Midfoot position, ROM and stiffness

Dan

Two seperate tests:

1. Forefoot Stability Test (described to me by Charlie Baycroft and is the one you mentioned; ie the direction the midfoot initially moves when raising on toes)
2. Lateral Forefoot Elevation Test (how far the patient can get the lateral column off the ground; Eric Fuller talks about it a lot)

 

Hi Craig,

Thanks for clarifying that.

Regards,

Dan

 

The concept of midtarsal joint stiffness is useful when discussing the load vs. deformation characteristics of the foot under the loads of weightbearing activities, or approximately 0.5X body weight (BW) in bipedal standing, 1.2X BW in walking, 3X BW in running and up to 7X BW in high jumping. Until I can manually exert 500 pounds of force on the foot of one of my 167 pound runner patients in order to determine the stiffness of their MTJ during running then I will not be able to adequately determine MTJ stiffness in an open-kinetic-chain examination. Manual examination techniques are simply unable to duplicate the magnitudes of forces which our patients' feet experience during their daily weightbearing activities. Watching the foot function during weightbearing activities is a more reasonable way to determine MTJ stiffness in most instances.

 

Thank you Kevin, very much.

Rebecca

PS: Hi Mark, David and Dan, its nice to put a Bootcamp face to some Podiatry Arena names

 

Hi all

I would just like to clarify this ROM / stiffness thing a bit further. :morning:

I can see that getting an accurate assessment of stiffness requires large forces to be applied and is just not possible in a nonweightbearing examination.

However, I'm sure I'm right in saying that by putting the 3 axes of the midfoot through their ROM in a nonweightbearing examination (as I described earlier) gives us an educated guess of the joints' stiffness. Granted, gait examination and weightbearing tests such as the lunge test would be more suitable in providing a more functional picture of the joint ROM and stiffness as the forces applied are more like 'real life'.

My main question here is: Is ROM directly proportional (not sure if that's the right word, maybe directly related) to stiffness. That is, if a joint has a large ROM, then it is less stiff. Can the two words be used almost interchangeably?

Rebecca

 

Hi Rebecca,

As soon as got back from bootcamp, I've been thinking about the exact same concept as you have just mentioned, "How do we assess for MTJ stiffness?" My initial thoughts were to logically perform NWB ROM tests of the MTJ (as you’ve just pointed out) to try and get a feel for the degree of stiffness in the MTJ. My assumption would be that if there is reduced stiffness in the MTJ when performing NWB ROM tests, then it would be logical to assume that there is reduced MTJ dorsiflexion stiffness during WB activities (but I may be wrong, Kevin might need to jump in here).

I was thinking, perhaps in conjunction with NWB ROM tests, the FPI might be a good way to assess MTJ stiffness, as we are assessing the foot in all planes, which if I stand correct, would correlate to the A-P, M-L, and Y MTJ reference axis. What say you?

Cheers,

Dan

 

Rebecca:

The concept of midtarsal joint (MTJ) stiffness is a concept that I am introducing to podiatry to better explain the mechanics of the foot, however, I have not measured it yet. In fact, I don't think anyone has measured it yet. The way it would be measured would be to apply a known loading force on the forefoot and then measure the motion of the forefoot relative to the rearfoot that occurs as a result of the applied force. If this is then measured again and again with increasing loading forces, with resultant increased forefoot to rearfoot motion, the slope of that resultant plotted load vs deformation curve would give us the stiffness for the MTJ at any loading level or deformation level.

To more specifically answer your question, if there is greater range of motion of the MTJ than normal when you apply a given load to the forefoot, then, yes, the MTJ passive stiffness is less than normal. If there is less than normal range of MTJ motion, then the MTJ passive stiffness is greater than normal.

I see the beauty of talking in terms of stiffness for the MTJ as being four fold:

1) Saying the MTJ is "less stiff" gets away from terms that can't be quantified such as "hyperflexible" or "hypermobile" and saying the MTJ is "more stiff" gets away from the unquantifiable terms such as "rigid" or "fixed".

