# Biomedical Comparison of Salvage Pedicle Screw Augmentations Using Different Biomaterials
## 4 Discussion  
When revision spine surgery was performed under various conditions, coexisting pedicle screw loosening was often encountered intraoperatively. Screw augmentation using different biomaterials is an important salvaging strategy to obtain adequate spinal stability and avoid extension of a fused segment. This study performed a biomechanical evaluation using four common biomaterials for screw augmentation to simulate primary and revision surgery with loosened pedicle screws. The results demonstrated that the mean maximal pullout strength after biomaterial filling was indeed increased compared with the control group, and bone cement offered the most significantly augmented effect in each pilot-hole size and bone-density condition. Moreover, the maximal pullout strength of different materials increased with increasing bone density. The strength of our study was that we simulated screw loosening in revision surgery using a 7.0 mm pilot hole and systematically investigated the effects of various biomaterials utilized for salvage screw stabilization, which has rarely been reported in the previous literature to our knowledge. From a clinical point of view, PMMA, HA/\(\beta\)-TCP and bone chips (from either autologous or allogenous bone) are materials that are available in a timely manner during surgery. Pedicle screw augmentation with PMMA can quickly provide good initial stability because the whole curing time is relatively low (of the order of 10 min or less). However, the risks of PMMA used for screw augmentation are cement leakage, which increases the difficulty and complexity of the next potential surgery. Commercial HA/\(\beta\)-TCP and ABP were also easily used during the surgery. We could use the prepared screw path with HA/\(\beta\)-TCP or ABP and then insert the screw. Although the initial augmentation via HA/\(\beta\)-TCP and ABP may not be as good as that of PMMA, HA/\(\beta\)-TCP and CS had osteoconductive properties that served scaffolds onto which bone cells can attach, migrate, and grow. CS has osteoconductive ability as well and can serve as a bone graft for bone defects, but the preparation time needed is approximately 30-45 min [22], which leads to limitations in spine surgery. In the present study, the mean maximal pullout strength in descending order was as follows: PMMA, CS, HA/\(\beta\)-TCP, ABP and control. PMMA and CS are paste-like materials that are distributed more homogeneously around the screw than particle-like materials and can offer increased stability. Among these materials, PMMA had the highest Young's modulus and mechanical strength, so the pullout strength was obviously higher than that of the other materials. Our findings were in line with other studies with similar procedures, e.g., there was a close correlation between bone mineral density and screw pullout strength [30; 31]. With the increase in the bone density, the mean maximal pullout strength of different materials increased in each group of different filling materials. This finding may be caused by the following: the area of the augmented screw\(-\)bone interface in the high mineral density bone was larger than that in the low mineral density bone. When comparing the difference in the pullout strength between the 3.2 mm and 7.0 mm pilot holes, we found that the mean maximal pullout strength in samples in the 7.0 mm group was similar (slightly larger or smaller) to that in the 3.2 mm group, which illustrated that pedicle screw augmentation with these materials was certainly helpful. Moreover, the mean maximal pullout strength of the PMMA-augmented salvage screw (7.0 mm) was apparently elevated in the 7.5 pcf test block. That is, PMMA bone cement augmentation should be considered to have more initial stability during revision surgery, especially in osteoporonic bone quality. [PERSON] et al. published a study [17] in which the authors compared pullout strength after pedicle screw augmentation with hydroxyapatite, calcium phosphate, and PMMA bone cement.