Cervical Disc Arthroplasty Compared with Arthrodesis for the Treatment of Myelopathy

Riew, K. Daniel MD; Buchowski, Jacob M. MD, MS; Sasso, Rick MD; Zdeblick, Thomas MD; Metcalf, Newton H. BS; Anderson, Paul A. MD

Journal of Bone & Joint Surgery - American Volume: 01 November 2008 - Volume 90 - Issue 11 - p 2354–2364
doi: 10.2106/JBJS.G.01608
Scientific Articles

Background: Although there have been case reports describing the use of cervical disc arthroplasty for the treatment of myelopathy, there is a concern that motion preservation may maintain microtrauma to the spinal cord, negatively affecting the clinical results. As we are not aware of any studies on the use of arthroplasty in this scenario, we performed a cross-sectional analysis of two large, prospective, randomized multicenter trials to evaluate the efficacy of cervical disc arthroplasty for the treatment of myelopathy.

Methods: The patients in the current study were a cohort of patients who were enrolled in the United States Food and Drug Administration Investigational Device Exemption studies of the Prestige ST and Bryan disc replacements (Medtronic, Memphis, Tennessee). The inclusion criteria were myelopathy and spondylosis or disc herniation at a single level from C3 to C7. Clinical outcome measures were collected preoperatively and at six weeks, three months, six months, twelve months, and twenty-four months postoperatively.

Results: A total of 199 patients were included in the present study; 106 patients (53%) underwent arthroplasty, whereas ninety-three (47%) underwent arthrodesis. The Neck Disability Index, Short Form-36 scores, and specific arm and neck pain scores improved significantly from baseline at all time points. Patients in all four groups had improvement in the postoperative neurological status and gait function; at twenty-four months after surgery, 90% (95% confidence interval, 77.8% to 96.6%) of the patients in the arthroplasty group and 81% (95% confidence interval, 64.9% to 92.0%) of those in the arthrodesis group had improvement in or maintenance of the neurological status in the Prestige ST trial and 90% (95% confidence interval, 75.8% to 97.1%) of the patients in the arthroplasty group and 77% (95% confidence interval, 57.7% to 90.1%) of those in the arthrodesis group had improvement in or maintenance of the neurological status in the Bryan trial.

Conclusions: We found that patients in both the arthroplasty and arthrodesis groups had improvement following surgery; furthermore, improvement was similar between the groups, with no worsening of myelopathy in the arthroplasty group. While the findings at two years postoperatively suggest that arthroplasty is equivalent to arthrodesis for the treatment of cervical myelopathy for a single-level abnormality localized to the disc space, the present study did not evaluate the treatment of retrovertebral compression as occurs in association with ossification of the posterior longitudinal ligament, and we cannot comment upon the treatment of this condition.

Level of Evidence: Therapeutic Level II. See Instructions to Authors for a complete description of levels of evidence.

1Department of Orthopaedic Surgery, Washington University in St. Louis, 660 South Euclid Avenue, Campus Box 8233, St. Louis, MO 63110. E-mail address for J.M. Buchowski: buchowskij@wustl.edu

2Indiana Spine Group, 8402 Harcourt Road, Suite 400, Indianapolis, IN 46260

3Department of Orthopaedics and Rehabilitation, University of Wisconsin, 600 Highland Avenue, Madison, WI 53792-7375

4Medtronic, Inc., 1800 Pyramid Place, Memphis, TN 38132

Article Outline

Cervical disc arthroplasty is a motion-preserving alternative to arthrodesis for the treatment of cervical degenerative disease1-19, and several studies have shown equivalent results following the use of cervical disc arthroplasty and arthrodesis for the treatment of cervical radiculopathy20-24. Although there have been case reports describing the use of cervical disc arthroplasty in the setting of myelopathy25-27, there is a concern that motion preservation in the setting of myelopathy may lead to continued microtrauma to the compromised spinal cord, leading to progression of myelopathy and negatively affecting the clinical result. As we are not aware of any studies on the use of cervical disc arthroplasty in the setting of myelopathy, we performed a post hoc subgroup analysis of two large, prospective, randomized multicenter trials to evaluate the efficacy of cervical disc arthroplasty as compared with arthrodesis for the treatment of myelopathy.

