A meta-analysis of the outcomes of semi-constrained, unconstrained, and constrained cervical artificial disc designs

Thomas Cho; Nipun U. Jayatissa; David Yatsonsky; Hossein Elgafy; Jiayong Liu

doi:10.1016/j.jor.2025.11.030

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72 (); 332-339

doi:

10.1016/j.jor.2025.11.030

A meta-analysis of the outcomes of semi-constrained, unconstrained, and constrained cervical artificial disc designs

Thomas Cho, Nipun U. Jayatissa, David Yatsonsky, Hossein Elgafy, Jiayong Liu

Department of Orthopedic Surgery, University of Toledo Medical Center, Toledo, OH, 43614, USA

⁎Corresponding author: Jiayong Liu. jiayong.liu@utoledo.edu

Received: 2025-11-16, Accepted: 2025-11-26, Published: 2025-11-27

Disclaimer:
This article was originally published by Reed Elsevier India Pvt. Ltd. and was migrated to Scientific Scholar after the change of Publisher.

Abstract

Cervical disc arthroplasty (CDA) is a surgical treatment indicated for symptomatic pathologies of the cervical spine. Cervical disc designs fall under one of the three categories: semi-constrained, unconstrained, or constrained. This meta-analysis aims to provide an overview of differences in postoperative outcomes and functional scores among the three types of cervical discs.

A literature search was conducted on PubMed and Embase through June 2025. Comparison studies that included at least two of the disc categories and reported outcomes of interest were included in this study. Review Manager 5.4. was utilized for statistical analyses, and a P-value ≤0.05 was considered statistically significant.

Fifteen studies were included, with a total of 1341 patients. Comparing semi-constrained to unconstrained, a significant difference in operative time was found in favor of semi-constrained, while the incidence of heterotopic ossification (HO) was found in favor of unconstrained. Comparing semi-constrained to constrained, a significant difference in incidence of HO was found in favor of semi-constrained. Lastly, comparing unconstrained to constrained, significant differences were found in the changes of Visual Analog Score-Neck, Neck Disability Index, 12- Item Short Form Survey Physical Component Score, and Functional Spinal Unit (FSU) total range of motion in favor of unconstrained, while the change of FSU angle was found in favor of constrained.

Unconstrained cervical discs seem to provide better functional scores compared to constrained discs, with an advantage in the rate of HO compared to semi-constrained discs. There does not seem to be many significant differences between semi-constrained compared to unconstrained and constrained cervical discs in terms of functional scores and complications. Future studies should be carried out to identify the most effective disc design.

Keywords

Cervical disc arthroplasty

Cervical disc degenerative disease

Cervical radiculopathy

Cervical myelopathy

Disc herniation

Total disc replacement

Unconstrained

Semi-constrained

Constrained

Outcomes

Show Related Articles from PubMed

1 Introduction

Conditions concerning the cervical spine can lead to extreme pain, stiffness, and overall weakness that can have a debilitating effect on patients. Some common pathologies include but are not limited to cervical disc degenerative disease, herniated cervical discs, and symptomatic cervical radiculopathy and myelopathy, with these conditions frequently being seen together or in relation to one other due to the underlying pathology. Regarding treatment methods, anterior cervical discectomy and fusion (ACDF) is considered the gold standard procedure for cervical pathologies, with multiple long-term studies showing success in relieving cervical radiculopathy and other associated symptoms, although high rates of complication, such as recurrence and adjacent segment disease, have been reported.¹ Cervical disc arthroplasty (CDA) is an alternative treatment method that puts an emphasis on maintaining the range of motion of the affected spinal level while also decreasing overall stress on the adjacent segments, increasing its popularity among surgeons and patients alike for its motion sparing properties.²

There are a variety of different cervical artificial disc designs in the market. Modern CDA implants can be split into three different categories: unconstrained, semi-constrained, constrained. Unconstrained disc implants are designed to have no limitation to mechanical motion, aiming to replicate the normal kinematics of the cervical spine and relying more on the facet joints and ligamentous structures for stability. Semi-constrained disc implants have some physical motion stops in place, reaping the benefits of unconstrained mobility while also offering greater stability. Constrained disc implants have the highest resistance to motion, with its main goal of stabilizing the spine and preventing extreme motion.³^,⁴ To decide which category a disc implant belongs to, the degrees of freedom (DoF) allowed by each disc design have been used for classification. When applying the DoF to the cervical spine, specifically the functional spinal units (FSU), there is a total of six DoF, with three directions of translation, which include vertical, AP, and lateral, and three angles of rotation, which include flexion-extension, lateral bend, and axial rotation.⁵ If a certain DoF is omitted from a disc design, the movements associated with that DoF are not allowed, resulting in that implant being more constrained. Studies in the literature have described constrained devices having 3 DoF, semi-constrained devices having 4–5 DoF, and unconstrained devices having 6 DoF.³^,⁶

