Clinical outcome 24 months after cervical disc arthroplasty with a new articulating and viscoelastic disc prosthesis

Introduction

Various therapeutical options exist for the treatment of cervical degenerative disc disease. In cases of progressive functional impairment or failure in conservative treatment surgery may be an option, as several studies indicate that surgical procedures are an effective therapeutic tool (1,2).

Open microsurgical anterior cervical discectomy (ACD) is established as the standard procedure for the surgical removal of nerve root compression (1). In order to avoid postoperative segmental instability and kyphosis, an interbody support (bone graft or cages made of various synthetic materials) and fusion (ACD-F), e.g., with additional anterior plating, can be added (3,4).

As an alternative to fusion and to maintain mobility in the operated cervical spine segment, ACD with cervical disc arthroplasty (CDA) instead of fusion has become increasingly important over the last decades (5,6). CDA is performed through the same approach as an ACD-F but instead of graft material, a prosthesis is placed in the decompressed disc space (7-10). Several studies and meta-analyses have shown comparable or even superior clinical results for CDA (11-13). Other studies support the hypothesis that CDA may also help to prevent adjacent segment disease (10,14,15).

Since the first description of CDA in the 1960s and the first implantation by Fernstrom in 1966, different design concepts have been developed (14,16,17). Most types of prosthesis currently available on the market can be grouped either in (I) an articulating multi-component design with a non-viscoelastic core or (II) a non-articulating design with a viscoelastic core. Group A prostheses allow for 2 to 5 degrees of freedom (DOF), whereas Group B prostheses additionally allow for axial compression/bumping with 6 DOF (Figure 1).

Figure 1 CDA design concepts. (A) Articulating multi-component design with a non-viscoelastic core. (B) Non-articulating design with a viscoelastic core. (C) Combines the articulating features of (A) design with the viscoelastic core of (B). CDA, cervical disc arthroplasty; DOF, degrees of freedom.

A newly designed type of prosthesis combines the articulating features of group A prostheses with the viscoelastic core of group B implants (Figure 1). Combining articulation and viscoelastic absorption aims to address the limitations of traditional ball-and-socket implants by better replicating physiological motion in all six DOF, including axial compression. While traditional first generation CDA devices provided mobility, they often lacked shock absorption, leading to abnormal facet joint overload and resulting in motion-dependent neck pain that often worsens with extension and rotation.

The aim of this study is to present the first clinical results in a prospective, mono-centric, non-randomized trial to investigate the safety and efficacy of CDA with such a newly designed articulating and viscoelastic prosthesis. We present this article in accordance with the STROBE reporting checklist (available at https://jss.amegroups.com/article/view/10.21037/jss-25-162/rc).

Methods

Study design

The presented study was designed as a prospective, non-randomized trial in a mono-centric setting at the DRK Spine and Neurotrauma Center, Berlin-Westend, Germany, with 3 specialized spine surgeons. Forty-five patients with one- or two-level cervical degenerative disc disease or a herniated disc that has not responded to conservative treatment were included for one- or two-level cervical disc arthroplasty starting in 01/2021. Clinical and radiological follow-up (FU) exams took place 6 weeks, as well as 6, 12 and 24 months after surgery. Data collection was closed in 01/2025. The study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. This study was approved by the DRK Kliniken Berlin Ethics Committee (KEK/Eth-47/20). Written consent was given by all patients for their clinical data to be stored in the hospital database and used for research purposes.

Implant design

The intervertebral cervical disc MOVE-C (NGMedical, Nonnweiler, Germany) is a prosthesis for the dynamic stabilization of one or two cervical segments between the C3 and C7 levels after anterior discectomy. The MOVE-C prosthesis is classified as a Class III medical device under the Medical Device Regulation 2017/745 (Appendix VIII Rule 8) (18). The prosthesis is comprised of a cranial and caudal titanium (Ti6Al4V) end plate with a core of polycarbonate urethane (PCU) molded into the caudal plate (Figure 2). The cranial end plate is coated on the inside with titanium niobium nitride (TiNbN). The material meets the requirements of the ASTM F136 standard for biocompatibility and stability. Due to the manufacturing method, the surface shows a microscopic rough topography for bone ingrowth. Additionally, the end plates have pins on the bone-turned side, which are used for primary fixation.

Figure 2 Prosthesis design. (A) Side view of the MOVE-C prosthesis with [1] articulating cranial plate, [2] viscoelastic PCU core, [3] cranial surface with anchor pins, and [4] caudal surface with anchor pins. (B) Inside view of the cranial plate with TiNbN-coated articulating gliding surface [5]. (C) Inside view of the caudal plate and the injection-molded PCU core with [6] ellipsoid gliding surface and [7] viscoelastic center. PCU, polycarbonate urethane; TiNbN, titanium niobium nitride.