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Materials and Methods

The patients in the present study were a subset of patients enrolled in the United States Food and Drug Administration Investigational Device Exemption studies for two different artificial cervical disc replacements, namely, the Prestige ST and Bryan artificial discs (both manufactured by Medtronic, Memphis, Tennessee). The Prestige ST disc prosthesis was developed in 2002 and was approved by the Food and Drug Administration in 2007; it is a stainless steel, metal-on-metal, ball-and-trough semiconstrained disc device. Its surfaces are grit-blasted to promote long-term osseointegration at the bone-implant interface. Locking screws are inserted through a 2.5-mm-thick anterior flange to achieve immediate implant fixation above and below the device. The prosthesis has a constant width of 17.8 mm, two different implant depths (12 and 14 mm), and variable heights (6, 7, 8, and 9 mm). The Bryan cervical disc was first implanted in 2000. It is made of two titanium-alloy shells with an intervening polyurethane nucleus, supplied as a single preassembled unit. The nucleus is surrounded with a polyurethane sheath to prevent soft-tissue ingrowth. It is a metal-on-polymer, unconstrained disc device and has porous surfaces coated with 250-μm titanium beads to promote osseointegration. Anterior lips on the shells prevent posterior migration of the prosthesis and serve as anchor points for device removal. The prosthesis has a fixed height and is available in five diameters (14, 15, 16, 17, and 18 mm).

In both studies, patients were randomized in a 1:1 ratio to artificial cervical disc replacement or anterior cervical discectomy and arthrodesis with allograft and a plate. Data were collected from multiple centers in a prospective fashion. The inclusion criteria included the presence of cervical myelopathy as evidenced by hyperreflexia, clonus, or a Nurick grade28 of ≥1 due to cervical spondylosis or disc herniation at a single level from C3 to C7. General exclusion criteria included the presence of infection, metabolic bone disease, osteoporosis, ankylosing spondylitis, and rheumatoid arthritis. In addition, patients with previous cervical surgery, patients with obesity, and patients who were receiving corticosteroids and/or daily insulin were also excluded. Last, certain specific radiographic findings led to exclusion from the study; these findings included moderate to advanced cervical spondylosis, bridging syndesmophytes, marked reduction or absence of segmental spinal motion, disc space collapse of >50%, moderate or severe facet arthropathy, cervical kyphosis or reversal of lordosis, and ≥2 mm of spondylolisthesis and/or ≥11° of angular instability relative to an adjacent segment or segments.

Of the 1007 patients in the two disc trials (including a total of 542 patients in the Prestige ST trial and a total of 465 patients in the Bryan trial), 199 had evidence of cervical myelopathy and were included in the present study. Of these 199 patients, 106 (53%) were randomized to cervical disc arthroplasty (including fifty-nine patients who received the Prestige ST prosthesis and forty-seven who received the Bryan prosthesis) whereas ninety-three (47%) underwent anterior cervical discectomy and arthrodesis (including fifty-two patients in the Prestige ST trial and forty-one patients in the Bryan trial). Case examples are shown in Figures 1-A through 2-C. Demographic information is shown in Table I. With the numbers available, there were no significant differences between the groups with regard to age, sex, tobacco use, alcohol use, preoperative work status, Workers' Compensation status, and unresolved spinal litigation status.

Clinical outcomes were examined on the basis of a number of validated outcome measures: specific disease outcomes were measured with use of the Neck Disability Index29, whereas general outcomes were measured with use of the physical and mental components of the Short Form-36 (SF-36)30. A numerical rating scale was used to evaluate the intensity and frequency of neck and arm pain. Physical examination included manual muscle testing, sensory testing to both light touch and pin prick, and reflex testing. In addition, the Nurick grade was used to assess the severity of myelopathy clinically. Patients with nerve root signs (that is, radiculopathy) but no spinal cord compression signs or symptoms were rated as grade 0, patients with spinal cord compression signs but no gait difficulty were rated as grade 1, patients with slight gait difficulty but who were working full time were rated as grade 2, patients with gait difficulty preventing work but who were able to walk unassisted were rated as grade 3, patients who were able to walk with assistance only were rated as grade 4, and patients who were bedridden and/or wheelchair-dependent were rated as grade 5. The results were examined preoperatively and at six weeks, three months, six months, twelve months, and twenty-four months postoperatively.

