. Our excitement to participate in the wave of this emerging technology must be met with great caution as we evaluate potential candidates for disc replacement. Strict adherence to inclusion and exclusion criteria benefits everybody involved in the process. Most importantly, this ensures that patients have the greatest chance of expecting a positive outcome and surgical success. Positive outcomes help to ensure that the technology will become more widely appreciated and accepted, whether by our patients, by the federal agencies funding the procedure, or by private insurers.

As the population continues to age and remain active, the demand from our patients has moved toward expectations of maintaining function. Patients who are chronologically aged but remain physiologically young may be appropriate candidates for disc replacement. Age alone may not be an appropriate exclusion criterion in isolation, but should be taken in context with the many other factors we have discussed in the chapter. In other words, the “aging spine” may still be deserving of this new technology. In fact, data on patients enrolled in the IDE study of the Charité Artificial Disc were analyzed based on age, with groupings of patients aged 18 to 45 years compared with those aged 46 to 60 years.¹⁶ At 2-year follow-up, there was no significant difference between the groups with respect to changes in ODI scores, VAS scores, or SF-36 component scores compared to baseline values. Patient satisfaction was equivalent in both groups (87% and 85%, respectively), and no significant differences were noted as far as adverse events or reoperation. This reflects the fact that, given judicious application of inclusion and exclusion criteria, patients who are chronologically older can still expect equivalent outcomes to their younger counterparts. Bertagnoli et al prospectively evaluated a series of patients aged 60 years or older (range 61 to 71 years) who underwent TDR for discogenic low back pain.¹⁷ They noted statistically significant improvement in patient satisfaction and ODI scores by 3 months after surgery and maintenance of these improvements throughout the 24-month follow-up. Although the authors recommend cautious use of TDR in this population, their results suggest that if patients otherwise meet indications for TDR, with particular attention to spinal stenosis and bone quality, age greater than 60 years alone is not a factor that should preclude them from having this procedure.

There are many questions for which we do not yet have answers. What happens to the patient with a TDR who develops osteoporosis? Will the prosthesis subside or will Wolff’s law protect the endplates? Will the prostheses function for the 40 years for which they have been biomechanically tested or will the TDRs give way to a slow fusion? With more time and with the accumulation of more long-term data from the population of total disc arthroplasty patients, these questions and others will be answered, and more stringent inclusion and exclusion criteria will be defined. Until that point, it is our hope that the discussion presented in this chapter will provide enough of a framework for surgeons who are currently performing disc replacements or those who are interested in pursuing this procedure to be able to provide the best possible outcomes for our patients, with consideration of their safety as our primary goal.

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