Acetabular Index (AI) (AC Angle)

Description

Hilgrenreiner used the acetabular index (AC angle) to evaluate infants or young children for congenital hip dysplasia.

Radiograph: anteroposterior (AP) of the pelvis with the patient supine and the legs in the neutral position

For reproducible results, care needs to be taken in positioning the patient. The angle maybe distorted if the pelvis is inclined or rotated. The diameter of the right and left obturator foramina should be the same or nearly the same (see page 112, Tonnis, 1984).

Lines on the radiograph:

(1) Hilgenreiner's line = horizontal line through the centers of the triradiate cartilages

(2) line from the point on the ilium to the superior acetabular rim (on a point lying on the sclerotic line of the acetabular roof)

The acetabular index is the angle where the 2 lines intersect.

Interpretation:

• An angle less than 1 SD above the mean are considered normal.

• An angle greater than 2 SD above the mean for age and gender is severely dysplastic.

• An angle >= 1 SD above the mean for age and gender is considered as borderline or mildly dysplastic.

Female, Left Hip

Age in Months	Mild Dysplasia (mean + 1 SD)	Severe Dysplasia (mean + 2 SD)
1 or 2	36	41.5
3 or 4	33	38.5
5 or 6	29.5	34
7 or 9	27	31.5
10 to 12	27	31.5
13 to 15	27	31.5
16 to 18	26	30.5
19 to 24	25.5	30.5
25 to 36	23.5	27
37 to 60	21	25.5
61 to 84	20	23.5

Female, Right Hip

Age in Months	Mild Dysplasia (1s)	Severe Dysplasia (2s)
1 or 2	36	41.5
3 or 4	31.5	36.5
5 or 6	27.5	32
7 or 9	25.5	29.5
10 to 12	24.5	29
13 to 15	24.5	29
16 to 18	24.5	29
19 to 24	24	28
25 to 36	22	25.5
37 to 60	18	22.5
61 to 84	18	23

Male, Left Hip

Age in Months	Mild Dysplasia (1s)	Severe Dysplasia (2s)
1 or 2	31	35
3 or 4	29	33.5
5 or 6	27	31.5
7 or 9	25.5	29.5
10 to 12	25	29
13 to 15	24	27.5
16 to 18	24	27.5
19 to 24	23	27
25 to 36	22.5	27
37 to 60	20	24
61 to 84	19	23.

Male, Right Hip

Age in Months	Mild Dysplasia (1s)	Severe Dysplasia (2s)
1 or 2	29	33
3 or 4	28	32.5
5 or 6	24.5	29
7 or 9	24.5	29
10 to 12	23.5	27
13 to 15	23	27.5
16 to 18	23	26.5
19 to 24	21.5	26.5
25 to 36	21	25
37 to 60	19	23.5
61 to 84	17	21

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Description

Specialty

Objective

ICD-10

Purpose: To evaluate the hips of an infant or young child for congenital hip dysplasia using the acetabular index of Hilgenreiner.

Description

Specialty

Objective

ICD-10