Three-dimensional hindfoot alignment measurements based on biplanar radiographs: comparison with standard radiographic measurements.

OBJECTIVE: To establish a hindfoot alignment measurement technique based on low-dose biplanar radiographs and compare with hindfoot alignment measurements on long axial view radiographs, which is the current reference standard.
MATERIALS AND METHODS: Long axial view radiographs and low-dose biplanar radiographs of a phantom consisting of a human foot skeleton embedded in acrylic glass (phantom A) and a plastic model of a human foot in three different hindfoot positions (phantoms B1-B3) were imaged in different foot positions (20° internal to 20° external rotation). Two independent readers measured hindfoot alignment on long axial view radiographs and performed 3D hindfoot alignment measurements based on biplanar radiographs on two different occasions. Time for three-dimensional (3D) measurements was determined. Intraclass correlation coefficients (ICC) were calculated.
RESULTS: Hindfoot alignment measurements on long axial view radiographs were characterized by a large positional variation, with a range of 14°/13° valgus to 22°/27° varus (reader 1/2 for phantom A), whereas the range of 3D hindfoot alignment measurements was 7.3°/6.0° to 9.0°/10.5° varus (reader 1/2 for phantom A), with a mean and standard deviation of 8.1° ± 0.6/8.7° ± 1.4 respectively. Interobserver agreement was high (ICC = 0.926 for phantom A, and ICC = 0.886 for phantoms B1-B3), and agreement between different readouts was high (ICC = 0.895-0.995 for reader 1, and ICC = 0.987-0.994 for reader 2) for 3D measurements. Mean duration of 3D measurements was 84 ± 15/113 ± 15 s for reader 1/2.
CONCLUSION: Three-dimensional hindfoot alignment measurements based on biplanar radiographs were independent of foot positioning during image acquisition and reader independent. In this phantom study, the 3D measurements were substantially more precise than the standard radiographic measurements.