Background: Universal ultrasound screening has led to overtreatment and higher follow-up rates than are found with clinical examination alone because of high incidence of physiologically immature hips (type IIa) in the first weeks of life. The
ability to predict future acetabular development in physiologically immature hips (type IIa) would therefore help to reduce
overtreatment and unnecessary follow-up.
Methods: We described the g-angle to assess the femoral head coverage by the acetabular roof, which is measured between the baseline defined by Graf and the cartilaginous edge line connecting the inferior point of the iliac bone (lower limb) to the
medial corner of the acetabular labrum. We retrospectively analyzed ultrasonographic findings of infants with developmental
dysplasia of the hip diagnosed in our hospital and infants with normal hips screened in our hospital. Group 1 (35 hips) consists
of type IIa hips at initial examination and went on to develop into dysplastic hips at follow-up. Group 2 (279 hips) consists of
type IIa hips at initial examination and went on to develop into normal hips (type I) at follow-up.
Results: The g-angles of type IIa hips that developed into type I hip at follow-up ranged between 77 and 82. The g-angles of type IIa hips that developed into hip dysplasia ranged between 72 and 78. All type IIa hips that had g-angles >78 degrees developed into normal hips. We also observed that all type IIa hips that had g-angles <77 degrees developed into dysplasia.
Conclusions: The amount of cartilage mass covering the femoral head, which is a part of the acetabular roof, can therefore
provide information about future acetabular development. This paper describes a new method of measurement (the g-angle) that assesses the extent of the cartilage coverage of the femoral head, which can predict acetabular development. Its use would decrease the rates of unnecessary follow-up and treatment.