I am grateful to Kevin Kirby and Precision Intricast for permission to reproduce this June 2000 Newsletter (you can buy the 2 books of newsletters off Precision Intricast):
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EFFECT OF SUBTALAR JOINT AXIS LOCATION ON ARCH IRRITATION IN ORTHOSES
Foot orthoses made for abnormally pronated feet are designed to reduce the pronated position, reduce the magnitude of pronation motion, or reduce the pronation velocity of the subtalar joint (STJ) during weightbearing activities. In order to reduce the STJ pronation of the foot, the properly constructed foot orthosis must be able to generate increased supination moment across the STJ axis. The increased STJ supination moment caused by the foot orthosis acts to counterbalance the increased STJ pronation moment which is due to the abnormal medially deviated STJ axis which is common in this type of foot (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).
Reaction force acting on the plantar foot (i.e. plantar reaction force) which is medial to the STJ axis causes a STJ supination moment and plantar reaction force (PRF) which acts lateral to the STJ axis causes a STJ pronation moment. In order to attempt to reduce or counterbalance the excessive STJ pronation moments which arise from a foot with a medially deviated STJ axis, the orthosis must shift the PRF from a more lateral location to a more medial location. By shifting the PRF from a more lateral to a more medial location, there will be, in turn, more PRF medial to the STJ axis and less PRF lateral to the STJ axis than there had been without the foot orthosis. The result of the increased STJ supination moment and decreased STJ pronation moment which occurs with this medial shift in PRF is a net increase in STJ supination moment which is mechanically necessary to reduce pronation related symptoms (Kirby KA: The medial heel skive technique: improving pro-nation control in foot orthoses, JAPMA, 82: 177-188, 1992).
In feet with posterior tibial dys-function, there will be always be a severely medially deviated STJ axis (Fig. 1). As a result of the severely medially deviated STJ axis, the PRF acting on the medial aspects of the plantar foot will have a shorter moment arm by which to generate STJ supination moment than it would have in a foot with a normal STJ axis location. This large medial shift in the STJ axis location is the reason that orthosis modifications which increase the PRF medial to the STJ axis, such as the medial heel skive technique, are critical to achieving optimal increases in STJ supination moment in the severe flatfoot deformity (Kirby, JAPMA, 1992).
Figure 1. Orthosis reaction force (ORF) acting at the navicular-medial cuneiform joint (NMCJ) will have different effects on the compression of the soft tissues plantar to the NMCJ and on the rotational effects across the subtalar joint (STJ ) axis depending on the relative spatial location of the STJ axis
The effect which the reaction force from the orthosis (i.e. orthosis reaction force) has on the soft tissue and osseous structures of the MLA will be very dependent on the spatial location of the STJ axis in relation to the specific anatomical area of the MLA (Fig. 1). For example, the effects that orthosis reaction force (ORF) acting plantar to the navicular-medial cuneiform joint (NMCJ) have on the local soft tissue structures, and on the foot as a whole, will differ widely depending on whether the foot has a normal STJ axis location, a medially deviated STJ axis or a laterally deviated STJ axis.
In the foot with a normal STJ axis location, ORF plantar to the NMCJ is medial to the STJ axis and in the foot with a laterally deviated STJ axis, ORF plantar to the NMCJ is even more medial to the STJ axis (Fig. 1). However, with a medially deviated STJ axis, ORF plantar to the NMCJ is close to being directly plantar to the STJ axis, which greatly changes the effect of ORF in this area of the foot. The reason that the spatial location of the STJ axis has such a significant influence on the mechanical effects of ORF on the plantar foot is due to the mechanical principle that a force will have different effects on an object depending on the location of the force relative to the axis of rotation of that object.
In order to better understand how an external force acting on an object can cause different mechanical effects on that object, a model will be used (Fig. 2). In this model, three forces are given to act on an object which is non-deformable, but which also may rotate about an axis of rotation. In biomechanics, an object may be considered to be a rigid body as long as the deformations of the parts of the structure of interest are insignificant relative to the motion of the structure as a whole (Nigg, B.M.: “Mechanics”. In Nigg, B.M., Herzog, W. (eds): Biomechanics of the Musculo-skeletal System. John Wiley and Sons, New York, 1994.).
Figure 2. In the model above, a rigid body object which has a constant axis of rotation is acted upon by an external force (FEXT) which is comprised of two component forces: the component force (FC) which causes compression of the rigid body at its joint axis, and the component force (FR) which tends to cause clockwise rotation of the object about its axis of rotation. When the external force is toward the left side of the object (A), the rotation force is of greater magnitude compared to the compression force. When the external force is moved toward the center of the object (B), the rotation force decreases in magnitude and the compression force increases in magnitude. When the external force is moved nearly in line with the axis of rotation (C), the rotation force is minimal with nearly all the force acting to compress the rigid body against its joint axis.
Every time an external force acts on an object with an axis of rotation, the force may have the possibility of having two actions on that object (Fig. 2). The external force may cause a rotational force (i.e. moment) which tends to cause the object to rotate about its axis of rotation. In addition, the external force may cause a compression force which tends to compress the object more
forcefully against its axis of rotation.
