Worrisome and Incidental Signs on Knee Radiographs SA–CME REVIEW Figure 5. Patella alta. (A) Lateral radiograph of a patient with acute blunt trauma from a fall shows a knee effusion (asterisk) and a > 20% difference (ratio > 1.2) between the maximum oblique distance across the patella (black line, 4.5 cm) and the distance from the patellar inferior pole to tibial tuberosity (white line, 7.0 cm). (B) Sagittal T1 MR image shows an acute patellar tendon tear (arrow). A A B B Figure 6. Transient patellar dislocation. (A) Sunrise radiograph of a patient with pain and instability shows a small ossific fragment (arrow) in the patellofemoral compartment. (B) Axial T2 fat saturated MR image shows the classic bone marrow contusion pattern centered at the medial patellar pole (short arrow) and lateral aspect of the lateral femoral condyle (long arrow), allowing for definitive diagnosis. An effusion is also present (asterisk). the tibiofemoral articulation. Most cases represent a developmental anomaly, but acquired patella alta is suspicious for a ruptured patellar tendon. Acutely acquired patella alta typically presents with new-onset knee pain, swelling, and decreased range of motion. 22,23 Chronic patella alta presents with a more insidious onset of patellofemoral pain and recurrent patellar subluxation or dislocation. 24,25 Patella alta is most common- ly evaluated on the lateral knee radiograph using the Insall-Salvati method (Figure 5). Patella alta is diagnosed when the distance from the inferior pole of the patella to tibial tuberosity is more than 20% of the maximum oblique distance across the patella. 26 Thus, a ratio of more than 1.2 indicates patella alta on a lateral knee radiograph. The condition is managed orthopedically Applied Radiology 11 January / February 2023