Histopathology of the Superficial Zone in Human Articular Cartilage

Background: The surface and superficial zone (SZ) of human knee articular cartilage (hK-AC) exhibits variable patterns of deterioration with aging and osteoarthritis. A standardized grading system for SZ cartilage would help elucidate the early stage pathogenesis of age-related cartilage degeneration. Objectives: To establish such a grading system, the objectives were to (1) record key histological features from current grading systems, (2) introduce standards for digital histology images, (3) collect and evaluate images of hK-AC from a digital histology base, and (4) provide clear examples of each feature grade for a comprehensive atlas. Methods: Key histological features for hK-AC were collected from the original reports of major grading systems for cartilage degeneration (Mankin, OARSI, ICRS). Properties (field of view (FOV), resolution) of traditional microscopy images were determined and guided an acquisition protocol for the images. Digital images with comparable properties were collected from a digital database on SlidePath (Leica Biosystems, IL). Images of 1, 3, and 20X were collected from each of 15 donor knee cartilage, across 4-6 sites on one medial femoral condyle, from n=6 young (21-40yrs), grade 1 and n=9 old ([greater than] 61yrs), grade 1-3 samples. Images were assessed for clarity, and further processed under standardized cropping and resizing to achieve representative images for an atlas. Results: An hK-AC image atlas was created. It contains representative images of SZ features with a standardized field of view and resolution according to the magnification. A table listing each feature to be graded was included on every image. Conclusion: With the creation of a standardized grading system for the SZ of hK-AC, local features of cartilage degradation can be assessed. The same approach can be used to extend the grading system into deeper zones of cartilage. Utilizing a standardized FOV and resolution, researchers are guaranteed a consistent image for grading that will ultimately help us better understand the early pathogenesis of cartilage degradation.