An 18 year old male futsal player presents following an acute injury to his knee. He describes twisting on his planted right foot, then saw and felt his patella dislocate laterally. This was reduced by a teammate. His knee became rapidly swollen and very painful.
On examination (five days after the injury) he had an antalgic gait, restricted knee range of motion and a large effusion. He also had localised tenderness about the medial patella and lateral femoral condyle as well as marked patella apprehension. He had no evidence of generalised ligamentous laxity. This is his fourth patella instability episode.
The skyline x-ray image shows a shallow femoral trochlear groove with elongation of the lateral femoral trochlea facet. There is also an ossicle related to the medial patella. This is likely to reflect an old avulsion fracture of the anterior attachment of the medial patellofemoral ligament. These findings are further demonstrated on the MRI.
The axial MRI image shows characteristic impaction injuries involving the anterolateral aspect of lateral femoral condyle and medial aspect of the patella. There are associated articular cartilage changes in these regions without a significant osteochondral injury or loose body. An Insall-Salvati ratio of 1.36 has been measured on the sagittal sequence.
In summary the imaging shows evidence of a recent patella instability episode. There is also evidence of femoral trochlea dysplasia and patellar alta as predisposing factors.
There are a number of radiological factors that can be associated with an increased risk of patella instability. The two most readily seen are trochlear dysplasia (where the trochlea appears relatively flat) and patella alta (a high riding patella). Lateralisation of the patella is another useful radiological sign.
Patella alta is often measured by calculating the Insall-Salvati ratio. This involves measuring the length or the patella tendon (A) and dividing this by the height of the patella (B). A normal ratio should be between 0.8 and 1.2. An alternative method (that is more gross) involves extending Blumensaat’s line (on a standing lateral x-ray). Blumensaat’s line is seen corresponds to the roof of the intercondylar fossa of femur. In a normal knee this line should touch the inferior pole of the patella. If the patella sits more than 10mm above this line then patella alta should be suspected.
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