Background Bone erosion is a central feature of rheumatoid arthritis (RA). While the detection and semi-qualtitiative evaluation of erosion size is well established, the quantitative assessment of erosion size is less developed, but essential for precisely determining potential longitudinal changes in erosions
Objectives To develop a new method to precisely calculate the volume of erosions using High Resolution-peripheral Quantitative Computed Tomography (HR-pQCT)
Methods HR-pQCT (XtremeCT, Scanco, Switzerland) scans of ACPA positive RA patients were assessed. Erosions were defined as previously described.  Additionally we distinguished between regular shaped erosions, defined as having an uninterrupted surface (Fig 1a) and irregular erosions, those with multiple excrescences creating different sections (Fig 1b). After identification, erosions were contoured automatically using the XtremeCT evaluation program (Fig 1c). This allows a segmentation of the erosion, which means 3D reconstruction and volume calculation (Fig 2a-c). This method was applied by two, blinded, independent assessors. To validate the volume erosion measurement, 125 mm3 cubes were printed using a high resolution 3D printer (Stratasys Objet30) and the same segmentation technique was applied in this geometric printed figure. The statistical analysis was performed with IBM SPSS V.21.0. Categorical variables were presented as numbers and percentages, continuous variables are provided as mean ± standard deviation. Cohen's kappa coefficient was used to determine the image reading agreement of the two independent and blinded readers.
Results A total of 65 RA patients (45 female; 20 male), all ACPA positive, were included and 101 erosions were analyzed. The mean age of all patients was 54.46±12.56 years and the disease duration was 8.4±8.3 years.Inter-reader reliability for the method was 0.997 (CI: 0.996-0.998). The mean erosion volume of all erosions was 30.7±59.6 mm3. Irregular shape of erosions was found in 61.39% of all evaluated erosions. When erosions were separated by their shape (regular/irregular) the mean volume for regular erosions was different compared to the irregular with regular erosions being smaller (5.99±7.00 vs. 46.29±71.89mm3, p<0.001). Using the 3D printed cube for validation our method we detected an average deviation of 2.9%.
Conclusions This method allows to measure the exact erosion volume in a feasible way. We achieved high inter-reader reliability and good agreement with the gold-standard, which was the direct measurement of erosion in a 3D print of the erosion.
Stach, C.M., et al., Periarticular bone structure in rheumatoid arthritis patients and healthy individuals assessed by high-resolution computed tomography. Arthritis Rheum, 2010. 62(2): p. 330-9.
Disclosure of Interest None declared