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Forefoot varus and valgus: Intrinsic or extrinsic posting?

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Foot Doc, Apr 7, 2009.

  1. Foot Doc

    Foot Doc Active Member


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    Hi all,
    When trying to treat a fully compensated or uncompensated forefoot varus or valgus.................does it matter if the forefoot post is intrinsic or extrinsic? aren't they one and the same thing? One is just intrinsically posted and the other is extrinsically posted, right? or am i wrong? I guess thats what im trying to figure out.
    Any help would be much appreciated!
    Thanks in advance,
     
  2. Phil Wells

    Phil Wells Active Member

    Doc

    Couple of points that can make a difference-
    1. Extrinsic posts look bulkier and can impact on patient compliance
    2. Extrinsic posts have a larger surface area from which to interact with the shoe - intrinsic posts have been known to dig into the sole unit of the shoe and damage it / lose there effectiveness.
    3. Intrinsic posts usually need to be made on a rigid material as otherwise they will flex under load.

    Lots of other issues but these are my top 3.

    Phil
     
  3. Foot Doc

    Foot Doc Active Member

    Thanks phil,
    Intrinsic and extrinsic posts theoretically function in the exact same way, right? I mean, the principle is the same, right?
    Thats what my main question was. Im sorry to mislead.
    Any help would be much appreciated.
     
  4. I'd say emphatically not! Different animal.

    Firstly you need to define whether you mean forefoot or rearfoot intrinsic posting. I'm going to assume you mean forefoot (as in, pound a nail into the 1st met, build up with plaster and shape this into the arch). The method for intrinsic rearfoot posting is rather obscure unless one considers a Kirby skive to be an intrinsic modification (which it is, although not a classical rearfoot post!). If you have the urge, try JW Philps "the functional Foot Orthoses" published by the fine people at Churchill Livingstone which gives a reasonable description.

    With a forefoot post of this type much depends on what you do with the heel. If you have already added your expansion to the heel, flattened it with a rasp, or if it is already flattish because its a partially WB cast then inverting the forefoot with an intrinsic forefoot post will also create a rearfoot post (assuming the orthotic is balanced). If the heel is left rounded the this effect is much attenuated because inverting a round heel cup by a few degrees actually does not change the shape very much.

    An Extrinsic forefoot post will not change the morphology of the arch, particularly if it is extented into the true forefoot (under the mets) as opposed to the distil bit of the MLA. One takes the shape of it as cast and then inverts that shape. An Intrinsic post relatively increases the arch height. This might seem like pedentry especially if one is using lots of arch fill on a nwb cast, but if one is using a PWB cast with a lesser amount of arch fill this is a significant difference.

    There's a few other points. I would consider casts taken in the NWB method and "corrected" by the lab to have already undergone a great deal of "Intrinsic modification". Consider the angle at which the heel expansion is added, is this not an intrinsic wedge based on where we consider the bisection to fall? Then there is the arch fill which in many senses is the opposite of the intrinsic forefoot wedge (making the arch lower rather than higher). If one is using a PWB casting method where the expansion of the soft tissue takes place in the casting room rather than being added by the lab then less needs to be done by the lab.

    Bit garbled but I hope this helps!

    Robert
     
  5. Foot Doc

    Foot Doc Active Member

    Hi Robert, thanks for the reply it was a big help....... but i just want to clarify a few things.

    Lets say we have captured a rare forefoot varus in a cast that was done NWB in the neutral postion. We only want to bring the rearfoot to a neutral postion (ie:vertical. neither inverted or everted). Now, we want to accomidate the forefoot varus. What is the difference between using an intrinsic post and an extrinsic post (assume the lab used minimal plaster expansions and arch fill)?
    Look forward to your reply.
     
  6. Jeff Root

    Jeff Root Well-Known Member

    Being related to inventor of intrinsic forefoot posting, I would like to take advantage of the opportunity to help clarify this subject. In theory, the dorsal contour of an orthotic shell should be identical with both intrinsic and extrinsic posting. For example, a positive cast with a 10 degree everted forefoot to rearfoot relationship will have a platform added to the forefoot and a medial and lateral expansion added to the cast with both techniques. The difference between these two techniques has to do with the angle of the platform.

