Abstract
Study Design. A calcium triglyceride bone cement (CTBC) was compared with the gold-standard polymethylmethacrylate (PMMA) to assess the stability of augmented sacral screw fixation under cyclic loading.
Objective. To determine whether CTBC augmentation of a pedicle screw would provide a similar level of fixation in the S1 pedicles compared with PMMA augmentation.
Summary of Background Data. Numerous studies have shown the advantages of using PMMA to augment screw fixation; however, its biomechanical properties are not ideal. CTBC offers potential benefits such as being low exothermic, a modulus of elasticity closer to bone, and the potential for osteoconductivity, but its comparative performance in this situation has not been previously evaluated.
Methods. Six cadaveric sacra were used in this study; 3.0 mL volumes of PMMA (Simplex P) and CTBC (Kryptonite (TM) Bone Cement) were injected into contralateral screw tracts, with the screw immediately inserted after cement injection. After a 12-hour setting period, the sacrum was potted in a custom fixture and mounted to the frame of a materials testing machine. Alternating flexion and extension bending moments were applied at 1 Hz. Flexion moments were applied starting at 0.5 Nm and increased by 1 Nm after every 1000 cycles until the screw had reached 6 degrees of rotation relative to its starting position. Extension moments were maintained at 0.5 Nm. Screw rotation relative to bone was determined in real time by a custom optical tracking system and was analyzed using two-way repeated-measures analyses of variance (ANOVAs) and post hoc Student-Newman-Keuls tests (alpha = 0.05).
Results. To reach 6 degrees of screw rotation, the PMMA-augmented screw required more loading cycles (15,464 +/- 2526 vs. 10,277 +/- 1762 cycles; P = 0.006) and a larger applied moment (15.3 +/- 2.2 vs. 10.5 +/- 1.7 Nm; P = 0.010) than CTBC-augmented screw.
Conclusion. The PMMA augmentation provided increased resistance to cyclic loading compared with the CTBC augmentation for sacral pedicle screw fixation, but both augmentations well exceeded previously published findings for nonaugmented screws.