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Where Should the First Ray be When Casting for Orthoses?

Discussion in 'Biomechanics, Sports and Foot orthoses' started by admin, Jul 13, 2005.

  1. admin

    admin Administrator Staff Member


    Members do not see these Ads. Sign Up.
    The latest newsletter from Larry Huppin at ProLab was about Where Should the First Ray be When Casting for Orthoses?:
    What say you?
     
  2. efuller

    efuller MVP

    VooDoo biomechanics just might work

    How does plantarflexion of the first ray when casting have anything to do with the position of the first ray when standing on a piece of plastic made from a cast with the first ray plantar flexed? Does the orthosis reach up and grab the the first met and pull it down into a plantar flexed position? The idea that the casting position (STJ neutral or first ray) will move the foot into a certain position is what I like to call voo doo biomechanics.

    To look at positions and motions of the foot you have to examine the forces and moments that cause those positions and motions. Plantar flexion of the first ray does two very significant things to the cast. It increases the amount of forefoot valgus (=decreases varus) and it raises the arch height.

    A cast with increased forefoot valgus that is balanced to heel vertical will tend to lift the lateral forefoot off of the ground. This will tend to increase lateral load and hence decrease medial load. This may help the first ray plantar flex.

    An increased arch height will have many explanations of increased supination moment. My favorite explanation is pain avoidance. If it hurts to pronate and lower the arch height of the foot because a piece of plastic is contacting the arch then the posterior tibial muscle and/or other supination moment muscles will contract and the foot will supinate to a point where the arch is comfortable. This will help pronation related symptoms except those related to PT dysfunction.

    It is possible to go too far with forefoot valgus correction. The gist of John Weed's teachings was that you should not try to evert the foot farther than it can go. A large forefoot valgus wedge may attempt to evert the MTJ and STJ farther than the available range of motion.
     
  3. pgcarter

    pgcarter Well-Known Member

    Sticking with the idea that after you cast a foot you have to "voodoo" up a pair of devices, I would offer the following;
    Where ever you have the 1st ray matters less than whether you know where it is or not...because what you do with your plaster additions and forefoot balance will actually determine the shape of the devices, and this is what will determine the net effect of them on the particular foot.

    I agree with some of what Craig has said over the years in that a lot of what we do is poorly understood. Although that seems to be improving slowly.

    It is a miracle that a device made by a tech in a factory who has had no contact with the foot concerned and minimal understanding of feet in general can do something to whatever cast comes in the door and generally it will help to some extent.
    To me this means there must be some pretty common themes that can be improved by fairly generic responses.....but this sort of system does not deal well with the cases that fall outside the common themes.

    One of the things I say to my students at Latrobe is that the ability to recognize similarities between cases will help, but only lead to generic responses. It is the ability to recognize increasingly subtle differences between cases that leads to the ability to treat each case in the uniquely best way for that individual...and lab produced devices are less likely to be able to provide this subtlety of response.

    A little off the topic...but related.
    Regards Phill Carter
     
  4. There are a number of problems with the statements above, but you must also understand that the "update" that ProLab puts out by Larry Huppin is meant to be fairly nontechnical so that the average podiatrist can understand it and have a "take to clinic" message. I have found many of Larry's updates to be quite helpful to my practice.

    However, if you want to speak more accurately biomechanically about first ray and first MPJ biomechanics, I would say the following about the above:

    First of all, the first ray does not need to be plantarflexed for the foot to be "stable" and for the first MPJ to have full range of motion since many cavus feet are very "stable" and always have full first MPJ range of motion at any first ray position.

    Second, to ensure that the first ray is plantarflexed with an orthosis, one does not need to rely solely on plantarflexing the first ray in the negative cast. One could also do a first ray cutout in the orthosis plate, add a 2-5 forefoot extension to the front of the orthosis or do a cast sectioning procedure to plantarflex the first ray of the positive cast. However, I do agree that plantarflexing the first ray during negative casting is easiest.

