KeywordsWeight bearing CTPedCATPedographyCentre of gravity (COG)Foot center (FC)
Weight bearing CT (WBCT) is a technology that allows 3D imaging with full weight bearing which is not influenced by projection and/or foot orientation [1, 2]. In the first published study, specific bone position (angle) measurements using WBCT were compared with conventional weight bearing radiographs and conventional non weight bearing CT . The angles differed between radiographs, CT, and WBCT, indicating that only WBCT is able to detect the correct angles, i.e., bone position . In a subsequent study, the correlation between 3D bone position and pedographic measurements, i.e., force and pressure (distribution), has been investigated . In that study, 3D bone position did not correlate with force and pressure distribution under the foot sole during simultaneous WBCT scan and pedography . Consequently, the bone positions measured with WBCT did not allow conclusions about the force and pressure distribution in this static configuration . Vice versa, pedography parameters did not allow conclusions about the 3D bone position . One conclusion was that further investigations with higher case number and more other parameters should be carried out to further validate these surprising findings . Meanwhile, center of gravity (COG) and foot center (FC) were discussed to be important parameters for biomechanical assessment around foot and ankle and consequently as basis for diagnostics and planning of corrective surgeries and/or joint replacement [4, 5]. In particular, a semiautomatic system (TALAS, CurveBeam, Warrington PA, USA) designed to measure hindfoot alignment as a 3D biometric uses the anterior midline of the forefoot (which joins the FC with the midpoint between the first and the fifth metatarsal heads) as a landmark for hindfoot alignment . The aim of this study was to analyze the difference between morphology- and anatomy (bone/WBCT)-based FC, calculated as the intersection of the median lines of the triangular-based pyramid model of the foot and force/pressure (pedography)-based COG. Motion of COG during WBCT/pedography scan should also be assessed as potential source for bias. For this study, a customized pedography sensor (Pliance, Novel, Munich, Germany) was inserted into a WBCT as described previously . Our hypothesis was that the FC should be a good predictor of mediolateral position of the COG but not longitudinal since the anatomy of the hindfoot allows free anteroposterior movement but limited mediolateral.
Inclusion and Exclusion Criteria, Ethics
The inclusion criteria were age ≥18 years, presentation at the local foot and ankle outpatient clinic, and indication for WBCT. The indication for WBCT was defined according to local practice as described previously . These indications have recently evolved to include all the patients presenting at our institution except initial postoperative follow-up radiographs without weight bearing.
The exclusion criteria were age <18 years, no indication for WBCT imaging, and participation in other studies.
Approval from the local ethical committee was granted based on the indications as described above. Informed consent was obtained from all subjects.
Image Acquisition: Foot Center (FC)
Pedography: Center of Gravity (COG)
Comparison of FC/COG
The statistical analysis was performed with Microsoft Excel 2016 (Microsoft, Redmond, WA, USA) and SPSS 24.0 (IBM, Rochester, MN, USA). The data (distances/shift between FC and COG) was successfully tested for normal distribution with a Shapiro-Wilk test. A bilateral paired t-test was used to compare data from the left to the right foot. One-way ANOVA with potential post hoc Scheffe test was used for data comparison between different pathologies. Pearson test (two sided) was used for correlation of BMI with measured data (distances/shift between FC and COG). Correlation was defined as significant when p < 0.05 and when significant then sufficient when r > 0.5 or r <−0.5.
Registered foot and ankle pathologies in 180 feet in 90 patients
Isolated hallux valgus
Complex forefoot deformity
Other combined deformities
Osteoarthritis without relevant deformity