Biomechanical Analysis of a Novel Prosthesis Based on the Physiological Curvature of Endplate for Cervical Disc Replacement

Biomechanical properties of a novel cervical spine implant with elastic deformation: a cadaveric study

Introduction: Anterior cervical discectomy and fusion (ACDF) is a most frequently used surgical procedure for treating cervical radiculopathy and myelopathy. However, there is concern about the high adjacent segment degeneration (ASD) rate after ACDF surgery. We creatively designed an elastically deformable cervical implant to reduce the postoperative stress concentration. In this study, we aimed to investigate the biomechanical performance of this novel cervical implant and compare it with the commonly used cervical devices. Methods: Biomechanical test was conducted on twelve fresh-frozen human cadaveric cervical spines (C2-C7) and randomly divided into four groups according to implant types: intact group, Zero-P VA fusion (ACDF) group, the novel cervical implant group and Pretic-I artificial cervical disc (ACDR) group. An optical tracking system was used to evaluate the segmental range of motion (ROM) of the C4/C5, C5/C6, and C6/C7 segments and micro pressure sensor was used to record the maximum facet joint pressure (FJP), maximum intradiscal pressure (IDP) at the C4-5 and C6-7 segments. Results: There were no significant differences in the ROM of adjacent segments between the groups. Compared with the intact group, the ACDR group essentially retained the ROM of the operated segment. The novel cervical implant decrease some ROM of the operated segment, but it was still significantly higher than in the fusion group; The maximum FJP and IDP at the adjacent segments in the ACDF group were significantly higher than those values in the other groups, and there were no differences in the other groups. Conclusion: While the newly developed elastically deformable cervical implant does not completely maintain ROM like the artificial cervical disc, it surpasses the fusion device with regards to biomechanical attributes. After further refinement, this novel implant may be suitable for patients who are prone to severe adjacent segment degeneration after fusion surgery but no indication for artificial cervical disc surgery.

Biomechanical evaluation of a novel biomimetic artificial intervertebral disc in canine cervical cadaveric spines

Background Context

Cervical disc replacement (CDR) aims to restore motion of the treated level to reduce the risk of adjacent segment disease (ASD) compared with spinal fusion. However, first-generation articulating devices are unable to mimic the complex deformation kinematics of a natural disc. Thus, a biomimetic artificial intervertebral CDR (bioAID), containing a hydroxyethylmethacrylate (HEMA)-sodium methacrylate (NaMA) hydrogel core representing the nucleus pulposus, an ultra-high-molecular-weight-polyethylene fiber jacket as annulus fibrosus, and titanium endplates with pins for primary mechanical fixation, was developed.

Purpose

To assess the initial biomechanical effect of the bioAID on the kinematic behavior of the canine spine, an ex vivo biomechanical study in 6-degrees-of-freedom was performed.

Study Design

A canine cadaveric biomechanical study.

Methods

Six cadaveric canine specimens (C3-C6) were tested in flexion-extension (FE), lateral bending (LB) axial rotation (AR) using a spine tester in three conditions: intact, after C4-C5 disc replacement with bioAID, and after C4-C5 interbody fusion. A hybrid protocol was used where first the intact spines were subjected to a pure moment of ±1 Nm, whereafter the treated spines were subjected to the full range of motion (ROM) of the intact condition. 3D segmental motions at all levels were measured while recording the reaction torsion. Biomechanical parameters studied included ROM, neutral zone (NZ), and intradiscal pressure (IDP) at the adjacent cranial level (C3-C4).

Results

The bioAID retained the sigmoid shape of the moment-rotation curves with a NZ similar to the intact condition in LB and FE. Additionally, the normalized ROMs at the bioAID-treated level were statistically equivalent to intact during FE and AR while slightly decreased in LB. At the two adjacent levels, ROMs showed similar values for the intact compared to the bioAID for FE and AR and an increase in LB. In contrast, levels adjacent to the fused segment showed an increased motion in FE and LB as compensation for the loss of motion at the treated level. The IDP at the adjacent C3-C4 level after implantation of bioAID was close to intact values. After fusion, increased IDP was found compared with intact but did not reach statistical significance.

