These are two structural disorders that may result in recurrent dislocation of the kneecap.
Patella alta is a condition where the kneecap is in a higher position than normal. In this position it may not engage in the groove of the underlying tibia in the first few degrees of bending, and may sublux or even dislocate. The patellar tendon may be abnormally long or the tibial tubercle, to which the patellar tendon attaches, may be in an abnormally high position.
In patellar dysplasia, another condition where the patella may be unstable, the patella is an abnormal shape.
Trochlear dysplasia is often found in correlation with 'patellar dysplasia', ie when the patella, too, is of an abnormal shape.
Trochleoplasty - or re-shaping of the groove in which the patella tracks - is an operation which may, in expert hands, be done to prevent patellar dislocation related to a very abnormal trochlea (eg trochlear dysplasia).
Some people are born with a flattened or abnormal trochlea, that is abnormal walls of the groove in which the patella rides. This renders them prone to recurrent subluxation and dislocation.
Trochleoplasty is a procedure which attempts to restore function to the trochlea by raising the side walls to better contain the patella from dislocation. It is a specialist procedure.