Keel slot process

[chasswen]

mark
please explain the keel slot cutting process. from my understanding they are cut at the same time or before the fixture is removed?
thanks
chuck

[mmglobal]

Chuck, I'll just address lumbar discs that I've seen implanted with a keel (ProDisc, MobiDisc, Activ-L, Kineflex). The procedures are strikingly similar, yet there are also substantial differences. With some of these devices I've only seen a few surgeries (Kineflex = 1), so I'll keep the discussion quite general. My understanding may not be fully baked. Also, here is the obligatory "I'm not a doctor and I may be completely wrong" statement.

After the level is exposed, a window is cut into the anterior ligament and annulus. The disc space mobilized and evacuated, then disctracted more, cleaned out more. Posterior decompression and cleanup is done as needed. (removal of osteophytes, remnants of old protrusions, etc...) Endplates are prepared - cartilagenous material moved. They expose bleeding bone to enhance integration.

Using flouroscopy, the midline is determined and marked. Trial implants (like blank prostheses) are inserted to verify proper sizing, angles, fit, etc... Once they have the proper fit, they are ready to make the cut. The keel cutter is a big chisel. It's very sharp and goes through the bone quite easily. The instrumentation varies greatly from one device to another. Remember that once the keel cut is done, it's done. There is no relocating it.

With the ProDisc, the trial implant... very snugly fitting (as the distraction is released)... location, size, angles all verified by flouro... the trial implant is the guide for the keel cutter. The chisel is advanced into the vertebral body. When they get near the back, they are watching it move very closely as they advance it. There is a stop to keep the keel cutter from going too far, but I've never seen that in play. The process is very easy to control. The surgeon is determining how far to go just by watching it. As he approaches the desired depth, he moves it with finer and finer control with lighter and lighter taps.

I suppose that is may be considered that the end of the keel cut is a stop, but I don't think of it that way. the surface tension on the keel and endplates is substantial and will keep the prosthesis from moving (when properly implanted).

Chuck, I hope this helps...

Mark

[chasswen]

thanks very much for a very detailed expaination
now is the keel a press fit or does it slide easly into the slot?
chuck

[mmglobal]

Chuck,

I believe that it is a press fit for several reasons, however, I've not personally felt the resistance, so I don't know how tight it is. When they talk about fixation or integration of a prosthesis, they talk about primary and secondary fixation. Primary fixation is what you have on implantation. Secondary fixation is what you get after osteointegration of the device.

I believe that all of the coated metal devices will achieve excellent secondary fixation. I was in on Mr. Bee's revision surgery at Stenum to revise the undersized, poorly located and subsided Charite'. The revision took place at 10 weeks post-implantation. I got to see, first-hand, how tightly the plates had adhered to the vertebral bodies. You could see the bone that was grown into the coating at just 10 weeks. Pictures included below with Mr. Bee's permission.

The strongest primary fixation comes when you seriously attach the device to the bones. The old Bristol cervical disc with a flange and 2 screws at each level provides the most primary fixation I've seen in ADR. Now, Medtronic still opts for primary fixation, but has dropped one screw per level, so they don't interfere with potential multi-level procedures.

A keel provides more primary fixation because of all the surface area of the keel, adds to the total surface area that provides friction or surface tension that will resist migration. A loose fit would not provide as much primary fixation as a tight fit. Also, for it to be a loose fit, the keel cutter would have to chisel a slot that is wider than the keel. No motivation to do that. It would also increase the risk of damaging the body to use a very wide chisel. So opening the slot with a chisel, then opening just a little more with the keel.

I'm going to the NASS meeting Wendesday through Saturday. I'll look at the tools and talk it over with some of the surgeons. If I learn more, I'll post it here.

Mark

PS... sorry that this went from a question about the keel into fixation, but that is what the keel is about.

Charite' explanted at 10 weeks. See bony ingrowth. They were very strongly adhered to the vertebral bodies.

[chasswen]

thanks mark
that makes sence in which the keel surface area adds to total sq inches involed for secondary fixation.
chuck

[mmglobal]

Just from a design point of view, I'm not sure I understand the motivation to go with holes in the keel. It reduces the surface area. Potentially, bone grown into the hole will take more with it on the way out if explantation is required????

Any of you engineer types care to speculate?

