Transpedicular screw fixation.

Spinal fixation employing transpedicular screws has recently been the focus of increased attention at various institutions throughout the world, but concerns about the safety and efficacy of transpedicular screws linger. This study was undertaken to address some of these concerns. The study included evaluation of the internal and external morphology of the vertebral pedicles, which revealed that adequate bone stock is generally available at T2, T7, T12, and L1-L5 spinal levels to accept screws in the 4-7-mm diameter range. The pedicle was observed to be composed of abundant cancellous bone internally with relatively thick cortical walls. The method of pilot hole preparation for pedicle screws was also examined. Screws inserted in pilot holes prepared with a 3.4-mm blunt probe (ganglion knife) resulted in higher pullout forces in eight of 10 trials as compared with those with pilot holes prepared using a 3.2-mm drill. Furthermore, the probes afford greater control of hole depth and alignment. Fatigue studies on three screw designs revealed a graduation of strength between a 7.0-mm pedicle screw, a 5.5-mm pedicle screw, and a modified 6.5-mm cancellous lag screw. The modified cancellous lag screw has an inherent stress riser that affected fatigue life. It was noted that extreme care must be exercised to prevent bending of the pedicle screws during implantation. If bending occurs one can expect a 50% reduction in the number of cycles to failure.