Quantification of the coupled motion that occurs with axial rotation and lateral bending of the head-neck complex: an experimental examination

Biomechanics of Cervical Disc Arthroplasty-A Review of Concepts and Current Technology

Activities of daily living require the subaxial cervical spine (C2-C7) to have substantial mobility. Cervical degenerative changes can cause abnormal motions and altered load distribution, leading to pain and limiting the ability of individuals to perform activities of daily living. Anterior cervical discectomy and fusion (ACDF) has been widely used to treat symptomatic cervical spondylosis. Clinical studies have shown cervical disc arthroplasty (CDA) to be a viable alternative to ACDF for the treatment of radiculopathy and myelopathy. The benefits of CDA are based on the premise that preservation of physiologic motions and load-sharing at the treated level would lead to longevity of the index-level facet joints and mitigate the risk of adjacent segment degeneration.This review article classifies cervical disc prostheses according to their kinematic degrees of freedom and device constraints. Discussion on how these design features may affect cervical motion after implantation will provide the reader with valuable information on how disc prostheses may function clinically.The ability of a disc prosthesis to work in concert with remaining bony and soft tissue structures to restore physiologic motion and load-sharing is a function of the following design features and surgical factors: Kinematic degrees of freedom-Prostheses that allow translation independent of rotation allow, in theory, the spinal anatomy to dictate segmental motion after CDA potentially restoring physiologic motion and load-sharing. A 6-degrees-of-freedom disc prosthesis may be best equipped to achieve the intended function of CDA.Built-in stiffness-A disc prosthesis with built-in resistance to angular and translational motion may have an advantage in restoring stability to a hypermobile segment without eliminating motion.Surgical factors related to prosthesis implantation may influence cervical segments after CDA. These factors include the amount of disc space distraction caused by the prosthesis, prosthesis placement in the sagittal and coronal planes, and integrity of the soft tissue envelope.

Multicentre comparative study of Z-shape elevating-pulling reduction and skull traction reduction for treatment of lower cervical locked facets

PURPOSE

The aim of this study was to assess the clinical efficacy and safety of Z-shape elevating-pulling reduction as compared to that of conventional skull traction in the treatment of lower cervical locked facet.

METHODS

Patients with cervical locked facet (n = 63) were retrospectively enrolled from four medical centers and divided into two groups according to the pre-operative reduction method used: Z-shape elevating-pulling reduction (Z-shape elevating group; n = 20) or traditional skull traction reduction (skull traction group; n = 43).

RESULTS

The success rates, efficacy of reduction, and safety were compared between the two groups. The success rates were significantly better in the Z-shape elevating group than in the skull traction group: 87.5% (7/8) vs. 35.3% (6/17) for unilateral locked facet reduction (P = 0.03) and 100% (12/12) vs. 69.2% (18/26) for bilateral locked facet reduction (P = 0.04). There was no obvious change in American Spinal Injury Association (ASIA) grade after the reduction in either group. Combined surgery was necessary in 5% in the Z-shape elevating group vs. 27.9% in the skull traction group. Imaging showed that the segment angle and horizontal displacement were significantly improved after surgery in both groups, with no significant difference between the groups. Follow-up with radiography showed good recovery of the cervical spine sequence; all internal fixation sites were stable, with no loosening, prolapse, or breakage of internal fixators.

CONCLUSIONS

Halo vest-assisted Z-shape elevating-pulling reduction appears to be a simple, safe, and effective technique for pre-operative reduction of lower cervical locked facets.

Biomechanics of a fixed-center of rotation cervical intervertebral disc prosthesis

BACKGROUND

Past in vitro experiments studying artificial discs have focused on range of motion. It is also important to understand how artificial discs affect other biomechanical parameters, especially alterations to kinematics. The purpose of this in vitro investigation was to quantify how disc replacement with a ball-and-socket disc arthroplasty device (ProDisc-C; Synthes, West Chester, Pennsylvania) alters biomechanics of the spine relative to the normal condition (positive control) and simulated fusion (negative control).

METHODS

Specimens were tested in multiple planes by use of pure moments under load control and again in displacement control during flexion-extension with a constant 70-N compressive follower load. Optical markers measured 3-dimensional vertebral motion, and a strain gauge array measured C4-5 facet loads.

