X-ray of hip dysplasia

Projectional radiography ("X-ray") is one of the two main modalities of medical imaging to diagnose hip dysplasia, the other one being medical ultrasonography.^[1].^[2] Ultrasound imaging yields better results defining the anatomy until the cartilage is ossified. When the infant is around 3 months old a clear roentgenographic image can be achieved. Unfortunately the time the joint gives a good x-ray image is also the point at which nonsurgical treatment methods cease to give good results.

In children

Image quality checking

Image quality checking.

Reliability of measurements increases if indicators of pelvic alignment are taken into account:

• Tönnis introduced a quotient of pelvic rotation by dividing the horizontal diameter of the obturator foramen of the right side and that of the left. In neutral rotation the ratio is 1 but is considered to be acceptable when it is between 0.56 and 1.8.^[3]
• The symphysis os-ischium angle of Tönnis evaluates the pelvic position in the sagittal plane. Lines are drawn from the highest point of the ischium to the most prominent point of the symphysis, joining at the inside of the pelvis. The range of normal values is from 90 to 135° and is related to the infant’s age.^[3]

In adults

X-ray of the hips of a 40-year-old female, with dysplasia of her right hip.

In the adult hip there are important landmarks to be recognized on plain film radiographs (Figure 5):^[3]

• (A) Iliopectineal or iliopubic line is formed by the arcuate line of the ilium and the superior border of the superior pubic ramus up to the pubic symphysis. It conforms to the inner margin of the pelvic ring and it is part of the anterior column of the acetabulum.
• (B) The ilioischial line of Köhler begins at the medial border of the iliac wing and extends along the medial border of the ischium to end at the ischial tuberosity. It is part of the posterior column of the acetabulum.
• (C) Acetabular Floor. In normal conditions the floor of the acetabular fossa is lateral to the ilioischial line by 2 mm in men and 1 mm in women. When the acetabular floor overlaps or overpasses the ilioischial line, the diagnosis of coxa profunda can be made. Nevertheless, coxa profunda had been found in 76% of asymptomatic hips, mainly in women. Therefore, this as an isolate criterion is not enough to make the diagnosis of pincer-type impingement. A more severe condition is protrusio acetabuli, diagnosed when the femoral head overlaps or overpasses the ilioischial line (Figure 5).
• (D) The teardrop represents a summation of shadows. Its medial aspect corresponds to the inner cortex of the pelvis and the lateral edge with the acetabular notch and the anteroinferior portion of the quadrilateral plate. It is not present at birth but gradually develops due to pressure of the femoral head.

In the adult hip, normal joint space ranges from 3 to 5 mm and must be uniform. Values under 2 mm are consistent with joint space narrowing. The most important measurements are detailed in Figure 6 and Table 1.^[3]

Figure 5 (a). Iliopectineal line (red), ilioischial line (yellow), tear drop (blue), acetabular fossa (brown), and anterior (white) and posterior (green) wall of the acetabuli showing mild upper crossover sign.^[3]

• 
  Figure 5 (b). Coxa profunda.^[3]
• 
  Figure 5 (c). Protrusio acetabuli.^[3]

Figure 6:

• 
  (a) Acetabular depth ratio.^[3]
• 
  (b) Center-edge angle of Wiberg.^[3]
• 
  (c) Femoral extrusion index.^[3]
• 
  (d) Tönnis angle.^[3]
• 
  (e) Sharp angle.^[3]
• 
  (f) Crossing ratio.^[3]
• 
  (g) Alpha angle measured in 45° Dunn view.^[3]
• 
  (h) Offset percentage measured in cross-lateral view.^[3]
• 
  (i) Cervical diaphyseal angle.^[3]

Acetabular depth value under 250 characterizes the dysplastic hip.^[3]

In normal conditions the acetabulum covers 75% of the femoral head. This coverage can be determined by three different measurements: lateral center-edge angle of Wiberg, anterior center-edge angle, and femoral extrusion index. Femoral extrusion index measures the percentage of the femoral head that lies outside of the acetabular roof. This percentage must be inferior to 25% in adults.^[3]

