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What is Articular Cartilage

What is Articular Cartilage

Articular Cartilage can be the cause of knee problems. What is the articular cartilage, and what treatments are available to relieve pain and other symptoms of damaged articular cartilage.

When it comes to knee problems, damage to the tendons (tendinitis), ligaments or the meniscus cartilage (the C-shaped wedge of cartilage that act as the knee’s shock absorbers) account for many cases, however there is another form of cartilage that can also be the cause of knee problems – the articular cartilage.

Articular cartilage is found in all moving joints in the body (synovial joints). Its primary role is to protect the bones by keeping the surfaces apart from each other, absorbing shock and enabling smooth articulation.

What Is Articular Cartilage

Articular cartilage does this by providing a protective, wear resistant surface to the end of the moving bones, this particular type of cartilage is called hyaline cartilage (derived from the Greek word hyalos, meaning glass).

Hyaline cartilage is a hard, white shiny material with a unique structure that has one of the lowest coefficients of friction known for any surface to surface

The amount of hyaline cartilage on the knee joint varies between individuals but is generally between 2 and 4mm in depth.

In the knee articular cartilage covers the ends of the femur (medial femoral condyle, lateral femoral condyle and the trochlea), the top of the tibia (tibial plateau) and the back of the patella.

How Can You Damage Articular Cartilage?

Damaged Articular Cartilage

Articular cartilage injuries are very common; there are several ways in which your articular cartilage can become damaged.

Sudden direct blow to the cartilage
– Damage to the articular cartilage can occur from high-energy impact, perhaps from a fall, or a direct hit from a moving object.

Degenerative Joint Disease or DJD – This refers to general wear and tear on the cartilage, this is more common in people with malalignment or instability in the joints, and/or those who are overweight.

Post Traumatic – If you have had a previous injury to your ligaments and/or menisci then you are at greater risk of articular cartilage damage. Even if the original injury has been successfully repaired, the cartilage is more prone to damage due to the altered mechanics of the joint.

Immobilization – The articular cartilage is nourished by joint movement so immobilization for long periods of time can also result in damage.

Osteochondritis Dissecans – This is a condition that is thought to occur due to poor blood supply to the underlying bone resulting in a piece of cartilage and subchondral bone separating from the articular surface.

A common misconception is that articular cartilage damage and osteoarthritis only affects people over 50. Although there is a greater incidence of articular cartilage problems in this age group, young people often damage their articular cartilage too, especially in sports or via osteochondritis dissecans.

It is possible to just damage the articular cartilage on its own, but in most cases, injuries to the articular cartilage are in conjunction with damage to other tissues in the knee.

Symptoms of Damaged Articular Cartilage

This condition can be particularly difficult to diagnose. At present there isn’t a particular test that can reliably diagnose an articular cartilage injury; however the history of your problem can offer many clues.

Where and how the injury occurred, which area is painful, what makes the pain worse, these are useful questions when diagnosing articular cartilage damage.

Symptoms vary between individuals however they often mimic other knee injuries. It is possible that you may experience;

  • Slight to moderate pain in the knee
  • Slight or moderate swelling of the knee joint
  • If a piece of articular cartilage has broken off it may remain as a loose body in the joint. In this instance you may experience catching, locking or giving way
  • Because these symptoms are consistent with other knee injuries, diagnosis usually comes after other problems have been ruled out e.g.

    “well if it isn’t your meniscus or ligaments, what else could it be, perhaps we should look at the articular cartilage?”

    Unfortunately a plain X-ray is not usually able to diagnose articular cartilage damage. Because X-rays only show bone injury, articular cartilage injuries can not be found unless there is some bony malalignment or bony damage.

    MRI scans are increasingly being used to help to diagnose articular cartilage injuries and new cartilage specific protocols are emerging that are becoming very valuable in the evaluation of articular cartilage. However, MRIs are expensive tools and in some countries, waiting lists for these machines are long.

    Currently the standard way to diagnose articular cartilage problems is using a minimally invasive surgical procedure called arthroscopy. The procedure is performed by inserting an arthroscope (a type of endoscope – small surgical probe with a tiny camera) into the joint through a small incision. Another incision is then made; through which other surgical instruments are inserted to operate on the damaged areas.

    What Happens When You Damage Your Articular Cartilage?

    Degeneration of Articular Cartilage

    In most tissues it is blood that delivers the essential nutrients for tissue regeneration. Articular cartilage doesn’t have a blood supply (it is avascular) and therefore it has an extremely limited capacity for self-repair. In general, partial thickness defects do not heal by themselves and can often get worse over time.

    When the articular cartilage is damaged it can begin to expose the sensitive nerve endings in the subchondral bone. Minor damage to the cartilage (grade 1 or 2) may only result in slight pain as the bone is still covered.

    A recent study found that an over whelming amount of patients who had undergone ligament surgery, also had articular cartilage damage that they had previously been unaware of.

    If, on the other hand, the damage happens to go all the way down to the bone (grade 4), the nerve endings become exposed and the pain can be excruciating.

    In this instance, when a chondral defect goes all the way down to the bone, the blood supply to the subchondral bone begins to heal this defect. This results in the formation of scar tissue made up of a type of cartilage called fibrocartilage.

