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MumrraifiiE

MumrraifiiE

Chart 1 Comparison between measurements of the physeal torsion on plain radiographs and the same measurements using 3D methodology (n = 45 hips; in 1 of the 23 cases a lateral image of the healthy side was not avaliable).

"frogleg-position" images. The angular accuracy of measurements on radiographs depends strongly on correct and reproducible positioning of the patient. Accurate knowledge of the slippage angle of the femoral head is essential for correctional osteotomy. However, the torsional angular measurement of the capital femoral epiphysis on radiographs differs considerably from that obtained from 3D models in most cases (a standard deviation of + 21.0°), without showing a systematic error. The main reason for this seems to be the difficulty in positioning. In a review of the plain radiographs, inaccurate positioning was clear, especially on "frogleg" images used to interpret the slippage in the transversal plane. On virtually all images an asymmetric position of left and right hip was clearly apparent.

Some angles that are essential for an accurate preoperative planning of correctional osteotomies, such as acetabular anteversion, cannot be obtained on plain radiographs. Although a few articles describe acetabular anteversion based on 2D CT data sets [11,15,18], axial CT images do not allow assessment of the acetabular inclination.

Slippage of the femoral head leads to an unphysiologic position of the head relative to the hip cup and therefore to an impingement between the acetabulum and the femoral neck. The consequence of this impingement may be an early degenerative joint disease. In severe cases of SCFE the femoral neck is regularly deformed [6,10]. In contrast to conventional preoperative planning on plain radiographs, 3D computer models provide all essential geometric information — femoral anteversion, physis anteversion, acetabular anteversion, shaft-neck angle, shaft-physis angle, and acetabular inclination — and further important information about the deformity of crucial anatomical structures.

There is still a debate about preventive fixation of the contralateral hip in cases of unilateral SCFE. Shear forces caused by changed femoral geometry depending on the level of activity may lead to a slippage of the femoral epiphysis [4]. A changed geometry of the unaffected proximal femur should be taken into account when considering preventive fixation of the unaffected contralateral epiphysis. Former 2D-CT studies reported that SCFE is associated with a reduced femoral anteversion [5, 16]. In this manner the lack of an appropriate measurement method for the unaffected hip may cause unnecessary surgery, or early stages of slippage may be overlooked.

Pathoanatomical analysis and preoperative planning based on 3D computer models provide accurate measurements, determine essential angles that are not considered in traditional planning, and present further information on the deformity of crucial anatomical structures.

Thus far, 3D reconstruction on CT/MR consoles has limited use because of very simple segmentation algorithms; in general, only intensity thresholding and connectivity checking are available.

More advanced segmentation algorithms can be found in the Segmentation section of this Handbook. Depending on the anatomy of interest, a combination of segmentation algorithms may be appropriate.

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