Articular cartilage is difficult to diagnose. There isn't a particular test at present that can be carried out as part of a physical examination that is able to reliably diagnose an articular cartilage injury in the same way as you can, for instance, for a ligament injury.
The history of your problem can provide many clues. Was there a specific incident when the injury occurred or has it developed over time? When and where are you getting your pain, what makes your pain worse and what eases it, is the problem getting better or worse - this type of information is useful guidance. There are a variety of symptoms that you may experience but they vary considerably between individuals and can often mimic other common knee injuries. You may or may not have pain and you could have no, or limited, swelling. Yes you may experience locking if a piece of articular cartilage has broken off and is a loose body in the joint or you may experience catching or giving way. You are likely to have some muscle wasting and difficulty in activities such as going up and down stairs, walking or running but then people with any moderate knee injury are likely to as well. This is why articular cartilage damage is the 'cinderella' of knee problems. It tends to be diagnosed only after other structures have been ruled out - 'well if it isn't your meniscus or ligaments, what else could it be, perhaps we should look at the articular cartilage?'
Plain X-rays aren't very helpful in diagnosing articular cartilage defects, especially early stage ones, as they only show bony injury. Articular cartilage injuries are often not seen on x-ray unless there is some bony malalignment and/or bony damage.
MRI scans are increasingly being used to help to diagnose articular cartilage and new cartilage specific protocols are emerging that are becoming very valuable in the evaluation of articular cartilage. However, MRIs are expensive tools and articular cartilage scans need longer imaging time which has cost implications. In addition waiting times for MRI scans can also be lengthy in some countries.
At present the 'gold standard' for diagnosing articular cartilage problems is to take a look directly inside your knee with a small camera in a 'keyhole' procedure called an arthroscopy. Often articular cartilage damage is only identified after an MRI scan or when your surgeon takes a look inside your knee joint with an arthroscope. This can be a real problem due to the length of time it takes to identify that the articular cartilage has been damaged and the implications of this delay.
If you have an MRI scan or an arthroscopy and it shows articular cartilage damage your surgeon will often refer to a grade of damage. The grading system that is commonly used is a scale between 0 and 4 where:
|0||cartilage is normal and intact|
|1||cartilage has some softening and blistering|
|2||partial thickness (less than 50%) defect or minor tears in the surface
of the cartilage
|3||deeper defect (more than 50%)|
|4||full thickness cartilage loss with exposure of the subchondral bone|
In addition to the grade of the damage doctors will also measure the size of each defect in square centimetres. Small defects are generally considered to be less than 2cm2. The amount of articular cartilage damage can therefore be calculated from the number of defects, their depth and their size. However, it is important to remember that although the amount of damage is an important factor the location of the defect(s) can also influence the symptoms you are getting in terms of pain and function and the repair options available to you. In particular 'kissing lesions' (articular cartilage damage on both the articulating joint surfaces) can pose a real problem as it is bone on bone with movement.
You may be surprised to hear that pain isn't a particularly good indicator of the extent of the articular cartilage damage. One person can have severe pain with a single small defect and then another person can have very little pain with several large full thickness defects. An important piece of the jigsaw is being missed out if we focus too closely on just the articular cartilage. It is important to look at what's happening at the underlying bone and consider the overall health of the knee joint as well in order to build the complete picture. Over the last five years researchers have been looking more closely at the whole osteochondral (articular cartilage and the underlying 'subchondral' bone) unit and there is increasing evidence to indicate that changes in one of the components of the osteochondral unit affects the other.
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. In addition to not having a blood supply articular cartilage also has no nerve supply (it is aneural). This is in contrast to the bone immediately below the cartilage which is abundantly supplied with both nerves and blood. If the articular cartilage damage is shallow (grades 1 or 2) you may not experience any pain from the defect at this stage as the sensitive nerve endings in the subchondral bone are still covered with some articular cartilage. Some people have articular cartilage defects and don't even know about them initially. A recent study found a large number of patients who underwent ligament reconstruction surgery had articular cartilage defects but had very few symptoms (Shelbourne et al 2003). On the other hand, when a chondral defect does go all the way down to the bone (grade 4) the nerve endings in the subchondral bone become exposed and you are more likely to feel pain especially when pressure is put on the nerves.
