PMMA-augmentation of the spinous process as an enhancing-protective measure against bone failure in "through the spinous process-vertebropexy".

BACKGROUND: Vertebropexy, a semi-rigid spinal stabilization technique, utilizes the spinous process(SP) as an anchor point for stabilizing tendon-grafts or flexible cerclages. In its primary form, it entailed drilling into the bone of 2 adjacent SPs and threading the materials through the holes. Biomechanical studies have identified the SP as the weakest part of the vertebrae, while cadaveric studies have demonstrated a higher bone failure rate with osteoporosis. We investigated whether cement augmentation of the SP could enhance the biomechanical strength and reduce the fracture-risk in the setting of first-generation Vertebropexy.

METHODS: Following computed tomographic analysis and measurement of the bone mineral density, 12 lumbar segments were divided in 2 groups (Osteoporotic/ Nonosteoporotic) and then fixed in custom-made 3D-printed clamps. The SPs of 6 segments underwent cement augmentation (PMMA-Group), and a CT scan confirmed adequate augmentation. The other 6 segments remained uncemented. (Native-Group). Posterior decompression, drilling, and instrumentation with bovine tendons were then conducted. Torque-to-failure stress tests were performed on a biaxial static testing machine.

RESULTS: The cement-augmentation of the SP significantly increases the torque-to-failure in flexion (p=.00037/ Median & IQR: 13.0 & 5.2 Nm in the Native-Group vs. 26.5 & 11.1 Nm in the PMMA-Group), regardless of the bone quality (p=.008). A statistically significant difference in torque-to-failure between Osteoporotic and NonOsteoporotic groups inside the PMMA and Native groups was determined (p=.015 and p=.025, respectively). A statistically significant correlation between bone density and failure torque was not detected in this cohort, possibly due to the limited sample size (Spearman 0.276, p=.192). A comparison between the torque-to-failure of the Native-NonOsteoporotic SPs and that of the PMMA-Osteoporotic showed no statistical significance (p=.240).

CONCLUSIONS: Based on the findings of this small-sample cadaveric study, cement-augmentation of the spinous processes can multiply the torque-to-failure/fracture in both osteoporotic and nonosteoporotic conditions and may be used as a salvage technique in first-generation vertebropexy procedures that compromise the spinous process.