Hello, I'm going to present our work on why is foot and ankle offset inadequate in assessing three-dimensional height-foot alignment on weight-balance CT scans with facts and mathematical evidence. I'm Shuyang Li, the corresponding author of this study, and we have nothing to disclose. The foot and ankle offset, abbreviated as FAO, was developed in 2017 as a 3D biometric tool to measure height-foot alignment on weight-balance CT scans. It corresponds to the offset between the height-foot to forefoot midline and the tailors and is reported as a percentage of the foot length. Recently, with increased availability of weight-balance CT in both research and clinical practice, the FAO has been widely used for height-foot alignment assessment. Here you can see I did a quick research in PubMed for foot and ankle offset, and you can see there are 61 publications with the majority of them came out in recent years, particularly in 2022. The FAO is based on the tripod concept with the following equation. You can see on the left side, that is the tripod concept, and four points were picked to represent the height-foot, the foot, and the ankle. The first one is the weight-bearing point of the first metatarsal, and then weight-bearing point of the fifth metatarsal, weight-bearing point of the calcaneus, and the center of the superior surface of the tailors. So then on the right side, the image illustrates how FAO is calculated. So the FAO is measured using the TALAS program in QPU. It's a commercialized program. However, after extensive review of weight-bearing CT scans, the authors of this study have noticed that FAO has a high false negative rate since it does not reflect deformities including the ankle, it does not include any information about the tibia, and nor does it reflect the central plane deformities of the foot and ankle. So the purpose of this study was to demonstrate why FAO is inadequate in assessing three-dimensional height-foot alignment using both case examples and mathematics algorithms. So this study has two parts. In part one, we are going to provide facts to show how FAO is not accurate. So it was a retrospective study with IRB approval, and among 260 patients, we found that 30 with weight-bearing CT scans, we found that 31 cases with various clinical diagnoses but with inaccurate FAO readings that did not correspond to their clinical deformities. So the deformities include velcros of various ankle deformities, height-foot deformities combined with sagittal plane deformities of the foot and ankle, rocker-bottom foot deformity, ankle equinus, metatarsus, perineus, elevators, and the first metatarsoplantar flexion. All of the latter deformities caused one or more parts of the triangular base for foot and ankle for FAO evaluation to remain off the floor. So that is either the first metatarsal or the fifth metatarsal, all the calcaneus was lifted above the floor. And the FAO was measured by two fellowship trained foot and ankle surgeons separately, and the clinical and radiographic diagnoses were compared with the FAO values to determine the accuracy and utility of the latter measurement. And you can see for each of these three, 31 cases examined the height-foot alignment information reflected by the FAO value was vastly different from the clinical and imaging studies. Using the reference value introduced for FAO, that is the normal range, the varus range, and the valve range, and we have found that 24 out of the 31 cases in the cohort had a normal FAO value. However, among them, there were seven with a valgus ankle or distal tibia plus a normal or cavernous height-foot. There were two cases with various ankles but a normal height-foot. There were one case with various ankle and a valgus height-foot, three cavernous feet, one clubfoot, one equinox ankle with cavernous foot, two equinox ankle with severe valgus height-foot, four severe valgus height-foot, and one varus height-foot with central plane deformities and two muller-wise disease patients. And five out of those 31 cases had a varus FAO value, but they all had a valgus ankle with either a varus height-foot or central plane deformities. And two out of the 31 cases had a valgus FAO value. However, both cases had a valgus ankle with a varus height-foot. So it was very interesting, and we can see that the FAO values didn't correspond to the clinical and the radiographic findings. So here's a table showing of all 31 cases. In the left column is a FAO value reading, and then in the FAO report, is it a normal height-foot, is it a valgus height-foot, or is it a varus height-foot? And in the very right column, there is a clinical and radiographic diagnosis with all sorts of complicated deformities that were not able to be reflected by the FAO value. So here's one case. It's a valgus ankle caused by deltoid rupture. And note the opening on the middle side of the ankle with anterior subluxation in image B. And although there is some compensation varus in the subtler joint, we can see it in image C, but the height-foot is still like falls on the lateral side of the ankle. So it is a valgus ankle. But the FAO measurement for this foot is within normal range. And if we only look at the FAO report, and one