# Biomedical Comparison of Salvage Pedicle Screw Augmentations Using Different Biomaterials
###### Abstract  
Alograft bone particles, hydroxyapatite/\(\beta\)-hydroxyapatite-tricalcium phosphate (HA/\(\beta\)-TCP), calcium sulfate (CS), and polymethylmethacrylate (PMMA) bone cement are biomaterials clinically used to fill defective pedicles for pedicle screw augmentation. Few studies have systematically investigated the effects of various biomaterials utilized for salvage screw stabilization. The aim of this study was to evaluate the biomechanical properties of screws augmented with these four different materials and the effect of different pilot hole sizes and bone densities on screw fixation strength. Commercially available synthetic bones with three different densities (7.5 pcf, 15 pcf, 30 pcf) simulating different degrees of bone density were utilized as substitutes for human bone. Two different pilot hole sizes (3.2 mm and 7.0 mm in diameter) were prepared on test blocks to simulate primary and revision pedicle screw fixation, respectively. Following separate specimen preparation with these four different filling biomaterials, a screw pullout test was conducted using a material test machine, and the average maximal screw pullout strength was compared among groups. The average maximal pullout strength of the materials, presented in descending order, was as follows: bone cement, calcium sulfate, HA/\(\beta\)-TCP, allograft bone chips and the control. In samples in both the 3.2 mm pilot-hole and 7.0 mm pilot-hole groups, the average maximal pullout strength of these four materials increased with increasing bone density. The average maximal pullout strength of the bone cement augmented salvage screw (7.0 mm) was apparently elevated in the 7.5 pcf test block. Salvage pedicle screw augmentation with allograft bone chips, HA/\(\beta\)-TCP, calcium sulfate, and bone cement are all feasible methods and can offer better pullout strength than materials in the non-augmentation group. Bone cement provides the most significantly augmented effect in each pilot hole size and bone density setting and could be considered preferentially to achieve larger initial stability during revision surgery, especially for bones with osteoporotic quality. p 2022 2022 2022 2022 2022 2022 2022 2022 2022 2022 2022 2022 2022 2022 2022 2022 2022 and tripled in the 1990s) and is expected to continue to increase [7]. A literature review reported a screw loosening rate of 0.6-11% and a breakage rate of 0.6-25% [8]. Moreover, the reported incidence of deep infection after lumbar instrumented fusion was 1.3-7.2% [9; 10], and the rate of concurrent screw loosening was up to 84% when a migrated interbody cage was noted [11]. Therefore, revision surgeries to correct failed instrumentation are progressively more common, and the surgical strategies for salvaging need to be handled more carefully. When revision surgery is performed under the circumstance of pedicle screw loosening, screw exchange with a larger length or greater diameter is often used initially [12; 13]. However, the increase in length and diameter has limitations because of the probability of vertebral anterior cortex penetration and pedicle fracture. There are some other strategies, including different directions of screw trajectory and pedicle screw augmentation using various biomaterials, to obtain adequate spinal stability and avoid extension of a fused segment [14; 15; 16; 17; 18]. Several biomaterials, such as allograft bone particles (ABP), hydroxyapatite/\(\beta\)-hydroxyapatite-tricalcium phosphate (HA/\(\beta\)-TCP), calcium sulfate (CS), and polymethylmethacrylate (PMMA) bone cement, were clinically used to fill the defective pedicle for pedicle screw augmentation. Allograft bone particles represent a suitable alternative to autogenous bone and are derived from humans as well. Allografts are osteoconductive and weakly osteoinductive (growth factors may still be present, depending on the processing). HA [Ca\({}_{10}\)(PO\({}_{4}\))\({}_{6}\)(OH)\({}_{2}\)] is the crystalline form of TCP.