Knee Cartilage Injury
What Is a Knee Cartilage Injury?
A knee cartilage injury is damage, softening, blistering or detachment of the articular cartilage that covers the joint surfaces of the thigh bone (femur), the shin bone (tibia) and the kneecap (patella). Articular cartilage is a smooth, slippery and resilient tissue: it spreads load, reduces friction to almost nothing and protects the bone underneath. Once this tissue is damaged, its capacity to repair itself is limited, which is why even a small lesion can lead over time to pain, swelling and mechanical symptoms.
Why Does Cartilage Not Heal on Its Own?
Articular cartilage has no blood vessels, nerves or lymphatic drainage; it is nourished by diffusion from the joint fluid. Because there is no blood supply, the cells and growth factors that normally start the healing response cannot easily reach the injured area. When the damage involves only the surface of the cartilage, the healing response is very weak. When it extends down into the bone beneath (the subchondral bone), cells from the bone marrow do form a repair tissue — but that tissue is usually fibrocartilage, which is mechanically less durable than the original hyaline cartilage.
What Causes It? Causes and Risk Factors
Cartilage damage arises in two main ways. The first is a single traumatic event: a direct blow to the knee, a twisting injury, a kneecap dislocation, or the sudden loading that accompanies a ligament or meniscus tear. The second is overload accumulated over years. The main factors that increase the risk are:
- A previous ligament or meniscus injury, especially untreated instability
- Malalignment of the leg (varus or valgus)
- A kneecap that is prone to slipping out of its groove
- Excess body weight and the increased load it places on the knee
- High-impact sports involving pivoting and jumping
In addition, conditions such as osteochondritis dissecans, in which the blood supply beneath the joint surface is disturbed, can cause a fragment of cartilage and bone to separate in younger patients. Because malalignment of the leg concentrates load on one compartment, assessing alignment is an inseparable part of the treatment plan.
How Do the Symptoms Appear?
Because cartilage itself contains no nerve endings, symptoms can be very mild in the early stage. They usually become clearer as irritation within the joint and mechanical mismatch increase:
- Deep knee pain that worsens with weight-bearing, on stairs or when squatting
- Recurrent swelling after activity, from fluid collecting inside the joint
- Grinding, crunching or a sense of catching
- Sudden locking or giving way when a loose cartilage fragment is present
- Reduced range of motion and stiffness after sitting for a long time
How Is Cartilage Damage Graded?
Cartilage damage is graded according to its depth within the joint surface. In the mildest grade the cartilage has softened but remains intact. Intermediate grades show fraying of the surface and fissures that extend either less than or more than halfway through the cartilage. In the most advanced grade the damage passes through the full thickness of the cartilage and exposes the bone beneath. This grading is one of the key pieces of information guiding treatment, but it is not sufficient on its own. The size and location of the lesion, the patient's age and activity level, and any accompanying ligament, meniscus or alignment problems all shape the decision.
The Difference Between Cartilage Injury and Osteoarthritis
An isolated cartilage injury is a focal lesion: damage with defined edges affecting a limited area within an otherwise healthy joint. Knee osteoarthritis, by contrast, is widespread wear involving a large part of the joint surface — often both opposing surfaces — and it progresses together with changes in the bone. This distinction matters because it changes the direction of treatment entirely: cartilage repair and transplantation methods are considered for focal lesions, whereas they are not a suitable option in widespread osteoarthritis.
What Can It Be Confused With? (Differential Diagnosis)
Knee pain and swelling have many causes. A meniscus tear can produce similar catching and locking; ligament injuries can be confused with it because of the sense of giving way. Pain around the kneecap may come from patellofemoral overload without any cartilage damage at all. An increase in joint fluid also occurs in inflammatory joint disease. For this reason the assessment is not limited to the cartilage; the knee joint is examined as a whole.
Diagnosis: Examination and Imaging
Diagnosis begins with a detailed history and physical examination. The doctor asks which movements worsen the symptoms, how often the knee swells and whether it has ever locked, then assesses joint-line tenderness, range of motion, the tracking of the kneecap and ligament stability. Weight-bearing X-rays show the joint space, bony changes and the alignment of the leg; they do not show cartilage directly but are critical for distinguishing widespread osteoarthritis. Magnetic resonance imaging (MRI) reveals the location and depth of the cartilage lesion, any bone marrow oedema beneath it and accompanying meniscus or ligament problems. In some cases the definitive assessment is made by viewing the joint surface directly during arthroscopy.
Non-Surgical Treatment
A significant proportion of cartilage injuries, particularly small and stable lesions, can be managed without surgery. The aim is to reduce pain, balance the load passing through the joint and strengthen the protective role of the muscles around the knee. The early phase focuses on activity modification, pain management and control of swelling. A graded strengthening programme then targets the muscles at the front and back of the thigh and around the hip, with range-of-motion and balance work added alongside. Weight loss is one of the most effective parts of treatment, because it directly reduces the load carried by the knee.
Injections and Biological Treatments
Injections into the joint can give selected patients a temporary improvement in pain and function. A corticosteroid injection provides short-term relief during an inflammatory flare, but repeating it frequently is not appropriate. Biological applications such as platelet-rich plasma (PRP) and stem cell-based methods are still being studied; there is no proof that these methods rebuild cartilage tissue, and they are not part of routine care. The decision to inject should be individual, taking into account the type of lesion and the patient's expectations.
When Is Surgery Needed?
Surgery comes into consideration for full-thickness lesions of significant size that do not respond to non-surgical treatment or that cause locking or giving way. A loose fragment of cartilage and bone may need to be removed or fixed back into place. Repair and transplantation methods — bone marrow stimulation, osteochondral autograft or allograft, and cell-based techniques — are selected according to the size of the lesion and the characteristics of the patient. Lasting results also require correcting the accompanying problems: if ligament instability, meniscus loss or malalignment are left untreated, the repaired cartilage is exposed to excessive load again. Where malalignment is marked, redistributing the load with an osteotomy may be considered.
Recovery and Return to Sport
Recovery varies markedly with the method used and the size of the lesion. In cases managed without surgery, symptoms may settle within weeks; after cartilage repair, the repair tissue takes months to mature and weight-bearing is increased gradually during that period. Regaining range of motion early is important, because movement supports the circulation of joint fluid that nourishes the cartilage. Return to sport should be planned around criteria — pain-free range of motion, muscle strength balanced with the other leg and passing functional tests — rather than around a fixed calendar. Returning early and without supervision can damage the repair tissue.
Prevention and When Should You See a Doctor?
The most effective ways to protect cartilage health are maintaining a healthy weight, strengthening the thigh and hip muscles regularly, increasing training load gradually, and treating ligament and meniscus injuries promptly. Knee pain that does not settle within a few weeks, recurrent swelling, and locking, catching or giving way should be assessed. Sudden widespread swelling after an injury, an inability to bear weight on the knee, or movement that is mechanically blocked all require prompt medical attention. You can find detailed information on other knee problems and contact Assoc. Prof. Serkan Sürücü for an assessment.