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Background: The treatment of ankle osteoarthritis (OA) 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 subpar reproducibility and reliability. 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 weightbearing CT (WBCT). Methods: Two patients with ankle OA and 1 healthy control who had all undergone WBCT of the foot and ankle were analyzed. The severity of OA in the ankle of each patient was scored using the Kellgren-Lawrence (KL) classification using plain radiographs. For each ankle, a volume of interest (VOI) was centered on the tibiotalar joint. Initial computation analysis used WBCT image intensity (Hounsfield units [HU]) profiles along lines perpendicular to the subchondral bone/cartilage interface of the distal tibia extending across the entire VOI. Graphical plots of the HU distributions were generated and recorded for each line. These plots were then used to calculate the joint space width (JSW) and HU contrast. Results: The average JSW was 3.89 mm for the control ankle, 3.06 mm for mild arthritis (KL 2), and 1.57 mm for severe arthritis (KL 4). The average HU contrast was 72.31 for control, 62.69 for mild arthritis, and 33.98 for severe arthritis. The use of 4 projections at different locations throughout the joint allowed us to visualize specifically which quadrants have reduced joint space width and contrast. Conclusion: In this technique report, we describe a novel methodology for objective quantitative assessment of OA using JSW and HU contrast. Clinical Relevance: Objective, software-based measurements are generally more reliable than subjective qualitative evaluations. This method may offer a starting point for the development of a more robust OA classification system or deeper understanding of the pathogenesis and response to ankle OA treatment.

In this work, we present a new experimental methodology that integrates magnetic tweezers (MT) with substrate deformation tracking microscopy (DTM) and traction force microscopy (TFM). Two types of MT-DTM/TFM experiments are described: force-control mode and displacement-control mode experiments. In model bead-on-gel experiments for each mode, an MT device is used to apply a controlled force or displacement waveform to a fibronectin-coated superparamagnetic bead attached to a fibrillar type I collagen gel containing a layer of covalently attached red-fluorescent microspheres. Serial fast time-lapse differential interference contrast and epifluorescence image acquisition steps are used to capture displacements of the bead and microspheres, respectively, in response to the applied force or displacement. Due to the large number of acquired images and the dynamic nature of the experiment, new quantitative approaches are implemented to adapt TFM for the analysis of the data, including (i) a temporospatial correction algorithm for improved tracking of microsphere displacements, (ii) a method for the objective determination of L2 regularization parameters for computing incremental traction stress solutions, and (iii) an empirical means for identifying time intervals within the data that can be approximated by elastostatic conditions. We also illustrate how force and energy balances in a force-control mode bead-on-gel experiment can be used to estimate the elastic modulus of a collagen substrate. Finally, in a proof-of-concept, bead-on-cell demonstration, measurements of incremental cell–matrix traction stresses are used to observe how a force applied to a focal contact on the apical surface of a keratinocyte is transmitted to the collagen substrate below the cell.

In this work we demonstrate the integration of magnetic tweezers (MT) with substrate deformation tracking microscopy (DTM) and traction force microscopy (TFM) for the investigation of extracellular matrix rheology and human epidermal keratinocyte mechanobiology in the context of human blistering skin diseases. Two model bead-on-gel experiments are described in which an MT device is used to apply a prescribed force or displacement waveform to a fibronectin-coated superparamagnetic bead attached to a type I collagen gel containing a layer of covalently attached red-fluorescent microspheres. Serial fast time-lapse DIC and epifluorescence image acquisitions are used to capture displacements of the bead and microspheres, respectively, in response to the applied force or displacement. Due to the large number of acquired images and the dynamic behavior of substrate microspheres observed during the experiment, new quantitative methods are developed for the tracking and filtering of microsphere displacement data, the selection of L2 regularization parameters used for TFM analysis, and the identification of time intervals within the overall image set that can be approximated as being subject to elastostatic conditions. Two major proof-of-concept applications are described in which integrated MT-DTM/TFM experiments are used to (i) estimate the elastic properties of a fibrillar type I collagen gel substrate and (ii) demonstrate how a force applied to a focal adhesion contact on the apical surface of a living keratinocyte is directly transmitted to basal cell-matrix anchoring junctions as observed by substrate deformations and incremental traction stresses that develop within the collagen subjacent to the cell.