Data collection

Patient reported outcome measures were collected on baseline pre-surgery and during 4 FU exams (6 weeks as well as 6, 12 and 24 months after surgery) using validated questionnaires and scores for pain, function and outcome:

Subjective arm and neck pain were measured as VAS arm/neck separately on a unidimensional numeric rating scale ranging from 0, indicating no pain, to 100 for worst imaginable pain (19,20). Denis Pain Scale (DPS) was used for examiner determined pain assessment. DPS is a 5-point scale (P1–P5) that includes information on pain intensity, need for medication and work status (19,21). Pharmacological pain management (PGPM) was documented according to the World Health Organization (WHO) Analgesic Ladder in four steps (no PGPM, WHO 1: non-opioid analgesics, WHO 2: weak opioids, and WHO 3: potent opioids) (22).

Function was evaluated using the Neck Disability Index (NDI), a validated 10-item-questionnaire for the assessment of neck pain (23,24). Each item was rated on a scale from 0 to 5 points. The final score of 0–50 points was transferred into percentage to allow for skipped or nonapplicable items. Lower scores indicated less disability with a minimal clinically important change of 21% (25,26). The modified Japanese Orthopaedic Association (mJOA) score was used as an investigator-administered tool to evaluate motor and sensory function in the upper and lower extremities, sensory function in the upper and lower extremities as well as bladder function (27,28). The combined score totals from 0 to 17 with a lower score indicating greater severity of disease. Mild myelopathy can be defined as a mJOA =15–17, moderate as mJOA =12–14 and severe as mJOA <12.

Outcome was assessed using the modified McNaab criteria (29), a 4-point rating scale: Patients were asked 6, 12 and 24 months after surgery to rate their subjective outcome as to be excellent, good, fair or poor. The number and type of adverse events (AEs) were recorded whenever noticeable.

Radiological measurement

All patients underwent magnetic resonance imaging (MRI) at baseline to display the disc herniation and to define the level of nerve root or dural sac compression. X-ray of the cervical spine was obtained at baseline (within 2 weeks before surgery) and for all patients at the 6 months, 1 year and 2 years FU (Figure 3). Radiographs were blindly analyzed by two surgeons and one radiologist. The amount of adjacent segment degeneration (ASD) was evaluated using the Goffin score (30,31). According to changes in disc height and the presence of osteophytes at the adjacent segment above or below, each of the 45 one- and two-level arthroplasties was scored as to have “no ASD” (grade 1), “mild ASD” (grade 2) or “moderate to severe ASD” (grades 3 and 4). For the evaluation of the postoperative shape of the cervical spine, a perpendicular line between the posterior inferior part of C2 and the posterior superior part of C7 was used. According to this lining, the position of the C3–C6 vertebra were judged as to be in a lordotic, straight or kyphotic shape.

Figure 3 Radiological imaging. MRI at baseline of a patient with C5/6 disc herniation [(A) sagittal; (B) axial]. X-ray of the cervical spine was obtained intraoperatively (C), at baseline (D) and at the 6 months (E), 1 year (F) and 2 years (G) follow-up. MRI, magnetic resonance imaging.

Statistical analysis

Statistical analyses were performed using SPSS, version 24.0 (SPSS, Chicago, Illinois, USA). Descriptive statistics are reported as the mean and standard deviation for continuous variables. The level of significance was defined as P<0.05. The repeated measures method was performed for variance analysis to compare clinical and radiographic data at baseline and the FU exams. Proportions and percentages are reported for categorical variables.

Results

In total, 21 (47%) male and 24 (53%) female patients with a mean age of 43.4±8.1 years (range: 30–62 years) were included in the study. All patients were followed over a minimum of 24 months. There were no lost to FU in this series. The baseline characteristics [including body mass index (BMI), American Society of Anesthesiologists (ASA) score, comorbidities, and duration of complaints] are detailed in Table 1. Thirty-eight (73%) patients had one-level surgery and 7 (27%) had two-level surgery, summing up to a total of 52 implants. Patients were operated on at the C4/5 level in 7 cases (14%), the C5/6 level in 22 cases (42%) and the C6/7 level in 23 cases (44%) (Table 1).

The VAS scores for arm and neck pain decreased from 72.4±13.1 and 44.3±28.1 at baseline to 11.9±23.1 and 27.9±18.3 (P<0.05), respectively, at FU 1. At the last FU after 24 months VAS arm was 8.2±12.1 (P<0.05) and VAS neck was 16.5±21.8 (P<0.05) (Table 2 and Figure 4).