will never know actually the real alignment of the foot and ankle for this case. Here's another case with a rocker bottom deformity. And note the flat calcaneus pitch angle and the elevated forefoot in the lateral view and the varus height-foot alignment in both lateral, and we can see the varus subtler joint and the height-foot view. But the FAO measurement for this foot is within normal as well. So as we just said, if we only looked at the FAO report, but it didn't look at the X-ray or vertebrate CT images, one will never know that the real alignment of the foot and ankle for this patient. And here's another patient. It is a varus ankle with a valgus subtler joint and a very flat arch. And note the opening on the lateral side of the ankle with the erosion and anterior shift of the lateral process of the talus and with the closing of the subtler joint and the flat arch. And the heel falling on the middle side of the tibia. So it is a slightly varus height-foot. But the FAO measurement for this foot is within the normal range. So also, like the FAO value didn't or doesn't reflect the real situation of this complicated foot and ankle deformity. So in part two of our study, we used math to show why FAO is not accurate. Because just now, we saw so many cases with completely complicated foot and ankle deformities, but on normal or inaccurate FAO value. Now we want to prove why it is not accurate. So we used a weight-variance CT scan of a normal foot. And we manipulated the foot by changing the position of the first metatarsal and fifth metatarsal calcaneus. And we changed the weight-variance point on either both underneath the ground and also elevated it above the ground level. And so we calculated the FAO value in each individual situation. And we want to provide a thorough algorithm and geometric evidence. We used both the TALAS program in PUPVU to calculate the FAO. And we also used the R program to replicate and to calculate what has really happened. So here you can see this is the normal foot. And the second one is with the first metatarsal elevated. And then with the first metatarsal lowered underneath the ground. Same thing with the first and fifth metatarsals elevated. And by doing math, we were able to crack open the black box of the TALAS program. So what we have found is in the TALAS program, how x, y, and z-axis was set up in the TALAS program. And we found that the zero point is on the proximal right limb side, on the lateral side, and it is on the anterior side. And we also found how the tripod system was recorded Actually, the triangle of the foot that is composed by the first metatarsal head, the fifth metatarsal head at the calcaneus weight-bearing point was recorded in real time. So that means when in weight-bearing CT scans, and when there's, so the x, y, and z-information the x, y, and z-information reflects the real information of where those weight-bearing points is. But the TALAS always project on the floor. So the TALAS in that program does not follow where the foot goes. So it is not, the TALAS is not projecting on the real triangle. So that's a problem. So in summary, FAO has a very limited capacity to reflect hind-foot deformities. Understandable when analyzing how the FAO is designed and measured mathematically, it has a lot of limitations. It is neither a 3D nor a semi-3D tool, using weight-bearing points to represent the first fifth metatarsal at the calcaneus, and the top center of the TALAS to represent the TALAS is not scientific, because it loses the majority of the 3D information of the bones. And moreover, the above four markers are not constantly stable at all. So any change of the foot position in the weight-bearing CT will change the four points dramatically. That means if you slightly rotate the foot a little bit, the weight-bearing point will change dramatically. And also for person A, his weight-bearing point could be totally different from the weight-bearing point found by person B. And also the real-time triangle recording system, it is real-time, but with TALAS always projecting on the floor, which cannot reflect sagittal plane deformities at all, nor coronal plane rotations. And false results are inevitably generated. And also look at the tripod system, it does not include the level of the ankle nor the distal tibia, so how can you use it to reflect high foot or ankle alignment? So therefore, FAO is a single numerical metric. It's not appropriate, not capable to be used as a high foot alignment evaluation tool, not to mention to use it to evaluate the ankle alignment. And as I said, there's a trend of using FAO to guide clinical practice. And you can see in this paper, it claimed that FAO, such a biometric tool, may enhance the pro-operative assessment and surgical planning for patients undergoing TAR, total ankle replacement, with the potential to optimize surgical outcome. So with what we had just now and what we have found just now, we really think FAO is not capable to be used as a tool to evaluate the high foot alignment, not to mention the ankle. Here are our references. Thank you.

foot and ankle offset

three-dimensional height-foot alignment

weight-balance CT scans

limitations of FAO

inadequacy of FAO