Subtalar Joint Offset: A 3D WBCT Measure to Distinguish Global Hindfoot Malalignment From Isolated Subtalar Alignment

Area of Science:

• Orthopedics
• Radiology
• Biomechanics

Background:

• Foot and Ankle Offset (FAO) is a 3D WBCT measurement for hindfoot alignment (HA).
• FAO provides a global view but does not isolate subtalar joint (SJ) alignment.
• Subtalar Joint Offset (SJO) is introduced as a novel 3D WBCT metric for SJ alignment.

Purpose of the Study:

• To introduce and assess the reliability of Subtalar Joint Offset (SJO) as a 3D WBCT measurement.
• To evaluate SJO's ability to distinguish subtalar alignment in neutral, valgus, and varus hindfoot alignment (HA).
• To compare SJO with Foot and Ankle Offset (FAO) measurements.

Main Methods:

• Retrospective study of 204 feet categorized by FAO into neutral, valgus, or varus.
• SJO calculated using WBCT software by identifying the SJ calcaneal middle facet position relative to the foot tripod.
• Intra- and interobserver reliability assessed using intraclass correlation coefficients (ICCs).

Main Results:

• Excellent intra- and interobserver ICCs for SJO (0.98 and 0.92, respectively).
• Significant differences in both FAO and SJO across neutral, valgus, and varus groups (P < .001).
• Moderate correlation between FAO and SJO in varus and valgus groups, low in the neutral group.

Conclusions:

• SJO is a reliable WBCT metric for distinguishing subtalar alignment in various hindfoot alignments (HA).
• The SJO-FAO difference may quantify subtalar contribution to overall HA.
• Further prospective, multicenter studies are warranted to validate SJO's clinical utility in surgical planning.