Biomechanical Study of Cervical Disc Arthroplasty Devices Using Finite Element Modeling

Many artificial discs for have been introduced to overcome the disadvantages of conventional anterior discectomy and fusion. The purpose of this study was to evaluate the performance of different U.S. Food and Drug Administration (FDA)-approved cervical disc arthroplasty (CDA) on the range of motion (ROM), intradiscal pressure, and facet force variables under physiological loading. A validated three-dimensional finite element model of the human intact cervical spine (C2-T1) was used. The intact spine was modified to simulate CDAs at C5-C6. Hybrid loading with a follower load of 75 N and moments under flexion, extension, and lateral bending of 2 N·m each were applied to intact and CDA spines. From this work, it was found that at the index level, all CDAs except the Bryan disc increased ROM, and at the adjacent levels, motion decreased in all modes. The largest increase occurred under the lateral bending mode. The Bryan disc had compensatory motion increases at the adjacent levels. Intradiscal pressure reduced at the adjacent levels with Mobi-C and Secure-C. Facet force increased at the index level in all CDAs, with the highest force with the Mobi-C. The force generally decreased at the adjacent levels, except for the Bryan disc and Prestige LP in lateral bending. This study demonstrates the influence of different CDA designs on the anterior and posterior loading patterns at the index and adjacent levels with head supported mass type loadings. The study validates key clinical observations: CDA procedure is contraindicated in cases of facet arthroplasty and may be protective against adjacent segment degeneration.

Comparative Finite Element Modeling Study of Anterior Cervical Arthrodesis Versus Cervical Arthroplasty With Bryan Disc or Prodisc C

ABSTRACT Introduction Cervical disc arthroplasty (CDA), a motion-preserving alternative to anterior cervical discectomy and fusion (ACDF), is used in military patients for the treatment of disorders such as spondylosis. Since 2007, the FDA has approved eight artificial discs. The objective of this study is to compare the biomechanics after ACDF and CDA with two FDA-approved devices of differing designs under head and head supported mass loadings. Materials and Methods A previously validated osteoligamentous C2-T1 finite element model was used to simulate ACDF and two types of CDA (Bryan and Prodisc C) at the C5-C6 level. The hybrid loading protocol associated with in vivo head and head supported mass was used to apply flexion and extension loading. First, intact spine was subjected to 2 Nm of flexion extension and the range of motion (ROM) was measured. Next, for each surgical option, flexion-extension moments duplicating the same ROM as the intact spine were determined. Under these surgery-specific moments, ROM and facet force were obtained at the index level, and ROM, facet force, and intradiscal pressure at the rostral and caudal adjacent levels. Results ACDF led to increased motion, force and pressures at the adjacent levels. Prodisc C led to increased motion and facet force at the index level, and decreased motion, facet force, and intradiscal pressure at both adjacent levels. Bryan produced less dramatic biomechanical alterations compared with ACDF and Prodisc C. Numerical results are given in the article. Conclusions Recognizing that ROM is a clinical measure of spine stability/performance, CDA demonstrates a more physiological biomechanical response than ACDF, although the exact pattern depends on the implant design. Anterior and posterior column load-sharing patterns were different between the two implants and may affect implant selection based on the anatomical and pathological state at the index and adjacent levels.

Biomechanical Study of Cervical Disc Arthroplasty Devices Using Finite Element Models

Various types and designs of artificial discs for cervical disc arthroplasty (CDA) have been introduced to overcome the disadvantages of the conventional anterior cervical discectomy and fusion (ACDF). The purpose of this study was to evaluate the effects of different CDA designs on the range of motion (ROM), intradiscal pressure (IDP), and facet force variables with different types of FDA-approved CDA devices under normal physiological loading conditions. A validated three-dimensional finite element model (FEM) of the intact cervical spinal column (C2-T1) was used in the present study. The intact spine model was modified and used for postoperative FE models simulating CDAs implanted at the C5-C6 intervertebral disc space. The normal surgical procedures were used in the simulations. The hybrid loading protocol (intact spine loading: 2 Nm) with a compressive follower force of 75 N was applied at the superior end of the spine. The inferior endplate of C7 vertebra was constrained in all directions. Flexion, extension, and lateral bending loading conditions were simulated in all models: intact spine and models with different CDA devices. At the index level, all CDAs except the Bryan disc showed an increase in motion, and the range of motions at the adjacent levels decreased in flexion, extension, and lateral bending modes. The largest increase in motion occurred during lateral bending. The Bryan disc reduced the segmental motion at the index level under flexion, extension, and lateral bending, and had compensatory increases in motion at the adjacent levels. The intradiscal pressure reduced at the adjacent levels with Mobi-C and Secure-C devices. The Bryan and Prestige LP devices showed increases in the intradiscal pressure at the adjacent levels due to the reduced index level motion (Bryan disc) and the metal-on-metal design (Prestige LP). The facet force increased at the index level in all CDAs, with the highest force with Mobi-C, and this was attributed to its unrestrained design. The facet force generally decreased at the adjacent levels with CDAs, except for the Bryan disc, due to reduced index level motion, and the Prestige LP in lateral bending, likely due to its metal-on-metal design. The present study demonstrates the influence of different CDA designs on the anterior and posterior loading patterns at the index and adjacent levels. In addition, the study validates key clinical observations: CDA procedure is contraindicated in cases of facet arthropathy; and CDA may be protective against adjacent segment degeneration.