Mark

[chasswen]

prodisc has holes?
one would think the holes are there for mfg proposes or that they are trying to get bone to grow thru the holes.
or sometimes additional holes are needed durning the machining process.

[mmglobal]

No, Prodisc keel is a fin... no holes. Kineflex and Maverick have holes. That's a Kineflex in the center of the iSpine header (currently). See holes in Keel. Activ-L on the right has no holes. Mobidisc has no holes, but there is a gap between the keel and endplates that may create a similar issue.

Mark

[mmglobal]

Chuck,

I spoke with Dr. Delamarter today... ProDisc keel is definitely a press-fit.... the keel is wider than the chisel.

I looked at some other designs today with interesting differences. More later.

Mark

[chasswen]

interesting must be a very fine line of how much of a press fit due to the stresses that would be created on the vertbra.
neat stuff
one would think that any device with a keel surely would stay in place all by its self due to the muscle and weight of the upper body. as the charite only has small teeth for a anchor.
chuck

[Michelle]

I was under the understanding that once Prodisc is implanted, it cannot be removed or replaced. I read that if the implant fails, the two levels must be fused. Is that incorrect?

[chasswen]

if needed they fuse the area with the disc in place.
prodisc can be revised early on per dr. b
chuck
ps mark would be able to do alittle better with a anwser

[mmglobal]

There was much talk about revisions at NASS. One session included a discussion about revisions and revision approaches by Dr. Salvador Brau. He's one of the top ADR approach surgeons who works with both Regan and Delamarter. Bertagnoli also presented 2-5 year results during this session and participated in a wonderful discussion about revisions.

I know that Bertagnoli has revised Charite to ProDisc. He has also revised ProDisc to another ProDisc. The original implantation was not his... a subsided, undersized ProDisc-L was replaced by a properly sized and placed ProDisc, using the same keel slot. Note that the vertebral body was not destroyed in removing the ProDisc.

Dr. Brau has removed ProDiscs and Charite's... some many months after implantation.

In anterior surgery, a serious problem is the inability to mobilize the great vessels so you have enough room to remove the prosthesis straight out the front. With a Charite' you can remove the core, then remove the plates individually, from an oblique angle... they do not have to come straight out the front.

With a ProDisc, you can also remove the core separately. Once that is done, the plates can be broken loose individually. There must be enough distraction applied to allow you lift the plate enough to clear the keel. Once that is done, the plate can be removed from an oblique angle. The short keel of the ProDisc lends itself well to easy removal... even at L4-5 - the problem level. (The bifurcation of the great vessels makes this level the toughest to deal with.)

I asked Dr. Brau to comment on his L4-5 ProDisc removal and speculate about removing a Maverick. Note that a Maverick does not have a core to remove. It will not be possible to apply enough distraction to clear the keels and remove a Maverick from an oblique angle. It can be impossible to mobilize the great vessels enough to allow the prosthesis to be slid forward - straight out the keel slots. If this is the case, the vertebral bodies will need to be seriously damaged to get the device out.

Mark

[mmglobal]

Michelle, I should probably answer your specific question.

Explanting the device is a pretty extreme measure. In the first few years I was researching ADR, it was cavalierly said, "you can try ADR and if it doesn't work, you can always do a fustion!"

As more and more people got disc replacements and we started to see revisions, it became clear that the cavalier, "just pop-em out and do a fusion" is a little understated. If you need revision surgery, whether it's from fusion, ADR or any other spine surgery, you are in trouble. However, as we learn more revisability becomes more paramount.

Even for the ADR's that can be removed obliquely, like the Charite', there is high motivation not to explant the prosthesis. For most failure modes, fusion will mean adding posterior instrumentation - pedicle screws and rods... leaving the ADR as an interbody spacer. More and more, we are talking about adding motion preserving posterior hardware, instead of just fusing... but the jury is still out on this as experience is very limited. Also, each case is something of a one-of-a-kind surgery.

If the prosthesis must be removed, then there is a big difference from one device to the next. As mentioned above, an all metal device with a tall keel will be difficult or impossible to explant without destroying a vertebral body. Next most invasive will be a keeled device with a tall keel and a poly core. If the keel is short enough to clear the keel without too much distraction, the keel is not an issue. Then, short keel devices... then no-keel devices.

Revisions without fusion may be possible... but as I said above... each surgery is a one-off.

Mark