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The Facet Joint Loading Profile of a Cervical Intervertebral Disc Replacement Incorporating a Novel Saddle-shaped Articulation

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The Facet Joint Loading Profile of a Cervical Intervertebral Disc Replacement Incorporating a Novel Saddle-shaped Articulation

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Abstract

Biomechanical study. To evaluate the cervical facet loading profile of an intervertebral cervical disc prosthesis incorporating a unique saddle-shaped articulation and to determine the effect of implantation of the prosthesis on both the loading curve and area distribution of facet loading, when compared with that of an intact specimen. This is the first study that examines the effect of implantation of a cervical disc replacement on the loading of the facet joints. Fresh-frozen ovine cervical spine specimens were used. Specimens were prepared and disarticulated to yield 6 functional spinal units, which were secured and mounted in a custom fixture on a material testing apparatus. A novel pressure sensor was constructed by combining a resistive ink thin film sensor with Fuji pressure-sensitive film. The sensor was calibrated before unilateral insertion into the cervical facet joint via a small arthrotomy. Specimens were tested in both intact condition and after surgical intervention consisting of discectomy and implantation of an intervertebral cervical disc prosthesis (CerviCore, Stryker Spine, Allendale, NJ). Specimens were tested in flexion, extension, and lateral bending to 3 Nm and in torsion to 2.5 Nm. At the termination of each loading profile, the load was maintained for 30 seconds to permit full exposure of the Fuji film. For each configuration, the maximum resultant load, load rate, and contact area pressure at the endpoint of the loading profile were determined. A paired Student t test was used to determine the differences between the intact specimen and the specimen with an implanted intervertebral disc prosthesis. There were no statistically significant differences in mean or maximum pressures between the intact specimen and the specimen with an intervertebral disc in all loading configurations. Similarly, there was no significant difference in the total measured force between the groups in all loading configurations. There was no significant difference in contact areas between the groups in flexion, lateral bending, and torsion. When evaluated in extension, the intact specimen had a mean contact area of 0.8 cm compared with 0.5 cm for the intervertebral disc (P<0.02). Biomechanical testing in an ovine model demonstrated no significant difference in measured facet pressures and forces between an intact native cervical disc specimen and a cervical intervertebral disc prosthesis using a saddle-shaped articulation. Peak and mean pressures were not demonstrated to be significantly different between the implanted and intact disc conditions. Implantation of the prosthesis resulted in a significant reduction of contact area under extension moments.

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... Existing experimental methods for measuring ex vivo facet joint apposition area alter the joint structures and kinematics, 12,13,23,33,36,39 and typically overestimate true apposition area, 2,14 but computational models of experimental kinematics data can provide a method of estimating joint apposition without compromising the joint. Kinematic data, combined with specimen-specific geometries of spine segments, has been used to estimate facet cartilage apposition area in the lumbar 7 and cervical spine 30 Unlike finite element (FE) modelling, where external forces are applied to a model to predict the propagation of stress and strain in the tissue(s), kinematic modelling applies measured kinematic data to the modelled (rigid) anatomic geometry to derive geometric relationships (such as facet joint articulation area) that cannot be measured directly. ...
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... In contrast, a similar study conducted by Steiber et al. [24] using ovine spines demonstrated no significant increase in the facet loading following TDR. Our data suggests that under hybrid loading conditions TDR maintains total facet loads similar to the intact facet loads—with lower facet loads for the SPH-I and OVL-I designs. ...
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