Nearly all spine companies with extensive product portfolios have some type of dynamic stabilization system. Nearly all use conventional pedicle screws for fixation. One can certainly understand why. The companies had their screws and simply developed a flexible element to replace the rod in a fusion construct, and voilà a brand new state of the art system for the rapidly emerging dynamic fixation market. Trouble is, pedicle screws won’t work very well in this application. Why, you ask? How do you know? Let’s check it out. We’ll take a little journey through time.
Just in my lifetime, we have tried to use threaded elements as permanent fixation in nearly all joints. It seemed like a natural solution. After all, screws tightly hold most things together for the life of the materials of which they are made. It was not to be for bones.
Several designs of screw-in hip stems saw use during the 1980s. To my knowledge, none exist today in any civilized nations of the world. Figure 1 shows one of the more notorious ones.
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| Figure 1: The Femora stem by Thackray, showing its threaded stem. |
In one study, the revision rate for all non-cemented stems examined was 4.5 percent at 4.5 years. The rate was 13.6 percent for the Femora stem. Further, the Femora stem needed revision in 20 percent of right hips and only four percent of left ones.
We don’t mean to rip on this one product. Other threaded stems performed equally dismally. Many of you might remember the threaded sleeve for the S-ROM. Whi