One thing I often explain to patients is that a cartilage injury is never only about the cartilage. This is because articular cartilage itself is aneural, meaning it has no nerve supply of its own. Even so, a defect can leave the knee painful, swollen, and weak, because the symptoms reflect the response of the whole osteochondral and synovial unit, particularly the highly innervated subchondral bone, synovium, and joint sensory pathways.
The bone is affected too, because cartilage, calcified cartilage, and subchondral bone function together as an osteochondral unit, which means damage to one layer alters the mechanical and biological environment of the others. Its position beneath the cartilage is also why I sometimes describe the bone as being a little like teeth. Teeth look solid, but anyone who’s had an exposed dental root knows how extraordinarily sensitive the deeper tissue can be. The analogy isn’t literal anatomy, but it helps patients understand why an apparently small osteochondral lesion can be very symptomatic.
How the Knee Responds to a Cartilage Injury
To explain how these symptoms come about in practical terms, I think of the irritated knee as doing three things, with each one building on the last.
1. Swelling: The Knee Calls for Help
Once the joint surface is damaged, the knee can’t simply regenerate normal mature articular cartilage on demand. Instead, the injury can trigger synovial inflammation and increased joint fluid, producing an effusion that patients might notice as swelling.
In simpler terms, I tell patients, “The knee cannot grow the cartilage back for you, but it reacts to the injury and tries to protect itself”. This is why I sometimes describe the extra fluid as the knee trying to “cushion the blow”. It’s a teaching metaphor rather than a proven biomechanical function. Medically, effusion is best understood as part of the inflammatory and synovial response to joint injury.
2. Pain: The Knee Sends a Signal
Alongside the swelling, the knee can send a pain signal, even though the cartilage surface itself doesn’t contain pain fibres. The tissues around and beneath it do, so synovitis, subchondral pathology, and bone marrow lesions are all recognised contributors to knee pain. Because synovitis is inflammation of the joint lining, the same response that produces an effusion can also add to the pain.
Beneath the cartilage, the subchondral bone is richly innervated, which is why I used the dental analogy earlier. Once the protective surface is lost and the deeper, sensitive layer is repeatedly loaded, the pain can be striking, rather like touching an exposed tooth root.
3. Weakness: The Knee Tells the Muscles to Back Off
With the knee swollen and painful, patients often tell me, “My leg suddenly feels weak”. This weakness might not simply come from loss of bulk. The irritated joint might also prompt the nervous system to hold the muscle back, preventing normal voluntary activation.
This can happen when pain and even relatively modest joint effusion alter sensory input from the knee, reflexively reducing activation of the quadriceps at the front of the thigh. The medical term for this is arthrogenic muscle inhibition (AMI). Once the muscle is inhibited, control and loading of the knee become poorer, which can irritate the joint further.
Because the weakness stems from the joint as well as the muscle, rehabilitation isn’t simply about strengthening the quadriceps. We often first need to settle the joint, reduce effusion and pain, and restore activation, before progressively rebuilding strength and load tolerance.
What Is the Logical Way to Treat a Cartilage Injury?
When it comes to treating the cartilage injury itself, the logic of cartilage restoration becomes much clearer once you think of cartilage and subchondral bone as one osteochondral unit. Within this unit, I describe the cartilage layer to patients as the bone’s shield. This shield isn’t simply a covering, as articular cartilage is part of a highly specialised load-bearing interface that distributes force and protects the underlying osteochondral unit. When this surface is lost, one aim of cartilage restoration is to reconstruct a durable biological interface over the defect, so the bone can, metaphorically, say, “Finally, I have my shield back”.
Scientifically, this means re-establishing the structure and function of the osteochondral unit, where cartilage, calcified cartilage, and subchondral bone work together. Scaffold-based cartilage restoration is built on this principle, with the defect carefully prepared and a biological scaffold used to support formation and maturation of repair tissue across the damaged articular surface.
Importantly, the term AMIC (autologous matrix-induced chondrogenesis) has traditionally described a one-stage scaffold procedure performed together with marrow stimulation. Contemporary cartilage surgeons though, might use modified scaffold-based strategies with different approaches to the subchondral plate, the thin layer of bone just beneath the cartilage. With this range of approaches, cartilage restoration can and should be bespoke, as each defect is different. This is also why the precise operative technique should always be stated rather than assuming every procedure labelled “AMIC” is identical.
What About Microfracture?
Alongside AMIC, patients might also hear about microfracture, whether through their own reading or as an option raised in an earlier consultation. As one form of marrow stimulation, microfracture deliberately perforates the subchondral plate so that marrow elements enter the defect and form repair tissue. The technique has an established role in selected indications, particularly smaller lesions, but it also intentionally alters the subchondral compartment.
Since I’ve already compared the underlying bone to a sensitive tooth root, my patient analogy here is simple: I ask whether we should drill into this surface unless there’s a good reason to do so. It’s a comparison though, not a claim that microfracture itself is proven to cause greater pain. Rather, my principle is to respect and preserve the subchondral bone whenever the biological objective can be achieved without unnecessary violation of it.
Beyond the analogy, experimental and clinical literature shows that marrow-stimulation procedures can produce structural changes in the subchondral bone. These findings are one reason I view the bone as an active part of knee cartilage surgery rather than merely the floor beneath a defect. Ultimately, cartilage restoration isn’t simply about filling a hole. It’s about restoring the protective osteochondral interface and creating the best biological and mechanical environment for the knee to function again.
Note: The analogies in this article are intended to help patients understand complex joint biology. They are not literal descriptions of tissue physiology and do not replace individual clinical assessment.
Assessing Knee Swelling and Weakness Before Treatment
When swelling keeps returning, pain is felt deep within the knee, or the leg suddenly feels weak, a specialist assessment can help identify what’s causing the symptoms and whether the cartilage surface is affected. Because cartilage and subchondral bone function together as one unit, any defect should also be assessed alongside the condition of the bone beneath it and the joint around it, particularly when planning rehabilitation and treatment, including the precise technique if surgery is considered. At Oxford Cartilage & Sports Centre, symptoms, examination findings, imaging, and personal goals are reviewed as a whole before the best options are recommended for each patient. To arrange a consultation with Dr Francis Wong, please contact the clinic to book an appointment.
References:
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2. Rice DA, et al. Quadriceps arthrogenic muscle inhibition: the effects of experimental knee joint effusion on motor cortex excitability. Arthritis Res Ther. 2014. PMID: 25497133.
3. Fan X, et al. Macro, micro, and molecular changes of the osteochondral junction in osteoarthritis. 2021. PMID: 34041240.
4. Choe R, et al. Biomechanical aspects of osteochondral regeneration. 2022. PMID: 34409874.
5. Lee YHD, Suzer F, Thermann H. Autologous Matrix-Induced Chondrogenesis in the knee: a review. Cartilage. 2014. PMID: 26069694.
6. Beck A, et al. Treatment of articular cartilage defects with microfracture and AMIC causes changes in the subchondral bone. Am J Sports Med. 2016. PMID: 27436718.
