Defining Normal Articular Characteristics of the Primary Joints of the Foot and Ankle: A 3D Hounsfield Algorithm Weight Bearing CT Study
Authors: Stebral Hannah J. , Tazegul Tutku , Jasper Ryan , Mallavarapu Vineel , Schmidt Eli , Carvalho Kepler A.M. , Mansur Nacime Salomao Barbachan , Chrea Bopha , Anderson Donald D. , de Cesar Netto Cesar 
Published: December 2023

Category: Ankle Arthritis; Hindfoot Introduction/Purpose: To properly treat osteoarthritis-(OA) patients, a grading-system is used to diagnose the stage-of-the- disease. The current gold-standard system, which relies on plain-2D-radiographs, is subjective, categorical, and lacks reliability. Using WBCT-images, 3D-Hounsfield-Unit-(HU) algorithms have been developed to measure the intensity of each image voxel across the joint-space, highlighting transitions between cancellous/subchondral bone and joint-space. The purpose of this study was to analyze and define normal standard values of joint-space-width (JSW) in the four essential joints of the foot-and-ankle (tibiotalar (ankle), subtalar (ST), talonavicular (TN), and first-metatarsophalangeal- (MTP) -joints) using an objective computational WBCT-HU algorithm in healthy non-arthritic feet. We hypothesized that the measurements of JSW and HU distributions across each of the four-essential-joints of the foot-and-ankle would be significantly different from each other, respecting local anatomy and unique functional characteristics of each joint. Methods: Retrospective-comparative-study, we evaluated WBCT-scans of 30 healthy ankle-joints, 28 ST-joints, 26-TN joints, and 30 1st-MTP-joints of control volunteers with no radiographic signs of foot-and-ankle OA. For each-joint, we used dedicated software to define a volume-of-interest (VOI) cube centered on the joint space. Five HU linear search arrays were then defined within this 3D VOI perpendicular to the articular-surface of each-joint, including four projections in each quadrant, and one in the center of the VOI (Figure 1). Image intensity profiles were generated for each search array crossing the transition from cancellous- to-subchondral bone, across the joint-space, back to subchondral-and-cancellous-bone (Figure 2). This profile was used to calculate JSW and to measure HU contrast in the region. Comparisons between the JSW of each joint and within each-joint were accordingly performed using paired t-tests or paired Wilcoxon. Significance was considered for p-values < 0.05. Results: The median-value and 95%-Confidence-Intervals-(CI) for JSW were 4.07mm [CI:3.73–4.20] for the ankle-joint; 4.07mm [3.95–4.44] for the ST-joint; 3.24mm [3.19–3.46] for the TN-joint; and 3.70mm [3.64–4.12] for the 1st-MTP-joint. The TN JSW was significantly narrower than the JSW in the ankle (p=0.0007), ST (p < 0.0002), and 1st-MTP-joints (p=0.0034) (Figure-3). JSW- values were similar across the entire ankle, ST and 1st-MTP-joints. In the TN-joint, the dorsal aspect of the joint was found to be slightly but significantly wider-than the plantar-aspect (p < 0.001). Regarding HU-contrast, we found a progressive increase in the overall contrast from proximal-to-distal (p < 0.001), with a mean HU contrast-value and 95%-CI of respectively 71.8[67.3–76.3] for ankle, 92.4[87.8–97.1] for ST, 84.1[79.2–88.9] for TN, and 101.3[96.9 – 106.8] for 1st-MTP-joints. The only joints with similar HU-contrast were ST and TN. Conclusion: We utilized a novel WBCT-3D-HU measurement algorithm to assess the normal JSW and HU contrast of the four- essential mobile joints of the foot-and-ankle. We found the JSW to be similar (~4mm) in the ankle, ST, and 1st-MTP joints. The TN-joint however, demonstrated a significantly narrower JSW when compared to the other 3 joints. HU-contrast increased progressively from proximal-to-distal, being less prominent in the ankle, similarly increased in TN and ST, and maximum at the 1st-MTP joint. This study's joint characteristic normality data provide a foundation for future-work developing an objective WBCT-based 3D HU-algorithm staging-system for OA-disease-progression in the foot-and-ankle joints.