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TECHNICAL NOTE

Method to Evaluate the Sagittal Plane Mobility of the Medial Arch of the Shod Foot with Foot Orthosis

Papadopoulos, Anthony PhD, CPed

Author Information
Journal of Prosthetics and Orthotics: October 2019 - Volume 31 - Issue 4 - p 262-266
doi: 10.1097/JPO.0000000000000262
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Abstract

The displacement (δ) of the medial height of the dorsal surface of the midfoot is a valid and reliable measure of the sagittal plane mobility of the medial arch of the foot1,2:

where Hsit is the medial height measured from the floor to the dorsal surface of the foot at 50% of foot length as the foot bears 10% of body weight achieved by sitting, and Hstand is the medial height measured from the floor to the dorsal surface of the foot at 50% of foot length as the foot bears 50% of body weight achieved by standing.1,2 Equation 1 plays an important role in foot orthotic therapy.3 Any foot orthosis that is designed to support a hypermobile medial arch reduces the value of δ.3 This reduction, however, could be either too large (which would disrupt the natural shock absorption mechanism of the medial arch) or too small (which would facilitate the existing hypermobility of the medial arch). Either way, there is a potential risk of injury or an unsuccessful therapeutic outcome if δ takes a value beyond the reference range.3 Consequently, Equation 1 has been used to evaluate the sagittal plane mobility of the medial arch of only the unshod foot with a foot orthosis to improve the orthotic treatment of foot pathomechanical disorders, such as flexible pes planus and abnormal foot pronation.3 Because foot orthoses are in-shoe devices, and shoes can affect medial arch function4 and foot deformation at the orthosis-foot interface,5 it is necessary to evaluate the sagittal plane mobility of the medial arch of the shod foot with a foot orthosis to optimize in-shoe orthotic effectiveness. Equation 1 should thus be adapted to the in-shoe condition:

where is the medial height measured from the floor to the dorsal surface of the shoe at 50% of shoe length as the shod foot bears 10% of body weight achieved by sitting, and is the medial height measured from the floor to the dorsal surface of the shoe at 50% of shoe length as the shod foot bears 50% of body weight achieved by standing. Like Equation 1, Equation 2 can be evaluated via the caliper method as described in Williams and McClay.1 However, because the caliper method involves various types of hardware not readily available in most clinics, a digital photographic method, involving a different setup from the one addressed in Pohl and Farr,6 will be presented in this technical note due to the accessibility of digital photographic devices.

METHOD

MINIMIZING PERSPECTIVE DISTORTION

When using photography to capture an image of an object on which measurements will be taken, it is important to account for perspective distortion of the image due to the difference in the angle of view of the object with respect to the photographic device. Perspective distortion translates to measurement error when geometrically analyzing a photographic image. Thus, to minimize perspective distortion, the shoe was aligned with a digital camera (Cannon PowerShot A590) in the same plane of reference as follows:

Using one thin sheet of rectangular paper (279.4 mm × 431.8 mm) that was taped to the hard surface floor, the medial axis of the shoe, defined by the anterior and posterior medial protrusions of the outsole, was aligned with the short side of the rectangular paper sheet (Figure 1). The posterior end of the digital camera was centrally aligned on the opposite short side of the rectangular paper sheet approximately 431.8 mm away from the shoe (Figure 1). This alignment places both the shoe and digital camera in the same plane of reference (that is, in the same transverse plane as determined by the rectangular paper sheet and in the same frontal and sagittal planes as determined by the hard surface floor), thus making it possible to accurately measure and from the digital images of the shoe with minimal perspective distortion. Any device, including a smart phone, that has digital photographic capabilities could be substituted for the digital camera, as long as the posterior end of the device can be aligned with the rectangular paper sheet as illustrated in Figure 1.

F1
Figure 1:
The shoe and digital camera were aligned in the same plane of reference to minimize perspective distortion. The shoe rested on the floor, and its medial axis, defined by the posterior and anterior medial protrusions of the outsole, was aligned with the short side of the rectangular paper sheet (279.4 mm × 431.8 mm). The digital camera rested on the thin paper sheet opposite from the shoe, and its posterior end was aligned with the short side of the paper sheet such that the medial axis of the shoe and the posterior end of the camera were parallel to each other. The thin rectangular paper sheet, which is taped to the hard surface floor on each of its long sides, was used to place the shoe and digital camera in the same transverse plane. The fact that the shoe, digital camera, and thin paper sheet all rested on the same hard surface floor placed both the shoe and camera in the same frontal and sagittal planes. The shoe and digital camera were thus in the same plane of reference, which minimized perspective distortion of the image of the shoe captured by the digital camera.

