Role of cortical and cancellous bone of the vertebral pedicle in implant fixation.

An experimental study was conducted to determine the role of cancellous and cortical bone of vertebral pedicles in the fixation of pedicular screws. Ten segments of the lumbar spinal column removed from adults at autopsy were used. Direct measurements were made to obtain the total diameter of the pedicles from L1 to L5 plus the diameter of cancellous bone and the diameter of cortical bone. For the study, the vertebra fragments were divided into two groups, each consisting of five sets of lumbar spine segments. In the first group, the pedicle was perforated using the orientation points employed in surgical procedures (articular facet and transverse process). In the second group, a transverse cut was made in the middle portion of the pedicle so that perforation and introduction of the screws could be performed under direct view. After pedicle perforation using a 3-mm drill, the screws were introduced into the vertebral pedicle in increasing order of diameter size, until structural alteration of the pedicle was observed (plastic deformation or rupture). These parameters were evaluated by means of successive measurements with a pachymeter and by direct observation. The screws used ranged in diameter from 3 to 12.5 mm, with progressive 0.5-mm increments. The analysis of the alterations in pedicle structure was performed by considering total diameter, spongy bone diameter, cortical bone diameter, and diameter of the screw used to produce the structural alteration. The results showed that the screws in general presented fixation mainly in the cancellous bone portion of the vertebral pedicle, whereas a small percentage of cortical bone was used for fixation.