During scoliosis instrumentation surgery, it is difficult for surgeons fully to track vertebral motion in 3D, because only the posterior elements of the spine are exposed. Different intra-operative modelling approaches are evaluated using a registration technique that matches intra-operative measurements with a 3D preoperative model of the spine. Two tracking systems (magnetic digitiser and mechanical arm) and two pre-operative reconstruction techniques (multiplanar radiography and CT scan) are sequentially combined to build four intra-operative models. Their accuracy is assessed by comparison with the pre-operative geometry. The most minimally invasive approach (multiplanar radiographic reconstruction and magnetic digitiser) has an accuracy of 5.9 mm in translation, and errors on vertebral rotations are 4.4o, 6.7o and 5.0o in the frontal, sagittal and transverse planes, respectively. With CT scan reconstruction, the accuracy is significantly increased by about 2 mm in translation and as much as 4.5o for vertebral rotations in the sagittal plane. For the mechanical arm, the accuracy is increased by less than 1 mm in translation and 1o for vertebral rotations. CT scan is the most accurate reconstruction technique, but its use for long spinal segments is generally not allowed because of the high radiation exposure. Multiplanar radiographic reconstruction may be an alternative solution for long spinal segments when great accuracy is not necessary. Considering the small increase in accuracy and its awkwardness, the use of the mechanical arm may not be appropriate during surgical manoeuvres.
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