    If the cast is intrinsically corrected (i.e. intrinsically balanced or posted depending on your choice of terminology), then the plantar plane of the balance platform will be perpendicular to the bisection of the heel. Therefore, when the plantar surface of the cast is placed on the supporting surface, the heel bisection will sit vertical, the plantar plane of the platform will parallel the supporting surface, and the plane of the met heads will be 10 degrees everted to the supporting surface. A slight amount of transitional fill is usually added immediately proximal to the balance platform to blend or transition the shell into the platform. This transitional fill creates a radius at the distal aspect of the device. In a cast with an everted forefoot condition, the radius is greater laterally. In a cast with an inverted forefoot condition, the radius is greater medially. The greater the deformity, the greater the radius.

    If the cast is extrinsically corrected (extrinsically posted forefoot or shell), then the plantar plane of the platform will be 10 degrees everted to the bisection of the heel. When you place the modified positive cast on the supporting surface, the heel will rest 10 degrees inverted and the plane of the forefoot will be parallel to supporting surface. The plantar plane of the platform will parallel the plane of the mets and the supporting surface. In other words, the positive cast should rest on the supporting just as if the forefoot had never had a platform applied to the cast since the "correction" will be added extrinsically on the orthotic shell.

    Let’s assume that both devices are to have a 16 millimeter heel cup and no extrinsic rearfoot post. In the intrinsically corrected (i.e. balanced cast ) device, the orthotic shell is pressed and ground with the forefoot cut to the proximal edge of the balance platform and the heel cup is cut to 16 mm. However, the device with extrinsic posting must be wedged laterally to evert the shell in order to bring the 16 mm heel cup to level. The heel cup is then marked and ground. If you place this device with the plantar surface of the device resting on the supporting surface, the shell will rest 10 degrees inverted and the superior aspect of the cup will be angled to the supporting surface.

    After adding a 10 degree everted (valgus) extrinsic forefoot post to this device, the superior aspect of the heel cup will parallel the supporting surface. The distal edge of the device will have an abrupt drop-off at the distal edge, which is much greater laterally than medially. The distal, dorsal edge of the orthotic shell is then ground in an effort to create the same radius on the anterior, superior orthotic shell that exists in the intrinsically posted device. Therefore in theory, the dorsal shape of the orthotic shell should be identical with intrinsic and extrinsically corrected orthoses.

    In reality, there tends to be differences between the shapes of intrinsically and extrinsically posted orthoses. These shape differences range from very slight (clinically insignificant) to highly significant. These differences are dependent on the manufacturer of the device. For example, some manufactures use more liberal filler on intrinsically posted devices than on extrinsically posted devices. As a result, there may be more forefoot support built into their extrinsically corrected device than their intrinsically corrected devices. Our goal is to make the shape of both shells as close as humanly possible. The presence of an extrinsic post may have other influences such as making the orthotic shell relatively stiffer and it will create greater plantar surface area for support against the shoe. A non-corrective, extrinsic forefoot post can be added, which in theory would make intrinsically and extrinsically posted devices essentially identical.

    Needless to say, these techniques are dependent on the practice of the Root theory biomechanics. I’m not sure how those using non-Root methods deal with this issue since they do not recognize the validity of heel bisections or the value of measuring the forefoot to rearfoot relationship in the foot or cast.

    Hope this helps.

    Jeff Root
    www.root-lab.com
     
  7. That makes perfect sense. Thanks Jeff:drinks.

    As Jeff says a lot depends on how the individual lab makes their insoles. Personally I tend to sweep the intrinsic correction radius much higher into the midfoot, trying to model a dorsal surface of the orthotic which mimics the desired planter conformation of the foot (yes Simon I know! But I did'nt say it WOULD conform to the orthotic). Since the mets cannot bend mid shaft to fit around a sharper radius, I prefer to deepen it somewhat.

    You are right, I don't measure those things, nor to I recognise the validity of them. Even if we ignore the error potention in getting the insole just so in the first place, given the inherent inaccuracy of a system with 3 interfaces, any one of which often change angulations I'm not sure if such measurements would be significant even could we derive them.

    That said I may still observe gross angular relationships and prescribe accordingly! I just don't apply a mathmatical value to them. I'm lucky enough to manufacture most of my own devices so I can use however much I feel appropriate. However if using an external lab or if my technician is making them I will use one of 3 or four increments of posting.