Conclusion

This study indicates that the bioAID can mimic the kinematic behavior of the replaced intervertebral disc and preserves that for the adjacent levels better than fusion. As a result, CDR using the novel bioAID is a promising alternative treatment for replacing severely degenerated intervertebral discs.

An update on animal models of intervertebral disc degeneration and low back pain: Exploring the potential of artificial intelligence to improve research analysis and development of prospective therapeutics

Animal models have been invaluable in the identification of molecular events occurring in and contributing to intervertebral disc (IVD) degeneration and important therapeutic targets have been identified. Some outstanding animal models (murine, ovine, chondrodystrophoid canine) have been identified with their own strengths and weaknesses. The llama/alpaca, horse and kangaroo have emerged as new large species for IVD studies, and only time will tell if they will surpass the utility of existing models. The complexity of IVD degeneration poses difficulties in the selection of the most appropriate molecular target of many potential candidates, to focus on in the formulation of strategies to effect disc repair and regeneration. It may well be that many therapeutic objectives should be targeted simultaneously to effect a favorable outcome in human IVD degeneration. Use of animal models in isolation will not allow resolution of this complex issue and a paradigm shift and adoption of new methodologies is required to provide the next step forward in the determination of an effective repairative strategy for the IVD. AI has improved the accuracy and assessment of spinal imaging supporting clinical diagnostics and research efforts to better understand IVD degeneration and its treatment. Implementation of AI in the evaluation of histology data has improved the usefulness of a popular murine IVD model and could also be used in an ovine histopathological grading scheme that has been used to quantify degenerative IVD changes and stem cell mediated regeneration. These models are also attractive candidates for the evaluation of novel anti-oxidant compounds that counter inflammatory conditions in degenerate IVDs and promote IVD regeneration. Some of these compounds also have pain-relieving properties. AI has facilitated development of facial recognition pain assessment in animal IVD models offering the possibility of correlating the potential pain alleviating properties of some of these compounds with IVD regeneration.

Motion capture evaluation of sagittal spino-pelvic biomechanics after lumbar spinal fusion

PURPOSE

The spine and pelvis coexist as a dynamic linked system in which spinal and pelvic parameters are correlated. Investigation of this system can inform the understanding and treatment of spinal deformity. Here, we demonstrate the use of motion capture technology to measure spine biomechanical parameters using a novel testing apparatus.

METHODS

Three complete cadaveric spines with skull and pelvis were mounted into a biomechanical testing apparatus. Each lumbar vertebra was monitored by motion capture cameras as the spines underwent maximal anterior and posterior pelvic tilts about two sagittal axes at a controlled speed and applied force. These axes were defined as the sacral axis which passes transversely through the ilium and S1, and the acetabular axis which passes transversely through both acetabula. The experiments were repeated after L4-L5 fusion, and then, after both L4-L5 and T12-S1 fusion with pedicle screw instrumentation. Data were collected for total range of motion and for coupled translation at each functional spinal unit (FSU).

RESULTS

Total range of motion and coupled translation within functional spinal units (FSUs) was decreased after spinal fusion. The displacement of each individual FSU was captured and summarized along with the observed patterns under each experimental condition.

CONCLUSION

Lumbar fusion decreases spinal motion in the sagittal plane in both overall ROM and individual coupled translations of lumbar vertebrae. This was demonstrated using motion capture technology which is useful for quantifying the translations of individual FSUs in a multisegmental spinal model.