Figure 4 VAS arm and neck values separately measured on a unidimensional numeric rating scale [0–100] at baseline and during FU. FU, follow-up; VAS, visual analog scale.

There was significant improvement in the NDI from 41±12 at baseline to 17±11 (P<0.05) at FU 1. Mean NDI scores stayed improved at FU 2, 3 and 4 between 11±10 and 12±13 (P<0.05, compared to baseline) (Table 2 and Figure 5).

Figure 5 NDI values during FU. FU, follow-up; NDI, Neck Disability Index.

For the modified JOA score, improvement from 14.5±0.9 at baseline to 16.4±0.9 (P=0.1) at FU 4 was observed (Table 2 and Figure 6).

Figure 6 mJOA values at baseline and during FU. FU, follow-up; mJOA, modified Japanese Orthopaedic Association.

The need for PGPM dropped from a WHO step 1 (27/45) and WHO step 2 (18/45) treatment pre-operatively to “no PGPM” in 30/45 cases and WHO step 1 in 15/45 cases after 24 months. Major reduction of oral pain medication intake was observed between 6 weeks and 6 months after surgery (Table 2).

At baseline, most patients were categorized into groups P3, P4 and P5 of the DPS (P3: 15/45, P4: 16/45, P5: 14/45). At FU 4, there was no patient in P5 left, 23/45 patients improved to a P1 scoring and 14/45 to a P2 scoring, respectively (Table 2).

According to the modified Macnab criteria, the overall results were “excellent” in 49%, “good” in 38%, and “fair” in 13% after 6 months with slight further improvement at 24 months (53%, 42% and 5%) (Table 2).

Overall, 4.4% of patients experienced post-operative medical AEs, including one case of dysphagia (2.2%) and one case of laryngeal recurrent nerve palsy (2.2%); both relieved through conservative treatment within 4–6 weeks. There was no mechanical problem or malfunction of the prosthesis to be noticed in any of the cases and no revision surgery was required during the FU period of 24 months (Table 3).

Using the Goffin score, “mild” (grade 2) ASD was observed in one case (2.2%) after 6 months at the adjacent upper level. After 1 year, 3 cases of grade 2 ASD were found (2 upper, 1 lower level), and after 2 years, one of these three mild ASDs (upper level) aggravated to a grade 3 ASD, summing up to 4.4% ASD grade 2 and 2.2% ASD grade 3 at the final FU. A total of 42 out of 45 cases (93.4%) showed no signs of ASD (Table 4).

Subsidence into the endplates of the index segment of more than 2 mm was observed in 5 of the 52 prostheses (9.6%) after 6 months and in a total of 6 prostheses (11.5%) at the 1- and 2-year FUs (Table 4). Heterotopic ossification at the index level (McAfee grades 3 and 4) were found in 5 (9.6%) of the operated segments at FU 2, in 7 (13.5%) at FU 3 and 9 (17.3%) at FU 4, respectively. Eight of the 9 patients with high grade ossification did not present with poorer clinical outcomes, even though segmental motion at the index level was restricted in all of them. VAS neck pain scores trended higher in only one case with McAfee grade 4 ossification on FU 3 and 4.

The cervical curvature was evaluated as to be lordotic in 80% (30/45) and 18% as to be straight 2 years after surgery. One patient (2%) developed ongoing kyphosis with onset 1 year after surgery (Table 4).

Discussion

Since the implementation of the first artificial disc implants in the 1960s, CDA has gone through several design changes (5,7). Currently, a variety of prostheses and designs are on the market. Meanwhile, some of them have received Food and Drug Administration (FDA) approval (32).

Disc prostheses are generally classified and subdivided by their type of articulation and the location of the center of motion, as well as by design features such as the kind of anchorage, use of endplate materials or surface coating, and friction couple (33,34).

Primary anchorage between implant and bone can be achieved by screws, pins or a certain surface or macrostructural features (35). As surface texture or coating additionally supports long term fixation via osseous integration, several materials, or coatings, such as hydroxyapatite, tricalcium phosphate, porous titanium or chrome-cobalt are often used in arthroplasty. Different types of friction couple include metal on metal, metal on polymer, ceramic on polymer or ceramic on ceramic (34,36). However, current FDA approved prostheses are either metal on metal or metal on polymer designs, like the MOVE-C prosthesis investigated in this study.