    Although fibrocartilage is able to fix the defect, its structure and properties are significantly different to that of hyaline cartilage. Fibrocartilage is denser and can not stand up to the demands of everyday activities therefore, it has a higher risk of breaking down.

    Articular Cartilage Repair Procedures

    In the past there have been many attempts to develop clinically useful procedures to repair damaged articular cartilage. To date, there are a number of articular cartilage repair procedures in general use and there are also a lot more in development. The selection of a procedure can be quite confusing as the different procedures often overlap, this means there may be more than one option available to you.

    Patients tend to opt for articular cartilage repair for four main reasons, to provide pain relief, improve function, slow the progression of damage and/or delay joint replacement surgery.

    The type of procedure you choose must relate to your particular needs. Important things to take into consideration include your age, activity level (including work, lifestyle and sports), size and location of your defect(s), how long you’ve had your symptoms plus any previous surgery you may have had.

    Before the procedure your surgeon will ask you to sign a form consenting to the operation and you must be willing and able to commit to and complete the rehabilitation programme.

    Articular cartilage repair procedures can be broadly categorized into four groups as follows:

  • Group 1:Arthroscopic lavage and debridement
  • Group 2:Marrow stimulating techniques
  • Group 3:Osteochondral autografts and allografts
  • Group 4:Cell-based repairs including autologous chondrocyte implantation
  • Arthroscopic Lavage and Debridement

    These two procedures use the minimally invasive technique, arthroscopic surgery.

    Lavage refers to the washing out of the knee joint and debridement refers to the removal of any unstable or degenerative articular cartilage flaps.

    The treatments are often used to reduce pain and irritation but benefits may are usually short term. Both techniques should not be viewed as procedures to repair the joint, simply because they do not actually repair but rather wash and clean the joint.

    Patients with smaller defects in the articular cartilage are more likely to benefit from these procedures.

    Marrow Stimulation Techniques

    Marrow stimulation techniques have been around for almost half a century. The techniques that fall under marrow stimulation are all based on two key steps.

    The removal of damaged cartilage to expose the underlying bone.

    Penetration of the subchondral bone to expose the blood vessels and generate a blood clot within the defect.

    Microfracture techniques were introduced around 20 years ago and involve using a small bone pick called an awl, to punch into the subchondral bone.

    Microfracture is frequently used as a first line treatment for articular cartilage defects in younger patients 30-40, who have smaller (less than 2cm2) isolated chondral defects.

    The reason for this is that microfracture is generally an arthroscopic procedure and if the microfracture doesn’t work it is possible to move onto another, usually more invasive, cartilage repair technique.

    Studies have shown that microfracture techniques may not fill the chondral defect fully, leaving fibrocartilage to heal in its place. Even still microfracture repairs can last 1 -2 years before symptoms begin to return.

    Osteochondral Autografts and Allografts

    Osteochondral Autografts and Allografts

    Sometimes referred to as OATS ‘osteoarticular transfer system’, this procedure involves taking Cylindrical plugs of articular cartilage from a donor site on the knee and then inserting these plugs into matching holes drilled in the chondral defect. Once the plugs then comprise of hyaline cartilage and the small spaces between the plugs and the donor site fill in with fibrocartilage.

    In effect OATS is like ‘robbing peter to pay paul’ and consequently the procedure is often limited to chondral defects less than 4cm2 in size.

    Autograft refers to donor cartilage which comes from your own knee, Allografts refer to those that came from ‘someone else’s’. Allografts taken from a deceased donor present one advantage, larger areas of osteochondral tissue can be taken. However allografts can be hard to come by in most countries.

    Osteochindral autografts and allografts can be completed during a single session, either by an arthroscopic procedure, or via a small arthrotomy (open incision) if required.

    Autologous Chondrocyte Implantation

    Autologous Chondrocyte Implantation ACI ACT

    Autologous Chondrocyte Implantation basically means ‘to get cartridge cells (chondrocyte) from yourself (autologous)’ and is sometimes referred to as Autologous Chondrocyte Transplantation.

    In the past articular cartilage repair procedures have tended to generate fibrocartilage repair tissue or a combination of hyaline and fibrocartilage repair tissue. In the 1980’s the autologous chondrocyte implantation/transplantation or ACI/ACT was introduced. This procedure enabled healing with a hyaline repair tissue and over the last 20 years has become more widespread and well researched.

    At present most ACI procedures are in two stages:

    • Stage 1: The first stage involves an arthroscopic assessment of the chondral defect. If the defect is suitable for ACI then a small sample of cartilage is taken from a non-weight bearing area of the femur. This sample is sent to a specialist laboratory where the cartilage cells (chondrocytes) are cultivated and encouraged to multiply into the millions. This usually takes between 3 and 6 weeks.
    • Stage 2: The second stage involves trimming back the chondral defect to a point where all surrounding edges are healthy cartilage and the underlying bone is exposed. The cells are then reintroduced back into the defect. Unlike the first stage, this stage is not arthroscopic – it is via an open arthrotomy.

    The Importance of Rehabilitation

    Rehabilitation following any articular cartilage repair procedure is long and demanding. There are many variations in rehabilitation protocols which differ from patients to patient. Your doctor will discuss in-depth the guidelines you must follow, be sure to follow these guidelines as any lapse in your recovery program may result in longer recovery time.

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