Radiographic evaluation of patellar instability

Radiographic evaluation of patellar instability

–rule out fracture or loose body:

medial patellar facet (most common)

lateral femoral condyle

–AP views

best to evaluate overall lower extremity alignment and version

–lateral views

best to assess for trochlear dysplasia

—crossing sign

trochlear groove lies in same plane as anterior border of lateral condyle

represents flattened trochlear groove

----double contour sign

anterior border of lateral condyle lies anterior to anterior border of medial condyle

represents convex trochlear groove/hypoplastic medial condyle

-----supratrochlear spur

arises in proximal aspect of trochlea

–evaluate for patellar height (patella alta vs. baja)

1-Blumensaat’s line should extend to inferior pole of the patella at 30 degrees of knee flexion.

2-Insall-Salvati method

The Insall-Salvati ratio or index is the ratio of the patella tendon length to the length of the patella and is used to determine patellar height.

normal between 0.8 and 1.2

3-Blackburne-Peel method

The Blackburne-Peel method measures the ratio of the articular surface length of the patella ( a ) to the height of the lower pole of the articular surface above a tibial plateau line ( b ) The aim of this prospective study was to evaluate the patellar height in 90° knee flexion.

normal between 0.5 and 1.0

4-Caton Deschamps method

The Caton-Deschamps index relies upon the length of the patellar articular surface and its distance from the tibia, reducing erroneous measurements in those with long patella bodies, as measured in the Insall-Salvati index.

normal between 0.6 and 1.3

Plateau-patella angle

normal between 20 and 30 degrees

–Sunrise/Merchant views

-----best to assess for lateral patellar tilt

–lateral patellofemoral angle (normal is an angle that opens laterally)

angle between line along subchondral bone of lateral trochlear facet + posterior femoral condyles normal > 11°

------congruence angle (normal is -6 degrees)

-------sulcus angle

evaluate for trochlear dysplasia

values > 140 degrees indicate flattening of the trochlea concerning for dysplasia