For nearly as long as spinal arthrodesis has been used to treat lumbar spine disorders, concerns have been raised regarding the long-term biomechanical and anatomic ramifications of successful fusion, particularly adjacent-segment disc degeneration and loss of lumbar range of motion. In many ways, the specter of adjacent-segment disc degeneration has been most concerning as it can lead to a deterioration in function that was gained following the initial procedure as well as to symptoms that may require further surgery. As a result, new surgical devices and procedures are being developed with the intention of treating lumbar degenerative conditions while maintaining near-normal biomechanical and anatomic parameters. Without question, the total disc replacement is one such "motion-sparing" device that has received substantial attention within the spine surgery community.
Despite the fact that total disc replacement has been performed in Europe since the mid-1980s, there have been few scientifically rigorous investigations capable of providing high-grade evidence regarding optimal indications, effectiveness, and longevity1,2. Recently, randomized prospective studies on the short-term benefits of single-level total disc replacement have demonstrated that the beneficial results of this procedure have been maintained for up to five years following surgery1,2. Yet, until now, no comparable investigation was available regarding the use of total disc replacement as treatment for lumbar degenerative disorders at two contiguous levels.
The authors of the present study are to be commended for undertaking the Herculean effort of performing a randomized, prospective, multicenter, Food and Drug Administration (FDA) trial intended to address the effectiveness of total disc replacement performed at two contiguous levels between L3 and S1. In this investigation, lumbar total disc replacement at two levels was compared with open 360° lumbar fusion. Ultimately, at the time of the two-year follow-up, 148 patients receiving total disc replacement were compared with sixty-seven who had undergone fusion. Non-inferiority was demonstrated between the two interventions and, as the authors point out, there was no significant difference between treatments in terms of the composite end point (p = 0.0874). However, when considering individual clinical outcomes, total disc replacement appeared to convey several advantages over fusion, including significant improvement in terms of Short Form-36 physical component scores, significant reduction in narcotics usage, and an increased percentage of patients demonstrating the Oswestry Disability Index success criterion. In their conclusion, Delamarter et al. maintain that two-level total disc replacement is a viable alternative to fusion that may confer clinical advantages of enhanced recovery and improved pain relief in the short term.
Ostensibly, as this investigation is a randomized, prospective trial, it may be assumed to provide high-level evidence and, as such, to represent some of the best-available literature on this topic in the annals of spine surgery. However, there are a number of limitations to this study, some of which I believe limit the widespread endorsement of these results. Possibly one of the most glaring limitations of this research is the fact that it was conducted as a FDA non-inferiority trial. While this study is still certainly randomized, prospective, and multicenter in nature, there are important features that distinguish such a study from other comparative randomized, controlled trials like the Spine Patient Outcomes Research Trial (SPORT).
Foremost, as a non-inferiority trial, the study design allowed for randomization in a 2:1 ratio, in favor of total disc replacement. Secondly, as required by the FDA, patients who went on to have secondary surgical procedures were not included in the results but were instead reported separately. The fact that these patients, who potentially may have had suboptimal outcomes, were excluded from the final analysis might indicate that the findings presented by Delamarter et al. are a best-case scenario and are not truly representative of the outcomes of these interventions across the board. Additionally, one must consider the fact that, although patients were blinded to treatment prior to surgery, neither they nor their care providers were blinded to the intervention after it was performed. This could have contributed to an inherent evaluation bias within the study, a fact recognized by the authors.
It is also important to appreciate that total disc replacement in this study was compared with open 360° fusion, with anterior and posterior arthrodeses being performed through separate incisions. Therefore, it should come as no surprise that the single-incision total disc replacement demonstrated statistical advantages in terms of operative time, estimated blood loss, and length of postoperative hospitalization. Previous studies on disc arthroplasty have been criticized for comparing results between total disc replacement and outmoded interventions such anterior fusion with the Bagby and Kuslich (BAK) cage1. A similar critique may be warranted here as open anterior-posterior lumbar fusion is a substantial surgical intervention and has not been declared to be the gold standard for the treatment of two-level lumbar disorders in any high-level scientific investigation. It would be interesting to see how the results of total disc replacement would compare, for example, with other, less-invasive fusion procedures, such as transforaminal lumbar interbody fusion (TLIF) or trans-psoas interbody fusion (XLIF/DLIF) with percutaneous posterior instrumentation.
Last, the study by Delamarter et al. only follows patients for two years after surgery and does not consider cost-effectiveness, two factors of considerable importance in the current practice environment. Although one of the main theoretical advantages of total disc replacement is the ability to limit the incidence of adjacent-segment disc degeneration, thereby improving the longevity of postoperative results, such clinical benefits could not be evaluated given the relatively short duration of the trial. It should be noted that results of long-term investigations of total disc replacement are quite sobering. For example, Putzier et al. examined outcomes for fifty-three patients at an average of seventeen years following disc arthroplasty3. Twenty-three percent of the cohort required subsequent lumbar fusion at the initial operative level, whereas 60% demonstrated autofusion at the total disc replacement site. Seventeen percent of patients exhibited adjacent-segment disc degeneration despite undergoing the motion-sparing procedure.
In conclusion, the authors of this report are to be commended for an important work that contributes substantially to the growing literature regarding total disc replacement. While it has limitations, the work of Delamarter et al. should be recognized as the first prospective, randomized study on two-level total disc replacement and one that highlights short-term clinical advantages for the procedure, such as accelerated rehabilitation and enhanced pain relief.