X-ray of hip dysplasia

File:Dislocated hip.jpg

Hip dysplasia with right-sided hip dislocation.

Author: Mikael Häggström
Hip dysplasia (also termed developmental dysplasia of the hip (DDH) or, in older texts, congenital dislocation of the hip (CDH)) is where the socket portion does not fully cover the ball portion, resulting in an increased risk for joint dislocation.

Planning

Choice of investigation

Ultrasonography of hip dysplasia 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 at which the joint gives a good x-ray image is also the point at which nonsurgical treatment methods cease to give good results.

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. 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.

Measurements

The most useful lines and angles that can be drawn in the pediatric pelvis assessing DDH are as follows:

• X-ray of measurements on a normal hip.jpg

  Normal hip.

• X-ray of measurements in hip dysplasia.jpg

  Hip dysplasia.

• (A) Hilgenreiner Line. It is considered a basal line joining the top aspect of the triradiate cartilages. This line is used to measure the acetabular angle and as a reference for Perkin line.
• (B) Perkin line is perpendicular to Hilgenreiner line, touching the lateral margin of the acetabulum. This leads to four quadrants and a normal femoral head has to be located in the inferomedial quadrant. We can measure the lateral displacement of the femoral head with regard to the Perkin line by dividing the width of the head that crosses the Perkin line by the diameter of the head. The value for patients under 3 years must be 0 and in older children this ranges from 0 to 22%.
• (C) Shenton line is a continuous arc drawn from the inner edge of the femoral neck to the superior margin of the obturator foramen. This should be smooth and undisrupted; otherwise it may indicate a fracture or hip dysplasia.
• (D) The acetabular index measures the acetabular roof slope. It is the most useful measure of acetabular dysplasia until 6 years of age. It is formed between Hilgenreiner line and the acetabular roof. In newborns, values of in males and in females are considered normal. Gradually this angle becomes smaller, with a mean value of in males and in females at 1 year of age.
• (E) The medial articular joint space is measured between the medial border of the femoral head or neck (when epiphysis is not ossified) and the acetabular platform. Normal values range between 5 and 12 mm. Differences greater than 1.5 mm between the two sides are considered abnormal.