    Third, "first ray hypermobility" should be discarded as a term and I propose the term be replaced by "decreased first ray dorsiflexion stiffness". A paper describing the proposed change in terminology, coauthored with Tom Roukis, DPM, will be published in the next few weeks in Biomechanics Magazine. First ray dorsiflexion stiffness is not affected by the orthosis plate but is instead affected by structural and functional factors of the foot such as arch geometry, ligamentous tensile stiffness and muscle contractile activity. However, the motion of the first ray will be affected by the orthosis. Importantly, the orthosis, as Eric mentioned, cannot "reach up and grab" the first ray and haul it plantarward.

    The best way to prevent excessive first ray dorsiflexion motion is to (1) unload the distal first metatarsal and increase the load of the lesser metatarsals with a 2-5 extension, (2) increase the STJ supination moment with the orthosis by inverting the orthosis and adding a medial heel skive, (3) increase the medial forefoot plantarflexion moment by decreasing the medial expansion plaster depth in the positive cast and stiffening the longitudinal arch of the orthosis to prevent arch deformation of the orthosis during late midstance.
     
  5. Paul R. Scherer D.P.M.

    Paul R. Scherer D.P.M. Welcome New Poster

    Eric’s term biomechanics voodoo is about as inappropriate as the terms bio-magic and biomechanics mumbo jumbo used to describe speakers who know their material but can’t get their point across. It doesn’t help the situation and I believe is part of the reason less than 1% of our profession understands the known mechanics of the foot.

    That said let’s discuss the issue and the terms. Kevin is right – Larry is attempting to explain, in language our profession might understand, a concept of improving the function of a dysfunctionial foot through techniques that are available to the average podiatrists in practice. We do know from Tom Roukis' work that, if an orthotic or a piece of korex raises the first metatarsal head by 4mm the range of motion of the big toe decreases, in a weight-bearing foot. This restriction is proportionate if the elevation is increased to 8mm. The assumption can be made that, if the negative casted position of the first ray reproduces this elevated position. The resulting piece of plastic will hold the first ray elevated and produce a functional hallux limitus or at best a tight plantar fascia.

    For the purpose of explaining this to a large population that uses orthoses, I support Larry’s use of terminology of dorsiflexed and plantarflexed. I have even used the term “up” and “down” to support the concept. I have also been criticized for this, but if I can get my point across I will bear this burden. By the way Eric, I truly believe it’s the little gremlins of Vern Inman that reach up and pull the first ray down. He used them very successfully in his text.

    Although I understand completely Kevin’s and Tom’s term “decreased first ray dorsiflexion stiffness” and find it totally logical, I believe it will throw additional mud on the computer screen. I applaud your paper proposing the change in terminology but this term will add to the confusion. I hate “first ray hypermobility” terminology and I would hope someday we could produce a more digestible and descriptive terminology.

    Kevin used the term excessive first ray dorsiflexion motion at the end of his posting and I think this is the best term for podiatrists. Preventing this by increasing load on the lesser metatarsal heads is effective but often a Peter and Paul payment issue. First ray cut outs are also effective but this decreases the size of the distal edge of the orthotic which is a major stabilizing structure. Cast sectioning procedure by the lab is a very inconsistent and controllable method. This leaves Larry’s recommendation to plantarflex the ray while casting to prevent excessive dorsiflexion.

    I have some thoughts on the first ray as you know and hopefully my paper on the subject will be published in JAPMA this year. I will present it at PFOLA in November in Vancouver.

    I avoided Kevin’s #2 suggestion of increasing the supination movement by adding a medial skive on purpose. I included this technique in last years experiment which attempted to increase to ROM of the 1st MPJ by using an orthotic. A small part of the investigation included comparing the change in RCSP with and without, to an increase in the fist MPJ ROM. I was very very surprised to find that a change in RCSP, due to an orthoses, did not produce correlation to the resulting increase in range of motion. Actually, a greater increase in 1st ROM resulted in individuals who did not get a change in their RCSP. I was sure it would! If anyone wants to see the data before PFOLA in November, I would be pleased to send it.

