MPFL Reconstruction With or Without Tibial Tubercle Transfer

Inclusion Criteria: * recurrent patellar dislocation or instability,patients with TT-TG value equal to or greater than 20 mm,patients with closed epiphyseal plate TT-TG：Two lines drawn perpendicular to posterior bicondylar line, one line through middle of the trochlear groove(TG) and the second through tibial tuberosity (TT). Distance between the lines is measured. Exclusion Criteria: * bilateral patellar dislocation, a previous surgery in the involved leg,patients with open epiphyseal plate,patients with severe trochlear dysplasia