Comparative Articular Cartilage Thickness Measures using High-Frequency Ultrasound versus Histology

Abstract

Osteoarthritis (OA) is the most common form of arthritis, affecting millions worldwide. Although research into treatments is continuously developed, scientists have not been able to find a way to reverse bone alterations due to this condition. Previous studies have shown that an initiating event in OA may involve remodeling of the tidemark, the cartilage-bone interface. Knowledge of factors that induce tidemark remodeling could help further design treatments to slow disease progression. The purpose of this study was to validate the use of Scanning Acoustic Microscopy (SAM) of mature bovine articular cartilage-bone explants to image the tidemark. Bovine explants were SAM scanned by high-frequency ultrasound (HFUS) before and after 1 week of in vitro culture, and cartilage thickness was measured using MATLAB (n=8). Imaged explants were frozen unfixed in OCT (optimal cutting temperature), cryosectioned in tissue arrays and submitted to enzyme staining for diaphorase, alkaline phosphatase, and chemical staining with hematoxylin and von-Kossa before and after DMMB to visualize the cartilage and tidemark. Enzyme staining showed that chondrocyte viability was preserved in the calcified cartilage layer during the 1 week culture period, with variable alkaline phosphatase staining in the cartilage deep zone and calcified cartilage layer. Quantitative histomorphometry using ImageJ (N=8) showed an average cartilage thickness of 1.02 mm (alkaline phosphatase), 1.07 mm (diaphorase) and 1.04 mm (von Kossa). HFUS measures of cartilage thickness were within 110 µm of histomorphometry thickness measures for 7 out of 8 explants before and after 1 week of in vitro culture. This study highlights the potential for HFUS to detect remodeling of the tidemark and calcified cartilage layer in a bovine explant tissue culture model.