Skinning cats: rearfoot posts revisited

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If we assume the fundamental purpose of a foot orthosis is to alter the forces acting beneath the foot. And that for arguments sake we say that for condition X a medial shift in the Centre of Pressure is desirable. How can we achieve this?

I believe that this can be achieved in a variety of ways:
1. Add an external rearfoot post
2. Add an internal rearfoot post
3. Relatively stiffen the medial heel cup/ longitudinal arch area of the orthosis
4. Increase the height/ curvature of the medial longitudinal arch area of the orthosis

Any others?
Why should any one of these methods be "better" than another?
Food for thought anyone? :drinks

 

Simon:

I just knew when I read the title of this thread "Skinning cats: rearfoot posts revisited", that you were the one that was the author of this thread title.

To add to your list of orthosis modifications that would increase the magnitude of external subtalar joint supination moment.

5. Increase the varus wedging plantar to the metatarsal heads.
6. Decrease the height and stiffness of the lateral longitudinal arch.
7. Decrease the stiffness of the lateral heel cup.

Here are my questions:

Why would an orthosis lab offer only a non-rearfoot posted orthosis when a rearfoot post can offer so much mechanical resistance to eversion of the foot inside the shoe?? How much pronation control can a non-rearfoot posted orthosis offer when the medial edge of the apex of the longitidunal arch of the non-rearfoot posted orthosis lies outside the base of support of the heel contact points and anterior contact points of the foot orthosis?:craig:

 

Simon,

I think the reason we have such a diverse range of options within orthotic therapy with the common goal of altering the forces acting beneath the foot, is that they all work to some degree.

Most Podiatrists find success of some degree using orthotic therapy. I find this a little confusing really, as a person who puts allot of thought and effort into orthotic prescription.

How is that some Podiatrists put a huge amount of effort into their prescriptions and others don't but still get results? I think this is a huge question.

Trent

 

True, a rearfoot post can offer mechanical resistance to eversion of the foot inside the shoe, but can't the other modifications achieve this too? From a physics perspective should any one of these methods be more effective at achieving the goal than another?

 

All of the following orthosis modifications may help offer mechanical resistance to eversion of the foot inside the shoe:

1. Add an external rearfoot post
2. Add a varus wedge to the heel cup (e.g. medial heel skive)
3. Increase the stiffness of the medial heel cup
4. Increase the stiffness of the medial longitudinal arch
5. Increase the height/curvature of the medial longitudinal arch
6. Decrease the stiffness of the lateral heel cup
7. Decrease the stiffness of the lateral longitudinal arch
8. Decrease the height/curvature of the lateral longitudinal arch
9. Increase the varus wedging plantar to the metatarsal heads
10. Add a pad plantar to the hallux (e.g. Cluffy wedge)

All of the above orthosis modifications offer mechanical resistance to eversion of the foot inside the shoe by either increasing the external subtalar joint (STJ) supination moment and/or decreasing the external STJ pronation moment (#1-9) or by increasing the internal dorsiflexion stiffness of the medial forefoot (#10).

The problem of designing the best orthoses for a patient is not just to eliminate pronation with an orthosis, as a few orthosis labs claim. The problem is that the orthosis must be designed to not only optimize gait function but also must be designed to attempt to eliminate the patient's symptoms and prevent other symptoms/pathologies from occurring. This is a huge leap of understanding for many of those who are promoting higher longitudinal arched, non-rearfoot posted orthosis designs for all patients.

Is gouging the medial longitidinal arch with an extremely high medial longitudinal arch (MLA) a unique or new concept? No. It was 120 years ago now, in 1888, when Royal Whitman, an orthopedic surgeon, first described his metal foot brace which had a medial and lateral flange and which was designed to undergo an inversion motion once the patient stepped down on it. The inversion motion of the plate would tend to press the medial flange rather vigorously into the area of the navicular thus causing control of pronation motion either by force or by pain (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).

Did the Whitman brace control pronation? It probably did very well. However, in controlling pronation by just pressing hard on the MLA did it also cause other pathology or pain? Certainly it did.

If the goal was just to control pronation with an orthosis, without any regard to the patients comfort or developing other pathology, then why not stick 3-4 tacks into the dorsal apex of the medial longitudinal arch of the orthosis and call it the SpikeThotic? Certainly this type of orthosis design would produce kinematic reductions in rearfoot pronation that are far superior to any other orthosis design!:bang:

 

Can i offer a number 11?

1 - 10 in Kevins list are all concerned with affecting moments in a "normally functioning" foot.

