When a functional foot orthosis rests on a flat surface, the weight-bearing foot at the orthosis-foot interface assumes the reference plantar morphology. But when the orthosis rests inside a shoe in which the shank is a curved surface, the weight-bearing foot at the orthosis-foot interface can assume a different plantar morphology from the reference via deformation along a geometrically incongruent orthosis-shoe interface.1 Because a functional foot orthosis should not deform along the curved shank of a shoe,1 a change in the plantar morphology indicates that the plantar deformation is abnormal. Screening for abnormal deformation should then be implemented to determine the suitability of footwear with a functional foot orthosis in refractory cases of functional foot disorders. A procedure to screen for abnormal deformation, however, has not been reported in the literature. The procedure would involve defining the deformation as the difference between the reference and the in-shoe morphology of the weight-bearing foot at the orthosis-foot interface. The devices (e.g., cameras, scanners) that are conventionally used to capture the morphology of the weight-bearing foot are not applicable to obtain the in-shoe morphology of the weight-bearing foot at the orthosis-foot interface because orthoses and shoes are generally opaque surfaces. Goonetilleke and Witana2 use a technique of casting the weight-bearing foot at an interface, with which the supporting surface is opaque, to obtain a plaster mold of the plantar foot. This technical note addresses a similar technique and describes a procedure to screen for abnormal deformation of the weight-bearing foot at the orthosis-foot interface.
FOOT MORPHOLOGY AT THE ORTHOSIS-FOOT INTERFACE
The approximate locations of the pternion and second metatarsal head were marked as points on the plantar surface of each foot using a skin-safe ink pen (Viscot). These two anatomical loci characterize the plantar foot axis3 as the baseline for the two-point registration,4 which is a superimposition method to be discussed in the subsequent section. A water-soaked plaster of Paris bandage (BSN Medical) was then molded to the entire plantar surface of each non–weight-bearing foot, including the sulcus but not the toes. Note that the plaster bandage must not restrict the foot from increasing in length during weight-bearing. When the plaster bandage molded to each foot was semidry, each plaster-bandaged foot was dressed with a thin nylon stocking to minimize the exposure of the semiwet plaster on the orthosis and inside the shoe. A pair of functional foot orthoses was then placed on a flat surface and was joined by both weight-bearing plaster-bandaged feet (Figure 1). The pair of orthoses was placed on the flat surface in a position in which the individual felt most comfortable. While the individual was standing still, each plaster bandage was allowed to dry completely until it became a rigid mold of each foot at the orthosis-foot interface. When both bandages were dry, the feet were placed in a non–weight-bearing position, the stockings were removed, and then the plaster casts were carefully detached from the feet. Each plaster cast is a mold of the reference morphology of the weight-bearing foot at the orthosis-foot interface (Figure 2). On the topside of each plaster cast (i.e., on the surface where the foot and plaster bandage were in contact), there should be two points indicating the approximate locations of the pternion and second metatarsal head, as they were transferred from the ink markings made on the foot to the plaster bandage (Figure 2A). While shining light into the topside of each plaster cast to illuminate the two points on the underside plantar surface, the illuminated points were marked on the underside using the Viscot ink pen (Figure 2B). The plaster cast of each foot was then placed plantar-side down on a flatbed scanner (Mustek A3 1200S) to capture the image (grayscale, whiten background, 900dpi) of its underside plantar surface (Figure 3A). Note that it may be necessary to place an opaque container (e.g., cardboard box) over the plaster cast during the scanning process to avoid interference of any external light with the scan.
Repeat the steps above using a plaster bandage to cast each weight-bearing foot at the orthosis-foot interface inside a pair of shoes (Figure 4). Subsequently, each plaster cast is a mold of the in-shoe plantar morphology of the weight-bearing foot at the orthosis-foot interface. After marking the two points on the underside plantar surface of the mold, the plaster cast of each foot was then placed plantar-side down on a flatbed scanner to obtain the image of the in-shoe plantar morphology (Figure 5A), which was used together with the reference plantar morphology to screen for abnormal deformation.
