Introduction: Flatfoot, a well-known foot deformity, is reportedly a risk factor for some musculoskeletal disorders. While conventionally evaluated by clinical assessments such as radiographic measurements and the Foot Posture Index (FPI-6), there are limitations that require clinical specialists and depend on the examiner's skill. Such assessments thus lack reliability and versatility. In contrast, assessments using the foot shape index (FSI) with FPI-6 as the norm are practical without requiring the examiner's expertise. Therefore, we investigated whether or not the FSI is more useful than clinical assessments for assessing flatfoot.
Methods:
Group 1 consisted of 208 feet from 104 adult women with foot diseases (58.1±11.8 years old), while Group 2 consisted of 44 feet from 22 healthy adults (39.3±10.6 years old). In both groups, eight FSIs were extracted from a three-dimensional foot scanner, including the following in different planes: Frontal, heel angle and leg heel angle; Sagittal, arch height ratio, medial arch angle, and instep angle; and Horizontal, medial protrusion area, heel center angle, and curve angle. In Group 1, a shoe fitter evaluated seven radiographic indexes in different planes under the full guidance of a podiatrist. In Group 2, the FPI-6 was also evaluated by two shoe fitters.
Results:
Group 1 showed strong correlations (r = 0.56-0.81) between FSIs and radiographic indexes in the sagittal plane and weak-to-moderate correlations in the frontal and horizontal planes (r = 0.22-0.63). For Group 2, FSIs in the sagittal plane showed strong correlations (r = 0.73-0.80) with the FPI-6 (congruence of the medial longitudinal arch [CMLA]). In addition, significant weak-to-moderate correlations were found in the frontal and horizontal planes (r = 0.31-0.51), except for the curve angle. The inter-rater reliability of the CMLA was found to be moderate (intraclass correlation coefficient [ICC](2,1) = 0.56), whereas the ICCs of the other FPI-6 values were below 0.5.
Discussion:
These findings suggest that these eight FSIs are useful for assessing flatfoot, particularly in the sagittal plane, where three indexes were shown to be effective. In addition, the instep angle is particularly notable and highly practical, as it can be easily obtained using a lateral camera shot without a marker attachment. However, the other indexes in the frontal and horizontal planes showed only weak-to-moderate correlations, possibly due to the required examiner's skill and the lack of reliability in both the radiographic measurement and FPI-6. In contrast, the reliability of the FSI depends on the measurement accuracy of the scanner used to automatically calculate them from the three-dimensional foot shape, without requiring the examiner's expertise. This approach may minimize human errors in the measurement process.
Impact/Application to the field:
These eight FSIs may be valid and practical for assessing flatfoot. FSIs are expected to have a wide range of applications, including health promotion, improvement of athletic performance, and prevention of foot disorders.
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