    Also I tend to used SWB foam box casts. With that method one can use the ground as the base plane rather than the bisection. Intrinsic modifications can be done by the clinician in the foam so the morphology of the dorsal surface is entirly under their control (wyciwyg, like wysiwyg but different) I'm sure its still subject to considerable error but foam has been shown to me more repeatable than POP at taking casts (even before one starts drawing lines on the positive.)

    If it really was an honest to goodness, genuine, bona fida forefoot varus? I'd use a forefoot varus extention rather than a FFvarus wedge. Remember, on a 2/3rds (pre met) length shell, a forefoot varus extention will NOT hold the forefoot in vaurs, that would involve holding the 1st met off the ground entirely! You are NOT "bringing the ground up to the varus" unless the varus post is under the met HEADS, the WB area. Rather it could be thought to increase forefoot inversion moment from force in the arch and, perhaps, decrease forefoot inversion moment from force under the medial forefoot. Increased inversion moment in the Forefoot should (emphasis should) create increased supination moment in the STJ.

    Regards
    Robert
     
  8. efuller

    efuller MVP

    I'd like to shorten Jeff's and Robert's excellent posts.

    In a forefoot varus intrinsic post a lab will often add medial fill to lower the arch height. Apparently, my foot is rare in that having an orthotic that has a higher arch than my foot really hurts. It seems that there are a lot of people prescribing very high arched devices and claiming that "patient's love 'em". So, an extrinsically posted forefoot varus orthotic will often have a lower arch than an extrinsically posted devide.

    I agree with Robert that if the problem the patient was having was not enough load on the first met head in static stance, (too much load on the lateral forefoot) then I would use a forefoot varus extension under the metatarsal heads so the support would be maintained after heel off.

    A forefoot valgus post works by lifting the lateral column higher off of the ground than if there had been no post. An intrinsic post achieves this by a downward curvature at the distal end. (Jeff called this a radius). Imagine a flat piece of plastic. Now curve one end and place the concavity downward. The top of the piece of plastic is now further from the surface because of the curve at one end. You can decrease the amount of intrinic or extrinsic forefoot valgus post by grinding the distal lateral plantar edge.

    Regards,
    Eric Fuller
     
  9. Asher

    Asher Well-Known Member

    Eric,

    I think the following paragraph needs correction ...

    Thank you to all for this thread, it is most helpful !!

    Rebecca
     
  10. Given the same shell material and thickness the two techniques will result in different shell stiffness between devices, even if the two devices have identical shell geometry. The extrinsic post will result in a stiffer device. However, if Eric's assumption regarding shell geometry is correct, in that an intrinsic posted device will be more highly curved than an extrinsically posted device (I tend to agree), the higher curvature of the intrinsic device will also add stiffness to the shell, but not necessarily to the same extent or at the same area of the shell as the extrinsic post. The question is: how important is this to outcomes?
     
  11. delpod

    delpod Active Member

    I have always taken the point of view that if the patient has enough range of motion for the FF to reach the floor, then an intrinsic FF post is more suitable (thus you are left with a device that holds the RF in its corrected position, allowing the FF to come down to ground - this is on the basis that there is adequete MTJ ROM).

    And in the case of say a true FF varus (i.e. rigid, not a supinatus) you would extrinsicly post, bringing the device up to the FF (opposite of above) whilst still maintaining whatever RF control is neccessary.
     
  12. Phil Wells

    Phil Wells Active Member

    Delpod

    I make all my orthoses using CAD software. This allows extrinsic and intrinsic posting to effect the shell in the same way - i.e. you don't have to change the heel alignment at all and the profile does not have to be any different either
    Simon's comments about the intrinsic change to shell properties is very relevant along with the extrinsic post having a larger contact area with the shoe.
    CAD software allows you to use methods similar to the cast sectioning principle. The shell can be rotated smoothly to apply forces to specific areas of the foot that previously could not be done with traditional methods.
    I think we may need to define orthoses design in the future to allow for the differences that CAD introduces.

