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 Table of Contents  
ORIGINAL ARTICLE
Year : 2018  |  Volume : 4  |  Issue : 1  |  Page : 27-34

Effect of a semiconstrained elastic integrated cervical artificial disc on the cervical motion


1 Department of Orthopaedic Surgery, Institute of Digital Medline, Nanjing First Hospital, Nanjing Medical University, Nanjing, Jiangsu Province, China; Department of Orthopaedic Surgery, David Geffen School of Medicine at University of California, Los Angeles, CA, USA
2 Department of Orthopaedic Surgery, Institute of Digital Medline, Nanjing First Hospital, Nanjing Medical University, Nanjing, Jiangsu Province, China
3 Department of Orthopaedic Surgery, David Geffen School of Medicine at University of California, Los Angeles, CA, USA
4 Keck Medical Center, University of Southern California, Los Angeles, CA, USA

Date of Web Publication18-May-2018

Correspondence Address:
Liming Wang
Department of Orthopaedic Surgery, Nanjing Medical University Nanjing Hospital, Nanjing Medical University, Nanjing 210006, Jiangsu Province
China
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/digm.digm_1_18

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  Abstract 


Background and Objectives: Cervical total disc replacement (TDR) is a novel dynamically stabilizing technique for the symptomatic cervical intervertebral segment. While the long-term effect of mainstream cervical nonconstrained artificial disc group (CNAD) does not match the theoretical effects of mobility preserving and neural decompression. The cervical semiconstrained elastic integrated artificial disc (CSID) may be a more reasonable design. However, beneficial or adverse effects of this design have not been measured and data for biomechanical effect are unavailable. The aim of this study is to assess the biomechanical effect of CSID on the segmental motion at implanted and adjacent levels. Methods: This study was supported by medical science developmental funding of Nanjing (20,000 dollars). Eight cadaveric C3–T1 specimens were loaded in flexion/extension (F/E), axial rotation (AR), and lateral bending (LB) with CSID, CNAD, and anterior fusion (AF) implanted at C5–C6 level alternatively. The range of motion (ROM), neutral zone (NZ), and elastic zone (EZ) at implanted and adjacent levels were measured. The mean values of parameters in the intact specimen group (INT), CSID group, CNAD group, and AF group were compared statistically (n = 8). Results: There was no significant difference of ROM, NZ, and EZ at implanted and adjacent levels between CSID and INT in F/E, AR, and LB (P > 0.05). CNAD caused a significant change of EZ in F/E and LB and ROM in LB at implanted level. Meantime, CNAD caused ROM increasing at adjacent levels (P < 0.05). AF caused the most significant changes of ROM, NZ, and EZ in F/E, AR and LB, compared to CSID and CNAD (P < 0.05). Conclusions: The semiconstrained elastic integrated design of cervical artificial disc may mimic of physiological disc's biomechanical effects on segmental kinematics at implanted and adjacent levels more closely, compared to nonconstrained discs and AF. CSID disc may reduce the acceleration of postTDR degeneration at the implanted and adjacent levels due to this promoted biomechanical performance. CSID disc could be a potential candidate for future cervical artificial intervertebral prosthesis studies.

Keywords: Artificial intervertebral disc, biomechanics, cervical total disc replacement, nonconstrained disc, segmental kinematics, semiconstrained artificial disc


How to cite this article:
Yao Q, Zhou Z, Li J, Shamie AN, Alshuaib YW, Chen J, Burser Z, Wang JC, Wang L. Effect of a semiconstrained elastic integrated cervical artificial disc on the cervical motion. Digit Med 2018;4:27-34

How to cite this URL:
Yao Q, Zhou Z, Li J, Shamie AN, Alshuaib YW, Chen J, Burser Z, Wang JC, Wang L. Effect of a semiconstrained elastic integrated cervical artificial disc on the cervical motion. Digit Med [serial online] 2018 [cited 2018 Sep 19];4:27-34. Available from: http://www.digitmedicine.com/text.asp?2018/4/1/27/232714




