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In Thought Experiment #5, a foot with a medially deviated STJ is illustrated (left) along with the same foot, now with a foot orthosis underneath its plantar arch (right). The foot has a medially deviated STJ axis so that the medial weightbearing surface of the foot is 3 cm medial to the STJ axis and the lateral weightbearing surface is 5 cm lateral to the STJ axis. As in Thought Experiments #4, included within this foot model is a buttress of bone on the foot that is 2.0 cm lateral to the STJ axis in the area of the sinus tarsi of the foot. This buttress of bone on the foot acts as a "bumper" to prevent further STJ pronation past the maximally pronated position and is meant to model the approximate anatomical location of where the lateral process of the talus abuts against the floor of the sinus tarsi of the calcaneus when the maximally pronated position of the STJ is reached.

Both the medial and lateral weightbearing surfaces of the foot have a ground reaction force of 200 N in the foot without an orthosis (left). In the foot with a foot orthosis (right), the orthosis has been designed to shift the reaction forces on the plantar foot to a more medial location. The orthosis reaction force (ORF) can be thought to act with a magnitude of 200 N at a point 0.9 cm medial to the STJ axis (i.e. the center or pressure of the orthosis is located 0.9 cm medial to the STJ axis). Note that since the foot orthosis is now bearing half of the plantar forces from the foot, the medial and lateral weightbearing surfaces of the foot now each have 100 N of GRF (right).

The vertically-directed compression force acting through the STJ axis is F1 and the vertically-directed compression force acting through the sinus tarsi is F2 in the foot without an orthosis (left). In the foot with an orthosis acting on its plantar aspect (right), the vertically-directed compression force acting through the STJ axis is F3 and the vertically-directed compression force acting through the sinus tarsi is F4. Both feet are given to be in translational and rotational equilibrium.

Here are my questions for Thought Experiment #5:

1. What is the magnitude of compression forces acting at the STJ axis (F1) and sinus tarsi (F2) in the medially deviated STJ axis foot without an orthosis (left)?

2. When a foot orthosis is added to the plantar aspect of the foot with a medially deviated STJ axis (right) so that it shifts the reaction force on the plantar foot in a medial direction, what is now the magnitude of compression forces acting at the STJ axis (F3) and at the sinus tarsi (F4)?

3. Why does the foot orthosis cause a change in the compression forces acting at the sinus tarsi (F2 and F4)?

4. Why does the foot orthosis cause a change in the compression force acting across the STJ axis (F1 and F3)?

5. Has the foot orthosis altered the rotational position of the STJ axis of this medially deviated STJ foot (i.e. has the foot moved out of the maximally pronated STJ position as a result of putting an orthosis underneath it)?

6. If the foot orthosis has not caused STJ supination (i.e. has not moved the foot out of its maximally pronated position), how can it possibly cause a change in the magnitude of compression forces within the sinus tarsi and possibly reduce the symptoms within the sinus tarsi in a patient that has a medially deviated STJ axis?

7. What does this mechanical analysis seem to indicate regarding the common notion among clinicians and past and present biomechanics researchers that attempt to use solely foot joint rotational position or foot joint motion (i.e. kinematics) in an attempt to predict injury within the human foot?

8. What do clinicians and researchers miss in regard to understanding mechanically-based foot and lower extremity injuries when they don't take into account the magnitude and locations of external forces acting on the plantar foot relative to spatial location of joint axes in the human foot?

Attached Images

Thought Experiment 5.jpg (88.3 KB, 560 views)

__________________
Sincerely,

Kevin

**************************************************
Kevin A. Kirby, DPM
Adjunct Associate Professor
Department of Applied Biomechanics
California School of Podiatric Medicine at Samuel Merritt College

e-mail: kevinakirby@comcast.net

Private Practice:
107 Scripps Drive, Suite 200
Sacramento, CA 95825 USA
My location

Voice: (916) 925-8111 Fax: (916) 925-8136
**************************************************

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Hi Kevin

I've been doing my maths again and have come up with some answers (hopefully right ones) to your questions...

