Objective To biomechanically compare the maximum pull-out strengths among two pedicle screws and three salvage techniques using poly methylmethacrylate (PMMA) augmentation in osteoporotic sacrum, and to determine which PMMA augmentation technique could serve as the salvage fixation for loosening sacral pedicle screws. Methods Eleven sacra were harvested from fresh adult donated cadavers, aged from 66 to 83 years (average 74.4 years) and included 5 men and 6 women. Radiography was used to exclude sacra that showed tumor or inflammatory or any other anatomic abnormal ities. Following the measurement of bone mineral density, five sacral screw fixations were sequentially establ ished on the same sacrum as follows: unicortical pedicle screw (group A), bicortical pedicle screw (group B), unicortical pedicle screw with the traditional PMMA augmentation (group C), ala screw with the traditional PMMA augmentation (group D), and ala screw with a kyphoplasty-assisted PMMA augmentation technique (group E). According to the sequence above, the axial pull-out test of each screw was conducted on a MTS-858 material testing machine. The maximum pull-out forces were measured and compared. The morphologies of PMMA augmented screws after being pulled-out were also inspected. Results The average bone mineral density of 11 osteoporotic specimens was (0.71 ± 0.08) g/cm2 . By observation of the pull-out screws, groups C, D, E showed perfect bonding with PMMA, and group E bonded more PMMA than groups C and D. The maximum pull-out forces of groups A, B, C, D, and E were (508 ± 128), (685 ± 126), (846 ± 230), (543 ± 121), and (702 ± 144) N, respectively. The maximum pull-out strength was significantly higher in groups B, C, and E than in groups A and D (P lt; 0.05), and in group C than in groups B and E (P lt; 0.05). There was no significant difference in pull-out strength between groups A and D, and between groups B and E (P gt; 0.05). Conclusion For sacral screw fixation of osteoporotic patients with bone mineral density more than 0.7 g/cm2, bicortical pedicle screw could acquire significantly higher fixation strength than the unicortical. Once the loosening of pedicle screw occurs, the traditional PMMA augmentation or ala screw with kyphoplasty-assisted PMMA augmentation may serve as a suitable salvage technique.
Objective To evaluate the effects of sacrectomy extent on the stabil ity of lumbo-il iac fixation using single or dual il iac screw technique, and to determine which conditions require the dual il iac screw technique. Methods 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 il iac fixation using bilateral single il iac screw (Single) wereconducted 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) sacroil iac joint resection and Single; in group D, total sacrectomy and Single; and in group E, the single il iac screw was replaced by dual il iac screws based on group D. Biomechanical testing was performed on a material testing machine under 0-800 N compression and —7-7 N•m torsion loading modes for construct stiffness evaluation. Results 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 gt; 0.05), however, the four groups showed significantly higher compressive stiffness than group D (P lt; 0.05). Although group E exhibited a comparable compressive stiffness with groups A, B, and intact state (P gt; 0.05), it displayed markedly higher compressive stiffness than groups C and D (P lt; 0.05). The torsional stiffness of intact state was (3.22 ± 1.23) N•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 gt; 0.05). However, groups C and D showed remarkably lower torsional stiffness than groups A, E, and the intact state (P lt; 0.05). Importantly, group E offered remarkably higher torsional stiffness than group B (P lt; 0.05). Conclusion After under 1/2 S1 partial sacrectomy, single il iac screw technique could effectively restore local stabil ity; whereas it could hardly provide adequate stabil ity for further resection of one-side sacroil iac joint or total sacrectomy; in such situation, the use of dual il iac screw technique could obtain sufficient construct stabil ity. Therefore, in the surgical treatment of sacral tumor, the dual il iac screw technique should be considered for the unstable conditions of totalsacrectomy or under 1/2 S1 sacrectomy with one-side sacroil iac joint resection.