Abstract
Background A previous study investigated the likelihood of cervical artificial discs achieving post-cervical disc arthroplasty (CDA) range of motion (ROM) in the physiological range, defined as 5° to 16°. Here, we delve deeper into the capability of a disc prosthesis to restore physiological motion when the patient is preoperatively hypomobile (ROM < 5°) or hypermobile (ROM > 16°), and capability to preserve physiological mobility when the preoperative ROM falls in the 5° to 16° range.
Methods This is a secondary analysis of prospectively collected radiographic data from 131 patients who participated in a 1-level clinical trial of the M6-C (Orthofix) artificial disc. We chose this prosthesis for demonstration purposes due to the availability of pre- and postoperative flexion-extension ROM data on each individual patient. This allowed calculation of the number of implanted levels with preoperative and postoperative flexion-extension ROM in the following motion ranges: hypomobile (ROM < 5°), physiological mobility (ROM: 5°–16°), and hypermobile (ROM > 16°), resulting in a preoperative vs postoperative 3 × 3 mobility matrix.
Results Preoperatively, 35 of the 131 implanted levels were hypomobile (ROM: 2.2° ± 1.5°). Of these, physiological ROM was restored to 26 implanted segments by 2 years after CDA (ROM: 7.6° ± 2.0°). A total of 89 of 131 levels had physiological mobility preoperatively (ROM: 9.1° ± 3.3°). Of these, 73 segments preserved their physiological range 2 years after CDA (ROM: 10.2° ± 3.1°). Seven of 131 segments were hypermobile preoperatively (ROM: 18.5° ± 0.9°). Of these, physiological motion was restored to 4 segments by 2 years after CDA (10.9° ± 1.4°). Since the group of patients with preoperative hypermobility was small (n = 7), we combined preoperatively hypo- and hypermobile groups in calculating the restoration rate, resulting in a 2 × 2 mobility matrix. The calculated physiological motion restoration rate was 71% (30/[35 + 7] = 0.71), and preservation rate was 82% (73/89 = 0.82). The combined result was that the prosthesis had a 79% likelihood of yielding physiological mobility at 2 years post-CDA follow-up ([73 + 30]/131).
Conclusions The ability to restore and preserve physiological motion are 2 distinct mechanisms that combine to yield physiological motion after CDA. The cervical disc prosthesis analyzed in this study restored physiological motion to 71% of segments with preoperatively nonphysiological (hypomobile plus hypermobile) ROM, while preserving physiological mobility in 82% of segments with preoperative ROM in the physiological range.
Clinical Relevance Knowing the likelihood of achieving an ideal postoperative motion outcome for a range of preoperative mobilities can help in guiding patient expectations about CDA and give clinicians additional information to use during implant selection.
Level of Evidence 2.
- cervical disc arthroplasty (CDA)
- range of motion (ROM)
- physiological mobility
- restoration rate
- preservation rate
- Total Disc Replacement (TDR)
Introduction
Cervical disc arthroplasty (CDA) is an accepted alternative to fusion for radiculopathy and myelopathy in single- and 2-level cervical pathology.1–10 Cervical disc prosthesis designs have evolved over the past 25 years along with the choice of biomaterials.11,12 The functional goals of CDA are to restore and/or preserve range of motion (ROM) to reduce the risk of accelerated adjacent segment degeneration13,14 while limiting excessive motion to obtain a biomechanically stable segment; here termed “physiological mobility range.” To that end, postoperative hypermobility has been shown to be associated with the development of adjacent level degeneration.14 Contemporaneous clinical studies,15,16 combining 24-month radiographic data from multiple CDA clinical trials, have demonstrated the influence of prosthesis design on the likelihood of reconstructed segments to yield flexion-extension ROM (FE-ROM) in the physiological mobility range, defined as between 5° and 16° after CDA. Comparing 7 artificial disc prostheses, those authors found that prostheses, on average, had a two-thirds likelihood of achieving physiological ROM (5°–16°) after CDA, ranging from a low of 62% to a high of 79%.15
Biomechanical studies reviewed in previous publications11,12 have suggested the motion response of a cervical spinal segment after artificial disc replacement is likely to be influenced by several factors including (a) prosthesis design and (b) preoperative mobility of the index segment. The authors noted that disc arthroplasty using the fixed-core, ball-and-socket designs (ProDisc-C [Centinel] and PCM [Globus Medical]) yielded postoperative ROM that was positively and significantly correlated with preoperative ROM (R2 = 0.74; P < 0.01).12 If the preoperative motion was high, the post-CDA motion was likely to continue to be high. Arthroplasty with the mobile-core prosthesis (Mobi C [Highridge]) also showed postoperative ROM that was positively correlated with preoperative ROM (R2 = 0.36; P = 0.025). Conversely, the 6-degree-of-freedom design with artificial annulus (M6-C [Orthofix]) yielded post-CDA ROM that was not significantly correlated with the preoperative ROM (R2 = 0.17; P = 0.12).
