Ultrasound of Newborn Hip Joints by Graf: Norms of Alpha and Beta Angles
Standard Projection and Structures
Assessment by Graf's method is performed in the longitudinal coronal projection ('egg in spoon'). The transducer is placed parallel to the lateral surface of the infant's thigh; in usual practice, it is tilted 10–15° (usually posteriorly) to obtain a straight line of the iliac bone. The child is positioned on their side (or back), with the hip and knee flexed approximately 20%. The image should visualize: the femoral head, the bony and cartilaginous roof of the acetabulum, the labrum, the triradiate cartilage, the ischium, and the ossified metaphyseal part of the femur, as well as the straight contour of the iliac bone.
In the transverse projection, a normal joint gives the appearance of 'ice cream cone': the head is well-centered in the socket, surrounded by the metaphysis anteriorly and the ischium posteriorly, forming a U-shaped configuration.
Line Construction and Angle Measurement
The baseline is drawn along the anterior cortex of the iliac bone. The line drawn from the promontory along the roof of the acetabulum determines the alpha angle. The line drawn from the promontory to the apex of the labrum determines the beta angle. The alpha angle is located between the baseline and the bony roof line, the beta angle is between the baseline and the line to the labrum.
Angle Norms
| Angle | What it reflects | Norm |
|---|---|---|
| Alpha (α) | The bony part of the acetabulum; changes occur gradually | >60° |
| Beta (β) | The cartilaginous part of the socket (labrum); changes occur faster | <55° |
Physiological Immaturity (Type IIa/IIb)
In joint immaturity, the angles differ from the norm:
| Type | Age | Alpha | Beta |
|---|---|---|---|
| IIa | physiological immaturity <3 months | 50–59° | 55–77° |
| IIb | immaturity >3 months | 50–59° | 55–77° |
Morphological and Dynamic Assessment
Graf's method involves assessing the portion of the femoral head contained in the bony socket and the position of the head center relative to the iliac line. Coverage landmarks of the head by the bony part of the socket: capsular weakness — coverage ~50%; subluxation — coverage <50% at rest (in some centers <33% is defined as subluxation, and 33–50% as an indeterminate result); dislocation — the head is completely outside the bony socket at rest. Stress tests are applied with counterpressure to the infant's back.
Modified Method
Rosendahl's method (modified Graf) involves measuring only the alpha angle of the bony acetabulum in the coronal/flexion projection.
Technical Notes
Too anterior transducer position gives a false positive result, too posterior gives a false negative. Stress tests are not performed if the joint is examined in a Pavlik harness or other fixation device.
Frequently asked questions
What are the norms of alpha and beta angles in a mature joint?
The alpha angle is normally >60°, and the beta angle is normally <55°.
What do the alpha and beta angles reflect?
Alpha reflects changes in the bony part of the acetabulum (gradual), while beta reflects changes in the cartilaginous part/labrum, which occur faster.
What are the angle values for type IIa/IIb according to Graf?
For both types, alpha is 50–59°, and beta is 55–77°. IIa is physiological immaturity <3 months, IIb is immaturity >3 months.
How to correctly draw the baseline for measurement?
The baseline is drawn along the anterior cortex of the iliac bone; the alpha line is from the promontory along the roof of the socket, and the beta line is from the promontory to the apex of the labrum.
How is head coverage classified in subluxation and dislocation?
Subluxation is head coverage by the bony socket <50% at rest (in some centers <33%, 33–50% is an indeterminate result); dislocation is when the head is completely outside the bony socket at rest.