The use of pedicle screws as fixation devices for posterior spinal fusion has become increasingly popular worldwide. The pedicle screws offer rigid segmental fixation after decompression and arthrodesis for various disorders of the spine, including scoliosis, spondylolisthesis, fractures, tumor and iatrogenic or degenerative instability (Chadha et al., 2003). The initial methods of pedicle screw fixation were described by Raymond Roy (Boos and Webb, 1997) . Roy-Camille et al. (1973) described the use of the posterior plate with screws positioned sagittally through the pedicle and articular processes. The screws were designed following recommendations from anatomical studies of the pedicle by Kabins and Weinstein (1991). The technique was
