[General Spine] Medical Device Engineer to answer questions you may have

[MDE]

Hello everyone. Some of you may remember me from a while back when I was active on these forums trying to answer some questions regarding medical device implants, instruments, surgeries etc. I've been pretty busy and wasn't able to get back on here for a while, but I'd love to answer any questions you may have.

I have two main points. I am very knowledgeable about spine surgery and the devices involved, however I am not a surgeon or a doctor. I'm an engineer. Any question I answer should not be taken as medical advice. If I don't know the answer, I'll try to research it for you, or at least point you in the direction of a resource that may be able to answer the question better.

I can answer questions on large range of topics including surgical approaches, devices, how the FDA works, how testing is done, and any questions regarding the literature that Justin posts here.

With that said, Ask away!

Greg

[Justin]

Thank you, Greg. We are happy to have you here at SPS.

[MDE]

BadGolfer asked me the following a while back:

MDE I was looking at your posts and don't see anything on DSS or any of the Dynamic stabilization systems. I am a Sr Mech Engr and like the basic design...but it seems fiddly. I am about to have a single level DSS installed here in Perth (by a Dr that does it "monthly").

Questions you might broaden to the Forum
What are the functional differences between the DSS, DYNSES, and other dynamic stabilization system?
How complicated is installation of the DSS and others?
How well do the installation tools and procedures suit the devices (easy to do to easy to screw up)?
That should be a good start,
Dan

Dynamic rods are a very interesting concept. The original thought process behind them was for "topping off" fusion constructs. It's been proven that if you were to fuse L3/L4, than the adjacent levels (L2/L3, and L4/L5) end up receiving more load. This is commonly thought to be the reason for those levels to degenerate (sometimes referred to as adjacent level disc disease, or adjacent level degeneration) This typically is more of an issue with the superior disk (the one above the fused levels) so the thought process was "We don't need to fuse, but we want to support the disc so that it doesn't prematurely degenerate.

In the US, in order to get FDA approval the "easy" way (510(k) approval), you had to say that these rods were for fusion, when they really were designed to not fuse. The goal was to design a rod that could pass ASTM 1717 testing (the typical mechanical construct testing for pedicle screw systems) with enough strength/stiffness that the FDA would allow them to be submitted "as an adjunct to fusion" similar to regular pedicle screw systems.


Dynesys did this in 2004. Unfortunately, it was so stiff that it actually did what they told the FDA it did, which was fuse. They have had relatively high post-op failure rates, and the technique different enough that it can be cumbersome intraoperatively. They've been doing an IDE study since 2003 in the US with mediocre results which is why it's not approved for "dynamic stabilization" yet.
Scient'x had already done the same thing with the "Isobar TTL" back in 99 but again, it fused, and really was a very poor design mechanically (one rotation point in the middle, allowed some compression, but very little actual motion once implanted) They were bought out by Alphatec, and I they don't really try to hide the fact that it really is a fusion rod that allows a little bit of motion rather than a true dynamic rod.

Then in 2007/2008 a bunch more came to the market including DSS, Globus Transition, Synthes N-spine, Ulrich medical Cosmic, Medtronic Legacy PEEK, and CD Horizon Agile, and Globus Accuflex.

Of these, Medtronics horizon Agile was recalled and some of the N-Spine rod lengths were recalled due to post-op breakage. The PEEK rods break as well, but you can't tell because PEEK is radiolucent so there are lower reported rates of failure, even though it's relatively common.

The Globus Transition is a direct copy of Dynesys and has the same problem (fuses).


Of all of these, the best design that has a long clinical history is the DSS. It offers the best range of physiologic motion, is not too stiff, and really was well engineered. It's biggest downside is that the thing is HUGE. Takes up lots of space, and if the surgeon is not used to it, facet impingement can be an issue. It's also pretty fiddly as far as implantation goes.

So to answer your questions:
1) Functionally, there are few dynamic rods out there that actually function as dynamic rods. Dynesys and Transition allow almost 0 extension, and compression is so stiff that you're not retaining much motion. DSS is the best in this regard, however expect some 2nd/3rd gen dynamic rods to come out soon, but not in the US. The FDA has eliminated the "dynamic rod as an adjunct to fusion" option due to poor results with Dynesys mostly. Because the market hasn't really proven itself, it's unlikely companies will pursue the IDE route due to excessive costs with limited gain.
2) Most dynamic rods are more difficult, but not excessively, than solid rods. The problem comes in the need to know exactly how far apart the screws are after implantation to appropriately size the implant. The DSS threads in and out, Dynesys requires you to pick the correct size of silicone bumper etc. The N-Spine is by far the easiest.
3) The instruments for surgery are typically what makes the difference. Lots of surgeons pick implants based on the instrumentation to put it in, because it can greatly effect the ease of implantation.

[Badgolfer]

BadGolfer asked me the following a while back:

Excellent That is exactly the insight and understanding I was looking for. And now that I actually have a DSS installed I feel better about the decision.

BTW, for the first week or two after it was installed I "felt" when I reached the limits but now the healing and accommodation I don't notice any longer.

Thanks again,
Dan

[Gilbert P]

Hello MDE

What can you tell us about Coflex devices, Their uses and success rates

Thanks

Gil

[SPC70]

How do you feel about the M6???