There are studies in the literature that have compared different cervical artificial disc designs to each other as well as to ACDF. However, the authors found that there is limited literature concerning the differences in functional scores and mobility between different cervical disc implants based on categorization with DoF. Therefore, the purpose of this meta-analysis study was to investigate the differences in post-operative functional score and complications between semi-constrained, unconstrained, and constrained cervical disc designs to see which type proves to be the most beneficial.

2 Materials and methods

The preferred reporting items for systematic reviews and meta-analyses (PRISMA) guideline was used for this study.⁷

2.1

2.1 Publication search

A publication search was performed on PubMed and Embase up until June 2025. The following keywords were utilized to construct the preliminary search results: “cervical disc degenerative disease”, “cervical radiculopathy”, “cervical myelopathy”, “cervical disc herniation”, “cervical disc arthroplasty”, “disc replacement”, “cervical disc”, “comparison”.

2.2

2.2 Inclusion and exclusion criteria

The following inclusion criteria was set in order to decide whether a study was to be incorporated in this meta-analysis or not: cohort study or randomized controlled trial (RCT), patients had at least one of the diagnoses of interest (herniated cervical disc, symptomatic cervical radiculopathy, cervical myelopathy, cervical disc degenerative disease), compared at least two of the interested cervical disc design categories, reported at least one pertinent outcome. Outcomes of interest to this study included intraoperative blood loss (mL), operative time (minutes), incidence of fusion, incidence of heterotopic ossification (HO), incidence of adjacent segment disease (ASD), infection, reoperation, and changes in Visual Analog Score (VAS)-Arm, VAS-Neck, Neck Disability Index (NDI), 12- Item Short Form Survey (SF-12) Mental Component Score (MCS), SF-12 Physical Component Score (PCS), C2-C7 Cobb angle, FSU angle, C2-C7 range of motion (ROM), FSU total ROM, FSU total ROM of the upper adjacent segment, FSU total ROM of the lower adjacent segment, and segmental ROM compared from pre-operation to 12–36 months post-operation. Articles that did not meet the inclusion criteria and/or were meta-analyses, case reports, biomechanical studies, expert opinions, or did not have full texts available were excluded from this study. Each author independently applied the inclusion/exclusion criteria to the received studies from the preliminary search.

2.3

2.3 Assessment of study quality

To assess the study quality of RCTs, the Cochrane Risk of Bias Tool was used, which can be found in the Review Manager 5.4 software. The following parameters were used, and each study was scored as a low risk, unclear risk, or high risk of bias: random sequence generation (selection bias), allocation concealment (selection bias), blinding of participants and personnel (performance bias), blinding of outcome assessment (detection bias), incomplete outcome data (attrition bias), selective reporting (reporting bias), and other bias.⁸ For all other non-randomized studies, the Newcastle-Ottawa scale was used.⁹

2.4

2.4 Data collection

After the number of included studies were finalized, alongside the outcomes of interest that were listed previously, the following additional data was collected and stored in an Excel sheet: first author, year of publication, journal, study type, treatment groups, sample sizes. The authors used the following classification of specific cervical disc designs, based off of studies from the literature, in order to make the semi-constrained, unconstrained, and constrained treatment groups.³^,⁶ The following cervical disc designs were considered semi-constrained: Prestige LP, Prestige ST, Mobi-C, Synergy disc, Secure-C, Pretic-I, Baguera, Kineflex. The following cervical disc designs were considered unconstrained: Bryan Disc, M6-C, CP-ESP. The following cervical disc designs were considered constrained: PCM, Discover, Prodisc-C, Prodisc vivo, ActivC. Due to the variety of follow-up times seen in the included studies regarding post-operative functional scores and outcomes, this study used a 12–36 months post-operative time interval to maximize inclusivity.

