Abstract
Introduction
Pedicle screw pullout or loosening is increased in the osteoporotic spine. Recent studies showed a significant increase of pullout forces especially for PMMA-augmentation. With application of conventional viscosity PMMA the risk of cement extravasation is associated. This risk can be reduced by using radiofrequency-responsive, ultrahigh viscosity bone cement.
Method
11 fresh-frozen lumbar vertebral bodies (VB) from 5 cadavers were collected and freed from soft-tissue and ligaments. By DEXA scan (Siemens QDR 2000) 8 VB were identified as severely osteoporotic (BMD 0.8 g/cm3), 3 VB were above this level. Two screws (6×45 mm, WSI-Expertise Inject, Peter Brehm, Weisendorf, Germany) were placed in the pedicles. Through the right screw 3ml of radiofrequency-responsive bone cement (StabiliT® ER2 Bone Cement, DFine, Germany) were injected via hydraulic cement delivery system (StabiliT® Vertebral Augmentation System, DFine, Germany). As control group, left pedicle screws remained uncemented. After potting the whole VB in technical PMMA (Technovit 4004, Heraeus Kulzer, Germany) axial pullout test was performed by a material testing device (Zwick-Roell, Zmart-Pro, Ulm, Germany).
Results
The mean BMD of all specimen was 0.771 g/cm3 (min./max. 0.615/1.116, SD ± 0.170). Due to the definition of osteoporosis 8 specimens had a BMD lower than 0.8 g/cm3 (mean 0.677, min./max. 0.615/0,730, SD ±0.045). The non-osteoporotic group consisted of 3 specimens with a mean BMD of 1.020 g/cm3 (min/max 0.928/1.116, SD ±0.094). Overall we observed an increase in the mean axial pullout strength of 284% when using cement augmentated screws (non-cemented 385 N vs. cemented 1029 N, p 0,001). In the osteoporotic group the mean pullout force of the non-cemented screws was 407 N vs. 1022 N for the cemented screws (p 0.001). Similarly the pullout force rose in the non-osteoporotic group from 325 N for the non-cemented screws to 1048 N for the cemented screws p 0,001). All surgical procedures could be performed without technical problems.
Conclusion
This cadaver study demonstrates the efficacy and effectiveness of pedicle-screw augmentation with ultra-high viscosity cement. Pullout forces are significantly increased, especially in osteoporotic bone. No complications like clogged in cannulated pedicle-screws or extravasation of bone cement were observed.
In daily clinical routine radiation exposure to operator during cement delivery is reduced due to remote-controlled, automated delivery of radiofrequency-responsive bone cement. Furthermore availability of longer time to work with the cement (up to 30 min) is achieved; hectically injection or multiple-cement-mixing is not necessary anymore. The WSI-Expertise cannulated pedicle screws can be inserted and also augmented in a minimal-invasive technique. The PMMA then is injected through an extension adapter. PMMA augmentation of pedicle screws is already well established in spine surgery. With the combination of radiofrequency-responsive, ultra-high viscosity bone cement and cannulated pedicle screws this established procedure could become more controlled and safer for both patient and physician.
