The effect of lateral wall perforation on screw pull-out strength: a cadaveric study

BACKGROUND Lateral pedicle wall perforations occur frequently during pedicle screw insertion. Although it is known that such an occurrence decreases the screw pull-out strength, the effect has not been quantified biomechanically. MATERIALS AND METHODS Twenty fresh cadaveric lumbar vertebrae were harvested, and the bone mineral density (BMD) of each was evaluated with dual-energy radiography absorptiometry (DEXA). Twenty matched, 6.5-mm pedicle screws were inserted in two different manners in two groups, the control group and the experimental group. In the control group, the pedicle screw was inserted in a standard fashion taking adequate precaution to ensure there was no perforation of the wall. In the experimental group, the pedicle screw was inserted such that its trajectory perforated the lateral wall. Group assignments were done randomly, and the maximal fixation strength was recorded for each screw pull-out test with a material-testing system (MTS 858 II). RESULTS The average BMD for both groups was 0.850 g/cm(2) (0.788-0.912 g/cm(2)). The average (and standard deviation) maximal pull-out forces were 1,015.8 ± 249.40 N for the experimental group and 1,326.0 ± 320.50 N for the control group. According to a paired t-test, the difference between the two groups was statistically significant (P < 0.001). CONCLUSION The results of this study confirm that the maximal pull-out strength of pedicle screws decreases by approximately 23.4% when the lateral wall is perforated.