Relationship between Feline Knee Joint Osteoarthritis and Bone Mineral Density Quantified Using Computed Tomography and Computed Digital Absorptiometry

Osteoarthritis (OA), including knee joint OA, is a common chronic condition in cats. In both cats and humans, knee joint OA is characterized radiographically by the presence of osteophytes, enthesiophytes, subchondral sclerosis, and joint space narrowing. However, only in humans have these radiographic signs been reported to increase bone mineral density (BMD). Therefore, this study aims to quantify the volumetric (vBMD) and relative (rBMD) BMD measures of the feline knee joint and compare BMD measures between various severities of OA to test the hypothesized OA-BMD relationship in the knee joint in cats. The 46 feline knee joints were imaged using computed tomography (CT) and conventional radiography supported by the computed digital absorptiometry (CDA) method to obtain vBMD and rBMD, respectively. Both BMD measures were assessed in three regions of interest (ROIs): the distal femur (ROI 1), patella (ROI 2), and proximal tibia (ROI 3). In all locations, vBMD and rBMD showed moderate (ROI 2: r = 0.67, <i>p</i> < 0.0001) to strong (ROI 1: ρ = 0.96, <i>p</i> < 0.0001; ROI 3: r = 0.89, <i>p</i> < 0.0001) positive correlations. Due to differences (<i>p</i> < 0.0001) in the width of the distal femur (17.9 ± 1.21 mm), patella (8.2 ± 0.82 mm), and proximal tibia (19.3 ± 1.16 mm), the rBMD was corrected (corr rBMD) using the thickness coefficient of 0.46 ± 0.04 for ROI 2 and 1.08 ± 0.03 for ROI 3. Regardless of the quantification method used, BMD measures increased linearly from a normal knee joint to severe OA, with differences in BMD between normal and mild to severe knee joint OA. The OA-BMD relationship in the feline knee joint can be preliminarily confirmed.