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I am grateful to Kevin Kirby and Precision Intricast for permission to reproduce this February 2002 Newsletter (you can buy the 2 books of newsletters off Precision Intricast):




Within the podiatric biomechanics community during the past fifteen years, there has been a gradual shift away from using the SJN Theory as a theoretical basis for mechanical foot therapy. One of the reasons why many podiatrists have moved away from the SJN Theory is due to some of the inherent problems and inconsistencies with this theory of mechanical foot therapy. One large problem with the SJN Theory relates to the reliability of the measurement procedures used within the standard biomechanical examination techniques proposed by Root et al over thirty years ago (Root, M.L., W.P. Orien, J.H. Weed and R.J. Hughes: Biomechanical Examination of the Foot, Volume 1. Clinical Biomechanics Corporation, Los Angeles, 1971). These examination techniques have been found to have only fair to poor intertester reliability and, therefore, can not be considered reliable from one examiner to another (McPoil, T.G. and G.C. Hunt: Evaluation and management of foot and ankle disorders: Present problems and future directions. JOSPT, 21:381-388, 1995.)

Another criticism of the SJN Theory is that the criteria for normalcy proposed by Root et al are not clinically practical since they are so restrictive that few individuals have “normal” foot and lower extremity structure (Root et al, 1971). In addition, the idea of Root et al that the subtalar joint should supinate through neutral position during the midstance phase of walking gait has been questioned by research by McPoil and Cornwall on 100 healthy, asymptomatic feet in which the subjects were more likely to have a rearfoot motion pattern which correlated to their resting calcaneal stance position than to their neutral calcaneal stance position (McPoil, T.G. and M.W. Cornwall: The relationship between subtalar joint neutral position and rearfoot motion during walking. Foot Ankle Intl., 15:141-145, 1994.) McPoil and Hunt have provided an excellent review of the problems associated with the SJN approach to mechanical foot therapy, including those listed above, and also have proposed a new model, the tissue stress model, for the approach to mechanical foot therapy (McPoil and Hunt, 1995).

McPoil and Hunt have chosen to use the tissue stress model “as the basis for developing an examination and management paradigm for treating individuals with foot disorders”. They claimed that the tissue stress model is not a novel idea since it is based on the same ideas that are already in current use in the treatment of parts of the body other than the foot and lower extremity. In addition, one of the benefits claimed for the tissue stress model is that it doesn’t rely on the use of the “unreliable measurement techniques” currently in use within the podiatric profession (McPoil and Hunt, 1995).

There have also been others that have also advocated the use of the tissue stress approach to mechanical foot therapy. Eric Fuller, DPM, has recently described the effects of rearfoot and forefoot wedging and how he uses the tissue stress approach in the clinical setting as a basis for mechanical foot therapy (Fuller, E.A.: Reinventing biomechanics. Podiatry Today, 13-3), December 2000). Dr. Fuller has also reviewed the concept of tissue stress and how computerized gait evaluation techniques along with the concept of modeling of the foot and lower extremity can help predict the stress in a specific anatomical structure (Fuller, E.A.: Computerized gait evaluation. pp. 179-205, in Valmassy, R.L. (editor), Clinical Biomechanics of the Lower Extremities, Mosby-Year Book, St. Louis, 1996). In addition, in two articles on future directions for podiatric biomechanics, I have also described the important concept of modeling of the foot and lower extremity and how modeling can be used to predict the loading forces, or stresses, which occur in the structural components of the foot and lower extremity during weightbearing activities (Menz, H.B. (moderator), Kirby, K., Cornwall, M., Rome, K., Tinley, P., Murphy, N., Keenan, A.: Clinical measurement of the lower extremity-where to from here? Australasian J. Pod. Med., 31 (3):95-99, 1997; Kirby, K. A.: What future direction should podiatric biomechanics take? Clinics in Podiatric Medicine and Surgery, 18 (4):719-723, October 2001).

