Chondral loss in the knee means that the layer of cartilage covering the ends of your bones inside the knee joint has worn away or been damaged. This cartilage, called articular cartilage, is a smooth, slippery tissue only 2 to 4 mm thick. Its job is to let the bones glide against each other with almost no friction and to absorb the force of every step, squat, and jump. When that layer thins or disappears, the joint loses its cushion, and you start to feel it.
If you’re reading this, you probably saw the phrase on an MRI report. Here’s what it actually means for your knee and what typically comes next.
How Cartilage Loss Is Graded
Doctors use a scale called the Outerbridge classification to describe how far the damage has progressed. It runs from Grade 0 (normal cartilage) to Grade IV (bone fully exposed), and where you fall on it shapes your treatment options.
- Grade I: The cartilage feels soft and swollen but looks intact on the surface. This is often only detectable when a surgeon probes it during a scope procedure.
- Grade II: Shallow cracks or fissures appear, but they don’t go deeper than halfway through the cartilage and span less than half an inch across.
- Grade III: The cracks extend deeper and wider than half an inch, reaching down to the bone underneath.
- Grade IV: The cartilage has worn completely through, leaving the underlying bone exposed inside the joint.
Your MRI report may describe chondral loss in slightly different language, such as “partial-thickness cartilage defect” or “full-thickness chondral defect.” Partial thickness generally corresponds to Grades II or III, while full thickness means Grade IV.
What Causes It
Chondral loss happens through two main pathways: a single traumatic event or gradual wear over time. In younger, active people, the damage often traces back to a specific injury. ACL tears and meniscus injuries are the most common triggers, and sustaining a major knee injury leads to cartilage breakdown in 25 to 50 percent of patients over the following years. The initial impact can kill cartilage cells directly in the areas that absorb the greatest force, and the inflammation that follows continues to degrade the tissue for weeks or months afterward.
The second pathway is slower and more common overall. Healthy joints can handle repetitive loading from normal daily activities without breaking down. But when loading becomes abnormal, whether from excess body weight, joint instability after an old injury, or repetitive occupational stress like frequent deep knee bending, the cartilage’s ability to repair itself falls behind the rate of damage. The collagen network deteriorates, the tissue swells with excess water, and it gradually thins. This is the process that eventually becomes osteoarthritis.
What It Feels Like
Early chondral loss can be surprisingly quiet. Many people have cartilage thinning visible on MRI without noticing much at all. As the damage progresses, the most common symptom is pain tied to activity, particularly loading activities like stairs, squats, or running. You might also notice swelling after activity that wasn’t there before, or a limp that develops toward the end of the day.
Deeper defects can produce mechanical symptoms. A catching or locking sensation happens when a loose flap of cartilage or scar tissue interferes with smooth joint motion. You may feel grinding (crepitus) when bending the knee, especially if the damage sits behind the kneecap. Lesions on the kneecap or in the groove it slides through tend to cause pain during activities that load the front of the knee: going downstairs, sitting for long periods, or getting out of a chair.
On examination, tenderness is often localized. If the damage is on the main weight-bearing surface of the thighbone, pressing just above the joint line reproduces the pain. Kneecap-related damage shows up as tenderness along the sides of the kneecap or pain when the kneecap is compressed and shifted.
How It’s Diagnosed
MRI is the primary tool for detecting chondral loss without surgery. Standard MRI can pick up moderate and severe cartilage defects, but early-stage thinning is harder to catch. Newer sequences performed on high-strength 3-Tesla MRI machines improve sensitivity. T2 mapping, for example, can detect changes in the cartilage’s water content and collagen structure before visible thinning appears on standard images. Three-dimensional sequences like 3D-DESS allow the cartilage to be reconstructed in multiple planes, giving a detailed view of defect size and depth.
X-rays are less useful for early detection because they only show bone. By the time cartilage loss appears on an X-ray as narrowed joint space, the damage is already advanced.
What Happens if It’s Left Alone
Cartilage has very limited ability to heal on its own because it has no blood supply. Research tracking patients with untreated focal cartilage defects shows they are more likely to develop additional cartilage damage in the same compartment over time. One study found that knees with a full-thickness defect on the main weight-bearing surface had 2.7 times the odds of developing cartilage damage elsewhere in that compartment. Defects behind the kneecap carried even higher risk, with 5.8 times the odds of spreading damage.
Obesity and repetitive knee bending both accelerate progression. One study found that frequent knee bending increased the odds of worsening cartilage damage by roughly four times over three years. Obese patients showed a statistically significant increase in the number of affected areas over the same period. That said, studies tracking patients for two years after diagnosis did not find that focal defects alone reliably produced the kind of widespread joint changes seen on X-ray as full osteoarthritis. The progression is real, but it’s typically slow.
Non-Surgical Treatment Options
For mild to moderate chondral loss, or for patients who aren’t candidates for surgery, management focuses on reducing symptoms and slowing progression. Strengthening the muscles around the knee, particularly the quadriceps, helps stabilize the joint and distribute load more evenly across the cartilage surface. Low-impact exercise like cycling and swimming maintains fitness without the repetitive pounding of running.
Joint injections are widely used, though evidence for any single type remains mixed. An international consensus panel found that hyaluronic acid, platelet-rich plasma (PRP), corticosteroids, and stem cell-based injections all may play a role, but experts could not reach full agreement on which works best. In practice, corticosteroid injections provide short-term pain relief, hyaluronic acid aims to improve joint lubrication, and PRP is thought to promote a healing response. Weight loss, when applicable, is one of the most consistently effective interventions for reducing knee cartilage stress.
Surgical Repair Options
When a cartilage defect is well-defined, symptomatic, and not responding to conservative care, several surgical techniques can fill or resurface the damaged area.
Microfracture is the simplest approach. The surgeon pokes tiny holes in the exposed bone to trigger bleeding. The resulting blood clot contains stem cells that can form a replacement surface. The catch is that this new tissue is fibrocartilage, a tougher but less smooth and durable material than the original. Microfracture works best for smaller defects.
Osteochondral autograft transplantation (OATS) involves harvesting small plugs of healthy cartilage and bone from a low-stress area of the knee and pressing them into the defect. The plugs are typically about 15 mm long and come from areas like the upper outer edge of the groove between the thighbone’s condyles. This is a single-surgery procedure, and the transplanted cartilage is true hyaline cartilage, not the fibrocartilage produced by microfracture.
Autologous chondrocyte implantation (ACI) and its newer form, MACI, use your own cartilage cells. In the first stage, a small sample of cartilage is harvested and the cells are grown in a lab over several weeks. In the second stage, those expanded cells are implanted into the defect. The advantage is the potential to regenerate a surface closer to native cartilage. The tradeoff is two procedures and a longer recovery.
Recovery After Cartilage Surgery
Recovery from cartilage repair is notably slower than from many other knee surgeries because the new tissue needs time to mature under protected conditions. Most protocols start partial weight-bearing within one to four weeks after surgery, typically limiting you to about 20 percent of your body weight on crutches. Full weight-bearing generally comes between six and twelve weeks, depending on the procedure. MACI protocols tend toward the slower end, with one common protocol progressing from 20 percent of body weight at two weeks to 60 to 80 percent at six to seven weeks, reaching full weight-bearing around 10 to 12 weeks.
Return to activity follows a staggered timeline. Low-impact, non-contact sports like cycling or swimming are typically cleared around four months. Full-contact or high-impact sports can take 12 to 24 months. The long timeline reflects how slowly cartilage matures. Rushing the process risks damaging the repair before it has developed enough strength to handle real-world loading.