Normal Acetabular Angle Size in Girls: Radiography & CT
The acetabular angle (also called the acetabular index) is a measurement of acetabular roof inclination on anteroposterior (AP) pelvic radiographs, reflecting the degree of lateral acetabular coverage of the femoral head. In girls, this angle is normally slightly larger than in boys and decreases progressively with skeletal maturation as the acetabulum deepens. Accurate measurement is essential for the early detection and monitoring of developmental dysplasia of the hip (DDH).
Normal Reference Values
| Orientation | Age | Measurement |
|---|---|---|
| Anteroposterior | Newborn | 24.0-33.6¡ |
| Anteroposterior | 3 months | 21.5-28.5¡ |
| Anteroposterior | 6 months | 19.2-27.2¡ |
| Anteroposterior | 1 y/o | 17.4-25.0¡ |
| Anteroposterior | 2 y/o | 14.0-22.0¡ |
Clinical Significance
The acetabular angle decreases with normal development, reflecting progressive ossification and remodeling of the acetabular roof. In girls, values are characteristically slightly higher than in boys at all age groups. An elevated acetabular angle indicates inadequate acetabular coverage and is a key radiographic marker for developmental dysplasia of the hip (DDH). Values persistently above approximately 30° after 3 months of age, or failing to show the expected age-related decline, warrant further evaluation and orthopedic consultation.
Common pitfalls include patient rotation on the AP radiograph — even mild obliquity falsely alters angle measurement. The Hilgenreiner line must be drawn correctly and symmetrically for reproducible results. In infants under 4–6 months, the femoral head is largely cartilaginous and not visible on radiograph, making the acetabular angle the primary objective measurement available.
- Developmental dysplasia of the hip (DDH) — elevated or non-declining acetabular angle
- Neuromuscular hip dysplasia — e.g., cerebral palsy, spina bifida
- Skeletal dysplasia — achondroplasia, spondyloepiphyseal dysplasia
- Post-septic arthritis sequelae — secondary acetabular remodeling abnormalities
- Normal variant / positioning artifact — always verify radiograph quality before interpreting
Reference: Ozonoff MB. Pediatric orthopedic radiology. W B Saunders Co. p. 181. (1992).
Imaging Notes
On AP pelvic radiographs, the acetabular angle is measured by drawing the Hilgenreiner line (horizontal line through the triradiate cartilages) and a second line along the acetabular roof from the medial acetabular corner to the lateral acetabular edge. The angle formed between these two lines is the acetabular index. Optimal technique requires a true AP projection with the pelvis level and the hips in neutral position; rotation or hip flexion significantly degrades measurement accuracy. A grid and standard exposure parameters appropriate for infant pelvis size are recommended.
On CT, the acetabular angle can be measured on coronal reformats using the same geometric principle, offering superior bony detail and eliminating the superimposition seen on plain radiographs. CT is generally reserved for preoperative planning or equivocal cases where ultrasound and radiography are inconclusive, given the ionizing radiation dose in the pediatric population. Multiplanar reformats should be standardized to true coronal orientation before measurements are obtained.