The effectiveness of cTDR has been under investigation for many years. One of the major challenges for cTDR includes the highly satisfactory clinical results of the “gold standard” treatment ACDF. At short term FU, no significant differences can be found between both technologies as long as the same adequate decompression techniques are performed. Numerous studies have demonstrated, however, that both cTDR and ACDF achieved comparatively similar improvements in all clinical outcome parameters [3, 4, 6, 8, 9, 13, 14]. However, a number of studies demonstrated that at short- and mid-term FU, the rates of post-operative revision surgeries were significantly lower and in favour of cTDR [1, 3, 5, 15]. The underlying reasons for this observation included implant and technique-related issues, which were linked to the fusion of the segments (i.e. non-union, implant subsidence and others). However, the ultimate proof that motion preservation with cTDR devices may result in lower rates of adjacent segment degeneration as well as lower reoperation rates is still pending.
Long-term follow-up of clinical outcome parameters
The present study cohort revealed a significant and maintained improvement of all clinical outcome parameters (NDI, VAS arm, VAS neck) at all post-operative stages in comparison to baseline levels up until the 10-year FU examination.
Similar observations were verified in a number of studies, which demonstrated comparable post-operative scores at different long-term observation periods, ranging between 4 and 30 years, and irrespective of the type of prosthesis used [2,3,4, 16,17,18,19].
However, a systematic review which compared cTDR with ACDF did not detect any relevant differences in disability, pain, general state of health, neurological success and satisfaction parameters at 2 years of FU .
These findings can, however, no longer be upheld based on the data from current comparative long-term studies, as previously published RCTs demonstrated statistically significantly superior clinical results for the cTDR group in comparison to the patients treated with ACDF [3, 4]. As an explanation for these findings, the significantly lower long-term revision rates, especially at the adjacent segment, should be taken into consideration. Janssen et al. published their data from a prospective randomised U.S. FDA investigation device exemption study of the Prodisc C with 7 years of follow-up. The authors found similar clinical outcomes between the Prodisc C (n = 103) and ACDF group (n = 106), however, with lower rates of second surgery observed in the cTDR cohort. Loumeau et al.  assessed the data from an RCT and compared the results from monosegmental treatments between Prodisc C versus ACDF with 7 years of FU. Revision surgeries had to be carried out in 27% of all cases in the cohort of fusion candidates, 9% thereof at the adjacent segment and 18% at the index segment due to pseudoarthrosis. Conversely, however, no revision surgery was required in the cTDR cohort. In a similar randomised study setting comparing monosegmental fusion with plate versus Prodisc C, Delamater et al.  reported somewhat lower reoperation rates at the 5-year FU. The ACDF group demonstrated a five times higher reoperation rate (16/106), mainly due to pseudoarthrosis (6/106) and symptomatic adjacent segment degeneration (8/106).
In comparison, the reoperation rate following total cervical disc replacement appeared to be low with only 2.9% (2/103 revision cases performed at the index segment; 1/103 at the adjacent segment), which was one of the main findings of the study.
In the current study, after an average FU of 10.2 years, only one reoperation was required in a patient with persisting neck pain and concomitant psychological distress, which was referred to the prosthesis and which therefore necessitated a conversion to an ACDF. There were no further cases, which required reoperation due to implant failure or adjacent segment disease. Hisey et al. reported the similar results of a RCT study comparing Mobi C with ACDF at 48 month of follow-up with only 0.6% of patients that underwent subsequent surgery to treat symptoms arising from the adjacent level to the index level in the TDR group in comparison to 3.7% in the fusion group .
Therefore, those patients treated with disc prosthesis demonstrated a 2% reoperation rate in this present long-term FU investigation. Including the two cases which required an interbody fusion with a cage resulting from intraoperative complications due to technical failures, the overall “intent to treat” reoperation rate was 3/50 (6%).
This low incidence of revision cases over a long-term FU in particular is one of the key arguments, which promotes the use of cTDR, irrespective of potentially concomitant radiological changes or the occurrence of heterotopic ossifications.
Adjacent segment degeneration
Up until the last FU, after an average of 10.2 years, adjacent segment degenerative changes such as disc height reduction, endplate sclerosis or development of spondylophytes were observed in 35.7% of all patients that were available for radiological assessment (n = 13/38). This rate of adjacent level degenerative changes does not differ substantially from the repeatedly cited work from Hilibrand et al.  with respect to the annual incidence of 2.9% of an adjacent segment degeneration following fusion.