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Statistical Analysis

Statistical comparisons between the disc arthroplasty and arthrodesis groups were performed for each study independently. To compare demographic, preoperative, and surgical measures, p values for continuous variables were determined with use of analysis of variance, and p values for categorical variables were determined with use of the Fisher exact test. To compare continuous outcome measurements such as the Neck Disability Index score, analysis of covariance was used, with the preoperative score as the covariate. To assess the significance of postoperative improvement in outcome scores from the preoperative status within each treatment group, a paired t test was used. To compare success or event rates, the Fisher exact test was used. Gait improvements were compared between the treatment groups with use of the nonparametric Wilcoxon rank-sum test. Confidence intervals were calculated with use of the exact binomial method.

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Results

Operative Results

All patients underwent surgery at a single level. The operative time, blood loss, length of stay, and the levels at which surgery was performed were similar in all four groups of patients (Table II), with the exceptions that the operative time was longer in the disc arthroplasty groups (in both the Prestige ST and Bryan trials) and that the blood loss was greater in the arthroplasty group than in the arthrodesis group in the Bryan trial. There were no revisions in the arthroplasty groups; however, there was one instance of removal of a plate and one foraminotomy for the treatment of continued symptoms in the arthrodesis group in the Prestige ST trial.

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Disease-Specific and General Health Outcomes

During the follow-up period, the Neck Disability Index improved significantly from baseline at all time points in all four patient groups (Table III). While there were no significant differences in the Neck Disability Index between the arthroplasty and arthrodesis groups in the Prestige ST trial, the Neck Disability Index improved to a greater extent in the arthroplasty group than the arthrodesis group in the Bryan trial (Table III). In both studies, when an improvement in the Neck Disability Index of ≥15 points was defined as a clinical success, the vast majority of patients had a successful surgical outcome regardless of whether they underwent an arthroplasty or an arthrodesis and regardless of which arthroplasty device was used (Table III). Similar findings were seen when SF-36 outcome measures were analyzed (Fig. 3) with use of both the Physical Component Summary (PCS) and the Mental Component Summary (MCS) scores. Given the improvement in the Neck Disability Index and SF-36 scores, it is not surprising that the arm pain and neck pain scores also improved significantly from baseline at all time points in all four groups (p < 0.001) (Figs. 4-A and 4-B). As with the Neck Disability Index and the SF-36 PCS and MCS scores, which showed no significant differences between the arthroplasty and arthrodesis groups in the Prestige ST trial and showed a greater improvement in the arthroplasty group than in the arthrodesis group in the Bryan trial, patients in the arthroplasty group in the Bryan trial tended to have greater improvement in terms of arm and neck pain twenty-four months after surgery in comparison with those in the arthrodesis group (p < 0.013 and p < 0.002, respectively).

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Adverse Events and Reoperations

During the Prestige ST trial, there was a single possibly implant-related adverse event in one (1.7%) of fifty-nine patients in the arthroplasty group and six possibly implant-related adverse events in six (11.5%) of fifty-two patients in the arthrodesis group (p < 0.049). One (1.7%) of fifty-nine patients in the arthroplasty group required reoperation (removal of the device). Two (3.8%) of fifty-two patients in the arthrodesis group required reoperation: one patient required removal of a plate, and one patient required removal of a plate and placement of a bone growth stimulator. During the Bryan trial, there were no possibly implant-related adverse events among the forty-seven patients in the arthroplasty group and two possibly implant-related adverse events in two (4.9%) of the forty-one patients in the arthrodesis group (p < 0.214). One of the forty-seven patients in the arthroplasty group required reoperation. One of the forty-one patients in the arthrodesis group required reoperation (supplemental fixation).

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Patient Satisfaction

The majority of patients in all four groups were satisfied with the results of surgery. In the Prestige ST trial, 89.8% of the patients in the arthroplasty group (forty-four of forty-nine patients) and 91.9% of those in the arthrodesis group (thirty-four of thirty-seven patients) were satisfied with the result at twenty-four months after surgery. In the Bryan trial, 94.7% of the patients in the arthroplasty group (thirty-six of thirty-eight patients) and 90.0% of those in the arthrodesis group (twenty-seven of thirty patients) were satisfied with the result at twenty-four months after surgery. There were no significant differences in patient satisfaction between the arthroplasty and arthrodesis groups in either the Prestige ST study or the Bryan study. When asked, the vast majority of patients in all four groups indicated that they would choose to have the surgical procedure again (data not shown).