Something i see a lot of in peads is patients with tight calf muscles / limited Talocrural dorsiflexion. In these cases the dorsiflexion moments exerted on the TC / ST complex cannot be absorbed by the TC joint and must be accomodated in the ST joint. We then see "escape pronation" taking place where the STJ is forced to pronate to accomodate sagital plane movement.

I therefore offer number 11, a heel lift, for patients with limited TC range.

One of the best answers i have seen to this question was offered by Dave Smith. Why, if Pre fabs / Neutral casts for everybody work for most, do we complicate things with a biomechanical assessment.

The answer he offered

Regards
Robert

PS

That would be a proprio / extero ceptive device would it not?

 

Kevin


SpikeThotic! --- I like it, very Niche market. Advertise in the Sado-Masochist Weekly.

How about the SpikeThotic 2 with Double sided spikes so whilst standing on tacks protruding exquisitly painfully into the soft underside of the foot one can also stamp vigorously over one's partner's bare back with an almost unbearable pleasure pain sadomasochistic feedback response ratio. All at once Improving both lumbar lordosis and MLA height. (cool)

OOH! I must go and lie down for a while. It's all so emotionally exhausting.:dizzy:


LoL Dave

 

Have rearfoot posted devices never caused other pathology or pain then?

Just trying to sharpen the argument up Kevin

 

Simon, Kevin and all,

A medial shift in center of pressure from shoe or orthotic or anything can increase symptoms. The medial shift of pressure is also relative to the knee. I can recall two patients off the top of my head who got knee pain from medially posted/ skived orthoses. One was quite remarkable in that in gait you could see about 10 degrees of frontal plane motion of the knee at heel contact, he also had a large amount of tibial varum. I could swear the frontal plane knee motion was faster with the orthotic in the shoe. His knee hurt with the orthoses. The other was a patient who had sinus tarsi syndrome with quite a bit of tibial varum. He came back several times with trading off of knee and sinus tarsi pain. I'd add a little varus wedge under his post and his sinus tarsi pain would feel better, but his knee would hurt more. Remove the wedge and you would get the opposite situation.

Reasoning:
With tibial varum, in the frontal plane, body weight on top of the knee is lateral to center of pressure on the foot. These two forces will cause a force couple that will tend to increase the tibial varum. The internal resistance to tibial varum is compression on the medial joint and tension in the structures on the lateral side of the knee joint. Any shift of force under the foot will increase the magnitude of moment on the tibia and cause a need for increased forces at the knee to resist the moment from body weight and ground reaction force.



There was earlier comment about the usefulness of prescription writing. If you were looking at neutral position related measurments (forefoot to rearfoot etc.) and did not pay attention to transverse plane position of the STJ axis you may not make a device that would be much different than the over the counter devcice. An OTC device is a piece of plastic and a custom device is a piece of plastic and if their shape is the same there should be no difference in how well they work. Now, if you choose to add a small, medium, or large medial or lateral heel skive you have a much greater chance of creating a device that will be different from an OTC device.

Cheers,

Eric Fuller

 

It is very easy to get a positive effect with just about any type of orthosis in a large number of patients... this is why there are so many types of OTC devices, and why there are an increasing number of different types of practitioners 'prescribing' orthoses- even students in sports stores are doing it....:bang:
You can see why though- a person comes into a store to buy a shoe, and mentions they are seeing a Podiatrist who is recommending orthoses 'that are very expensive'. They see some OTC devices for a fraction of the cost and decide to try them- they help, and they think they have saved a packet...
I think the difference is that a conscientious Pod will put the effort into designing something which will help the symptoms... and also try to optimize foot function- these things do not always go together. This may not make any difference in some patients, but I can guarantee that it makes a big difference in many.

In addition, they will also evaluate their effectiveness and will have a better idea of what the problem is if the results are not satisfactory (such as in Eric's example) and will have the knowledge to do something about it.

I like Dave's point above (thanks Robert)-
Remember that an orthosis is a tool. It is not the orthosis that helps the patient- it is what you are doing with it that helps... or hinders if inappropriate.

 

Kevin and Simon;

the trouble with the suggestion #5 comes back to DFion stiffness and the ability of the foot to supinate away from a FF varus post.

#6 would be more likely to mediallize the CoF if you increased the height and stiffness of the lateral long arch.

I won't argue with the others except to say that "a foot's response to an orthosis is "like a box of chocalates... you never know what you are going to get.""

cheers
Bruce

 

Eric,

So you are referring to the plantar aspect on the lateral heel cup. I can agree to this.
You are then not referring to the lateral aspect of the lateral heel cup, the part that is mostly vertical.

Regards,

Stanley