Should Podiatrists Think More Like Engineers?

Author(s): 
Kevin A. Kirby, DPM

Biomechanics and sports medicine have always been of interest to me since I entered podiatry school. After my first year of surgical residency, I was fortunate to do the Biomechanics Fellowship at the California College of Podiatric Medicine (CCPM). Even though the year of my Biomechanics Fellowship was the period of my life when I probably experienced the greatest intellectual growth, I ended the year with more questions than I had answers.

   I saw that all the traditional biomechanical measurements based on subtalar joint neutral that I had been trained to perform as a podiatry student did not necessarily correlate to my patients’ symptoms. Even though most patients were getting better with the orthoses that all the clinicians and myself made for them, none of our measurements of “foot deformities” exactly agreed with each other.

   Toward the end of my Biomechanics Fellowship, I began reading the medical and biomechanics literature that was outside the podiatric sphere. The most influential references I remember reading from the CCPM library were a book on knee biomechanics by a Belgian orthopedic surgeon, Paul Maquet, MD, and a series of papers on foot and ankle biomechanics by John Hicks, MD, an orthopedic surgeon in England, and Benno Nigg, Dr.sc.nat., Dr.h.c., a biomechanist in Canada.

   In their book and papers, Drs. Maquet, Hicks and Nigg all used an engineering approach to discuss and illustrate the internal forces and stresses within the structures of the foot and lower extremity. Their physics-based engineering approach was a revelation to me after spending years at CCPM learning how to properly measure subtalar joint neutral, measure “forefoot to rearfoot deformities” and “rearfoot deformities,” and make foot orthoses to “prevent compensations for forefoot and rearfoot deformities.”

   By using easily understandable models of the foot and lower extremity, these three men effortlessly discussed what was mechanically occurring inside the foot and lower extremity during weightbearing activities. I became hooked on this idea that we, as podiatrists, could also learn more about the internal forces and stresses that each structural component of the foot and lower extremity was subject to by “thinking like an engineer.”

   Upon reading more about engineering principles during my early years of practice, I began to learn more about the concepts that engineers use to build, for example, bridges, buildings, automobiles and ships. I realized that, unlike podiatrists who seemed to concentrate on static clinical measurements and static radiographic images of the foot and lower extremity, engineers focused on trying to understand the internal forces and stresses on each component of the structure they were designing in order to keep the bridge, building or machine from mechanically failing.

   Now, over 28 years since I graduated from my Biomechanics Fellowship, I am even more convinced that the way forward for our profession is for all of us to think more like engineers. If our desire as a podiatry profession is to remain as the medical specialty that most fully understands the mechanically-based etiologies of the various pathologies of the foot and ankle, we need to move forward intellectually.

   We must start focusing our podiatry school education and podiatry seminar content more on how abnormal stresses generate within the tissues of the foot and lower extremity, and how we can best reduce those pathological stresses to more physiologic levels, whether with stretching, strengthening, strapping, padding, bracing, foot orthoses and/or surgery. We must start focusing our energy and education more on how shoes, foot orthoses, braces and our foot and ankle surgeries affect the forces and stresses acting on and within the foot and lower extremity so our patients can heal faster and suffer fewer post-treatment sequelae. I believe this “tissue stress approach” will be the key to more effective conservative and surgical treatment for our patients, and to the continuing success and development of our profession.

Comments

This article makes a valid and pertinent point.

My concern is it will substitute one set of notional values for another and the goal of attaining an understanding how movement of the body facilitates staying balanced upright and moving so the ability to determine an outcome will still be just as elusive.

The key is an understanding of and working with the body’s nervous system and then fully involving the individual with the problem in his or her own treatment.

An integral part of such an approach is an orthotic that can accommodate change. We all know our body is in a state of constant change. So it all hinges on sharing knowledge with those individuals that have an imbalance causing pain. Then it is their choice as to how well they work with their body. Without that, the long-term prospect is bad (i.e. pain will return albeit in a different location).

In a nutshell, a balance has to be struck between mind brain and body in order to attain and maintain a state of equilibrium. When that has been achieved, comfort reigns supreme.

While I agree with Dr. Kirby's insightful argument guiding us towards a more scientific, EBM-based biomechanical approach, as I have followed Kevin's career, I have watched him simultaneously reduce the importance of morphology, posture, architecture and foot type of the biological structure that we are engineering.

