While changes of the mechanical and anatomical axes are likely to occur in derotational osteotomy, unintended changes on axes should be minimized. Regarding the femur, mechanical and anatomical axes differ on a frontal plane radiograph, whereas the mechanical anatomical angle (AMA) depends on the femoral torsion. For more accuracy and confirmability, the amount of valgus deviation is usually measured as the angle between the mechanical axis of the tibia versus the mechanical axis of the femur. Valgus malalignment is defined as a lateral deviation of the mechanical axis (line from the femoral head to the upper ankle joint), which is referred to the center of the knee joint on a frontal plane radiograph. In the author’s clinical observation, valgus deformities often appear in conjunction with torsional deformities in cases of patellofemoral instability. Increased lateral facet pressure and increased medial retinaculum strain was shown to be correlated with increased femoral antetorsion. Correction osteotomy at the distal femur has been shown to be a reliable option for correction of torsional pathologies. Increased femoral antetorsion is one important risk factor for patellofemoral instability and anterior knee pain syndrome in teenagers and young adults.
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