[MDE]

Hello MDE

What can you tell us about Coflex devices, Their uses and success rates

Thanks

Gil

Let me get back to you. A surgeon I know uses it quite a bit and I think he has some recent study data that I may be able to share, if not, I'll go into more depth and explain the pearls and pitfalls of such devices.

How do you feel about the M6???

I like the idea behind the device. As an engineer, I love the design as it is the very first disc to try and mimic all of the behaviors of a natural disc. It contains some interesting technology and I've heard they did some excessive wear testing that proved successful, which is not easy considering the nature of all of the wear points on the device. Seriously though, I'd like to shake the hand of the engineer who made this idea work. Everyone who's ever worked on an ADR has thought about this style of implant, but getting it to pass testing is more difficult than you'd think.

If I was a patient though... I wouldn't push to have it implanted in me. The problem with devices like this is that we (as engineers, as humans etc) tend to think we've got it all figured out too soon. I have no doubt that the engineers designing the device did their due diligence (alliteration much?), however, there are 5-6 more potential failure points on this device than a current generation arthroplasty device. As much as we try to test every condition, there are always things that happen in the human body that are not thought of ahead of time... which would normally lead me to reference literature on the device... Except there really isn't much out there yet. The problem is that the devices have started their clinical trials in the US, but there really aren't any decent studies that have been published about them with good data supporting them, much less supporting them over other ADRs.

Overall the implant design is pretty cool. That said It has some negatives though. The actual endplate design is not very anatomical, and manufacturing costs of this thing must be massive. The instruments are mediocre at best, but I feel like that was caused by the implant being designed by itself prior to instrumentation really being considered.

Overall though, I think this could be a great ADR in 3-4 years once quality clinical data is presented. Until you could show me that this device truly provides better patient outcomes over another launched disc, I'm not sure I would want it.

[MDE]

Hello MDE

What can you tell us about Coflex devices, Their uses and success rates

Thanks

Gil

Interspinous devices were all the rage with some surgeons about 2-3 years ago. Unfortunately, the dynamic style ones like the original coflex have really proven to be less than successful in some studies.

The issue is really patient selection. All the interspinous spacers do is displace the posterior elements of one level away from another. Unfortunately, while this does take some of the load off of the anterior column, it shifts the load to the very anterior section of the disc, which doesn't solve any disc caused stenosis.

There are really two types of interspinous devices, those that only effect extension, and those that effect extension and flexion.
Devices like coflex can help relieve peripheral nerve compression, however realistically may not be the best approach as it doesn't allow the surgeon direct access to see this happen. The best indication of whether the device may help is if there are radicular symptoms that improve during flexion.

Contraindications where interspinous devices should not be used (according to Tamburrelli et al, Eur Spine J (2011) 20: s27-S35)
Severe lumbar stenosis
Herniated discs
Degenerative disc disease
Scoliosis

So the "newest" generation of interspinous spacers are actually fusion devices, meant for older people who cannot withstand a more invasive surgery, and do not load their spines enough to warrant a "proper" stabilization using pedicle screws. Coflex F is their 'constrained' device for fusion.

So if we look at how these can be used we see a variety of "constructs".

A) Dynamic IS with a microdiscectomy
B) Dynamic IS with a complete discectomy and interbody fusion
C) Solid IS with minimal discectomy.
D) Solid IS with a complete discectomy and interbody fusion

A was the method originally used and while successful in a very specific indications (basically if the disc is minimally degenerarated causing loss of segmental height, but not causing neurological deficit due to stenosis, the devi

B is a bad idea. Basically it's not enough stabilization, and fusion rates are lower. Again, very old patients where stabilization is not as necessary a surgeon might think to do it this way, but an interspinous fusion device would be a better option

C Is not a great idea either. It's really only applicable if the patient is very old and does not need very much stabilization in the first place. Again, contraindications would be

D The interbody fusion device takes most of the load so that the interspinous device isn't loaded excessively. I think that this is probably the best application, and again, mostly indicated as a less invasive option for older people. In general though, if you're active, pedicle screws are a better way to go.

[laid up doc]

MDE -

I myself have an M6 - I'm not an engineer, but am an MD and have a good understanding of trials.

Unfortunately, it seems the M6 and likely many future devices will never see clinical trials or therefore use in the US because 1. the FDA can't seem to decide whether it wants to fast track anything and take any risk or make the study time so long that the device is obsolete before decent #'s are obtained, 2. US patent law makes it perilous to bring anything in - that's my understanding of what happened w/ SpinalKinetics and Synthes, and 3. insurers are so discordant in what they'll cover that any given manufacturer doesn't know if their device will be paid for! Add in the uncertainty of the healthcare system and what of "Obamacare" will stay in effect and how those laws will affect patient care/spending/etc are totally unknown.

the other unfortunate matter is that as far as I know, the M6 and Freedom discs are the only ones made of titanium... aka not of chromium. I was highly advised NOT to get a chromium implant due to my other metal allergies AND because I'm a female of childbearing age. also, the prodisc goes in in 3 pieces and one could theoretically come out...

i'm sure there will be issues w/ the M6 that come to life... but for those of us who have no other options, it's a potential lifesaver. i hope to put a lot of years on mine, so will try to take it easy on it while getting as much "life" as i can.

I guess I don't really have another question - just I guess pointing out some of the additional device issues as a clinician and a patient.

[mwolf2022]

MDE

What are your thoughts on the stalif device for fusion.For example what are it benefits , downside , and is a standalone better then a posterior pedicle fusion.

Thank's , Mike .