KeywordsFoot ankle offsetPedCATTALASHindfoot alignmentWeight bearing computed tomography
Hindfoot malalignment is a common cause of foot and ankle disability [1, 2]. A reliable and precise measurement of hindfoot alignment (HA) is paramount for the diagnosis and treatment of various foot and ankle pathologies, although the best way to reach a correct assessment is still a matter of debate. Traditionally, HA can be assessed from weight bearing measurements on plain radiographic views, such as the calcaneal long axial view and HA view. These methods are used worldwide for preoperative planning and postoperative evaluations [2–5]. However, radiography has many flaws such as perspective distortion due to lower limb rotation, bone superimposition because of two-dimensional (2D) visualization and some operator related bias, which all affect the method of measurement making it imprecise and at times unreliable [5–8].
The recent development of new radiographic imaging equipment such as 3D weight bearing computed tomography (WBCT) has the potential to provide more accurate anatomical information on full bipedal weight bearing scans, allowing for more precise measurements with a similar total amount of radiation [9–13]. Recent studies have shown how the different angles used to assess HA may differ substantially between radiographs, traditional CT, and WBCT, suggesting that 3D weight bearing measurements represent a major step forward to detect the “true alignment” [2, 14, 15]. In addition, this technology can provide a digitally reconstructed radiograph, similar to traditional radiography with the advantage of no rotation bias [9, 16]. Common measurements, such as the long axial view (termed HACT), can be taken using this reconstruction to evaluate HA.
A novel parameter used to assess HA on WBCT images is foot ankle offset (FAO) [5, 17]. Technically, this is a semiautomatic 3D biometrics and is calculated using semiautomatic software: Torque Ankle Lever Arm System or TALAS™ (CurveBeam®, Warrington, PA). This measurement considers the forefoot rather than the tibia as the reference since the forefoot interacts with the ground like the hindfoot and contributes to ankle biomechanics [5, 18]. In a previous retrospective study, Lintz et al. found that FAO had excellent inter- and intraobserver reliability and concluded that it was a precise and discriminating tool for measuring HA . However, the clinical efficiency of FAO and the reference values for normal, varus, and valgus hindfoot have not yet been confirmed.
The aim of this study was to assess FAO and HACT on a large population of patients. We hypothesized that FAO would be a clinically relevant and reproducible method and that the values would be comparable to previously published data in normal, varus, and valgus cases.
Materials and Methods
Study Population and Design
Between September 2017 and April 2018, all consecutive patients undergoing a WBCT investigation in our department as part of their standard care were enrolled in this prospective comparative study.
Based on a standard physical examination, the patients were divided into three groups: clinically normally aligned hindfoot group (G1); valgus alignment group (G2); and varus alignment group (G3). This division was based on clinical examination by two experienced foot and ankle specialist orthopedic surgeons (one of the two). When the assessment of the hindfoot alignment was unclear, a goniometer was applied on the posterior aspect of the ankle and the hindfoot, measuring the standing tibiocalcaneal angle (angle between the bisectors of the calf and the calcaneus). Values between 0 and 7 degrees of valgus were considered normal. If greater than 0 degrees, the hindfoot was varus. If lower than −7, it was valgus.
Clinical indications to perform bilaterally WBCT scans were the same as with a conventional radiographic setup (obtaining comparative images).
The study was approved by the relevant Ethics Committee and was compliant with the Health Insurance Portability and Accountability Act (HIPAA) and the Declaration of Helsinki. Informed consent was signed by all participants to be included in the study.
Our analysis included only patients who had a bilateral WBCT, in order to minimize the risk of a nonphysiological static loading of the foot. Among these, feet which had a previous surgery with some important metal artifacts were excluded.
Quantitative variables are reported as mean ± standard deviation (SD), range, and 95% confidence intervals [95% CIs]. The Kolmogorov-Smirnov test was performed to investigate the distribution of the normal foot group (G1). Regression analysis was performed to calculate the correlation between FAO and HACT using Spearman’s coefficient and visualization of a corresponding scatterplot.
To assess the reproducibility of FAO and HACT, inter- and intraobserver variability expressed by the intraclass correlation coefficient (ICC) was calculated. ICC values range from 0 to 1, with a higher value indicating better reliability. An ICC value >0.7 indicates good reliability and a value >0.9 excellent reliability. All analyses were carried out using SPSS 20.0 software (Standard version; IBM, Armonk, NY, USA). The significance level was set at p < 0.05.
Measurements of the foot and ankle offset (FAO) and hindfoot alignment through a long axial view (HACT) in the three groups, expressed as mean value, standard deviation (SD), 95% confidence interval (95%CI), and range of values