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Neurological Function

Patients in all four groups had improvement in terms of postoperative neurological status and gait function (Figs. 5 and 6) (see Appendix). More specifically, in the Prestige ST trial, 89.8% (95% confidence interval, 77.8% to 96.6%) of the patients in the arthroplasty group (forty-four of forty-nine patients) and 81.1% (95% confidence interval, 64.9% to 92.0%) of those in the arthrodesis group (thirty of thirty-seven patients) had improvement in or maintenance of neurological status, including motor function, sensation, and reflexes, at twenty-four months after surgery. In the Bryan trial, 89.7% (95% confidence interval, 75.8% to 97.1%) of the patients in the arthroplasty group (thirty-five of thirty-nine patients) and 76.7% (95% confidence interval, 57.7% to 90.1%) of those in the arthrodesis group (twenty-three of thirty patients) had improvement in or maintenance of neurological status, including motor function, sensation, and reflexes, at twenty-four months after surgery.

In the Prestige ST trial, at twenty-four months after surgery, 47.9% (95% confidence interval, 33.3% to 62.8%) of the patients in the arthroplasty group (twenty-three of forty-eight patients) and 37.8% (95% confidence interval, 22.5% to 55.2%) of those in the arthrodesis group (fourteen of thirty-seven patients) had improvement in gait function, no patient in either group had deterioration in gait function, and 52.1% (95% confidence interval, 37.2% to 66.7%) of the patients in the arthroplasty group (twenty-five of forty-eight patients) and 62.2% (95% confidence interval, 44.8% to 77.5%) of the patients in the arthrodesis group (twenty-three of thirty-seven patients) had maintenance of gait function. In the Bryan study, at twenty-four months after surgery, 46.2% (95% confidence interval, 30.1% to 62.8%) of the patients in the arthroplasty group (eighteen of thirty-nine patients) and 26.7% (95% confidence interval, 12.3% to 45.9%) of those in the arthrodesis group (eight of thirty patients) had improvement in gait function. No patient in either group had deterioration of gait function, and 53.8% (95% confidence interval, 37.2% to 69.9%) of the patients in the arthroplasty group (twenty-one of thirty-nine patients) and 73.3% (95% confidence interval, 54.1% to 87.7%) of those in the arthrodesis group (twenty-two of thirty patients) had maintenance of gait function. At twenty-four months, there were no significant differences in neurological function or in improvement in gait function between the arthroplasty and arthrodesis groups in either study.

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Discussion

The current approved indication in the United States for cervical disc arthroplasty is the treatment of radiculopathy and/or myelopathy due to neural compression caused by a disc herniation or spondylotic changes (that is, retrodiscal compression). Although early data from several different Investigational Device Exemption studies have suggested that arthroplasty appears to be a promising alternative to arthrodesis for the treatment of cervical spondylotic disease1,4,9,12,13,15,31, aside from case reports and small case series, we are not aware of any studies that have been performed to evaluate the role of cervical disc arthroplasty in the treatment of myelopathy caused by retrodiscal compression. There is concern that preserving motion in patients with myelopathy may theoretically maintain microtrauma to the compromised cord, resulting in adverse long-term results. Also, the preservation of motion may prevent osteophyte remodeling, which can occur in association with a fusion in turn leading to a decrease in residual compression once a fusion occurs. Furthermore, cervical disc arthroplasty, by maintaining motion, may be associated with late osteophyte formation1,3,32. Finally, there is concern that, in patients with kyphotic alignment or in those in whom the arthroplasty device is placed in a kyphotic alignment, progressive kyphosis may develop, leading to recurrence or worsening of myelopathic symptoms25.

Fortunately, despite these theoretical concerns, the current study showed clinical improvement in patients with disc-based spinal cord compression due to disc herniation and/or spondylotic disease. We found that all functional outcome measures studied were similar between the arthroplasty and arthrodesis groups. More importantly, however, we noted similar improvement in neurological status and gait function in the arthroplasty and arthrodesis groups, and not a single patient in the arthroplasty group had deterioration in neurological function during the study period. This finding suggests that, at least at two years postoperatively, with a thorough decompression, arthroplasty appears to be similar to arthrodesis for the treatment of cervical myelopathy due to retrodiscal compression at a single level secondary to a disc herniation and/or spondylotic changes localized to the disc space. It must be emphasized, however, that the present study did not address the use of cervical disc arthroplasty for the treatment of myelopathy due to neural compression behind the vertebral body (that is, retrovertebral compression), such as from ossification of the posterior longitudinal ligament, as the abnormality in the present study was limited to the disc space. Additionally, patients with small spinal canals and multilevel disease were not assessed in the present study. Last, as the present study was a secondary, exploratory, unplanned pooled analysis, the results should be regarded as hypothesis-generating and not as conclusive and should not be interpreted to mean that cervical disc arthroplasty is superior to arthrodesis.

