Hello, today I am going to present our work on the spatial orientation of the foot, a critical step in hindfoot alignment evaluation on weight-bearing CT scan. I am Shuyuan Li, and this collaborative work was done by researchers from University of Colorado, Steps to Walk, and Beijing Turing Hospital in China. The authors have nothing to disclose. As we all know, weight-bearing CT has been proved effective in evaluating the foot and ankle alignments and guiding clinical procedures. However, until now, all measurements used in analyzing weight-bearing CT scans are all based on two-dimensional concepts. Therefore, when using weight-bearing CT scans for alignment evaluations, obtaining specific perspectives, or we can call it views, is very critical. It is not different from X-ray analysis. So you can see on the left side, when we take a hindfoot alignment view X-ray, we need to position the X-ray and the foot in a specific angle. And on the right hand, in weight-bearing CT scan examination, although the scanner is taking 3D data from the foot and ankle, but at a later stage, we still need to position the foot to get a specific angle for analysis. And we see here all the parameters we are currently using for weight-bearing CT scan analysis are from the two-dimensional measurements world, and we think that they do not resist rotation. Several studies have pointed out that foot positioning is very important on hindfoot alignment evaluation when using X-rays, as it has been proven in this paper that with rotating the foot from externally 10 degrees gradually into neutral position and into internal rotation 10 degrees or 15 degrees, and the hindfoot alignment changes from a varus hindfoot into neutral and then into a belgus hindfoot. It has been noticed that in weight-bearing CT evaluation, positioning of the foot has not been sufficiently prioritized. Therefore, false data could easily be generated and further affect diagnosis. Here are some images illustrating that by positioning the foot differently, one can manipulate a neutral hindfoot into a belgus or varus hindfoot. You can see on the lateral side is a neutral hindfoot, and with internal rotation, the hindfoot turns into belgus, and with external rotation, the hindfoot turns into varus. So the purpose of this study was to demonstrate the importance of positioning of the foot in weight-bearing CT scans, and our hypothesis was that changing the foot orientation will significantly affect the alignment of the hindfoot, which will in turn affect further clinical decision-making. It was a retrospective study with IRB approval. We scanned 10 feet with no foot and ankle deformities. We positioned the foot in a neutral position by aligning the second metatarsal head with the posterior base of the heel. That is exactly how the foot is positioned for taking X-rays. And the weight-bearing CT scan images were then rotated on the transverse plane for different degrees, both externally and internally, to simulate malpositioning. At each position, the semi-three-dimensional hindfoot moment arm, which was reintroduced to the weight-bearing CT field by a group in 2021 as we show here in reference 1, but modified, re-modified by us, later was used. For the hindfoot moment arm value, a varus hindfoot was recorded by a negative value and a valgus hindfoot was recorded by a positive value. Here are our results, and you can see there was a linear relationship between the HMA values and the rotation angles. We can see that when rotating the foot from externally 20 degrees to internally 20 degrees, the HMA demonstrated a linear change from a varus alignment into a valgus hindfoot. And also, from external rotation to internal rotation, every 1 degree increase in the rotation angle has caused a 0.552 mm increase in the HMA value and with statistical significance. Our results also showed that these HMA measurements had excellent intra-observer and inter-observer reliability. And we can see from both weight-bearing CT images and the measured values, they all confirmed that external rotation has changed a neutral hindfoot into varus and internal rotation could change a neutral hindfoot into valgus. Conclusion, when using current two or semi-three-dimensional methods to evaluate hindfoot alignment on weight-bearing CT scans, positioning the foot in the correct orientation is critical. Mal-rotation and mal-position of the foot will cause significant impact on measurement and generate false data as well as misdiagnosis. This might also apply to many other two-dimensional measurement parameters in addition to the hindfoot alignment. Although this study was performed on normal feet, but it is clinically relevant when applied to deformities, in particular those associated with abduction or adduction of the midfoot and forefoot. Since current knowledge about deformed foot positioning is limited, this is an important topic for future investigation. And also this study indirectly demonstrated that developing advanced three-dimensional measurements with more rotation stability for using weight-bearing CT is necessary. Here are our references. Thank you.

Shuyuan Li

spatial orientation

foot

hindfoot alignment evaluation

weight-bearing CT scans