In-vitro Biomechanics of the Cervical Spine: a Systematic Review

In-vitro testing has been conducted to provide a comprehensive understanding of the biomechanics of the cervical spine. This has allowed a characterization of the stability of the spine as influenced by the intrinsic properties of its tissue constituents and the severity of degeneration or injury. This also enables the pre-clinical estimation of spinal implant functionality and the success of operative procedures. The purpose of this review paper was to compile methodologies and results from various studies addressing spinal kinematics in pre- and post-operative conditions so that they could be compared. The reviewed literature was evaluated to provide suggestions for a better approach for future studies, to reduce the uncertainties and facilitate comparisons among various results. The overview is presented in a way to inform various disciplines, such as experimental testing, design development, and clinical treatment. The biomechanical characteristics of the cervical spine, mainly the segmental range of motion (ROM), intradiscal pressure (IDP), and facet joint load (FJL), have been assessed by testing functional spinal units (FSUs). The relative effects of pathologies including disc degeneration, muscle dysfunction, and ligamentous transection have been studied by imposing on the specimen complex load scenarios imitating physiological conditions. The biomechanical response is strongly influenced by specimen type, test condition, and the different types of implants utilized in the different experimental groups.

Spontaneous Fusion After Cervical Disc Arthroplasty: A Case Report and Literature Review

Objective

To report a rare case of spontaneous fusion (SF) following cervical disc arthroplasty (CDA), to review the related literature, and to propose a new measure to prevent it.

Methods

The course of a patient with SF is described here. The potential causes, risk factors, and preventive measure of SF after CDA published in previous studies have also been reviewed and discussed.

Results

A 63-year-old man presented with a 6-month history of progressive neck pain and developed left C-7 radiculopathy 4 years ago. Magnetic resonance imaging revealed disc herniation at the C6–C7 levels resulting in compression of the left C-7 nerve root. The patient underwent CDA at the C6–C7 levels, during which a PRESTIGE cervical disc device was implanted. He failed to follow-up regularly as recommended postoperatively because he was completely free from the pain in his neck and left upper limb. Four years later, he was readmitted with a 2-month history of occasional neck stiffness. Plain radiographs indicated complete radiographic fusion of the C6–C7 levels with trabecular bone bridging surrounding the cervical disc prosthesis, and dynamic imaging showed no motion. He was seen at regular follow-up visits for up to 60 months without special treatment, as his symptoms of neck stiffness were minor and his symptom has not worsened since then.

Conclusion

SF after CDA is a rare condition that can be attributed to patient- or prosthesis-related causes, and its risk factors are diverse. SF after CDA did not affect the patient’s clinical outcome, and no special treatment was required for it. Practitioners should be aware of this rare complication and advise patients of the risks before performing CDA.

Does preoperative inferior endplate morphology influence the outcomes of cervical disc arthroplasty at C5-C6 level: A retrospective study

OBJECTIVES

Despite encouraging clinical results, the postoperative segmental kyphosis was observed in many cervical disc arthroplasty (CDA) studies. Most studies discussed technical factors related to kyphosis. However, the cervical endplate, which contacted directly with prostheses and distributed the compressive load across the vertebral body, was seldom investigated. The purpose of the study was to investigate the effects of the inferior endplate morphology on clinical and radiological outcomes after C5/C6-level CDA with Prestige-LP Disc.

PATIENTS AND METHODS

One hundred and thirty-two patients with C5/C6-level CDA were retrospectively reviewed. Their preoperative inferior endplates on the mid-sagittal plane were visually classified into 3 types: type I with the endplate concavity apex located posteriorly, type II with the apex located in the middle and type III with the apex located anteriorly. Sagittal diameter of endplate, endplate concavity depth and endplate concavity apex location were measured. Japanese Orthopedic Association (JOA), visual analogue scale (VAS) and Neck Disability Index (NDI) scores were used to evaluate clinical outcomes. Range of motion (ROM) and sagittal alignment including C2-7 and C5-6 angle were assessed.

RESULTS

The type I, type II and type III endplates accounted for 25.00 %, 51.52 % and 23.48 % of 132 individuals. Most patients achieved significant reduction in VAS scores and NDI but the significant increase in JOA scores. No substantial differences in clinical outcomes were found among the 3 endplate types. Compared with preoperative values, the C5-6 ROM, C2-7 ROM and C2-7 angle were preserved, while the C5-6 angle increased significantly from kyphosis to lordosis. At the last follow-up, type I endplate had the least C5-6 angle (2.03°, 3.94° and 4.46° for type I, II and III endplates) and the highest incidence of segmental kyphosis at C5-C6 level (33.30 %, 14.70 % and 9.70 % in the same order, P = 0.028).

