Histological grading scheme (McNulty et al., 2011)*.

Articular cartilage structure score (ACS)
Grade (key feature)	Associated criteria (tissue reaction)
Grade 0	Articular surface morphology is intact and smooth
Grade 1-3	Fibrillation/clefts and loss of cartilage involving one by fourth or less of articular cartilage thickness
Grade 4-6	Fibrillation and/or clefts and/or loss of cartilage involving one half or less of articular cartilage thickness
Grade 7-9	Fibrillation and/or clefts and/or loss of cartilage involving more than one half of articular cartilage but less than full thickness
Grade 10-12	Fibrillation and/or clefts and/or loss of cartilage involving the full thickness of articular cartilage

*McNulty MA, Loeser RF, Davey C, Callahan MF, Ferguson CM, Carlson CS. A Comprehensive Histological Assessment of Osteoarthritis Lesions in Mice. Cartilage. 2011; 2:354–63. https://doi.org/10.1177/1947603511402665 PMID:3741515