2) Using stiffness to describe the MTJ makes clinicians realize that the MTJ can not be "locked" since the MTJ is a spring-like mechanism that will deform more with increasing load. The MTJ can be said to be "stable to dorsiflexion load" instead of being "locked".

3. Using stiffness to describe the MTJ gives clinicians and biomechanics researchers a common language so that a better understanding of foot biomechanics and better therapeutic measures for mechanically-related foot and lower extremity pathology may be developed over time.

4. Stiffness is progressively being used within modern biomechanics research papers to describe the load vs deformation characteristics of the foot and lower extremity during weightbearing activities. It is time we start using this more accurate and less ambiguous terminology for all joints of the foot and lower extremity.

I'm just trying to point podiatry in the direction in which it should be headed.

 

Kevin,

You beat me to it. Since submitting my last post, and hearing from Dan B, I have realised that stiffness is not something we necessarily have to measure. We know what it is, we see the effects of it in gait. It is a concept that allows us to move away from the old MTJ theory and towards describing what happens with more appropriate terminology.

Thanks for staying with me Kevin. At no time have I doubted anything you said. I am very grateful to be able to speak freely to such an expert and receive the most current advice. :drinks

I am a relatively remote private practitioner and certainly on my own by many hundreds of kms. But thanks to Podiatry Arena and its contributors I am by no means at a disadvantage. I'm pretty sure you know that means a lot to me and others like me Craig, but it has to be said.

Regards
Rebecca

PS: what do you think Dan?

 

Hi Rebecca,

Yes, I am in complete agreement with you and Kevin. I am very grateful that you started this thread and I am also very appreciative of Kevin's (& eveyone elses) input. To be honest, I was thinking the exact same thing as you since bootcamp, so I was really glad when I discovered you had started this thread.

I am in a similar proposition to you in terms of working in a solo private practice, so I know how you feel. It would be nice to work alongside another colleague to discuss these concepts. I don’t know where I would be if Podiatry Arena did not exist!

Thanks again,

Dan

 

I circulated this thread to other staff in the Dept as an eg of the sort of thinking that the Boot Camp stimulated, but also pointed out that this is the kind of deeper thinking that we try to instill in the undergraduates (with mixed success) so they can become more critical thinkers on graduation (with mixed success). ... so appreciate the comments :dizzy:

 

So there are two components to stiffness: passive stiffness (due to the tautness of ligaments and joint morphology) and active mediated stiffness (from intrinsic and extrinsic muscular contraction and integrity).

What can we do if we need to increase passive stiffness in a planus foot:
I wouldn't think you can do much conservatively. But you could increase passive stiffness by surgically repairing ligaments or fusing joints.

To increase active mediated stiffness in a planus foot you can:
Use strengthening exercises to improve the function of muscles
Surgically repair musculotendious units eg: posterior tibialis in PTTD
Dry needling or trigger point therapy for inhibited muscles eg: peroneals

To increase stiffness via external measures for a planus foot we can use shoes and orthoses (and other in-shoe devices). Just putting a shoe on or a pretty basic arch support in your shoe, I expect will be increasing midfoot stiffness. Then in regard to prescription variables for orthoses stiffer shell materials, EVA arch fills and other methods of increasing shell stiffness (higher heel cups, extended rearfoot posts), increased arch contouring, others?

To reduce passive stiffness in a cavus foot you could:
Use mobilisations and manipulations to release stiff joints
Surgically lengthening of tight ligaments or arthroplasty of joints.

To reduce active mediated stiffness in a cavus foot you can:
Use stretching exercises to reduce the tension in a musculotendinous unit
Surgically lenthening musculotendious units
Dry needling or trigger point therapy for tight muscles

To reduce stiffness via external measures of a cavus foot, apart from going barefoot, I guess we choose footwear or prescribe orthoses that don't increase stiffness in the midfoot unecessarily eg: Brooks Beast or the orthosis prescription variables mentioned above.

