New goals for Podiatric Biomechanics

Simon -- we we typing at the same time!!! -- you must be quicker on the keyboard than I and got yours done first!!! -- I had a long day here - its still early in the AM for you.

 

Not so early here anymore. Some of the members of this forum are clearly descended from owls, badgers and other creatures of the night

As for my typing speed- working on a lap top at the moment and they don't call me "three fingered Spooner" for nothing

 

Simon,
GREAT stuff thankyou.....getting your perspective on some of these issues. I can look facts and theories up in books, can't get the perspective of experience from those sources as well. Thanks for your time ...sorry you aren't sleeping better.
regards Phill

 

I agree with Craig's contention re: risk. Further, what Craig is basically saying here is that the interaction between the genotype and the environment is key in determining pathology. This is a unique set of circumstances for the individual in question and is explained by Spooners Quantitative Genetic Theory of Foot Function (OK- I didn't invent the equation, but I'm definitely the first to apply it to global foot function) viz:
P= afunction of: G +E +(GxE) + i

Where P = a quantitative measure of foot function
G = genotype
E= environment (all non-genetic factors)
i = measurement error

So if we take two identical environmental factors (or risk factors), for example: shoes, hard flat surfaces or orthoses etc. and apply them to two different individuals over time, the outcomes in terms of their foot function will be dependent upon the test subjects genotype and previous environmental exposures. Thus, the outcomes could be very different.

What this also tells us though is that classification of "foot-types" is a fruitless endeavour because everyone is different, even Mz twins- sorry Eric.

I know some people will love this, and some will hate it, but if you're a student- go bug your lecturer with it. Unfortunately, it doesn't help you treat patients any better, other than tell you to treat all patients as individuals. Six years of PhD and these are the fruits....

 

risk factors and cause

Yes, previous environmental exposure will be a factor in what pathology develops. I still take exception with the idea that everything is a risk factor. Something has to cause the pathology. Yes, LDL is a risk factor for atherosclerosis. However, when you look at the artery, there is gunk stuck on the walls of the artery. That gunk is the pathology that causes diminished blood flow distal to the plague. We may not know all of the factors that must be combined with a high LDL to create a plaque, but there must be something about LDL that helps create/ cause plaque. Something, or a bunch of somethings cause the plaque.

In the foot there may be genetics that create a "type" of foot that is more predisposed to certain pathology. All we need is the correct measurement of that type. STJ pronation is probably not it. A large percentage of patients with HAV did not have it when they were born. Therefore some forces, or lack of forces, caused the toe to change position. Perhaps a shoe is a risk factor for HAV because it creates a force from medial to lateral at the tip of the hallux. This force would accelerate an existing predisposition toward HAV. It has been shown (Snijders) that plantar flexion moments on the hallux will increase the medial deviation of the first metatarsal, at least temporarily. Plantar flexion moment may also create a moment that would "cause" the hallux to rotate toward the second toe. We may be able to find measurements that will be associated with high plantar flexion moments at the 1st MPJ and these would be our risk factors.

So, I have disagree with the notion that classification of the foot into types is fruitless. A foot with a medially deviated STJ axis will behave differently, and be at higher risk for certain pathology, than a foot with a laterally deviated STJ axis. A foot with a paritally compensated rearfoot varus will be at higher risk for some pathology than a foot with plenty of eversion range of motion available. These particular risk factors are certainly part of the cause of the pathology.

Perhaps this differentiation of cause and risk facotrs is just semantics. This distinction becomes important in treatment. If posterior tibial tendon dysfunction is associated with a high pronation moment from ground reaction force, then treatment should be succesful if the pronation moment from the ground is reduced. On the other hand if your treatment of high LDL is to find a way to lower the LDL by getting more of it to stick to the walls of arteries then you may not be treating the problem. So, it is important to look at risk factors and it is important to look for the cause of pathology.

Cheers,

Eric Fuller

 

Kevin, can you forward me to "how you specifically locate the STJ axis on a subject/patient". Is it reliable and how well does it correlate with any other testing procedure such as supination resistance testing and FPI?

I have been shown the point pressure and STJ motion response, but I am not convinced about my own testing procedure. Maybe my thumb is too big and covers too broad an area.


