Intrinsic posting

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Hi I am recently qualified and am new to the site so forgive me if this question has already been asked or if it is a stupid question.

When a patient is over pronating their foot at the STJ should you post the orthotic device intrinsically to the measured calaneal stance position or should you use an extrinsic post such as a medial wedge to control the rearfoot?

 

:welcome: :drinks

That is theoretically correct and if applied in clinical practice patients to tend to get better if you do it that way. However, in reality the theory works really well at achieving that with inanimate objects (eg plaster casts) and does not really achieve that with animate objects (eg the human foot) ... but clinicaly the patient does get better ... don't figure

 

Thanks for the welcome Craig.

When posting should I intrinscally post the device or extrinsically post it?

 

Davey

These are really two different methods to control pronation 'moments' and achieve the same result. if you read some of the posts here regarding the STJ, pronation and posting, you will find that both methods achieve those goals but differently . It is mostly a preference.

Personally, if I were trying to affect STJ pronation moments I would become familiar with Dr. Kevin Kirby's papers on medial skives, the STJ joint axis location and rotational equilibrium across the STJ and consider using the medial skive technique.

The Kirby Skive is an intrinsic posting method and it is very effective. I had mediocre success with medial wedges comparatively.

Regards

 

G'day Davey,

To pick up on a point raised by David (and one drummed into my head by Craig when I was at uni), it's about understanding how much force you need and where you need to apply it. Kevin Kirby's papers are great, especially his stuff on the longitudinal subtalar joint axis, (being medially or laterlaly deviated). For a medially deviated STJ axis, more force is required in a smaller area, so intrinsic posting and additions such as a medial heel skive are most useful in applying sufficient control to a specific area. An extrinsic post will give control over a more generalised area, and can be added later to enhance what the intrinsic additions are doing (if necessary). But at the end of the day, every foot is different. Sometimes it's not the STJ that needs correcting, but another area in the foot. Look at the foot as a whole complex machine, and address it as such.
Good luck!

 

If the forefoot needs posting, then I don't think it matters which way. There are pros and cons of both approachs. I hardly ever use either.

 

I agree. I've found that most forefoot posting is negated by the time it reaches the midfoot due to the amount of complex joints that adapt to the different forefoot positioning. Feet are designed to addapt to all sorts of terrain. I really only every post midfoot/rearfoot and let the forefoot adapt to the more proximal posting.

 

Thanks for the replies, I will look at those Kirby papers. When posting the rearfoot would you use a wedge or have it built into the device?

 

Like forefoot posting, there are pros and cons of doing it either way. See these threads:
How much skive to prescribe?
Question about intrinsic rearfoot posting
Do Rearfoot posts Work?
Skives and posts
Pros and Cons of DC wedge orthoses
Medial Heel skives

 

I've found that there is a regional difference in terminology regarding intrinsic posting. I've heard a reference to intrinsic rearfoot posting. It was taught at the California College of Podiatric Medicine as "balancing the cast inverted". There is intrinsic forefoot posting, but not intrinsic rearfoot posting.

I'll assume you are talking about the rearfoot. It is important to understand how the prescription changes the orthotic. Taking a positive cast and invering it will tend to make the heel cup of the orthosis behave like a varus wedge. As Kevin pointed out in his medial heel skive article, this does not always happen and that is why he came up with the medial heel skive.

To skive or not to skive: I make this decision based on location of the STJ axis. Medially deviated get the skive avg, to lateral do not get the Medial heel skive. There are some people who pronate in the late stance phase of gait, who have laterally deviated STJ axes. They pronate because of muscular activity. The medially deviated axis foot will have a large pronation moment from ground reaction force and a varus heel wedge will help reduce the pronation moment from the ground. So, there are different reasons why people pronate.

Regards,

Eric Fuller

 

Hi Davey,

Since you are a rookie, I thought I would enlighten you a little, as to intrinsic forefoot posting vs. extrinsic forefoot posting.
As far as I know, intrinsic posting was first done with steel devices (pre Root) by hammering down the medial forefoot. The result was raising the arch and inverting the heel cup.
The same effect is done by balancing a cast, so in effect the front of the device is being bent down.
If the device ends proximal to the plantar most part of the metatarsal heads, then the metatarsal heads are not moved, and the intrinsic post is not affecting the forefoot.
Extrinsic forefoot posting was done also pre Root on steel devices using leather wedges, and attached with rivets. This also raised the arch and inverted the heel cup. Unless the leather wedge was placed under the plantar most aspect of the metatarsal heads, the wedge did not affect the metatarsals, and hence did not affect the forefoot. The main difference between extrinsic and intrinsic posting is two things. The shape of an orthosis with an extrinsic forefoot post is more similar to the cast. Also, the post shortens the span of the orthosis, and therefore stiffens it. In the case of a theoretically perfectly rigid material, this stiffening is not a factor.
Now in regards to rear foot posting, it makes more sense to look at the extrinsic rearfoot post as two posts: the lateral half and the medial half. The lateral half should increase the pronatory moment, and hence increase the rate of pronation of the device to the vertical. However, since the posterior aspect of the rearfoot post is circular, it has minimal effect on what the device does. (As an experiment, lift the forefoot of a rearfoot posted device so that only the posterior aspect of the orthosis [post] is touching the ground. Now touch the medial forefoot and then the lateral forefoot, and see what happens.)
The medial half of the rearfoot post theoretically prevents eversion of the device when the forefoot is not in contact with the ground. In reality it does not do this. When the forefoot is not in contact with the ground, the posterior aspect of the rearfoot post is in contact with the ground. Since the posterior part of the rearfoot post is circular, it is incapable of providing support at this part of the gait cycle (do he same experiment as above). The rearfoot post also stiffens the medial aspect of the orthosis by shortening the span. Again, in a perfectly rigid device the stiffening of the device by the rearfoot post is not necessary. Once the forefoot contacts the ground, the distal medial side of the orthosis prevents the device from everting.
Intrinsic posting of the rearfoot is done by grinding into the heel cup of the device. This was done as a way of getting around Langer laboratory’s patent of the Halfthotic. The concept of the Halfthotic was thinning pf the orthosis to fit better in the shoe (especially pumps). The Halfthotic resulted in no material under the center of the heel, while the intrinsic posting resulted in less material under the center of the heel.
The other threads that were referenced do an excellent job of exploring the Kirby skive.

Regards,

Stanley