Relationship between foot posture index and weight bearing computed tomography 3D biometrics to define foot alignment

Background: The Foot Posture Index (FPI) was originally validated only against two dimensional radiographic imaging at the time of its inception since weight bearing three dimensional (3D) imaging did not exist. This technology is now widely available but it is not known if the FPI will continue to correlate well against it. Research question: How does the clinical assessment tool of FPI correlate against 3D biometrics of foot alignment assessed on weight bearing computerised tomography (WBCT)? Methods: The FPI and 3D biometrics of 66 feet in 33 patients presenting to a single foot and ankle unit were assessed by two observers independently. All measurements were assessed for intra-rater and inter-rater reliability, and the association between the clinically assessed FPI and radiologically assessed 3D biometrics was identified. Results: The FPI demonstrated excellent intra-rater and good inter-rater reliability. All 3D biometric measures demonstrated excellent intra-rater and inter-rater reliability. A strong and significant correlation was identified between the FPI and 3D biometrics overall although subgroup analysis revealed the strength of association dropped when the hindfoot had a valgus alignment. Significance: This study confirmed that the FPI is a valid tool for assessing foot position. However, correlation between the FPI and 3D biometrics fell in the valgus aligned hindfoot so caution must be used when the FPI is used for assessment in this population.