Spondy increase chances of ProDisc keel breaking?

[ans]

I wonder about this as I have spondy-whatever (actually retrosthilesis about 3-4 mm. lateral displacement). Two ADR docs who recommend fusion and ADR at L4-L5 say this shouldn't be a serious concern. But I wonder how strong the keel really is. I haven't researched this enough, admittedly.

http://www.spineuniverse.com/display...rticle114.html

Any input would be greatly appreciated.

Allan

[chasswen]

it would take a very great force to break the keel lenght wise.
and ans did you view the keel post from a few months ago?
chuck

[mmglobal]

ProDisc is cobalt chrome (with titanium spray coating). I've never heard of it breaking. The instability you describe would be in a plane that would not be adding stress to the keel. I can put you in touch with patients who have ProDisc done in spite of substantial instability.

Mark

[ans]

Thanks for your help.

Also Mark, I'd like to be in touch with spondy pts. who've had ProDisc. I do not know the cut-off margin e.g. 4mm displacement and Regan and Lauryssen seem unconcerned about it. Yet, the literature shows some concern re: fusion.

Hmm, so if the keel is nicely attached to the bone, I wonder to what degree instability/torsion can break it from the bone. This sounds like a question worth pursuing - maybe Drs. Delamarter, Dr. B., etc.

Best ~ Allan

[Jim M2]

Allan,

Mark's stress analysis is right on. The stresses aren't in the direction to be of concern to the keel. Beyond that the strength of this type of alloy known as a "super alloy" in the world of jet and rocket engines is so strong your bones will give way long before the prodisc.

Here's some dicey info on retro vs spondo (I'll let Mark comment or delete if it's too off the mark). I always thought a mild spondo condition could often be associated with par defect. A more severe spondolis... could be associated with fractures as seen in the figures and x-rays in the spineuniverse article. Slight retro is good in that (i would think) it means you don't have notable bone defects. As a disc collapses the facets will guide a healthy upper vertebral body slightly backward relative to the lower body. I've tried to explain this observation to a few people but Laura (annapurna) is the only person who ever understood immediately what I was talking about.

changing the subject now, We're in our 4th night with no electricity and 20 deg temps here in the northwest. I'm at work just to stay warm and have internet. It's mostly dark if you drive at night. Gas stations with power to pump are out of gas. It's turning into a mad max movie.

see ya,
Jim

[ans]

Hi Jim,

Sorry you're freezing in WA (couldn't find this on Google's News). I know what that's like and brrr...

Not all spondy is associated with a pars defect and this I lack. With 3-4 mm. retro I wonder how strong my bone will be to absorb the torsion stresses.

One way to find out.

Be well ~ ans

[mmglobal]

I don't recall hearing of any problems that would lead me to believe that instability and ADR issues have to do with stresses on the prosthesis or on the bone/plate interface. The structures that keep a healthy spine from being unstable are also required to keep the ADR implanted spine stable. I believe that problems associated with spondy and ADR have to do with protection of the posterior elements... just as in a non-ADR implanted spine. Watching this evolve over the years, there has been much discussion about degenerative instability vs. isthmic instability. Degenerative instability comes from collapsing disc spaces... imagine the disc as a tire on it's side. As you let the air out, it becomes wobbly and unstable. ADR is tolerant of degenerative instability because as the disc space is restored, the system is 're-tensioned' and the instability is resolved. Isthmic instability is the result of a true structural issue (like a pars fracture). This will not be resolved by ADR and the abuse heaped on the posterior elements will not be halted.

Here is where a discussion of constrained vs. unconstrained prostheses is important. A more constrained prosthesis with a stationary joint like a ProDisc or Maverick may provide more centering force that will help to keep the joint stable even with source of instability unresolved. The patient that I'm asking to come and respond had a ProDisc implanted in spite of substantial spondy. It was a calculated risk and, I believe, one that paid off. Time will tell. I look forward to her update.

At this years SAS, Dr. Marnay presented positive results from a series of patients that had repairs of Pars defects in conjunction with ProDisc. I know a patient who had this done early in Marnay's learning curve... unsuccessfully. It will be interesting to see where this goes... how much instability is too much.... how will we know. As with all other things spine, the answer will not be clear. Some patients with gross instability will be successful... others with minor instability will not.

Here comes the "I'm not a doctor" disclaimer... take everything I write with a grain of salt...

Mark

[ans]

Hi Mark,

Thanks for the incisive answer!