Cartilage

There are two types of cartilage within the knee. There is the ‘lining’ (chondral) cartilage and the ‘cushion’ (meniscus) cartilage.

Chondral Cartilage

Most joints in the body have a smooth lining of cartilage which covers the end of the two bones as they meet. There is also joint fluid (synovial fluid) that aids lubrication. This hyaline cartilage is very efficient at decreasing friction within the joint, however it has no capacity to regenerate once damaged or if worn away.
Damage can be considered in two broad categories. Gradual wear and tear (Osteoarthritis) and sudden damage (Chondral Defect)
Osteoarthritis.

This is a condition where the layers of cartilage in the knee slowly wear away. It gradually exposes the bone underneath to increasing amounts of abnormal pressure, and this results in pain. There are many reasons why the cartilage begins to wear away. The joint can be overloaded, for example if the patient is overweight. If the joint is out of alignment, one side of the knee can take an abnormal amount of load compared to the other side. This increased load leads to the degeneration of the cartilage. The cartilage may also have been damaged in the past, so is less robust as the patient gets older. The absence of all or part of a meniscus is another reason for developing osteoarthritis (see Meniscus). As explained in the ACL section, abnormal movement within the knee caused by a ruptured cruciate ligament can lead to osteoarthritis as well.

The treatment options vary. Broadly the options are either an operation, or to treat without an operation. Non-operative or “conservative” treatment involves strategies such as losing weight, using painkilling tablets, using a stick or crutch, modifying activity and strengthening exercises with the physiotherapists. Operative treatment involves a variety of options. These include osteotomy, mini joint resurfacing, partial knee replacement and total knee replacement.

Chondral Defect

When a knee is injured, a fragment of the lining cartilage (or chondral surface) can sometimes be damaged, leaving a defect. Sometimes, if the injury is picked up early enough, the dislodged fragment can be fixed back in the hole it came from. However, usually either the fragment is in several pieces or the defect has only been noticed too late to rectify it acutely. If the defect is symptomatic (usually pain is the main symptom), then the options can be limited. The reason for this is because of the nature of hyaline cartilage that the chondral surfaces are made of. Most other tissues in the body (eg skin or bone) can regrow when damaged. Hyaline cartilage has no capacity to do this.

The first, and main, option is microfracture. This is where small holes are made into the bony base of the defect, causing bleeding. This bleeding forms a blood clot that fills in the gap. The idea is that this clot will form new cartilage, but it will form fibrocartilage rather than the hyaline cartilage that was originally there. Hyaline cartilage has no capacity to regenerate however. The fibrocartilage is not as durable as the hyaline cartilage, but it is better than the defect.

One of the many areas of active research within knee surgery is the attempt to develop a method that will grow hyaline cartilage in the defect, or to make the fibrocartilage that develops from the microfracture clot more durable. One of the problems with developing this technology is that currently it is extremely expensive. The main technology to grow hyaline cartilage involves harvesting cells from elsewhere in the knee, then expanding the numbers of the cells in the lab, and implanting them at a later date. To make fibrocartilage more durable, artificial coverings over the clot have been developed. Most of these are still experimental, however, they look promising.

Meniscus

The meniscus is the cartilage between the femur and tibia. Its main role is to provide a cushion between the femur and the tibia, within the knee. The end of the femur is in two halves (called condyles) that are rounded in shape. The ACL and PCL are between these two condyles. The shape of the top end of the tibia opposite the femur does not match the femur. The meniscus is there to ensure that there is a good fit between the femur and tibia. If the meniscus is removed, the contact pressures within the knee increase, and osteoarthritis can develop. The importance of the meniscus in preventing osteoarthritis has only been appreciated relatively recently.

When the meniscus tears, it can cause symptoms such as catching, locking and most commonly pain. These symptoms are often cured by removing the torn portion of the meniscus. However sometimes the tear is so big that removing the torn fragment results in loss of almost all the meniscus. This can therefore lead to future long term problems. It seems therefore obvious to repair rather than resect the meniscus. However, the blood supply within a joint is usually very poor. How well a tissue heals is directly related to its blood supply. Cuts to the head, for example, will often bleed profusely but will heal very quickly. The poor blood supply of the meniscus is at the heart of why most meniscal tears are resected.

However, some tears are suitable for repair. As interest as grown in preserving the meniscus, techniques have developed to improve the chances of a repair healing. This is particularly important on the lateral side of the knee (the same side as the little toe). The shape of the tibia is very different to the medial side (the same side as the big toe). A large amount of meniscal tissue removed from the lateral side means that osteoarthritis is virtually inevitable. If the tear happens to someone at a young age, this has obvious implications.

The meniscus is fixed using sutures that are passed into the knee using an arthroscope to see (keyhole surgery). Techniques are used to try and encourage the edge of the tear to bleed. Strong, permanent sutures are used and a rehabilitation regime, often involving a knee brace, is used. The chance of getting the meniscus to heal is around 80%, although there are many other factors that influence this. For example, if an ACL reconstruction is taking place at the same time then the chances of the meniscus healing are much higher.

Anterior knee pain syndrome

Pain towards the front of the knee is very common. It can be caused by damage to the cartilage in the knee, usually under the patella (the patellofemoral joint). However, pain here is more often present when there is no obvious cause. The way the knee moves, in particular the way the patella moves, is due to a complex interplay of the different muscles in the lower limb. Weakness in some of these muscles can lead to an imbalance and this can in turn result in pain. Once damage to the joint has been ruled out, focused physiotherapy exercises are often what is needed to cure this condition.

Patellofemoral Instability

Patellofemoral Instability is where the patella (knee cap) either dislocates from the knee joint, or feels like it is going to. The patella is normally very mobile, but is held in place mainly by the soft tissues surrounding it. These soft tissues are disrupted when the patella dislocates, but if they heal in the same position then further problems are less likely. If the patella dislocates, it almost always goes to the outside (lateral) side of the knee. This means that it is the tissues on the opposite (medial) side that have been disrupted. If they heal in a stretched position, then an operation to augment this side is often done.

Sometimes the patella has dislocated because the anatomy around the knee is slightly unusual. The groove (the trochlea) that the patella sits in as the knee bends can form incorrectly, so there is either no groove, or sometimes even a bump. The patella tendon can be longer than normal, so the patella sits much higher. It therefore spends less time in the trochlea groove, so there is more opportunity to dislocate. Finally, where the patella tendon joins the tibia is called the tibial tubercle. If this tubercle is more over to the lateral side than normal, then there is always a force pulling the patella out. If any of these three possible abnormalities is excessive, then an operation to correct them can be done.