    It doesn’t matter what message you send Caesar, what matters in if he gets it.
    Marcus Brutus, 44 BC

    Paul R. Scherer
     
  6. First ray hypermobility vs. decreased first ray dorsiflexion stiffness

    Good to see you join in on the discussion, Paul. I always am interested in your thoughts on subjects such as these.

    I just received my July 2005 issue of Biomechanics Magazine that has the article that Tom Roukis and I wrote on the problems with the term "first ray hypermobility". The paper can be accessed with references (but, unfortunately, without the five drawings I did for the paper) at the following URL: http://www.biomech.com/showArticle.jhtml?articleID=165700382

    It is my firm belief that the term that we propose in the article, decreased first ray dorsiflexion stiffness, will ultimately replace "first ray hypermobility" in the podiatric and foot biomechanics literature. I will be lecturing on this subject at the PFOLA meeting in November in Vancouver that Paul is organizing (by the way, for all of those interested, I'm not again lecturing on "abductory twist" this year).

    In order to understand first ray biomechanics, the term describing it must include not only motion (i.e. mobility) but also force. "Hypermobility" only describes motion (i.e. kinematics) not force (i.e. kinetics). "Stiffness" is the standardly used term within the international biomechanics literature for describing the load vs. deformation characteristics of joints. Therefore, the term decreased first ray dorsiflexion stiffness is the most scientifically accurate and unambiguous term that can currently replace the imprecise, ambiguous and innaccurate term "first ray hypermobility".

    Actually, the paper just published in Biomechanics Magazine is a condensed version of a series of six Precision Intricast Newsletters that I did on the concept of stiffness and compliance, dorsiflexion stiffness in metatarsals and dorsiflexion stiffness of the first ray. I think that once the significance of the material in my newsletters and our article is digested further by the profession, then it will become clear that the term decreased first ray dorsiflexion stiffness is the only logical choice for a term to describe the load vs deformation characteristics of the first ray. By understanding my reasoning for my suggested change in terminology, it is my hope that this will elevate our level of understanding of first ray biomechanics and allow podiatrists, orthopedists, physical therapists and pedorthists to discuss foot biomechanics with the same scientific accuracy that the biomechanists and engineers do.

    In other words, this is one of my goals ....to help build a bridge of accepted and unambigous terminology between clinicians and researchers involved in foot and lower extremity biomechanics.
     
  7. Bruce Williams

    Bruce Williams Well-Known Member

    Gentleman;
    Excellent discussion as always. Great to see you posting Paul! Nice article Kevin, I've almost finished it... all these new definitions to understand! ;)

    I would like to make a few comments re: the casting position, as I have probably progressed thru most of the "usual" techniques and have ended up devoloping a new one that I feel is much improved.

    First, I am a strong proponent of nwbing STJ casting and plantarflexing the 1st ray. I re-learned this from a ProLab seminar and it made a huge difference in my orthotic outcomes!

    Second, I of couse regularly use 1st ray cutouts and strongly support them and verify their benefit many time weekly w/ the use of F-scan.

    I will say that initially I used many reverse morton's extensions, a la ProLab (or whoever coined the term). They do work and they work much much better than w/o a 1st ray cutout. In fact they work best with a cutout and a back fill of 1/16" or 1/8" ppt to delay the plantarflexion of the first ray, and to increase the, sorry Craig, proprioception of the 1st mpj.

    Third, I will use medial heel skives on patients who have a RCSP calcaneal valgus, and have calcaneal supinatory ROM. I mention this because many patients with advanced pronatory changes will have little to no supinatory or inversion ROM of the calcaneus - try it sometime when they are in STJ neutral nwbing. You'll see what I mean, and you'll realize what a disaster it would be to use a medial skive on them. It hurst too damn much.
    In those patients, usually there will be some form of fixed FF varus, and this will better work with a fully inverted post of the entire device. At a minimum, equally posted FF and RF 2-4 degrees inverted. IMHO!!!

