A study of the developing hip joint compared x-ray findings to MRI findings. The infant hip has more cartilage than bone and can be imaged with ultrasound, but ultrasound is not very useful after the age of six months because bone forms in the cartilage as the infant becomes more mature. Doctors have relied on x-rays to determine the need for treatment, or the success of treatment after the age of six months. MRI is used in some cases to help make decisions, but this has not been correlated with the x-ray findings. A recent report by Chinese researchers compared MRI and x-rays in a large number of children with hip dysplasia and a large number of normal hips in children who had MRI and X-rays for other reasons.
The research confirmed much of our current information, but did add some new information. The slope of the outer margin of a baby’s hip is only slightly more than an adult. The hip develops rapidly by age two years and stays fairly constant from age two to adulthood. By age four years the period of rapid bone development is complete. This correlates with x-ray findings for healthy children. However, children with hip dysplasia on one side are more likely to have subtle hip dysplasia on the opposite side. This may be visible on the MRI but not on the X-rays for several years. Thus, a child with hip dysplasia usually has both hips treated with bracing or immobilization when the affected side is being treated, but the child should probably be followed for several years to evaluate the opposite hip as well as the affected hip.
Another finding was that children with hips slightly out of the socket wear down the outer margin of the socket more than children with hips that are totally out of the socket. The authors suggested this is because the slightly dislocated hip rests against the outer margin of the socket instead of being totally above the socket. The significance of this finding is not clear, but it may suggest that very high dislocations may be more difficult to reduce back into the socket. However, these more severe dislocations may benefit from surgical treatment at a slightly older age when the dislocation is detected after walking age because the outer margin of the socket is more intact.
Comment: MRI is unnecessary for most children because the x-rays are reliable for most decisions. Also, an MRI for a child usually requires general anesthesia to keep the child perfectly still for the 20-30 minute procedure. Until MRIs become faster the x-ray remains the best option for evaluating the child with hip dysplasia after the age of six months. However, the MRI is demonstrating increasing usefulness for children between the ages of one and eight years to evaluate borderline cases of hip dysplasia.
Development of the osseous and cartilaginous acetabular index in normal children and those with developmental dysplasia of the hip: A cross-sectional study using MRI. L.Y. Li, et al. J Bone Joint Surg Br 2012;94(12):1625-31