The above-referenced biomechanical responses of preoperative motion vs motion after CDA have implications for the ability of these cervical disc prostheses to restore and preserve physiological motion—2 mechanisms of action that combine to improve the likelihood of achieving physiological motion after CDA. The current clinical study described in this manuscript is an attempt to apply these biomechanical concepts to clinical data.
More specifically, we aimed to assess the likelihood of physiological motion preservation and physiological motion restoration by evaluating the preoperative and postoperative radiographic data on 131 individual patients who participated in an investigational device exemption (IDE) clinical trial of the M6-C disc prosthesis,10,17 a 6-degree-of-freedom device with an artificial annulus. Although the M6-C artificial disc prosthesis has been withdrawn from commercial sales in the US by the manufacturer as of February 2025, this prosthesis was chosen for demonstration purposes due to the availability of pre- and postoperative FE-ROM data on each individual patient.
Methods
Patient Sample
The patient sample consisted of 131 patients with a mean age of 43.6 ± 9.1 years (range: 22–68); 51% of the patients were men.17 Index level preoperative ROM was 8.3° ± 5.0°. The average preoperative disc height was 3.2 ± 0.7 mm. The Neck Disability Index was 54.8 ± 14.1; visual analog scale for neck pain was 7.3 ± 1.9. The most frequently operated level was C5 to C6 (51.3%), followed by C6 to C7 (40.0%), C4 to C5 (6.3%), and then C3 to C4 (2.5%).
Physiological Mobility Range
The range of segmental physiological mobility has been defined as 5° to 16°.15,16 The 5° lower bound of the physiological mobility range was based on clinical observations of significantly reduced incidence of progressive radiographic adjacent level degeneration in patients with 5° or greater ROM after CDA.13 The 16° upper bound was based on laboratory data on 133 C5 to C6 and C6 to C7 segments from 102 cervical spines with mild-to-moderate degeneration evaluated in the biomechanical studies reported in the literature, which showed a mean FE-ROM of 12.1° ± 4.0°.18–21 The upper ROM bound was derived from the sum of the standard deviation and average ROM of the 133 segments.
Combinations of Pre- and Postoperative Motion Categories—The 3 × 3 Mobility Matrix
We analyzed preoperative FE-ROM data and 24-month post-CDA FE-ROM data from each of the 131 patients who participated in the 1-level IDE clinical trial of the M6-C artificial disc prosthesis. The flexion and extension radiographs were analyzed by an independent core laboratory (Medical Metrics, Houston, Texas, USA), which has validated the accuracy and reliability of their motion measurement methodology,22 which is widely used in US Food and Drug Administration–approved clinical trials of numerous implants and artificial disc prostheses.
