Abstract
Research Type:
Level 3 - Retrospective cohort study, Case-control study, Meta-analysis of Level 3 studies
Introduction/Purpose:
Foot and Ankle Offset (FAO) is a semi-automatic 3D biometric tool designed to assess Hindfoot Alignment (HA) using weight-bearing CT (WBCT). FAO is calculated by identifying a reference point on the talus and determining its relationship with the foot tripod. In the original description, the most proximal, central aspect of the talar dome is used, as it represents the ankle joint center. However, in cases of severe deformity, instability, talar tilt, or joint incongruency, this reference point may become off-loaded, making it less reliable. This study aimed to assess the impact of alternative ankle reference points in FAO calculations when tibio-talar incongruency is present. We hypothesized that FAO would overestimate HA in the presence of joint misalignment.
Methods:
This retrospective comparative study included 39 patients (39 feet; 22 males, 17 females; age 64.9 years, BMI 33.37 ± 6.6 kg/m²) who underwent weightbearing CT (WBCT) for foot and ankle deformities. Overall, 25 feet were varus (FAO < 0.6%) and 14 were valgus (FAO >5.2%). Three FAO measurement methods were evaluated: Contact Point FAO (CP-FAO), using the tibio-talar contact midpoint in sagittal and coronal planes; Tibial Plafond Center FAO (TM-FAO) using the midpoint of the tibial plafond as it likely represents the closest approximation to the normal ankle center; and the classic FAO (TD-FAO), derived from the talar dome center.Traditional radiographic parameters (medial distal tibial angle, tibiotalar angle, talar tilt angle, tibiocalcaneal angle, calcaneal moment arm) were measured manually and assessed for comparison. Student t-test and Wilcoxon Rank-Sum test were used for two-way analysis, and Friedman’s test was used for multi-group comparison. Statistical significance was set at p< 0.05.
Results:
FAO values varied significantly across reference points and alignment type (p < 0.001). Intra- and interobserver reliability for CP-FAO and TM-FAO was excellent (CP-FAO ICC: 0.98/0.97; TM-FAO ICC: 1.0/0.94). In valgus alignment, mean CP-FAO (10.86 ± 7.97) was significantly lower than TD-FAO (14.78 ± 5.33) (p = 0.009). Both CP-FAO (p = 0.001) and TD-FAO (p = 0.006) differed significantly from TM-FAO (15.83 ± 4.77), with CP-FAO demonstrating the greatest difference. In varus alignment, CP-FAO (–9.77 ± 6.04) deviated more from TM-FAO (–16.69 ± 4.17, p< 0.001), while TD-FAO remained comparable (-17.1 ± 3.97 p=0.23). On three-way comparison, there were significant differences between CP-, TD-, and TM-FAO (p < 0.001).
Conclusion:
The absolute value of CP-FAO was consistently lower than TD-FAO and TM-FAO for both varus and valgus deformity. Our findings suggest that FAO measurements using the traditional talar dome center (TD-FAO) may overestimate hindfoot alignment deformities in cases of joint incongruency. The reference talar dome point for the traditional FAO measurement increases the offset from the foot tripod, which can inflate FAO values. In contrast, CP-FAO accounts for deformity compensation, which may be more biomechanically representative in cases of joint incongruency. Future studies should correlate CP-based FAO to surgical planning and outcomes to clarify its role versus conventional methods.
Figure 1 (A–E) Landmarks for FAO calculations, illustrating key reference points used in measurement. (F) Comparative representation of FAO calculated using the talar dome (TD-FAO) and the contact point (CP-FAO), demonstrating differences in alignment assessment. (G) Three-way comparison test between CP-FAO, TD-FAO, and tibial plafond midpoint FAO (TM-FAO) in both varus and valgus alignments.