Transient Patellar Dislocation - MSK Radiology
Imaging Findings:
• Hyperintense marrow edema of the anterolateral femoral condyle consistent with bone contusion.
• There is also a bone contusion of the inferomedial patella.
• The medial patellofemoral ligament has intrinsic signal and is disrupted at the anterior patella attachinent.
• Increased Insall-Salvati ratio (shortest patellar tendon/maximum length of patella > 1.2) and lateral patellar tilt (on axial).
Case description:
• Lateral dislocation of the patella out of the femoral trochlear groove, usually from twisting injury of the knee.
• Marrow contusions in classic distribution
- Inferomedial patella
- Lateral aspect of lateral femoral condyle
• Sprain or disruption of the medial patellofemoral ligament
• Can get chondral fractures (however articular cartilage is intact on this case). Look closely for cartilage injury and possible intra-articular bodies!
• Chronic injury can result in patellar maltracking and instability.
Differential diagnosis:
• Patella fracture: Marrow edema with or without a fracture line. However usually not with this classic pattern and MPFL injury.
• Quadriceps Tendon Tear: Can be seen with TPD, but if isolated will not present with the above findings.
• Chondromalacia Patella: Chondral defects and can have subchondral marrow edema. However, no soft-tissue swelling and the MPFL is intact.
Dr. Donald von Borstel @DrvonBorstel
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