The Effect of Arch Height and Body Mass on Plantar Pressure

Carine HM van Schie, MSc, PhD; , Andrew JM Boulton, MD, FRCP


Wounds. 2000;12(4) 

In This Article

Arch Index

The arch index was measured using the method as described by Cavanagh and Rogers,[7] as this has been reported to be an easy and reliable method to quantify the arch height. In addition, the arch index has also been shown to be a valid predictor of arch height by McCrory, et al.[8] She reported that the arch index was strongly associated with navicular height as measured by radiography (r = 0.67).

To determine the arch index, a dynamic footprint was first obtained during walking, using a Podotrack footprint mat.[29] The footprint was made during walking as measures of foot-type should be evaluated in the dynamic rather than the static situation,[30] as it is dynamic foot function that is believed to be related to pathology. In addition, it has been demonstrated that static measures are not good predictors of dynamic foot function,[31,32] and dynamic footprints have shown greater reproducibility than static footprints,[33] regardless the inherent variability of the dynamic situation. The footprint mat was placed directly upon the pedobarograph pressure plate, and the same starting position was used for the foot pressure measurements. Thus, all footprints and pressure measurements were made of the same footstep after initiation of walking (second or third step). Any trials during which the subject aimed for the mat were not used for analysis.

In the footprint, a line defined as 'foot axis' was drawn from the tip of the second toe to the centre of the heel (Figure 1a). Two lines perpendicular to the foot axis were drawn tangential to the foot, one on the most posterior aspect of the heel and one on the most anterior aspect of the footprint excluding the toes, in front of the metatarsal heads. The part of the foot axis between these two lines was trisected into equal parts, dividing the footprint into forefoot, midfoot, and rearfoot regions. The arch index was defined as the area of the midfoot divided by the total area of the footprint, ignoring the toes (Figure 1b). All footprint outlines were digitized using Vids Image Analysis software (Synoptics, Cambridge, UK) and arch index was calculated accordingly. All obtained arch indexes were categorized into three different foot types using a classification system as suggested by Cavanagh and Rogers.7 An arch index =< 0.21 was indicative of a high arch, an arch index between 0.21 and 0.26 indicated a normal arch, and an arch index >= 0.26 indicated a flat or low arch.

Measurement of the arch index. The foot axis line from the 2nd toe to the middle of the heel was drawn in the footprint (Figure 1a). Two lines, one at the most posterior end of the heel and one at the most anterior aspect of the forefoot (excluding the toes) were drawn perpendicular to the foot axis. The foot axis was then divided into three equal lengths to divide the footprint into a heel, midfoot and forefoot area (Figure 1b). The arch index was determined as the ratio midfoot area divided by the whole footprint area (excluding toes).
Figure adapted from: McCrory et al. (1997)


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