2D versus 3D imaging of hip displacement in children with cerebral palsy

In population-based studies, hip displacement was found to affect approximately one-third of children with cerebral palsy and was directly related to functional limitations as classified according to the Gross Motor Function Classification System (GMFCS), but not the type of movement disorder. For many years, hip development in children has routinely been assessed and monitored using plain radiography, i.e. two-dimensional (2D) imaging. In children with cerebral palsy, an anteroposterior X-ray of the pelvis has been used as a basis for hip surveillance and to design a classification of hip morphology at skeletal maturity as an outcome measure. Two-dimensional imaging of the hip can yield reliable images on which decisions can be made for referral and for intervention such as adductor releases, and to monitor the outcome of such surgery. It is widely available and with attention to positioning of the child and measurement technique, is capable of producing reliable results. In addition the dose of radiation is small, typically ranging from 0.1 to 0.6mSv per image. Gose et al. have conducted a careful and detailed study of three-dimensional (3D) imaging of the hip using a multidetector computed tomography (CT) protocol with ‘ageand weight-specific pediatric protocols’ to limit the dose of radiation. The authors report a substantial body of new information, which can be helpful in understanding the pathology of hip displacement in children with cerebral palsy, including precise assessment of acetabular morphology, and proximal femoral geometry. This is the largest published study of 3D imaging of the hip in children with cerebral palsy to date and the information gained may be important on many levels. This study also demonstrates the relationship between functional status according to GMFCS level and hip morphology. It also validates the classification of hip morphology reported by Robin et al., based on 2D imaging. However, the question remains as to when 2D imaging of the hip is appropriate and when 3D imaging should be utilized? The principal disadvantages of 3D imaging are the more limited accessibility of CT in some hospitals, the need for sedation to gain images of good quality for children unable to keep still, and the increased dose of radiation received by the child. Typical radiation doses for CT scans of the hips are in the range of 3 to 4mSv, which is greater than the world average annual background of radiation of about 2.4mSv per annum. The radiation dose for a CT scan of the hips is between 10 and 20 times the radiation dose for a single anteroposterior radiograph of the pelvis. The difference will vary with the CT imaging parameters, the volume of tissue scanned, and the radiographic technique for the pelvic X-ray. So the question therefore is when is the additional patient time, expense, and radiation dose justifiable in terms of the much more detailed information available from the procedure? Two-dimensional, plain radiography seems to be adequate for hip surveillance in the younger child, so long as correct positioning, technique, and measurement are undertaken. Two-dimensional imaging is also adequate for assessing the outcome of soft-tissue surgery by adductor releases and as a simple measure of hip morphology at skeletal maturity. However, in older children, with severe hip displacement, the study by Gose et al. demonstrates a range of deformities affecting both the acetabulum and the proximal femur which may give useful information to orthopaedic surgeons treating an individual patient. This information adds to those from earlier work by Brunner et al. which reported the magnitude and direction of acetabular deformity in older children with cerebral palsy and severe hip displacement. These and other authors have commented on the benefits of this additional information in planning reconstructive surgery. In addition, the same authors have noted the inaccuracy of the migration percentage, measured from 2D radiographs of the hip in predicting femoral head displacement in six children with cerebral palsy who had anterior displacement of the hip. However, the direction of hip displacement in cerebral palsy is posterolateral or posterior in the majority of children. Pure anterior displacement is rare and can be recognized clinically by progressive limitation of hip flexion. In our opinion, 2D radiography of the hips would seem to be the criterion standard for initial diagnosis of hip displacement and for hip surveillance. Three-dimensional imaging with CT may be indicated if there is clinical suspicion of anterior hip displacement, obscure hip pain, and when there is established deformity in the acetabulum or proximal femur, prior to bony reconstruction. By this means important information for the surgeon, such as the direction of the dislocation and the morphology of the acetabulum, can be obtained. In the interests of child safety and parental education, it is increasingly important for clinicians to be aware of the risk versus benefit of diagnostic procedures and in this context, a greater radiation dose associated with CT compared to 2D plain radiology. The alternative modality for 3D imaging of the hip is magnetic resonance imaging. However, the scanning times are much longer than for CT and the benefit of no ionizing radiation may be outweighed by the need for general anaesthesia, in the majority of children with cerebral palsy.