Does this seem like a reasonable way to discuss this? Anything to add or refute about the above?

Also, I have asked this previously, but in regard to the old MTJ theory, supinating the STJ reduces MTJ ROM and pronating the STJ increases MTJ ROM. So unless this is wrong, another way to increase or reduce midfoot stiffness via external measures would be to use the prescription variables that supinate of pronate the STJ eg: rearfoot post, medial or lateral heel skive etc etc.

It might seem like I am being a smartarse but I am just trying to make sure I am not getting it wrong. It is very satisfying to actually get what goes on at the midfoot.

Rebecca

 

Agree with the sentiments- a great thread! :drinks

Just to go off on a slightly different tangent with this...
We have often talked about the idea of the midfoot 'locking' (I know, I know,- this is controversial... don't take me task on it). Perhaps it may be more valid to say that there is a change in passive stiffness of the foot as the STJ supinates and there is subsequent midfoot pronation to keep the foot on the ground.
Encouraging this via an orthosis would be a form of increasing passive stiffness via external measures. This would be independent of the stiffness of the orthosis itself.
Does this make sense??? What say you all??

 

Rebecca and Colleagues:

I basically agree with most of the above. By supinating the STJ away from a maximally pronated position, the passive dorsiflexion stiffness of the medial-lateral midtarsal joint (M-L MTJ) axis will be increased due to the increased dorsal-plantar thickness of the MTJ-midfoot. There is also a correlation of STJ axis spatial location to M-L MTJ axis dorsiflexion stiffness. As the STJ axis location becomes more medially deviated, in general, there is a decrease in M-L MTJ dorsiflexion stiffness. In addition, as the STJ axis location becomes more laterally deviated, in general, there is an increase in M-L MTJ dorsiflexion stiffness.

The decrease in M-L MTJ dorsiflexion stiffness is caused by the distal talar head being located more medial and plantar relative to the distal calcaneus as the STJ axis becomes more medially deviated. The increase in M-L MTJ dorsiflexion stiffness is caused by the increase in "stacking of the talus on top of the calcaneus" as the STJ axis becomes more laterally deviated. In other words, in general, medial STJ axis deviation will decrease the dorsal-plantar thickness of the midfoot and lateral STJ axis deviation will increase the dorsal-plantar thickness of the midfoot. Increased thickness of the midfoot means, in general, increased M-L MTJ dorsiflexion stiffness. This is an observation that I made about 20 years ago but have never before written about it...you heard it first here on Podiatry Arena!

By the way, you can forget about Elftman's idea of "converging or parallel MTJ axes" as causing the reduction in apparent motion in the MTJ with STJ supination, it is just not possible to have imaginary axes restrict or allow motion in the foot, or anywhere else, for that matter (Elftman, H.: The transverse tarsal joint and its control. Clin. Orthop., 16:41-44, 1960). It is interesting that Elftman's ideas are still taught to this day in podiatry medical colleges around the world. We have a lot of work to do in this profession!

 

Poor Craig, sorry you don't get the opportunity to really kick back.

Nevertheless, your project does sound fantastic!

Regards

Rebecca

 

In regard to this passive stiffness of ligaments:

If you get two people who are exactly the same, except one has a planus foot and one has a cavus foot:

Are the properties of their ligaments the same? That is, are they capable of stretching the same, will they fail at the same point? Not counting the effects of functioning planus or cavus over time. So if it was possible to take off a ligament and examine it, all other things being equal, would the ligements have the same properties??

Rebecca

 

Rebecca:

One reasonable hypothesis could be that an individual that congenitally has increased tensile stiffness in their ligaments may end up with a higher arched foot as an adult when compared to the individual with ligaments with decreased tensile stiffness. Another hypothesis is that all individuals have the same stiffness of ligaments and the higher arched foot and lower arched foot is due to different osseous morphologies. I believe it is probably a combination of factors such as these that determine the ultimate arch morphology of the foot in the adult.