The lower and upper bounds of the physiological ROM (5°–16°) were superimposed onto the preoperative vs postoperative index-level motion graph (Figure 1). This subdivided both the preoperative and postoperative FE-ROM into the following motion ranges: hypomobile (ROM: 0°–4°), physiological mobility (ROM: 5°–16°), and hypermobile (ROM > 16°). Thus, there were 9 cells in the 3 × 3 preoperative vs postoperative mobility matrix, each cell representing a unique combination of preoperative and postoperative motion categories: hypo-hypo; hypo-physiological; hypo-hyper; physiological-hypo; physiological-physiological; physiological-hyper; hyper-hypo; hyper-physiological; and hyper-hyper (Figure 1). The number of implanted segments in each cell was counted and expressed as a percent likelihood of occurrence (Table 1).
Pre- vs postoperative motion for each of 131 patients from a US IDE study.10,17 A grid dividing both preoperative and postoperative range of motion (ROM) data into 3 motion groups is superimposed, yielding 9 cells in the 3 × 3 preoperative vs postoperative motion matrix. The blue bands represent the range of physiological motion (5°–16°); the vertical band is for preoperative motion, and the horizontal band is for motion after cervical disc arthroplasty (CDA). The number of segments in the horizontal blue band divided by the total number of reconstructed segments gives the likelihood of achieving physiological motion after CDA. The intersection of the horizontal and vertical blue bands is the square region (highlighted in light green), which contains those reconstructed segments in which the preoperative physiological motion is preserved after CDA. The reconstructed segments that lie in the horizontal blue band but outside the central (green) square are those in which preoperatively nonphysiological (hypomobile or hypermobile) motion has been corrected and physiological motion restored.
Influence of preoperative ROM on likelihood of achieving postoperative physiological mobility: implant count and likelihood (%).
Motion Restoration vs Motion Preservation
The blue bands in Figure 1 represent the range of physiological motion (5°–16°); the vertical band is for preoperative motion, and the horizontal band is for motion after artificial disc replacement. The number of segments in the horizontal blue band divided by the total number of reconstructed segments gives the likelihood of achieving physiological motion after CDA. This concept was applied to clinical data in comparing the performance of 7 artificial cervical disc prostheses in a previous study.15 The intersection of the horizontal and vertical blue bands is the square region (highlighted in green), which contains those reconstructed segments in whom the preoperative physiological motion is preserved after CDA. The reconstructed segments that lie in the horizontal band but outside the central (green) square region are those in whom preoperatively nonphysiological (hypo or hypermobile) motion has been corrected and physiological motion restored.
Thus, the likelihood of achieving physiological mobility after CDA is a combined result of 2 functional abilities of the artificial disc: (a) physiological motion restoration and (b) physiological motion preservation. Physiological motion restoration rate was defined as how many of the preoperatively hypomobile (ROM < 5°) or hypermobile (ROM > 16°) segments became physiologically mobile (ROM between 5° and 16°) at 2 years after the disc replacement surgery. Physiological motion preservation rate, on the other hand, was defined as how many of the preoperative segments that were physiologically mobile (ROM between 5° and 16°) retained physiological mobility 2 years after disc replacement surgery.
Average Post-CDA ROM of Segments in the 3 × 3 Mobility Matrix
We also calculated the average post-CDA ROM for the segments in each of the 9 cells of the 3 × 3 mobility matrix. This was done by summing the postoperative ROM magnitudes of all implanted segments belonging to a cell of the mobility matrix and dividing the result by the total number of implanted segments in that cell, yielding the average ROM for all segments that fell in a particular cell of the mobility matrix. This calculation was made for each of the 9 cells of the preoperative vs postoperative mobility matrix (Table 2).
Average post-CDA ROM of segments in the 3 × 3 mobility matrix.
Statistical Analysis
Mean values of preoperative segmental ROM in the 3 motion groups (hypomobility, physiological, and hypermobility) were compared using paired t tests with Bonferroni corrections to confirm whether the 3 preoperative motion groups were significantly different from each other.23 Similar analyses of postoperative segmental ROM in the 3 motion groups were performed to establish whether 3 postoperative groups were significantly different from each other.
