Developmental Dysplasia of the Hip (DDH): Radiographic Lines, Imaging, and Diagnosis

What lines are used to diagnose DDH?

Developmental Dysplasia of the Hip (DDH) is primarily diagnosed using a combination of clinical examination and imaging, where specific radiographic lines drawn on X-rays serve as crucial indicators of hip joint anatomy and potential abnormalities, particularly in infants and young children.

Understanding Developmental Dysplasia of the Hip (DDH)

Developmental Dysplasia of the Hip (DDH), formerly known as Congenital Dislocation of the Hip (CDH), refers to a spectrum of conditions where the hip joint has not formed correctly. This can range from a subtle instability to a complete dislocation of the femoral head from the acetabulum. Early and accurate diagnosis is paramount, as timely intervention can prevent long-term complications such as limping, pain, early arthritis, and the need for complex surgical procedures later in life.

The Role of Imaging in DDH Diagnosis

While physical examination is the initial step in screening for DDH, imaging studies are essential for definitive diagnosis and monitoring. For infants up to approximately 4-6 months of age, ultrasound is the preferred imaging modality dueable to the largely cartilaginous nature of the infant hip, which is not well-visualized on X-ray. Ultrasound allows dynamic assessment of hip stability and morphology.

However, once the femoral head and acetabulum begin to ossify (typically after 4-6 months of age), X-rays become the primary diagnostic tool. On an anteroposterior (AP) view of the pelvis, specific anatomical lines are drawn and measured to assess the relationship between the femoral head and the acetabulum, providing objective criteria for diagnosing DDH.

Key Radiographic Lines for DDH Diagnosis

The following lines are fundamental to the radiographic assessment of DDH:

• Hilgenreiner's Line (Horizontal Baseline)

  • Description: This is a horizontal line drawn between the inferior aspects of both triradiate cartilages. The triradiate cartilage is the Y-shaped growth plate where the ilium, ischium, and pubis meet in the acetabulum.
  • Significance: It serves as a baseline for other measurements and provides a reference for the vertical position of the femoral head. It is crucial for calculating the Acetabular Index.

• Perkin's Line (Vertical Line)

  • Description: A vertical line drawn perpendicularly to Hilgenreiner's line, extending downwards from the most lateral aspect of the acetabular roof.
  • Significance: This line divides the acetabulum into medial and lateral quadrants. In a normally developed hip, the ossified nucleus of the femoral head (or its expected position if not yet ossified) should lie in the inferomedial quadrant (below Hilgenreiner's and medial to Perkin's). If the femoral head is lateral to Perkin's line, it indicates lateral displacement, a sign of subluxation or dislocation.

• Shenton's Line (Menard's Line)

  • Description: This is a continuous, smooth curvilinear line formed by the inferior border of the femoral neck and the superior border of the obturator foramen (specifically, the inferior margin of the superior pubic ramus).
  • Significance: In a healthy hip, Shenton's line should be a smooth, unbroken arc. Disruption or discontinuity of this line (e.g., a "step" or break in the curve) is a strong indicator of hip subluxation or dislocation, as the femoral head is displaced relative to the acetabulum.

• Acetabular Index (Ombrédanne's Line/Angle)

  • Description: This is an angle measured between Hilgenreiner's line and a line drawn from the medial aspect of the triradiate cartilage to the lateral aspect of the acetabular roof.
  • Significance: The Acetabular Index quantifies the slope or steepness of the acetabular roof. A higher angle indicates a shallower and more vertical acetabulum, suggesting acetabular dysplasia (underdevelopment of the socket). Normal values vary with age, but generally, an angle greater than 30 degrees in a newborn or 25 degrees by one year of age is considered abnormal.

• Tönnis Angle

  • Description: Also known as the Horizontal Acetabular Angle, this angle is formed by a line connecting the medial and lateral edges of the acetabular weight-bearing surface and a horizontal reference line (often Hilgenreiner's or a line through the tear drops).
  • Significance: The Tönnis angle assesses the inclination of the acetabular roof. An increased angle indicates a more horizontal or dysplastic acetabulum, signifying insufficient coverage of the femoral head. It is particularly useful in older children and adults to evaluate residual dysplasia.

Interpreting the Radiographs: Beyond the Lines

While these lines provide critical objective measurements, the diagnosis of DDH is not solely based on them. Radiologists and orthopedic specialists also look for other signs, including:

• Delayed appearance or small size of the ossification nucleus of the femoral head on the affected side.
• Lateral and superior migration of the femoral head relative to the acetabulum.
• Irregularity or hypoplasia (underdevelopment) of the acetabular roof.
• Widening of the medial joint space (the distance between the medial aspect of the femoral neck and the ilium).

Furthermore, clinical context, patient history, and physical examination findings (e.g., asymmetry of thigh folds, limited abduction, Galeazzi sign) are always considered in conjunction with imaging results for a comprehensive diagnosis.

Conclusion: The Interplay of Science and Clinical Acumen

The radiographic lines used to diagnose DDH—including Hilgenreiner's, Perkin's, Shenton's, and the Acetabular Index—are indispensable tools for objectively assessing hip joint morphology and stability. These lines, when meticulously drawn and interpreted, provide vital information that guides diagnosis, treatment planning, and monitoring of DDH. However, it is crucial to remember that these measurements are part of a larger diagnostic picture, which integrates clinical findings and the expertise of healthcare professionals to ensure optimal outcomes for individuals affected by this condition.