How do acetabular version and femoral head coverage change with skeletal maturity?

Abstract

Background: Normal changes in acetabular version over the course of skeletal development have not been well characterized. Knowledge of normal version development is important because acetabular retroversion has been implicated in several pathologic hip processes.

Questions/purposes: The purpose of this study was to characterize the orientation of the acetabulum by measuring (1) acetabular version and (2) acetabular sector angles in pediatric patients during development. We also sought to determine whether these parameters vary by sex in the developing child.

Methods: We evaluated CT images of 200 hips in 100 asymptomatic pediatric patients (45 boys, 55 girls; mean age, 13.5 years; range, 9-18 years) stratified by the status of the triradiate physis and sex. We determined the acetabular anteversion angle at various levels in the axial plane as well as acetabular sector angles at five radial planes around the acetabulum.

Results: For both genders, anteversion angle was greater for the closed physis group throughout all levels (p < 0.001) and both open and closed physis groups were more anteverted as the cut moved caudally away from the acetabular roof (p < 0.001). At the center of the femoral head, the mean anteversion angle (± SD) in girls was 15° ± 3° in the open group and 19° ± 5° in the closed group (p < 0.001). In boys, the mean anteversion angle increased from 14° ± 4° in the open group to 19° ± 4° in the closed group (p = 0.003). In the superior, posterosuperior, and posterior planes, the acetabular sector angles were greater in the closed compared with the open physis group for both boys and girls with the largest increase occurring in the male posterosuperior plane (approximately 20°) (all p < 0.05).

Conclusions: This study demonstrates that acetabular anteversion and acetabular sector angles in both male and female subjects increase with skeletal maturity as a result of growth of the posterior wall. This suggests that radiographic appearance of acetabular retroversion may not be attributable to overgrowth of the anterior wall but rather insufficient growth of the posterior wall, which has clinical treatment implications for pincer-type impingement.

Fig. 1

Coronal reformatted image from a MDCT scan demonstrates reformatting technique for axial plane images through the acetabulum. Contiguous axial images at 1.0-mm spacing and 0.6-mm thickness through both acetabula were performed.

Fig. 2

Coronal reformatted images through the center of the femoral demonstrate open and closed triradiate cartilage physes bilaterally (white arrows) are shown.

Fig. 3

Axial reformatted images demonstrate open and closed anterior (black arrows) as well as posterior rim secondary ossification center (white arrows) bilaterally.

Fig. 4

Axial reformatted CT image through the pelvis demonstrates the measurement technique for determining the anteversion angle. The anteversion angle is the angle between a line connecting the anterior and posterior acetabular rim and a line perpendicular to a centerline through both femoral heads.

Fig. 5A–D

Axial reformatted CT images through the center of both femoral heads demonstrate the technique for measuring anterior acetabular sector angle (AASA) (A), posterior acetabular sector angle (PASA) (B), posterior superior sector angle (PSSA) (C), and superior sector angle (SSA) (D).

Fig. 6A–B

(A) Acetabular version is based on the distance from the acetabular roof for females. (B) Acetabular version is based on the distance from the acetabular roof for males.

Fig. 7A–B

(A) Acetabular sector angles are shown at five different directions in females. Asterisks denote significant difference between open and closed triradiate physes (p < 0.05). (B) Acetabular sector angles are shown at five different directions in males. Asterisks denote significant difference between open and closed triradiate physes (p < 0.05). AS = anterosuperior; PS = posterosuperior.