I am all for teaching more engineering to our students and colleagues but not at the price of reducing or elimination architecture, carpentry and structural mathematics from the table.

Disclaimer: I am the patent holder for functional foot typing, foot centering pads and foot centering orthotics, and I have a financial interest in making these statements for my podiatry partners and I.

Dennis

Kevin A. Kirby DPM's picture

Dr. Shavelson:

Thank you for the kind comments regarding my article. However, I don’t understand how you arrived at the idea that I have reduced “the importance of morphology, posture, architecture and foot type of the biological structure that we are engineering."

Rather, my 29 years of being a podiatric educator has been devoted toward educating podiatrists on how to integrate standard biomechanics and engineering concepts into better understanding foot and lower extremity function, and into designing better conservative and surgical treatments for the mechanically-based pathologies of their patients’ feet and lower extremities.

Contrary to what you claim, I have never “reduced the importance of morphology, posture and architecture” over my long career as a podiatric educator. In fact, many of my papers and various sections of my four books go into great detail as to how certain foot morphologies and structural abnormalities or postures of the foot alter the external and internal forces acting on and within the body. Morphology, structure and posture of the foot play a key role in both the external and internal forces which can create pathology within the feet and lower extremities.
Engineering principles help us understand how changes in foot structures will alter the internal loading forces acting on and within the structural components of the foot and lower extremity during weightbearing activities and may eve help us better predict which pathologies may occur. Using such a scientifically-based approach will certainly allow us all to better comprehend which of our treatments will work most effectively to allow our patient’s feet and lower extremities to heal in the most rapid manner and with the least amount of disability.

I do not, however, see any reason why modern podiatrists should attempt to pigeonhole feet into “foot types," which artificially assigns various functioning properties to the feet of our patients when, in fact, we know that the human foot and lower extremity is extremely variable. Rather, podiatrists should be embracing the diversity of the human foot and understanding its function using known biomechanics and engineering principles, not regressing backwards in their intellectual development by categorizing feet into “foot types” that has, to my knowledge, not a thread of research evidence to support it.
Kevin A. Kirby, DPM
Adjunct Associate Professor
Department of Applied Biomechanics
California School of Podiatric Medicine

Good article, There isn't much to disagree with your concepts here, but I think the focus is in the wrong direction, particularly if you want to think like an engineer. There is too much focus on tissue stress without first understanding the structural stability better. Trying to understand the “internal” forces on the structural components, the structure must be stable first, otherwise, how else are you going to get quantitative measurements?

We've gotten away from looking at how the internal structure is moving moment to moment (coupled with the internal and external forces acting upon it), where and when over-pronation is becoming unstable (with AND w/o an orthotic).
Pressure scans offer more information than we're using, especially where engineering information is vital.

The measuring of axis or ROM applied to principles of engineering are used to support theories but can't be proven and there's no science behind it. Perhaps there are other technologies that offer more engineering applied to principles of physics that might be better and easier to engineer.

Don't you think, once you have true structural stability, tissue stress reduction will follow?

Kevin A. Kirby DPM's picture

Good article, There isn't much to disagree with your concepts here, but I think the focus is in the wrong direction, particularly if you want to think like an engineer. There is too much focus on tissue stress without first understanding the structural stability better. Trying to understand the “internal” forces on the structural components, the structure must be stable first, otherwise, how else are you going to get quantitative measurements?

We've gotten away from looking at how the internal structure is moving moment to moment (coupled with the internal and external forces acting upon it), and where and when over-pronation is becoming unstable (with AND w/o an orthotic). Pressure scans offer more information than we're using, especially where engineering information is vital.

The measuring of axis or ROM applied to principles of engineering are used to support theories but can't be proven and there's no science behind it. Perhaps there are other technologies that offer more engineering applied to principles of physics that might be better and easier to engineer.

Don't you think, once you have true structural stability, tissue stress reduction will follow?

Dr. Kiper:

Thank you for your kind comments regarding my article.

Even though you say that there “isn’t much to disagree with”, you do not seem to appreciate what engineers are concerned with when determining the internal loads on a structure (i.e. tissues for us as podiatrists) since you claim that “there is too much focus on tissue stress”. What are engineers concerned with when they are trying to determine how much internal load is occurring within a structure or machine that they are analyzing? They are concerned mostly with stress!