When a chondral defect goes all the way down to the bone the blood supply in the subchondral bone can start a healing process in the defect. This results in the formation of scar tissue made up of a type of cartilage called fibrocartilage. Although fibrocartilage is able to fill in articular cartilage defects its structure is significantly different to that of hyaline cartilage. Fibrocartilage is much denser and it isn't able to withstand the demands of everyday activities as well as hyaline cartilage and is therefore at a higher risk of breaking down.
If an area is unstable fragments of articular cartilage may break off from the subchondral bone and surrounding articular cartilage and become mobile in the knee joint. These fragments can not only cause mechanical problems, such as catching and locking, but can also induce chemical changes in the knee joint as the fragments breakdown and result lead to inflammation of the lining of the joint (synovitis).
This is the million dollar question that anyone with an articular cartilage defect wants answered. Is my articular cartilage defect going to get worse and progress to osteoarthritis?
A recent study has found that small articular cartilage defects do progress to osteoarthritis over time (Wang et al 2006). However, other studies have found that some defects repair with fibrocartilage and can last for many years.
Which defects do progress to osteoarthritis? It is not clear as yet exactly which factors are associated with any progression but there is an emerging concept of a process of articular cartilage loss over time (chondropenia). The important point is that it is a process. An articular cartilage defect that initially may be relatively small still has the potential to have a physical and chemical 'domino effect' on the surrounding 'normal' articular cartilage. Excessive stress on an articulating surface of a joint that already has articular cartilage damage may accelerate further degenerative changes. Correction of any malalignment or instability that is present can help to 'normalise' stress. There is also some evidence to indicate that articular cartilage defects that don't have clearly defined margins and are poorly shouldered have an increased risk of progression as well.
So is repairing articular cartilage just delaying the inevitable in terms of progression to osteoarthritis in the knee? The answer at the moment is that no one can really be sure. Currently there is no concrete evidence to show that articular cartilage repair can prevent the progression of a defect to osteoarthritis. Ongoing studies are showing promising results but most have only short follow up times at present so the longer term outlook for patients is still unclear. However, it does look increasingly likely that repairing articular cartilage defects can slow down the degeneration of the joint compared to if the defect was left untreated. Therefore, at present, articular cartilage repair procedures are treatments aimed at providing relief of your symptoms and improvement in your function. Articular cartilage repair procedures MAY slow down the progression of a defect to osteoarthritis later in life but the jury is still out on this one.
The ultimate goal of any treatment is to restore the surface of the joint to 'normal' hyaline articular cartilage. This has been the quest of surgeons for the last two decades and it has lead to the development of a range of surgical procedures that will be summarised later. Many of these procedures have been able to 'fill in the pot holes' caused by articular cartilage damage but even in the most promising procedures the new tissue isn't exactly the same as natural articular cartilage. The repair tissue may look like hyaline but it is not the same mechanically - it is hyaline-like. So at present we can't 'restore' the structure of normal articular cartilage but we can 'repair' a defect with a similar tissue.
If articular cartilage can't be restored to it's normal structure why then would you consider having surgery to repair a defect with a tissue that probably isn't as good? After all articular cartilage repair carries the risks of any surgical procedure and you should always have a good reason for consenting to any surgery. Is this hyaline-like repair tissue good enough to make it worth putting yourself through the surgery?
Well, articular cartilage damage isn't life threatening but it can, and often does, threaten quality of life as it can lead to not only pain but also swelling, reduced mobility and restrictions on your activity. By recovering the subchondral bone in an articular cartilage repair procedure the pain can be signficantly reduced and at the same time the articulating surface is reestablished.
The questions as to how long the repair tissue lasts for and how durable it is compared to normal articular cartilage are still being answered. Articular cartilage repair is a relatively new technology and there are still a lot of 'ifs' or 'possiblys' to be addressed. There are an increasing number of clinical trials that are following articular cartilage repair patients over time to see how well, and how long, the articular cartilage repairs last.