  Introduction Top


Cervical spinal degeneration is one of the most common diseases associated with neck pain, radiculopathy, and/or myelopathy.[1] Currently, treatment options for patients with symptomatic cervical spinal degeneration range from conservative management (medication, physical therapy, and bracing) to surgical intervention (decompression with or without fusion and instrumentation).[2] If conservative management for symptomatic cervical degeneration fails, operative intervention becomes the main treatment choice through nucleus gelatinous removal, decompression by discectomy and/or laminectomy, bone grafting, and internal fixation.[3] Anterior cervical discectomy and fusion (ACDF) is considered as a “well-accepted surgical option” as it promotes the release of neural elements, corrects local kyphosis, and decompresses the nerve root impinged by collapsed disc.[4],[5] The segmental mobility loss caused by ACDF has been shown to induce an adjacent-level effect, including hypermobility, and an increased stress on the adjacent segments.[6],[7] This adjacent-level effect has been shown to result in greater adjacent-level degeneration and recurrent radicular symptoms in about 25% of patients.[8],[9],[10],[11]

As a novel nonfusion alternative to ACDF, the cervical artificial disc has gathered increased interest for its theoretical effects of mobility preservation and neural decompression.[3],[12],[13],[14] An effective artificial disc should restore the normal segmental kinematics to reduce adjacent-level degeneration along with providing a comparable safety and efficacy to ACDF [15],[16],[17] and to reduce complications related to fusion,[18] such as pseudarthrosis and donor site morbidity.[2],[3],[19]

Cervical total disc replacement (TDR) has become increasingly common with various designs of artificial discs such as a substitute for the primary ball-shape design, a polyethylene inlay that may be fixed to the caudal endplate (Prodisc-L™, Synthes; Paoli, Pennsylvania), designs that include a ball that may be mobile in anterior/posterior direction (Activ-L™, B. BraunAesculap; Tuttlingen, Germany), or those mobile in a plane parallel to the endplate (Mobidisc™, LDR Me'dical; Troyes, France).[14],[20],[21] However, current literatures have shown that the long-term effect of index-level mobility failed to meet the hypothetical effects.[4],[7],[8],[22] Similarly, the direct link of the adjacent-level degeneration progression to TDR have been found, suggesting that these artificial disc designs need to more closely mimic physiological disc.[23],[24],[25]

We hypothesized that the design of artificial disc needs to be improved to match the physiological disc biomechanical characterization as a fiber-cartilage connection rather than a synovial joint. A newly developed cervical semiconstrained elastic integrated artificial disc (CSID) may be a reasonable choice in theory, which uses an elastic fiber structure to mimic the fiber-connecting property of the physiologic disc, titanium-alloy endplates with motion-constraining pillars to limit over-range motion, and a saddle-shaped PEEK core to simulate the nucleus pulposus [Figure 1].
Figure 1: The design of cervical semiconstrained elastic integrated artificial disc device cervical semiconstrained elastic integrated artificial disc was designed as an integrated prosthesis composed of endplates, ligaments that simulate the annulus fibrosus and a core that simulates the nucleus pulposus. (a) The design of endplates and restriction pillars by computer-aided design; (b) The schematic of cervical semiconstrained elastic integrated artificial disc structure by computer-aided design; (c) The transverse section view of cervical semiconstrained elastic integrated artificial disc with core, annulus ligaments, and endplates by three-dimensional computer-aided design; (d) The anterior three-dimensional computer-aided design view of cervical semiconstrained elastic integrated artificial disc without ligaments (cervical nonconstrained artificial disc group); (e) The anterior-posterior view of cervical semiconstrained elastic integrated artificial disc fibers; (f) The cross-section view of cervical semiconstrained elastic integrated artificial disc (g) The oblique view of cervical semiconstrained elastic integrated artificial disc (h) The anterior-posterior view of cervical semiconstrained elastic integrated artificial disc