Previous to the time that I first heard the concept of the “tissue stress model” in a lecture given by Tom McPoil, PhD in 1997 at the American Academy of Podiatric Sports Medicine Annual Meeting in Bellevue, Washington, I had independently developed a similar thought process and approach to mechanical foot therapy that I called “thinking like an engineer” (Kirby, K.A.: Thinking like an engineer. March 1992 Precision Intricast Newsletter. In Foot and Lower Extremity Biomechanics: A Ten Year Collection of Precision Intricast Newsletters. Precision Intricast, Inc., Payson, Arizona, 1997, pp. 267-268). In the newsletter, I described how it is more important for the podiatrist to focus on the internal loading forces, or stresses, which cause injury when treating mechanically related pathology than to just focus on determination of “deformities”. I also described how a structural engineer might use a similar approach when analyzing the stresses within the structural components of a building or bridge.

The tissue stress model is another way of stating the idea that podiatrists would be more effective at treating their patients if they would only use some of the basic mechanical concepts that have already been used for decades by structural engineers. The model is based on the concept that any mechanical therapy designed for the patient should be based not only on the specific anatomical site of injury of the patient, but also on the nature of the pathological loading forces that are causing the injury and how to most effectively design a mechanical therapy program to reduce these pathological loading forces so that healing may be optimized. Podiatrists who use the more logical and biomechanically sound approach to mechanical foot therapy inherent in the tissue stress model are much more likely to efficiently and effectively heal the mechanically based pathology of their patients. The podiatrist that only uses the concepts advocated by the proponents of the SJN Theory, where treatment of externally apparent “deformities” guides the design of the mechanical foot therapy, likely will be less effective at treating the wide range of foot and lower extremity pathology that can be treated with foot orthoses.

[Reprinted with permission from: Kirby KA.: Foot and Lower Extremity Biomechanics II: Precision Intricast Newsletters, 1997-2002. Precision Intricast, Inc., Payson, AZ, 2002, pp. 13-14.]

How does a podiatrist use the tissue stress approach when seeing patients? Here is way it should be done:


1. Accurately identify the anatomical structure which is injured or symptomatic.

2. Determine the structural and functional characteristics of the individual's foot and lower extremity.

3. Determine the most likely type of abnormal tissue stress which is causing the pathology within the injured anatomical structure (i.e. compression, tension or shearing stress).

4. Design a treatment protocol to reduce the abnormal tissue stresses on the injured structure and reduce the local inflammatory response so that more normal gait and weightbearing function can occur.


This is certainly very different from the way I was taught to treat patients with foot orthoses (i.e. STJ neutral theory) since this was based nearly solely on trying to get the patient to function in STJ neutral position, with little regard to the injured structure that was being treated.

__________________
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
**************************************************

Forces damage tissue. Position and motion do not damage tissues. Treat the forces, not the motion and position .... (I have spent the last ~20 years treating motion and position becasue that what I thought we were supposed to do )

__________________
Craig Payne
Department of Podiatry
La Trobe University
Melbourne, Australia
http://www.latrobe.edu.au/podiatry
__________________________________________________ ___________________________________
God put me on this earth to accomplish a certain number of things - right now I am so far behind, I will never die.
The views expressed above are those of the author and not that of La Trobe University
This is where I am, where are you?

It's interesting that podiatrists trained at the California College of Podiatric Medicine in the 1970's and 1980's (including myself) were dogmatically instructed that foot orthoses were meant to "prevent abnormal compensations", "make the subtalar joint function in neutral position", "lock the midtarsal joint" and "bring the calcaneus to vertical". All of these ideas are wrong and should be signficantly modified or discarded if one is striving to achieve the goals of foot orthosis therapy utilizing the tissue stress approach which are to:

1. Decrease pathological loading forces on the injured structural components of the foot and lower extremity;
2. Make the patient asymptomatic, and;
3. Optimize their gait pattern.

The theories promoted by my biomechanics instructors at CCPM taught us that "preventing abnormal compensations" was meant to include preventing abnormal compensation motions with no mention that compensation should also include preventing abnormal compensation moments. The idea that foot orthoses should attempt to prevent abnmormal compensation is not a bad one as long as it is understood that any abnormal compensation motions directly results from abnormal componsation moments.

Since one can not have a change in rotational motion or have a change in rotational position in the foot and lower extremity without a change in moments, we should then rightly conclude that the term "compensation" does not need to include the concept of motion or position within its definition. Intead, the term "compensation", relative to foot and lower extremity function, should be defined as follows:

"Compensation" is an alteration in moments acting across the joint axes of the foot and/or lower extremity that is caused by the mechanical interaction between the foot and the ground in a weightbearing environment and which may be modified by structural, positional or functional abnormalities of the foot and/or lower extremity. (Kirby KA: "The Biomechanics of Compensation for Foot Deformities", in Kirby KA: Foot and Lower Extremity Biomechanics II: Precision Intricast Newsletters, 1997-2002. Precision Intricast, Inc., Payson, AZ, 2002, pp. 33-36).

