Fig. 1A–D.

The schematic drawings illustrate the effect of a VFO. (A) Before VFO, the hip shows subluxation with joint incongruity and a decreased weightbearing surface, and overload in the region of the acetabular corner in the AP plane. (B) The VFO aims to increase joint congruity (a) and the weightbearing surface; medialize the center of rotation (b); lengthen the lateral lever arm of the abductors (c); reduce overload in the region of the acetabular corner (d); and increase tension of the abductors (e). Lateral displacement of the distal femur in the coronal plane is required to maintain a normal relationship with the proximal femur (f). (C) In the lateral plane, before VFO, the hip shows a decreased weightbearing surface and an anteriorly protruding portion of the femoral head. (D) The flexion osteotomy increases the congruous weightbearing surface and decreases shear (a); moves the anteriorly protruding portion of the femoral head away from the acetabular rim (b); tightens the anterior portion of the capsule (c); and shifts the center of rotation posteriorly (d). Anterior displacement of the distal femur in the sagittal plane is required to maintain a normal relationship with the proximal femur (e).