Change of force and lever arm of the hip abductors after subtrochanteric de-/rotational osteotomy

Abstract

Purpose: Previous studies have shown that rotational osteotomy of the femur reliably corrects anatomical torsion but may result in a ±1 cm antero-posterior shift of the greater trochanter (GT) per 10° of torsional correction. It is unclear whether an inadvertent displacement of the GT following surgical treatment of femoral maltorsion affects the hip abductors in a clinically relevant manner and needs to be addressed by compensatory digastric trochanteric osteotomy. The aim of this study was to investigate the influence of rotational subtrochanteric osteotomy on hip abductor force generation using three-dimensional (3D) surface models.

Methods: A 3D statistical shape model (SSM) of a hip/femur was used. From a baseline value of 12°, the SSM was derotated/rotated in 10° increments, creating five different scenarios of anatomical femoral torsion: 32°, 22°, 12° (baseline), 2°, -8° (corresponding to 8° of retro torsion). The origins and insertions of the gluteus medius (GMed) and minimus (GMin) muscles were created. The hip abductor moment was defined as the muscle force (of the hip abductor muscles) multiplied by the force ratio in the direction of abduction (FRDA) multiplied by the lever arm. All values were measured, whereas muscle strength was defined as the length of the muscle fibres in relation to their optimal length.

Results: Results indicated minimal changes in muscle length (±1.2%) and FRDA (1.7% to -7%) for GMed und GMin across all scenarios. However, lever arm demonstrated variations (21% to -10%), with an increase observed for derotational osteotomies and a decrease for rotational osteotomies.

Conclusion: Subtrochanteric osteotomy in the management of femoral maltorsion affects the hip abduction moment solely through the altered lever arm. A 20° derotation for increased femoral torsion (FT) corresponds to a 20% increase in abduction force of the GMed (improved lever arm), whereas a 20° rotation for decreased FT reduces the abduction force by 10%.

Level of evidence: Level III, diagnostic.

Keywords: 3D analysis; abductor force; subtrochanteric osteotomy.

Figure 1

(a) Hip abductors divided into separate subbranches (gluteus medius in purple and gluteus minimus in orange). (b) Force vectors of the hip abductors. The hip abductor moment is calculated as multiplication of the muscle force (of the hip abductor muscles) multiplied by the force ratio in the direction of abduction (FRDA—green arrow) multiplied by the lever arm (blue arrow). FRDA, force ratio in the direction of abduction.

Figure 2

Anatomical femoral torsion is defined as the two‐dimensional angle between the femoral neck axis (shown in red) and the retrocondylar plane (shown in blue), projected onto the axial plane of the hip coordinate system. In contrast, functional femoral torsion uses the greater trochanter centre instead of the femoral neck centre as an anatomical reference (not shown). The lever arm (averaged across all subbranches) of the gluteus medius muscle (GMed) is illustrated in purple and that of the gluteus minimus muscle (GMin) in orange.

Figure 3

Axial view showing simulated anatomical femoral torsion of 32° (red), 12° (yellow) and −8° (blue).

Figure 4

Relationship of the lever arm of the hip abductor muscles and anatomical femoral ante torsion. The maximum lever arm for gluteus medius and minimus muscles was 8° retrotorsion and 2° antetorsion, respectively.