__________________
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
**************************************************

Hi,
Kevin has explained the tissue stress approach to managing foot pathology very well, and I agree that "position and motion" do not damage tissue, but rather forces do... However, if we were then going to treat the "abnormal forces" how would we go about it any differently than by attempting to alter the position and motion?
The tissue stress model gives us a better understanding of foot pathology, but my real question is - how do we relate that to changing our biomechanical assessment and orthotic presciption? Wound't we still be doing the same assessment and prescription at the end of the day, just with a different understanding of what we were doing?

Adam

Hi Kevin,
The points you make above I believe are very valid, and we do need to shift away from the old Root version of "motion and position" when dealing with foot pathology and biomechanics. Points 1 & 2 above descibe the "biomechanical and clinical assessment" side of the equation, and point 4 the treatment. However, what I do not understand is point 3, how do we determine which type of force it is? and if it is a compression type force affecting certain structures due to end-range pronation for example. isn't the treatment protocol going to attemt to make a device which affects the "position and motion" of the STJ anyway so as end range compression forces do not occur?
Apologies if I'm a little behind on this topic, but any of your thoughts may help me get a better grasp of the new model, and how to alter our clinical practice.
Thanks,
Adam

Adam:

We can best determine the most likely type of tissue stress (i.e. compression, tension, torsion, shearing) by knowing the function of that tissue, its anatomic location and how externally generated and internally generated forces are affecting it. For example, in a patient with plantar heel pain, which we commonly call plantar fasciitis, the pain can be caused by compression stress due to ground reaction force acting directly on the plantar medial calcaneal tubercle or due to tension stress from the fibers of the central component of the plantar aponeurosis on the medial calcaneal tubercle. If it is due to compression stress, then reducing the compression stress would be the most appropriate means of relieving pain. If it is due to tension stress, then reducing tension stress would be the most appropriate means of relieving pain. Of course, probably many cases of "plantar fasciitis" are caused by a combination of compression and tension stress so that therapy will need to include provisions for both.

Another example is tenderness in the midsubstance of the central component of the plantar aponeurosis, which can be caused by both tension stress and compression stress. Tension stress would typically be caused by Achilles tendon tension during weightbearing activities and may respond to more arch support from an orthosis. Compression stress could be caused by a foot orthosis that is pressing too firmly on the medial arch of the foot during weightbearing activities and may respond to less arch support from an orthosis. Therefore treatments will vary, using the tissue stress approach, depending on the mechanical nature of the forces acting on the structural component in question.

__________________
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
**************************************************

Kevin,
So we've identified the tissue, lets take your example of a patient with plantar heel pain, due to tension stress from the fibers of the central component of the plantar aponeurosis on the medial calcaneal tubercle, how do I make an orthotic for this patient? How should I cast the foot? How should I balance the forefoot to rearfoot on the positive cast? What degree of rearfoot wedging should I add? In other words, how do I arrive at my orthotic prescription in the tissue stress paradigm?


Also, when I have dispensed the devices, how do I know whether I have altered the moments favourably? Now some bright spark is going to say, "because the patients symptoms improve". But we all know that this doesn't answer the question because symptoms can improve for a multitude of reasons, without necessarilly changing moments.

Best wishes,
Simon

__________________
Science is the antidote to the poison of enthusiasm and superstition

My location

You can still alter forces without changing position and motion. A change in position/motion of the rearfoot is not associated with outcomes (we have had a thread on this, but sorry I can not link to it as on a really slooooooow connection here at JFK and it will take forever to find it)

__________________
Craig Payne
Department of Podiatry
La Trobe University
Melbourne, Australia
http://www.latrobe.edu.au/podiatry
__________________________________________________ ___________________________________
God put me on this earth to accomplish a certain number of things - right now I am so far behind, I will never die.
The views expressed above are those of the author and not that of La Trobe University
This is where I am, where are you?

Simon,

The tissue stress theory of mechanical foot therapy is not comprehensive enough to provide an exact prescription protocol for all foot pathologies. However, it will allow guidance of the clinician toward an orthosis prescription that is quite likely to accomplish the goal of making the patient less symptomatic.

