Hip ultrasound for developmental dysplasia: the 50% rule

Abstract

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.Early in the development of hip ultrasound, there was a need to establish its validity in comparison with established techniques like the pelvic radiograph. For acetabular development, the radiographic "acetabular index (angle)" was, and still is, the gold standard. In our quest to validate the coronal flexion sonogram as a reflection of acetabular development, we enlisted Christian Morin, a pediatric orthopedic fellow from France, to compare femoral head coverage as seen by sonogram with a standard pelvic radiograph obtained at the same time. It is important to note the ultrasound technology of the time (1984) was the mechanical sector transducer, which showed a curved configuration of the iliac bone and had less resolution. Comparing bony acetabular depth (d) with the diameter of the cartilaginous femoral head (D) produced a fraction easily converted to a percentage. When comparing percentage of coverage with measure of acetabular angle, we found that 58% or greater coverage always correlated with an accepted normal value. Similarly, coverage of less than 33% always reflected an abnormal radiographic acetabular angle [7]. Many of our sonograms fell in the 33-58% coverage range, where the radiograph measure could be either normal or abnormal. Terjesen et al.[8] also compared radiography and ultrasound using a measure of acetabular coverage called "bony roof %." This measure used different landmarks on a coronal neutral view and found some cases with normal ultrasound and dysplasia by radiographic index [8].When sonography transitioned to linear-array transducers, thereby achieving better resolution a...

The natural history of abnormal ultrasound findings in hips of infants under six months of age

“…In the context of the slight differences in the definition of abnormal between different authors and the different age groups in the FHC classification, the coverage of 50% was defined as the border between normal and abnormal in the articles that reported on the follow-up of FHC. 17 The time period of non-treatment in these reports varied between two weeks and eight years. The times of follow-up of different groups varied depending on the start of treatment of hips that did not develop to Graf I or a FHC of more than 50% at the different times of follow-up.…”

An audit of referral time frames for ultrasound screening of developmental hip dysplasia in neonates with a normal antenatal clinical examination

“…The results of this audit support DDH screening ultrasounds be performed at 6 weeks of age as per the set guidelines by the Australasian Society for Ultrasound in Medicine . Current literature suggests initial examinations between 4 and 8 weeks of age, which can allow for some flexibility with bookings and permit slightly later examinations with prematurity. By booking screening DDH examinations in this time frame, there will be a lower incidence of follow‐up required without an increased risk of long‐term negative outcomes.…”

“…The Graf method was used to calculate alpha and beta angles . The Morin technique was used to calculate femoral head coverage, and the Harcke method was used to dynamically assess the hips for laxity . All examinations were performed by specialist paediatric sonographers and formally reported by paediatric radiologists.…”

“…Any patient that had a suboptimal result from their initial scan also had their subsequent scan results reviewed. Examinations were considered subnormal if the Graf classification was 2A or lower, there was laxity present or femoral head coverage was less than 50% …”

Interobserver agreement and clinical disparity between the Graf method and femoral head coverage measurement in developmental dysplasia of the hip screening

“…[ 22 ] The FHC ratio was calculated as the distance between the base line and the parallel line connected to the most medial femoral head divided by the distance between the two lines parallel to base line and connected to the most lateral and medial femoral head. [ 19 , 23 ] . FHC < 50% was defined as abnormal.…”