χ2 analysis was performed to assess whether preoperative mobility had a significant influence on the proportions of implanted segments in the postoperative physiological motion range. Multiple paired comparisons with Bonferroni corrections were performed to assess whether preoperative mobility had a significant influence on the magnitude of postoperative motions of implanted segments in the physiological mobility group.
Results
The Pre- and Postoperative Motion Groups
Preoperatively, the 3 motion groups were distinctly different from each other (P < 0.05; Figure 2A). The average preoperative motion of segments in the hypomobile (n = 35), physiologically mobile (n = 89), and hypermobile (n = 7) groups was 2.2° ± 1.5°, 9.1° ± 3.3°, and 18.5° ± 0.9°, respectively. Bonferroni-corrected paired comparisons showed statistically significant differences between the ROMs of these 3 preoperative motion groups (P < 0.05).
Range of motion (ROM) histograms. (A) Preoperative index level ROM. (B) Twenty-four-month postoperative index level ROM.
Similar analysis showed the 3 postoperative motion groups were also distinctly different from each other (P < 0.05; Figure 2B). The average postoperative motion of segments in the hypomobile (n = 20), physiologically mobile (n = 103), and hypermobile (n = 8) groups was 2.3° ± 1.0°, 9.5° ± 3.1°, and 19.1° ± 1.8°, respectively. Bonferroni-corrected paired comparisons showed statistically significant differences between these 3 postoperative motion groups (P < 0.05).
Influence of Preoperative ROM on Likelihood of Achieving Postoperative Physiological Mobility
Preoperatively, 35 of the 131 segments (27%) were hypomobile with a ROM of 2.2° ± 1.5°. Of these 35 segments, 26 (74%) achieved physiological mobility (between 5° and 16°; 7.6° ± 2.0°) by 2 years after cervical disc replacement, while 6 segments (17%) continued to display hypomobility (pre: 1.2° ± 1.9°; post: 2.2° ± 0.8°) and 3 segments (9%) became hypermobile after disc replacement (pre: 1.8° + 1.7°; post: 18.2° ± 1.4°; Tables 1 and 2).
Eighty-nine of 131 segments (68%) had physiological ROM (mean: 9.1° ± 3.3°) preoperatively. Of these, 73 (82%) maintained their physiological range 2 years after disc replacement (10.2° ± 3.1°), while 13 of 89 segments (15%) lost motion and became hypomobile (2.3° ± 1.2°), and 3 segments (3%) became hypermobile (19.1° ± 2.2°; Tables 1 and 2).
Seven of 131 segments (5%) were hypermobile preoperatively (mean: 18.5° ± 0.9°). Of these, 4 segments (57%) yielded physiological motion (10.9° ± 1.4°) 2 years after CDA, 2 remained hypermobile, and 1 was hypomobile postoperatively (Tables 1 and 2).
χ2 analysis of the implant counts showed that preoperative mobility did not have a significant influence on the proportions of implanted segments in the postoperative physiological motion range (P = 0.09). The group of patients with preoperative hypermobility was small, with n = 7. A nonsignificant P value in the χ2 analysis would indicate that there is no dependence of physiological ROM outcome on the preoperative motion group (hypo, physiological, or hyper). However, because the P value of 0.09 is borderline, the statistical power of this statement is questionable.
As a potential solution to account for such a data limitation, we combined hypo- and hypermobile groups into the nonphysiological group when calculating the restoration rate. This resulted in a 2 × 2 mobility matrix (Table 3). The χ2 analysis showed a nonsignificant P value of 0.17, indicating that the postoperative frequencies of physiological motion were not significantly influenced by the preoperative motion groups (physiological vs nonphysiological).
Influence of preoperative ROM on likelihood of achieving postoperative physiological mobility: (2 × 2) mobility matrix. Implant count and likelihood (%) of physiological motion preservation vs restoration.
