MRI is unparralleled when it comes to imaging of the knee. It is the most reliable non-invasive tool that is available to doctors today to diagnose disorders of the knee.
The knee is the largest joint of the body, as well as probably the most frequently injured and thefore the most commonly imaged. Frequent indications for imaging include for the assessment of meniscal tears, ligament sprains/tears, cartilage and bone injuries, fluid masses, inflammatory arthritis and surrounding tendons.
Ultrasound has a limited role in the assesment of the knee, typically to visualise superficial structures, such as the extensor mechanism (quadriceps and patellar tendons) masses and also utilised to perform any procedures, such as joint aspiration (arthrocentesis) and injection based therapies.
The most frequent reason for MRI assessment of the knee is to diagnose a meniscal tear.
The menisci are triangular fibrocartilaginous structures interposed between the femur and tibia (the two bones that make up the main compartment of the knee joint) in order to cushion the forces transmitted between these two. With trauma (and in the non-paediatric setting, age related degeneration), the menisci are prone to shear forces and therefore tearing, resulting in painful movement of the knee, a sensation of instability, catching and locking. If a tear is large enough, then the meniscus can lose most if not all of its function, thus resulting in early arthritis.
MRI assessment of the knee following trauma in order to exclude a medial meniscal tear of medial collateral ligament sprain demonstrates bone marrow oedema of the medial femoral condyle consistent with a bone contusion (“bruise”). Further supportive of direct trauma is a loculated fluid collection within the overlying soft tissue, consistent with a collection of blood (haematoma). The knee was otherwise structurally intact and the patient treated with a short period of rest, pain relief and graduated return to normal activities.
MR imaging of the knee following an episode of twisting and instability reveals contusion of the medial aspect of the patella (P) and lateral femoral condyle (F), diagnostic of an episode of lateral patellar dislocation. This occurs when the medial patellar retinaculum that holds the patellar in its position is torn (double arrows), resulting in unopposed forces from the lateral side dragging the patella, eventually resulting in dislocation from its groove (G) when the patella relocates to its normal position (as shown below), collision occurs between the two bones to result in the pattern of bone contusions described.
Juvenile Chronic Arthritis (JCA).
MRI assessment of the knee in a child with a painful swollen joint demonstrates a large collection of fluid within the joint known as an effusion.
The joint is lined by specialised tissue that produces fluid known as synovium or synovial cells. When these cells become inflamed, this is known as synovitis which results in net increased production of synovial fluid and thus the accumulation of an effusion.
Inflamed synovium can be caused by many conditions, such as inflammatory arthritis (rheumatoid arthritis for example) as well as infections and degeneration. In this case, the likely diagnosis was Juvenile Chronic Arthritis (JCA).