Grade 0 — Radiographically Normal Knee Joint
No osteophytes, narrowing, or sclerosis are visible on plain film.
- None: Osteophytes (no marginal lipping)
- Normal: Joint space (~5.6mm medial)
- Absent: Sclerosis (no subchondral change)
- Baseline: Prevalence (reference population)
Defining the normal reference film
Short placeholder: normal films anchor the whole KL comparison scale.
Placeholder highlight: normal grade serves as the atlas baseline image.
Grade I — Doubtful Narrowing and Possible Osteophyte
Equivocal findings that may or may not represent early disease.
- Doubtful: Osteophytes (questionable lipping)
- ~4.6mm: Joint space (borderline)
- None: Sclerosis (not yet present)
- Low: Reader agreement (high inter-rater variance)
Why Grade I is the least reproducible category
Short placeholder: doubtful features drive most inter-reader disagreement.
Placeholder highlight: Grade I overlaps heavily with normal aging changes.
Grade II — Definite Osteophytes, Preserved Joint Space
Clear osteophyte formation marks definite radiographic osteoarthritis.
- Definite: Osteophytes (small marginal spurs)
- ~3.4mm: Joint space (possible narrowing)
- Minimal: Sclerosis (early subchondral change)
- Variable: Symptom link (often asymptomatic)
Osteophytes as the defining Grade II feature
Short placeholder: definite osteophytes are the diagnostic threshold for OA.
Placeholder highlight: Grade II is the minimum grade termed "definite OA".
Grade III — Moderate Narrowing with Subchondral Sclerosis
Multiple osteophytes and sclerosis accompany moderate joint narrowing.
- Multiple: Osteophytes (moderate size)
- ~2.0mm: Joint space (definite narrowing)
- Moderate: Sclerosis (subchondral whitening)
- Common: Symptoms (pain and stiffness)
Sclerosis emerges alongside cartilage loss
Short placeholder: sclerosis reflects bone remodeling under thinning cartilage.
Placeholder highlight: Grade III correlates strongly with clinical symptoms.
Grade IV — Severe Narrowing, Sclerosis, and Deformity
End-stage changes include bone-on-bone contact and joint deformity.
- Large: Osteophytes (severe marginal spurs)
- <1mm: Joint space (near-complete loss)
- Marked: Sclerosis (dense subchondral bone)
- TKA: Surgical (often indicated)
End-stage deformity and surgical thresholds
Short placeholder: deformity and bone-on-bone contact define Grade IV.
Placeholder highlight: Grade IV is the classic total knee arthroplasty indication.