In acute trauma, the Ottawa Knee Rules determine if x-rays should be obtained (see Text Box 1). A normal horizontal beam lateral (Fig.1) can reassure fracturing is unlikely.
If there is significant arthritis, which is best demonstrated on AP weight bearing x-rays (Fig.2), knee replacement may be more appropriate than arthroscopy, obviating the need for further imaging. For post-surgical cases or fracture follow-up, x-rays screen for complications and confirm fracture healing.
MRI gives a gold standard overview of all knee structures. It can enable more rapid return to work or sport through exclusion of a significant injury and triage injured patients for arthroscopy versus conservative management. A twisting or ‘dislocating’ type injury with rapid onset effusion/haemarthrosis suggests a potential ACL tear or patella dislocation. Often difficult to differentiate clinically, MRI can confirm the diagnosis (Fig.4) and frequently reveal and characterise other important intra-articular injuries, such as meniscal tears. In isolation, meniscal tears tend to cause more gradual onset effusion, while displaced meniscal fragments may cause a block to full extension.
MRI can also reveal articular cartilage injuries and marrow oedema due to arthritis/stress or insufficiency fractures that are usually occult on other imaging modalities. In children, a rebateable MRI can be obtained irrespective of trauma history, helping exclude a range of important conditions e.g. osteochondritis dissecans (Fig.5) or juvenile arthritis.
A Medicare rebate applies for knee MRI scans in adults (16-49 years) following acute trauma where inability to extend the knee suggests a possible acute meniscal tear or clinical findings suggests acute anterior cruciate ligament tear, and in those under 16 for suspected internal joint derangement.
CT has a limited role in acute knee trauma, being primarily used for operative planning of comminuted or intra-articular fractures (Fig.6). CT may reveal an occult fracture if lipo-haemarthrosis on x-ray, but is insensitive to other intra-articular injuries identified on MRI. CT plays a key role in surgical planning for computer or robot-assisted TKRs.
CT arthrograms are not routinely performed if MRI is available, due to their inferior sensitivity for soft tissue pathology and bone marrow oedema, radiation (albeit low dose on modern scanners) and the invasive component to the scan. It is reasonable for detection of meniscal tears as long as adequate contrast coating of the menisci is achieved (problematic in acute trauma with a haemarthrosis). CT arthrogram does have better sensitivity for full thickness fissuring than MRI, so is occasionally requested by orthopaedics for work up of uni-compartmental knee replacement.
Ultrasound is limited to assessment of superficial soft tissues; collateral ligaments, quadriceps and patella tendons, bursae and Baker’s cyst, confirming an effusion or synovitis if inflammatory arthritis is suspected.
- Plain films: first line in trauma and arthritis
- MRI: gold standard especially in suspected cruciate or meniscal injury
- CT arthrogram: in over 50 without significant OA where MRI not rebateable
References available on request.
Questions? Contact the editor.
Author competing interests: the author works for a group providing imaging.
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The Ottawa Knee Rules – Developed by Dr Ian Stiell
A knee x-ray series is only required for knee injury patients with any of the following findings
- Age 55 or older
- Isolated tenderness of the patella (no other bony tenderness)
- Tenderness of the head of the fibula
- Cannot flex to 90 degrees
- Unable to bear weight both immediately and in ED for 4 steps.