FOOT DEFORMATION AT THE ORTHOSIS-FOOT INTERFACE
To screen for abnormal deformation of each weight-bearing foot at the orthosis-foot interface, the scanned images of the two pairs of plantar foot surfaces are needed: the first pair shows the reference plantar surface contour of each weight-bearing foot at the orthosis-foot interface (Figure 3A), and the second pair shows the in-shoe plantar surface contour of each weight-bearing foot at the orthosis-foot interface (Figure 5A). Screening for abnormal deformation requires an outline shape analysis of the closed plantar surface contour of the weight-bearing foot at the orthosis-foot interface. Because the contoured edge of the plaster cast at the plantar sulcus is not homologous in structure, it was not intended to close the plantar surface contour. A standardization technique to close the plantar surface contours was thus required: the section of the plantar surface contour proximal to the straight line that is perpendicular to the foot axis at the point of the second metatarsal head is an essential area of functional foot orthotic correction, as it encompasses the entire surface area of the plantar midfoot and plantar rearfoot (Figures 3B and 5B).
Selecting the right foot to describe the procedure to screen for abnormal deformation, the image of the reference plantar surface contour of the right weight-bearing foot at the orthosis-foot interface was imported first into the digitization software program, tpsDig version 1.40.5 The image displays two points on the plantar surface. Using the tool, “Digitize landmarks,” the first point was digitized as landmark 1 (approximate to the second metatarsal head) and the second point as landmark 2 (approximate to the pternion). The plantar surface contour was then digitized using the tool, “Draw background curves,” which interpolates the contour with a set of coordinates (Figure 6A). Note that the contour starting point, which is the first coordinate, was digitized at landmark 1. The contour coordinates and the two landmark points were saved together as a TPS file. This was repeated for the image of the in-shoe plantar surface contour of the right weight-bearing foot at the orthosis-foot interface (Figure 6B), and its interpolated set of contour coordinates and two landmark points were saved together in the same TPS file as the reference plantar surface contour. With this TPS file, the two sets of contour coordinates and landmark points were imported into the morphometrics software program, Morpheus et al.,6 after which some commands in the program were executed. Bookstein4 two-point registration was performed in the program to standardize the two contours for location and orientation. Scale was not standardized because the reference and the in-shoe plantar surface contour of the right weight-bearing foot at the orthosis-foot interface are from the same individual, so the same weight-bearing foot. Executing the command, “set superimposition Booksteinscaling none,” ensures that the scales of the two contours are preserved. Then entering the command, “superimposition bookstein 1 2,” executes the two-point registration, which superimposes the two contours (i.e., reference and in-shoe) with respect to the plantar foot axis or the baseline defined by the two landmarks (Figure 7A).
Repeat the above procedure for the left foot using a separate TPS file for the two sets of contour coordinates and landmark points. Screening for abnormal deformation of the left weight-bearing foot at the orthosis-foot interface (Figure 7B) shows the difference in the deformation compared with the right weight-bearing foot. It is thus important to screen for abnormal deformation of each weight-bearing foot separately by using two separate TPS files.
Unlike an accommodative foot orthosis, a functional foot orthosis should not depend on the shank of a shoe to resist deformation.1 If a functional foot orthosis deforms along the curved shank of a shoe, then the consequent plantar deformation is abnormal. The deformation could reduce the effectiveness of the orthosis in treating a functional foot disorder. Although orthosis-shoe interactions and the clinical importance of these interactions to foot orthotic function are known (see Kirby1 for details), no morphology-based method has been reported to screen for abnormal deformation of the weight-bearing foot at the orthosis-foot interface. The procedure described in this note can be used to screen for the deformation. It is important to keep in mind that there is a significant amount of variation in orthotic surface morphology and footwear shank curvature. It is thus possible to observe abnormal deformation with some shoes and not with others. Unfortunately, there are no a priori selection criteria for such shoes, including those that claim to be “orthotic friendly.” If abnormal deformation is observed (such as the case shown in this note; see Figure 7), then the clinician should either fabricate an insert that fits in between the orthosis and shoe to correct for the deformation or determine suitable footwear, in which the deformation is negligible or absent, via the procedure described in this note. Another option that could minimize or eliminate abnormal deformation (albeit with a particular shoe) is to take a mold of the foot inside the shoe and then to fabricate a functional foot orthosis from the in-shoe casting. This alternative casting technique could be an excellent option for those who wear primarily one pair of shoes during their daily activities.
Clinicians will undoubtedly encounter cases in which functional foot orthotic therapy has not been effective. These refractory cases may simply be the result of abnormal deformation of the weight-bearing foot at the orthosis-foot interface. The deformation should thus be screened for in each orthosis-shoe combination to determine footwear suitability before considering other treatments.
The author thanks the participant for taking part in the demonstration of the procedure described in this note. The author also thanks the reviewers for providing comments and suggestions that greatly improved the manuscript.
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