Non-Destructive and Failure Responses of Cervical Spine Artificial Disc Surgery for Military Applications

Ossification patters, non-destructive and failure responses of cervical artificial discs were determined using an in vivo Caprine model. The animals were anesthetized, discectomies were done at C3–C4, and Bryan disc, ProDisc-C, or anterior discectomy and fusion with plating (ACDF) was done. They were euthanized after monitoring for six months. Non-destructive loads were applied to the excised cervical columns in flexion, extension and lateral bending, combined with compression. The failure test was done under compression-flexion. X-rays and computed tomography scans were used to determine fusion. Force-displacement data for both artificial discs were grouped based on the presence or absence of heterotopic ossifications. They were compared with the ACDF specimens. Heterotopic ossifications occurred in both discs. Patterns of ossification were similar to those reported in civilian patients. Force-displacement responses of ossified spines were stiffer in all modes for both discs. However, differences were non-uniform. Biomechanical corridors are presented for all cases. The present preliminary study should be extended to discern the role of the disc type, loading mode and spinal level in future research.

Prospective randomized study of cervical arthroplasty and anterior cervical discectomy and fusion with long-term follow-up: results in 74 patients from a single site

Object The purpose of this study was to evaluate the long-term results of cervical total disc replacement (TDR) and anterior cervical discectomy and fusion (ACDF) in the treatment of single-level cervical radiculopathy. Methods The results of 2 separate prospective, randomized, US FDA Investigational Device Exemption pivotal trials (Bryan Disc and Kineflex|C) from a single investigational site were combined to evaluate outcomes at long-term follow-up. The primary clinical outcome measures included the Neck Disability Index (NDI), visual analog scale (VAS), and neurological examination. Patients were randomized to receive cervical TDR in 2 separate prospective, randomized studies using the Bryan Disc or Kineflex|C cervical artificial disc compared with ACDF using structural allograft and an anterior plate. Patients were evaluated preoperatively; at 6 weeks; at 3, 6, and 12 months; and then yearly for a minimum of 48 months. Plain radiographs were obtained at each study visit. Results A total of 74 patients were enrolled and randomly assigned to either the cervical TDR (n = 41) or ACDF (n = 33) group. A total of 63 patients (86%) completed a minimum of 4 years follow-up. Average follow-up was 6 years (72 months) with a range from 48 to 108 months. In both the cervical TDR and ACDF groups, mean NDI scores improved significantly by 6 weeks after surgery and remained significantly improved throughout the minimum 48-month follow-up (p < 0.001). Similarly, the median VAS pain scores improved significantly by 6 weeks and remained significantly improved throughout the minimum 48-month follow-up (p < 0.001). There were no significant differences between groups in mean NDI or median VAS scores. The range of motion (ROM) in the cervical TDR group remained significantly greater than the preoperative mean, whereas the ROM in the ACDF group was significantly reduced from the preoperative mean. There was significantly greater ROM in the cervical TDR group compared with the ACDF group. There were 3 reoperations (7.3%) at index or adjacent levels in the cervical TDR group; all were cervical laminoforaminotomies. There were 2 adjacent-level reoperations in the cervical TDR group (4.9%). There was 1 reoperation (3.0%) in the ACDF group at an index or adjacent level (a second ACDF at the adjacent level). There was no statistically significant difference in overall reoperation rate or adjacent-level reoperation rate between groups. Conclusions Both cervical TDR and ACDF groups showed excellent clinical outcomes that were maintained over long-term follow-up. Both groups showed low index-level and adjacent-level reoperation rates. Both cervical TDR and ACDF appear to be viable options for the treatment of single-level cervical radiculopathy.