Ron

 

Eric the problem is that factors such as the position of the STJ axis aren't, in isolation, necessarilly predictors of pathology. That is someone could have a medially deviated STJ axis but no pathology. Furthermore, as a predictor, it lacks specificity. That is a medially deviated axis may be a "risk factor" in a whole host of conditions, so if we try to predict which pathology someone will experience by axial position classification alone we may be wrong.

Also, when does an axis become "medially deviated? Wherever you draw that delinaetion, it is an artificial delineation because the two feet exhibiting axes either side of that delineation may or may not function completely differently. Common sense tells us there is more to this than simply classifying the axial position. Not least, I'm sure you of all people will recognise that the position of the CoP relative to this axis is a key determinant.

In isolation foot type classification tells us very little. For example, two feet with medially deviated axes- which one gets the post tib dysfunction and which one gets the plantarfasciitis? Which one gets the hallux limitus and which one gets the HV? Need more info? Yes in order to build a good predictive model we need to identify the "predictors" of these pathologies. That is the independent variables which can be used statistically to account for the variance in the dependent. I personally prefer the term predictor to risk factor. But even with good predictive models we still get false positives and false negatives.

 

Ron:

Sorry I didn't respond sooner. We just moved from our home of 16 years to a new home and we are still living out of boxes to some extent.

There are three different ways to clinically determine the STJ axis spatial location:

1. STJ palpation technique: First described 19 years ago in my paper in JAPMA (Kirby KA: Methods for determination of positional variations in the subtalar joint axis. JAPMA, 77: 228-234, 1987). If you would like a pdf copy of the paper, please give me your e-mail address.

2. STJ grid dot rotation technique: First described 12 years ago by two of my biomechanics professors from CCPM, Jack Morris, DPM and Lester Jones, DPM (Morris JL, Jones LJ: New techniques to establish the subtalar joint's functional axis. Clinics Pod Med Surg., 11(2):301-309, 1994). A grid of skin markings are made at the approximate exit points of the STJ axis on the posterior-lateral calcaneus and anterior talar neck. Then the point of least rotation is marked on the skin after putting the STJ through range of motion. These points should approximate the STJ axis anterior and posterior exit points. Simon Spooner and I use this method to position the STJ Axis Locator on subjects (Spooner SK, Kirby KA: The subtalar joint axis locator: A preliminary report. JAPMA, 96:212-219, 2006).

3. Standing STJ approximation technique: The best way to approximate the STJ axis in standing is to do the grid dot rotation technique and then have the patient stand so that you can see where the axis will pass over the forefoot. However, a very nice little shortcut technique, the standing STJ approximation technique, which I have been using for about the past 5 years (I haven't published it yet), is to place a pencil or pen tip on the bisection of the anterior talar neck and then angulate it toward the posterior-lateral calcaneus so that the pencil or pen approximates the anterior spatial location of the STJ axis. This seems to correlate well to the STJ axis spatial location and is quick to perform in the clinical setting. I train the podiatric surgical residents that rotate through my office on this technique and they seem to pick it up fairly easily.

Craig Payne and coworkers (Payne C, Munteaunu S, Miller K: Position of the subtalar joint axis and resistance of the rearfoot to supination. JAPMA, 93(2):131-135, 2003) have studied STJ axis location correlation to the supination resistance test (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). I don't know of any studies that try to correlate STJ axis location to Tony Redmond's Foot Posture Index (FPI). However, I would think that the FPI would not correlate to STJ axis spatial location since the FPI is a composite measure of the foot morphology and not a functional measure of the STJ moments that may occur during weightbearing activities.

 

Thanks Kevin.

sportsconditioning@bigpond.com

 

Ron:

I think that once you start to objectively analyze STJ axis spatial location in the patient's foot, you will find, like I have, that it correlates quite well to the symptoms that the patient complains of. For all those that are interested in the research, theory and clinical usefulness behind STJ axis location, here is a list of references for your further reading pleasure:

Kirby KA: Foot and Lower Extremity Biomechanics: A Ten Year Collection of Precision Intricast Newsletters. Precision Intricast, Inc., Payson, Arizona, 1997.

Kirby KA: Foot and Lower Extremity Biomechanics II: Precision Intricast Newsletters, 1997-2002. Precision Intricast, Inc., Payson, AZ, 2002.

Kirby KA: Methods for determination of positional variations in the subtalar joint axis. JAPMA, 77: 228-234, 1987.