    Finally, much of the need for modification w/ reverse morton's extensions, Cluffy wedges, and plantarflexed 1st rays can be eliminated thru the use of a change in the nwbing STJ neutral casting technique.
    It is a simple matter to place these feet in a slightly pronated neutral position nwbing and to plantarflex the 5th ray during casting. What you will find is that when you do this, and then attempt to plantarflex the 1st ray, that the 1st ray will already be in its maximally plantarflexed position 99% of the time! Again, try it on your patients and what you will see is a tremendous difference.
    Also, you will find that most of the casts from this technique will already be balanced FF to RF w/in 1-2 degrees of perpindicular. Little or no correction or posting will be necessary to the devices! Plus they fit great and are instantly accepted by the patients!

    So, I agree very much with what Larry wrote, though I feel it does need to be updated, again IMHO!

    Cheers to you all, and I hope to see you in the fall in Vancouver! :)
    Respectfully;
    Bruce Williams
     
  8. efuller

    efuller MVP

    Being helpful

    Paul, I have to disagree with you about the appropriateness of the term voodoo biomechanics in this forum. Craig asked for opinions on the statements made in your Lab’s newsletter. I’m assuming that the people who read the biomechanics section on Podiatry Forum are in the percentage of the profession who understand that there are multiple theories on the biomechanics of the foot. At least they are willing to think about and debate the topic. I don’t think that you can blame people who are critical of one the many theories for the lack of interest in understanding biomechanics in our profession. I also disagree with notion that calling the statement that casting position for a functional orthosis will help determine the position of the foot on the orthosis voodoo is not helpful.

    It is important that a practioners believe that what they do will help the patient. Some people believe that voodoo medicine works. Biomechanics can work at that level. You can follow a cookbook approach to biomechanics and achieve success. I’m sure there are a lot of people out there who do biomechanics and regard it as bio-magic. I recall Bill Orien using the term magic shovels in reference to orthoses. He is a very entertaining lecturer. I don’t think that the magic shovel comment made people not want to understand biomechanics.

    So why do a such a small percentage of podiatrists understand the, in your words, known mechanics of the foot. I think a major part of this is that we think we know more than we do. We have made observations, casting the foot in neutral position and casting the foot in neural position with the first ray plantar flexed and then making an orthosis from this cast seems to make patients better. This is an observation and not really mechanics. The podiatric biomechanics that I was taught in school is a series of brilliant observations that are not really tied together into a coherent theory. (A good example is that the concept of partially compensated varus is a brilliant observation. However, the attempt to combine this observation with neutral position confuses the issue. A partially compensated varus can be explained without using neutral position.) A series of observations does not make a coherent theory. We tell students to cast in neutral position without a really good rationale for doing so. This can lead to skepticism and reluctance to use orthoses clinically. I really feel that skepticism of the statement that plantar flexing while casting will lead to plantar flexing on top of an orthosis is a good and natural thing. I feel that this skepticism is why such a small percentage of podiatrists “understand” biomechanics. The purpose of my post “Voodoo biomechanics just might work” was to provide an additional theoretical explanation as to why plantar flexing the first metatarsal works. Hopefully, coherent theories should lead to a better understanding of foot mechanics.

    I apologize if you took offense at the voodoo comment. I really don’t have a problem with people practicing biomechanics at level where you do something just because you were taught to do it that way, as long as it works. However, in an academic setting I feel that it is important to have a plausible reason why you do what you do. You can teach what to do and why to do it, and if the student only remembers what to do that’s ok, most of the time.

    Paul continued:
    I don't quite understand the logic behind your assumption. Nor do I understand it for plantarflexion during casting. An orthosis usually ends behind the metatarsal heads and the original work that you and Tom did was placing pressure directly under the met head.

    An orthotic made from a cast that had the met dorsiflexed will tend to have a lower arch than an orthotic made with cast plantarflexed, assuming the same arch fill. What is the difference between the former with minimal arch fill and the later with maximal arch fill? I agree with notion that you should avoid too much arch height in the orthosis with an eqinous, because the orthotic will hurt.

