New approach for quantitative assessment of articular cartilage degeneration in horses with osteoarthritis

OBJECTIVE: To evaluate a modified digital imaging technique for quantitative assessment of the grade of osteoarthritis across the proximal articular surface of the first phalanx in horses. SAMPLE POPULATION: 6 metacarpophalangeal (fet-lock) joint specimens from 6 horses with various stages of osteoarthritis. PROCEDURE: First phalanx specimens, together with 4 gray scale reference calibration targets, were positioned in a bath with the proximal articular cartilage surface submerged in saline (0.9% NaCl) solution. Digital images were obtained from the articular surface before and after staining with Indian ink. Computer-controlled gray level analysis of the nonstained and Indian ink-stained cartilage surfaces and gray scale reference calibration targets was performed by use of the mean pixel value (based on 255-gray scale). An increase in the mean pixel value after staining was used to calculate the cartilage degeneration index (CDI). RESULTS: The CDI of the proximal articular cartilage surface of the first phalanx specimens ranged from 9.2 +/- 5.7 (early stage osteoarthritis) to 41.5 +/- 3.6% (late stage osteoarthritis). The effect of repeating the measurement 6 times in nonstained (including repositioning) and stained specimens (including repositioning and restaining) was not significant. Up to 10 measurements of nonstained specimens could be made without refreshing the bath solution. In stained specimens, mean gray level increased significantly after the sixth measurement. CONCLUSIONS AND CLINICAL RELEVANCE: The modified digital imaging technique allowed quantitative assessment of cartilage degeneration across the articular cartilage surface. The CDI is the first quantitative measure for osteoarthritis-induced cartilage degeneration over an entire joint surface in horses.