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The intent of the current study is to understand changes in the segmental three-dimensional (3D) “load and motion” biomechanical characterization and segmental stiffness that occur after placement of the implant. Our previous finite element (FE) study [26] revealed that compared to nonconstrained artificial disc, the semiconstrained elastic design might be a more reasonable design due to the protection of segmental zygapophyseal joints in the lumbar spine. In this study, we compared the effects of the physiologic disc, CSID, cervical nonconstrained artificial disc group (CNAD), and anterior fusion (AF) on the implanted and adjacent segmental kinematics to evaluate the rationality of the CSID design.


  Methods Top


Eight fresh-frozen human cadaver cervical spine specimens C3–T1, with a median age of 54 years (range 51–60) were tested. Radiographs were taken before preparation to exclude damage, cervical diseases, and severe cervical degeneration. The specimens were freshly sealed and stored at −28°C until thawed for testing. Polymethylmethacrylate (Vertex Self-curing; Hj Zeist, The Netherlands) was used to embed half of each specimen's cranial (C3) and caudal (T1) vertebral body.

Polyethylene sheets were used to wrap these specimens separately, which may keep specimens hydrated during the experiment. Then, the specimens were thawed at room temperature (22°C) and a computer-controlled electronic three-dimensional motion testing machine KD-101 (Shanghai Institute of biomechanical engineering, Shanghai University, China), which could mimic the cervical physiological load and motion, was chosen to measure these specimens.[27]

Before the test, the three-dimensional XYZ axial coordinate was established. Then, each specimen was fixed onto the KD-101 foundation bed with the C3 upper endplate connected to the load-bearing plate. A 10 min and 50 N compressive force following the direction of gravity line in neutral position was applied on each specimen to precondition these specimens and reduce creep effects.[8],[27] This was done once for each specimen.

First, specimens were tested nondestructively in the intact condition using a 1.5 Nm torque, which could apply the cervical load and motion without the injury of these specimens.[15],[16],[27] Pure movement of 1.5 Nm maximum was applied sequentially about the three primary anatomical axes to induce flexion/extension (F/E), axial rotation (AR), and lateral bending (LB). After the pure motion applied, a displacement-controlled load was performed on each specimen by a 70 N load at the midsagittal plane. The 1.5 Nm torque plus the load were applied gradually to induce F/E, AR, and LB until each test reached the maximum C3–T1 angle (sum of C3–C4, C4–C5, C5–C6, C6–C7, and C7–T1 angles).

The three-dimensional cervical load-motion performance at C4–C5, C5–C6, and C6–C7 levels of these eight specimens were measured with a range of motion (ROM), neutral zone (NZ), and elastic zone (EZ) recorded. In each test, the 1.5 Nm torque was repeatedly applied three times to reduce the viscoelasticity. The frequency of each load/unload circle was 30 s to eliminate creep deformation. During all tests, three-dimensional motion was recorded and analyzed using the image camera analysis system (Optical Fringe Pattern Analysis System, Shanghai University, China). Marker coordinates were converted to the angles of each anatomical axis.

Then CSID and CNAD prosthesis were implanted into the C5–C6 level in sequence after discectomy performed, and the same load and torque were applied to each specimen in F/E, AR, LB position with ROM, NZ, and EZ recorded. The available CSID and CNAD prosthesis sizes were 4 mm–7 mm in heights and 1, 14, and 16 mm in widths. The size chosen was based on the anatomy of these specimens, in which one specimen received 4 mm × 12 mm prosthesis, three specimens received 5 mm × 14 mm prosthesis, three specimens received 6 mm × 14 mm prosthesis, and one specimen received 7 mm × 16 mm prosthesis [Figure 2].
Figure 2: The implant of cervical semiconstrained elastic integrated artificial disc (a) the picture of the cervical semiconstrained elastic integrated artificial disc implanted specimen; (b) the anterior-posterior X-ray view of the cervical semiconstrained elastic integrated artificial disc implanted specimen

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After artificial disc removed, AF was performed using a wedge graft (PEEK Interbody, Wego, China) and an anterior locking plate (Cervi-Lock, Wego, China) with fixed screws of each specimen. The heights of wedge grafts and anterior plates were determined by the heights of the disc prosthesis. The same load and torque were performed to AF specimens with same parameters recorded.

