This content is sponsored by Titan Spine.
Q: In your opinion, what is the role of "surface technology" for interbody spinal fusion implants?
Adam Bruggeman, MD, Texas Spine Care, San Antonio: Surface technology is the new frontier for today's spine surgeons. For decades we have thought of interbodies as spacers designed to hold two bones in anatomic space from each other, but not designed to participate in the fusion. Surface technology changes the paradigm of interbody devices by creating a device that is actually a participant in the fusion as opposed to being a bystander or inhibitor of fusion.
David DeWitt, MD, NeuroSpine Center of Wisconsin, Appleton: Anything implanted in the body triggers a reaction. This reaction starts when the body's cell receptors, called integrins, interact with the surface of the implant. This takes place at the nano-structure level and triggers an internal cellular response. Technology has now advanced to the point that we are able to study the cellular responses to different implant nano-structure surfaces. The creation of spinal implants that utilize surface nano-technology to harness the body's cellular response to promote fusion will optimize the surgeon's ability to provide patients with the desired outcome.
Q: How do devices that incorporate this type of technology function differently from others, for instance, like those made of PEEK?
Kade Huntsman, MD, Salt Lake Orthopaedic Clinic, Salt Lake City: There is very good basic science indicating PEEK is hydrophobic, causes local cell death and the creation of a fibrous tissue layer around the implant which inhibits fusion. A specialized surface using titanium will signal the local cells to become osteoblasts, stimulate them to be highly functional and use the local hosts biology for fusion. It does not make sense to use implants that inhibit our goal of fusion, when implants that will help us are available.
Dan Bradley, MD, Texas Back Institute, Plano: Bone is known to have a natural affinity for titanium and has a natural attachment. We've seen this for decades in the dental industry. There is quite a bit of science and literature and experience with surface technology and titanium there. However, as we really study PEEK, it's hardly used in any other spine or orthopedic implant than for fusion. We know it's hydrophobic and has a fibrous layer that repels bone and diminishes fusion aspects. Some of the more recent research suggests PEEK inhibits bone growth locally.
Q: When did the cellular reaction to implant surfaces first become important to you?
Raphael R. Roybal, MD, Chatham Orthopaedic Associates, Savannah, Ga.: Once I found out about it, basically. This is a technology that has actually been around for a while, but the spine community has taken a while to catch up. In general orthopedics there has always been references to bony ingrowth and titanium coated stems used for hip ingrowth or tooth implants, and that's a hostile environment. It's been a well-known technology for years that we have ignored in the spine world until now.
AB: The cellular reaction became important to me as the latest YODA studies were being published regarding BMP and its use in spine surgery. The last eight to 10 years have seen tremendous focus on biologics as opposed to the implants themselves. Implant surface technology provided an alternative pathway to fusion that didn't involve the expensive biologics that were being placed on the market, many without significant scientific research. The research surrounding cellular biology and implant surfaces is impressive and overwhelming when compared to the research seen in the biologic industry with regards to spinal fusion.
Q: Describe how this cellular reaction impacts spinal fusion surgery and outcomes.
DD: In my practice, harnessing this cellular response has given me greater confidence that I will get a fusion every time. I don't utilize adjunctive therapies such as internal or external bone stimulators. I don't use any braces. I have used a variety of different bone grafts and synthetics that all work with these nano-scaled titanium implants. My graft choice is based on lowering cost and decreasing morbidity. I am more aggressive about letting patients return to activity and in some cases have let patients perform activity as tolerated immediately postop without compromising their outcome.
AB: The outcomes data is still being accumulated, but my personal experience is earlier return to function when compared to previous techniques I used with biologics and PEEK cages. Hopefully surgeons currently using surface technology implants will be publishing data soon to allow for more discussion revolving around the effectiveness of this technology specific to spinal fusion and outcomes.
What is interesting to me is the way that the bone seems to be reacting to these implants on early CT scans. I am seeing trabecular bone that appears to be condensing around the stress lines of the endplates, particularly on the edge of the implant where there is significant endplate to implant contact. This is similar to what is seen on proximally-coated total hip implants in general orthopedics.
KH: In my practice, on a daily basis, I see patients that I have fused using titanium cages with a subtractive surface technology, and the patients do better quicker. This is anecdotal in my practice, but many studies have now been completed, and many more underway, that confirm my clinical practice experience.
Q: What benefits have your patients experienced?
DD: Lower cost of care. Improved early postop outcomes and accelerated return to activity with higher fusion rates.
DB: It’s my impression that my patients experience higher fusion rates using this implant with earlier clinical results. It's been a win-win. They feel better sooner and heal more thoroughly with a lower risk of complications. These days you also have to look at the value proposition, and this is affordable technology for the type of procedure we’re doing.
RR: You can see the implant very conclusively during intraoperative X-rays and you can place the implant more accurately. You can have a better designed implant with more structural integrity so you're able to do more with it. I think in the future, there will be more sophisticated implants fabricated in more anatomically correct ways or extension capabilities for a natural interbody fit. The benefit would be to promote arthrodesis and promote it in a way that we have a higher percentage of success in the material use.
Q: Why do you think there is a growing trend away from PEEK and toward titanium?
KH: PEEK is an antiquated technology that was probably sufficient when we used large doses of BMP, although that combination had a high subsidence rate. As surgeons moved away from BMP, fusion rates decreased, and the choice of implant became a priority. Now, the surgical community is beginning to understand PEEK is suboptimal for bone growth and fusion.
We were taught the modulus of elasticity was better in PEEK, which is a marketing ploy not based in science. The design of the implant, surgical technique, and placement of the implant on more structurally-sound areas of the endplate need more consideration from surgeons than the modulus of elasticity.
