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Biomechanics of Subtalar Joint Arthroereisis

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Kevin Kirby, Mar 31, 2011.

  1. Here are some better quality illustrations that I drew for the paper.

    Figure 5: With the STJ arthroereisis implant in place (left), the rotational position of the STJ is reset into a new maximally pronated position that causes a more dorsiflexed position of the talar head and a more dorsiflexed position of the STJ axis than without the implant in place (right).

    Figure 6: The STJ axis is always medially deviated in a flatfoot deformity (left). The STJ arthroereisis implant prevents the talus from adducting abnormally relative to the calcaneus which causes the STJ axis to become more normally located (right).

    Figure 3: When the STJ is in the neutral position with implant in place (right), there will be no compression force from the talus and calcaneus acting on the implant. However, with maximal STJ pronation (left), the arthroereisis implant places large compression forces on the talar lateral process and floor of the sinus tarsi to limit excessive STJ pronation.

    Attached Files:

  2. drsarbes

    drsarbes Well-Known Member

    Hi Kevin:
    I enjoyed the article, very complete and well written; as usual.
    You remind me of Rush Limbaugh in that once you discuss a topic, there really isn't too much to add!

    One question i have; you state .............
    "all STJ arthroereisis implants are “direct impact” implants, which create compression forces between the talus and calcaneus".............

    I always assumed the forces were more or less a "de-rotational" force applied to the talus. I always felt that there was no real direct pressure downward on the implant proper from the weight of the super structure (unless the implant is too large). It does seem that the larger implants are all put into the pediatric patient rather than the adult.

    My question: As the pediatric foot with a large size implant begins to grow, do you feel that the forces acting on the implant are different that those of an adult patient?

    Thanks again

  3. David Wedemeyer

    David Wedemeyer Well-Known Member

    Kevin thank you for that article. I have been trying to understand this procedure from a biomechanical perspective and this really helped. We have a DPM local to me who has a boutique practice mainly performing the HyproCure implant. I met with him and wanted to better understand what he was doing. I bet you can guess who he is, he appears famous on PM news ;)

  4. Steve:

    All subtalar arthroereisis implants are subjected to compression forces which will, in effect, be a "de-rotational force", as you say. The contact forces created on the implant by the talus and calcaneus do not need to be "downward" in order to limit STJ pronation motion....they just need to occur and act both the talus and calcaneus to prevent further talo-calcaneal rotation/translation in order to limit STJ pronation motion.

    The compression forces acting on the subtalar implant will likely change as the child grows. Factors that would increase implant compression forces would be increased body weight, increased running and jumping activities, tighter gastrocnemius/soleus/Achilles tendon, increased medial deviation of STJ axis, and decreased muscle strength of the STJ supinators. I would not recommend STJ implants be used for isolated correction of moderate to severe adult acquired flatfoot deformity since the implant or bone surrounding it would likely become damaged due to the tremendous forces that would be placed on it over time.
  5. drsarbes

    drsarbes Well-Known Member

    Hi Kevin:

    I guess the point I was trying to make is this; Normally in adult flat foot the small size implants are usually used. In peds, the largest are used. I have never used an 11 or 12 on an adult, but I have in peds. Once the pediatric becomes an adult, now we have a situation with an adult having a very large implant.
    Do we know the effects?

    On a side bar note: In adults with PTTD and acquired flexible Flat foot, I normally try to implant the absorbable implant along with tenodeisis or other tendon work. Seems to work very well.

  6. I was recently fortunate to hear an excellant talk by Greg Quinn, in which he spoke of adaptive plasticity, and the remodeling of bone under stress.

    I think we will not know the true success rate, nor fully the risks, of the hyprocure implant until we start seeing them 5, 10, 15 or more years down the road. But given the plasticity of a juvenile talus I have substantial concerns over the use of these implants in children.

    For some reason the term "boutique practice" chills me slightly, and the number of Podiatrists and even surgeons who still think that a child who does not have a calc and leg with a straight line drawn up them needs "correcting" for their "overpronation" does nothing to alleviate that concern!
  7. drsarbes

    drsarbes Well-Known Member

    Hi Robert:
    I don't use the hypocure. I have used the original MBA design for - I'm guess-timating...8 -10 years.
    Almost all of the early ones I did were on pediatric patients. I assure you, all the peds that received an implant from me needed them, badly.

    The hypocure is relatively new to the scene and, apparently, they really market this in Europe. I get the feeling from posts on the arena that Non-USA podiatrists almost use HYPOCURE synonymously with STJ Arthroereisis.

  8. I don't doubt that for a minute Steve. My concern is that not everybody would be so discerning. I've seen at least one case where an arthroesis was suggested after a very poor neurectomy outcome to correct the "overpronation" deemed to be the reason for the persistant post operative pain.

    Out of interest, do you still see your paediatric arthroesis patients for follow up? A 10 year post arthoesis xray would be very interesting. Have you removed any at skeletal maturity or do you leave them in situ?

    You are correct when you say hypocure is marketed strongly in europe. I know its not the only game in town, but it does seem to be the big one.
  9. Steve:

    Sounds like a good research question for someone to answer in the future. The best long term STJ arthroereisis study only left the implant in for about 12 months in the children studied, with a 12+ year average followup (Koning PM, Heesterbeek PJC, Visser ED. Subtalar arthroereisis for pediatric flexible pes planovalgus. J Am Podiatr Med Assoc. 2009;99(5):447-453). This study used a screw inserted into the sinus tarsi to prop up the lateral talar process....probably cost a fraction of the current implants now being used.
  10. Admin2

    Admin2 Administrator Staff Member

  11. I got an Mike question/statement re the size of the implant seems all back to front to me.

    Kids get the bigger implant, adults the smaller should it not be the other way .

    problems with Compression Bone - stent - Bone

    If pressure is a similar or the same type of in seems to be if its engineering stress.