All implants were inserted into each specimen by the manufacturer's recommended tools and procedures with consistent marker calibration, and position checking were performed using lateral fluoroscopy as would be done clinically. In this study, ROM was determined in each plane as the angle during maximum load in the positive-loading direction relative to angle at maximum load in the negative direction, NZ is the range of cervical motion from zero load position to the neutral position, and EZ is the range of cervical motion from zero load to maximum spinal loads.[16],[27] From the raw data, the rate of ROM (RR), a representation of the segmental stiffness, was calculated and recorded. RR is the ratio of Rf and Ri, in which Rf represents the mean value of implanted specimen's ROM, and Ri represents the mean value of intact specimen's ROM (RR = Rf/Ri*100%).

All the data were obtained and recorded, a total of 384 measurements data were analyzed using SPSS 13.0 software (SPSS Inc, Chicago, IL, US). The one-way analysis of variance (ANOVA) was performed after error analysis be tested to get a satisfactory valuation and setting rang. All data were made using individually paired t-tests and Fisher's exact test analysis, each with 0.05 considered as the level of statistical significance.


  Results Top


Three-dimensional motion of implanted level

There was no significant difference between ROM in F/E and AR position between CNAD and CSID. Significant differences between CNAD and CSID were observed in EZ at F/E (1.64 ± 0.89, 3.56 ± 1.28, and P < 0.05), AR (3.32 ± 1.61, 5.42 ± 1.63, and P < 0.05), and LB (5.18 ± 1.56, 2.53 ± 1.71, and P < 0.05). The ROM in LB using CNAD was also apparently higher than using CSID (ROM 13.42 ± 1.86, 9.25 ± 1.94, P < 0.05, and EZ), but there were no apparent difference of NZ in LB between intact specimen, using CNAD, and CSID (7.28 ± 1.27, 8.24 ± 1.10, 7.62 ± 1.58, and P > 0.05).

There were no apparent difference of ROM between intact specimens and CSID implanted specimens in three-dimensional kinematics (F/E 14.12 ± 1.61, 12.78 ± 1.32, P > 0.05, AR 25.60 ± 2.70, 22.87 ± 2.42, P > 0.05, LB 10.18 ± 1.12, 9.25 ± 1.94, and P > 0.05). Similarly, NZ and EZ were not apparently changed using CSID than intact specimens.

The most significant reduction of ROM and NZ were observed using AF in segmental kinematics. Compared to physiological disc, C5–C6 level RR using CSID and CNAD in F/E and AR were slightly decreased, but RR was decreased significantly using AF in F/E, AR, and LB [Table 1], [Figure 3].
Table 1: Mean range of motion, neutral zone, elastic zone at the C4-5 implanted level and adjacent levels for intact, cervical semiconstrained elastic integrated artificial disc, cervical non-constrained artificial disc, and anterior fusion specimens in flexion/extension, axial rotation, and lateral bending

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Figure 3: Average range of motion and standard deviation of implanted segment in flexion/extension, axial compression, and lateral bending. The range of motion in lateral bending using cervical nonconstrained artificial disc group was also apparently higher than using cervical semiconstrained elastic integrated artificial disc. Significant differences between cervical nonconstrained artificial disc group and cervical semiconstrained elastic integrated artificial disc were also observed of elastic zone in three-dimensional motion. The average range of motion were smallest using anterior fusion than intact group, cervical semiconstrained elastic integrated artificial disc, or cervical nonconstrained artificial disc group implanted in flexion/extension, axial compression, and lateral bending