1. F1 = 200N, F2 = 200N

2. F3 = 390N, F4 = 10N

3. From thought experiment 4... subtalar joint compression force + sinus tarsi compression force = GRF force medial + GRF lateral (GRF total)
or F1 + F2 = 200 + 200 = 400
or F3 + F4 = 100 + 200 + 100 = 400

And... Torque Plantar Medial to STJ = Torque Plantar Lateral to STJ - sinus tarsi compression force:
ie. for the foot without orthoses:
200 x 0.03 = 200 x 0.05 - F2 x 0.02
6Nm = 10Nm - 0.02 x F2
F2 = 4Nm/0.02m = 200N

for the foot with orthoses:
100 x 0.03 + 200 x 0.009 = 100 x 0.05 - F4 x 0.02
3Nm + 1.8Nm = 5Nm - 0.02 x F4
F4 = 0.2Nm/0.02m = 10Nm

So in the foot with orthoses, the sinus tarsi compression force is reduced because the orthoses increase the supination moment acting medially, and lessen the pronation moment acting laterally. The less pronation moment, the less compression at the sinus tarsi...

4. The STJ compression force changes because STJ Compression + Sinus Tarsi Compression must always equal the Total GRF, so...
F1 + F2 = 400 and F3 + F4 = 400

So the STJ axis (F1 without orthotic and F3 with orthotic) will have more compression force in the foot with an orthotic:
F1 = 400 - F2 = 400 - 200 = 200N
compared with F3 = 400 - F4 = 400 - 10 =390N

5. No because the STJ has remained in exactly the same place because the model is in equilibrium.

6. The change in compression forces has occurred because of the addition of the ORF, which has changed the magnitude of the GRFs. The ORF (200N)is acting medially to the STJ axis, which is helping to even the balance by reducing the GRFs (now 100N each).
The sum of the torques medial to the STJ are:
GRF medial torque + ORF torque= 100 x 0.03 + 200 x 0.009 = 3Nm + 1.8Nm = 4.8Nm
And the sum of the torques lateral to the STJ are:
GRF lateral torque - sinus tarsi torque = 100 x 0.05 - 10 x 0.02 = 4.8Nm.

So the magnitude of compression force in the sinus tarsi without orthoses is 200N, and the magnitude of compression force in the sinus tarsi with orthoses is 10N!

7. The mechanical analysis indicates that clinicians are more likely to use foot joint rotational position or kinematics (eg. STJ neutral approach) for predicting foot injury, rather than looking at exactly how the forces, eg. compression, shearing and tension, are causing injury to the feet...I am definitely guilty of this but not any more

8.

Regards

Donna

Donna:

You have again demonstrated your superior problem solving skills and all the questions you answered are correct. Great job! It is very satisfying for me to see your progress. Hopefully, others are also becoming more comfortable with these types of problems along with you.

Question 8: What do clinicians and researchers miss in regard to understanding mechanically-based foot and lower extremity injuries when they don't take into account the magnitude and locations of external forces acting on the plantar foot relative to spatial location of joint axes in the human foot?

They miss out on being able to better understand the direction and magnitudes of internal forces acting within the structural components of the foot and lower extremity which are the causes of the injuries that they are treating or researching.

__________________
Sincerely,

Kevin

**************************************************
Kevin A. Kirby, DPM
Adjunct Associate Professor
Department of Applied Biomechanics
California School of Podiatric Medicine at Samuel Merritt College

e-mail: kevinakirby@comcast.net

Private Practice:
107 Scripps Drive, Suite 200
Sacramento, CA 95825 USA
My location

Voice: (916) 925-8111 Fax: (916) 925-8136
**************************************************

Thanks Kevin...my brain only had half a meltdown this time!

Kevin:

As has been shown, orthotics are designed to increase the plantar force medial to the STJ axis which then decreases the GRF under the medial aspect of the foot and also decreases the compression forces at the sinus tarsi. Could you explain to me how the medial heel skive works to decrease these medial GRF's even more then what an orthotic without a medial heel skive would.

Mike

First of all, not all foot orthoses are designed to shift the plantar reaction force (i.e. that force exerted by either shoe, ground or foot orthosis on the plantar foot) to a position medial to the subtalar joint (STJ) axis. Traditional Root style orthoses, made with the heel vertical, probably do little to move the plantar reaction force (PRF) more medial to the STJ axis. The Blake Inverted, medial heel skive and DC Inverted Wedge orthosis techniques all are designed to shift the PRF more medial to the STJ axes. Some foot orthoses, such as those I make for patients with lateral ankle instability and chronic peroneal tendinitis, are designed to increase the external STJ pronation moment (e.g. lateral heel skive, increased medial expansion plaster thickness, everted cast balancing positions and valgus forefoot extensions).