The paper by Davis et al on the spring ligament complex showed a 6.45x difference in elastic modulus of the 8 specimens tested. That is a huge difference in elastic modulus of ligaments between individuals!! (Davis WH, Sobel M, DiCarlo EF, et al: Gross, histological, microvascular anatomy and biomechanical testing of the spring ligament complex. Foot Ankle Int. 17:95-102, 1996.)

 

Thanks Kevin, that makes sense.

Considering the larger tensile forces applied to the ligaments of planus midfoot compared to the cavus midfoot whose ligaments are subject to less lensile forces:

Would it be fair to say that over time, the ligaments of a planus foot stretch to a longer length and the ligaments of a cavus foot don't, or even contract?

When I say ligaments, I include the plantarfascia.

Rebecca

 

I would be fair to say that the ligaments of a pes planus foot are subjected to much larger magnitudes of tensile forces than the ligaments of a pes cavus foot and that this may eventually allow gradual flattening of the arch over time due to the ligamentous creep that may occur, especially if the muscles do not intermittently reduce the tensile force on the plantar ligaments during weightbearing activities. The only extrinsic foot muscles that have the capacity reduce the plantar ligament tensile forces during weightbearing activites are the posterior tibial, flexor digitorum longus, flexor hallucis longus and peroneus longus. The gastrocnemius-soleus-Achilles tendon complex is the major muscle which increases the tensile force in the plantar ligaments.

By the way, Rebecca and others, check out the paper by Davis et al I just added to my last posting. This is probably the best paper ever written on plantar ligament biomechanics and this paper has been very influential in my understanding of this subject over the past 10 years.

 

OK Kevin, many thanks. Rebecca

 

Craig:

Glad you are enjoying the topic. As the subtalar joint (STJ) supinates, the increased "stacking" of the talar head relative to the distal calcaneus will increase the passive dorsiflexion stiffness of the medial-lateral midtarsal joint (M-L MTJ) axis. If supinating the STJ in closed kinetic chain causes an increase in ground reaction force (GRF) under the lateral metatarsals and decrease in GRF under the medial metatarsals, this will cause increased dorsiflexion moment at the lateral midfoot-MTJ joints and decreased dorsiflexion moment at the medial midfoot-MTJ joints. The forefoot will remain plantigrade as long as the STJ supination is not so great that the medial forefoot is dorsiflexed away from the supporting surface by the STJ supination motion.

When a foot orthosis is placed under a foot, the orthosis is not increasing either the passive stiffness or actively mediated stiffness of the M-L MTJ. Rather, M-L MTJ dorsiflexion stiffness is a measure of the intrinsic load vs. deformation characteristics of the foot within the sagittal plane. The addition of an orthosis to the foot does not change the intrinsic stiffness of the foot, it simply changes the magnitudes and plantar locations of the loading forces occurring on the plantar foot, what I have been calling for the past 15 years orthosis reaction force (Kirby KA, Green DR: Evaluation and Nonoperative Management of Pes Valgus, pp. 295-327, in DeValentine, S.(ed), Foot and Ankle Disorders in Children. Churchill-Livingstone, New York, 1992). [See PDF copy of chapter below.]

In other words, if the orthosis pushed on the foot the same way that GRF does without the orthosis, then the MTJ stiffness would be unchanged. However, since the orthosis pushes with more ORF plantar to the MTJ and the orthosis causes less GRF plantar to the metatarsal heads, the forefoot dorsiflexion moments will be decreased with the orthosis. However, that is not to say you could not come up with another measurement parameter that measured the change in arch height of the foot to varying load and how that arch height changed with orthosis intervention. Possibly you could call it "Orthosis Mediated Forefoot Dorsiflexion Stiffness"?.

