Biomechanical advantages of dual iliac screws over single ones in lumbo-iliac fixation construct

Objective To evaluate the biomechanical effects of single short, single long and dual short iliac screws on the stability of lumbo-iliac fixation construct. Methods L2-pelvic specimens were harvested from 7 fresh human cadavers. Following the test of the intact state simulated by L3-L5 pedicle screw fixation, the destabilization by total sacrectomy and L3-iliac screw-rod reconstructions with 3 placements of iliac screws were conducted as follows: bilateral single short iliac screws in Group A; bilateral single long iliac screws in Group B; bilateral dual short iliac screws in Group C. The 3 iliac screw fixations were preformed in the same specimens in a random order, and the lengths of short and long iliac screws were 70 mm and 130 mm, respectively. Biomechanical testing was performed on a material testing machine under 800 N compression and 7 Nm torsion loading modes for construct stiffness evaluation. Results In compression, the constructs in Groups A, B, C obtained 73%, 76%, and 108% of the initial compression stiffness, respectively. No significant differences were demonstrated between Groups A and B ( P > 0. 05), but the 2 groups showed obviously lower construct compression stiffness than the intact state and Group C ( P 0. 05). However, Groups A and B provided markedly lower construct torsion stiffness than the other 2 conditions (P < 0. 05). Conclusions Following total sacrectomy, it is rather difficult to restore the initial local stability by using bilateral single long iliac screws. However, the dual iliac screws when compared with the single ones can provide much higher construct stability comparable to the initial state. Key words: Lumbosacral region; Joint instability;  Lumbo-iliac reconstruction; Bone nails; Biomechanics