Internal rotation gait: a compensatory mechanism to restore abduction capacity decreased by bone deformity.

Children with excessive femoral anteversion frequently walk with abnormal internal rotation of the hip. The authors hypothesized that excessive anteversion decreases the abduction moment arm of the gluteus medius and that this moment arm is restored with internal rotation; hence internal rotation may be a compensatory mechanism to preserve abduction capacity. To test this hypothesis a three-dimensional computer model of an adult lower limb was developed to determine how changes in femoral anteversion angle, neck-shaft angle, and hip internal rotation angle affect the abduction moment arm of the gluteus medius. Analysis of the model revealed that anteversion and valgus deformities of the femur can decrease the abduction moment arm of the gluteus medius substantially. In particular, increasing the anteversion angle of the model by 30 to 40 degrees caused a 40 to 50% decrease in the abduction moment arm of the gluteus medius - enough to impair walking. Internal rotation of the hip by 30 degrees restored the abduction moment arm of the gluteus medius to within 5% of the moment arm of the model in its normal, undeformed state. These results support the authors' hypothesis and are consistent with the theory that internal rotation may be a compensatory mechanism adopted by children with femoral deformities to achieve the abduction moment arm needed for walking.