Motion Restoration vs Motion Preservation
Preoperatively, there were 35 segments that were hypomobile, of which 26 gained physiological mobility, and 7 segments that were hypermobile, of which 4 gained physiological mobility after cervical disc replacement. To account for the small number of preoperatively hypermobile patients, the physiological motion restoration rate was calculated as how many of the preoperatively hypomobile plus hypermobile segments (35 + 7), termed the nonphysiological motion group, became physiologically mobile postoperatively (26 + 4).
Physiological motion restoration rate = (26 + 4)/(35 + 7) = 0.71 = 71%.
Physiological motion preservation was calculated as how many of the preoperative levels that were physiologically mobile (89) retained physiological mobility (73) 2 years after the disc replacement surgery.
Physiological motion preservation rate = 73/89 = 0.82 = 82%.
Discussion
A previous study investigated the likelihood of cervical artificial discs in achieving post-CDA ROM in the physiological range, defined as between 5° and 16°.15 Those authors performed a secondary analysis of radiographic post-CDA FE-ROM data from 7 IDE trials and found 62% to 79% of implanted segments achieved physiological mobility at 2 years, depending on the prosthesis design. In the current study, we delve deeper into the capability of a disc prosthesis to restore physiological motion when, preoperatively, the patient has nonphysiological mobility (hypomobility or hypermobility), and its capability to preserve physiological mobility when the preoperative ROM falls in the physiological range. These are 2 distinct mechanisms that combine to provide physiological mobility after CDA.
The lower physiological ROM bound of 5° came from the disc arthroplasty literature, both cervical and lumbar, that separated patients who went on to develop ASD compared from those who did not. The upper bound of physiological ROM was derived from cadaveric specimens with mild-to-moderate degeneration, not in vivo spines. This was done for 2 reasons: first, the in vivo data reported in the literature come from a variety of sources, not all with comparable reliability and accuracy. The cadaveric data from our laboratory have been collected over the years with identical, validated methodology (including load limits) with proven reliability and accuracy that we could trust; second, the in vivo data likely come from patients with unknown degenerative status. On the other hand, all laboratory data used to come up with the ROM upper boundary came from specimens whose magnetic resonance imaging degenerative status was in the narrow “mild-to-moderate” range, and considered practically equivalent to healthy or asymptomatic in vivo spines.
The vast majority of patients in this and other clinical trials have the index levels at C5 to C6 and C6 to C7 (51% and 40%, respectively, in the M6-C IDE trial), with C4 to C5 and C3 to C4 combined making up the remaining index level percentages. In a study comparing 7 IDE trials of cervical disc prostheses,15 the disc replacement surgery occurred at C3 to C4, C4 to C5, C5 to C6, and C6 to C7 segments on average at 2%, 8%, 53%, and 37% of the times, respectively. The ROM values reported in in vivo studies22 and in published cadaveric studies21 differ very little between C4 to C5, C5 to C6, and C6 to C7, and therefore, to keep the analysis simple, we chose to use the same definition of physiological ROM across all index levels commonly encountered in CDA.
The primary goals of the study presented in this manuscript were (a) to present a framework of preoperative-vs-postoperative mobility matrix that allows us to make a distinction between physiological motion preservation vs restoration, adding granularity to the concept of likelihood of physiological motion outcomes of CDA reported by Patwardhan et al15; and (b) to demonstrate the application of this secondary analysis and its interpretation using prospectively collected radiographic FE-ROM data on each individual patient.
The dataset that was available to the authors at the time of this writing was the 2-year follow-up data on patients enrolled in the M6-C IDE study. Naturally, we do not intend to draw any conclusions regarding the long-term biomechanical or clinical behavior of this implant.
