Hip ultrasound for developmental dysplasia: the 50% rule

Excerpt

For those who perform infant hip ultrasounds, there has always been the issue of measurement as part of the sonographic assessment. The 2013 American Institute of Ultrasound in Medicine (AIUM) practice parameter states: “Validation by angle and femoral head coverage measurement is optional” [1]. The “Graf” followers have subscribed to the tables of alpha and beta angles and find publications illustrating their use and results [2, 3]. Those who do not wish to use angles and rely on the “Dynamic (Harcke)” technique for position and stability [4, 5] have other criteria for acetabular development [6]. Our group directed attention to the coverage of the femoral head by the bony acetabulum [7] as well as to the slope and configuration of the iliac bone. As a consequence, we are referenced as the source of the 50% rule: “In a normal hip, the acetabulum should cover 50% (one half or more) of the femoral head.” While we have not discouraged the use of this guideline, we have not formally published it as dictum. This review seeks to clarify the rule, present data that support its use, and note pitfalls that can occur with its application. As a caveat, this discussion assumes that when reference is made to a coronal sonogram, whether the hip is extended (neutral) or flexed, the image correctly shows: (1) a straight iliac line, (2) the junction of the iliac bone and tri-radiate cartilage and (3) the echogenic tip of the labrum. …