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

Abstract

The development of iliac screws has provided a markedly easier way for spino-pelvic instrumentation than the classical Galveston technique. However, high rates of iliac screw loosening and breakage are usually reported, especially in cases where bilateral single iliac screws are used. Therefore, there is a need for exploring more stable iliac fixation techniques. Thus, the objective of this study was to compare the biomechanical effects of bilateral single and dual iliac screws on the stability of L3-iliac fixation construct under total sacrectomy condition. In this study, L2-pelvic specimens were harvested from seven fresh human cadavers. After biomechanically testing the intact state simulated by L3-L5 pedicle screw fixation, destabilization was introduced by total sacrectomy. Upon destabilization, L3-iliac screw-rod reconstructions were performed by four different techniques as follows: (1) bilateral single short iliac screws (Single-Short); (2) bilateral single long iliac screws (Single-Long); (3) bilateral dual short iliac screws, placed in the upper and lower iliac columns (Dual-UL); and (4) bilateral dual short iliac screws, all placed in the lower iliac column (Dual-Lower). These four iliac screw fixation techniques were sequentially preformed in the same specimen, and the lengths of the short and long iliac screws were 70 and 130 mm, respectively. Biomechanical testing was performed on a material testing machine under 800 N compression and 7 Nm torsion loading modes to evaluate the construct stiffness. In compression, the stiffness of the L3-iliac fixation constructs of Single-Short, Single-Long, Dual-UL, and Dual-Lower techniques were 73, 76, 98, and 108% of the intact state, respectively. No significant differences were detected between Single-Short and Single-Long (P = 0.589) techniques. However, the compressive stiffness of these two techniques was significantly lower than the intact state, and the Dual-UL and Dual-Lower techniques (P < 0.05). There was no statistical difference between the intact condition and the Dual-Lower technique (P = 0.109). Interestingly, Dual-Lower exhibited notably higher compressive stiffness than Dual-UL (+10.3%, P = 0.049). In torsion, the stiffness of Single-Short, Single-Long, Dual-UL, and Dual-Lower techniques were 72, 79, 105, and 109% of the intact condition, respectively. No significant differences were detected between Single-Short and Single-Long techniques (P = 0.338), and also among Dual-UL, Dual-lower techniques, and the intact state (P > 0.05). However, Single-Short and Single-Long techniques provided markedly lower construct torsional stiffness than the other three groups (P < 0.05). For lumbo-illiac reconstruction after total sacrectomy, even the use of bilateral single, long iliac screws do not help in restoring the local stability to the intact condition. However, dual iliac screws provide much higher construct stability than single iliac screw techniques. Therefore, dual iliac screw technique should be preferred for treating the unstable situation caused by total sacrectomy.

Fig. 1

Photograph of the experimental setup. The cranial and caudal sites of the specimen were mounted in polyester resin, by which the specimen was firmly clamped between the upper and lower grips of the MTS machine; insertion of the screws for reconstruction and the biomechanical testing were performed under the displacement control mode of the MTS machine

Fig. 2

X-ray images showing the four iliac screw techniques used for lumbo-iliac fixation after total sacrectomy in this study. Single-Short bilateral single short iliac screws, Single-Long bilateral single long iliac screws, Dual-UL bilateral dual short iliac screws (each placed in the upper and lower iliac columns), Dual-Lower bilateral dual short iliac screws (all placed in the lower iliac column)

Fig. 3

Sketch showing the three tracts used for iliac screw insertion in this study. Tract A (red color): the passage from the inferior insertion point of PSIS to AIIS; Tract B (yellow color): the passage from the superior entry site of PSIS towards AIIS; Tract C (blue color): the passage from the superior entry point directing to the point of 10 mm below the highest portion of the iliac crest

Fig. 4

Two representative load–displacement curves plotted using the data from the 4th loading cycle. The range of motion (ROM) was defined as the linear (a) or angular (b) displacement under the maximum load

Fig. 5

Histogram showing the mean values and standard deviations of compressive stiffness for the five types of lumbo-iliac constructs: Intact, Single-Short, Single-Long, Dual-UL, and Dual-Lower. *P < 0.05 in comparison with Single-Short and Single-Long; ^#P < 0.05 in comparison with Dual-UL

Fig. 6

Histogram showing the mean values and standard deviations of torsional stiffness for the five types of lumbo-iliac constructs: Intact, Single-Short, Single-Long, Dual-UL, and Dual-Lower. *P < 0.05 in comparison with Single-Short and Single-Long