Developing Objective Computational Methods for Quantifying Ankle Osteoarthritis using Low-Dose Weight Bearing CT

Abstract

Category: Ankle Arthritis; Ankle; Other Introduction/Purpose: Decision regarding ankle osteoarthritis (OA) management varies depending on the severity and distribution of the associated joint degeneration. Disease staging is typically based on subjective grading of appearance on conventional plain radiographs, with reported sub-par reproducibility and reliability. Weight-bearing computed tomography (WBCT) offers clinical advantages in the setting of OA, where thinning of the ankle cartilage, softening of the cartilage, and other deformities become more apparent under load. WBCT also provides a better geometric representation of the ankle and allows for more accurate measurements when compared to a conventional radiograph. The purpose of this study was to develop and describe computational methods to objectively quantify radiographic changes associated with ankle OA apparent on low-dose WBCT images. Methods: We analyzed two patients with ankle OA and one healthy control that had all undergone WBCT of the foot and ankle. The severity of OA in the ankle of each patient was scored using the Kellgren-Lawrence (KL) classification by plain weight-bearing radiographs. For each ankle, the subsequent analysis focused on a volume of interest (VOI) centered on the tibiotalar joint. Within the VOI, the initial computational analysis focused on measuring the 3D joint space width (JSW). Subsequent analyses utilized WBCT image intensity (Hounsfield Unit, or HU) profiles along lines perpendicular to the subchondral bone/cartilage interface of the distal tibia extending across the entire VOI. The HU intensity profiles were recorded, and graphical plots of the HU distributions were generated for each line. These plots were then used to calculate the HU contrast, a novel measure of the regional variation in bone density. Results: The average JSW was 3.89 mm for the healthy control ankle, 2.69 mm for the mildly arthritic ankle (KL 2), and 1.57 mm for the severely arthritic ankle (KL 4). The average HU contrast was 72.31 for the healthy control ankle, 62.69 for the mildly arthritic ankle, and 33.98 for the severely arthritic ankle. The use of four projections at different locations throughout the joint allowed us to visualize specifically which quadrants have reduced joint space width and contrast. One projection in the severely arthritic ankle had JSW and contrast values of 0 due to complete joint space loss along with projection 4, which corresponds in this case to the posterolateral part of the joint (Figure). Conclusion: We presented a novel computational assessment of ankle osteoarthritis using low-dose WBCT imaging. We were able to demonstrate differences between normal ankles and ankles with mild and severe OA using JSW and HU contrast measurements. This methodology represents an important step towards a more reliable OA assessment when compared to the current standard qualitative evaluations, potentially serving as a starting point for the development of a more robust osteoarthritis staging system. Additional studies are needed to assess the algorithm more rigorously over a variety of radiographic presentations.