Computer Aided Design Used to Study Cervical Artificial Disc ReplacementsThe use of cervical artificial disc replacements (CADR) is still in the first decade (10 years) of study. Before conducting studies on humans, computer simulation can be used to assess the biomechanical effects on movement. This concept is referred to as kinematics.
Computer Aided Design (referred to as CAD models) is widely used for many areas of study from theatre lighting to engineering bridges and now surgical procedures like the cervical artificial disc replacement (CADR).
In this study, mathematicians joined mechanical engineers and orthopedic surgeons to compare different types of cervical disc implants using a computer aided design model. They used the CT scans of a young, healthy 21-year-old man to build the spinal model. With a computer-aided model, everything is done in three-dimensions. The effect of force and load with motion is calculated by the computer rather than on a live subject.
Two different types of artificial disc implants were compared. Both were made of cobalt-crome with a polypropylene (plastic) core (inner piece). The artificial device mimicked the natural anatomy of a disc with its outer layer (annulus fibrosus) and the inner core (nucleus propulsus). Placement was at the C56 level where the majority of disc implants are currently placed.
The first implant was a fixed-core type. This means the inner piece did not move, shift, slide, or glide intentionally. The second implant type was a mobile core unit meaning the inner plastic core could move above the lower plate that formed the bottom half of the device.
All material and mechanical properties of the implanted device were programmed into the computer aided design model. Likewise, force, load, friction, tension, and angles were included in the model. In this way, the model could be put through thousands of spinal movements normally available in the human body and the results could be studied without subjecting a live human to that kind of experimentation.
Range-of-motion and load placed on the facet (spinal) joints were measured. The effects of load on spinal flexion-extension were evaluated. Data was also collected on the effects of side-bending load, rotational (twisting) load, force on the spinal (facet) joints, and stress on the spinal ligaments. Tension on the polyethylene core was also measured.
In general, range-of-motion increases are observed with all types of artificial disc implants. The reason for this is the fact that the anterior longitudinal ligament (down the front of the spine) is cut away in order to remove the disc. In this study, they found that range-of-motion was greater in all directions with the more mobile disc implant. There was more load and force on the spinal joints at the level of disc replacement (but not above or below that level).
Stress applied to the six spinal ligaments was tested and compared with and without the implant. There were significant increases in tension on the posterior longitudinal ligament (PLL), posterior capsule, and ligamentum flavum during flexion (forward bending motion) of the spine. These are soft tissue structures along the back of the spine. There was no significant change in the motion of segments above or below the artificial level.
In summary, the use of this newly developed model (computer-aided design (CAD) for the cervical artificial disc) provides a biomechanical comparison between two different types of implants.
That's helpful because comparing prostheses with fixed versus mobile core structures is a little bit like comparing apples to oranges. The differences in design create a slightly different center point for the implant. These mechanical differences could affect load and pressure both on the implant as well as on the facet (spinal) joints.
And as it turned out, the fixed core with its smaller motion and lower pressure on the joints and ligaments did put more pressure on the core itself. On the other hand, with less pressure on the mobile core, it may hold up better while still providing good motion. Long-term human studies will be needed to know for sure.
Sang-Hun Lee, MD, et al. Comparison of Cervical Spine Biomechanics After Fixed- and Mobile-Core Artificial Disc Replacement: A Finite Element Analysis. In Spine. April 20, 2011. Vol. 36. No. 9. Pp. 700-708.
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