    On dorsiflexion stiffness vs hypermobility. Is decreased stiffness (hypermobility) a cause or result? Does decreased stiffness cause pronation or does high medial column load lead to damage of ligaments that then is expressed as decreased stiffness? So, if we see decreased stiffness what should we do?


    Fine discussion,
    Eric
     
    Last edited by a moderator: Jul 25, 2005
  9. GarethNZ

    GarethNZ Active Member

    I'm with Eric on the lack of evidence for plantarflexing the 1st ray while in neutral. If I make a modified Root orthoses that is anything of reasonable prescription, what ever happens to the 1st ray during casting will not alter the mechanics of the 1st ray. It will depend on how much arch fill is placed on the positive cast while at the lab. I would agree that a fore-foot valgus pad/wedge (reverse mortons extension) will help create 1st ray function.
     
  10. bev Durrant

    bev Durrant Welcome New Poster

    I am not sure that we are talking about 1st ray hypermobility anyway as this implies that there is an increased range of motion and what we are talking about is directional motion ocurring at the wrong time. The 1st ray is designed to have the motion that it has....just not to function in a dorsiflexed position during propulsion. So maybe hypermobility was an inappropriate term in the first place??
     
  11. Craig Payne

    Craig Payne Moderator

    Articles:
    6
    There may be a lack of direct evidence for plantarflexing the 1st ray (its certainly logical and intuitive and an indirect case can be made using available evidence, eg Roukis mentioned aove)..... but we have data from a project we abandoned for ethical reasons in which for the orthoses we deliberately added a foefoot varus post in those who did not need it (ie dorsiflex the first ray) --- we gave up for safety reason -- the orthoses hurt and people wearing them had balance problems.
     
  12. This is becoming an interesting discussion regarding plantarflexion of the first ray during negative casting. I don' use this negative casting technique as much as I previously did during my Biomechanics Fellowship 20 years ago.

    Plantarflexing the first ray in the negative cast will increase the medial arch height and increase the everted forefoot deformity or decrease the inverted forefoot deformity in the cast. However, whether that change in first ray plantarflexion angle in the negative cast actually causes increased first ray plantarflexion moment in the weightbearing foot (versus not performing that negative cast modification) is very much dependent on how the positive cast is built and the various permutations of foot orthoses that can be made from that positive cast. Suffice it to say that I don't believe that plantarflexing the first ray during negative casting necessarily increases the plantarflexed position of the first ray during weightbearing activities.

    Just like we speak of rotational equilibrium of the subtalar joint, we must be careful when we speak of first ray position and moments acting on the first ray. Making an orthosis that puts more orthosis reaction force (ORF) at the first metatarsal-cuneiform joint and less ORF at the first metatarsal neck would certainly cause increased external first ray plantarflexion moment (or decreased first ray dorsiflexion moment). However, whether this modified orthosis would actually plantarflex the first ray during weightbearing activities would be dependent on the other external and internal forces/moments that are simultaneously acting on the first ray during these activities.

    The physics concept of rotational equilibrium tells us that if the first ray has not moved when a new or additional moment has been applied to it (i.e. such as when standing on an orthosis made with a plantarflexed first ray), then there must be some other moment acting at the same time that is acting in an opposite direction to oppose that moment. If this were not the case, then the additional moment acting on the first ray would tend to cause rotation acceleration of the first ray in the direction of the moment acting upon it.

    In addition, if we are to speak just in terms of moment arms, any orthosis modification added at the metatarsal head level of the orthosis (i.e. as a forefoot extension) has significantly longer moment arm to produce either an increase or decrease in first ray dorsiflexion or plantarflexion moments. Therefore, the traditional Root orthosis, that rarely had any forefoot extensions, have much less potential to increase or decrease first ray sagittal plane moments than orthoses that rely on the use of forefoot extensions to either increase or decrease the ORF under one or more of the metatarsal heads.