RESULTS

Range of motion and lax zone after disc replacement were not significantly different from normal values except during lateral bending, whereas plating significantly reduced motion in all loading modes (P < .002). Plating but not disc replacement shifted the location of the axis of rotation anteriorly relative to the intact condition (P < 0.01). Coupled axial rotation per degree of lateral bending was 25% ± 48% greater than normal after artificial disc replacement (P = .05) but 37% ± 38% less than normal after plating (P = .002). Coupled lateral bending per degree of axial rotation was 37% ± 21% less than normal after disc replacement (P < .001) and 41% ± 36% less than normal after plating (P = .001). Facet loads did not change significantly relative to normal after anterior plating or arthroplasty, except that facet loads were decreased during flexion in both conditions (P < .03).

CONCLUSIONS

In all parameters studied, deviations from normal biomechanics were less substantial after artificial disc placement than after anterior plating.

In vitro radiographic characteristics and biomechanical properties of the canine lumbar vertebral motion unit after lateral corpectomy, mini-hemilaminectomy and hemilaminectomy

OBJECTIVE

The purpose of this study was to assess the effect of three surgical procedures (left lateral corpectomy [LC], LC plus mini-hemilaminectomy [LC-MH], and LC plus hemilaminectomy [LC-H]) on the biomechanics and intervertebral collapse of a lumbar vertebral motor unit (VMU).

METHODS

Six canine cadaveric first and second lumbar vertebrae (L1-L2) VMU were retrieved. Range-of-motion (ROM) was measured while a custom-built mechanical simulator applied 3 Nm torque in lateral bending, flexion and extension to the intact VMU and following the three surgical procedures (LC, LC-MH, LC-H) performed sequentially. Radiographs were taken with and without 3 kg axial compression at each step.

RESULTS

Left lateral corpectomy and LC-MH significantly increased the ROM in left lateral bending and total lateral bending. A LC-H significantly increased the ventral, left, right, total lateral, and total dorsoventral ROM. Significant intervertebral collapse was observed after LC-H with and without axial compression, and after LC and LC-MH, but only with axial compression.

CLINICAL SIGNIFICANCE

A LC induces significantly increased ROM in lateral bending to the side of the surgery and in total lateral ROM. Extending the LC to a LC-MH does not change the spinal column stability compared to LC alone, while it provides better access to the spinal canal. The LC-H further destabilizes the VMU. The finding of intervertebral collapse following these surgical procedures confirms the importance of the intervertebral disc and articular facet in the maintenance of spatial integrity.

Primary and coupled motions after cervical total disc replacement using a compressible six-degree-of-freedom prosthesis

This study tested the hypotheses that (1) cervical total disc replacement with a compressible, six-degree-of-freedom prosthesis would allow restoration of physiologic range and quality of motion, and (2) the kinematic response would not be adversely affected by variability in prosthesis position in the sagittal plane. Twelve human cadaveric cervical spines were tested. Prostheses were implanted at C5-C6. Range of motion (ROM) was measured in flexion-extension, lateral bending, and axial rotation under ± 1.5 Nm moments. Motion coupling between axial rotation and lateral bending was calculated. Stiffness in the high flexibility zone was evaluated in all three testing modes, while the center of rotation (COR) was calculated using digital video fluoroscopic images in flexion-extension. Implantation in the middle position increased ROM in flexion-extension from 13.5 ± 2.3 to 15.7 ± 3.0° (p < 0.05), decreased axial rotation from 9.9 ± 1.7 to 8.3 ± 1.6° (p < 0.05), and decreased lateral bending from 8.0 ± 2.1 to 4.5 ± 1.1° (p < 0.05). Coupled lateral bending decreased from 0.62 ± 0.16 to 0.39 ± 0.15° for each degree of axial rotation (p < 0.05). Flexion-extension stiffness of the reconstructed segment with the prosthesis in the middle position did not deviate significantly from intact controls, whereas the lateral bending and axial rotation stiffness values were significantly larger than intact. Implanting the prosthesis in the posterior position as compared to the middle position did not significantly affect the ROM, motion coupling, or stiffness of the reconstructed segment; however, the COR location better approximated intact controls with the prosthesis midline located within ± 1 mm of the disc-space midline. Overall, the kinematic response after reconstruction with the compressible, six-degree-of-freedom prosthesis within ± 1 mm of the disc-space midline approximated the intact response in flexion-extension. Clinical studies are needed to understand and interpret the effects of limited restoration of lateral bending and axial rotation motions and motion coupling on clinical outcome.