Indications to perform an ultrasound evaluation for DDH:
- Abnormal findings on physical examination of the hip;
- Newborns/infants with a normal physical examination of the hip but having risk factors for DDH.
Risk factors for DDH:
- Family history of DDH;
- Breech birth;
- Postural molding conditions (torticollis,
skull or foot deformities);
- Primiparity;
- Macrosomia;
- Oligohydramnios;
- Female sex.
In the screening setting,
the ultrasound evaluation for DDH must be performed between 4 and 6 weeks of life.
Technique
Ultrasound evaluation of the hip is performed with a linear transducer using the highest frequency that provides adequate penetration of soft tissues to the required depth.
The infant should be positioned in lateral decubitus,
although supine position may also be used.
Graf’s method
Many methods are described,
but the most used is the one proposed by Graf,
which is a static method based on the acetabular morphology in a standard coronal plane.
This view can be obtained with the hip in either the physiologic neutral position (15°-20° flexion) or the flexed position.
The ultrasound transducer is placed over the lateral aspect of the hip and its superior edge is rotated posteriorly by 10 to 15º in order to identify a plane which fulfills the following criteria:
- the osseous contour of the ilium appears as a horizontal echogenic line;
- the midportion of the acetabulum is visualized to its maximal depth;
- the triradiate cartilage is seen;
- the middle of the fibrocartilaginous labrum is visualized.
Normal anatomic findings in the standard coronal plane are shown in Fig. 1.
After obtaining the standard coronal plane,
some measurements must be performed:
- alpha angle: measured between the bony acetabular roof and the vertical cortex of the ilium.
The normal value is greater than or equal to 60 degrees.
- beta angle: measured between the vertical cortex of the ilium and the fibrocartilaginous labrum.
Fig. 2: Coronal ultrasound image depicting how to measure alpha and beta angles.
References: Radiology, Centro Hospitalar Universitário do Algarve - Algarve/PT
These measurements,
mainly the alfa angle,
are used to classify the hip into different types,
according to Graf ( Fig. 3 ).
Femoral head coverage (Fig. 4)
Additionally,
the standard coronal plane may be used to assess the femoral head coverage,
corresponding to the degree of lateralization of femoral head.
This method was first proposed by Morin and is based on a ratio – the Morin Index.
Three lines are drawn:
- one through the contour of the ilium (as in the Graf’s method);
- two lines paralleling the line first described - one tangent to the lateral part of the femoral head and another one tangent to the medial junction of the femoral head and acetabular fossa.
Then,
it is necessary to measure the perpendicular distances between medial and iliac lines and between medial and lateral lines.
Finally,
the ratio between the first and the second measurements multiplied by 100 results in the percentage of femoral head covered by bony acetabulum.
This calculation is semi automatically performed by some ultrasound equipment,
provided the performer radiologist draw the line through the contour of the ilium and a circumference adjusted to the femoral head contour.
Values equal to or greater than 50 % are normal.
Dynamic evaluation
In dynamic evaluation,
stress maneuvers corresponding to the Ortolani and Barlow’s tests are performed while the hip is being scanned in the coronal and transverse planes.
The infant is positioned in lateral decubitus or supine position,
with the hip flexed,
while posterior stress is applied to the knee.
In the normal hip,
the femoral head is well seated within the acetabulum,
both at rest and during provocative stress.
If the hip is lax or dislocatable,
the femoral head moves lateral and posterior during provocative stress.
In these situations,
femoral head coverage also decreases compared to the rest position.
In our institution,
a combination of the three methods described is used in the ultrasound evaluation for DDH.
Graf’s classification and illustrative cases
Graf’s classification is mainly based on the alpha angle,
although other parameters,
such as the age of the patient and the beta angle are used to differentiate between some categories.
Fig. 3: Infant hip types according to Graf’s classification.
- Type I refers to a normal,
mature hip,
with an alpha-angle ≥60°.
Fig. 5: Normal/mature hip – Graf type I hip (alpha-angle>60º).
References: Radiology, Centro Hospitalar Universitário do Algarve - Algarve/PT
- Type IIa refers to an immature hip in an infant less than 3 months old,
with an alpha-angle of 50-59°.
Fig. 6: Physiological immature hip in a 5-week-old boy – Graf type IIa hip. Both alpha angle and femoral head coverage are slightly decreased. Revaluation at 3 months of age showed mature hip findings.
References: Radiology, Centro Hospitalar Universitário do Algarve - Algarve/PT
- In type IIb,
the alpha-angle is also 50-59°,
but the infant is older than 3 months,
thus representing a delay in hip development and a sign of dysplasia.
Fig. 7: Dysplastic hip in a 3-month-old infant – Graf type IIb (alpha-angle 50-59º). Femoral head coverage is abnormal (<50%).
References: Radiology, Centro Hospitalar Universitário do Algarve - Algarve/PT
- In type IIc the bony rim is very rounded or flat,
with an alpha angle of 43-49°,
but the cartilaginous roof and labrum still cover the femoral head.
Fig. 8: Dysplastic hip in a 2-week-old girl – Graf type IIc (alpha-angle 43-49º; beta-angle < 77º). Femoral head coverage is also abnormal (<50%).
References: Radiology, Centro Hospitalar Universitário do Algarve - Algarve/PT
- Type D is similar to type IIc except for the displaced cartilage roof that increases the β-angle (>77°).
Fig. 9: Decentered hip in a 5-week-old girl – Graf type D hip (alpha-angle 43-49º; beta-angle > 77º).
References: Radiology, Centro Hospitalar Universitário do Algarve - Algarve/PT
- Type III refers to an eccentric hip,
with an alpha angle < 43° and the labrum displaced/pressed upwards.
Fig. 10: Eccentric hip in a 8-week-old girl - Graf type III hip. The bony acetabulum is flattened and the labrum is pressed upwards.
(Courtesy of Dr. Miguel Oliveira e Castro)
References: Radiology, Centro Hospitalar Universitário do Algarve - Algarve/PT
- In type IV the femoral head is dislocated in a way that presses the cartilaginous roof and labrum against the bony acetabular edge.
Thus,
the labrum is displaced downwards.
Fig. 11: Eccentric hip - Graf type IV hip. Labrum - yellow arrow - is inverted and compressed against the bony acetabular edge by the femoral head. Bony rim is flattened. Note the pulvinar’s hypertrophy - green arrow.
(Courtesy of Dr. Miguel Oliveira e Castro)
References: Radiology, Centro Hospitalar Universitário do Algarve - Algarve/PT