The term “DOF” is usually used to describe the number of independent motions a disc can move in three-dimensional space. A healthy disc has six DOF: three translational movements (anterior-posterior, medial-lateral, and superior-inferior) and three rotational movements (flexion-extension, lateral bending, and twist) (37,38). Based on that physiological concept, CDA prostheses can be subdivided into “non-constrained” implants with six DOF, “semi-constrained” implants with a free nucleus and five DOF, or “constrained” implants with a fixed nucleus and three DOF (39-41). Due to the combination of articulating components with a viscoelastic core, the MOVE-C design can be categorized as a non-constrained implant with six DOF.

Clinically, the current FDA approved cervical prostheses are indicated in bone-healthy patients (in the absence of trauma, infection, malignancy, or osteoporosis) for the reconstruction of a single cervical disc level between the C3/4 and C6/7 segments (17,42,43). Clinical indications are intractable radiculopathy with or without neurological deficit, as well as some cases of myelopathy in the absence of dynamic instability and without major cord compression. In contrast to lumbar disc replacement, cervical disc arthroplasty is not recommended for isolated axial neck pain (44,45). Compressive pathologies, such as hypertrophies from facet joint arthrosis, OPLL, segmental instability with sagittal plane translation of >3.5 mm on dynamic radiographs or sagittal imbalance with kyphotic deformity are considered contraindications (46). Other contraindications are allergy or intolerance to the implant materials, as well as several systemic diseases, such as insulin-dependent diabetes mellitus, human immunodeficiency virus (HIV) and hepatitis B or C, and various types of autoimmune diseases (46). All patients of our study group were screened pre-operatively according to these inclusion and exclusion criteria. Beyond that, age limitations are under ongoing discussion, as some of the contraindications to CDA (such as facet joint arthrosis, segmental instability, osteoporosis, and disc space collapse) become more prominent with aging. Therefore, some surgeons hesitate to implant prostheses in patients over the age of 50 years. The mean age of our study group was 43.4±8.1 years. The range was between 30 and 62 years with 11 patients over the age of 50 years. However, subgroup analysis did not show better results in the group of patients younger than 50 years of age.

Overall, CDA is considered to be a rather safe surgical procedure with complication rates comparable to published complication rates following fusion techniques, like ACD-F (2,47). Due to pre-vertebral soft-tissue distraction, dysphagia and recurrent laryngeal nerve palsy are the most reported complications associated with an anterior cervical approach (32). These complications were seen in one case of our study group, respectively. Infection is an uncommon complication of CDA and did not occur in our study group.

The most common hardware-related complications are subsidence with axial migration of the implant into the endplates of the adjacent vertebrae (32,48). Settling of components, such as anchor-pins must be differentiated from real subsidence due to osteolysis or bony compression (49). As most pins have a length of approximately 1 mm, a loss in disc space height of up to 2 mm between the intra-operative imaging and post-operative radiographs can occur, most of which are asymptomatic. However, subsidence of more than 2 mm may result in kyphosis. Several studies have shown that subsidence is more often seen in undersized implants than in oversized (50). In our study, we observed 5 cases of subsidence >2 mm after six months and a sixth case 1 year after surgery. All six cases were clinically asymptomatic and did not need any surgical revision. However, four of these six cases developed advanced heterotopic ossification (defined as McAfee grades 3 and 4) as well, leading to restricted range of motion in two of them without a new neurological deficit or new onset of neck pain. Migration anteriorly along the surgical approach is rare and not observed in any of our cases, as well as lytic loosening. Undersized implants and off-axis positioning may predispose to segmental deformity, mostly in kyphosis. Only one patient in our study group developed a kyphotic deformity, first seen at the 1-year FU with a persisting kyphotic angle of 14 degrees at FU 4. Anterior subsidence of 3.5 mm and heterotopic ossification, leading to anterior osteophytes were seen as reason.

While many artificial discs have successful long-term records, others were already removed from the market after clinical data revealed high revision rates, i.a., due to severe osteolysis, implant migration, or material wear debris.

Devices that produce excessive wear particles (e.g., polyethylene or metal-on-metal) can trigger an immune response leading to localized inflammation and osteolysis (51,52). Due to these risks, manufacturers chose to discontinue the M6-C in early 2025.

Other CDA devices failed to maintain stability after implantation, leading to dislocations causing persistent neck pain or swallowing difficulties (51,52). The Porous Coated Motion (PCM) disc was withdrawn after long-term studies showed high rates of anterior migration. The ball-and-socket design caused excessive stress during neck extension, leading to the device slipping out of intervertebral space.

