Ultrasound measurements of the patella

Ultrasound measurements of the patella were followed by CT/MRI

The concept of Patella alta refers to the high position of the patella relative to the trochlea of the femur, resulting in progressive pain in the suprapatellar area, which is caused by inadequate contact between the quadriceps of the patella and the condyle of the femur in the flexion state of the high patella [when the quadriceps of the femoral condyle is in the flexion state of the normal knee joint, the pressure on the surface of the patellofemoral joint is relatively dispersed]. This leads to increased pressure on the articular surface of the patella. Long-term increased pressure will produce relative wear on the articular surface of the patella at an earlier stage. Further cartilage wear will lead to articular dysfunction of the patella, including pain during progressive aggravation of activities and limited range of motion of the knee joint. Several conditions are known to be associated with high patella, including:

·Idiopathic retropatellar pain

·Recurrent dislocation of patella

·Chondromalacia patella

·Effusion knee joint

High patella can also be the result of spastic cerebral palsy.

The Insall-Salvati ratio is probably the most commonly used measure to assess patellar height, partly based on its simplicity, but may be influenced by abnormalities in the tibial tubercles. Insall-Salvati ratio was originally measured at the sagittal position on 30° flexion knee radiographs and later applied to sagittal MRI.

·Patellar tendon length (TL) : The length from the lower pole of the patella to the attachment point on the tibia

·Patellar length (PL) : the maximum length between the upper and lower poles of the patella

Insall-Salvati ratio = TL/PL

Figure 1 X-ray measurement of Insall-Salvati ratio. A: Length of patellar tendon (TL, red line); B: patella length (PL, yellow line).

The normal value of the Insall-Salvati ratio on the X line is 0.8 to 1.2. Greater than 1.2 can diagnose high patella. However, on MRI, due to differences in measurement techniques, visualization of patellar tendon makes the length of the lateral branches of patellar tendon generally larger than the TL value below the X-ray line. Therefore, different thresholds are used on MRI, and usually high patella is diagnosed only when the Insall-Salvati ratio is greater than 1.5.

The scan section of the ultrasound examination is limited and does not directly show the total length of the patella and patellar tendon. However, with the application of ultrasonic panoramic imaging technology and the improvement of measurement accuracy, ultrasound can also be used to evaluate the high patella. Also, when the flexion was 30 degrees, panoramic imaging of longitudinal fracture was performed along the anterior knee area, and the length of patellar tendon (TL) and patellar length (PL) were measured on the panoramic ultrasound image to obtain the nsall-Salvati ratio. The measurement threshold of MRI was also used, and the diagnosis of high patella was only when the Insall-Salvati ratio was greater than 1.5.

Figure 2 Ultrasonic measurement of high patella. The nsall-Salvati ratio of 1.58 was obtained by measuring the length of patellar tendon and patella on the panoramic ultrasonic image of longitudinal section of anterior knee.

Figure 3 Ultrasonic measurement of high patella. The nsall-Salvati ratio of 1.82 was obtained by measuring the length of patellar tendon and patella on the panoramic ultrasonic image of longitudinal section of anterior knee.

Ultrasound is relatively convenient, non-radioactive and can be used as an alternative to X-ray and MRI to measure and evaluate the patella of the high position.