In this issue of The Journal, Riew et al. present a high-quality retrospective study of prospectively collected data regarding the use of total disc arthroplasty in patients with cervical myelopathy resulting from a single-level disc herniation or degeneration. Prior reports on cervical disc arthroplasty have largely described its use in patients with isolated radiculopathy1-5. This study contributes new data regarding the application of this technology in the setting of spinal cord compromise.
In assessing the importance of this paper, we must recognize the limitations in the data presented. Primary among these is the fact that these data were not collected with the intention of this analysis in mind. While the data were collected as part of a prospective, randomized clinical trial of cervical total disc replacement, a post-hoc retrospective analysis of the data may be subject to bias. Thus, from the perspective of the evidence-based medicine hierarchy, this study is not at the highest level-I evidence represented by a prospective randomized clinical trial. To their credit, the authors recognize and comment on this limitation.
In fact, the results do suggest that a bias may exist in the dataset. Nonblinded randomized controlled trials involving new technology carry the risk that enthusiasm for the studied procedure can be conveyed to the enrolled patients receiving the device, resulting in an enhancement of patient-reported outcomes. Conversely, disappointment among patients randomized to the "control procedure" can potentially negatively impact their results.
This effect is suggested by the Neck Disability Index scores presented in Table III. In the Bryan prosthesis arm of the study, especially, there appears to be a substantially greater improvement in pain and disability scores among the patients who received total disc arthroplasty as compared with the patients who underwent fusion. This effect is absent in the Prestige ST arm of the study. As the two arms were likely carried out at different centers, this contrast suggests that the patients in the Bryan arm of the study may have been influenced by their perceptions of the likely benefit of total disc replacement compared with discectomy and fusion. In my opinion, it is otherwise difficult to attribute the differences in reported outcome to real differences in function not only between the Bryan prosthesis and fusion, but also between the Bryan prosthesis and the Prestige ST prosthesis.
These concerns are further compounded by a review of the neurological data regarding gait improvement, which is shown in the electronic appendix (Table E-1). Again, there are differences in the reported outcomes, which suggest that greater improvements occurred for the patients who received the Bryan prosthesis as compared both with the patients who underwent fusion and the patients who received the Prestige ST prosthesis. As suggested by the authors, it is difficult to conceive of a mechanism by which total disc arthroplasty would improve neurological dysfunction to a greater degree than fusion, given the persistence of motion and the possible retention or redevelopment of neurological compression at the involved level. In addition, the gait assessments used in the data collection appear to be based on physician or investigator assessment rather than on self-reported patient assessments. This pattern in the reported data is again suggestive that some bias may have existed in the data-collection process, especially in the Bryan arm of the study.
Despite these concerns, the study presents the best data to date regarding the application of total disc arthroplasty in the setting of cervical myelopathy due to spinal cord compression limited to a single disc level. The data do support the conclusion that total disc replacement is equivalent to anterior discectomy and fusion in this clinical setting at the time of the two-year follow-up. The authors' interpretation of their data as "hypothesis-generating and not…conclusive" and their final conclusion that "studies with at least five to ten years of follow-up are required" are, in my view, a fair and balanced interpretation of the level of evidence provided by this report.
*The authors did not receive any outside funding or grants in support of their research for or preparation of this work. Neither they nor a member of their immediate families received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. A commercial entity (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.
1. Coric D, Finger F, Boltes P. Prospective randomized controlled study of the Bryan Cervical Disc: early clinical results from a single investigational site. J Neurosurg Spine. 2006;4:31-5.
2. Hacker RJ. Cervical disc arthroplasty: a controlled randomized prospective study with intermediate follow-up results. Invited submission from the Joint Section Meeting on Disorders of the Spine and Peripheral Nerves, March 2005. J Neurosurg Spine. 2005;3:424-8. Erratum in: J Neurosurg Spine. 2006;4:189.
3. Porchet F, Metcalf NH. Clinical outcomes with the Prestige II cervical disc: preliminary results from a prospective randomized clinical trial. Neurosurg Focus. 2004;17:E6.
4. Robertson JT, Papadopoulos SM, Traynelis VC. Assessment of adjacent-segment disease in patients treated with cervical fusion or arthroplasty: a prospective 2-year study. J Neurosurg Spine. 2005;3:417-23.
5. Sasso R, Rouleau J. Cervical kinematics in ACDF and disc replaced subjects. Spine J. 2005;5:87S.