EVALUATING THE ARCH HEIGHT MOBILITY OF THE SHOD FOOT WITH FOOT ORTHOSIS

A 40-year-old woman diagnosed with flexible pes planus and abnormal foot pronation in both feet was recruited as a participant for demonstrating the procedure described in this note. The participant’s shoes (New Balance, 860v7, 10 mm heel-to-toe drop, cushioned stability) and custom functional (semirigid) foot orthoses were fitted according to clinical pedorthic standards (Figure 2). Starting in the sitting position, in which 10% of body weight was assumed by the left shod foot, the upper and lower left leg formed an approximate 90° angle (Figure 3). The shoes were tied and tightened to the feet according to the optimal comfort level of the participant. A digital image of the left shoe, in which the custom functional foot orthosis supported the foot, was then captured (Figure 4). The digital image was imported into the digitization software program, tpsDig,7 for geometric analysis. First, the distance (279.4 mm) between the two corners of the short side of the rectangular paper sheet, with which the medial axis of the shoe was aligned, was used to set the pixel scale of the software program to millimeters (scale factor, 0.232). Then the shoe length was measured horizontally from the posterior tip of the topline to the anterior tip of the toe box, and 50% of that length was calculated and projected perpendicularly from the floor (that is, from the thin paper sheet taped to the floor) to the dorsal surface of the shoe to measure (Figure 5). The posterior tip of the topline and the anterior tip of the toe box are landmarks that are easily identified on any closed shoe and were thus the selected points from which the shoe length was measured.

F2
Figure 2:
The properties of the pair of shoes and foot orthoses worn by the participant are addressed. The pair of shoes was manufactured by New Balance (model: 860v7; cushioned stability running shoe) with a firm heel counter, dual-density medial post, and 10 mm heel-to-toe drop. The pair of functional (semirigid) foot orthoses was custom fabricated to fit the plantar surface of each foot, and designed to treat two pathomechanical disorders with which the participant was diagnosed: 1) flexible pes planus and 2) abnormal foot pronation. The pair of orthoses was engineered with a full forefoot extension, metartarsal pad, deep heel cup, and high medial flange. The orthosis top cover was made of leather over 0.0625 of an inch of Poron and Spenco soft foams.
F3
Figure 3:
The left shod foot supported 10% body weight achieved in the sitting position. The upper and lower left leg was positioned in an approximate 90° angle with the left shoe tied and tightened to the foot according to the optimal level of comfort of the individual.
F4
Figure 4:
The image of the left shoe, in which the foot orthosis supported the foot, was captured by the digital camera as the left shod foot supported 10% of body weight achieved in the sitting position as illustrated in Figure 3.
F5
Figure 5:
The digital image in Figure 4 was imported into the digitization software program, tpsDig,7 for geometric analysis. The distance () between the two corners of the short side of the rectangular paper sheet is 279.4 mm. The short side of the paper sheet was aligned with the medial axis of the shoe, and its distance (279.4 mm) was used to set the pixel scale of the program to millimeters (scale factor, 0.232). The shoe length () is the horizontal distance measured from the posterior tip of the topline to the anterior tip of the toe box. A point on the floor (that is, on the thin paper sheet taped to the floor) was digitized at half of the shoe length. A line () was then drawn perpendicularly to the floor from that point to another point that was digitized at the dorsal surface of the shoe. The line, which is described by the two digitized points, represents (88.16 mm).

The above procedure was repeated, but in the standing position in which 50% of body weight was assumed by the left shod foot (see Figure 6), to measure (Figures 7 and 8). Substituting the measured values of (88.16 mm) and (85.14 mm) into Equation 2, δ* equals 3.02 mm. Cross-referencing this value (δ* = 3.02 mm) from Table 5 in McPoil et al.2 (left foot, diff dorsal arch hgt = 11.9 mm ± 2.5 mm), the sagittal plane mobility of the medial arch of the participant’s left shod foot with left foot orthosis can be assessed as being very low because it is less than three standard deviations from the mean. Equation 2 should also be evaluated for the right shod foot with right foot orthosis following the same procedure as described previously.

F6
Figure 6:
The left shod foot supported 50% of body weight achieved in the standing position. The left shod foot and digital camera remained in the same position on the floor from sitting to standing during the photographing of the shoe.
F7
Figure 7:
The image of the left shoe, in which the foot orthosis supported the foot, was captured by the digital camera as the left shod foot supported 50% of body weight achieved in the standing position as illustrated in Figure 6.
F8
Figure 8:
The digital image of Figure 7 was imported into the digitization software program, tpsDig,7 for geometric analysis. The same scale, as determined in Figure 5, was used to set pixels to millimeters (scale factor, 0.232). The shoe length was then measured exactly in the same way as performed in Figure 5, and a point on the floor was digitized at half that length. A line was then drawn perpendicularly to the floor from that point to another point that was digitized at the dorsal surface of the shoe. The line, which is described by the two digitized points, represents (85.14 mm).

