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.