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BIOM101 query - orthoses posts motion

Discussion in 'Biomechanics, Sports and Foot orthoses' started by markjohconley, Feb 16, 2012.

  1. markjohconley

    markjohconley Well-Known Member

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    "...and possibly consider making the rearfoot post flat, with no motion."
    Prof Kirby, part of a great post, as usual, I, like most probably, can never get enough (unless 'stiffness' is mentioned).
    My query is, and was always, why grind any post movement??
    The orthoses has been shown not to necessarily change the kinematics to get its desired effect, and do we want the foot moving in the frontal plane in the shoe, or do we want the foot - shoe unit to be moving.
    If this is a load of bollocks I really don't mind being corrected, as i'm sure there may be others with similar thinking, thanks, mark
  2. efuller

    efuller MVP

    Post motion is a concept that probably works for a different reason than it was originally attempted. Fortunately, it works. The original idea was that a flat post on the bottom of the orthotic would not allow the heel to evert and the orthotic should promote motion. Hence, the bevel was on the bottom on the rearfoot post so that the orthotic would rock the amount of the bevel put into the post. There are many problems with this. The foot can move relative to the orthotic, so a flat posted orthotic will not prevent the motion. The shoe hits inverted to the ground and therefore the platform that the orthotic is on is moving already, so the amount of motion will not be caused by the amount of the bevel. Also, after a period of time, the socklliner of the shoe will conform to the shape of the bottom of the post, and you won't be able to create the motion relative to the shoe. So, it doesn't work as originally thought.

    What it does do, in my opinion, is shift the center of pressure more medial. If you wanted to go to an extreme, you could use a half post on the medial side of the device if you wanted to achieve maximum shift of the location of the center of pressure. (Thinking about center of pressure brings up another reason the original explanation was wrong. To get the post to rock on to the lateral side of the post the center of pressure from the foot above the orthotic has to be on the lateral side of the line created by the intersection of the two planes on the underside of the post in the ortotic with "motion". If you look at center of pressure paths, there is so little time, that center of pressure under the foot is on the lateral side of the line that it wouldn't have time to invert the heel. The location of center of pressure line is on the medial side of the line before the lateral forefoot hits the ground.

    I had this discussion many times at CCPM. However, they still taught the concept. It was frustrating.

  3. Mark:

    You may consider that when Drs. Mert Root, John Weed and Tom Sgarlato all worked together at CCPM to create the first dental acrylic post on a Rohadur orthosis (probably back in the early 1970s) the standard shoe being worn was a hard rubber or leather heeled shoe with minimal to no cushioning in the shoe heel. As I remember the story being told by John Weed and Mert Root, they discovered that a flat rearfoot post often caused back pain or lower extremity discomfort. However, they soon discovered that if they added a 4 degree grind on the lateral aspect of the rearfoot post to allow 4 degrees of motion during the contact phase of gait, then this would eliminate the patient's symptoms caused by the rearfoot post. This scenario likely occurred because the dental acrylic post was acting on the relatively uncompressible surface of the leather or hard rubber shoe heel.

    Now, fast forward to 1980-1981 when we (myself along with many of the Pod Squad Running Club members including Matt Fettig, Steve Palladino, Dave Hannaford, and Jeff Christensen) were all helping Rich Blake to make his newly created "Blake Inverted Orthoses" at the Mechanical Orthopedics (MO) Lab at the California College of Podiatric Medicine (CCPM). Dr. Blake had us always use a flat dental acrylic rearfoot post on the Blake Inverted Orthoses that were always made of Rohadur at the time. Dr. Blake believed that the rearfoot post motion wasn't necessary or may even have been undesirable since the vast majority of the patients he made the Blake Inverted Orthoses for were runners wearing softer soled running shoes and he felt the flat rearfoot post gave "better control of pronation".

    So, I saw both extremes from no post to the standard 4 degree/4 degree post to the flat rearfoot post while I was a student at CCPM. Probably today, a flat rearfoot post would not create the same postural problems in patients that they did when it was first created in the 1970s since shoe soles now are such such lower durometer (i.e. more cushiony) than they were 40 years ago. That being said, I still put 4 degree/4 degree rearfoot posts on most of my orthoses but also commonly use flat rearfoot posts and no rearfoot posts in other situations.

    Hope that this information gives some better historical context to the discussion.
  4. markjohconley

    markjohconley Well-Known Member

    Thanks Kevin,
    ... the young ones don't know what they're missing, that beautiful aroma when grinding Rohadur!

    Has it been shown whether it did allow that motion??
  5. Personally, I think that the grinding of "post motion" is completely unnecessary given modern manufacturing materials of both shoes and orthoses.
  6. I do this quite a lot especially with skive posting
  7. No. But the 4 degrees of post motion does seem to work well in most patients. This topic obviously needs further research.
  8. phil

    phil Active Member

    I'd love a quick diagram which illustrates what is being discussed here. what does a post motion grind look like?
  9. some info no picture but might help ?




    taken from here

  10. Berms

    Berms Active Member

    Hi Mike, what exactly is "skive posting".

  11. same as a medial skive -

    the reason for the posting bit - skive is a type of intrinsic rearfoot post

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