Acetabular dysplasia is a condition where the hip socket is abnormally shallow or misaligned. Insufficient coverage of the femoral head can lead to instability and uneven weight distribution within the hip. Over time, these mechanical stresses can cause accelerated wear and tear on joint cartilage, potentially resulting in early onset osteoarthritis, pain, and reduced hip function. Accurate identification through imaging is important for timely intervention and to prevent progressive joint damage.
Imaging Modalities
Radiological diagnosis relies on various imaging techniques, suited to different age groups and clinical scenarios. X-rays are a common initial tool for older children and adults, once significant ossification of the femoral head and acetabulum has occurred. An anterior-posterior (AP) pelvis view provides a two-dimensional assessment of hip joint anatomy.
For infants up to approximately six months of age, ultrasound is the preferred imaging method. At this early stage, the hip joint is largely cartilaginous and not visible on X-rays. Ultrasound visualizes cartilaginous structures, allowing dynamic assessment of hip stability and femoral head coverage without ionizing radiation.
For detailed three-dimensional anatomical information, such as for complex cases or pre-surgical planning, computed tomography (CT) scans are utilized. CT provides cross-sectional bone images, offering a comprehensive view of acetabular and femoral contours. CT scans involve a higher radiation dose than X-rays. Magnetic resonance imaging (MRI) visualizes soft tissues like cartilage and the labrum, the rim of cartilage surrounding the acetabulum. MRI offers detailed anatomical assessment without radiation, useful for evaluating cartilage damage or other soft tissue abnormalities.
Key Radiological Measurements
Radiologists use specific measurements and visual indicators on imaging to diagnose acetabular dysplasia. The Acetabular Index (AI) is a key measurement in infants and young children, quantifying the slope of the acetabular roof. An increased acetabular index indicates a shallower hip socket. Normal AI values generally decrease with age, typically less than 30 degrees at birth and reducing to less than 25 degrees by one year.
The Center-Edge (CE) Angle of Wiberg is another important measurement, used in older children and adults, to assess lateral femoral head coverage by the acetabulum. This angle is formed by a vertical line through the femoral head’s center and a line extending from the center to the acetabular roof’s lateral edge. A normal CE angle ranges from 25 to 40 degrees. Values below 20 degrees indicate severe dysplasia; 20 to 25 degrees suggests borderline dysplasia.
The Tönnis Angle, also known as the acetabular roof angle, evaluates the medial coverage and slope of the acetabular weight-bearing surface (sourcil). Measured on an AP pelvic radiograph, a normal Tönnis angle falls between 0 and 10 degrees. An angle greater than 10 degrees indicates acetabular dysplasia.
Beyond angular measurements, other visual signs contribute. Shenton’s line, an imaginary curved line formed by the inferior border of the superior pubic ramus and the inferomedial border of the femoral neck, should be continuous and smooth. An interruption signals femoral head subluxation or dislocation. Femoral head position relative to the acetabulum, including subluxation or displacement, is also assessed. In infants, ultrasound measurements like the alpha angle, indicating acetabular depth, are used; a normal alpha angle is typically greater than 60 degrees.
Diagnostic Interpretation and Age Considerations
Radiologists synthesize imaging findings, combining quantitative measurements with qualitative observations, for a comprehensive diagnosis. This process involves more than evaluating individual angles; it requires a holistic assessment of hip joint morphology and congruity. Interpretation is nuanced, considering the interplay of bony and cartilaginous structures.
Diagnostic approach and interpretation vary significantly with patient age due to progressive hip joint ossification. In infants, where the hip is largely cartilaginous, ultrasound is the primary tool. The alpha angle, measured on ultrasound, provides information about acetabular depth and femoral head coverage. Early detection in infancy is important, as many minor hip abnormalities visible on ultrasound can resolve with management.
As children grow and hip bones ossify, X-rays become the standard imaging modality for evaluating dysplasia. For older children and adults, measurements like the CE angle of Wiberg and the Tönnis angle assess bony femoral head coverage. These measurements help determine acetabular deficiency and guide treatment decisions.
Radiological findings are interpreted with the patient’s clinical presentation. Symptoms like hip pain, limping, or limited hip movement provide context for imaging findings. This clinical correlation helps radiologists and clinicians understand the functional impact of observed anatomical abnormalities. Radiological findings contribute to classifying dysplasia severity, ranging from subtle acetabular insufficiency to significant hip instability or dislocation.