Center-edge Wiberg’s angle measures the superior-lateral coverage of the femoral head. It is useful in children older than 5 years and in adulthood. For children between 5 and 10 years the minimum normal value is 15°, and in adults it is about 20°, although after 55 years this minimum increases to 24°. Values over 40° indicate overcoverage.^[3]

Anterior center-edge Lequesne’s angle can be measured in a false profile view of the hip or in a sagittal CT scan. In this case the tangent line touches the anterior rim of the acetabulum. Values under 20° indicate undercoverage of the femoral head.^[3]

The acetabular slope can also be measured by different methods. The Tönnis angle quantifies the slope of the sourcil (the sclerotic weight-bearing portion of the acetabulum). Values over 10° are considered a risk factor for instability, while values under 0° are considered a risk factor for pincer impingement.^[3]

The Sharp angle is a global way to measure the acetabular slope. Angles over 45° are indicative of acetabular dysplasia.^[3]

Normal acetabulum is oriented in anteversion. Its value ranges from 15 to 20° in the equatorial plane of the acetabulum and decreases gradually towards the acetabular roof, where normal values range from 0 to 5°. Retroversion of the upper part of the acetabulum has been related with pincer type impingement. In radiography the presence of a “crossover sign” is produced when the posterior wall of the acetabulum crosses the anterior wall before reaching the acetabular roof. It is a sign of acetabular retroversion and it has been linked with overcoverage and pincer impingement. Nevertheless, this sign has been described in 6% of the normal population. Therefore, more important than its presence is the percentage of crossing. This ratio is considered significant if it is over 20%.^[3]

Other signs associated with acetabular retroversion are the sciatic spine and posterior wall signs. The first one is considered positive when the sciatic spine is projected medial to the iliopectineal line in an AP radiography of the spine, indicating that it is not just the acetabulum but the whole hemipelvis that is twisted into retroversion. The second sign is considered positive when the posterior wall edge is medial to the center of the femoral head, indicating deficiency of the posterior wall.^[3]

In normal conditions there is a symmetric concave contour at the junction of the anterior and posterior profile of the femoral head and neck. Loss of this concavity or bone bulging may lead to cam type impingement. The degree of this deformity can be measured by the alpha angle. Although it can be measured in the cross-lateral view, the 45° Dunn view is considered more sensitive and the frog leg view more specific in determining pathologic values. Debate about which values are considered normal is still in progress. Based on the Copenhagen Osteoarthritis Study, a recent work defined three ranges of values for the α-angle: pathological (≥83° in men and ≥57° in women), borderline (69° to 82° in men, 51° to 56° in women), and normal (≤68° in men and ≤50° in women).^[3]

The offset between the neck and femoral head can also be calculated in the lateral projection of the hip. A value of less than 10 mm is considered pathologic. The percentage is calculated by dividing the distance between the femoral head and the neck lines by the femoral head diameter. If this percentage is under 0.18 there is high probability of cam type impingement.^[3]

The angle formed between the femoral neck and femoral diaphysis ranges from 120° to 140°. Coxa valga is diagnosed if the angle is higher and coxa vara if the angle is lower than this normal range.^[3]

Although femoral version or torsion can be measured by radiographs, CT overcomes the inconsistencies demonstrated in the measurements made by biplane radiography.^[3]

Crowe classification

In 1979 Dr. John F. Crowe et al. proposed a classification to define the degree of malformation and dislocation. Grouped from least severe Crowe I dysplasia to most severe Crowe IV.^[4] This classification is very useful for studying treatment results.

Rather than using the Wiberg angle because it makes it difficult to quantify the degree of dislocation they used 3 key elements to determine the degree of subluxation: A reference line at the lower rim of the "teardrop", junction between the femoral head and neck of the respective joint and the height of the pelvis (vertical measurement). They studied anteroposterior pelvic x-rays and drew horizontal lines through the lower rim of a feature called "teardrop". The distance between this line and the middle lines of the junction between femur head and neck gave them a measure of the degree of femur head subluxation. They further established that a "normal" diameter of the femur head measures 20% of the height of the pelvis. If the middle line of the neck-head junction was more than 10% of the pelvis height above the reference line they considered the joint to be more than 50% dislocated.^[4]

The following types resulted:^[4]

References