One of the most sophisticated and current methods by which engineers may predict internal loads and internal stresses occurring within a given structure is to create a computer replica of the structure of interest, along with the known Young’s modulus of each component part, and then run that model through a “finite element analysis” computer program.

In finite element analysis (FEA), the stresses and strains acting at any section of a structure or machine (or foot or any other part of the body) may be predicted by mathematical computations by a piece of sophisticated computer software. FEA is of prime importance in predicting where the high stress areas are within any structure or machine. This information may, in turn, may be used to predict which components of the machine or structure may first fail due to excessive stress and load. FEA is used routinely in engineering and is increasingly being used to analyze stresses in the foot and lower extremity.

If you want to do static analysis of a structure, then by definition that is when the structure is most stable and the structure is also said to be in both translational and rotational equilibrium. A typical engineering analysis of a “stable structure”, as you call it, could be done quite simply with a technique called a “free body diagram analysis” where, again, the internal loads and stresses may be predicted by using simple geometric models of the foot. Therefore, contrary to your statement, engineers are very concerned with abnormal stresses, as podiatrists should also be when they consider the mechanical factors that produce tissue injuries in the patients that present to their offices.

In addition, you are mistaken that there is “no science behind” the use of joint axes to determine the internal and external moments within the joints of the foot and lower extremity. The science behind these concepts began with Sir Isaac Newton and his wonderful treatise “Philosophae Naturalis Principia Mathematica”, published in 1687, in which he describes his three Laws of Motion. Much of the science of engineering is based on Newton’s Laws of Motion. Therefore, for you to say that there is “no science behind” the use of joint axes to determine the internal and external moments that drive all motions of the foot and lower extremity is erroneous.

As I said in an earlier comment, focusing on tissue stress and engineering does not preclude the podiatrist from using other intellectual tools by which to determine the best treatments for our patients. I happen to believe that “thinking like an engineer” would benefit the majority of podiatrists practicing around the world and would greatly clarify their understanding of the mechanical factors which produce injuries in their many patients. It is the definitely the way forward for our profession.

Kevin A. Kirby, DPM
Adjunct Associate Professor
Department of Applied Biomechanics
California School of Podiatric Medicine

Dr Kirby,

First, let me say I certainly do appreciate and marvel at what engineers are concerned with and the complexity of their projects around the world. What I'm saying is looking at forces and internal stress loads before having a stable platform leads to poor outcomes. This is well documented in the literature and clinical trials of present day technology. I've never seen a building built where they put the phone lines. plumbing and electrical lines in before they have a stable foundation. And that is what you're talking about when you try to figure out the internal loads on the foot before you can prove Newton's Laws of Motion apply to a shell orthotic. Besides, how can you accurately measure and quantify any results without a stable platform first?

I think along scientific and engineering principles as well, and in my haste, while I erroneously stated “there isn't any science behind trad orthotics,” I meant to say there isn't “enough” science behind the principles applied to your theories. Your reference to Newton and the use of “axis” doesn't mean your platform of a shell orthotic works on those principles without identifying how. Instead, you've just fast forwarded to figuring out internal stresses??

So, in addition to figuring out lever arm forces and axis, let's use those laws of motion to discuss why a shell orthotic cannot prevent intrinsic pronation, thus causing destabilization within the gait cycle. Of all the engineering and use of scientific laws (however you apply them), this in my opinion stands the tallest in figuring out first. This is why traditional technology has been unable to quantify a prescription, and unable to figure out the secondary biomechanical and kinetic effects of destabilization.

“Mathematical computations by a piece of sophisticated computer software, used to predict which components of the machine or structure may first fail due to excessive stress load”
does not explain the momentum of the biomechanics in motion (Newton would love this). So while FEA may be useful, especially in conjunction with pressure scans which easily define the stress loads of the forefoot on the ground with and w/o orthotic placement (and clearly demonstrates the stability of the MTJ and STJ), the engineering goal to seek is “balance” and “stability” (of the entire foot) under a full load. Couple that with the dynamics and momentum of motion and still maintain “balance and stability” through the stance phase through toe off. This is where traditional technology used today is in my opinion inadequate and faulty.

So, thinking like an engineer is an excellent idea, provided you start at the beginning rather than the middle.

Kevin A. Kirby DPM's picture

Dr. Kiper:

Does your opinion that "traditional technology used today is in my opinion inadequate and faulty" have anything to do with you marketing your "Silicone Dynamic Orthotic" on the Internet at http://www.drkiper.com/ ?

