Do Iliac Screws Placed Close to the Sciatic Notch Have Greater Pullout Strength?

BACKGROUND AND

OBJECTIVES

Optimal iliac screw position in relation to the sciatic notch remains unknown. In 12 cadavers undergoing S2 alar-iliac (S2AI) screw placement, we tested the pullout strength of screws placed in proximity to the sciatic notch (≤5 mm) vs farther away from the sciatic notch (>5 mm).

METHODS

A biomechanical, cadaver-based study was performed on 12 cadavers undergoing bilateral S2AI screw insertion. The position of the S2AI screw regarding the sciatic notch was dichotomized as ≤5 mm from the sciatic notch on the right side and >5 mm on the left side, confirmed using c-arm fluoroscopy. The primary outcome was the pullout strength of the screw (N). Secondary outcomes were stiffness (N/mm), yield force (N), and work to failure (N mm). Ischial tuberosity was embedded into polymethyl methacrylate and secured to a custom 3-axis vise grip mounted to a 14.5-kN load cell. Pullout testing was performed at 5 mm/min. Force and displacement data were collected at 100 Hz and evaluated using MATLAB. The Mann-Whitney test was performed.

RESULTS

Of 24 S2AI screws, 3 screws could not be tested because of cement-bone interface failure. A positive though nonsignificant trend of screw pullout strength was found for screws close to the notch compared with those farther from the notch (861.8 ± 340.7 vs 778.7 ± 350.8 N, P = .859). Similarly, screws close to the notch demonstrated a higher trend of stiffness (149.4 ± 145.4 vs 111.34 ± 128.2 N/mm, P = .320) and force to yield (806.9 ± 352.0 vs 618.6 ± 342.9 N, P = .455). Conversely, screws farther from the notch had a higher but similarly nonsignificant area under the force-displacement curve (10 867.0 ± 9565.0 vs 14 196.6 ± 9578.3 N mm, P = .455), which might be due to excess sheer/translation force that could not be reliably quantified.

CONCLUSION

Although placing S2AI screws ≤5 mm of the sciatic notch provided stronger fixation in 3 of 4 biomechanical testing categories, these results were not statistically significant. Therefore, placing S2AI screws ≤5 mm of the sciatic notch did not provide stronger fixation.