The majority of the literature on cervical arthroplasty has been focused on the outcomes of the procedure in a mixed cohort of patients with both radiculopathy and myelopathy7,20-22,24,33. Although several studies have been published on the use of cervical arthroplasty for the treatment of myelopathy25-27, those studies have been limited by a small number of patients and short follow-up. For example, Sekhon reported on a series of eleven patients who presented with cervical myelopathy resulting from anterior spinal cord compression secondary to spondylotic stenosis or acute disc herniation who were managed with Bryan disc replacement26. Although only five patients were followed for eighteen months or more, all patients had radiographic documentation of a decompressed spinal cord and, on the average, improvements of one Nurick grade and of 45% on the Neck Disability Index were observed postoperatively. The other studies demonstrated similar results25,27. Our results further validate those preliminary findings that arthroplasty appears to work reasonably well in the short term for the treatment of myelopathy due to retrodiscal compression.

One limitation of the present study is the relatively short duration of follow-up. A minimum of five to ten years of follow-up has been recommended for such procedures6,7. There are general concerns related to the use of disc arthroplasty with regard to material wear31,34-36 and the ability of these disc prostheses to maintain motion37-39. Another concern is whether disc arthroplasty may lead to the recurrence of syndesmophytes. There is a concern that motion preserved by cervical disc arthroplasty may lead to recurrence of these osteophytes after the initial decompression1,3,32. Finally, there is concern that in patients with kyphotic alignment or in those in whom the arthroplasty device is placed in a kyphotic alignment, progressive kyphosis may occur following cervical disc arthroplasty, leading to recurrence or worsening of myelopathic symptoms as occurred in one patient in the study by Sekhon26. These questions cannot be answered without long-term follow-up and a specifically designed study to address such issues. Last, we were not able to critically evaluate radiographic features that may have affected the outcome in myelopathic patients such as canal size or whether the cord compression was caused by a disc herniation or spondylosis.

In conclusion, spinal motion maintained by cervical disc arthroplasty does not appear to have an adverse effect in the setting of myelopathy due to retrodiscal compression at up to two years after arthroplasty. It must be emphasized, however, that the present study does not address the use of cervical disc arthroplasty in the setting of retrovertebral compression, and, therefore, we believe that patients with myelopathy due to cord compression behind the vertebral body should not undergo cervical disc arthroplasty. In addition, although short-term results of cervical disc arthroplasty appear encouraging, studies with at least five to ten years of follow-up are required before cervical disc replacement can be viewed as a standard treatment of disc-based cervical myelopathy.

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Appendix Cited Here...

A table summarizing Nurick grade changes is available with the electronic versions of this article, on our web site at jbjs.org (go to the article citation and click on “Supplementary Material”) and on our quarterly CD/DVD (call our subscription department, at 781-449-9780, to order the CD or DVD).

Disclosure: In support of their research for or preparation of this work, one or more of the authors received, in any one year, outside funding or grants in excess of $10,000 from Medtronic. In addition, one or more of the authors or a member of his or her immediate family received, in any one year, payments or other benefits in excess of $10,000 or a commitment or agreement to provide such benefits from a commercial entity (Medtronic). Also, commercial entities (Medtronic) paid or directed in any one year, or agreed to pay or direct, benefits in excess of $10,000 to a research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which one or more of the authors, or a member of his or her immediate family, is affiliated or associated.

A commentary is available with the electronic versions of this article, on our web site (www.jbjs.org) and on our quarterly CD-ROM/DVD (call our subscription department, at 781-449-9780, to order the CD-ROM or DVD).

Investigation performed at Washington University in St. Louis, St. Louis, Missouri; Indiana Spine Group, Indianapolis, Indiana; and University of Wisconsin, Madison, Wisconsin

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