Reliability and correlation of weight-bearing cone beam CT and Foot Posture Index (FPI) for measurements of foot posture: a test-retest study

OBJECTIVE: To assess test-retest reliability and correlation of weight-bearing (WB) and non-weight-bearing (NWB) cone beam CT (CBCT) foot measurements and Foot Posture Index (FPI) MATERIALS AND METHODS: Twenty healthy participants (age 43.11±11.36, 15 males, 5 females) were CBCT-scanned in February 2019 on two separate days on one foot in both WB and NWB positions. Three radiology observers measured the navicular bone position. Plantar (ΔNAV(plantar)) and medial navicular displacements (ΔNAV(medial)) were calculated as a measure of foot posture changes under loading. FPI was assessed by two rheumatologists on the same two days. FPI is a clinical measurement of foot posture with 3 rearfoot and 3 midfoot/forefoot scores. Test-retest reproducibility was determined for all measurements. CBCT was correlated to FPI total and subscores. RESULTS: Intra- and interobserver reliabilities for navicular position and FPI were excellent (intraclass correlation coefficient (ICC) .875–.997). In particular, intraobserver (ICC .0.967–1.000) and interobserver reliabilities (ICC .946–.997) were found for CBCT navicular height and medial position. Interobserver reliability of ΔNAV(plantar) was excellent (ICC .926 (.812; .971); MDC 2.22), whereas the ΔNAV(medial) was fair-good (ICC .452 (.385; .783); MDC 2.42 mm). Using all observers’ measurements, we could calculate mean ΔNAV(plantar) (4.25±2.08 mm) and ΔNAV(medial) (1.55±0.83 mm). We demonstrated a small day-day difference in ΔNAV(plantar) (0.64 ±1.13mm; p<.05), but not for ΔNAV(medial) (0.04 ±1.13mm; p=n.s.). Correlation of WBCT (WB navicular height - ΔNAV(medial)) with total clinical FPI scores and FPI subscores, respectively, showed high correlation (ρ: −.706; ρ: −.721). CONCLUSION: CBCT and FPI are reliable measurements of foot posture, with a high correlation between the two measurements. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s00256-023-04352-1.