Computed methods to convert conventional 2D radiological hindfoot measurements to a 3D setting using weightbearing CT

Reliability and correlation analysis of computed methods to convert conventional 2D radiological hindfoot measurements to a 3D setting using weightbearing CT, by Burssens et al. Int J CARS (2018).

Abstract:

Purpose
The exact radiographic assessment of the hindfoot alignment remains challenging. This is reflected in the different measurement methods available. Weightbearing CT (WBCT) has been demonstrated to be more accurate in hindfoot measurements. However, current measurements are still performed in 2D. This study wants to assess the use of computed methods to convert the former uniplanar hindfoot measurements obtained after WBCT towards a 3D setting.

Methods
Forty-eight patients, mean age of 39.6 ± 13.2 years, with absence of hindfoot pathology were included. A WBCT was obtained, and images were subsequently segmented and analyzed using computer-aided design operations. In addition to the hindfoot angle (HA), other ankle and hindfoot parameters such as the anatomical tibia axis, talocalcaneal axis (TCA), talocrural angle, tibial inclination (TI), talar tilt, and subtalar vertical angle were determined in 2D and 3D.

Results
The mean HA2D was 0.79∘ of valgus ± 3.2 and the HA3D was 8.08∘ of valgus ± 6.5. These angles differed significantly from each other with a P<0.001. The correlation between both showed to be good by a Pearson correlation coefficient (r) of 0.72 ( P<0.001). The ICC3D showed to be excellent when compared to the ICC2D, which was good. Similar findings were obtained in other angles. The highest correlation was seen between the TI2D and TI3D (r = 0.83, P<0.001) and an almost perfect agreement in the TCA3D ( ICC3D=0.99). Conclusion This study shows a good and reliable correlation between the HA2D and HA3D. However, the HA3D overcomes the shortcomings of inaccuracy and provides valuable spatial data that could be incorporated during computer-assisted surgery to assess the multiplanar correction of a hindfoot deformity.

subtalar-vertical-angle
Measurement of the subtalar vertical angle in 3D (SVA3D). a The surface of the posterior facet of the subtalar joint was marked (red contour). The most posterior and anterior point of the marked surface was calculated in the direction of the AP (x-) axis (blue dots). This allowed to determine the length of the posterior facet by a software operated connection of both points. The mid-point of this distance was calculated and used as an origin to fit a plane parallel to the coronal plane at a distance of -5mm, 0mm, and +5mm to mimic, respectively, the posterior, middle, and anterior SVA as described by Colin et al. [9].b The contour of the posterior facet running in themiddle subtalar plane was used to determine the inclination (dashed line) by connecting the calculated most medial with the most lateral point. c The intersection of this subtalar axis with the vertical (z-) axis became the middle SVA. d Depiction of the middle SVA in a 3D hindfoot configuration

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