For example, if it is determined by the clinician that the pain in the plantar heel is caused by increased tensile force within the medial fibers of the central component of the plantar aponeurosis pulling on its origin at the medial calcaneal tubercle, then I would design the orthosis with specific modifications that would tend to reduce the tensile force within the medial fibers of the central component of the plantar aponeurosis. This may include designing the foot orthosis with a 5 mm polypropylene shell, 4/4 degree rearfoto post, a 3-4 mm heel contact point thickness, 2 mm medial heel skive, 16 mm heel cup, minimal medial expansion thickness, a plantar fascial accommodation, and a 2-5 forefoot extension of 3 mm thick korex.

To answer your other questions, the foot would be casted in STJ neutral position using neutral suspension casting technique described by Root et al. The forefoot to rearfoot of the positive cast would be balanced so that I am not creating either an excessive STJ supination or excessive STJ pronation moments with the orthosis. I don't use "rearfoot wedging" in the orthosis since all the correction is made into the orthosis....BTW, is "rearfoot wedging" some form of British podiatric orthosis therapy??

If these specific modications then are shown at followup examination to have resulted in improvement of the condition (i.e. by increased subjective comfort and decreased tenderness on plantar heel), then one could logically conclude that the foot orthosis has reduced the tensile stress within the medial fibers of the central component of the plantar aponeurosis, which was the original mechanical etiology of the patient's complaints. Of course, other potential explanations are possible, but I think those questions are best left to the researchers who have the time to explore such ideas.

By the way, the orthosis likely accomplishes the goal of reducing the tensile stress within the medial fibers of the central component of the plantar aponeurosis by decreasing the net forefoot dorsiflexion moment and decreasing the net first ray dorsiflexion moment since the function of the medial fibers of the central component of the plantar aponeurosis is to increase forefoot plantarflexion moment and increase the first ray plantarflexion moment.

This tissue stress approach to mechanical therapy is all quite logical and mechanically coherent, as long as one understands the principles of modelling and free-body diagram analysis. I see that the biggest problem with many podiatrists using the tissue stress approach successfully is their relatively weak physics and biomechanics backgrounds. In other words, most podiatrists don't know the difference between a stress and a strain, a moment and a force and don't understand how modelling approaches may be used to determine internal forces within the foot using only a knowledge of the anatomy of the foot and the external forces being applied to the foot. The podiatrists who were engineers will be using the approach with no problem at all since it makes total sense to them. The podiatrists who struggle with basic mechanics concepts will never completely grasp these ideas so that they will likely achieve only mediocre results with their foot orthosis therapy.

__________________
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
**************************************************

You've made a start within this reply Kevin, I'm sure all across the land patients will be receiving this prescription tomorrow Another way of looking at this is that perhaps the prescription does not need to be as exact as previously thought to achieve the desired outcome?

Funnily enough I made a couple of pairs of these today , except I used 3mm EVA for my forefoot extension and I didn't put in a plantar fascial accommodation as I haven't found them necessary in the past (I made these before I read your post-honest). Now back to playing Devil's advocate Kevin, why a 4/4 post and not a 5/4 post or 6/4 post? Why a 2mm heel skive (presumably inclined at 15 degrees?- Why 15 and not 10 or 12 or some other number of degrees?) Why a 16mm heel cup and not 15?

Kevin, you know that you and I are pretty much on the same page when it comes to this stuff, but you must see it from the perspective of someone coming from the dogma of old, these are the things they want answering. I think, Howard (or maybe it was Craig) talked about viewing one model through the lens of another, and this is inevitably what is occurring, rightly or wrongly. Rootian mechanics gave a nice easy recipe to follow, and lets not forget, has helped a lot of people overcome their symptoms.

I think the point is that if we did alter the prescription you gave above, lets say we did put a 10 degree, 3mm heel skive, we, changed the rearfoot posting angle by a few degrees +/-, it may not make a whole lot of difference to our outcome. I used to describe this to students as the "treatment envelope": we treat a patient with an orthotic device and they get better thus we know our prescription worked, but we don't know just how "accurate" the prescription was to the patients requirements, it could be that we could alter the prescription quite markedly and the patient would still get better, it could be that if we alter the slightest detail ,i.e. rearfoot post by 1 degree the prescription would have failed to alleviate the symptoms. Obviously this works vice versa, if a device fails to alleviate symptoms, we don't know how far away from the success zone we are, could be that 1 degree either way in the rearfoot post would have made all the difference.

