Purpose: Dislocation of hips with developmental dysplasia within an hip spica cast is not infrequent although preceding closed reduction of the affected hip was satisfactory.
Methods and Materials: In 91 infants (mean age 5 months, male: n=19, female: n=72) with developmental dysplasia of the hip (DDH) the diagnostic capabilities of MRI to control hip reduction within a spica cast were evaluated. By a 1.5-Tesla Philips Gyroscan/ACS magnet transverse and coronal T1 and T2 images were acquired after spica cast application. All children prior to MRI were sedated by rectal application of chloral hydrate. It was proven whether all relevant hip structures could be identified and assessed by MRI (femoral head (FH), labrum, acetabulum (Ac), acetabular roof (AcR), triradiate cartilage (TC)). To quantify hip dislocation anterior-posterior, caudo-cranial, and lateral displacement was measured in the appropriate image. In addition it was checked how often anatomical obstacles (e.g. lig. teres, adipose tissue) preventing an exact reduction could be found.
Results: In nearly all hips the relevant structures could be identified (FH 100 %, labrum 98 %, Ac 97 %, AcR 99 %, TC 100 %). Image quality was good in 80 %, moderate in 19 % and limited in 1 % of the MRI examinations. Within the spica cast of the 182 hips 44, 31, and 30 showed a displacement in the respective direction. In total 35 hips a minor displacement could be observed. 29 hips (11.5 %) showed a more severe displacement that required at least cast revision. In children with minor displacement MRI follow-up was recommended. In 40 hips we found obstacles (ligamentum teres n=16, adipose tissue n=24) for an exact hip reduction resulting in a hip displacement in different directions.
Conclusion: MRI is an exact method to assess residual or recurrent dysplasia of a hip during spica cast therapy. Due to spica casting there is no degradation of MR images provided that sedation is sufficient. Therefore MRI is recommended for early characterizing and confirmation of an incorrect reducted hip in spica cast.
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