Registration and geometric modelling of the spine during scoliosis surgery: a comparison study of different preoperative reconstruction techniques and intra-operative tracking systems

During scoliosis instrumentation surgery, it is difficult for surgeons full y to track vertebral motion in 3D, because only the posterior elements of t he spine are exposed. Different intra-operative modelling approaches are ev aluated using a registration technique that matches intra-operative measure ments with a 3D preoperative model of the spine. Two tracking systems (magn etic digitiser and mechanical arm) and two pre-operative reconstruction tec hniques (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 (mul tiplanar radiographic reconstruction and magnetic digitiser) has an accurac y of 5.9 mm in translation, and errors on vertebral rotations are 4.4 degre es, 6.7 degrees and 5.0 degrees in the frontal, sagittal and transverse pla nes, respectively. With CT scan reconstruction, the accuracy is significant ly increased by about 2 mm in translation and as much as 4.5 degrees for ve rtebral rotations in the sagittal plane. For the mechanical arm, the accura cy is increased by less than 1 mm in translation and 1 degrees for vertebra l 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 alter native solution for long spinal segments when great accuracy is not necessa ry. Considering the small increase in accuracy and its awkwardness, the use of the mechanical arm may not be appropriate during surgical manoeuvres.