 

... pretty sure I'm right to make this correction (in bold)

As the subtalar joint (STJ) supinates, the increased "stacking" of the talar head relative to the distal calcaneus will increase the passive dorsiflexion stiffness of the medial-lateral midtarsal joint (M-L MTJ) axis. If supinating the STJ in closed kinetic chain causes an increase in ground reaction force (GRF) under the lateral metatarsals and decrease in GRF under the medial metatarsals, this will cause increased dorsiflexion moment at the lateral midfoot-MTJ joints and decreased dorsiflexion moment at the medial midfoot-MTJ joints. The forefoot will remain plantigrade as long as the STJ supination is not so great that the medial forefoot is dorsiflexed away from the supporting surface by the STJ supination motion.

 

Thanks for watching my 6, Captain Asher!

 

Craig P

You have mentioned midfoot stiffness and the need to match it to orthotic shell stiffness (I can't find on PA where you mentioned it):

high midfoot stiffness - high shell stiffness
low midfoot stiffness - low shell stiffness.

My thoughts on this would be if the midfoot is not stiff, we need to stiffen it up with a high stiffness orthotic shell.
And if the midfoot is stiff, maybe not to add to the stiffness, so a low orthotic shell stiffness.

What do you think?

Rebecca

 

Its still a learning curve, but as I mentioned at Bootcamp, when you try to analyse some of the proposed "newer" clinical biomechanical theories (without getting into the debate about if they really are new or worthwhile), such as Neoteric biomechanics, Bottom Block theory etc ... midfoot stiffness (they still call it flexibility) seems to be important in them. The concept is to match midfoot stiffness with orthotic shell stiffness (though they all use different terminology).

This is such a new area (we have just started measuring orthotic shell stiffness), that its hard to say what way its going.

Some now try to argue that the orthotic shell should be more conforming to the arch (less plaster fill), but have more flex built into it, in which case, the orthotic stiffness should probably match the foot stiffness.

Orthotics stiffness vs midfoot stiffness may also play a role in orthotic tolerance.

Others may argue as you have suggested

....so much to do and so little time....

 

My preliminary thinking on this would be if we are increasing the orthotic stiffness on a foot which has low midfoot stiffness, wouldn't the orthotic/ground reaction force at the midfoot be increased, exacerbating the problem? Similarily in a foot with increased midtfoot stiffness if we had an orthotic shell with a low stiffness this would not be creating the dorsiflexion force required at the midfoot. Furthermore isnt the increased forced needed due to the correlation between increased midfoot stiffness and equinus?

That thought is confusing me a little. Can anyone clear that up?

Dean

 

Hi Dean

The forces created by the orthotic on the plantar aspect of the foot are referred to as, orthotic reaction forces. In terms of stating that the GRF/ORF would exacerbate the problem, depends on what the problem is your referring to.

If you look at it intuitively, if for example you are treating a pt with proximal plantar faciitis and there are excessive pronation moments occurring in the foot, how are you going to treat this pt mechanically? Even if you don’t fully understand the concept of MTJ stiffness yet, you are probably still applying this clinically. For example, your orthotic prescription would probably include (I hope) a stiffer shell, reduced medial arch fill, 4/4 RF post etc etc. All of these prescription modifications would help stiffen the MTJ. So therefore, how would this make the problem worse?

However, the only problem (at this stage) I could see potentially happening, would be excessive compression forces acting on the plantar medial arch i.e. medial arch irritation, due to a high medial arch in the orthotic prescription.

A foot with increased dorsiflexion stiffness would actually function more effectively than a foot with lower dorsiflexion stiffness, because the medial arch is more resistant to arch flattening moments. I think it would be detrimental if you were to create more of a dorsiflexion force in the forefoot as you would be encouraging more arch flattening moments to occur.

In terms of addressing an equinus deformity, wouldn’t a heel raise incorporated into the orthotic design and stretches be more appropriate in alleviating the tensile stresses in the gastrocnemius-soleus muscle group complex? (Providing its not caused by an osseous malformation).

