P16 The quarter-ellipsoid foot: a clinically accessible 3-dimensional composite measure of foot deformation

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Study DesignDescriptive Laboratory.ObjectivesTo assess change in foot morphology across loading using innovative, clinically accessible 3-dimesional composite measurements of surface area (SA), volume (V), and SA/V ratio in recreationally-active young adults.BackgroundThe foot is quarter-ellipsoid shaped and normally deforms during loading. Foot pliability varies by morphotype and is clinically important when managing foot and ankle pathology. Current clinical composite measures of foot deformation are limited to 2-dimensions.Methods and MeasuresThe feet of 106 recreationally-active young adults (54 right, 52 left, 46 males, 60 females, mean age:21.1±4.2, BMI:24.2±5.0, FPI:4.0±4.0) were evaluated. Foot posture index (FPI) was assessed and categorised as <-1=supinated, 0–5=normal, 6–9=pronated, and ≥10 = highly-pronated. Foot length, width, and height were measured in sitting and bipedal standing and used to calculate the SA and V of a quarter-ellipsoid. SA and V changes characterise foot widening, lengthening, and arch deformation during loading. The SA/V (%) represents changes in both foot size and shape. ANOVAs were performed to compare the effects of foot morphotype and loading on the composite measures. Post-hoc Fisher’s least significant differences (LSD) were calculated for significant interactions.ResultsLoading significantly decreased foot volume across all foot types (mean ±95% CI: supinated:624.4±61.1 cm3 to 615.7±58.2 cm3, normal:564.1±29.8 cm3 to 556.7±31.3 cm3, pronated:565.7±57.7 cm3 to 541.6±57.3 cm3, highly pronated:554.8±76.9 cm3 to 539.8±76.5 cm3; p<0.001, η²=0.16). There was a significant loading by morphotype interaction for SA/V measures (mean ±95% CI: supinated:36.6±1.2% to 37.0±1.2%, normal:38.0±0.7% to 38.6±0.7%, pronated: 38.2±1.3% to 39.4±1.4%, highly-pronated: 38.9±2.1% to 39.9±2.4%; p=0.005, η²=0.12, LSD=0.2%). Significant progressive increases in SA/V were observed the more pronated a foot was. No other significant differences were observed for the three composite measures.ConclusionsQuarter-ellipsoid measures calculated from morphologic measurements may have clinical utility in quantifying 3-dimensional foot changes during loading and likely advantageous over 2-dimensional measures when assessing need and response to intervention.

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