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 maybe 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 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. Examination should also include standard techniques for excluding any co-existent pathology such as meniscus tears, ligamentous injury or misalignment of the knee.
A standard series of knee radiographs are taken and assessed to identify the bony component of an OCD. A number of classifications have been devised on the basis of the plain x-rays such as that by Cahill 29 and Berg29. This classification is more apt for research purposes than for clinical practice. Further staging systems have been devised on the basis of additional use of technisium bone scanning.
The bone scan can have some predictive value on the ability of the lesion to heal, when distinguishing between juveniles and adults. Paletta et al30 have reported that four out of four patients with open growth plates, had increased activity on a bone scan healed with non-surgical treatment, whereas the two patients without increased activity did not heal. In contrast, in adults with closed growth plates, only two out of six healed despite having similar increased activity within the lesion.
The current imaging technique of choice is magnetic resonance imaging (MRI), with high resolution 3T scanners being even more sensitive. These have been used to try and predict the stability of the lesion. For example if the MRI scan demonstrates fluid behind the lesion, this indicates that the flap is partly or completely detached and would probably require surgical intervention.
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 .
Natural History & Prognosis
The literature on this condition contains no randomised controlled clinical trials for either surgical or non-surgical interventions for OCD of the knee.
A large recent multi-centre review of the European Paediatric Orthopaedic Society study, provides some information5. This study included 509 knees of which 318 were juvenile and 191 were adult. Their conclusions included:
– When the fragment is stable, the prognosis is better than it is when there are signs of instability of the fragment.
– Pain and swelling are poor indicators of loosening and instability of a fragment.
– Plain radiograph and CT scans are not useful to predict stability.
– Sclerosis (thick white line along the crater or the defect) on plain x-ray predicts poor outcome to drilling.
– Lesions bigger than 2 cm in diameter have a worse prognosis than smaller ones.
– When there is evidence of instability and looseness of the fragment, surgical results are better than non-surgical.
– Lesions in the classical location have a better outcome.
Using MRI scans, it has been suggested that older patients with one or more signs of chondral destruction are more likely to fail non-surgical treatment. Younger patients with no MRI criteria for instability of the fragment were the most likely to recover with non-surgical treatment .
The goal of non-surgical treatment is to promote the healing of the lesion and prevent its displacement. The goal of surgical treatment is to salvage native cartilage if possible, but if not possible to undertake a restorative procedure.
It is important for the patient and the family to understand the nature of the disease and the potential long-term implications of the condition, including the fact that treatment is usually not short term. Symptoms that are exacerbated by activity should be identified, including limiting sporting and high impact activities.
This primarily involves activity modification and possibly periods of limited or non-weight bearing. There is no good evidence for the use of braces or casts.
Symptom relief can also be gained with the use of a Cryocuff, ice, compression, analgesia and anti-inflammatories. It is however important for the patient to understand that this symptom relief does not mean that they can resume normal activities immediately.
For patients who continue to have pain and episodes swelling, the next treatment would be surgical.
In adults without evidence of loose fragments or unstable lesions, management can employ pain medication, activity modification, strengthening using non-impact loading exercises and weight control. It is important to treat any concurrent pathology such as instability or malalignment, which is more of an issue in adults.
The first stage of surgical management usually involves arthroscopy to classify the lesion, primary to assess the integrity of the underlying cartilage and the stability of the lesion. This will give one of three results, either the lesion is intact as is the cartilage, or that the cartilage is not intact but that the lesion is stable, or finally that the cartilage is not intact and the lesion is unstable.
Surgical treatment for stable lesions with normal articular cartilage involves drilling the subchondral bone with the intention of stimulating inflow of blood and blood vessels to the subchondral bone and allowing this to heal. The average time for healing using this technique is about four months. Drilling however, does work better in skeletally immature patients than in older adults, but is still worth attempting in all patients with symptomatic lesions and intact articular cartilage.
Surgical treatment for the unstable lesion which is not displaced, usually involved fixing the fragment back on. There are various implants that can be used for this, but there can be complications associated with the fixation, the implants and also the risk of having to remove the implants. It has been suggested that simply removing the loose fragment is not an adequate technique, as this can lead to increased loading to the surrounding cartilage and propagation of damage to the cartilage.
The final salvage option for full thickness defects of articular cartilage, include autologous chondrocyte implantation, mosaicplasty and osteochondral allograft.
The two principle factors in OCD are skeletal maturity at symptom onset and the contiguity of the subchondral and bone cartilage surface, are the most important determinants in choosing treatment.
OCD is not a benign condition, even in the skeletally immature knee and can progress to arthritis and degeneration in the long term.