What is Osteochondritis Dissecans?
Osteochondritis Dissecans is a condition that affects synovial joints , resulting in localised abnormalities of the articular cartilage, such that the cartilage softens and can eventually dislodge and become a floating loose body within the knee.
The cause of OCD is much debated. A number of theories have been proposed. The condition was first described and named by Konig in 1888 who suggested that the primary problem may be inflammatory in nature to explain the loose bodies in the joint. This theory has now been further universally discarded.
Other theories include repetitive micro-trauma, possibly a secondary infection, of insufficient blood supply and finally potentially inherited factors.
What is known now is that the primary pathology is not to the articular cartilage itself, but to the underlying sub-chondral bone with a secondary effect on the overlying articular cartilage.
The incidence of OCD has been estimated at between 0.02% and 0.03% based on knee x-rays and adds 1.2% based on knee arthroscopies.
The highest incident rates appear among patients between the ages of 10-15. It is a condition that is more common in males than females approximately 2:1 respectively. Importantly, bilateral lesions can occur which typically are in different phases of development, but have been described in 30% of cases. This indicates that both knees should be assessed in patients presenting with this diagnosis.
Classification of osteochondritis dessicans of the knee, involves identification of a specific location, potential fragmentation and/or displacement and the status of the growth plates. Skeletal age at the onset of symptoms, appears to be most important determinant of prognosis and remains an important factor in deciding the timing and type of treatment. (Table 1 page 91. Imaging and Arthroscopic criteria for classifying OCD in the knee).
It has been emphasised in the literature that there should be a distinction between the juvenile and adult type of OCD based on the osseous (bone), age of the patient at the time of symptom onset. Those with open growth plates are considered to have juvenile-onset OCD whereas those with skeletal maturity are considered to have the adult form. It has been theorised that the adult-onset of OCD may simply be a delayed onset of previously asymptomatic juvenile OCD that has failed to heal and presented later with loosening and joint degeneration.
Early presentation usually involves fairly poorly defined complaints. Pain is generalised to the anterior knee with variable amounts of swelling and is usually intermittent. It may be noted that there is an association by the patient of periods of increased activity and episodes of swelling. In patients with more advanced OCD, persistent swelling may be accompanied by catching, locking, or giving way. In the later stages of the disease, there is usually a loose body sensation described.
Physical examination findings may be correlated with the area of the lesion. Wilson27 described external rotation of the tibia during walking as signifying compensation for impingement of the tibial eminence on an OCD lesion of the medial femoral condyle (the classical sight for an OCD is the lateral part of the medial femoral condyle). (Show picture). Wilson’s test involves the reproduction of pain by the examiner by internally rotating the tibia during extension of the knee between 90° and 30° and then relieving pain with tibial external rotation. However, recent literature suggests that this test has a poor predictive value. The examination should also include standard techniques for excluding any co-existent pathology such as meniscus tears, ligamentous injury, or misalignment of the knee.
Classification and Characterisation
Distribution of OCD lesions in the knee are most commonly associated with the lateral aspect from the medial femoral condyle. The OCD can however, affect any part of the joint, although it is very rare that it affects the tibial surface.
Plain knee radiographs provide the initial basis for assessment of growth plate maturation, but also lesion location and stability. Berndt and Harty35 have described four stages of chondral lesion based on plain x-rays of the talus in the ankle, but this has been used in the knee:
Stage 1. Involvement of a small area of compression of the subchondral bone.
Stage 2. Partly detached osteochondral fragment.
Stage 3. Completely detached fragment that remains in the underlying crater.
Stage 4. Complete detachment/ loose body.
The size of the lesion has been suggested as being a method of determining which ones could be treated non-surgically. Lesions less than 400 mm square may heal non-surgically, whereas lesions greater than about 440 mm square are associated with a poor outcome. The presence of marked sclerosis (white line demarcating the edge of the crater), has also been suggested as a poor predictor of non-surgical management.
As alluded to above, MRI scans are much more sensitive at demonstrating potential stability of the lesion and the overlying articular cartilage.
The gold standard however, is the arthroscopic findings, and these have been described by Guhl37:
Type 1. Softening of the cartilage but no breach.
Type 2. Breached cartilage that is stable.
Type 3. Definable fragment that remains partially attached (Flap lesion).
Type 4. A loose body and osteochondral defect of the donor site .