# Biomedical Comparison of Salvage Pedicle Screw Augmentations Using Different Biomaterials
## 4 Discussion
The mean pullout strength, compared with that of controls, increased by 12.5% in HA-augmented screws (\(p\) = 0.600) and by 14.9% in calcium phosphate-augmented hemivertebrae (\(p\) = 0.234). The pullout strength of polymethylmethacrylate-versus hydroxyapatite-augmented pedicle screws was 60.8% higher (\(p\) = 0.028). Even though the pullout strength of HA was not significant, they concluded that HA was likelya better clinical alternative to PMMA, as HA augmentation, unlike PMMA augmentation, stimulates bone growth and can be revised. In our opinion, both HA (or bone chips) and PMMA augmentation are feasible methods that depended on intraoperative screw purchase, the etiology of screw loosening, and surgical planning. For example, patients who are young or concurrent with spine infection should not undergo bone cement augmentation due to a long life expectancy and foreign body consideration. For patients who simultaneously underwent anterior surgery with additional anterior support, the requirement for the strength of the pedicle screw fixation was not as strict as those who underwent posterior revision surgery alone. [PERSON] et al. [32] investigated pedicle screw fixation augmented with allograft bone particles (ABPs) in osteoporortic vertebrae. Their results showed that trajectory augmentation with allograft bone particles can significantly increase the strength of the augmented screws. Full-trajectory augmentation was suggested compared with half-trajectory augmentation. The authors also analyzed proper ABP size and recommend using 1 mm ABP (0.3 g ABP for a 5.5 mm \(\times\) 40 mm screw and 0.5 g ABP for a 6.5 mm \(\times\) 45 mm screw) for augmentation. In addition, the optimal filling volume of the bone cement was 75% of the trajectory volume (approximately 1.03 mL) for osteoporortic vertebrae in the primary surgery, which was suggested by [PERSON] et al. [33]. Rescue augmentation with cement was studied by [PERSON] et al. [34], who reported that the fatigue load reached 207 \(\pm\) 75 N for the non-augmented screws, 301 \(\pm\) 96 N for the initial cement augmentation, and 370 \(\pm\) 87 N for the rescue augmentation. The volume of cement used for the rescue augmentation ranged between 2 and 5 mL, depending on the radiological assessment of the distribution. They concluded that cement augmentation of initially loosened pedicle screws was a promising option to restore adequate screw stability. Inevitably, this study had some limitations. The first limitation was that the homogenous bone density of the test blocks cannot represent the real structure of the vertebrae in the clinic. However, the test blocks were made of uniform polyurethane foam, which reduced the influence of the variability of properties and the morphometry of cadaveric bones and provided an effective platform to compare the mechanical characteristics of various filling materials in different degrees of bone mineral density. The second limitation was that ABP, HA/\(\beta\)-TCP, and CS were absorbable and osteoconductive biomaterials. In the process of bone formation and remodeling, the effects of these osteoconductive biomaterials on the mechanical properties of screws may change. Thus, this issue should be explored through further animal experiments. Third, only one brand of bone cement and one design of pedicle screws were used in this study. This was because the purpose of this experiment focused on comparing the characteristics of filling biomaterials, so the influence of different bone cement and screw designs was not investigated in this study. Finally, only a static loading (screw pullout test in synthetic bone) was conducted without considering other physiological loading modes. In actual physiological conditions, the screw/biomaterials/bone interfaces are subjected to complex dynamic loadings, which may have an impact on long-term screw-fixation strength due to the effects of fitting and stress shielding.