Normative values of the alpha angle and triangular index measured from the hip radiographs of an African population

Undetected femoroacetabular impingement (FAI), caused by asphericity of the femoral head, has been implicated as a leading cause of hip osteoarthritis in later stages. Two known types of impingement of the femur head with the acetabulum include the cam-type and pincer type, and the alpha angle and triangular index are indices applied in determining the presence of FAI in a hip radiograph.

To determine the normal range and upper limit of normal of the alpha angle and triangular index of an African population using non-pathological hip radiographs.

Hip radiographs of 104 subjects reported to be normal were retrospectively studied. The images were reassessed jointly by three radiographers and afterwards by three consultant radiologists. The alpha angle and triangular index of selected radiographs were measured with appropriate electronic callipers. Values obtained were correlated with the age and gender of the patients.

The mean ± SD of the alpha angle for the population studied was 46.70 ± 12.34⁰ with a range of 33.30⁰-96.82⁰ (males: 45.09 ± 12.03⁰, females: 47.32 ± 12.37⁰), and the upper limit of normal obtained was 79.16⁰. The mean triangular index was 1.71±1.38 with a range of 0.03-5.98 (males: 1.77 ± 1.39, females: 1.65 ± 1.36), 4.12 being the upper limit of normal. There was no statistically significant difference between measurements of both indices with age and gender. However, there was excellent agreement between and within raters for both measurements (alpha angle: 0.992; triangular index: 0.924).

The upper limits of normal for the alpha angle and triangular index of the studied population are 79.16⁰ and 4.12. Age and gender were independent of the values measured, and the measurements demonstrated excellent inter-rater and intra-rater agreement. A normal range is thus provided, with upper limits beyond which a possible femoroacetabular impingement may be diagnosed.