Effect of iliac screw insertion depth on the stability and strength of lumbo-iliac fixation constructs: an anatomical and biomechanical study.

Abstract

Comparison of feasibility and safety of the placement of short and long iliac screws by anatomic and biomechanical evaluations as they apply to lumbo-iliac fixation construct.
To compare the stability of the short and long iliac screw fixations for lumbo-iliac reconstruction by anatomic and biomechanical evaluations.
Spinopelvic reconstruction remains a challenge to spine surgeons. Despite the advent of many fixation methods, the use of iliac screws seems most favorable so far. Various lengths of iliac screws are applied in surgical treatments; however, no biomechanical comparison has been reported based on the screw length.
For anatomic observation, CT scan data of 60 Chinese adults were used to measure the details of the iliac spine structures. For biomechanical evaluation, 7 adult human cadavers (L3-pelvis) were observed. L4-S1 pedicle screw fixation was performed with posterior spinal fixation system. On the basis of the lengths of iliac screws, 2 groups were tested (short screw group using 70 mm screws and long screw group using 138 mm screws). In this study, short and long iliac screws were placed in the same specimen. Biomechanical testing was performed on a material testing machine under 800 N compression and 7 Nm torsion loading modes for stiffness evaluations. Finally, pullout testing was performed for all the iliac screws to measure the maximum pullout force.
The length of the line between posterior superior iliac spine and anterior inferior iliac spine was 140.6 +/- 1.1 mm, and the distance between this line and the greater sciatic notch was 18.3 +/- 0.8 mm. The length of the line between posterior superior iliac spine and the second narrowest point was 67.1 +/- 0.62 mm in men and 70.1 +/- 1.4 mm in women. Insertion lengths of the short and long iliac screws were 70 +/- 2 mm and 138 +/- 4 mm, respectively. The lumbo-pelvic reconstruction using short and long iliac screws restored 53.3% +/- 13.6% and 57.6% +/- 16.2% of the initial stiffness in compression testing respectively. In torsion testing, the use of short and long iliac screws harvested 55.1% +/- 11.9% and 62.5% +/- 9.2% of the initial stiffness, respectively. No significant difference was detected between the 2 reconstructions in terms of compressive and torsional stiffness (P > 0.05). However, the maximum pullout strength of long iliac screw group was significantly higher than the short screw group (P < 0.05).
The local stability is rather difficult to be restored to the original levels regardless the length of iliac screws. Obviously, long iliac screws resisted significantly greater axial pullout force. However, under physiologic, torsional, and compressive loading conditions, the mechanical stability of lumbo-pelvic fixation construct with short iliac screws was comparable with that of the long ones. Therefore, the use of short iliac screws, which are only about half the length of the long iliac screws, could reduce the implantation risk without significantly compromising on the stability of the construct.