Anterior Cervical Discectomy and Fusion (ACDF) is a standard procedure for the treatment of degenerative cervical disc disease. Based on the notion that preserving motion reduces the risk of Adjacent Segment Disease (ASD) (Chang et al., 2007; Robertson et al., 2005), Anterior Cervical Discectomy with Arthroplasty (ACDA) has been introduced as an alternative to fusion in the 1990´s. Although clinical outcome is well documented for both techniques (Anderson et al., 2012a; Fallah et al., 2012), some patients will experience persistent or increasing symptoms over time.
Adjacent Segment Disease
Despite its well documented benefits, ACDF may cause ASD in mid- and long-term follow-up (Goffin et al., 2003; Ishihara et al., 2004; Katsuura et al., 2001; Kim et al., 2009; Song et al., 2011; Yao et al., 2015). Biomechanical studies have shown increased intradiscal stress and motion compensation in the levels adjacent to the fusion site (Eck et al., 2002; Schwab et al., 2006) with a change of the center of rotation in adjacent levels postoperatively (Dvorak et al., 1993) (Eck et al., 2002) (Anderson et al., 2012b). Although this is considered by some authors to be the underlying cause for ASD, it is still controversial if this is attributable to the biomechanical effects of fusion or to the natural history of cervical degeneration (Anderson et al., 2012a; Bartels et al., 2010; Seo & Choi, 2008).
Reoperation due to ASD has been documented at a rate of 2.9% annually after ACDF (Hilibrand et al., 1999). ACDA is considered as an alternative to ACDF preserving normal cervical kinetics and biomechanics (Goffin et al., 2002). Thus, the rate of additional surgeries may be reduced with less stress on adjacent levels using ACDA (Upadhyaya et al., 2012) (Traynelis, 2006). The pooled surgery rate for ASD after disc prosthesis (ACDA) was 3.8% (0.9 -7.6%) within a follow-up of up to 84 months summarizing 13 randomized controlled trials (RCT) (Kang et al., 2015). Although clinical short-term results are satisfactory (Goffin et al., 2003; Heller et al., 2009; Murrey et al., 2009), there are only a few studies reporting mid- to long-term results (Ding et al., 2012; Garrido et al., 2010; Goffin et al., 2010; Walraevens et al., 2010). Garrido et al (Garrido et al., 2010) reported improved functional outcome for ACDF and ACDA on 24 and 48 months follow-up with no degradation of the outcome measures between 2 and 4 years after surgery. This is in concordance with the results of Goffin et al. (Goffin et al., 2010), who reported consistent if not improved clinical results at 4- and 6-years follow-up compared to the 1- and 2-years postoperative results. Our study supports these data that patients improved significantly after surgery and the clinical results remained stable on mid-term follow-up.
The protective effect of ACDA on the adjacent discs remains controversial. In the single level arm of their prospective cohort study, Kim et al. (Kim et al., 2009) observed ASD in 13% of all patients treated with ACDA compared to 23% in the ACDF group at a median follow-up of 19 month. Walraevens et al. reported ASD in the adjacent upper and lower segment to the operated site for up to 8 years after ACDA (Walraevens et al., 2010). Similar observations were made by Ding et al. (Ding et al., 2012). They observed mild ASD in the adjacent levels in approx. 23% of all patients. The degeneration mainly manifested as new formation or enlargement of an anterior osteophyte. However, no degeneration in clinical outcome occurred due to the lack of a direct relation between radiographic and clinical ASD (Ding et al., 2012). In our cohort with the new ROTAIO® prosthesis, no patient required adjacent level surgery within 2 years.
Quality of motion
After ACDA emphasis is often placed on presence and magnitude of motion as assessed by ROM, while quality of motion by parameters like instantaneous COR, COR, and instantaneous axis of rotation has just recently been identified as important for evaluating changes in the cervical motion pattern (Anderst et al., 2013; Guo et al., 2019; Jonas et al., 2018; Penning, 1988). Anderst et al. demonstrated that the instantaneous COR was generally fixed in the longitudinal direction, but it translated in the anterior-posterior direction during flexion-extension (Anderst et al., 2013). If translation is not adequately possible, non-physiologic stress on the facet joints at the index level ensues, which may cause facet joint syndrome, as it has commonly been seen in lumbar disc arthroplasty. Liu et al. evaluated the instantaneous COR located at the superior half of the lower vertebral body height and the posterior half of its width, and changing with age (Liu et al., 2014). It has been postulated that these further findings should be considered in clinical practice and when designing disc prostheses (Guo et al., 2019).
Although the overall effectiveness of ACDA has already been demonstrated, the kinematic properties of the various designs differ substantially (Galbusera, Bellini, Brayda-Bruno, et al., 2008; Kang et al., 2010). The Bryan disc prosthesis with its almost unconstrained design retained kinematic motion adequately (Pickett et al., 2005; Powell et al., 2010; Ryu et al., 2013), (Fleck et al., 2017) (Kowalczyk et al., 2011), yielding a near-physiological rotation at the index level (Galbusera, Bellini, Raimondi, et al., 2008). Ball-and socket designs like the Prestige LP (semiconstrained design) and the Prodisc-C (semiconstrained with fixed axis of rotation), however, did not fully restore normal mobility in view of ROM and COR, which may cause secondary problems over time (Rousseau et al., 2008). Particularly neck pain can be an ongoing problem after ACDA as a result of abnormal forces and load on the facet joints.
As standard ball and socket designs and (semi-) constrained devices do not allow uncoupled translation and are thus thought to force the facet joints into non-physiologic movements as mentioned above. As this may interfere with successful outcome over time, our group focused on an unconstrained disc prosthesis with uncoupled translation. The low revision rate and the stable clinical results over time in this series seem to support these considerations.
Clinical Outcome
In 2016, preliminary clinical and radiographic results with the ROTAIO® cervical prosthesis demonstrated excellent results (Obernauer et al., 2016). Our present results with more than 100 patients in a multicenter prospective trial confirm these findings with excellent clinical outcome. Pain relief, reduction of analgesics consumption, functional improvement, reduction of disability, patient satisfaction and quality of life were found to be very high and at least comparable to previous IDE trials. Revision rate was very low and no implant failure was observed. No surgical procedure due to ASD was performed within 2 years.
Nevertheless, longer follow-up is necessary to prove durability and functionality of the prosthesis. In view of our current data, however, the ROTAIO® prosthesis is a suitable alternative to ACDF and other available prostheses. The particular biomechanical characteristics with uncoupled translation and a variable center of rotation may allow physiological cervical spine motion with low fusion and ASD rates.