CONCLUSIONS

Patients achieved satisfactory clinical outcomes after C5/C6-level CDA without significant differences among three endplate types. Type I endplate had a high incidence of segmental kyphosis at C5-C6 level.

Biomechanical analysis of cervical range of motion and facet contact force after a novel artificial cervical disc replacement

An ideal implantation of artificial cervical disc replacement (ACDR) prosthesis should preserve the cervical biomechanics and maintain normal cervical biomechanics. In this study, we designed a novel prosthesis based on the physiological curvature of the cervical endplate, and conducted an in vitro test with cadaveric cervical specimens to study its effect on cervical range of motion (ROM) and facet contact force. Eighteen cadaveric cervical specimens (C2-C7) were evaluated with a 50 N follower load and a moment of ± 2.0 N-m. Testing conditions were as follows: control (group 1), C5-C6 ACDR with a novel prosthesis (group 2), C5-C6 ACDR with the Prestige LP prosthesis (group 3) and C5-C6 cervical discectomy and fusion (ACDF) with cage internal fixation (group 4). The range of motion (ROM) of all segments and facet joint contact force were measured and analyzed. The results showed that there was no significant difference among the group 1, 2 and 3 in terms of ROM (P>0.05). The ROM of C5-C6 in the group 4 showed a significant decrease compared with the group 1, 2, and 3 (P<0.05). The group 2 had a similar facet joint force with the group 1, and there is no statistical difference among the group 1, 2 and 3 (P>0.05). The facet joint force of C5-C6 in the group 4 reduced significantly compared with the other groups (P<0.05). In conclusion, both novel cervical prosthesis and Prestige LP could better preserve the normal cervical ROM and maintain facet joint force than ACDF.

In vitro biomechanical comparison after fixed- and mobile-core artificial cervical disc replacement versus fusion

In vitro biomechanical analysis after cervical disc replacement (CDR) with a novel artificial disc prosthesis (mobile core) was conducted and compared with the intact model, simulated fusion, and CDR with a fixed-core prosthesis. The purpose of this experimental study was to analyze the biomechanical changes after CDR with a novel prosthesis and the differences between fixed- and mobile-core prostheses.Six human cadaveric C2-C7 specimens were biomechanically tested sequentially in 4 different spinal models: intact specimens, simulated fusion, CDR with a fixed-core prosthesis (Discover, DePuy), and CDR with a mobile-core prosthesis (Pretic-I, Trauson). Moments up to 2 Nm with a 75 N follower load were applied in flexion-extension, left and right lateral bending, and left and right axial rotation. The total range of motion (ROM), segmental ROM, and adjacent intradiscal pressure (IDP) were calculated and analyzed in 4 different spinal models, as well as the differences between 2 disc prostheses.Compared with the intact specimens, the total ROM, segmental ROM, and IDP at the adjacent segments showed no significant difference after arthroplasty. Moreover, CDR with a mobile-core prosthesis presented a little higher values of target segment (C5/6) and total ROM than CDR with a fixed-core prosthesis (P > .05). Besides, the difference in IDP at C4/5 after CDR with 2 prostheses was without statistical significance in all the directions of motion. However, the IDP at C6/7 after CDR with a mobile-core prosthesis was lower than CDR with a fixed-core prosthesis in flexion, extension, and lateral bending, with significant difference (P < .05), but not under axial rotation.CDR with a novel prosthesis was effective to maintain the ROM at the target segment and did not affect the ROM and IDP at the adjacent segments. Moreover, CDR with a mobile-core prosthesis presented a little higher values of target segment and total ROM, but lower IDP at the inferior adjacent segment than CDR with a fixed-core prosthesis.