In order to determine how much the resultant external force tends to cause either compression and/or rotation of the object at the axis of rotation, the resultant external force vector is commonly broken down into two component vectors (i.e. a compression component force vector and a rotation component force vector) which are directed 900 to each other (Fig. 2). The compression component vector is that part of the resultant external force vector whose line of action is directed toward the axis of rotation of the object and therefore produces only an increase in compression force at the axis of rotation of the object. The rotation component force vector is the part of the resultant external force vector which is directed 900 from the compression component vector and which produces only an increase in rotational force (i.e. moment) by acting across a moment arm. Therefore, each external force acting on an object with an axis of rotation may have varying magnitudes and ratios of compression and rotation effect depending on the direction, line of action, point of application and magnitude which that external force has on the object.
In the model described, the rotation and compression effects of three different forces acting on a rigid body object with identical magnitudes and directions, but with varying lines of action and points of application will be compared. The axis of rotation is rigidly fixed outside the object, so that external forces acting on the object will not displace the axis of rotation. In the first example of the model, the external force acting on the object is well to the left of the axis of rotation so that there is a relatively large magnitude of rotation force component acting on the object and a relatively small magnitude of compression force component acting on the object (Fig. 2A).
In the second example, the external force is moved toward the right on the object so that there is decreased magnitude of rotation force component and increased magnitude of compression force component acting on the object (Fig. 2B). In the third example, the external force is moved nearly directly in line with the axis of rotation so that nearly all of the effect of the external force is to compress the axis of rotation with very little of the effect to cause a rotation force on the object (Fig. 2C).
Therefore, moving the external force more directly in line with the axis of rotation on the object increases the compression effect and decreases the rotation effect which the force has on the object.
In order to further explore these mechanical principles in a situation which is more applicable to the real-life condition of a foot orthosis acting on a foot, a second model will be used (Fig. 3). In this second model, the object now has a relatively non-deform-able inner core to approximate the bones of the midfoot which is surrounded by a deformable outer layer to approximate the soft tissue of the plantar foot. The object has a single axis of rotation, but the axis can move within the object to different spatial locations, with the object being able to rotate freely about the axis, independent of its spatial location. The axis of rotation is rigidly fixed outside the object, so that external forces acting on the object will not displace the axis of rotation, but may either deform the soft covering of the object and/or rotate the object. The object has only one external force acting on it, with the force having a constant magnitude, line of action, point of application and direction.
Figure 3. In the model above, an object with a relatively non-deformable inner core and a deformable outer layer with a variable axis of rotation is acted upon by an external force (FEXT). When the external force is far to the left of the axis of rotation (A), there is a relatively greater rotation force component (FR) than a compression force component (FC). As the axis of rotation moves to the left (B), the external force has an increased compression effect and a decreased rotation effect. As the axis moves even further to the left, nearly in line with the external force (C), the rotation force is minimal with nearly all the force acting to compress the deformable outer layer against the non-deformable inner core
Mechanical analysis of the actions of the external force on the object demonstrates that the spatial location of the axis of rotation within the object in relation to the line of action of the external force is quite important. In the first example, the axis of rotation is a relatively large distance from the line of action of the external force so that there is a relatively large rotation force component and a relatively small compression force component (Fig. 3A). In the second example, the axis of rotation is moved to the left causing the external force to have a relative decrease in its rotation force component and a relative increase in its compression force component (Fig. 3B). In the third example, the axis of rotation is moved even further to the left, causing the external force to have a relatively small rotation force component and a relatively large compression force component (Fig. 3C). Therefore, as the axis of rotation of the object moves closer to being directly in line with the external force, not only does the force have less ability to cause clockwise rotation of the object but also has an increasing compression effect on the object. The increased compression effect both deforms the material covering the object and compresses the object against its axis of rotation.
It is a common clinical observation that as the foot becomes more pronated, there is more likelihood that the foot orthosis will cause irritation to the medial longitudinal arch (MLA) of the foot. Irritation to the MLA from foot orthoses is commonly seen in the severe flatfoot deformity, such as would be seen in posterior tibial dysfunction. In consideration of the mechanical analyses which have been explored above, it now becomes obvious that the most likely cause of the MLA irritation from an orthosis in a severe flatfoot deformity with a medially deviated STJ axis is due to the spatial location of the STJ axis in relation to the ORF. In the example of the NMCJ, as the STJ axis becomes increasingly more medially deviated, the ORF acting on the plantar NMCJ will cause increasing compression effect on the soft tissues plantar to the NMCJ and decreasing supination rotation effect across the STJ axis (Fig. 1). If the STJ lies directly over the NMCJ, then ORF plantar to the NMCJ causes only soft tissue compression, with no STJ rotational effects being produced. However, if the STJ is in its normal location, significantly lateral to the NMCJ, then ORF plantar to the NMCJ causes much less soft tissue compression and much more STJ supination moment.
Therefore, the medial deviation of the STJ axis which occurs in flatfoot deformity can greatly change the specific mechanical effects which ORF has on the compression of the soft tissues of the plantar foot and on the rotational force across the STJ axis. The soft tissue structures located plantar to the medial osseous structures of the midfoot, such as the NMCJ, are much more likely to be injured by a foot orthosis as the STJ axis is more medially deviated, than when it is in a more normal location. These mechanical analyses explain the very common observation that medial orthosis edge irritation is much more likely to occur in flatfoot deformities with medially deviated STJ axes than in feet with more normal STJ axis locations.
[Reprinted with permission from: Kirby KA.: Foot and Lower Extremity Biomechanics II: Precision Intricast Newsletters, 1997-2002. Precision Intricast, Inc., Payson, AZ, 2002, pp. 29-32.]
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