By using an inverted heel cup orthosis, such as a medial heel skive orthosis, there is increased external STJ supination moment added to the rearfoot by the orthosis. The increased external STJ supination moment of the medial heel skive will tend to produce the following mechanical effects on the foot:

1. Decrease the interosseous compression force within the sinus tarsi if the foot was originally in the maximally pronated position of the STJ (Kirby KA: Rotational equilibrium across the subtalar joint axis. JAPMA, 79: 1-14, 1989).

2. Shift the PRF from medial to lateral on the forefoot. In other words, the lateral metatarsal heads will have increased PRF and the medial metatarsal heads will have decreased PRF with application of the external STJ supination moment at the rearfoot with the medial heel skive. [I may be helpful here to realize that the shift in PRF to a more lateral position of the forefoot increases the external STJ pronation moment acting on the forefoot in response to the increase in external STJ supination moment from the medial heel skive to maintain rotational equilibrium, assuming no STJ rotational motion has occurred.]

If the medial heel skive is used in a foot that has a normal STJ axis location, then it is more likely that this varus wedging in the heel will produce STJ supination motion than if the foot has a significantly medially deviated STJ axis where it may not cause any STJ supination to occur. However, in both of these examples, the medial heel skive is still causing an external STJ supination moment.

Hope this helps.

__________________
Sincerely,

Kevin

**************************************************
Kevin A. Kirby, DPM
Adjunct Associate Professor
Department of Applied Biomechanics
California School of Podiatric Medicine at Samuel Merritt College

e-mail: kevinakirby@comcast.net

Private Practice:
107 Scripps Drive, Suite 200
Sacramento, CA 95825 USA
My location

Voice: (916) 925-8111 Fax: (916) 925-8136
**************************************************

Donna,

In your answer to 3 I would like to change the wording a bit. The orthosis shifts the center of pressure under the foot to a more medial location. The center of pressure is a weighted average of all the forces applied to the foot. So, when you have some forces causing supination moments and other forces causing pronation moments you can average them together into one single force. The software for pressure distribution devices does this and gives you a little dot on the screen that allows you to see the relative pronation moment from ground reaction force.

On designing orthoses to shift center of pressure. The classic root device often created an orthosis with a higher medial arch than the patient has when they stand barefoot. This should thoeretically increase the pressure more on the medial arch. However, in looking at the wear pattern on the top covers of some of these orthoses you will see a lot of wear lateral and very little wear medial. Let's take this observation as a lateral shift in center of pressure. For an additional thought experiment think of an explanation that uses moments to explain this observation. If the center of pressure is more lateral, what does this do to moment from ground reaction force. Can you explain why you see increased supination motion when the patient wears this device?

The center of pressure gives you the pronation moment from the ground. There may be a supination moment from some other source (muscles) that creates a higher supination moment than the pronation moment from ground reaction force. In this situation you would see supination motion and a laterally positioned center of pressure.

In an orthosis made with a medial heel skive, or when standing on a varus wedge, you would expect the center of pressure under the heel to be more medial. I have some unpublishted data agreeing with this. So orthoses can work by shifting center of pressure or they can work by changing muscle activity, or both. It makes it very difficult to do research with center of pressure as a variable.

Eric

Hi efuller,

The questions posed by Kevin Kirby in TE #5 did not ask me to calculate the centre of pressure...so that is why I didn't include COP in my answer! :p I'm sure I'm not alone in saying that if COP was also added to the problem it would probably be a little too difficult to understand the concepts... Unless you're a physics guru looking at these problems, the calculations would just cause a lot of this >>>

The biggest piece of information I gained from this Thought Experiment was that:

Kevin has spent a great deal of time and energy simplifying the models so that little by little, as each problem is solved, we can gain more and more understanding of these basic mechanical principles and how they affect the foot. I personally have found it quite interesting and beneficial to see how such small changes in the STJ axis position can change the way GRFs act on the foot. Prior to seeing these TE's, I had never spared a thought for how physics *specifically* affects foot function, ie. I had never actually sat down and thought about how different STJ positions and GRFs and ORFs can affect how sinus tarsi pathology for example arises...

So thanks to Kevin for making me a bit cleverer!

Regards

Donna