Additionally, the goal of this study was not to advocate for a particular implant (M6-C, for example) but to demonstrate the concept of preoperative-vs-postoperative mobility matrix (3 × 3 or 2 × 2). To demonstrate the methodology, we used preoperative and post-CDA radiographic data from a cohort of patients who participated in the IDE clinical trial of single-level cervical disc replacement using the M6-C disc prosthesis. This particular prosthesis was chosen for demonstration purposes due to the availability of individual patient ROM data. It should be noted that the results of this motion analysis are valid only for the design of the artificial disc whose pre- and postoperative radiographic data were analyzed in this study and may not be generalized to other prosthesis designs.
In the present study, we introduced the concept of “physiological motion restoration” vs “physiological motion preservation.” The M6-C disc prosthesis demonstrated the ability to restore physiological motion (ROM between 5° and 16°) to 71% of preoperatively hypomobile (ROM < 5°) plus hypermobile (ROM > 16°) segments, while preserving physiological mobility in 82% of segments with preoperative ROM in the 5° to 16° range. The combined result was that M6-C had a 79% likelihood of achieving physiological mobility at 2 years post-CDA (Tables 1 and 3), a result that matches the previously published work.15 Knowing the likelihood of good postoperative motion outcomes for a range of preoperative mobilities and for all available implant designs can help in guiding patient expectations about disc replacement surgery, as well as help surgeons to choose individual implant designs that may better address a specific patient’s motion requirements.
The group of patients with preoperative hypermobility was small, with n = 7, resulting in a borderline P value of 0.09. This is a limitation associated with the patients enrolled in the IDE study of the M6-C disc (there were not many preoperatively hypermobile patients in the study). As a potential solution to account for such a limitation, we combined hypo- and hypermobile groups in calculating the restoration rate. This resulted in a 2 × 2 mobility matrix, and the χ2 analysis showed a nonsignificant P value of 0.17, indicating that the postoperative frequencies of physiological motion in this dataset were not significantly influenced by the physiological vs nonphysiological preoperative motion characteristics.
Finally, the impact of postoperative ROM after cervical artificial disc replacement on patient-reported clinical outcomes is yet to be conclusively demonstrated using long-term clinical evidence. While clinical outcomes take priority over post-CDA mobility, we argue that maintaining physiological motion should also be an important functional goal of artificial disc replacement in order to mitigate adjacent segment pathology that is so often associated with spinal fusion.
Conclusions
The ability to restore and preserve physiological motion represents 2 mechanisms that together contribute to the likelihood of a disc prosthesis achieving physiological motion after CDA. Knowing the likelihood of achieving an ideal postoperative motion outcome for a range of preoperative mobilities can help in guiding patient expectations about disc replacement surgery.
The goal of this study was not to advocate for a particular implant (M6-C) but to demonstrate the concept of mobility matrix using an available dataset. The goal of our future work will be to report on long-term radiographic results of all cervical disc prosthesis designs that have been FDA-approved to potentially give clinicians additional information to use during implant selection.
Footnotes
Funding The authors received no financial support for the research, authorship, and/or publication of this article.
Declaration of Conflicting Interests The authors report no conflicts of interest in this work.
Disclosures Avinash Patwardhan reports stock options from 3Spine. Alexander Satin reports grants/contracts from the Cervical Spine Research Society and Kuros (both paid to the institution); royalties or licenses from Degen Medical and Circinus; consulting fees from Degen and Orthofix; payment or honoraria from Kuros, Highridge, and Centinel; payment for expert testimony from the US Attorney’s Office; support for attending meetings or travel from Degen; serving on committees for the CSRS and NASS; and stock or stock options from AGADA and Circinus. Jack Zigler reports consulting fees from Centinel Spine and Highridge (hourly compensation for teaching and consulting). The remaining authors have nothing to disclose.
- This manuscript is generously published free of charge by ISASS, the International Society for the Advancement of Spine Surgery. Copyright © 2026 ISASS. To see more or order reprints or permissions, see http://ijssurgery.com.
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