    The dorsiflexion stiffness of the first ray will determine whether that forefoot extension under the first ray (i.e. Morton's extension) will tend to simply dorsiflex the first ray relative to the second ray or tend to cause STJ supination. Feet with higher arch height will generally have increased first ray dorsiflexion stiffness and will tend to have more STJ supination moment with a Morton's extension than a foot with decreased first ray dorsiflexion stiffness. Increased dorsiflexion stiffness of the first ray means that greater force will be necessary to produce a given dorsiflexion motion of the first ray. Therefore, in the foot with increased first ray dorsiflexion stiffness, the first ray will be more likely to resist dorsiflexion relative to the second ray when a Morton's extension is added to the orthosis than in a foot with decreased first ray dorsiflexion stiffness where the same Morton's extension would easily cause first ray dorsiflexion.

    I strongly believe that the concept of dorsiflexion stiffness of the first ray will open up new avenues of intellectual growth for podiatrists in biomechanics just as the concept of STJ axis deviation and rotational equilibrium of the STJ has done so in the past.
     
  13. efuller

    efuller MVP

    Stiffness a result of load?

    Kevin, I agree that dorsiflexion stiffness will be a part of what determines whether or not the STJ supinates in response to increased force on the first ray. There are many contributers to moment about the STJ and the moment from ground reaction force is one of them. The moment from ground reaction force is determined by the location of the center of pressure relative to the STJ axis. (Direction of ground reaction force and position of the axis in both the sagittal and transverse plane are important in this calculation.) The stiffer the first ray the greater probability that the center of pressure will be located force medially and hence a more medial center of pressure. That said the first met head is often medial to the STJ axis and the stiffest first ray will not create a supination moment from ground reaction force with a medially positioned axis. Reduced pronation moment yes, but there is still a pronation moment in this instance. For this reason I think STJ axis location is more important to look at as a factor in foot function when compared to the importance of first ray dorsiflexion stiffness.

    What do people think about the idea that decreased stiffness of the first ray is the result of repettative high loading of the first ray? When treating first ray problems do you want to increase load on the first ray to increase its load to shift the center of pressure more medially or do you want to try and decrease the load because there is evidence that there has been high stress on the first ray? I lean toward the latter. The experiment craig had to halt is interesting in this regard. On the other hand if you have an overloaded 2nd met I have no problem increasing load on the first.

    I'm enjoying the discussion.

    Eric
     
    Last edited by a moderator: Jul 27, 2005
  14. Bruce Williams

    Bruce Williams Well-Known Member

    I think the point is not whether the orthotic will increase the plantarflexed position of the 1st ray, as much whether the casted position, and any cutouts used (extensions as well) will delay the dorsiflexion of the ray.

    So, in that vain, casting without plantarflexion of the ray should tend to increase dorsiflexion of the 1st ray and cause more FnHL and more STJ pronation, or prolonged position. But, if you add a cutout, or groove, and/ or plantarflex the 1st ray, you will have less of a negative dorsiflexion moment on the 1st ray, and therefore decrease the effects of FnHL and increase the supination moments at the STJ.
    Did I get the moments right Kevin? I always screw that up! ;-)

    Bruce Williams
     
    Last edited by a moderator: Jul 27, 2005
  15. Bruce Williams

    Bruce Williams Well-Known Member

    Eric;
    I do not think that repetitive high loading of the 1st ray will lead to decreased stiffness of the 1st ray. I think the problem is much more complex than just that. To me that is counter-intuitive, unless a structural hallux limitus is created. But then, with a structural limitus, you will get increased stiffness in many of these casses as the met-cun and mp joints become arthritic and dorsiflex towards end ROM and stay there.

    My inclination with adequate 1st mpj extension and FnHL is to attempt to increase the load on the 1st metahead and increase the foreces under the 1st ray. Especially w/ lesser met pain. Big cutouts, more malleable foot, larger and taller metapads, medial skives etc. These patients tend to have much less pressure sub 1st ray on f-scan.

