Dymicron’s novel cervical replacement design is the only hard-on-hard device that emulates the natural motion of the spine while being both energetically stable, and aligning both the axis of load and the center of rotation. This is made possible by our cervical total disc replacement (cTDR) device’s unique Tri-Lobe articulation mechanism: three spherical lobes that mate with three non-congruent, spherical pockets.
The motion of the three lobes within their corresponding pockets makes Triadyme-C a self-centering device which allows motion in a variety of axes. These physiological characteristics are very similar to those of the natural spinal disc—and they can’t be achieved with the ball-and-socket joint used in today’s top-selling devices.
The human spine is a marvel of biomechanical engineering. Unlike the fixed center of rotation seen in a ball-and-socket joint like the shoulder or hip, the motion between vertebrae is actually a complex coupling of rotations and translations made possible by the intervertebral discs that connect the vertebrae.
A natural disc contributes to the stability of the spinal column: The fibrous annulus and gelatinous nucleus resist motions away from center, helping to prevent hypermobility and protect the many critical structures of the spine and surrounding areas.
But injury or even the aging process can lead to a variety of problems with natural intervertebral discs. As these discs degenerate, the resulting pain limits function and decreases quality of life.
For years, the standard surgery for treating cervical disc disease was to remove the offending disc and then fuse the bones of the spine together. Unfortunately, the loss of motion imposed by fusion places additional stress on the joints above and below the fused vertebrae, which can cause the adjacent discs to deteriorate even faster—a dynamic that sends many patients back for more fusion surgeries at the adjacent spinal segments.
Now, however, surgeons increasingly prescribe spinal arthroplasty, or total disc replacement (TDR). This procedure replaces the diseased disc with an artificial device to remove the cause of pain while preserving motion. An ideal disc replacement device is therefore one that behaves like a natural, healthy disc.
Dymicron’s next-generation Triadyme-C is unique in the marketplace because it comes closest to this ideal, thanks to its self-centering energetic stability and controlled range of motion.
Figure 1
The section near the center of Figure 1 where the load curve is mostly linear is called the neutral zone, which is important to note since this is where the disc spends most of its time. The positive slope of the curve indicates the stability of the motion.
The energetic stability and self-centering tendency of the natural disc are essential properties because they help protect the critical structures around the spine while keeping the vertebrae from settling into an off-center position.
One way to characterize the stability of a disc is to measure the rotation of the vertebrae while applying a torque. An increasing load is needed to rotate the spine farther from its center position, and stiffness rises dramatically as the spine nears its limit of motion. Scientists often plot this data in graphs similar to Figure 1.
Figure 2
Another way to look at this is by plotting the increase in energy as the motion segment moves away from the neutral position. The green line in Figure 2 shows the energetic stability of a natural disc.
Little energy is needed to move within the neutral zone, and more energy is required as it nears the limits of motion. The curve is always increasing as the disc is moved from center, even in the neutral zone. This increasing energy will provide a counterforce to further movement and help to push the disc back to its neutral position.
Instead of self-centering, typical replacement discs require energy to be pushed back to the neutral point. These devices may show a negative slope in the neutral zone on the load-displacement graph, or at least make a negative contribution to the overall stability.
Dymicron’s Triadyme-C, by contrast, is the only hard-on-hard articulating TDR device that is both energetically stable and self-centering; its energy profile is similar to the green curve in Figure 2.
Figure 3
Both the body’s natural disc and Triadyme-C exhibit this energetic stability, which limits motion appropriately and protects critical surrounding structures.
In cadaveric studies conducted at the Orthopedic Research Laboratory at the University of Utah (Figure 3), treatment with the Triadyme-C device resulted in only a small decrease in neutral zone stability at the motion segment attributable to the transection of the anterior longitudinal ligament (ALL), which is necessary to access the disc space. A test of a generic ball-and-trough design showed a far greater decrease in stability.
Both the body’s natural disc and Triadyme-C exhibit this energetic stability, which limits motion appropriately, protects critical surrounding structures, and keeps the vertebrae from settling into unnatural positions.
Figure 4
Range of motion tests
Manufacturers touted the large range of motion offered by early TDR devices. But missing from those devices’ designs was an adequate appreciation for the importance of stability.
The selling point of these devices seemed to be “more is better.” And that’s understandable when one remembers that these devices were being marketed as an alternative to the default treatment—fusion—which permits zero motion. But allowing completely unrestrained motion allows for unnatural motion between the vertebrae, motion which the body is not equipped to handle.
Triadyme-C was specifically designed to restore both the motion and the stability that are lost to disc degeneration. Its range of motion as measured in cadaveric tests is presented in Figure 4.
Compared with healthy intact discs, Triadyme-C showed a slight increase in range of motion (also likely influenced by the transected ALL). The variation in range of motion, however, was significantly less, demonstrating the predictability of motion after treatment with Triadyme-C.
The generic ball-and-trough, on the other hand, showed a significant increase in the range of motion and a corresponding loss of stability. It is easy to imagine how such excessive motion of the spine, with its many surrounding nerves, vessels, and other structures, could produce negative consequences. Beyond that, unconstrained devices can also settle into unnatural positions, in severe cases even encouraging a de facto spinal fusion.