AB: Many trends are driven by marketing and industry, but I think this is a case where science is driving the trend. PEEK was clearly driven by industry and was sold as a way to better match the stiffness and modulus of elasticity of bone to prevent stress shielding and subsidence. In reality, PEEK still showed subsidence and the subsidence rate of titanium was related to implant design, location of implant placement, and surgical technique.
A side benefit of PEEK was the ability to see fusion through the cage and limit scatter seen in older MRI and CT scans. The growing scientific evidence showing the downsides of PEEK combined with the upregulation of osseous integration seen in titanium is driving the trends today.
Q: But didn’t the industry primarily move away from titanium implants in the late 1990’s? How are the new titanium designs different?
KH: Titanium implants have changed dramatically in the past 20 years. Earlier designs, such as threaded cages, were subject to subsidence because of their design limitations. They would violate the endplates, usually be placed centrally in the vertebral body on the weakest structural area, and had relatively small footprints. Imaging of the neural structures around these dense implants was difficult, with the design of the implant and the poor quality of MRI and CT technology at that time.
Now, new designs, spearheaded by Titan, are very easy to image around. The comparison of earlier titanium implants to those manufactured by Titan is like apples to oranges. New cages have better designs, use far less titanium, so they are less dense and easier to image, spare the endplates, and are placed on better load bearing surfaces of the endplates. Also, Titan has the only true nanotechnology implant that has been proven to interact with osteoblasts and stimulate them to form bone. Again, MRI and CT scan technology has dramatically improved to allow better imaging with less scatter.
Q: There also appears to be a trend to coat PEEK implants with titanium, which appears to combine the best of both materials: the osteogenic benefits of titanium with the radiolucency and modulus of elasticity of PEEK. How do you view these types of implants?
DD: I think the new composite devices are an admission of the failure of PEEK. The high rates of fusion and low subsidence rates with new titanium implant designs clearly debunks the myths put forth regarding the modulus of elasticity and the fact that PEEK cages are now being coated with titanium is an admission of the value of implant surface. The reality is that the coated surfaces are creating more potential problems with delimitation of the surface and third body wear debris.
The titanium coatings that are being applied have not even been studied in term of a cellular response. The data in the literature on roughened titanium surfaces and cellular responses to it are almost entirely based on a proprietary surface which is different than the surface coating being applied to PEEK. Touting that an unstudied surface application has the same osteogenic properties appears to be another attempt to confuse the spine community as they did successfully with their modulus of elasticity campaign.
DB: In my view, these types of devices are an unnecessary compromise with significant limitations. They have a complex manufacturing process that was abandoned decades ago in the dental industry because of potential failures. Because the titanium-coated PEEK is bonding two materials together, there will always be risks of particulate debris, which isn’t good in the neural elements.
Q: In researching titanium coated implants, I discovered at least one instructions for use (IFU) warned that “excessive insertion forces may cause damage to the implant,” which seems to shift liability away from industry and the FDA to the surgeon. What are your thoughts on this?
DB: I hadn't seen that in the IFU before and it worries me. When I look at that statement it makes me wonder what specifically constitutes excessive insertional force and how am I as a surgeon supposed to know how much that is. When there is a choice of implants, why would I want to use one prone to mechanical failure and has warnings? I would be putting myself and my patients at risk.
DD: Similar to “off label use” this IFU shifts liability to the surgeon using the product. To my knowledge, "excessive insertion forces" is not even defined in the warning. "Excessive force" failure may be within the range of force required for routine intra-operative implantation. There have already been anecdotal cases of de-lamination during routine implantation.
I am aware of a preliminary study that simulated interbody implantation stresses on titanium coated PEEK cages using a foam block model. Results demonstrated not only de-lamination of plasma sprayed titanium from the PEEK but also shear failure within the titanium layer itself. Surgeons using these implants need to be aware of the possible complications related to surface debris particles.
Q: How are acid-etched titanium surfaces different than coated surfaces?
KH: Acid etched titanium surfaces are much different because they use a subtractive technology, which leaves powerful molecular bonds that will not leave debris behind at insertion. This is very different than an additive surface. Coatings can come off and leave large debris or microscopic debris, and subsequent failures. This does not happen with implants that have utilized subtractive technology, because the titanium is still bonded to the adjacent titanium molecules by powerful bonds.
RR: The acid-etched surfacing is a reduction technology. You're starting with a solid block of titanium and you are removing part of that titanium to create the macro, micro, and nano-scaled textures. The coated implant would be PEEK that is either sprayed or amalgamated with a titanium coating, so you have less accuracy as far as replicating the environment osteoblasts like and you have the uncertainty between the titanium coating and whatever material you are coating.
The coating is subject to de-laminating, fracturing or creating debris during insertion that might be counterproductive to bony fusion. If the interface breaks and coating comes off, that could induce an osteolytic reaction that lessens the chance of arthrodesis.
Q: How do you see surface technology continuing to evolve in the future?
AB: I see further studies being done to see what can be done on this microscopic and nanoscopic level to further induce bone formation and fusion. In addition, there will likely be a focus in the biologic industry on developing complementary technologies to assist the fusion process. Many current biologics induce initial osteolysis or cause inflammation, both of which are counterproductive to the process that occurs when an appropriate handshake happens at the microscopic level between titanium and bone.
RR: I think surface technology is going to be used more. It's apparent with everyone who is trying to coat their pre-existing implants with some kind of titanium surface that the device is trying to promote fusion and would benefit from nanotechnology that provides this microscopic architecture for bone growth. I foresee this might be utilized in surfacing titanium screws as well as cages and implants in the interbody space. It's possible they’ll surface other devices used in fusion such as interspinous devices and anterior plates. It’s something that derives a very friendly environment that all implants could benefit from if the goal is fusion.