What is the common imaging method to diagnose developmental dysplasia of the hip in infants less than 4 months old?

Ultrasound is the preferred imaging method for diagnosing developmental dysplasia of the hip in infants less than 4 months old because it is safe, does not involve radiation, and is particularly effective in evaluating the hip joint and surrounding soft tissues in this age group. The cartilaginous structures of the hip are not well visualized on X-ray in very young infants, making ultrasound a superior choice for accurate assessment.

Ultrasound allows for dynamic assessment of the hip joint and can help identify hip dislocation or instability, which are critical in the early detection and management of developmental dysplasia of the hip. In older infants, X-ray may become more useful due to the increased ossification of bone structures, but for those under 4 months, ultrasound remains the gold standard for evaluation.

Other imaging modalities, such as MRI and CT, are generally reserved for situations where more detailed anatomical evaluation is necessary or when other diagnoses need to be ruled out, as they are often more complicated, less accessible, or involve more risk compared to ultrasound for this particular application.