Biomechanical comparison of long iliac screw and short iliac screw augmenting with PMMA fixations.

Objective To compare the strength of long iliac screw and short iliac screw augmenting with PMMA fixations after a fatigue simulation and evaluate the biomechanical effect of the short iliac screw. Methods Eleven adult corpse embalmed pelves (7 males, 4 females, with the average age of 60.7 years, ranged from 41 to 78 years) were used. Using dual energy X-ray absorptiometry (DEXA), bone density was measured to characterize bone quality, the mean value of BMD was 0.68±0.17 g/cm2, long screw was difined as the length of 2 mm around anterior inferior iliac spine, and short screw was difined as the length of exceeding no more than 5ram over ischial notch. Long screw [7.0 mm diameter and (138±4) mm length] was randomly inserted in one side of each pelve, and short screw [7.0 mm diameter and (70±2) mm length] following augmentation with PMMA was inserted in the other side. The PMMA was inserted 2-5 ml (mean 3.6 ml). Cyclic loading from 30 N to 300 N was applied to each screw at a frequency of 2 Hz up to 2000 cy-cles. Pullout tests were conducted at 5 mm/min after completion of the fatigue tests, and the maximum pull-out strength was recorded. Results Initial stiffness of the long screw fixation and the short screw fixation with augmentation were (249±101) N/mm and (253±122) N/mm respectively. Pullout force of the short screw fixation with augmentation and the long screw fixation were (2436±915) N and (2529±1055) N respectively, no significant difference was found between the two groups. The pullout forces of two fixations following cyclic loading showed linear correlations with the bone mineral density of pelvis. Conclusion The short ili-ae screw fixation following augmentation with PMMA will be a viable clinical option for spino-pelvic recon-struction, especially in osteoporotic patients. Key words: Ilium;  Bone screws;  Bone cements;  Biomechanics