[Biomechanical effects of sacrectomy extents on stability of lumbo-iliac fixation using iliac screw techniques]

To evaluate the effects of sacrectomy extent on the stability of lumbo-iliac fixation using single or dual iliac screw technique, and to determine which conditions require the dual iliac screw technique.

Nine fresh L2 to pelvic specimens were harvested from donated adult cadavers. After testing the intact state simulated by L3-5 pedicle screw fixation, sequential partial sacrectomies and L3 to iliac fixation using bilateral single iliac screw (Single) were conducted on the same specimen as follows: in group A, under S1 partial sacrectomy and Single; in group B, under 1/2 S1 partial sacrectomy and Single; in group C, one-side (left) sacroiliac joint resection and Single; in group D, total sacrectomy and Single; and in group E, the single iliac screw was replaced by dual iliac screws based on group D. Biomechanical testing was performed on a material testing machine under 0-800 N compression and 7-7 N x m torsion loading modes for construct stiffness evaluation.

The compressive stiffness of intact condition was (392 +/- 119) N/mm, groups A, B, C, D, and E obtained 106.4% +/- 9.5%, 102.7% +/- 8.0%, 92.2% +/- 10.1%, 72.7% +/- 8.0%, and 107.7% +/- 10.7% of intact condition, respectively. No significant differences were found among groups A, B, C, and the intact state (P > 0.05), however, the four groups showed significantly higher compressive stiffness than group D (P < 0.05). Although group E exhibited a comparable compressive stiffness with groups A, B, and intact state (P > 0.05), it displayed markedly higher compressive stiffness than groups C and D (P < 0.05). The torsional stiffness of intact state was (3.22 +/- 1.23) N x m/deg. Groups A, B, C, D, and E acquired 105.4% +/- 10.1%, 89.8% +/- 12.3%, 75.9% +/- 10.6%, 71.2% +/- 10.2%, and 109.1% +/- 16.9% of intact state, respectively. No significant differences were detected among groups A, B, E, and the intact state (P > 0.05). However, groups C and D showed remarkably lower torsional stiffness than groups A, E, and the intact state (P < 0.05). Importantly, group E offered remarkably higher torsional stiffness than group B (P < 0.05).

After under 1/2 S1 partial sacrectomy, single iliac screw technique could effectively restore local stability; whereas it could hardly provide adequate stability for further resection of one-side sacroiliac joint or total sacrectomy; in such situation, the use of dual iliac screw technique could obtain sufficient construct stability. Therefore, in the surgical treatment of sacral tumor, the dual iliac screw technique should be considered for the unstable conditions of total sacrectomy or under 1/2 S1 sacrectomy with one-side sacroiliac joint resection.