2.5

2.5 Statistical analysis

Review Manager 5.4 was used to carry out all statistical analyses for this meta-analysis. Continuous variables were presented as mean ± standard deviation (SD), while dichotomous variables were presented as event rates. For the continuous variables, an inverse variance method with a mean difference (MD) was utilized, while for the dichotomous variables, a Mantel-Haenszel method with risk ratio (RR) was utilized. Regarding the assessment of heterogeneity, the I² statistic was used for each analysis with the following interpretation: 0 %–40 % (might not be necessary), 30 %–60 % (may represent moderate heterogeneity), 50 %–90 % (may represent substantial heterogeneity), and 75 %–100 % (considerable heterogeneity).¹⁰ If I² ≤ 50 %, a fixed effect analysis model was used. If I² > 50 %, a random effects analysis model was used. A P-value ≤0.05 was considered statistically significant, and significant results were presented as a forest plot with a 95 % CI.

3 Results

3.1

3.1 Summary of included study characteristics

15 studies were ultimately included in this meta-analysis (Fig. 1).^11–25 There were 587 patients in the semi-constrained group, 334 in the unconstrained group, and 420 in the constrained group, for a total of 1341 patients (Table 1). Two studies were RCTs while the other 13 were retrospective cohort studies. No risks of biases were found in any of the included studies.

PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) flowchart.

Table 1 Study characteristics.

Author (Year)	Journal	Study type	Discs included	Semi-constrained	Unconstrained	Constrained
Chin (2019)¹¹	Clin Spine Surg	RCS	Prodisc-C, Mobi-C, Secure-C, Prestige LP-C	41	NA	76
Coric (2010)¹²	J Neurosurg Spine	RCT	Bryan, Kineflex/C, Discover	16	21	16
Guo (2021)¹³	BMC Musculoskelet Disord	RCS	Pretic-I, Discover	32	NA	32
Hacker (2013)¹⁴	Spine (Phila Pa 1976)	RCS	Bryan, Prestige LP	19	28	NA
Hui (2019)¹⁵	J Spine Surg	RCS	M6-C, Mobi-C	3	10	NA
Kowalczyk (2011)¹⁶	J Neurosurg Spine	RCS	Bryan, Prodisc-C, Prestige LP	20	20	20
Ma (2024)¹⁷	J Orthop Surg Res	RCS	Bryan, Mobi-c	40	41	NA
Noriega (2016)¹⁸	World Neurosurg	RCS	Baguera, Prodisc, PCM	27	NA	39
Park (2015)¹⁹	J Spinal Disord Tech	RCS	Mobi C, Bryan, PCM, Prestige LP	25	20	13
Ryu (2010)²⁰	J Neurosurg Spine	RCS	Bryan, Prodisc-C	NA	19	17
Wang (2021)²¹	Neurosurgery	RCS	Prestige LP, Discover, Prodisc-C	303	0	91
Yanbin (2011)²²	J Spinal Disord Tech	RCT	Bryan, Prodisc-C	NA	20	26
Yang (2020)²³	Spine (Phila Pa 1976)	RCS	ActivC, Bryan	NA	48	35
Yi (2010)²⁴	Spine (Phila Pa 1976)	RCS	Mobi C, Prodisc-C, Bryan disc	61	81	28
Zhang (2015)²⁵	Arch Orthop Trauma Surg	RCS	Bryan, Prodisc-C	0	26	27

3.2

3.2 Semi-constrained versus unconstrained discs

When assessing the semi-constrained group relative to the unconstrained group, there was a significant difference in average operative time (MD = −6.02; 95 % CI: −9.13 to −2.91; P < 0.001; Fig. 2A) in favor of semi-constrained and a significant difference in the rate of HO (RR = 2.45; 95 % CI: 1.53 to 3.93; P < 0.001; Fig. 2B) in favor of the unconstrained group. All other outcomes showed no significant differences.

Significant results for Semi-constrained versus Unconstrained discs displayed as forest plots, including (A) Operative time, (B) Rate of HO.

3.3

3.3 Semi-constrained versus constrained discs

When assessing the semi-constrained group relative to the constrained group, there was a significant difference in the rate of HO (RR = 0.84; 95 % CI: 0.75 to 0.95; P = 0.004; Fig. 3) in favor of the semi-constrained group. All other outcomes showed no significant differences.

Significant result of the rate of HO for Semi-constrained versus Constrained discs displayed as a forest plot.

