Acetabular sector angle
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The acetabular sector angles are a set of angles, comprising the anterior acetabular sector angle (AASA), the posterior acetabular sector angle (PASA) and horizontal acetabular sector angle (HASA) and are used on cross-sectional imaging, especially pelvic CT, for the evaluation of acetabular coverage.
Acetabular sector angles have been used for the assessment of anterior, posterior and global acetabular undercoverage in the setting of acetabular dysplasia and are useful for surgical planning of acetabular reorientation surgery 3,4. There is also potential use in acetabular overcoverage 5.
The acetabular sector angles are determined based on a line connecting the centers of the two femoral heads 1-3.
- AASA: obtained by measuring the angle between a line connecting the center of the two femoral heads and the anterior acetabular margin
- PASA: obtained by measuring the angle between a line connecting the center of the two femoral heads and the posterior acetabular margin
- HASA: calculated by adding the anterior and posterior sector angles
The suggested level of measurement is the center of the femoral head 2.
The following mean and standard deviations were suggested 1,2 as well as cut off values for acetabular dysplasia 4:
- AASA: 63° ± 6.1° and ≤50° indicating anterior undercoverage
- PASA: 105° ± 7.9° and ≤90° indicating posterior undercoverage
- HASA: 168° ± 11° and ≤140° indicating global undercoverage
Potential measurement errors
A change of 1° in pelvic tilt leads to the following change 1,2:
- anterior acetabular sector angle (AASA): 0.7°-1.1°
- posterior acetabular sector angle (PASA): -0.5°
A change of 1° in pelvic obliquity leads to the following change 2:
- anterior acetabular sector angle (AASA): 1.9°
- posterior acetabular sector angle (PASA): 1°
- horizontal acetabular sector angle (HASA): 2.8°
History and etymology
The acetabular sector angles were first described by Svein Anda in 1986 1.
- 1. Anda S, Svenningsen S, Dale L, Benum P. The Acetabular Sector Angle of the Adult Hip Determined by Computed Tomography. Acta Radiologica Diagnosis. 1986;27(4):443-7. doi:10.1177/028418518602700415 - Pubmed
- 2. Anda S, Terjesen T, Kvistad K. Computed Tomography Measurements of the Acetabulum in Adult Dysplastic Hips: Which Level is Appropriate? Skeletal Radiol. 1991;20(4):267-71. doi:10.1007/bf02341662 - Pubmed
- 3. van Bosse H, Lee D, Henderson E, Sala D, Feldman D. Pelvic Positioning Creates Error in CT Acetabular Measurements. Clinical Orthopaedics & Related Research. 2011;469(6):1683-91. doi:10.1007/s11999-011-1827-9 - Pubmed
- 4. Beltran L, Rosenberg Z, Mayo J et al. Imaging Evaluation of Developmental Hip Dysplasia in the Young Adult. AJR Am J Roentgenol. 2013;200(5):1077-88. doi:10.2214/ajr.12.9360 - Pubmed
- 5. Valera M, Ibáñez N, Sancho R, Llauger J, Gich I. Acetabular Overcoverage in the Horizontal Plane: An Underdiagnosed Trigger of Early Hip Arthritis. A CT Scan Study in Young Adults. Arch Orthop Trauma Surg. 2017;138(1):73-82. doi:10.1007/s00402-017-2811-y - Pubmed