Patellar Instability

Lateral trochlear inclination: An inclination angle of less than 11° indicates trochlear dysplasia.

Trochlear facet asymmetry: ratio of the medial facet (M) length divided by the lateral facet (L) length expressed as a percentage. A trochlear facet ratio of less than 40% indicates dysplasia.

Trochlear depth: calculated as (A + C/2) − B. A trochlear depth of 3 mm or less indicates dysplasia.

Patellar height ratio: length of the patellar tendon (line A) is divided by the longest superoinferior diameter of the patella (line B). A patellar height ratio of more than 1.3 indicates patella alta.

Distance from the tibial tubercle to the trochlear groove: The distance (double-headed arrow) from the deepest point of the trochlea (line B) to the middle of the tibial tubercle (line A) is measured. A distance of less than 15 mm is considered normal. Distances between 15 and 20 mm are borderline, and distances of more than 20 mm indicate marked lateralization of the tuberosity.

Lateral patellofemoral angle is the angle of a line drawn along the lateral joint surface of the patella (line A) and a line drawn along the anterior aspect of the condyles (line B).

A patellofemoral angle opening laterally is considered normal. An angle opening medially indicates patellar tilt.

Four types of trochlear dysplasia, according to the classification of Dejour et al:

Type A dysplasia: Trochlear morphologic structures are preserved, but the sulcus is shallow.

Type B dysplasia: flat, horizon- tally oriented trochlear joint surface.

Type C dysplasia: flat, obliquely oriented trochlear joint surface with facet asymmetry.

Type D dysplasia: same as type C but with a prominent bone protrusion (arrow) on the parasagittal view (cliff pattern).