Note that because Equation 2 is a measure of displacement, it can also be used to evaluate the sagittal plane flexibility (C*) of the medial arch of the shod foot with a foot orthosis (see arch height flexibility in Zifchock et al.8):

where 0.4W is the applied force as a function of the difference in the percentage of body weight (W) between sitting and standing.8

DISCUSSION

Equation 1 has been used in numerous studies to evaluate the sagittal plane mobility of the medial arch of the foot.1,2 It has also been used to evaluate the sagittal plane mobility of the medial arch of the unshod foot with a foot orthosis to improve the orthotic treatment of foot pathomechanical disorders.3 For the shod foot, however, Equation 1 has not been evaluated, and this technical note describes a method using digital photography (see Pohl and Farr6) to evaluate Equation 2. This evaluation has a practical advantage in that all foot orthoses are in-shoe devices. More importantly, shoes can affect medial arch function4 and foot deformation at the orthosis-foot interface.5 Equation 2 should thus be evaluated for various types of footwear to optimize in-shoe orthotic effectiveness. One particular characteristic of shoes that may influence both medial arch mobility and orthosis deformation is a nonzero heel-to-toe drop, which is an inherent internal shoe heel lift. An excellent, albeit statistically limited, article by Shimizu and Andrew9 shows that increasing the heel height (or the heel-to-toe drop) increases the medial arch height via the activation of the windlass mechanism. They further discuss that activating the windlass mechanism, as the heel lifts above the toes during the gait cycle, inhibits the degree to which the medial arch deforms in response to applied loads.9 In fact, wearing a 10-mm heel-to-toe drop shoe stiffens the medial arch—a result purported in Kelly et al.,4 although the authors4 do not attribute the result to this particular shoe characteristic. With this in mind, it would not be surprising to see the value of δ* change with respect to different shoe heel-to-toe drops: δ* is expected to decrease as the shoe heel-to-toe drop increases. In theory, therefore, clinicians can select shoes with a specific heel-to-toe drop that would normalize δ*, thereby optimizing in-shoe orthotic effectiveness. As a case in point, the sagittal plane mobility of the medial arch of the participant’s left shod foot with left foot orthosis is very low (δ* = 3.02 mm). The reference range for the sagittal plane mobility of the medial arch of a female’s left foot is 11.9 mm ± 2.5 mm (see Table 5, left foot, diff dorsal arch hgt in McPoil et al.2). Thus, a lower heel-to-toe drop shoe (one that is less than 10 mm) selected for the participant should, in theory, increase δ* (3.02 mm) to within 11.9 mm ± 2.5 mm, thereby normalizing. Of course, Equation 2 is not limited to orthotic intervention. In fact, in the absence of foot orthoses, running shoes can be selected with a specific heel-to-toe drop that would normalize δ*, thereby optimizing in-shoe midfoot mobility.

LIMITATION

The weight-bearing percentages, 10% of body weight achieved by sitting and 50% of body weight achieved by standing, are assumed in this note but never actually measured with a weighing scale. Although these percentages are reasonable assumptions achieved by the two weight-bearing states,8 a thin weighing scale, on which the entire rectangular paper sheet would rest, would be ideal during the evaluation of Equation 2.

CONCLUSIONS

This technical note provides clinicians and researchers an accessible method to evaluate the sagittal plane mobility of the medial arch of the shod foot with a foot orthosis. For clinical applications, in which foot orthoses are prescribed for treating foot pathomechanical disorders, Equation 2 should be evaluated for optimizing in-shoe orthotic effectiveness. For nonclinical cases, in which foot orthoses are not prescribed, evaluating Equation 2 is useful for optimizing in-shoe midfoot mobility.

ACKNOWLEDGMENT

The author thanks the participant for taking part in the demonstration of the procedure described in this note. The author also thanks two reviewers for providing comments that significantly improved the manuscript.

REFERENCES

1. Williams DS, McClay IS. Measurements used to characterize the foot and the medial longitudinal arch: reliability and validity. Phys Ther 2000;80:864–871.
2. McPoil TG, Vicenzino B, Cornwall MW, et al. Reliability and normative values for the foot mobility magnitude: a composite measure of vertical and medial-lateral mobility of the midfoot. J Foot Ankle Res 2009;2:6.
3. Sheykhi-Dolagh R, Saeedi H, Farahmand B, et al. The influence of foot orthoses on foot mobility magnitude and arch height index in adults with flexible flat feet. Prosthet Orthot Int 2015;39:190–196.
4. Kelly LA, Lichtwark GA, Farris DJ, Cresswell A. Shoes alter the spring-like function of the human foot during running. J R Soc Interface 2016;13.
5. Papadopoulos A. Method to screen for abnormal deformation at the interface between foot and functional foot orthosis. J Prosthet Orthot 2016;28:83–87.
6. Pohl MB, Farr L. A comparison of foot arch measurement reliability using both digital photography and calliper methods. J Foot Ankle Res 2010;3:14.
7. Rohlf FJ. TpsDig [Digitization Software Program]. In: Version 1.40. State University of New York at Stony Brook: Department of Ecology and Evolution; 2004: Available at: http://life.bio.sunysb.edu/morph/.
8. Zifchock RA, Theriot C, Hillstrom HJ, et al. The relationship between arch height and arch flexibility. J Am Podiatr Med Assoc 2017;107:119–123.
9. Shimizu M, Andrew PD. Effect of heel height on the foot in unilateral standing. J Phys Ther Sci 1999;11:95–100.
Keywords:

arch; foot; mobility; orthosis; shoe

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