By Michael Mayer
In the last six years, cervical total disc replacement (CTDR) has become a popular surgical alternative to anterior cervical discectomy and fusion (ACDF) for patients suffering from cervical disc herniations and/or axial neck pain. The idea of preserving motion and thus decreasing the risk of accelerated adjacent segment degeneration was the main argument that drove CTDR technology forward.
A series of randomised controlled trials, as well as large case series, have provided evidence that CTDR is a safe procedure that has lower re-operation rates than ACDF and comparable complication rates. Additionally, the clinical success rates of CTDR are equal to or exceed the ones published for ACDF and extrusions. Subsidence or hardware removal is rare and generally reflect a “learning curve phenomenon” in most of the publications on the new therapy. Motion of the implanted segment is preserved in midterm outcomes and it correlates with preoperative range of motion.
However, two main concerns are dominating the current discussion about CTDR. The first is heterotopic ossification (HO) a frequent phenomenon? The second question is does it lead to fusion of the segment? Heterotopic ossifications were observed in some of the early published studies five to six years ago. They seemed to be attributed to some technical details, which included: immobile segment before disc replacement; non-irrigated bone dust from high-speed burring; non-sealing of open bony surfaces after osteophyte removal; and no prophylactic measures such as non-steroidal anti-inflammatory drugs to prevent HO.
Although a recent meta-analysis paper(1) described a pooled prevalence of HO of 44.6 % (rates between 2.4% and 77% in short- to midterm follow-up), it seems that HO does not influence clinical outcome in the vast majority of the cases. Mobility of the segment also seems to be preserved and the rate of complete fusion is low (<10%). Therefore, the unanswered questions today are: are we ending up with a slowly progressing fusion? (but, at least, this is not true for the midterm results—up to five years); if we do end up with a slowly progressing fusion, is it acceptable, from a medical and economical standpoint, to only achieve temporary motion preservation?; and will a slow progression fusion influence adjacent segment degeneration?
This brings me to the next “hot” unanswered question: is there really an adjacent segment preservation/protection? What we know from randomised controlled trials is that the rate of radiographic worsening in the adjacent segment is lower after CTDR than after ACDF.
Looking at “symptomatic” adjacent segments, the clinical results do not differ from ACDF—at least in midterm follow-ups. However, the re-operation rate of segments adjacent to CTDR seems to be lower than segments adjacent to ACDF.
Beside other less relevant issues, the rates and speed of spontaneous fusion as well as adjacent segment behaviour following CTDR will probably determine the fate of the whole technology. From the data available today, there is evidence for good intermediate clinical outcomes and safety of CTDR, which makes it an acceptable procedure within the spinal surgeons’ portfolio. According to the published results, there are neither reasons for nor evidence against using this technology. However, the truth of the initial chain of arguments: motion preservation = protection of adjacent segment still has to be proven in long-term results.
Michael Mayer is professor of Neurosurgery and head of the Spine Center at the FIFA Medical Center of Excellence, Schön Klinik München Harlaching, Munich, Germany.
1. Guèrlin P, et al. Acta Orthop Belg 2012; 78:80-6