Introduction
Cartilage is a precious commodity. As William Hunter, an 18th-century anatomist and physician, observed, an ulcerated cartilage “admits of a cure with more difficulty than a carious bone; and that, when destroyed, it is never recovered.” This matters because our bodies generally heal well, especially when we are young. For example, when bones are broken, they are put back into place and then they heal and become strong again. Likewise, when skin is cut, it bleeds, and with stitches or a simple plaster, it heals well.
However, cartilage is different. When cartilage is damaged, even a small crack puts it at a disadvantage because it has little blood supply and few cells to respond. As a result, the body’s repair is slow and unreliable. Knee pain then appears with load and is often felt during squatting or running. In response, many people rest and stop their sports or physical activity for a time. Then they try to get back to exercise, only to feel the same knee pain again. Over repeated attempts, this can become a vicious cycle. Each episode might feel more severe until the pain seems constant. This is where I normally see my patients. By then, there is often a hole or defect in the cartilage, and the underlying bone might already be involved. Why is this so? To understand what is happening, the next section shows how cartilage injuries progress and why healing is limited.
Cartilage and Joint: A Poor Recipe for Healing
So, what is cartilage exactly? In simple terms, cartilage is a thin, smooth layer that wraps the articulating surfaces within a joint, with firm support from the bone beneath it. Its main purpose is straightforward: to keep the joint surfaces smooth so movement remains effortless and low in friction. Crucially, unlike many other tissues, this cartilage layer is different. It has no blood supply, no nerve supply, and relatively few cells. To make that difference concrete, let me draw a comparison to skin: when there is a cut, blood flows to the area and forms a clot; nerves signal pain, prompting protection; and abundant cells, including keratinocytes, fibroblasts, and macrophages, clear debris and rebuild tissue as the clot remodels into new skin.
Now in comparison to skin, a direct impact that creates a small crack in cartilage often does not hurt much at the start because the cartilage layer lacks a nerve supply, as outlined above. As a result, the joint is usually used as normal, so the crack is not protected to heal. In the same way, the absence of a local blood supply means no stable clot forms to support early remodelling. Compounded by the fact that there’s only a limited number of local cells available to clear debris and rebuild tissue, the crack tends to remain open rather than heal.
By contrast, the underlying bone that supports the cartilage follows a different pattern. Because it has a blood supply, a nerve supply, and active cells, it mounts a response when cartilage support is lost. As cartilage damage becomes significant, the underlying (subchondral) bone senses the increased load and transmits nociceptive signals that are perceived as pain. At the same time, the joint attempts to cushion the area by producing more synovial fluid, which results in swelling after activity. So yes, by the time pain occurs with exercise and swelling follows each session, the cartilage damage is already significant. In more severe injuries, both the cartilage and the subchondral bone might be injured, and blood can enter the joint from the bone. However, as the joint is filled with synovial fluid and is always moving, any blood clot that starts to form is easily dislodged or washed away, making effective healing difficult.
Cartilage Injury: A Precursor to Osteoarthritis
Building on the points above, cartilage injuries as described, when left untreated, tend not to heal. With ongoing activity, the defect worsens. This then leads to a few issues, such as loose body formation. Simply put, a damaged cartilage fragment can dislodge from bone and become a loose particle within the joint, causing clicking or brief jamming when you are trying to move. As it migrates, it then rubs against other cartilage surfaces, creating new cracks, and a vicious cycle ensues. With continued use, local cartilage defects progress to joint-wide involvement. And when loss of cartilage becomes generalised across the articular surface, the condition is diagnosed as osteoarthritis
What Should Be Done Then?
Given the progression described above, early detection and treatment are key. First, seek advice from an orthopaedic surgeon, especially one with in-depth knowledge of cartilage treatment and surgery. Next, an MRI scan is usually performed to assess the joint’s cartilage and its other soft tissues in detail. Then, based on these findings, aggressive early treatment is instituted to protect and stabilise the defect, aiming to heal small cracks where possible. Otherwise, when indicated, the cartilage is best repaired through surgical means.
Speak to Oxford Cartilage & Sports Centre (Oxford Orthopaedics)
Cartilage injuries are difficult to detect, especially in the early stages. This is why confirming the diagnosis before deciding on treatment is sensible, particularly if pain returns with activity or swelling follows exercise. In a focused consultation, Dr Francis Wong will first review MRI findings and then explain how the location and size of the cartilage defect, the condition of the subchondral bone, and relevant biomechanical factors fit together in the clinical picture. From there, treatment discussions can outline a plan to get back to safe activity by prioritising cartilage-preserving options that match knee function goals and realistic recovery timelines, guided by the best available evidence for the specific injury type and size. To speak with Oxford Cartilage & Sports Centre by Oxford Orthopaedics, please contact the clinic to book an appointment.
References:
- Ow ZGW, Ting KJE, Wong KL. Single-Stage Arthroscopic Cartilage Repair With Chondrectomy and Implantation of a Templated Membrane Collagen Scaffold With Bone Marrow Aspirate Concentrate Augmentation (AMIC Plus). Arthrosc Tech. 2023 Oct 30;12(11):e2085-e2091. doi: 10.1016/j.eats.2023.07.030. PMID: 38094947; PMCID: PMC10714417.