3.4

3.4 Unconstrained versus constrained discs

When assessing the unconstrained group relative to the constrained group, there were significant differences in the change in VAS-Neck (MD = −0.90; 95 % CI: −1.52 to −0.28; P = 0.004), change in NDI (MD = −5.99; 95 % CI: −7.32 to −4.67; P < 0.001; Fig. 4A), change in SF-12 PCS (MD = 3.34; 95 % CI: 0.65 to 6.03; P = 0.01; Fig. 4B), and change in FSU Total ROM (MD = 3.36; 95 % CI: 1.22 to 5.50; P = 0.002; Fig. 4C) in favor of unconstrained, while a significant difference in the change in FSU angle (MD = −2.40; 95 % CI: −2.42 to −2.38; P < 0.001; Fig. 4D) was found in favor of the constrained group. All other outcomes showed no significant differences.

Significant results for Unconstrained versus Constrained discs displayed as forest plots, including (A) Change in NDI, (B) Change in SF-12 PCS, (C) Change in FSU Total ROM, (D) Change in FSU angle.

4 Discussion

Diagnoses stemming from cervical spine conditions can cause pain and weakness that could have a devitalizing effect on those who are affected. CDA is a surgical treatment method that can be used to treat these conditions. Due to the limited literature concerning the post-operative outcomes between different cervical discs, this study aims to bring forth an encompassing overview to see whether semi-constrained, unconstrained, or constrained cervical discs provide better outcomes in patients undergoing CDA.

Regarding the differences between semi-constrained and unconstrained discs, this study found that the semi-constrained group had a lower average operative time compared to the unconstrained group, with the unconstrained group having a lower rate of HO. A meta-analysis from Zavras et al. found that unconstrained devices had significantly greater segmental ROM compared to semi-constrained at a 2-year follow up, and no significant differences in rate of HO, NDI, VAS-Neck, VAS-Arm, rate of ASD, and rate of reoperation between the two groups.³ A meta-analysis from Wahood et al. found the unconstrained discs had the lowest incidence of HO and the largest change in NDI, VAS-Neck, and VAS-Arm scores compared to the semi-constrained discs, although there were minimal differences in the rate of ASD and rate of reoperation.²⁶ A finite element modelling study found that the semi-constrained disc led to supraphysiologic ROM and facet force at the index level and a decrease in both of these factors at the adjacent levels, while the unconstrained disc showed a reduction in ROM and facet force at the index level while showing an increase in flexion at adjacent levels but decrease in extension.²⁷ Unconstrained devices having better segmental ROM in the long term compared to semi-constrained is reasonable due to unconstrained devices having 6 DoF, allowing for better mimicking of the native motion of the cervical spine. Unconstrained discs having a lower rate of HO is also a sensible result due to the literature showing that restricted motion is associated with higher rates.²⁴^,²⁸ The differences seen in statistical significance between ROM and rate of HO between the results of this study and the other studies mentioned could be attributed to a variety of different factors, although one major factor could be that both Zavras et al. and Wahood et al. conducted an indirect comparison between the different cervical designs, while this study used a direct comparison analysis.³^,²⁶ An animal in vivo study found that the unconstrained group had a higher rate of anterior migration and extrusion out of the disc space compared to the semi-constrained group, which is something that could be taken into consideration.²⁹

When comparing semi-constrained discs to constrained discs, this study found that the semi-constrained group had a lower rate of HO compared to the constrained group. Zavras et al. found no significant differences in the rate of HO, segmental ROM, NDI, VAS-Neck, VAS-Arm, rate of ASD, and overall rate of reoperation between the two groups.³ Wahood et al. found that some constrained discs had a higher rate of HO, ASD, and reoperation at 2 years compared to other semi-constrained discs, although there did not seem to be a definitive difference in NDI, VAS-Neck, and VAS-Arm at 2 years between the two groups.²⁶ The reason for the significant difference seen in the rate of HO is similar to the one listed above. A finite element study found that the semi-constrained discs presented with a higher amount of flexion and extension at the index level compared to constrained discs, although the Prestige LP specifically had increased intradiscal pressures at both adjacent levels.³⁰ Another finite element study found a similar result regarding the Prestige LP, with the constrained disc showing increased intradiscal pressures at the adjacent levels.²⁷ This study did not find any significant differences regarding ROM. However, these finite element studies suggest that semi-constrained discs allow for better kinematics of the spine compared to constrained discs, which is most likely due to the greater DoF, although the increased demand on posterior structures with Prestige LP is a factor that should be taken note of.

