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Published on: February 10, 2014
Comparative assessment of sacral screw loosening augmented with PMMA versus a calcium triglyceride bone cement
Stewart D McLachlin1, Khalid Al Saleh, Kevin R Gurr
1Jack McBain Biomechanical Testing Laboratory, Thompson Engineering Building, The University of Western Ontario, London, Ontario, Canada.
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™ 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° 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 (α = 0.05).
Results:
To reach 6° 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.