    So compression = force/area.

    and Force = Mass * Acceleration

    So compression is Mass * Acceleration/area

    So in the adult foot we have higher mass with Possibly higher acceleration so therefore higher force.

    We also have a smaller area of stent used in adults so we have higher force divided by smaller area which would then equal higher compression force values in adults . right ?

    But if we used a larger stent in the adult the compression force values would be lower because of the greater surface area of the stent. right ?

    Would make more sense re compression forces to have large ( increased surface area ) stents in children and larger (greater surface area ) in Adults . No ?
  12. Hey Mike

    I'm sure you'll get a more cogent answer from our surgical colleagues, but meantime consider this. The juvenile foot has much, much higher adaptive plasticity than the adult foot. That is the bone is more likely to be deformed by the forces on it. This being the case one could make a good case for a larger surface area.
  13. Ok So we will get changes re Wolfs law- Maybe then the size of the stent should be relative to the surface are of the calc. So relatively speaking the stent size will always be large (greater surface area relative to bone surface area) but for different reasons.
  14. drsarbes

    drsarbes Well-Known Member

    "Kids get the bigger implant, adults the smaller should it not be the other way ."

    Off hand you would thing so, but that's not how it works.
    The largest sizes are always used with the kids.

    I have my theories but I do not know for sure.
    I assume, for various reasons, a 12 year old boy has a much larger sinus tarsi than a 50 year old.

    Another study?

  15. Robert:

    I think you have hit the nail squarely on the head. I believe that the expectation from the surgeon is for the child's medial column to be able to adapt into a more plantarflexed position after the use of the subtalar arthroereisis implant over time, whereas, in an adult, there is no expectation of substantial adaption of the medial column over time.

    I would be very interested in finding out from other surgeons whether they also agree with Steve's observation.
  16. drsarbes

    drsarbes Well-Known Member

    Hi guys
    This isn't just my observation. It's an accepted fact for those who put these in regularly.
    Even the sales reps comment on it.

    I have never seen an adult I could fit a size 12 into, whereas 10, 11 and even 12s are fairly common in peds.
    In fact, the Original MBA did not have an 11, they skipped form 10 to 12. 11's were added because so many surgeons wanted them, not for the adults, but for the peds.

    Like I said, it's the way it is.

  17. That raises another interesting question. If the ST is larger in children than in adults, that would imply that the ST shrinks when skeletal maturity happens. Unless of course there is a size 12 implant in the way, in which case one will presumably a size 12 sinus tarsi...

    What are the Mid and Long term implications of that?

    I spoke to a very pleasant DPM in Portugal this weekend. He described an arthroesis as "an internal orthotic". I thought that was a very intelligent observation.
  18. Robert:

    The term "internal orthotic" has been used to describe subtalar arthroereisis implants by podiatric surgeons from the time I first remember having these implants described to us as podiatry students during the early 1980s at CCPM during our podiatric surgical lectures. From what I can gather, these implants don't seem to have been discussed at all in the UK podiatric education system (is that true??), whereas, here in the US, subtalar arthroereisis implants have been part of the four year podiatric curriculum at the podiatry colleges for over 30 years.

    If you all are interested, I know Dr. Steve Subotnick pretty well, (who was the first podiatrist to ever describe using these implants in the medical literature) and I'll bet he would be happy to come on to Podiatry Arena to describe how he first started using these implants by carving up a piece of silastic to be placed into the sinus tarsi of a flatfooted individual (Subotnick SI. The subtalar joint lateral extra-articular arthroereisis: a preliminary report. J Am Podiatr Assoc. 1974; 64(9):701).

    Steve is a very interesting and very intelligent man and now also has his chiropractor license.
  19. Be great if you can get him on Kevin, in my eyes one of the pillars of Biomechancial/sports med. Podiatry - Ive had his book on Sports medicine of the lower extremity since I began at Pod school many moons ago now.
  20. drsarbes

    drsarbes Well-Known Member

    That's funny Kevin:
    I was just telling one of the reps a month or so ago how we had blocks of silicone and it was the residents job to cut it to shape, a little at a time. We always had two blocks ready in case we cut too much off the first one.
    I bet it was pretty cheap too.
    No intraoperative fluoroscopy either.
    Think we would even be able to implant that now with all the regulations on implantables?
    My guess is no.
  21. bob

    bob Active Member

    Hello Kevin,

    The UK podiatric education system is quite varied in how much surgery is discussed within the degree. Some podiatrists will have exposure to lectures regarding procedures for flatfoot surgery including arthroeresis and some may experience the surgery when on their surgical rotations (observations at BSc level). In podiatric surgery, all fellows and surgical pupils will have specific training and experience in putting arthroereses in via the Faculty of Podiatric Surgery training system. I recall someone on here stating that 'hyprocure' seemed synonymous with arthroeresis in the UK. I am not sure who said it, but that is thankfully not the whole truth. Hyprocure has been well marketed in the UK in recent years, but podiatric surgeons have been using arthroereses in the UK long before the 'invention' of the hyprocure.

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