[Biomechanical effects of insertion location and bone cement augmentation on fixation strength of iliac screw].

Objective: To compare the maximum pull-out strength of the upper iliac screw and lower iliac screw with and without polymethylmethacrylate (PMMA) augmentation, and to provide the experimental evidences for the rational use of iliac screws.

Methods: Ten intact human ilium from 5 donated cadavers with formalin embalmed were selected. The bone mineral density (BMD) of L1-4 of each cadaver was measured with a dual energy X-ray absorptiometry. The screws placed in the upper and lower iliac column were named as the upper and lower iliac screw, respectively. Using 70 mm length and 7.5 mm diameter screws with and without PMMA augmentation, 4 iliac screw technique models were sequentially established and tested as follows: upper iliac screw (group A), upper iliac screw with PMMA augmentation (group B), lower iliac screw (group C), and lower iliac screw with PMMA augmentation (group D). Each ilium was mounted on a material testing machine with its position similar to standing. Under 2 000 cyclic compressive loadings of 100-300 N to the screw, the maximum pull-out strength of iliac screw was measured.

Results: The BMD value of the 5 human cadavers was (0.88 +/- 0.06) g/cm2. All the iliac screws were inserted into the screw tracts accurately as expected. No screw penetrations of acetabulum or cortex was not observed through visual inspection. There was no "halo" ring sign surrounding any screw after the 2 000 cycle loading. The maximum pull-out strengths of groups A, B, C, and D were (964 +/- 250), (1462 +/- 266), (1537 +/- 279), and (1964 +/- 422) N, respectively. Group D exhibited the highest maximum pull-out strength among the 4 groups (P < 0.05). No significant difference was detected between groups B and C (P > 0.05); however, groups B, C showed higher maximum pull-out strength than group A (P < 0.05).

Conclusion: The lower iliac screw offers significantly higher fixation strength than the upper iliac screw; PMMA augmentation could effectively increase the fixation strength of iliac screws and therefore could be applied in the salvage of iliac screw loosening.