Kellgren-Lawrence Grading System: Understanding X-Ray Stages of Knee Arthritis
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By Dr. Jeffrey Peng, MD · Published March 5, 2026 · 6 min read
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If you have been told you have knee arthritis, your doctor likely used an X-ray to assess the severity of your condition. The most widely used classification tool for this purpose is the Kellgren-Lawrence (KL) grading system, which rates knee osteoarthritis on a scale from 0 to 4. A grade of 0 indicates no signs of osteoarthritis, while a grade of 4 represents severe, bone-on-bone arthritis.
Understanding where you fall on this scale can help you have more informed conversations with your physician about treatment options. However, as I will explain, the grade on your X-ray does not always predict the severity of your symptoms.
What Is the Kellgren-Lawrence Grading System?
The Kellgren-Lawrence (KL) system is a radiographic classification used by physicians worldwide to assess the severity of osteoarthritis. Developed in the 1950s, it evaluates X-ray images for four key features: joint space narrowing, osteophyte (bone spur) formation, sclerosis (increased bone density), and deformity of the bone contours. Each knee is graded from 0 to 4 based on the presence and severity of these findings.
KL Grade 0: No Osteoarthritis
A KL grade 0 X-ray shows no radiographic signs of osteoarthritis. The joint spaces between the medial compartment, lateral compartment, and patellofemoral compartment are all well preserved. There are no osteophytes and no evidence of bony sclerosis. This is what a healthy knee looks like on imaging.
KL Grade 1: Doubtful Osteoarthritis
Grade 1 suggests doubtful joint space narrowing with possible small osteophytes. At this stage, the edges of the bone may appear slightly pointed — these are early bone spurs caused by increased mechanical stress and the beginning of cartilage breakdown. The joint space may still appear preserved on X-ray, but the underlying cartilage has already started to weaken.
KL Grade 2: Mild Osteoarthritis
At grade 2, X-rays show definite osteophytes and early joint space narrowing. Grades 1 and 2 are collectively classified as mild osteoarthritis. Even at this early stage, the biochemical environment of the knee is deteriorating. Inflammatory markers, toxic enzymes, and destructive proteins are being released into the joint, progressively weakening the remaining healthy cartilage.
This is an important window for intervention. Most clinical studies examining osteoarthritis treatments focus on grades 1 and 2 because this is the optimal time to slow disease progression. Treatments such as platelet-rich plasma (PRP) injections, exercise therapy, and weight management have the greatest potential for benefit when the disease is still in its early stages.
KL Grade 3: Moderate Osteoarthritis
As osteoarthritis progresses to grade 3, the cartilage between the bones erodes more substantially. X-rays demonstrate multiple osteophytes and more pronounced joint space narrowing. Sclerosis of the underlying bone becomes apparent, and early deformity of the bone contours may be visible. This stage represents moderate disease and is often when patients first seek treatment due to increasing pain and functional limitations.
KL Grade 4: Severe (Bone-on-Bone) Osteoarthritis
Grade 4 represents severe osteoarthritis with very large osteophytes, significant or complete collapse of the joint space, marked sclerosis, and visible bone deformity. At this stage, treatment options are primarily palliative and focus on symptom control. If pain cannot be managed through non-surgical methods, a knee replacement surgery may ultimately be necessary.
How Many Knee Compartments Are Typically Affected?
When evaluating knee arthritis on X-ray, physicians assess each of the three compartments independently: the medial (inner) compartment, the lateral (outer) compartment, and the patellofemoral (kneecap) compartment. Arthritis does not necessarily affect all three compartments equally.
A systematic review and meta-analysis published in Osteoarthritis and Cartilage found that approximately 50% of people with knee osteoarthritis have disease in only one compartment, with the medial compartment being the most commonly affected. Around 33% have arthritis in two compartments, and only 17% have tricompartmental disease. This is significant because it means that three-quarters of patients do not have arthritis throughout the entire knee.
Do X-Ray Findings Predict Your Symptoms?
This is arguably the most important takeaway: X-ray severity does not reliably correlate with symptom severity. In my practice, I regularly see patients with moderate grade 3 arthritis who are still running half marathons. I also treat patients with grade 1 X-rays whose knees are so swollen they can barely walk.
The clinical implications of this disconnect are twofold. First, having mild arthritis on X-ray does not mean you should wait until your disease progresses to grade 3 or 4 before pursuing treatment. Early intervention is key to slowing progression and maintaining function. Second, having moderate or even moderately severe arthritis does not mean your only option is a knee replacement. Many patients with advanced radiographic changes continue to live active, fulfilling lives with the right combination of non-surgical treatments.
What matters most is treating the patient and their symptoms, not simply treating the X-ray. If you are experiencing knee pain regardless of your KL grade, I encourage you to schedule a consultation to discuss evidence-based treatment options tailored to your individual needs.
References
1. Stoddart JC, Dandridge O, Garner A, Cobb J, van Arkel RJ. The compartmental distribution of knee osteoarthritis - a systematic review and meta-analysis. Osteoarthritis and Cartilage. 2021;29(4):445-455. doi:10.1016/j.joca.2020.10.011
Medical Disclaimer: This content is for educational purposes only and does not substitute for the medical advice of a physician. Always consult your healthcare provider before beginning any new treatment program. The information presented reflects the opinion of Dr. Jeffrey Peng and does not represent the views of his employers or affiliated hospital systems.