Just do a search in JAPMA, you'll find the term wedging is used frequently in papers published in American journals How do you know when the forefoot to rearfoot of the positive cast is balanced so that excessive STJ supination or excessive STJ pronation moments are not created with the orthosis? OK I'll stop now

Enjoying myself again in a podiatry chat-room

Best wishes,
Simon

__________________
Science is the antidote to the poison of enthusiasm and superstition

My location

You are absolutely correct. An exact orthosis prescription may not be necessary for a foot orthosis to be therapeutically effective since there are thousands of orthosis design permutations that may render a patient asymptomatic. The problem is that there are also probably a million or more orthosis design permutations that will cause little therapeutic benefit and may, in fact, cause other injuries to occur. The basic idea, therefore, is to get close to the correct prescription that accomplishes the goals of orthosis therapy (i.e. get patient better, improve their gait and cause no other pathology or symptoms).

These are good questions because they point to the idea that orthosis prescription does not need to be exact in order for patients to improve and derive benefit from them. This is an important point that I try to make when training podiatrists, podiatry residents and students. I teach them that as long as they are headed in the right direction with orthosis design, they are more likely to achieve therapeutic success with the orthosis and there isn't one "magic" orthosis prescription that will work for each patient....there are multiple orthoses that may work for them.

Some pathologies require very precise orthosis design to make the patient better and not cause other pathologies, whereas other pathologies will respond well to widely varying orthosis designs. Some of this is also patient dependent. The patients that can feel the slightest orthosis adjustment and then get pain as a result of this slight adjustment I call my "Princess and the Pea Patients". They are commonly the same patients that will wear their socks inside-out to avoid irritation to their toes from the seam at the dorsal digits.

This is very similar to the way I view it. However, the clinician does need a treatment model by which to make specific treatment decisions. The better the treatment model, the better it will be at allowing a high percentage of clinicians to have a high percentage of treatment success with their foot orthoses. When it comes to the treatment models, I don't think any of them (sagittal plane facilitation, STJ neutral, etc) even come close to the tissue stress and STJ axis equilibrium model in being able to design foot orthoses that can treat the widest range of foot and lower extremity pathologies. However, for me to be chumming the waters now for further discussions in this regard may not be the wisest choice, but the bad boy in me just couldn't resist the temptation.

If the patient complains that they are having supination related symptoms (e.g. feel like their ankle is going to roll into inversion or walking on the lateral side of their foot) or pronation related symptoms (e.g. sinus tarsi pain or PT tendinitis) then one knows that the orthosis has created excessive STJ supination or excessive STJ pronation moments. There are other factors for determining proper forefoot to rearfoot "posting" in the orthosis, but this topic is far beyond the scope of this discussion.

Good discussion Simon. By the way, I feel sorry for your former instructors....hope they didn't run away from you like they ran away from me during my podiatry student days when they saw me walking toward them with a list of questions.

__________________
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
**************************************************

Hi Craig,
I understand that a change in position/motion of the rearfoot is not necessarily associated with outcomes... However, my question still remains - If we were then going to treat the "abnormal forces" how would we go about it any differently than by attempting to alter the position and motion of the foot with orthoses?
(ie wound't we still be doing the same assessment and prescription at the end of the day, just thinking about it in a different way?) BTW I would like to read the thread you mentioned on this topic in your last reply if you have time to provide me with a link.

Thanks again,
Adam

Adam

Its now 2.00AM and I made it to LAX ... still have another day of work tomorrow before heading back ... just come from the 5 degrees below and snow of Montreal .... I guess that better than the 42 degrees in Melbourne :p

The thread is Kinematic change and foot orthoses outcomes.

__________________
Craig Payne
Department of Podiatry
La Trobe University
Melbourne, Australia
http://www.latrobe.edu.au/podiatry
__________________________________________________ ___________________________________
God put me on this earth to accomplish a certain number of things - right now I am so far behind, I will never die.
The views expressed above are those of the author and not that of La Trobe University
This is where I am, where are you?

Sorry Kevin,
no entry into the bad boy club until you are mad as well as bad.
DaFlip