I am grateful to Kevin Kirby and Precision Intricast
for permission to reproduce this March 2002 Newsletter (you can buy the 2 books of newsletters off Precision Intricast):
THE BIOMECHANICS OF TISSUE STRESS
The tissue stress approach to mechanical foot therapy is based on the concept that the location, type and magnitude of stresses that cause mechanical symptoms of the foot and/or lower extremity should be strongly considered when specific treatments are being designed for a patient. The ideas inherent in the tissue stress approach should not be foreign to any clinician who routinely treats mechanically based symptoms of the foot and/or lower extremity. For example, if a patient has pain plantar to the second metatarsophalangeal joint (MPJ), the tissue stress approach simply states that treatment should be directed toward reducing any abnormal stresses in the injured structures of the plantar second MPJ so that healing may be accelerated and a normal gait pattern may be reestablished. This type of approach to mechanical foot therapy has a different emphasis than that advocated by the proponents of the subtalar joint neutral (SJN) theory in which mechanical treatment is directed at preventing “abnormal compensations” for “deformities” in the foot and/or lower extremity. When prescriptions for custom foot orthoses are considered, the SJN theory places much less emphasis on the anatomical site and type of pathological stresses which are causing the symptoms and/or injury in the patient than does the tissue stress approach to mechanical foot therapy.
In order for the clinician to become more familiar with the idea of tissue stress, it is important that the basic mechanical concepts of loading forces and stresses first be reviewed. The external forces that cause stresses to occur within a structure may be classified by the way they tend to deform the structure upon which they are acting. Axial loading can occur in either one of two ways: compression and tension. A structure is defined as being under compression if the axial loading force tends to make it shrink (Fig. 1). If the axial loading force tends to make a structure elongate, then it is defined as being under tension. Another form of loading on a structure is called shear loading. Shear loading tends to produce horizontal sliding of one layer of a structure over another. Tensile and compressive forces are commonly called normal forces while shearing forces are commonly called tangential forces (Ozkaya, N., Nordin, M.: Fundamentals of Biomechanics: Equilibrium, Motion and Deformation. Van Nostrand Reinhold, New York, 1991, p. 271).
Figure 1. External loading forces acting on a structure will result in the development of internal resistance to that load called stress. When compression loading forces (Fc) occur (top), the structure will develop compressive stress. When tension loading forces (Ft) occur (middle), tensile stress will be developed within the structure. When shear loading forces (Fs) occur (bottom), the structure will develop shear stress.
Any tissue in the body, when subjected to an external loading force, will develop an internal resistance to that load, which is called stress. If a tissue tends to have a large capacity to resist external loading forces, then the internal load, or stress, it can develop under load will be relatively large. However, if an anatomical structure has only a small capacity to resist the loading forces, then the internal load, or stress, it can develop under load will be relatively small. For example, a steel cable is able to develop higher stresses within its structure when compared to a rubber band since the steel cable has a larger capacity to resist tensile loading forces than does the rubber band (Whiting, W.C., Zernicke, R.F.: Biomechanics of Musculoskeletal Injury. Human Kinetics, Champaign, IL, 1998, p. 67).
Depending on the nature of the external loading force acting on it, a structure may develop compressive stress, tensile stress and/or shear stress (Fig. 1). In order to develop a compressive stress, the structure must develop an internal loading force that tends to resist being pushed together (e.g. the tibia develops compressive stress in response to supporting the mass of the body during weightbearing activities). In order to develop a tensile stress, the structure must develop an internal loading force that tends to resist being pulled apart (e.g. the Achilles tendon develops tensile stress in response to ankle joint dorsiflexion loads). Any forces acting parallel, or tangential, to the applied external loading force creates a shear stress. Shear stress is commonly developed in structures where torsional forces are applied to that structure. For example, the tibia will develop shear stresses directed perpendicular to its long axis when torsional forces are applied that tend to cause internal rotation of the tibia relative to the foot (Whiting, Zernicke, 1998, pp. 67, 75).
The amount of stress within a structure may be calculated by dividing the magnitude of the axial load, F, by the cross-sectional area over which the load is distributed, A. Axial, or normal, stress is commonly denoted by the Greek letter, sigma. Therefore, the formula for determining axial stress is as follows: σ = F/A. Forces that act tangential to the applied load create shear stress which is commonly denoted by the Greek letter, tau. Therefore, the formula for determining shear stress is as follows: τ = F/A. The standard unit used for the measurement of stress in biomechanics is the Pascal (Pa), which is defined as one Newton (N) distributed over one square meter (1 Pa = 1 N/m2). One Newton is equivalent to 0.225 pounds (Whiting, Zernicke, 1998, p. 67).
Since the magnitude of stress acting on a biological tissue is determined by the magnitude of loading force divided by the cross-sectional area over which the load is distributed, then cross-sectional size of the tissue is just as important as the absolute force acting on the tissue. For example, if a tensile loading force of 300 N is applied to a posterior tibial tendon with a cross-sectional area of 0.25 cm2, then the stress on the posterior tibial tendon would be as follows: σ = 300 N/0.0025 m2 = 120,000 Pa.
However, if the same loading force of 300 N is applied to a posterior tibial tendon where half of the tendon fibers have been ruptured, so that the cross-sectional area of the tendon is now 0.125 cm2, then the magnitude of stress acting on the remaining tendon fibers would be increased by two-fold as follows: σ = 300 N/0.00125 m2 = 240,000 Pa. Therefore, the loss or rupture of a percentage of fibers in a tendon can greatly increase the stress on the remaining tendon fibers that, in turn, will increase the likelihood of tendon rupture even under normal magnitudes of tensile loading forces on the tendon.
[Reprinted with permission from: Kirby KA.: Foot and Lower Extremity Biomechanics II: Precision Intricast Newsletters, 1997-2002. Precision Intricast, Inc
., Payson, AZ, 2002, pp. 15-16.]