Regards,

Dan

 

Hi Dean,

I'm a little confused by your questions, but thanks for your input!

To my way of thinking, you put any device under the foot, it is going to increase stiffness in the sagittal plane.

But if you have a really flat arch which is not stiff (Kevin, is it OK to say flexible or not?), if you put cotton wool or something with minimal stiffness, the foot is going to do its own thing and function the way it usually does. So the orthotic shell stiffness needs to be enough to change something.

Maybe its a matter of finding the mifdlle ground between not stiff enough and too stiff?

Rebecca

 

Definately in agreement.

Totally agree, sorry I tried to use an example and it wasn't a very good one.

I just confused myself there without thinking what I was saying! I actually feel quite stupid now!! I will go eat some of my humble pie in the fridge. I think I am in that annoying learning/questioning stage that Kevin has pointed out.

Thankyou for clarifying this all Asher and Dan
Dean

 

Hi Dean,

No need to be sorry. I'm glad you understand now. Once you get your head around it all, it does start to make a lot more sense. Sometimes the only way to learn, is to just bite the bullet and ask the question. It's the only way to learn.

I think the most important part is moving away from the old terminology we used to use, as it just doesn't make any good biomechanical sense. I would strongly suggest you go back through and reread Asher's and Kevin Kirby's responses. The depth they have gone into re: this topic is priceless.

Cheers,

Dan

 

Don't feel bad, Dean. Like you, I feel a bit like a pre-schooler playing on the big kids play equipment. I think its worth it though.

Why in a stiff midfoot do we need more midfoot dorsiflexion moment? I don't know if we do or don't.

I suggested earlier that putting anything under the foot will increase its stiffness. And I also suggested that probably a foot with high midfoot stiffness doesn't need any further stiffness. Although no doubt its very important concept (and I admit I don't fully get it yet), maybe its not only about MTJ / midfoot stiffness. Maybe the way an orthosis works in a cavus foot is more about redistributing moments on the plantar surface of the foot, not affecting stiffness. Does this sound feasible? I hope Kevin can help us out.

I don't understand your question Dean.


Dan B, I'm not sure we can assume that a foot with a high FF dorsiflexion stiffness necessarily functions better than than that with a low FF dorsiflexion stiffness. Maybe as far as eg: plantarfascial tension, but possibly not as far as FF pressure pathologies?

Regards

Rebecca

 

Yes I would definately agree there, Asher. I suppose the next question I would ask, is, how do you determine the "ideal" amount stiffness for a particular foot type.?

That probably wasn't a good example I used. My intentions were to try and make clear to Dean, that there is definitely a distinct differecne between increased and reduced stiffness, which will in turn mean, that our treatments will have to be in accordance with these findings.

Regards,

Dan

 

Sorry that was not a clear question and Dan has reviewed my approach to the midfoot stiffness. What I was saying is that there is a correlation between increased midfoot dorsiflexion stiffness and ankle equinus.

Why I said this was in regards to the previous quote of reduced ankle joint dorsiflexion in the foot which has reduced midfoot dorsiflexion stiffness. But as Dan readily pointed out we address this by other means such as stretching, heel raises etc.

Sorry for confusing the topic, I hope I have cleared up where I was coming from Rebecca.

And great analogy, I think it is quite fitting,

 

Just read all this, very interesting, now I understand why I felt a bit ahead of the pack at Uni as a student, we'd worked all this out 20 years ago fitting ski boots and variouys types of orthoses materials....our rule of thumb was just about exactly what Kevin finished up with.....I knew I had a feel for it, just did not have all the formal terminology. Have used these ideas constantly since registering as a podiatrist and find them far more usful clinicaly than other widely published theories.
regards
Phill Carter

 

Rebecca: The concept of midtarsal/midfoot dorsiflexion stiffness is my attempt to theoretically explain many of the observations I have made clinically on the feet I have examined and treated over the past quarter century that make sense using known mechanical principles.