    Phil
     
  13. delpod

    delpod Active Member

    Hi Phil,

    Im not sure what the abbreviation "CAD" stands for, however it sounds like the software you are describing is similair to what is used by the lab that makes my orthoses...the RF and FF can be corrected independently from each other without one affecting the other (i.e. a large intrinsic FF correction will not affect the arch height to the same degree that it often can when using traditional plaster filling methods. by "cast sectioning methods" im assuming you are referring to cutting the cast in half in order to sepertae the FF from the RF to achieve the independent correction we are talking about? I think we are on the same page.

    Phil, going back to extrinsic vs intrinsic posting: with the CAD software, this intrinsic correction of the FF (independently from the RF) provides a similar/same function to a traditional extrinsic FF post? Please jump in if you disagree. However, even with this softwear I believe its not always necessary to control the FF independently from the RF, particularly where the patient has enough ROM for the FF to reach the ground (whilst the RF is being properly controlled of course). For example, if a patient needs 5 degrees of eversion control in the RF and has a 4 degree FF supinatus (and adequate MTJ ROM), it would be quite reasonable to control the rearfoot by whatever means necessary and simply balance the FF to perpendicular, allowing the FF to come down onto the device.

    I havent had a great deal of experience with this sort of software as of yet but it seems to me that this "independent control" that we are talking about is mostly indicated with more rigid FF deformities (i.e. valgus/true varus).

    I apologise for rambling on, I hope you can make some sense from the point I am trying to make.

    Cheers
     
  14. Phil Wells

    Phil Wells Active Member

    Del

    I prefer to always have the forefoot parallel to the posted rearfoot as you desribe. E.g. a 5 degree rearfoot post will result in the forefoot edge of the device being 5 degree inverted also. I use the software to then pull the front edge of the shell down to the ground to give a more stable device. The method of this bend can vary - the arch may be lowered at the same time or only the front edge needs to be touched - it a personal preference. It also depends on the Mtjt ROM. I don't want to maintain a forefoot supinatus if I can help it.

    I think we are both approaching things from the same direction.

    Phil
     
  15. delpod

    delpod Active Member

    that makes sense to me, so basically the software enables you to have a lot more control/detail as to where you want your device to be acting on the foot, which is a huge advantage IMO and im sure yours. I havent had the opportunity to see the software work (from the lab's perspective) with my own eyes yet but look foward to this opportunity in the near future.

    cheers
     
  16. efuller

    efuller MVP

    Holds the rearfoot in corrected position??? You should check this assumption on the next few patients you see when they stand on their orthoses. I was really disappointed, when I was a student, when I looked at the heel bisection before and after the patient stood on their orthotics. There is very little change the majority of the time. I believe that the heel cup of the orthoses can change forces and moments within the foot, but rarely changes position.

    People have been succesfully treated using the "deformity" paradigm. However, there are some holes in the "deformity" theory.

    Regards,
    Eric Fuller
     
  17. delpod

    delpod Active Member

    I'll admit that from my experiences (as a student and relatively new graduate) that heel bisections arent always "corrected" when the patient is standing on the orthoses, however symptomatic relief/changes in gait that are evident with the use of orthtotics probably suggests that this is not the absolute basis of orthotic therapy (as you mentioned), and there is still some form of "correction" or limitation of "unwanted" forces? perhaps I was a little loose with my terminology in regards to RF correction, however it was a means for me to get my point across inregard to intrinsic FF posting.

    Cheers
     
  18. MR NAKE

    MR NAKE Active Member

    thank you all for contributing to this fantastic debate, i have a feeling that we all say the same thing but we phrase it differently,

    secondly when we do intristic/extrinsic posting it is quite interesting to note that some element of descrepancy(note my choice of context here for the lack of a better word) starts the moment casting starts, then the clinical input(practice) (in the name of justifiable proper professional orthotic making that has been passed from generation to generation as in the making of the Samurai swords), filling in on the positive cast has a lot of influence if manually done, i dont know as for the CAD system...at the end of the day we do both and i agree with most members we could put physics/calculus into it suprisingly it works.........our goal is to primarily bring the ground to the foot right??(ceteris paribus)noting the fact that the most proximal joint (STJ) has an influnce more to total functionality to reduce the symptoms of pain.

    Now i have always had a feeling of dejavu, when i meet a FF valgus in a cavovarus foot (rigid STJ/anterior caus with massive plantarflexed 1st met), as their relationship is quite extraordinary.how best then can i apply the principles of discussion above, please help


    whose foot is it anyway?
     
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