[Biomechanical effects of iliac screw plates on stability of lumbo-iliac fixation construct]

To evaluate the biomechanical effect of a self-made iliac screw plate on the stability of lumbo-iliac fixation construct before and after fatigue loading.

Twelve fresh lumbo-pelvic specimens from donated adult cadavers with formalin embalm were used in the study. According to whether use the iliac screw plate or not, the specimens were randomly assigned into group A (with iliac screw plate, n=6) and group B (without iliac screw plate, n=6). The bone mineral density (BMD) of L(t-4) was measured using dual-energy radiograph absorptiometry. The pedicle screw and iliac screw fixation were given at L3-5, and bilateral facetectomy and diskectomy at L5, S1 level were performed to prepare the model of the intervertebral destabilization. The biomechanical testing was conducted on a material testing machine under 0-600 N compression and -7-7 N.m torsion loading modes for the initial compressive stiffness and torsional stiffness evaluation. And then 20 000 cyclic compressive loading of 40-400 N was given to the specimen, the stiffness evaluation was repeated. Then the maximum pull-out strength of screws at every level was measured and compared. Gross observation and radiological observation were performed during experiment.

The BMD values of groups A and B were (1.15 +/-0.13) g/cm(2) and (1.12 +/-0.11) g/cm(2) respectively, showing no significant difference between 2 groups (t=0.428, P=0.678). All pedicle screws and iliac screws were inserted in good position; no loosening or breaking of screw was observed during loading. After fatigue loading, the incidence of halo ring around the iliac screws of groups A and B was 16.7% (1/6) and 50.0% (3/6), respectively. The compressive stiffness and torsional stiffness after fatigue loading were significantly lower than those in initial state in groups A and B (P < 0.05); there was no significant difference in compressive stiffness and torsional stiffness between groups A and B before fatigue loading (P > 0.05). However, group A had higher compressive stiffness than group B (t=2.664, P=0.024) after fatigue loading, and there was no significant difference in torsional stiffness between 2 groups (t=0.410, P=0.690). No significant difference was found in screw pull-out strength of pedicle screws at L3, L4, and L5 levels between groups A and B (P > 0.05); however, the pull-out strength of the iliac screws in group A was significantly higher than that in group B (t=3.398, P=0.007). In groups A and B, the pull-out strength of L3 screw was significantly lower than that of L4 and L5 screws (P < 0.05). In group A, pull-out strength of the iliac screws was significantly higher than that of L3, L4, and L5 screws (P < 0.05); in group B, the pull-out strength of iliac screws was significantly lower than that of L4 and L5 screws (P < 0.05).

In the lumbo-iliac reconstruction, the use of iliac screw plate could resist iliac screw loosening, therefore, it has the potential to increase the stability oflumbo-iliac fixation construct.