The moving surface of the knee is lined by articular cartilage. Generalised damage to the articular cartilage, as can appear with degenerative conditions, is termed osteoarthritis. However, isolated lesions of a non-degenerative nature can also occur. Traumatic damage to the lining surface of the joint is quite common. As can be seen here on this arthroscopic picture, the smooth white articular cartilage has been damaged and a piece knocked off to expose the underlying bone. Some blood clots can also be seen, indicating this to be an acute injury. The underlying meniscus cartilage is normal.
This can occur either from acute trauma which is common with injuries such as cruciate ligament injury, patella dislocation or direct impact injuries. It can also occur as a result of a pre-existing condition in the knee called osteochondritis dissecans (OCD).
Traumatic Osteochondral Damage to the Articular Surface of the Knee
These lesions occur associated with traumatic injuries to the knee. They can occur in any part of the knee joint, but most commonly are found on either the femoral condyles or in the patellofemoral articulation.
They can happen as a result of direction compaction injuries, such as can occur in the patellofemoral joint, if for example, the knee is struck against a car dashboard in a road traffic accident. Compaction injuries occur when two areas of the joint hit each other causing bleeding in the bone, as can be seen in the MRI scan below.
The more common mechanism for traumatic osteochondral defects is usually a rotational injury to the knee. The mechanism to cause an osteochondral lesion is usually a shearing force across the articular cartilage. For adequate shearing forces to occur, tearing of a major ligament must occur to the knee, the most common of which is an anterior cruciate ligament tear. The other common site of significant shearing forces developing, is at the patellofemoral joint, when a patella (knee cap) dislocation occurs.
The severity of damage to the articular surface can vary across a spectrum, with one end being a crushing injury to the articular cartilage with no actual loss of continuity of the articular cartilage. The other end of the spectrum is a piece of articular cartilage, with or without the underlying bone being physically knocked off from its original bed. The type of lesion is dependent on the magnitude of the force of injury and the directional vectors of the resulting force, as well as any associated injuries to the knee.
Osteochondral/chondral injuries are commonly associated with major ligament injuries to the knee and also with meniscal injuries.
A detailed history taken from the injured person will normally indicate the mechanism of injury. For example a direct compaction injury, associated with a road traffic accident, versus a rotational injury that can commonly occur, for example when changing direction in football. Depending on the severity of the injury, the injured person may present with a localised pain and a mild swelling all the way through to a tense hemarthrosis (blood filling the joint) and possibly a locked knee.
Localised compaction injuries are treated symptomatically with ice, elevation, anti-inflammatories and routine physiotherapy modalities. They can be diagnosed on MRI scanning, as below, although the scan below shows a compaction pattern that is typical for ACL tears. This MRI scan shows the typical bone compaction pattern of an ACL tear. The compacted areas appear white due to the fluid (blood) in the “bruise”.
If an actual piece of cartilage or bone and cartilage has been knocked off, the patient tends to get more bleeding and therefore a more tense haemarthrosis. If the loose piece floats around within the knee, it can actually become physically stuck and cause the knee to lock so as to prevent certain movements until the piece moves out and ‘unlocks’.
In the chronic situation, once all the acute symptoms have settled, the injured person will usually complain of intermittent locking and can also complain of instability symptoms, where the loose piece can get trapped in the knee and make the knee feel like it is going to give way, or may occasionally even truly give way.
The examination of the knee depends on whether the injured person is seen acutely following the injury or chronically at some later time. In the acute situation, the knee is usually swollen with a reduced range of motion and generalised pain. There may be associated ligament instabilities which are clinically detectable.
In the chronic situation, there may be a mild effusion (fluid within the joint). The injured person may complain of a feeling of a loose piece floating within the knee, which is sometimes actually physically palpable within the knee. If the knee is locked at the time of examination, the injured person will have a reduced range of motion with a true mechanical block to further motion. Once again associated ligament instabilities may be clinically detectable on examination.
All patients will routinely have a 4-view series of x-rays of the knee. This may reveal in the case of an osteochondral injury, an alteration in the continuity of the outline of the bone. Alternatively, the plain x-rays may be normal. It is important to assess the patellofemoral joint, as these can be the origin of the defect, which may not be visible on routine x-rays without a skyline view.
The diagnostic investigation for articular cartilage injuries to the knee is MRI scanning. MRI scans will commonly reveal a loose fragment or may reveal the area of damage to the articular cartilage, with the overlying cartilage still being intact. If there is a loose fragment, the origin of this will commonly be identified on the MRI scan and any additional ligament or meniscal damage can also be identified.
Prof Jari is fortunate to have access to a high-resolution 3 Teslar MRI scan which with appropriate sequences is ideal for showing articular cartilage damage much more clearly and with greater accuracy than a normal resolution scanner. It is not uncommon for articular cartilage injuries to be missed on a routine scan, to then be picked up on a high-resolution MRI scan.
Other investigations that occasionally are used include CT scanning and even Bone (SPECT) scanning.