Kirby, K.A., Loendorf, A.J., and R. Gregorio: Anterior axial projection of the foot. JAPMA, 78: 159-170, 1988.

Kirby KA: Rotational equilibrium across the subtalar joint axis. JAPMA, 79: 1-14, 1989.

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.

Kirby KA: The medial heel skive technique: improving pronation control in foot orthoses. JAPMA, 82: 177-188, 1992.

Kirby KA.: Biomechanics and the treatment of flexible flatfoot deformity in children. PBG Focus, J. Podiatric Biomechanics Group, 7:10-11, 1999.

Kirby KA: Biomechanics of the normal and abnormal foot. JAPMA, 90:30-34, 2000.

Kirby KA: Conservative treatment of posterior tibial dysfunction. Podiatry Management, 19:73-82, 2000.

Kirby KA: Subtalar joint axis location and rotational equilibrium theory of foot function. JAPMA, 91:465-488, 2001.

Kirby, Kevin A.: What future direction should podiatric biomechanics take? Clinics in Podiatric Medicine and Surgery, 18 (4):719-723, October 2001.

Lewis GS, Kirby KA, Piazza SJ: A motion-based method for location of the subtalar joint axis assessed in cadaver specimens. Presented at 10th Anniversary Meeting of Gait and Clinical Movement Analysis Society in Portland, Oregon. April 7, 2005.

Lewis GS, Kirby KA, Piazza SJ: Determination of subtalar joint axis location by restriction of talocrural joint motion. Gait and Posture. In press. 2006.

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.

Roukis TS, Kirby KA: A simple intraoperative technique to accurately align the rearfoot complex. JAPMA, 95:505-507, 2005.

Ruby, Patricia, Maury L. Hull, Kevin A. Kirby, and David W. Jenkins: The Effect of Lower-Limb Anatomy on Knee Loads During Seated Cycling, Journal of Biomechanics, 25 (10): 1195-1207, October 1992.

Spooner SK, Kirby KA: The subtalar joint axis locator: A preliminary report. JAPMA, 96:212-219, 2006.

Van Gheluwe B, Kirby KA, Hagman F: Effects of simulated genu valgum and genu varum on ground reaction forces and subtalar joint function during gait. JAPMA, 95:531-541, 2005.

Benink, R.J.: The constraint mechanism of the human tarsus. Acta Orthop Scand, 56: (Suppl) 215, 1985.

Fuller EA: Center of pressure and its theoretical relationship to foot pathology. JAPMA, 89 (6):278-291, 1999.

Klein, P., Mattys, S, and Rooze, M.: Moment arm length variations of selected muscles acting on talocrural and subtalar joints during movement: An in vitro study. J. Biomechanics, 29:21-30, 1996.

Lundberg A, Svensson OK: The axes of rotation of the talocalcaneal and talonavicular joints. Foot, 3:65, 1993.

Morris JL, Jones LJ: New techniques to establish the subtalar joint's functional axis. Clinics Pod Med Surg., 11(2):301-309, 1994.

Nester, C.J.: Review of literature on the axis of rotation at the subtalar joint. The Foot, 8: 111-118, 1998.

Payne C, Munteaunu S, Miller K: Position of the subtalar joint axis and resistance of the rearfoot to supination. JAPMA, 93(2):131-135, 2003.

Phillips, R.D., Lidtke, R.H.: Clinical determination of the linear equation for the subtalar joint axis. JAPMA, 82:1-20, 1992.

Van Den Bogert, A.J., Smith, G.D., Nigg, B.M.: In vivo determination of the anatomical axes of the ankle joint complex: an optimization approach. J Biomechanics, 27 (12):1477-1488, 1994.

Van Langelaan, E.J.: A Kinematical Analysis of the Tarsal Joints. Acta Orthop. Scand., 54:Suppl. 204, 135-229, 1983.

 

Mr Spooner some people would say this is a 'blunderbuss' approach to medicine.
Ref: 'In terms of theories...' surely it's better to use only the correct colours instead of all of them?
The 'blunderbuss' approach often characterises academics with an impaired understanding of their discipline.

 

David,

You just struck the heart of Posturology. Namely, applying a tactile input to the bottom of the foot affects the entire body, not just the foot. The key is learning how to stimulate the foot to achieve the desired outcomes.

Brian R