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Three-dimensional motion of adjacent levels

Compared to physiological disc, ROM at C4–C5 and C6–C7 levels after CSID inserted were slight increased, but there were no statistical differences among them. Using CNAD, ROM was significantly increased in adjacent levels than intact specimens in three-dimensional motion [Table 1], [Figure 4]a. EZ of C4–C5 and C6–C7 levels in AR were lower using CNAD than CSID (C4–C5 3.02 ± 1.76, 6.17 ± 1.48, P < 0.05, C6–C7 3.05 ± 1.87, 5.66 ± 1.71, and P < 0.05) [Table 1], [Figure 4]b. The most apparent increasing of ROM, NZ, and EZ were observed after AF in three-dimensional motion. The RR of adjacent levels after TDR were slightly decreased than physiological disc, and the most significant change of RR at adjacent levels were observed using AF in three-dimensional motion.
Figure 4: Average range of motion, elastic zone, and standard deviation of adjacent levels in flexion/extension, axial compression, and lateral bending. Using cervical nonconstrained artificial disc group and range of motion were significantly increased in adjacent levels than intact specimens in three-dimensional motion. Elastic zone of adjacent levels in axial rotation were lower using cervical nonconstrained artificial disc group than cervical semiconstrained elastic integrated artificial disc. The average range of motion and elastic zone were greatest using anterior fusion than intact group, using cervical semiconstrained elastic integrated artificial disc, and using cervical nonconstrained artificial disc group in three-dimensional motion

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  Discussion Top


ACDF has been a traditional treatment for cervical disc degeneration disease with apparent clinical symptoms. By maintaining mobility and preserving biomechanics postoperation, cervical TDR devices are believed to slow degeneration and the associated symptoms compared to ACDF.[3],[4],[5],[6],[28] Over the past decade, a growing body of Level 1 evidence has revealed the potential advantage of single-level TDR as an alternative to ACDF.[22],[23],[24],[25],[26],[28] Various artificial disc prosthesis have been designed and commercialized, including Bryan (Medtronic Sofamor Danek, Memphis, TN, USA),[20] Mobi-C (LDR Medical, Troyes, France),[29] and ProDisc-C (SynthesInc, West Chester, PA),[14],[28] which are metal–polythene-mental prosthesis that have undergone clinical experiments and registration in the US; Prestige (Medtronic SofamorDanek, Memphis, TN, USA)[28] and CerviCore (Stryker Spine, Allendale, NJ),[23] which are metal–metal (Co–Cr) interface prosthesis; porous coated motion,[30] which has a metal (Co–Cr)-UHMWPE structure.

Artificial discs are currently classified according to structure into three-component prosthesis, two-component-prosthesis, and integrated prosthesis. For instance, the components of the two-component prosthesis (Prodisc series prosthesis) perform the loading function of the upper and lower soleplates, respectively. Three-component prosthesis (Mobi-C series prosthesis) consists of upper and lower soleplates and the elastic core that form the articular facet of the prosthesis after composition. In the short-term clinical research, TDR has revealed significant improvement compared to ACDF. Davis et al.,[29] evaluated the safety and effectiveness of two-Level Mobi-C TDR compared to two-Level ACDF and found that at the primary end point of 24 months, the overall study success rates were 69.7% and 37.4% for the TDR and ACDF groups, respectively. Although the intermediate follow-up of cervical arthroplasty showed good clinical outcomes, Park et al.[31] found that there was a trend toward reduction in alignment and motion with 94.1% of the overall heterotopic ossification (HO) occurrence at 24 months. Jin et al.[32] found that both Type 1 and Type 2 HOs are related to biomechanical stress (compressive force for Type 1 HO and traction force for Type 2 HO). In addition, Chen et al.,[33] setup a meta-analysis of single-level cervical TDR based on the reports collected from PubMed, Medline, Cochrane library, and found that adjacent segment degeneration provided by TDR might not be as good as has been believed. Integrated prosthesis (Prestige series prosthesis) is most designed as the constrained structure with the restriction of ROM in the range of couple. However, both the cranial and caudal types of these ball-and-socket designs failed to fully restore the normal mobility regarding ROM.[34]

