[Choice of bone cement augmentation techniques when sacral pedicle screw loosening]

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.

Eleven sacral 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 sacral that showed tumor or inflammatory or any other anatomic abnormalities. Following the measurement of bone mineral density, five sacral screw fixations were sequentially established 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.

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 pullout strength was significantly higher in groups B, C, and E than in groups A and D (P < 0.05), and in group C than in groups B and E (P < 0.05). There was no significant difference in pull-out strength between groups A and D, and between groups B and E (P > 0.05).

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.