Hip joint instability after the neonatal period. I. Value of measuring the acetabular angle.

The acetabular angle was measured by two independent observers in a consecutive series of 29 infants with idiopathic instability of the hip. The diagnosis was made at the age of 1 1/2 to 21 months, and was confirmed by arthrography. The average difference in the measurement values between the two observers was less than 2 degrees. In infants with unilateral instability the acetabular angle was always greater on the unstable side. This difference decreased after treatment. Criteria for pathologic angles are discussed. Measurement of the acetabular angle is of greatest value for assessing the effect of treatment, but is of limited usefulness in diagnosing instability, especially during the first 6 months of life, as the cartilaginous component of the joint is not included in the measurement. Arthrography is recommended in cases difficult to evaluate.