Nunley et al.  previously reported similar observations. After a median of 4 years of FU, the authors reported symptomatic ASD following total cervical disc replacement with an annual incidence of 3.1%. Jawahar et al.  also reported a comparable incidence of ASD. In a RCT, 93 patients were included (59 cTDR versus 34 cases of ACDF). After a median FU of 37 months (24–49 months), ASD was diagnosed in 16% of the cTDR patients and in 18% of patients with ACDF.
However, it needs to be highlighted that a routine MRI investigation of the cervical spine was not included in the present study protocol. Conversely, the presence of ASD was determined based on lateral X-ray images by two independent observers according to previously established and well-defined criteria. Similar radiological criteria were used by Lee et al. . They found adjacent segment degeneration in 35.7% in the cervical disc arthroplasty group and in 57.1% in the fusion group at a mean follow-up of 43.4 resp. 44.6 months for the fusion group. Interestingly, the ASD appeared earlier in the fusion group. The risk factor analysis indicated a higher lordosis angulation from C3 to C7 for non-ASD-patients in both groups and the development of heterotopic ossifications in the arthroplasty group was associated with the presence of ASP. Contrary to this study, Di Martino et al.  conclude in a systematic review regarding the cervical spine alignment in disc arthroplasty that the overall cervical alignment after TDR tends towards a loss of lordosis. Some studies demonstrated that focal or global kyphosis after ACDF is associated with the appearance of adjacent segment disease [25, 26]. The topic of post-operative cervical alignment as an influencing factor for ASD has to be studied more in detail in further investigations.
As outlined in Fig. 5, lower ROM values preoperatively were already positively correlated with the occurrence of adjacent level degenerative changes at later post-operative FU stages.
One possible explanation of the present results could be that an impaired pre-operative mobility of the motion segment had already taken effect on the natural course of degeneration in the adjacent segment. In addition, the significant decrease in the ROM during the post-operative course may be interpreted as an indicator of the importance of maintained motion preservation of the index segment.
At later FU stages, an increase in high-grade ossifications of the prostheses was noted, which was associated with a significant reduction in range of motion. Despite the fact that this had no significant adverse effects on the clinical outcome parameters, as noted in other studies [27,28,29,30,31,32,33], efforts should be made to avoid the occurrence of HO.
Ament et al. conclude in a cost-effectiveness study regarding cTDR and ACDF for the treatment of two-level cervical degenerated disc disease that cTDR appears to be a highly cost-effective surgical modality compared with ACDF. Qureshi et al. analysed the cost-effectiveness comparing single level TDR and ACDF and conclude also that cTDR and ACDF are cost-effective procedures, but cTDR must remain functional for at least 14 years to establish greater cost-effectiveness than ACDF . The appearance of HO right up to solid fusion (Grade 4) is a well known complication in cervical total disc replacement which can affect the motion, respectively, the function of the prosthesis [31, 32, 35] regardless the implant-type [7, 11, 17, 27, 28, 36,37,38,39]. The progression of HO into higher grades over time was also seen in other studies [28, 32]. Up to now there is no study available that could demonstrate a significant effect of the use of NSAID in the early post-operative period to avoid or reduce the incidence of HO. Nevertheless, at least a trend toward less HO formation and fewer immobile discs in patients who used NSAID’s after could be observed [29, 39, 40]. Further possible underlying causes for the high incidence of heterotopic ossifications may include the preparation of the end plates as well as the opening of the cortex resulting from the chiseling of the keels during implant insertion [39, 41]. Future studies should therefore investigate whether further developments and design changes which serve to avoid damages to the vertebral body surface may ultimately reduce the incidence of heterotopic ossifications.
One of the limitations of this present investigation is a potential selection bias, which may be attributed to the fact that the data are derived from a non-randomised case series.
In addition, an adequate assessment of adjacent segment degenerative changes is challenging without post-operative MRI imaging of the cervical spine. However, due to the above mentioned and previously defined radiological criteria examined in conjunction with an additional clinical deterioration, an adjacent segment disease can still be diagnosed adequately.