The Ahlbäck classification of radiographic knee OA of the tibiofemoral joint and the Kellgren and Lawrence grading system (adapted) 3 4
| Ahlbäck grade | Ahlbäck definition | Kellgren & Lawrence grade | Kellgren & Lawrence definition |
| Grade 1 ‘Doubtful’ | Minute osteophyte, doubtful significance | ||
| Grade 2 ‘Minimal’ | Definite osteophyte, unimpaired joint space | ||
| Grade I | Joint space narrowing (joint space < 3 mm) | Grade 3 ‘Moderate’ | Moderate dimunition of joint space |
| Grade II | Joint space obliteration | Grade 4 ‘Severe’ | Joint space greatly impaired with sclerosis of subchondral bone |
| Grade III | Minor bone attrition (0–5 mm) | Grade 4 ‘Severe’ | Joint space greatly impaired with sclerosis of subchondral bone |
| Grade IV | Moderate bone attrition (5–10 mm) | Grade 4 ‘Severe’ | Joint space greatly impaired with sclerosis of subchondral bone |
| Grade V | Severe bone attrition (>10 mm) | Grade 4 ‘Severe’ | Joint space greatly impaired with sclerosis of subchondral bone |