When comparing unconstrained discs to constrained discs, this study found significant differences in the changes of VAS-Neck, NDI, SF-12 PCS, and FSU total ROM in favor of unconstrained, while a significant difference in the change of FSU angle was found in favor of constrained. Zavras et al. found a significant difference in segmental ROM in favor of unconstrained discs, although no significant differences were found in incidence of HO, NDI, VAS-Neck, VAS-Arm, incidence of ASD, or overall reoperation.³ The statistical significances in NDI, VAS-Neck, and SF-12 PCS seen in this study presents new findings that suggest that unconstrained disc might not only have better ROM, but also better patient-reported outcomes post-operation compared to constrained discs. Wahood et al. found that unconstrained discs had a lower incidence of HO, reoperation rate at 2 years, and larger changes in NDI, VAS-Neck, and VAS-Arm scores at 2 years compared to the constrained discs, which shows some similarity in results with this study regarding patient-reported outcomes.²⁶ The reason for the differences in patient-reported outcomes between these two groups is most likely due to the greater preservation of ROM in unconstrained discs, which would theoretically reduce stress on the adjacent segments and overall contributing to improved pain and functional outcomes. On the contrary, the change in FSU angle is most likely favored in constrained discs compared to unconstrained discs because by design, constrained disc reduces the risk of hypermobility that might be seen in unconstrained discs due to the lower DoF, which may help preserve the physiological alignment and segmental lordosis, thus maintaining the FSU angle.³¹^,³²

This study was not without its limitations. First, the majority of the included studies were cohort studies, with only two being RCTs. Cohort studies generally offer lower-quality evidence compared to RCTs due to the lack of randomization that could lead to increased risks of selection bias and confounding. Second, due to the overall lower amount of literature regarding differences between cervical disc designs, this resulted in some discrepancies in the number of studies in each analysis, with some studies having reported more interested outcomes than others. Lastly, the use of a time interval for post-operative functional scores and outcomes instead of a specific time point can be a potential limitation to this study. Due to the wide variety of follow-up times reported for outcomes across the included studies, the authors opted to use a post-operative time interval of 12–36 months to compare post-operative functional scores and outcomes. Although this is not ideal compared to a specific follow-up time, studies in the literature suggest that for many of these functional outcomes, they achieve maximum metrics and remain stable beyond 12 months.^33–36 Thus, the authors decided to use this time interval to both maximize study inclusivity and maintain clinical comparability.

5 Conclusion

Unconstrained cervical discs seem to offer preferable functional scores compared to constrained disc designs, while also showing superiority in the rate of heterotopic ossification compared to semi-constrained discs. There does not seem to be many significant differences between semi-constrained discs compared to unconstrained and constrained cervical discs in terms of functional scores and complications, although a significant difference in the rate of heterotopic ossification in favor of semi-constrained compared to constrained discs was noted. Future studies that incorporate larger sample sizes and high-quality methodology should be carried out in the near future in order to further contribute to the topic of discussion on the most effective cervical disc design for patients who need an arthroplasty.

Ethics approval and consent to participate

Not applicable. This article is a systematic review of previously published studies.

Consent for publication

Not applicable.

Availability of data and materials

All data generated or analyzed during this study are included in this published article.

Authors’ contributions

JL contributed to the conception and design of the study. TC, NJ, and DY performed the literature search and data extraction. TC & NJ conducted the data analysis, drafted the initial manuscript, and revised the manuscript. JL & DY critically revised the manuscript for important intellectual content. JL supervised the study, provided guidance throughout, and finalized the manuscript. All authors read and approved the final version of the manuscript and agree to be accountable for all aspects of the work.

Disclosure

All authors have nothing to disclose.

Investigation performed at statement

Investigation performed at The University of Toledo Medical Center, Toledo, Ohio.

Ethical statement

We confirm that all authors have read and approved the manuscript, and no other individuals meet the authorship criteria but are not listed. We have also agreed upon the order of authorship as presented. Furthermore, we assure you that this manuscript has not been submitted elsewhere.

Funding information

This research was conducted independently and did not receive specific funding from public, commercial, or not-for-profit agencies.

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Show Sections

[1] Leven D., Meaike J., Radcliff K., Qureshi S., . Cervical disc replacement surgery: indications, technique, and technical pearls. Curr Rev Musculoskelet Med. Jun 2017;10(2):160-169.
[Google Scholar]

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A meta-analysis of the outcomes of semi-constrained, unconstrained, and constrained cervical artificial disc designs

Abstract