Dr. Tempel: What we’re trying to figure out is the optimal degree of stiffness to maintain structural support. So, two separate camps. The tethers are more of a ligamentous augmentation technique, and the distributed loading rod technique is still about fixation. Maybe a titanium rod is still too much; where do we draw that line? Is there an option to start talking in the future about bringing peek rods back or incorporating 5.0 titanium?
Dr. Mullin: I don’t think there’s a holy grail. Maybe for your patient who needed a really major correction, you need to have more and go higher to T9 with transition rods. but a little old lady maybe doesn’t need quite as much correction, and you can use the tether. In the future we need to look at more patient-specific outcomes and how we can determine ideal optimization on patient-specific levels based on large clinical outcome.
Dr. Tempel: I agree. I think that solving that problem is going to rely on our partners here to help us do that. As I look at all these things on a scale, it’s all a gradation, starting from super stiff to less stiff. The question should not be: “What’s better a tie or a multiple material rodded construct?” It’s: “What’s the best thing for that individual patient?” That’s how we have to think moving forward.
Dr. Mullin: Reg, what are your thoughts on this debate regarding the gradients and materials here and the most accurate way to prescribe a construct for an individual patient?
Moderator Haid: These topics are near and dear to my heart. I’ve been doing this for about 30 years and I’ve always been concerned about types of metals. What we’re really talking about is a transition in stiffness in long constructs, a soft landing. We’ve tried hooks, we placed unilateral screws, and unilateral transverse process hooks. Chris Shaffrey, Justin Smith, Shay Bess and I have helped design a tether which got talked about; we talked about the finite element analysis as well as the clinical outcomes of stiffness.
We talked about rod stiffness and going from 3.5 mm rods to 6.5 mm rods. As you increase the radius of a rod, you cube the strength. To go from a 5.5 mm to a 6.0 mm rod may not sound like much, but that strength change is huge.
As we evolved years ago from stainless steel into titanium for MRI compatibility, we found that titanium had a spring to it. We started using larger rods and then cobalt chrome rods, which are stiffer and also stronger. If we talk a little bit more about stiffness and strength, stiffness is an elastic feature. Strength is when something bends and stays bent. Cobalt chrome is stronger, probably 30% stronger in yield strength and stiffness and it maintains the bend, while titanium springs back. In summary, what Zach’s talking about are very sophisticated principles that surgeons should use up and down the spine.
Zack and Jeff talked about using interlaminar tethers. The concept of tethers isn’t new. Before we had screws in the cervical spine, we used 18-gauge wire. Then we took 22-gauge wire and we braided it, which had the same strength as 18-gauge wire and was much more malleable. Then the Songer and Atlas cables, and a thing called SoftLayer came around, which were stainless steel and later titanium cables that allowed us to do spinous process, facet, or sublaminar wiring.
