Kellgren-Lawrence Grading of Osteoarthritis

Kellgren-Lawrence (KL) is the most widely used RADIOGRAPHIC grading system for osteoarthritis severity and the reference standard for defining OA in epidemiology and clinical trials. It scores a joint 0–4 from four features: OSTEOPHYTES (the earliest definite sign), JOINT-SPACE NARROWING, subchondral SCLEROSIS and bony DEFORMITY. GRADE 0 = normal; GRADE 1 = doubtful (possible osteophyte, no narrowing); GRADE 2 = definite osteophyte ± possible narrowing — the conventional THRESHOLD for definite radiographic OA; GRADE 3 = multiple moderate osteophytes, definite narrowing, some sclerosis ± deformity; GRADE 4 = large osteophytes, marked narrowing, severe sclerosis, definite deformity ('bone on bone'). The single most-tested point is that GRADE 2 is the cut-off for definite OA (grade 2 or greater is the inclusion threshold in most studies). The second is that radiographic grade correlates only MODESTLY with symptoms and function — many people with grade 3–4 changes have little pain and vice versa — so the grade must NEVER drive a surgical decision in isolation; symptoms, function and patient factors do (although baseline KL severity IS a useful prognostic biomarker for the future onset of frequent pain). It is applied to knee, hip, hand and spine, but was developed for the knee; reliability depends on using a standardised ATLAS, and it is relatively insensitive to early cartilage loss compared with MRI. For the HIP, the Tönnis and Croft systems are common alternatives; cartilage-specific grading uses Outerbridge/ICRS, and research increasingly adds OARSI-atlas joint-space scoring, MRI scores (WORMS, MOAKS) and quantitative joint-space width.