Another major concern after fusion surgery is degeneration of the adjacent segments (53-55). Several studies and meta-analysis have already shown that CDA has lower rates of ASD and lower rates of subsequent surgeries at adjacent levels compared with fusion techniques (53,56). In our study, mild ASD (Goffin grade 2) was seen in 6.6% (3/45) after 1 year. On FU 4, one of these cases deteriorates to a grade 3 ASD. Hence, the rate of moderate or severe ASD (grades 3 and 4) was 2.2% in our study group, equal to results of meta-analysis showing ASD grades 3 and 4 in 2–4% of CDA cases, compared to up to 9.5% in ACD-F cases.

All patient-reported outcomes of this study significantly improved from baseline through 2 years of FU. Surgical treatment was considered to be successful after two years if (I) neurologic function was equal or better compared to baseline with additional improvement in the mJOA and Denis scores; (II) if pain measures improved by at least 10/100 (minimal clinically important difference) in the VAS scores; (III) if the NDI score improved by at least 21%; (IV) if no serious treatment-related AEs occurred; and (V) if no revision surgery at the index level or an adjacent level became necessary. Forty-three of the 45 patients (95.6%) in our study group met these criteria. Only in two patients the VAS neck did not improve better than the minimal clinically important difference of 10/100. These results were confirmed with the modified Macnab criteria, showing that 95% of our patients evaluated their subjective clinical outcome as to be “good” or “excellent”, likewise.

Strengths and limitations

There are several limitations that should be considered. The single-center design of our study may impact generalizability of the results, although multiple surgeons were involved in the surgical treatment to strengthen variability.

Furthermore, the sample size of this study is relatively small, but the inclusion of multiple FU assessments over a 24-month period and the absence of any loss to FU contribute to the robustness and validity of the collected data.

Expanding data collection by long-term studies with a FU of more than 2 years would be desirable to better understand the biomechanical behavior of the new combined articulating and viscoelastic CDA design. Long-term FU studies (5 years+) of other CDA devices such as Mobi-C, Prestige LP, and Prodisc-C generally show that these devices are safe, effective, and durable alternatives to ACD-F. Key findings include maintained range of motion, high patient satisfaction, and lower rates of ASD compared to fusion. The Mobi-C 10-year post-market study showed significant improvements in NDI scores, pain reduction, and quality of life. Patients maintained segmental ROM, and the cumulative rate of adjacent-level reoperations was low (4.3% at 10 years, with no adjacent level surgeries occurring after 7 years) (57). The low-profile Prestige LP demonstrated superior results in overall success and NDI compared to ACD-F in 2-level cases at 84 months (7 years). 10-year FU data reported stable results with maintained motion, high satisfaction (>90%), and a 13.8% cumulative adjacent-level surgery rate (58). Prodisc-C demonstrated excellent long-term durability with an exceptionally low reoperation rate (approx. 2.1%) at the index level. Device mobility was maintained, though mean range of motion (ROM) declined slightly from 9.1° (1 year) to 7.6° (10 years) due to heterotopic ossification (HO), which did not impact clinical outcomes (59).

As the focus of this study was on clinical outcomes rather than biomechanical results, dynamic cervical radiographs were not available for all participants at any FU. Hence, no detailed statistical analysis on flexion-extension parameters or lateral bending can be presented for our study group at that point. These limitations underscore the necessity for further research, ideally by a multicenter study with a larger patient cohort.

Conclusions

This single-center multisurgeon study assessed the clinical and radiographic outcomes after CDA with a newly designed prosthesis. All subjects demonstrated significantly improved quality of life and reduced pain and decreased need for analgesics. The clinically tested articulating and viscoelastic design concept demonstrated both safety and effectiveness for the treatment of degenerative cervical disc disease.

Acknowledgments

The abstract was previously presented with a smaller sample size at the Eurospine 2024 congress in Wien, Austria.

Footnote

Reporting Checklist: The authors have completed the STROBE reporting checklist. Available at https://jss.amegroups.com/article/view/10.21037/jss-25-162/rc

Data Sharing Statement: Available at https://jss.amegroups.com/article/view/10.21037/jss-25-162/dss

Peer Review File: Available at https://jss.amegroups.com/article/view/10.21037/jss-25-162/prf

Funding: This work was supported by NGMedical GmbH, Nonnweiler, Germany, covering the costs for publication. The funder had no influence on authorship or the scientific content of this article.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://jss.amegroups.com/article/view/10.21037/jss-25-162/coif). All authors report receiving support from NGMedical GmbH, Nonnweiler, Germany, covering the costs for publication. The authors have no other conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. The study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. This study was approved by the DRK Kliniken Berlin Ethics Committee (KEK/Eth-47/20). Written consent was given by all patients for their clinical data to be stored in the hospital database and used for research purposes.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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