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):
TISSUE STRESS APPROACH TO MECHANICAL FOOT THERAPY
In last month’s newsletter, the subtalar joint neutral (SJN) approach to mechanical foot therapy was reviewed. To summarize, the SJN Theory is based on the premise that the structures of the foot and lower extremity can be accurately measured so that any deviation from an ideal or a “normal” structure would be considered to be a “deformity”. Using the SJN Theory as the basis for mechanical foot therapy, foot orthoses are designed to “prevent compensation for deformities” with the orthosis prescription being based on the “deformities” which are determined during the biomechanical examination of the patient. Proponents of the SJN Theory do not necessarily change the prescription variables of foot orthoses when different anatomical structures are injured or the mechanical nature of the pathological loading forces are different since it is assumed that by simply “preventing compensation for deformities”, more normal gait function will occur and the injured structure will eventually heal.
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.]