Table 4.

Osteoarthritis cartilage histopathology assessment classification system (Osteoarthritis Research Society International) [66].

Grade (Key Feature)	Subgrade (Optional)	Associated Criteria (Tissue Reaction)
Grade 0: Surface intact, Cartilage intact	No subgrade	Intact, uninvolved cartilage
Grade 1: Surface intact	1.0 Cells intact 1.5 Cell death	Matrix: superficial zone intact, edema and/or fibrillation; Cells:proliferation (clusters),hypertrophy; Reaction must be more than superficial fibrillation only.
Grade 2: Surfacediscontinuity	2.0 Fibrillation through superficial zone 2.5 Surface abrasion with matrix loss within superficial zone	As above +Discontinuity at superficial zone; ±Cationic stain matrix depletion (Safranin O or Toluidine Blue) upper 1/3 of cartilage (mid zone); ±Disorientation of chondron columns
Grade 3: Verticalfissures	3.0 Simple fissures 3.5 Branched/complex fissures	As above; ±Cationic stain depletion (Safranin O or Toluidine Blue) into lower 2/3 of cartilage (deep zone); ±New collagen formation (polarized light microscopy, Picro Sirius Red stain)
Grade 4: Erosion	4.0 Superficial zone delamination 4.5 Mid zone excavation	Cartilage matrix loss, cyst formation within cartilage matrix
Grade 5: Denudation	5.0 Bone surface intact 5.5 Reparative tissue surface present	Surface is sclerotic bone or reparative tissue including fibrocartilage.
Grade 6: Deformation	6.0 Joint margin osteophytes 6.5 Joint margin and central osteophytes	Bone remodeling; Deformation of articular surface contour (more than osteophyte formation only); Includes: microfracture and repair