    But, in patients with a structural problem you do something similar, but much less. They don't have the motion to gain at the STJ or the 1st ray, so you must be more subtle in the attack. Smaller cutouts with equal PPT fill ins, varus posts instead of skives, and more subdued metapads. Often these patients will have more pressure medial colum and sub 1st mpj on F-scan, not always, but often.

    It really depends on the stiffness of the 1st ray, the less stiff it is the lower the sub 1st mpj pressures, despite the arch height!

    Interesting question, with many answers Eric.
    Sincerely;
    Bruce
     
  16. You are right about the orthosis modifications and that they will delay dorsiflexion of the first ray. A better, more accurate, way of saying this would be to say that the orthosis modifications which increase first ray plantarflexion moment would also, tend to decrease the overall first ray dorsiflexion moments (i.e. rotational equilibrium about first ray axis). And, as a result, this decrease in first ray dorsiflexion moment would reduce the tendency for the first ray to dorsiflex during the late midstance phase of gait.

    I would not use the term "negative dorsiflexion moment" since this is confusing term and an unconventional method of describing moments acting across a joint axis. An orthosis modification or other external or internal force can only act in one of five ways across the first ray axis:

    1. Increase the magnitude of first ray dorsiflexion moment.
    2. Decrease the magnitude of first ray dorsiflexion moment.
    3. Increase the magnitude of first ray plantarflexion moment.
    4. Decrease the magnitude of first ray plantarflexion moment.
    5. Cause no change in magnitude of sagittal plane moments across the first ray axis.

    I believe then, it would be more correct and clear to say instead "But, if you add a cutout, or groove, and/ or plantarflex the 1st ray in the negative cast, you will have an increased magnitude of first ray plantarflexion moment from the orthosis , and therefore decrease the tendency to develop functional hallux limitus during gait and increase the supination moments at the STJ."
     
  17. I agree. Sounds like a quote from our chapter, doesn't it??

    I agree. The first ray with increased magnitude of dorsiflexion stiffness will decrease the magnitude of STJ pronation moment in the foot with a medially deviated STJ axis since it will tend to shift the center of pressure (CoP) more medially on the plantar forefoot. STJ axis spatial location and relative dorsiflexion stiffness of metatarsal rays both have a large effect on biomechanical function of the foot and lower extremity during weightbearing activities.

    This is where speaking in terms of first ray dorsiflexion stiffness adds considerably to the ability to precisely describe the mechanics of the foot during gait.

    Of course, over time, high loading forces in a first ray that is supported by ligaments that are within the plastic range of their load-deformation curve will tend to show a decrease in dorsiflexion stiffness with repetitive loading. However, in the foot with increased medial arch height, the plantar fascia and plantar intrinsic ligaments of the medial arch will likely be at a lower or more elastic range within their load-deformation curve that will allow them to not develop an decrease in dorsiflexion stiffness of the first ray with repetitive loading activities. Instead, these feet may develop sesamoiditis or plantar tylomas at the first metatarsal head.

    One other factor is that increasing medial forefoot pressure with an orthosis, such as Craig's experiment, will also tend to increase the STJ supination moment in late midstance where this may cause STJ supination instability during propulsion, thus causing the individual to shorten their stride length and shorten the propulsive phase of their walking gait. I have demonstrated this effect numerous times in gait lectures at seminars where I have added a 6 mm first metatarsal pad to the subject's foot and then measured their stride length versus the stride length with no pad. The addition of a 6 mm thick first metatarsal pad (that shifts the CoP more medially on the forefoot) will shorten stride length generally by over 1 cm per step in the average subject.
     
  18. Lawrence Bevan

    Lawrence Bevan Active Member

    dorsiflexion stiffness

    Kevin said
    "this may cause STJ supination instability during propulsion"

    Kevin could you expand on what you mean by this?

    Also would you predict that individuals such as those in the Kilmartin study eg juveniles with HAV would have perhaps had a measurable increased compliance in their 1st ray?

