Experimental pullout testing and comparison of variables in transpedicular screw fixation. A biomechanical study

The specific objective of this investigation was to assess and compare the relative performance of four types of pedicular fixation screws currently in use. All screws were tested under similar conditions. The results obtained have permitted the authors to rank the various screws tested in terms of performance as follows: 1) the Steffee screw, 2) the AO screw, 3) the Howmedica screw, and 4) the Roy-Camille screw. While the considerable variability of the bone material is recognized, the screw performance is also influenced by the geometric variables of screw design; improvements in the pullout strength can be achieved by an increase in the major diameter of the screw; screw displacement before failure appears related to the screw pitch, where an increase in the pitch of the screw will increase the amount of displacement before failure; screw angulation was found to have little effect on the pullout strength, but may have an effect on the screw displacement and energy absorption before failure.