The mechanism of the unsatisfactory long-term outcomes, including metal endplate-endplate impingement, secondary degeneration, and/or HO at the implanted and adjacent levels, might be caused by the ball-socket design. The ball-socket design cannot match the stretching-constraining function and has fewer load-buffering function compared to the physiological disc in motion.[35],[36],[37] Therefore, the newly developed disc needs to match the disc fiber-connecting function instead of a movable vertebral synovial joint.

In this study, we revealed that the ROM at implanted level using CSID or CNAD were close in most positions, which indicates that the CSID has similar physical properties to CNAD due to the same design of saddle-shaped joint and endplate pillars, while the difference of ROM and EZ in LB between CSID and CNAD may be caused by the fiber-connecting structure in CSID. The difference of effect on the cervical motion at implanted and adjacent levels between the two discs showed that compared to CSID, using CNAD might cause more inserted segmental instability and adjacent-level fatigue.

The difference of RR in CSID, CNAD, and AF showed that semiconstrained fibers perform the major function of stretching-constraining in segmental motion. Similarly, the NZ and EZ at inserted and adjacent levels using the CSID were more closely to intact than CNAD and AF, which shows that CSID provides a significant improvement of physiological disc simulation. The current study was performed under load control based on the assumption that patients tend to apply a constant load and movement. As a result of this load-moving modality, the magnitude of the three-dimensional motions measured was similar to those reported by others.[1],[4],[13],[30]

In this study, the biomechanical difference between CSID and CNAD was not symmetric in three-dimensional motion, especially in AR and LB. We believe this difference may be caused by the direction and location of the fibers in the CSID. In AR, all the fibers counteract the force of rotation, but in unilateral LB, only the fibers on the convex side counteract the bending force. This made the CSID more resistant to rotational forces than CNAD. Furthermore, the difference of RR might be caused by the coeffect of saddle-shaped joint articulation geometry, nonelastic restriction pillars, and elastic fibers. This study did not take into consideration the effects on the anterior and posterior longitudinal ligament, which contribute to the stability of the spine. All working conditions were simplified as in the disc replacement operation.

Based on the biomechanical analysis, compared to CNAD and AF, CSID may provide a superior biomechanical milieu on cervical kinematics, which may reduce the acceleration of post-TDR degeneration at implanted and adjacent levels. CSID disc could be a candidate for a cervical artificial intervertebral prosthesis. Meantime further clinical investigation between the CSID and commercial available nonconstrained designs is needed to determine its clinical efficacy along with providing a valuable contribution to current artificial disc research.


  Conclusions Top


The semiconstrained elastic integrated design of cervical artificial disc may mimic of physiological disc's biomechanical effects on segmental kinematics at implanted and adjacent levels more closely, thus reducing the acceleration of postTDR degeneration at the implanted and adjacent levels due to this promoted biomechanical performance. CSID disc could be a potential prosthesis for cervical surgeries.

Acknowledgment

The authors would like to thank Dr. Yijin Wang, Shanghai University, Shanghai, China for his assistance with biomechanical measurement. The authors also would like to thank Dr. Lei Jiang, Wego Orthopedic Device CO., LTD, Weihai, Shangdong, China for his assistance with CSID and CNAD sample manufacture. Support was provided by Key Research Program of Science and Technology Support Program of Jiangsu Province (BE 2015613, BE 2016763).

Financial support and sponsorship

This study was supported by Key Research Program of Science & Technology Support Program of Jiangsu Province (BE 2015613, BE 2016763).

Conflicts of interest

There are no conflicts of interest.



 
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