The tibial tubercle-trochlear groove distance (TT-TG) was first described four decades ago. Since then, a considerable body of research has become available on the pathophysiology of chronic patellar instability, whose diagnosis and treatment remain challenging. Tibial tubercle medialisation can correct an abnormal TT-TG. Preoperative planning based on the TT-TG and trochlear angle may avoid both under-correction inducing persistent instability and overcorrection responsible for pain. Preoperative planning should be patient-specific. With appropriate preoperative planning, compensation for moderate trochlear dysplasia can be achieved without any additional procedure.