We report on a case of bilateral medial patellar dislocation, studied with hip/knee/ankle TC for evaluation of torsional defects and treated four times with distal derotative femoral osteotomy and cuneiform subtractive osteotomy of the lateral part of the femoral trochlea.

The patient, a woman 18 years of age at the beginning of treatment, presented with femoral neck anteversion of 30° dx and 25° sx referred to the plane passing posteriorly to the distal femoral condyles; the trochlear angle was 140° dx and 144° sx with medial inclination of trochlea due to medial hypoplasia. The patellae were facing medially, and clinically the patient had suffered medial dislocation of the patella several times.

The treatment lasted 4 years: the femoral derotation of 10° was executed by Orthofix monoassial external fixator; the lateral bone wedge removed from the trochlea was about 10 mm and we used reabsorbable nails for fixation.

Current values are 17° of femoral neck anteversion bilaterally, and the trochlear angle is 151° dx and 150° sx. The patient, now 26 years old, has not had any more dislocations and her knees have complete mobility with no pain.

The good result demonstrates the importance of femoral neck anteversion in the genesis of knee disorders. In this case the absence of specific abnormalities of the extensor mechanism (valgus knee, lateralisation of tibial apophyisis) probably caused the hypoplasia of the medial part of the trochlea and the resulting, rare medial patellar dislocation.