Osteochondritis dissecans in adolescence

Osteochondritis dissecans (00) involves the adolescent radio-capitellar joint and is due to chronic, compressive forces on the capitellum, ultimately causing injuries ranging from an articular cartilage injury to an osteochondral avulsion fracture. Plain fi Ims are often negative, while CT, MRI and MR arthrography are the examinations of choice. 00 should not be confused with a pseudodefect of the capitellum or Panner's disease. Staging of the lesion is important for its management, which is conservative for stages1 and 2, while surgery is indicated for stages3 and 4. Osteoarthritis is a late complication in 500/0 of patients with advanced disease.


Case report
A lS-year-old elite downhill mountain-biker presented with a painful elbow after falling off his bike on a descent. On clinical examination he had a suspected fracture of the capitellum with associated soft tissue swelling. There was also limited range of movement, but no crepitus was present. On further questioning, there was a history of a few minor falls and intermittent pain.
Plain films show defect of the lateral aspect of the capitellum ( Figure 1). Axial CT showed a deep defect of the capitellum and loose bodies in the coronoid and olecranon fossae ( Figure  2). MRI T2 fat saturation ( Figure 3a) and Tl coronal images (Figure 3b) showed the capitellar defect with fluid partially surrounding the fragment, as well as associated rnicrotrabecular bone marrow oedema.
The clinician decided to treat the patient conservatively, as it was felt that the elbow is a non-weight-bear-topage37 Osteochondritis dissecans in adolescence frompage36 ing joint. Rest was advised and further weight-bearing and compressive trauma to the elbow discouraged.

Discussion
Repeated valgus stress producing lateral compartment compression may lead to osteochondritis dissecans of the capitellum 1 • Osteochondritis dissecans represents a trans chondral fracture secondary to a transmission of tangential shearing forces across the joint space. The stress compresses the bone and may result in a spectrum of injury ranging from articular cartilage injury to an osteochondral avulsion fracture. This injury is thought to be traumatic in origin, owing to either a single major event or repeated minor injuries." Abnormal stress owing to malalignment or ligamentous laxity may be a predisposing factor. Other proposed causes include a hereditary ossification defect, familial localised abnormality in cartilage nutrition, or micro emboli with peripheral necrosis of bone. The age range is four to 15 years, but patients are predominantly within the second decade oflife, Most children with this condition are active. Boys are three times more likely to have this entity than girls. The knee, elbow and talus are the most common sites in- volved. Gymnasts, pitchers and downhill mountain-bikers are predisposed to 0 D of the capitellum or radial head due to chronic lateral impaction.' Plain films may show a fracture line, a fragment with a surrounding lucent zone or a concave defect. The fragment may be dense, suggesting necrosis, or may resorb.l+' An anteroposterior plain film of the elbow with 45% flexion is also suggested to show the pathological process. S The use of ultrasound is also suggested by certain authors. Localised subchondral bone flattening with overlying cartilaginous thickening is seen in early disease.S CT, MRT and MR arthrography are used to diagnose and stage osteochondritis, as the staging affects the management.1,2,4 It is important to stress that different classifications for OD exist for the elbow and the talus. This often leads to confusion.'> MRT Stage 1 OD of the elbow demonstrates signal changes consistent with cartilage injury, without disruption, and a normal subchondral bone. Stage 2 shows high signal breach of articular cartilage, but with a stable subchondral fragment. Stage 3 demonstrates a partial chondral attachment associated with a thin bright T2 signal behind the osteochondral fragment, probably representing synovial fluid around the fragment. Stage 4 represents a loose body within the centre of the osteochondral bed or free in the joint space. 2

,s
Care should be taken not to confuse an osteochondral defect of the elbow with the pseudodefect on MRI, owing to the abrupt transition between the posterolateral margin of the capitellum and the non-articular portion of the lateral humeral condyle.' topage38 Crstecictioridritis dissecans in adolescence frompage37 Osteochondritis dissecans should also be distinguished from osteochondrosis of the capitellum, which is known as Panner's disease. Age is an important discriminator, because Panner's disease typically occurs in children five to Il years of age, compared with the 13 to 16 years of age for OD. Panner's disease is thought to represent avascular necrosis of the capitellar ossification centre that occurs secondary to trauma. Plain films may show subtle fragmentation, while MRI shows abnormally decreased signal intensity within the ossifying capitellar epiphysis. Subsequent scans reveal normalisation of these changes with little or no residual deformity of the capitellar articular surface.' Conservative treatment is indicated for stages 1 and 2, as the bone fragment may revascularise and heal. Surgery is indicated for stages 3 and 4, where the bone fragment is either fixed to the parent bone or removed and the osteochondral crater debrided or fixed. Osteoarthritis develops in 50% of patients with OD of the elbow. 1 ,2,4

Conclusion
Osteochondritis dissecans of the elbow is a post-traumatic injury to the radio-capitellar joint caused by either a single major traumatic event or chronic lateral compressive forces. Plain film findings (in contrast to the index case) are usually negative,while CT and MR! show the different stages of the pathological process optimally. The diagnosis and staging are important, as the management and long-term prognosis are dependent on accurate imaging. Aknowledgement I would like to thank Dr Andy du Toit for help regarding the subject matter in this paper.

Allied Health/Clinical Su ort Radiologists
Toowoomba Health Service District, Queensland, Australia. Remuneration value up to A$207 039 p.a. or up to A$215 900 p.a. for Director position, comprising salary between A$86 876 -A$116 354 p.a., employer contribution to superannuation (up to 12.75%), annual leave loading (17.5%), Option A private practice allowance (45%), private use of a fully maintained vehicle, communications package, study and conference leave on full pay with expenses paid and professional Indemnity cover. (MOl-I to M02-2) (two permanent full time positions available: one Director and one Staff Specialist) VRN: 1lISD 312/00. DutIes/AbilitIes: The Department of Medical Imaging provides general, emergency and procedural radiology, angiography (including interventional), ultrasound, CT services and general radiology. There are 14 radiographers including two full time sonographers, two staff specialists, visiting medical officers, six administrative officers and three registered nurses staff the Department. Over 45 000 examinations are performed in a financial year. Director's Allowance will be provided for the Director's position. Toowoomba Health Service District is the referral centre for southern inland Queensland. The City of Toowoomba is located I'J, hours west of Brisbane and has a reputation of being The Garden City, with excellent amenities/facilities available for recreation, cultural, educational, social and commercial activities. Toowoomba has a beautiful environment and excellent climate. Toowoomba Hospital will sponsor temporary visits for successful applicants, and will proceed to sponsor suitable, successful applicants and their families for permanent residency in Australia. EnquIries: Dr Andrew Cumming +61 7 4631 6151 or e-mail: andrew_cumming@health.qld.gov.au AppUcatIon Kit: +61 7 4631 6258. Closing Date: Open. Free intraperitonea I air on paediatric plain radiographs and differentiating this from a 'food mound" in the gastric fundus accumulates in a mound instead of as the expected airfluid level. When faced with a surgeon concerned about this finding, re-evaluation of the patient's clinical status is recommended together with re-assessment of the radiographs for free air in other recognised locations.We also keep examples of previous normal patients with the "food mound" sign, which we show to doubtful surgeons.
If the radiologist is familiar with this finding, such a radiograph will cause less distress to him/herself the surgeon and the parents of the recently fed child. May the "food mound" be with youl