On this website, you claim "The Silicone Dynamic Orthotic is the next generation prescription in arch support —". You further claim on your web site: "It is the mechanical action that separates the Silicone Dynamic Orthotic from any orthotic. It matches the way the foot walks."

You further claim on your web site: "Most orthotics operate in a “fixed" position (a static fulcrum) which does not allow the foot to move naturally. If it’s a traditional "flexible" support, the arch moves up and down, while the foot is moving forward. This does not prevent the front of the foot from becoming unstable."

Your web site comes complete with mail order instructions for your "Silicone Dynamic Orthotic" for $399.00.

I am curious. How do you know that "traditional technology used today is in my opinion inadequate and faulty" when, in fact, numerous research papers have clearly shown that custom foot orthoses, using the Root model, are very effective at treating a large number of foot and lower extremity pathologies?

Kevin A. Kirby, DPM
Adjunct Associate Professor
Department of Applied Biomechanics
California School of Podiatric Medicine

Ditto!
Dennis

Kevin A. Kirby DPM's picture

Dr. Kiper:

First of all, Newton’s Laws of Motion applies to an orthosis shell as well to all materials and physical objects in our world unless those objects are traveling close to the speed of light. Therefore, contrary to your assertion, Newton’s Laws of Motion can be applied to an orthosis shell. In addition, Newton’s Laws of Motion have, for years, been applied to orthosis biomechanics for the past two decades within the scientific literature.

Secondly, as I mentioned earlier in a previous posting, by using the time-trusted engineering load analysis techniques of free body diagram analysis and finite element analysis, the internal loads that occur within the human foot and lower extremity can be estimated quite well. In addition, finite element analysis can be used to determine how foot orthoses can affect the internal loads within the foot and lower extremity and how the orthosis is affected by the loads from the foot. Therefore, I don’t know why you think I have “fast forwarded” to figure out internal stresses in the foot when this has already been done for years by biomechanics researchers.

Here are just a few of the published papers which clearly show that finite element analysis has been used now for over a decade now at analyzing the internal loads within the foot and lower extremity, the effects of foot orthoses on internal loads, and the effect of foot loading forces on the internal loads within foot orthoses.

Barani Z, Haghpanahi M, Katoozian H: Three dimensional stress analysis of diabetic insole: a finite element approach. Technol Health Care, 13:185-192, 2005.

Chen WP;,Tang FT, Ju CW: Stress distribution of the foot during mid-stance to push-off in barefoot gait: a 3-D finite element analysis. Clin Biomech, 16:614-620, 2001.

Chen WP, Ju CW, Tang FT: Effects of total contact insoles on the plantar stress redistribution: a finite element analysis.Clin Biomech, 18:S17-24, 2003.

Cheung JT, Zhang M, Leung AK, Fan Y: Three-dimensional finite element analysis of the foot during standing--a material sensitivity study. J Biomech. 38:1045 -54, 2005.

Cheung JT, An KN, Zhang M: Consequences of partial and total plantar fascia release: A finite element study. Foot Ankle Int. 27:125-132, 2006.

Kristen KH, Berger K, Berger C, Kampla W, Anzböck W, Weitzel SH: The first metatarsal bone under loading conditions: a finite element analysis. Foot Ankle Clin. 10:1-14, 2005.

Liu X, Zhang M: Redistribution of knee stress using laterally wedged insole intervention. Finite element analysis of knee-ankle-foot complex. Clin Biomech. 28:61-67, 2013.

Wu L: Nonlinear finite element analysis for musculoskeletal biomechanics of medial and lateral plantar longitudinal arch of Virtual Chinese Human after plantar ligamentous structure failure. Clin Biomech. 22:221-229, 2007.

Kevin A. Kirby, DPM
Adjunct Associate Professor
Department of Applied Biomechanics
California School of Podiatric Medicine

I agree with Dr. Kirby’s sentiments that podiatrists should think more like engineers. Engineering is the field of study that will allow us to understand mechanically caused pain. One of my instructors was debating with a general surgeon about whether foot surgery or general surgery was harder. Jack Morris’ reply was that you don’t have to walk on your stomach. We have to understand and take into account the force of body weight and ground reaction force to understand how the foot works. Along these lines, I would like to make some comments on Dr. Kiper’s criticism of Dr. Kirby’s approach.

