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.
|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. While it hung in there until the 1990s, it eventually joined its counterparts in the museum of orthopaedic not-so-good ideas.
So the engineers and surgeons decided that the geometry of the stem somehow magically prevented threads from providing long term fixation. Surely this would not be the case for the acetabular side, where we had a “geometric advantage.” We began to see a proliferation of threaded cups, accompanied by giant lug wrenches to crank those babies in. (See Figure 2.)
|Figure 2: Typical threaded cup circa late-1980s.|
Needless to say, this didn’t work out too well. Here is a quote from one clinical series.
“The high rate of failure indicates that further use of these acetabular components cannot be recommended.”
“The use of smooth-surfaced threaded acetabular cups has been unanimously condemned.”
How do you handle that one on your résumé? “Worked as part of the design team for a technology cited as unanimously condemned.”
Threaded cups were thus abandoned in the States in preference of hemispherical porous-coated ones. It is ironic that during this time, threaded cups were cleared for use without cement, but porous-coated ones were not!
It is interesting to note that the European experience was more positive. The reasons remain unclear. Perhaps we were not as good with the lug wrenches. So, what gives?
One key overarching reason that threads are incapable of providing lasting fixation is that they become loose immediately due to the viscoelastic nature of all biomaterials. Bone exhibits both creep and stress relaxation. When bone is loaded, it changes shape over time in accordance with the direction of the load. This is illustrated in Figure 3.
|Figure 3: Creep-fracture curve of adult human bone at a constant stress of 60MPa.|
When a surgeon tightens a screw in a trauma case, and then places and tightens another one, the first one is loose already because the bone creeps in response to the stress. Although Figure 3 shows creep to fracture, if the stress is low enough, the phenomenon theoretically goes on forever, thus giving rise to the adage that all implants move through bone. Figure 4 shows a threaded component that has not only loosened, but has “walked” several millimeters northward, and might ultimately migrate to chitlinville if nothing is done.
|Figure 4: Typical loose threaded acetabular component.|
For this same reason, you are shorter at the end of the day than when you got up. If you stayed on your feet forever, you would continue to shorten, albeit at a diminishing rate.
By now you are getting the reason why the right Femora stems loosened dramatically more than the lefts. It was due to the handedness of the thread. Loads causing rotation tightened the left hip stems.
Hence, the only applications in which screws are still used are those wherein fracture healing is required, i.e.: where the healing of the surrounding bone obsoletes the hardware used for initial fixation.
So this brings us to those pesky pedicle screws. One of the reasons that pedicles were initially thought to provide superior fixation in the spine is that they are intermedullary canals, and we were able to fix hips stems reasonably well. Pedicle screws worked adequately in fusions, provided fusion occurred before the hardware failed. Dynamic stabilization requires the pedicle screw to provide permanent fixation that is not rendered redundant by fusion. Based on history and experience, we might have known it could have some issues.
Numerous studies of dynamic systems note loose screws, and one would predict that given their popularity, we will see higher incidence in larger numbers the further out patients are. Figure 5 shows a CT of a patient with loose pedicle screws. This image is eerily familiar to anyone who has studied the history of threaded implants.
|Figure 5: CT image showing loose pedicle screws.|
Some reading this will say that the problem is solved because we are now HA coating pedicle screws for use with dynamic systems. (We are HA coating because of concerns about loosening.) The joint replacement people learned long ago that screws are poor constructs for permanent fixation, and I will bet that very few on the joint replacement side believe that merely HA coating them will be a solution. What can we learn from these people?
The Graf ligament was one of the early dynamic devices. It was originally used with standard CD pedicle screws, and while the device was loaded only in flexion, loosening was still a concern. Henry Graf and the engineers began to apply lessons learned from hip replacement, and iterated a series of designs focused on providing more robust, permanent fixation. One of the later iterations is shown in Figure 6.
|Figure 6: The Graf Ligament System|
You will note some elements of contemporary joint replacement concepts. The screw utilizes threads for initial stabilization, but also includes a tapered, roughened section to closely appose cortical bone precision machined into the pedicle. (Think S-ROM.) Further iterations included porous coating and HA coating of the tapered portion to encourage a biological response and hence, long term fixation.
This is not to say that this is the solution for the problem, only a more informed approach that uses wisdom gained throughout the history of orthopaedics. Nor am I aiming to provide the solution in this article. I am, however, challenging every qualified person reading this to come up with a way to permanently fix dynamic elements in the spine, because conventional pedicle screws just ain’t optimal.
In fact, why use the pedicle at all?
John Engelhardt is a founding partner of ORTHOWORLD INC., a venture firm focused on the musculoskeletal industry. A former executive of AcroMed Corp. and DePuy, Mr. Engelhardt is a futurist and recognized authority on technology trends in orthopaedics. He holds 19 patents covering large and small joints, spine and trauma. Mr. Engelhardt is a Member of the College of Fellows, American Institute for Medical and Biological Engineering. He can be reached via email at firstname.lastname@example.org.
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Article first published in BONEZONE Winter 2008. Reprinted with kind permission from John Engelhardt and ORTHOWORLD.