First of all, putting an orthosis under a foot does not increase the MTJ dorsiflexion stiffness of that foot. The foot orthosis only changes the foot's deformation under load by applying the forces to the plantar foot closer to the midtarsal/midfoot joints than normal so that decreased forefoot dorsiflexion moments and decreased forefoot dorsiflexion occurs with the orthosis.

A foot with high MTJ dorsiflexion stiffness would tend to better resist forefoot dorsiflexion than a foot with low stiffness. However, what may be necessary in one foot in one weightbearing activity may not be the same in another foot in another weightbearing activity. That is why we have a central nervous system that allows us to actively mediate MTJ dorsiflexion stiffness to "fine tune" the MTJ dorsiflexion stiffness depending on the activity requirements of the individual at that point in time.

Foot orthoses need to be designed with the following three goals in mind:

1. Reduce the pathological loading forces on the injured structural components of the foot and/or lower extremity.
2. Optimize gait function.
3. Prevent other pathologies/symptoms from occurring.

This may mean that a high arched foot that pronates excessively during running may need a very high medial arched orthosis to heal the individual's medial tibial stress syndrome. It could also mean that a high arched foot with metatarsalgia needs a relatively stiff orthosis with an increased anterior edge thickness to decrease the individual's plantar forefoot pain. I don't I can make any generalizations about foot orthoses and matching their stiffness to the MTJ dorsiflexion stiffness as long as goals #1-3 above are met in each patient's orthoses.

Hope this helps.

 

Along this line I would say that what you may need to do to change some ones presenting problem may not actually be tolerated by their particular foot type, so although you may be able to make a few generalizations about what may or may not work to effect a particular problem, that particular individual may not be able to tolerate the solution. This is the amazing variety of human anatomy rearing its head again
regards Phill

 

Kevin I read this thread today and wonder if you could expound on the above modification for a cavus foot type.

 

Asher (Rebecca)

I would like to add one extra concept that may reflect on the choice of materials for orthoses. The properties of visco-elastic materials such as tendons etc is that there stiffness is dependent on the rate of loading. Therefore (theoretically) the running foot may function stiffer than the walking foot due to the increased loading rates = The amount of ORF (Orthotic reaction force) will vary based on the above.(?)

In practical terms this means that if I am prescribing an EVA or PU device for sports, I will make sure that it is shank independent (It must be fitted to the shoe it is used in to allow the amount of shank contact to be specified). This means that when the foot is less stiff and I need more ORF, the material/arch compresses until it hits the shoe and ORF increases. When the foot is more stiff (increased rate of loading of the soft tissues) then less deformation of the arch occurs and the orthotic stays above the shank of the foot.
As a manufacturer of orthoses, this has gone some way to reducing arch irritation in some sports patients.

Hope this makes sense

Cheers

Phil

 

The foot with a cavus structure, where the forefoot does not readily dorsiflex on the rearfoot under weightbearing loads, will tend to also have decreased surface area by which ground reaction force (GRF) may act on its plantar aspect. Because of this decrease in plantar surface area of contact for GRF in the cavus foot, when compared to the planus foot, the plantar pressures at the calcaneus and metatarsal heads will naturally be increased, for a given body weight, in the cavus foot versus the planus foot.

Making an orthosis that supports the medial arch and lateral arch of the cavus foot will reduce the GRF at the normal areas of high plantar pressure in the cavus foot, since a portion of the total GRF for the plantar foot will now be acting on the plantar medial and lateral arches. Making the orthosis slightly longer (3 mm longer) and much thicker (3-5 mm thicker) at the anterior edge of the orthosis will create what I call "an internal metatarsal bar effect" that will significantly reduce the GRF plantar to the metatarsal heads. I wrote about this modification about 15+ years ago in my first book (Kirby KA: Foot and Lower Extremity Biomechanics: A Ten Year Collection of Precision Intricast Newsletters. Precision Intricast, Inc., Payson, Arizona, 1997).

Hope this helps.