Classification system
Outerbridge Classification
Outerbridge Classification of Chondral Lesions
The Outerbridge classification grades articular cartilage damage from 0 to IV at arthroscopy, progressing from normal cartilage through softening, fissuring under and over 1.3 centimetres in diameter, to full-thickness erosion exposing subchondral bone.
What the Outerbridge category means
Outerbridge scores run from 0 to 4, where a lower score is better. The bands below are conventional reporting categories, not diagnostic cut-offs.
- Grade 00–0
- Grade I1–1
- Grade II2–2
- Grade III3–3
- Grade IV4–4
Published criteria and reliability
The published cut-offs that define this instrument, and the measured agreement between observers using it. Each is reported with the population it was derived in.
| Type | Value | Subscale | Population | Follow-up | Source |
|---|---|---|---|---|---|
| Criterion | Grade II vs III is defined by lesion diameter at 1.3 cm (half an inch) | — | Arthroscopic assessment | — | Outerbridge RE 1961 |
| Criterion | Grade IV corresponds to full-thickness loss with exposed subchondral bone | — | Arthroscopic assessment | — | Outerbridge RE 1961 |
| Criterion | Interobserver agreement is moderate, and lower than for the ICRS system | — | Arthroscopic graders | — | Dwyer T 2017 |
Criterion is a published cut-off, definition or performance figure belonging to a classification or decision rule — it is not a change score and should not be read as one.
How the score is calculated
Stated in full so the calculation can be independently checked, and reproduced by hand if needed.
Grade 0 is normal cartilage. Grade I is softening and swelling of the cartilage with an intact surface, identified by probing. Grade II is fragmentation and fissuring in an area less than half an inch — about 1.3 cm — in diameter. Grade III is fragmentation and fissuring in an area greater than half an inch in diameter. Grade IV is erosion of cartilage down to exposed subchondral bone. Grades II and III differ only in the diameter of the affected area, not in depth.
Instrument profile
- Developer
- Robert E. Outerbridge
- Institution
- University of British Columbia, Canada
- First published
- 1961
- Items
- 1
- Response format
- Single five-level arthroscopic grade
- Score range
- 0 to 4
- Direction
- Lower score = better
- Completion time
- About 1 minutes
- Age range
- Adults
- Regions
- Knee, General Health
- Conditions
- Chondral lesion, Chondromalacia patellae, Osteoarthritis, Cartilage defect
- Procedures
- Knee arthroscopy, Microfracture, Cartilage repair, Autologous chondrocyte implantation
- Observer agreement
- Interobserver kappa typically 0.5–0.7
- Validity
- Correlates with symptom severity and with subsequent progression to osteoarthritis, and remains the most widely reported chondral grading system in arthroscopic practice.
- Reliability
- Agreement is moderate. Grade I depends on probing and is frequently under-reported when the surface looks intact, and the grade II/III boundary requires measuring a diameter that is often estimated rather than measured. The ICRS classification is preferred in cartilage repair research because it grades by depth.
When to use the Outerbridge
Well suited to
- Describing chondral damage found at knee arthroscopy, where it is the most widely understood grading system.
- Operative documentation and communication between surgeons.
Limitations and poor fits
- Cartilage repair research and trials, where the ICRS system grades by depth and is the preferred standard.
- MRI-based cartilage assessment, for which MRI-specific grading systems exist and where the 1.3 cm diameter criterion does not transfer cleanly.
- As a sole indication for cartilage surgery — lesion size, location, depth, containment and patient factors all matter.
Outerbridge compared with other instruments
Instruments measuring a related construct, with the practical difference between them.
| Instrument | Items | Range | What it adds |
|---|---|---|---|
| Kellgren-Lawrence | 1 | 0–4 | The Kellgren-Lawrence classification grades radiographic osteoarthritis from 0 to 4 based on osteophyte formation, joint space narrowing, subchondral sclerosis and bone contour deformity, with grade 2 conventionally taken as the threshold for definite radiographic osteoarthritis. |
| KOOS | 42 | 0–100 | The KOOS is a 42-item patient-reported questionnaire that scores knee function as five separate subscales — Symptoms, Pain, Activities of Daily Living, Sport and Recreation, and Quality of Life — each from 0 (extreme problems) to 100 (no problems). |
Copyright and permitted use
Published openly because it is the question every clinician and researcher asks before using an instrument, and almost nobody answers it.
A published classification system. The grade definitions are freely reproducible and no licence is required.
References
- Outerbridge RE. The etiology of chondromalacia patellae. Journal of Bone and Joint Surgery (British). 1961.PMID 14038135
- Dwyer T, Martin CR, Kendra R, et al.. Reliability and validity of the arthroscopic International Cartilage Repair Society classification system. American Journal of Sports Medicine. 2017.PMID 28060521
Editorial status
This page has not yet been signed off by a named clinical reviewer. The scoring logic is stated in full above so it can be independently verified, and every threshold is traceable to its source. See our editorial methodology.