    Is it a possibility that those individuals who have a less than favourable outcome from HAV surgery eg recurrence, lesser met pain etc may also have a measurable mean increased compliance in their 1st ray?

    Is this a clinically measurable variable?

    Interesting stuff, thank you.
     
  19. Lawrence Bevan

    Lawrence Bevan Active Member

    dorsiflexion stiffness

    Just tried out the postioning technique suggested by Bruce - semipro + plantarflexing 5th ray

    I used my lab tech who has mild HAV and a very compliant 1st ray, higher pressure sub-2nd with F scan and and moderate ff valgus. She is also Peruvian !

    On examination from a supine position plantarflexing the 5th and maintaining the semi-pro position is technically sl tricky but seems to plantarflex all the rays including the 1st.

    From a prone position one observes that a semi-pro STJ position prevents one getting an adducted FF. Pulling down the 5th plantarflexes the entire ff. I think it is basically like plantarflexing the 1st ray in the casting position and "balancing" it at the same time. IE plantarflexing the 1st ray alone gives an everted FF that one might balance with a valgus post - the valgus post will increase the lateral arch in the orthotic. Bruces casting position achieves this without plaster work or extrinsic posting. V neat!

    One could refine this further? If you do the above and observe prone a parallel ff-rf and you want inc STJ moment add a RF post. If you do the above observe still an overall ff valgus and you want inc STJ moment leave it - it will invert the device slightly. If you dont want that remove some of the valgus whilst casting.

    Would one take the Kevins suggested terminology further? Medial STJ axis deviation in some could represent inc compliance/decreased stiffness in the transverse component of STJ/MTJ motion?
     
  20. Bruce Williams

    Bruce Williams Well-Known Member

    Lawrence;
    Glad you liked the new casting technique. I like your perspective on varus or valgus posting the cast as needed, I just have the lab balance it to perpindicular at the heel bisection. Some of the patients that can be cast this way will also stand with a slight valgus heel. You can use a 2 degree medial heel skive with them as well if they have available motion at the calcaneus. Beware if they do not, it will be poorly tolerated!
    Sincerely;
    Bruce Williams
     
  21. Increased medial positioning of the center of pressure (CoP) during late midstance will tend to increase the magnitude of STJ supination moment. At the initiation of propulsion, the gastrocneumius-soleus complex (GSC) must have strong contractile activity to increase ankle joint plantarflexion moment, but this contractile activity will also increase the magnitude of STJ supination moment (assuming normal STJ axis spatial location). Therefore, if the CoP is too medial during late midstance and early propulsion, the body will "shut down" the GSC earlier during propulsion to avoid supination instability of the ankle-STJ complex. This early cessation of GSC contractile activity will also be seen clinically to shorten the propulsive phase and decrease ankle joint plantarflexion during propulsion. Lateral forefoot wedging will eliminate some of this effect by producing a more lateral positioning of the CoP, thus counterbalancing the STJ supination moment from GSC contractile activity during propulsion.

    I haven't quite had the time to write this paper yet.
     
  22. Lawrence Bevan

    Lawrence Bevan Active Member

    U still use your Amfit Bruce?
     
  23. Bruce Williams

    Bruce Williams Well-Known Member

    Yes I use the Amfit machine every day. I no longer cast w/ plaster, except for Richie Brace's and Arizona/California Braces.

    I am able to see where I want the foot, position it well after manipulation, and add all of the prescription additions that I want and where I want them.
    I've even figured out how to build in a 1st ray cutout or groove about an 1/8" deep. We fill this back in w/ 1/8" ppt as a kinetic wedge modification, a la Dananberg! ;-)

    I can add the heel lifts, a medial heel skive, reinforce the heel cup or the entire device etc invert it, evert it whatever.

    Its an amazing machine and program, and now I can't complain about my devices being made wrong! If I do, then I have to go talk to the mirror about it!

    Also the neat thing about the medial / lateral skives, is that you can "see" and improved foot position when you add it into the device on the 3D imaging of the device.
    Bruce
     
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