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Radiographic assessment of pedicle hook placement
K M Cheung1, K D Luk, J C Leong
1Department of Orthopaedic Surgery, University of Hong Kong, Duchess of Kent Children's Hospital, Sandy Bay, Hong Kong.
Study Design:
A cadaveric study of the changes in the radiographic morphology of the thoracic pedicle with vertebral rotation.
Objective:
To study the effect of vertebral rotation on the assessment of pedicle hook placement by determining the radiographic anatomy of the pedicle and its relation to the pedicle hook.
Summary Of Background Data:
It is commonly assumed that the oval pedicle shadow seen on spine radiographs is formed by the waist of the pedicle. The postoperative assessment of pedicle hook position is based on this assumption.
Methods:
Thoracic cadaveric vertebrae between T5 and T8 were studied, with wire markers placed at the waist, the pedicle-facet junction, and the pedicle-body junction. Thoracic pedicle hooks were placed in the correct position and offset to one side. Radiographs of the vertebral body were taken from 0 degree to 30 degrees of rotation with reference to the sagittal plane.
Results:
The radiographic morphology of the pedicle changes with vertebral rotation; it also differs between the concave and the convex side of a scoliotic spine. With rotation of less than 20 degrees, the pedicle shadow is formed by the waist on both sides. With 30 degrees of rotation, the concave pedicle shadow is formed by the pedicle-facet junction, whereas on the convex side, it is over-lapped by the transverse process shadow. A laterally misplaced hook on the convex side and a medially misplaced hook on the concave side can appear correctly located with vertebral rotation.
Conclusion:
The oval pedicle shadow is not simply represented by the waist of the pedicle. An understanding of the changes in radiographic morphology with rotation is necessary for the assessment of thoracic pedicle hook location after surgery.