Dr. Kiper wrote: First, let me say I certainly do appreciate and marvel at what engineers are concerned with and the complexity of their projects around the world. What I'm saying is looking at forces and internal stress loads before having a stable platform leads to poor outcomes. This is well documented in the literature and clinical trials of present day technology. I've never seen a building built where they put the phone lines. plumbing and electrical lines in before they have a stable foundation. And that is what you're talking about when you try to figure out the internal loads on the foot before you can prove Newton's Laws of Motion apply to a shell orthotic. Besides, how can you accurately measure and quantify any results without a stable platform first?

My reply:
This is not a valid criticism because it is not possible to figure out the internal forces without knowing what the external forces are. It seems that Dr. Kipper is saying that Dr. Kirby is skipping over the external forces, but that is not possible when you use free body diagram analysis to estimate internal forces.

Dr. Kipper continued:
Your reference to Newton and the use of “axis” doesn't mean your platform of a shell orthotic works on those principles without identifying how. Instead, you've just fast forwarded to figuring out internal stresses?

My reply:
This is exactly why we need to think like engineers. The subtalar joint does not always pronate when the foot hits the ground. Sometimes it supinates and there is a sprained ankle when the foot hit the ground. We have the available engineering tools to understand which way the ground will make the STJ move. These tools are the calculation of the center of pressure of ground reaction force and knowing where the center of pressure is relative to the STJ axis. Dr. Kirby wrote an excellent paper describing how you can find the location of the STJ axis and why performing his test it is easy to see that force applied to one side of the axis will move the STJ in one direction and force on the other side of the axis will move the foot in the other direction. So, in answer to Dr. Kipper’s comments, we know how it works and we did not skip over anything.

Dr. Kipper continued:
So while FEA may be useful, especially in conjunction with pressure scans which easily define the stress loads of the forefoot on the ground with and w/o orthotic placement (and clearly demonstrates the stability of the MTJ and STJ), the engineering goal to seek is “balance” and “stability” (of the entire foot) under a full load. Couple that with the dynamics and momentum of motion and still maintain “balance and stability” through the stance phase through toe off. This is where traditional technology used today is in my opinion inadequate and faulty.

My reply:
One thing that engineers like to do is quantify things. Engineers would like to be able to measure stability. Dr. Kipper’s call for balance and stability demonstrates where engineers would have a problem. These terms have no definition when applied to the foot. When is a foot more balanced or less balanced? If we can’t define the term, it shouldn’t be used. Engineers also have a lazy side to them. Sometimes they need exact numbers to answer their questions and other times they just need a ballpark figure. If we decide that a foot has too much pronation, we only need to know which direction to shift the center of pressure to decrease pronation. So, it is possible to treat the foot like an engineer would without doing all the calculations. We just need to understand the anatomy and the forces applied to that anatomy.

Unfortunately, too many podiatrists think more like salesmen than engineers when it comes to podiatric biomechanics, Kevin. We all know who they are.

Simon,
Your comment sounds very personal (a dialogue with Kevin) and has me confused.

Can you and/or Kevin let us in on who the salesmen vs. engineers are as I for one am not sure who they (we) are?
Dennis

The fluid dynamics of human gait along with tensile strengths of structures involved in movement are rarely reflected in some specialty specific literature. It's not uncommon for general surgery and orthopedic residents to be fully educated in all facets of the act of "walking, jogging, and running," as they impact or are impacted by a myriad of pathologies, both genetic and acquired.

An abundance of user-friendly sources are available online through Orthopedics Hyperguide, Sermo, Medscape, ePocrates, with remarkably easy to comprehend general physics calculations. These refresher lectures, CME courses, are at no cost and essential to anyone providing care to these patients.

As an structural engineer, nursing a pair of sore feet, I have to agree with Dr. Kirby! Think like an engineer.

With today's FEA software and 3D printing (for instant orthotics if needed) a foot should be fairly easy to model. Pressure points (stress locations) could be mapped and compared with acceptable know stress values. Overstressed regions could be identified and load sharing could be used to reduce the pressure. You could even run the models under multiple conditions and predict failures.

(Frankly, as an engineer, I'm surprised at how difficult it is to get an orthotic that works but that is another story)

Good article, Dr. Kirby. Keep going!

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