Normal Lateral Acetabular Inclination Angle Size Radiography
The lateral acetabular inclination angle (also known as the acetabular roof angle or Sharp's angle variant) quantifies the obliquity of the acetabular weight-bearing surface relative to the horizontal on an anteroposterior (AP) pelvic radiograph. Accurate measurement is essential for evaluating hip joint congruency, load distribution, and susceptibility to osteoarthritis or instability.
Normal Reference Values
| Orientation | Measurement |
|---|---|
| Anteroposterior | 30-50¡ |
Clinical Significance
The normal lateral acetabular inclination angle on AP radiography falls between 30° and 50°. Angles below 30° suggest a horizontally oriented acetabulum, which may indicate acetabular overcoverage (coxa profunda or protrusio acetabuli) and is associated with femoroacetabular impingement (FAI). Conversely, angles exceeding 50° reflect a more vertical acetabulum, which reduces femoral head coverage and predisposes to instability and accelerated cartilage wear.
Precise measurement guides surgical planning for procedures such as periacetabular osteotomy (PAO) and total hip arthroplasty (THA) cup positioning. In THA, the acetabular component is commonly targeted within a "safe zone" near the lower end of the normal range to minimize dislocation risk and wear.
- Angle <30°: Acetabular overcoverage — evaluate for FAI (pincer type) or protrusio acetabuli
- Angle >50°: Acetabular dysplasia — consider developmental dysplasia of the hip (DDH)
- Asymmetric inclination: Post-traumatic deformity or asymmetric growth plate injury
- Progressive steepening: Erosive arthropathy (e.g., rheumatoid arthritis) with medial wall migration
- Post-surgical change: Assess component positioning following THA
Reference: Hoppenfeld S, M.D. MS. Orthopaedic Dictionary. Lippincott Williams & Wilkins. (1994).
Imaging Notes
Measurement is performed on a standing or supine AP pelvis radiograph with the patient positioned symmetrically — legs in neutral rotation and the pelvis level. The angle is formed between a horizontal reference line (connecting the inferior margins of the ischial tuberosities or the inter-teardrop line) and a line drawn along the lateral acetabular sourcil (weight-bearing roof). Consistent technique is critical; pelvic tilt and rotation significantly alter the measured angle and can lead to over- or underestimation of acetabular obliquity.
Ensure adequate radiographic exposure to clearly delineate the sourcil, acetabular teardrops, and the ilioischial line. When plain radiographic landmarks are obscured (e.g., obesity, overlying bowel gas), CT with multiplanar reformation provides more reliable measurement and three-dimensional characterization of acetabular orientation.