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 meghanism 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° 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° 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.
The purpose of this study was to determine the effects of superior displacement of the hip center and changes in three prosthetic parameters (neck length, neck-stem angle, and anteversion angle) on the capacity of muscles to generate force and moment about the hip. A three-dimensional model that calculates the maximum isometric forces and moments generated by 25 muscles crossing the hip over a wide range of body positions was used to evaluate the effects of a 2 cm elevation of the hip center and changes in the prosthetic parameters. After superior displacement of the hip center, the neck length was increased from 0 to 3 cm, the neck-stem angle was varied between 110 and 150 degrees, and the anteversion angle was varied between 0 and 40 degrees. Our analysis showed that a 2 cm superior displacement of the hip center would decrease the moment-generating capacity of the four muscle groups studied (abductors, adductors, flexors, and extensors) if neck length were not increased to compensate for decreased muscle length. In the computer model of an adult man that we used, a 2 cm increase in neck length restored the moment-generating capacity of the muscles by increasing muscle length and force-generating capacity. However, a 3 cm increase in neck length increased passive muscle forces substantially, which potentially could limit joint motion. An increased neck-stem angle (i.e. a valgus neck) decreased the abduction moment arm but increased the moment-generating capacity of the other muscle groups. A change in the anteversion angle from 0 to 40 degrees had a relatively small effect on the isometric moment-generating capacity of the muscles studied.
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